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Influence of Ammonia Stripping Parameters on the Efficiency and Mass Transfer Rate of Ammonia Removal

Influence of Ammonia Stripping Parameters on the Efficiency and Mass Transfer Rate of Ammonia... applied sciences Article Influence of Ammonia Stripping Parameters on the Efficiency and Mass Transfer Rate of Ammonia Removal Eun Ju Kim , Ho Kim and Eunsil Lee * Center for Plant Engineering, Institute for Advanced Engineering, Yongin-si 17180, Korea; ejkim@iae.re.kr (E.J.K.); kimh0505@iae.re.kr (H.K.) * Correspondence: les0302@iae.re.kr Abstract: This study analyzed the influence of different ammonia stripping parameters on ammonia removal efficiency and mass transfer rate. Ammonia stripping was performed on two devices, a column and a packed tower, with artificial ammonium hydroxide wastewater. First, ammonia concentration and pH were varied in a column without liquid circulation. At the same pH, the removal efficiency and mass transfer rate were constant, irrespective of initial ammonia concentration. When pH was increased, the ammonia fraction also increased, resulting in higher removal efficiency and mass transfer rate. Second, the effects of stripping were assessed using a packed tower with fluid circulation. The ammonium hydroxide concentration did not affect the removal efficiency or mass transfer rate. Furthermore, at apparatus liquid-gas ratios of 26.8–107.2 L/m , a lower liquid-gas ratio led to increased ammonia removal efficiency and mass transfer rate. Conversely, the lower the liquid-gas ratio, the greater the air consumption. In conclusion, considering the removal rate and volume of air supply, the range of optimal liquid-gas ratio was determined as 26.8–53.6 L/m . In particular, the 26.8 L/m condition achieved the best ammonia removal rate of 63.0% through only 6 h of stripping at 70 C and pH 8.5. Keywords: wastewater treatment; nitrogen removal; ammonia stripping; air stripping; stripping tower Citation: Kim, E.J.; Kim, H.; Lee, E. Influence of Ammonia Stripping Parameters on the Efficiency and 1. Introduction Mass Transfer Rate of Ammonia With industrial development and population growth, there has been a rapid increase Removal. Appl. Sci. 2021, 11, 441. in the amount of wastewater containing ammonia from various sources, including sewage https://doi.org/ sludge, cattle excrement, food waste, and biomass [1]. Ammonia containing wastewater is 10.3390/app11010441 also generated in the energizing process of waste resources [2,3]. Ammonia in wastewater should be removed because it causes eutrophication and ecotoxicity when it leaks into Received: 10 November 2020 ecosystems [4,5]. Accepted: 31 December 2020 There are various methods for removing ammoniacal nitrogen from solution, including Published: 5 January 2021 physical, electronic, and biological methods. Ammonia stripping is a particularly efficient Publisher’s Note: MDPI stays neu- method for ammonia recovery from high concentration wastewater [6,7], In ammonia tral with regard to jurisdictional clai- stripping system, ammoniacal nitrogen is removed as a gas by supplying gas such as air ms in published maps and institutio- or steam [6–9]. The reactor is usually a fixed-bed column or packed bed tower [6–9]. In nal affiliations. tower-type reactors, ammonia mass transfer is achieved by gas-liquid contact within the packing material [10]. Therefore, the liquid-gas ratio should be carefully considered to optimize the operating conditions of packed towers. In this regard, substantial research and development have focused on the operating Copyright: © 2021 by the authors. Li- conditions and other factors affecting stripping towers for different types of wastew- censee MDPI, Basel, Switzerland. ater [11–16]. For example, Ferraz et al. [12] performed ammonia stripping from land- This article is an open access article fill leachate using a packed tower at room temperature and pH 11 and achieved a re- distributed under the terms and con- moval efficiency of 98% after 24 h of stripping at a liquid-gas ratio (L/G) of 6.7–20 L/m . ditions of the Creative Commons At- Guštin et al. [13] used a continuous flow packed tower to strip ammoniacal nitrogen from tribution (CC BY) license (https:// anaerobic digestate at a liquid-gas ratio of 2.5 L/m , achieving a removal efficiency of 55% creativecommons.org/licenses/by/ at 50 C and pH 10. Liu et al. [14] evaluated the ammonia removal efficiency and rate 4.0/). Appl. Sci. 2021, 11, 441. https://doi.org/10.3390/app11010441 https://www.mdpi.com/journal/applsci Appl. Sci. 2021, 11, 441 2 of 13 for the urine with temperature, pH, concentration, and liquid-gas ratio. Zhu et al. [15] and Li et al. [16] also analyzed ammonia removal rate by temperature, pH and air supply condition. Conventional ammonia stripping processes are typically performed at a temperature between room temperature and 50 C and in a pH range of 10–12 [11–16]. These process require a large amount of reagents to adjust the pH [13,17]. In the ammonia stripping process, a basic reagent is used to increase the pH. After ammonia removal, the pH must be adjusted to neutral for wastewater treatment and discharge. One method for reducing the use of these reagents is high-temperature stripping; at a high temperature, the same removal efficiency can be achieved with a smaller quantity of reagent [17]. Although there is a problem that a heat is required for high-temperature stripping, the heat energy consumption can be minimized by using waste heat. For example, the hydrothermal thermal carbonization (HTC) reaction is carried out at 180–260 C [18,19], and it is possible to raise the temperature of the HTC wastewater up to 70–80 C by using HTC heat. In addition, the high-temperature process can be operated more economically by reducing the air supply [8]. The purpose of this study is to design a high-temperature stripping tower and derive the optimal operating factors that will reduce the use of reagents and air supply while maintaining the ammonia fraction at an operating temperature of 70 C. To this end, the ammonia removal efficiency and mass transfer coefficient are assessed at different temperatures, pH, ammonium hydroxide concentration, and airflow volume. Specifically, by deriving the optimal liquid-gas ratio and air supply volume for efficient liquid-to-gas contact in the packing layer, this study provides useful data on the key factors in the design of high-temperature ammonia stripping devices. 2. Theoretical Considerations 2.1. Ammonia–Water System In aqueous solution, total ammonia exists as both free ammonia (NH ) and ammonium ions (NH ), as shown in Equation (1), and the water is dissociated into ions, as shown in Equation (2) [10]. NH + H O $ NH + OH , (1) 3 2 H O $ H + OH , (2) The total ammonia concentration in the solution is expressed by Equation (3). [NH ] = [NH ] + NH , (3) 3,L 3,L where [NH ] , [NH ], and NH mean the molar concentration (mol/L) of total 3,L 3,L ammonia, free ammonia, and ammonium ion, respectively. The ionization constants for water (K ) and ammonia (K ) are known in Equations (4) H O NH 2 3 and (5), respectively. K = H OH , (4) H O NH [OH ] K = , (5) NH [NH ] 3,L where H and OH are the molar concentration (mol/L) of hydronium ion and hydrox- [ ] [ ] ide ion, respectively. In addition, the ionization constants are expressed as a function of absolute tempera- ture (K), as shown in Equations (6) and (7) [6,10,11,20]. ln K = 140.932 13445.9/T 22.4773 ln T, (6) H O ln K = 97.976 5930.7/T 15.063 ln T 0.01127T, (7) NH 3 Appl. Sci. 2021, 11, 441 3 of 13 2.2. Ammonia Stripping In aqueous solution, the free ammonia can be stripped through transfer from liquid to gas. Thus, the concentration ratio of the free ammonia to total ammonia is a major factor in stripping and is defined as the ammonia fraction ( f ) in Equation (8) [13]. NH [NH ] 3,L f =   , (8) NH [NH ] + NH 3,L The ammonia fraction is also expressed by Equation (9) from Equations (4) and (5). H O f = =  , (9) NH 3 + pH K + K [H ] 1 + K /K 10 H O NH NH H O 2 3 3 2 Since the ionization constants is described as a function of absolute temperature in Equation (6), the ammonia fraction is dependent on both temperature and pH. The percentage of ammonia removal (h) is calculated as the amount of ammonia removed compared to the initial ammonia and is expressed as Equation (10): T T [NH ] [NH ] 3,L 3,L 0 t h = , (10) [NH ] 3,L 2.3. Mass Transfer of Ammonia In the process of ammonia stripping, the mass transfer rate of ammonia (F ) from NH liquid to gas phase is given by Equation (11) [14,21,22]. [NH ] 3,L F = V , (11) NH L dt where t is time and V is the total volume of liquid. In a batch reactor, the mass transfer rate for liquid ammonia to gas is defined by Matter-Mueller, as shown in Equation (12) [14,22]: K aV L, NH L F = Q H [NH ] 1 exp , (12) NH G NH 3,L 3 3 Q H NH where Q is the gas flow rate and H is the dimensionless Henry’s law constant. K G NH L, NH 3 3 is the mass transfer coefficient of liquid ammonia. The interfacial area per unit volume between liquid and gas is indicated by a. Combining Equations (8), (10) and (12) and integrating the total ammonia concentra- tion over time gives Equation (13): [NH ] Q H f K aV 3,L G NH NH L, NH L t 3 3 3 ln = 1 exp t, (13) V Q H L G NH [NH ] 3,L 3 The slope for the logarithm of ammonia concentration ratio over time is then given by Equation (14): Q H f K aV G NH NH L, NH L 3 3 3 slope = 1 exp , (14) V Q H L G NH From Equations (13) and (14), the overall mass transfer coefficient for liquid ammonia to gas is calculated as follows: ( !) Q H [NH ] G NH 3,L 3 t K a = ln 1 + ln , (15) L, NH V Q H f t L G NH NH [NH ] 3 3 3,L 0 Appl. Sci. 2021, 11, 441 4 of 13 Q H G NH K a = ln 1  (slope) , (16) L, NH V Q H f L G NH NH Appl. Sci. 2021, 11, x FOR PEER REVIEW 4 of 13 3 3 The main variables in ammonia stripping are the ammonia fraction, the rate of change in the ammonia concentration, the volume of gas supplied, and the overall mass transfer coefficient according to the type of reactor as shown in Equations (12) and (14). Hence, transfer coefficient according to the type of reactor as shown in Equations (12) and (14). in this study, we analyzed the effect of the major stripping factors—ammonia concentra- Hence, in this study, we analyzed the effect of the major stripping factors—ammonia con- tion, fraction and gas flow rate—on the ammonia removal efficiency and mass transfer centration, fraction and gas flow rate—on the ammonia removal efficiency and mass coefficient. transfer coefficient. 3. Materials and Methods 3. Materials and Methods 3.1. Preparation 3.1. Preparation In this study, air stripping experiments were conducted and stripping factors were In this study, air stripping experiments were conducted and stripping factors were evaluated for high ammonia concentration wastewater such as HTC or anaerobic liq- evaluated for high ammonia concentration wastewater such as HTC or anaerobic liquids uids [1–4,18,19,23]. These wastewaters contain ammonia up to 4000 mg/L [1,3,4]. To [1–4,18,19,23]. These wastewaters contain ammonia up to 4000 mg/L [1,3,4]. To simulate simulate artificial wastewater, 1000–3700 mg/L ammonia solution was prepared by dilut- artificial wastewater, 1000–3700 mg/L ammonia solution was prepared by diluting a 28– ing a 28–30% ammonia solution (SAMCHUN) with tap water. To evaluate the physical 30% ammonia solution (SAMCHUN) with tap water. To evaluate the physical degassing degassing properties of ammonia, the effects on trace amounts of nitrate, nitrite were properties of ammonia, the effects on trace amounts of nitrate, nitrite were excluded. excluded. 3.2. 3.2. Stri Stripping ppingColumn Column A column-type ammonia stripping device was configured as shown in Figure 1. The A column-type ammonia stripping device was configured as shown in Figure 1. The capacity capacity of of the the column was 2 L, a column was 2 L, and nd a di a disk-type sk-type air air dif difuser ffuser was was in installed stalled at at the the bottom, bottom, thr through wh ough which ich extern external al air could be air could be supplied. suppliThe ed. The air r equir air required ed for stripping for stripping was contr was con- olled at a pressure of 30 Pa using a blower and regulator, and a flow meter was used to control trolled at a pressure of ≤30 Pa using a blower and regulator, and a flow meter was used to the volume. To control the pH of the solution, NaOH solution was supplied using an auto control the volume. To control the pH of the solution, NaOH solution was supplied using titration system (905 Titrando, Metrohm AG, Switzerland). Stripping was performed on an auto titration system (905 Titrando, Metrohm AG, Switzerland). Stripping was per- 1 L of artificial wastewater, and the air flow rate was set to 900 L/h. The stripping factors formed on 1 L of artificial wastewater, and the air flow rate was set to 900 L/h. The strip- were assessed while varying the ammonia concentration and the pH of the wastewater, ping factors were assessed while varying the ammonia concentration and the pH of the under the conditions shown in Table 1. wastewater, under the conditions shown in Table 1. Figure 1. Schematic view of the ammonia stripping column. Figure 1. Schematic view of the ammonia stripping column. Table 1. Experimental conditions of NH3 stripping using a stripping column. Temperature pH Initial NH3 Concentration (°C) (-) (mg/L) 8.9 2900 9.4 1500–2900 10.2 2900 10.8 2900 Appl. Sci. 2021, 11, 441 5 of 13 Table 1. Experimental conditions of NH stripping using a stripping column. Temperature pH Initial NH Concentration ( C) (-) (mg/L) 8.9 2900 9.4 1500–2900 Appl. Sci. 2021, 11, x FOR PEER REVIEW 5 of 13 10.2 2900 10.8 2900 3.3. Packed Tower 3.3. Packed Tower Figure 2 shows the packed tower system constructed to determine the effect of the Figure 2 shows the packed tower system constructed to determine the effect of the liquid-gas contact ratio on the stripping factors. The tower was designed with a diameter liquid-gas contact ratio on the stripping factors. The tower was designed with a diameter of 0.2 m and a height of 1.5 m to enable observation of the physical characteristics of NH3 of 0.2 m and a height of 1.5 m to enable observation of the physical characteristics of air stripping at the bench scale. The precise specifications are shown in Table 2. The inter- NH air stripping at the bench scale. The precise specifications are shown in Table 2. The nal packing material consisted of 18 mm PP pall rings, and the height of the packing was internal packing material consisted of 18 mm PP pall rings, and the height of the packing 0.48 m. The maximum capacity of the tank was 147 L; the effective capacity for the strip- was 0.48 m. The maximum capacity of the tank was 147 L; the effective capacity for the ping experiment was set to 100 L. The stripping temperature was set to 70 °C, and a heater stripping experiment was set to 100 L. The stripping temperature was set to 70 C, and a in the tank and a heat gun were used to control the stripping temperature. Air was sup- heater in the tank and a heat gun were used to control the stripping temperature. Air was plied using the heat gun into the bottom of the tower and was emitted from the top of the supplied using the heat gun into the bottom of the tower and was emitted from the top tower. The flow rate of air was controlled using a valve and measured through a flow of the tower. The flow rate of air was controlled using a valve and measured through a meter. The liquid was supplied into the top of the tower from the tank using a pump; after flow meter. The liquid was supplied into the top of the tower from the tank using a pump; spraying through a nozzle, the liquid passes through the packing layer, then cycled back after spraying through a nozzle, the liquid passes through the packing layer, then cycled back into the ta into the nk. tank. The cycli Then cycling g rate of the rate of liq the uid liquid and the flo and the w rate flow of the rate of gas the are gas specified are specified along along with twith he otthe her operat other operating ing condit conditions ions in Tabl ine Table 3. A te 3. mperature A temperatur -adjusted pH e-adjusted se pH nsor was sensor was instal ins letalled d inside the t inside the ank to moni tank to monitor tor the pH the an pH d t and emp temperatur erature, and e, t and he N the aOH NaOH solusolution tion was was suppli supplied ed into the ta into thenk using a tank using pump t a pumpoto co contr ntrol the pH. Ammonia ol the pH. Ammonia stripping stripping using using the the packed tower was performed at 70 C and pH 8.5 (NH fraction 76.1%). To ascertain the packed tower was performed at 70 °C and pH 8.5 (NH3 fraction 76.1%). To ascertain the optimal operating conditions for the tower, the liquid flow rate was fixed at 117.0 L/h, and optimal operating conditions for the tower, the liquid flow rate was fixed at 117.0 L/h, and the stripping efficiency was assessed according to the liquid-gas ratio by varying the gas the stripping efficiency was assessed according to the liquid-gas ratio by varying the gas flow rate. flow rate. Figure 2. Schematic view of the ammonia stripping packed tower system. Figure 2. Schematic view of the ammonia stripping packed tower system. Table 2. Design parameters of the stripping tower. Design Parameter Value Unit Tower diameter 0.2 m Tower height 1.5 m Packed layer height 0.48 m Packed materials PP pall ring - Liquid tank volume 147 L Appl. Sci. 2021, 11, 441 6 of 13 Table 2. Design parameters of the stripping tower. Design Parameter Value Unit Tower diameter 0.2 m Tower height 1.5 m Packed layer height 0.48 m Packed materials PP pall ring - Liquid tank volume 147 L Table 3. Experimental conditions of NH stripping using a packed tower. Initial NH Conc. Liquid Flow Rate Gas Flow Rate Liquid-Gas Ratio, L/G (mg/L) (L/h) (L/h) (L-Liquid/m -Gas) 1100–3700 117.0 26,200 4.5 1100 117.0 4367 26.8 1100 117.0 2183 53.6 1100 117.0 1092 107.2 3.4. Analysis The ammonia concentration in the aqueous solution was analyzed to assess the stripping efficiency. Solution samples of 10 mL were taken from stripping column and packed tower tank at regular time intervals. All samples were treated with sulfonic acid to pH 2.5, and the concentrations of ammonium ions were quantified using ion chromatography (930 Compact IC Flex, Metrohm AG, Herisau, Switzerland) equipped with a cation separation column (Metrosep C 4 150/4.0, Metrohm AG, Herisau, Switzerland). 4. Results 4.1. Air Stripping without Liquid Circulation Ammonia stripping was performed using the stripping column in Figure 1. In the column reactor, the contact surface between liquid and gas is the interface of the air bubble [14]. The mass transfer interfacial area is related to the size and number of air bubbles. Therefore, it is possible to compare the mass transfer coefficient in the constant conditions: total liquid volume; air flow rate; temperature [14]. As an experimental condition, the airflow rate was constant of 900 L/h on 1 L of artificial wastewater. The stripping efficiency and mass transfer rate of ammonia were analyzed according to initial concentration and pH. Ammonia removal efficiency and mass transfer coefficient were calculated by Equations (10), (15), and (16). Figure 3 shows the results of stripping at different ammonium hydroxide concentra- tions at 20 C and pH 9.4. Figure 3a shows the ammonia removal rate according to the concentration. The removal rate increased with increasing time. After 3 h, similar results of 38.4%, 38.5%, and 38.3% were observed for concentrations of 1500, 2200, and 2900 mg/L, respectively. Figure 3b shows the natural logarithm of the relative change in total ammonia con- centration over time. A linear regression analysis was conducted against time; the mean slope was 0.16  20.011. The R values were 0.980, 0.973, and 0.996 for each respective concentration, indicating that the results were reliable. The overall volumetric mass transfer coefficients on the liquid phase (K a) were calculated from the slope of Figure 3a and L, NH Equation (16). The mean K a was 0.319 h , which remained constant regardless of L, NH the initial ammonia concentration in the solution. Therefore, the ammonium hydroxide concentration had little effect on the removal efficiency or rate. The change of pH in the ammonia solution causes a change of ammonia fraction, which means that the higher pH, the more ammonia can be removed. pH is an important parameter affecting the stripping efficiency and mass transfer coefficient. Appl. Sci. 2021, 11, x FOR PEER REVIEW 7 of 13 Appl. Sci. 2021, 11, 441 7 of 13 100 1.6 o o 2 290 900 mg/L 0 mg/L 20 C, pH 9.4 20 20 C, pH C, pH 9.4 9.4 1.4 2 220 200 mg/L 0 mg/L Air flow rate = 900 L/h 80 A Ai ir f r fl lo ow w rate = 90 rate = 900 0 L L/ /h h 1 150 500 mg/L 0 mg/L 1.2 Initial NH slope K a 3 LNH3 1.0 (mg/L) (/h) (/h) 0.8 2900 0.173 0.309 2200 0.156 0.307 0.6 1500 0.158 0.340 0.4 0.2 0 0.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 0.00.5 1.01.5 2.02.5 3.0 Tim Time e (h (h) ) Time (h) (b) (a) Figure 3. Ammonia stripping performance according to initial concentration at 20 °C and pH 9.4: (a) Ammonia removal Figure 3. Ammonia stripping performance according to initial concentration at 20 C and pH 9.4: (a) Ammonia removal rate; (b) logarithm of the ammonia concentration ratio. rate; (b) logarithm of the ammonia concentration ratio. Figur The chang e 4 repre esents of pH the in t results he am of mo ammonia nia soluti stripping on causes accor a change ding to ofpH amat mo 2900 nia fr mg/L action, ammonia concentration and 20 C. The ammonia removal rate was shown as a linear which means that the higher pH, the more ammonia can be removed. pH is an important increase over time; after 2.5 h of stripping, the removal rate increased from 13.9% to 72.6% parameter affecting the stripping efficiency and mass transfer coefficient. when pH Figu was re 4 incr repr eased esents the r from 8.9 esults o to 10.8. f ammo To analyze nia stripping a the influence ccording of to pH pH onat ammonia 2900 mg/L removal, the ratio of removal rate to ammonia fraction (h/ f ) was compared. h/ f ammonia concentration and 20 °C. The ammonia removal rate was shown as a linear in- NH NH 3 3 was approximately constant for pH 9.4–10.8; The ratios were 72.0%, 71.2%, and 75.2% at crease over time; after 2.5 h of stripping, the removal rate increased from 13.9% to 72.6% pH 9.4, 10.2 and 10.8, respectively. From these results, it was confirmed that the ammonia when pH was increased from 8.9 to 10.8. To analyze the influence of pH on ammonia removal rate was linearly affected by the ammonia fraction according to the pH change. removal, the ratio of removal rate to ammonia fraction (𝜂/𝑓 ) was compared. 𝜂/𝑓 Figure 4b shows the natural logarithm of the relative change in total ammonia con- was approximately constant for pH 9.4–10.8; The ratios were 72.0%, 71.2%, and 75.2% at centration against time; the slopes for each pH value were obtained by linear regression pH 9.4, 10.2 and 10.8, respectively. From these results, it was confirmed that the ammonia (R = 0.997, 0.980, 0.993, 0.989). As a result, the slope and mass transfer coefficient increased removal rate was linearly affected by the ammonia fraction according to the pH change. proportionally with increasing pH. Figure 4b shows the natural logarithm of the relative change in total ammonia con- The amount of NaOH which was used to adjust the pH was determined. NaOH centration against time; the slopes for each pH value were obtained by linear regression Appl. Sci. 2021, 11, x FOR PEER REVIEW 8 of 13 consumption was 0.74, 1.44, 1.84, and 1.92 g of 100% NaOH/L liquid for pH 8.9, 9.4, 10.2, (R = 0.997, 0.980, 0.993, 0.989). As a result, the slope and mass transfer coefficient increased and 10.8, respectively, to maintain pH at 20 C. When pH increased, the ammonia removal proportionally with increasing pH. rate and mass transfer rate were increased, but the amount of chemical required also The amount of NaOH which was used to adjust the pH was determined. NaOH con- increased. sumption was 0.74, 1.44, 1.84, and 1.92 g of 100% NaOH/L liquid for pH 8.9, 9.4, 10.2, and 10.8, respectively, to maintain pH at 20 °C. When pH increased, the ammonia removal 100 1.6 rate and mass transfer rate were increased, but the amount of chemical required also in- pH slope K a pH8.9 20 C LNH3 1.4 (/h) (/h) pH9.4 creased. Air flow rate = 900 L/h 8.9 0.056 0.219 pH10.2 1.2 9.4 0.173 0.340 pH10.8 10.2 0.384 0.462 1.0 10.8 0.521 0.574 0.8 0.6 0.4 20 C Air flow rate = 900 L/h 0.2 0 0.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Time (h) Time (h) (a) (b) Figure 4. Ammonia stripping performance according to pH at 20 °C: (a) Ammonia removal rate; (b) logarithm of ammonia Figure 4. Ammonia stripping performance according to pH at 20 C: (a) Ammonia removal rate; (b) logarithm of ammonia concentration ratio. concentration ratio. 4.2. Packed Tower Air Stripping with Liquid Circulation To analyze the effect of liquid-to-gas contact efficiency on the stripping tower oper- ating factors, the stripping performance was assessed using the packed tower in Figure 2. In the packed tower, the mass transfer interfacial area is a water film on the surface of packed materials. In this study, tower design, packed layer height, and liquid flow rate were constant as shown in Tables 2 and 3. In consequence, a relative comparison of mass transfer coefficients is possible using overall volumetric mass transfer coefficients. Stripping was performed on 100 L of artificial wastewater at ammonia concentrations of 1100 and 3700 mg/L with the packed tower. The liquid flow rate was 117.0 L/h and the gas flow rate was 26,200 L/h. The stripping results for ammonia concentration in the packed tower were shown in Figure 5. Figure 5a shows the ammonia removal rate over time. After 1 h, the removal rate reached 30.1% and 18.4%, in 1100 and 3700 mg/L solutions, respectively. The reason for the difference in ammonia removal rate is that pH of the 1100 mg/L solution decrease to 8. Thereafter, NaOH was used to maintain the pH at approximately 8.5, and the ammonia removal rate showed similar trends over total stripping time. Figure 5b shows the natural logarithm of the change in the ammonia concentration ratio. As a result of linear regression analysis against time, the R values were relatively low (0.923–0.957), but reliable results were obtained. The slopes were similar between the two solution concentrations and the mass transfer coefficients of liquid ammonia were also similar. At 70 °C, pH 8.5, and 4.5 L/m L/G ratio, the mean mass transfer coefficient −1 was 0.192 ± 0.003 h . Stripping using the packed tower exhibited similar patterns regard- less of the solution concentration as shown in Figure 3. Finally, the ammonia concentra- tion of solution had little effect on the removal rate or mass transfer rate in both the strip- ping column and packed tower. NH NH r re em mova oval ( l (% %) ) 3 3 NH removal (%) T T -ln( [NH ] / [NH ] ) 3,L t 3,L 0 T T -ln( [NH ] / [NH ] ) 3,L t 3,L 0 Appl. Sci. 2021, 11, 441 8 of 13 4.2. Packed Tower Air Stripping with Liquid Circulation To analyze the effect of liquid-to-gas contact efficiency on the stripping tower operating factors, the stripping performance was assessed using the packed tower in Figure 2. In the packed tower, the mass transfer interfacial area is a water film on the surface of packed materials. In this study, tower design, packed layer height, and liquid flow rate were constant as shown in Tables 2 and 3. In consequence, a relative comparison of mass transfer coefficients is possible using overall volumetric mass transfer coefficients. Stripping was performed on 100 L of artificial wastewater at ammonia concentrations of 1100 and 3700 mg/L with the packed tower. The liquid flow rate was 117.0 L/h and the gas flow rate was 26,200 L/h. The stripping results for ammonia concentration in the packed tower were shown in Figure 5. Figure 5a shows the ammonia removal rate over time. After 1 h, the removal rate reached 30.1% and 18.4%, in 1100 and 3700 mg/L solutions, respectively. The reason for the difference in ammonia removal rate is that pH of the 1100 mg/L solution decrease to 8. Thereafter, NaOH was used to maintain the pH at approximately 8.5, and the ammonia removal rate showed similar trends over total stripping time. Figure 5b shows the natural logarithm of the change in the ammonia concentration ratio. As a result of linear regression analysis against time, the R values were relatively low (0.923–0.957), but reliable results were obtained. The slopes were similar between the two solution concentrations and the mass transfer coefficients of liquid ammonia were also similar. At 70 C, pH 8.5, and 4.5 L/m L/G ratio, the mean mass transfer coefficient was Appl. Sci. 2021, 11, x FOR PEER REVIEW 9 of 13 0.192  0.003 h . Stripping using the packed tower exhibited similar patterns regardless of the solution concentration as shown in Figure 3. Finally, the ammonia concentration of solution had little effect on the removal rate or mass transfer rate in both the stripping column and packed tower. 1.2 1100 mg/L Initial NH slope K a 3 LNH3 3700 mg/L (mg/L) (/h) (/h) 1.0 1100 0.147 0.189 3700 0.143 0.194 0.8 0.6 0.4 o o 70 C, pH 8.5 70 C, pH 8.5 Air flow rate = 26,200 L/h 0.2 Air flow rate = 26,200 L/h Liquid flow rate = 117.0 L/h Liquid flow rate = 117.0 L/h 0.0 0 12345 6 01 23456 Time (h) Time (h) (b) (a) Figure 5. Ammonia stripping performance according to ammonia concentration for the packed tower at 70 °C and pH 8.5: Figure 5. Ammonia stripping performance according to ammonia concentration for the packed tower at 70 C and pH 8.5: (a) Ammonia removal rate; (b) logarithm of ammonia concentration ratio. (a) Ammonia removal rate; (b) logarithm of ammonia concentration ratio. ToTo i identify dentify the opti the optimal mliquid-to-gas al liquid-to-gcontact as conta range ct range for the tower, for the tower, the stripping the strippi perfor ng per - - mance was compared at different gas flow rates in the range of 4.5–107.2 L/m . Figure 6a formance was compared at different gas flow rates in the range of 4.5–107.2 L/m . Figure shows 6a shows the the removal remova rate l ra forte 1100 for mg/L 1100 mg/ ammonium L ammonium hydr hydrox oxide solution ide solution at dif fer at different L/G ent L/G ra- tios. rati The os. The ammonia ammoni removal a remova rateltended rate tended to to increase incr asease as the L/G the L/G ratio decr ratio eased. decre Aadecr sed. A ease de- in the L/G ratio corresponds to an increase in the volume of air supplied per unit effective crease in the L/G ratio corresponds to an increase in the volume of air supplied per unit liquid capacity, which improves the ammonia removal efficiency [14,15]. However, when effective liquid capacity, which improves the ammonia removal efficiency [14,15]. How- the L/G ratio was excessively reduced (4.5 L/m level here), airflow rate was too much, ever, when the L/G ratio was excessively reduced (4.5 L/m level here), airflow rate was resulting in a reduction in ammonia removal efficiency. In Figure 6a, the removal rates too much, resulting in a reduction in ammonia removal efficiency. In Figure 6a, the re- moval rates after 6 h were 57.1%, 63.0%, and 57.1% for L/G of 4.5, 26.8, 53.6 L/m , respec- tively. The most effective removal performance was shown at L/G 26.8 L/m . Figure 6b shows the natural logarithms of the relative concentration according to L/G ratios. Figure 6b indicates that the slope and mass transfer coefficients increased with de- creasing L/G ratio, except for 4.5 L/m . The reason the mass transfer coefficient increases with reducing L/G is that contact efficiency improves at the liquid-to-gas interface due to increased airflow rate. Base on the L/G ratio of 107.2 L/m , when it decreases by 1/2 or 1/4, the gas flow rate increases by two or four times, and the mass transfer coefficient increased 1.4 or 1.6 times, respectively. However, when L/G reduced to 4.5 L/m , the gas flow rate increases significantly by 24 times, whereas the mass transfer coefficient increases by 1.5 times. As a result, it was derived that the mass transfer coefficient increased according to L/G decrease but decreases below a certain level. The gas flow rate and the mass transfer coefficient are not necessarily proportional, and the optimal point should be derived. The amount of NaOH consumed was 1.09 g of 100% NaOH/L liquid to maintain pH 8.5 at L/G 107.2 L/m for 24 h. When compared to the results by pH at 20 °C, the ammonia removal efficiency is excellent despite less air supply and NaOH consumption for the unit liquid. NH removal (%) T T -ln( [NH ] / [NH ] ) 3,L t 3,L 0 Appl. Sci. 2021, 11, 441 9 of 13 after 6 h were 57.1%, 63.0%, and 57.1% for L/G of 4.5, 26.8, 53.6 L/m , respectively. The most effective removal performance was shown at L/G 26.8 L/m . Figure 6b shows the natural logarithms of the relative concentration according to L/G ratios. Figure 6b indicates that the slope and mass transfer coefficients increased with decreasing L/G ratio, except for 4.5 L/m . The reason the mass transfer coefficient increases with reducing L/G is that contact efficiency improves at the liquid-to-gas interface due to increased airflow rate. Base on the L/G ratio of 107.2 L/m , when it decreases by 1/2 or 1/4, the gas flow rate increases by two or four times, and the mass transfer coefficient increased 1.4 or 1.6 times, respectively. However, when L/G reduced to 4.5 L/m , the gas flow rate increases significantly by 24 times, whereas the mass transfer coefficient increases by 1.5 times. As a result, it was derived that the mass transfer coefficient increased according to L/G decrease but decreases below a certain level. The gas flow rate and the mass transfer coefficient are not necessarily proportional, and the optimal point should be derived. Appl. Sci. 2021, 11, x FOR PEER REVIEW 10 of 13 The amount of NaOH consumed was 1.09 g of 100% NaOH/L liquid to maintain pH 8.5 at L/G 107.2 L/m for 24 h. When compared to the results by pH at 20 C, the ammonia removal efficiency is excellent despite less air supply and NaOH consumption for the unit liquid. 100 2.0 o o 70 C, pH 8.5 70 C, pH 8.5 Liquid flow rate = 117.0 L/h Liquid flow rate = 117.0 L/h 1.5 1.0 L/G slope K a LNH3 3 3 4.5 L/m (L/m ) (/h) (/h) 4.5 0.143 0.194 26.8 L/m 0.5 20 3 26.8 0.159 0.222 53.6 L/m 53.6 0.122 0.174 107.2 L/m 107.2 0.068 0.099 0 0.0 0 2468 10 12 14 16 18 20 22 24 0 2468 10 12 14 16 18 20 22 24 Time (h) Time (h) (a) (b) Figure 6. Ammonia stripping performance according to the liquid-gas ratio at 70 °C and pH 8.5: (a) Ammonia removal Figure 6. Ammonia stripping performance according to the liquid-gas ratio at 70 C and pH 8.5: (a) Ammonia removal rate; rate; (b) logarithm of ammonia concentration ratio. (b) logarithm of ammonia concentration ratio. Figur Figure e 77 shows shows the tota the total air l air supp supplied lied aga against insthe t the a ammonia mmonia r remov emovalarate l rate a at dif t di fer fferent ent L/G ratios. Here, the total air supplied refers to the total cumulative air for 1 L of artificial L/G ratios. Here, the total air supplied refers to the total cumulative air for 1 L of artificial wastewater. To compare the total air supplied according to L/G ratio, it was revealed that wastewater. To compare the total air supplied according to L/G ratio, it was revealed that the lower L/G ratio, the greater the total air consumed at the same removal rate. The total the lower L/G ratio, the greater the total air consumed at the same removal rate. The total 3 3 3 3 volume of air required was 240 L/L liquid at 53.6 L/m and 212 L/L water at 107.2 L/m volume of air required was 240 L/L liquid at 53.6 L/m and 212 L/L water at 107.2 L/m to to achieve 76.1%, where the ammonia removal rate was equal to the ammonia fraction achieve 76.1%, where the ammonia removal rate was equal to the ammonia fraction (h/ f = 1). At L/G ratios of 4.5–26.8 L/m , it is predicted that a greater volume of air (𝜂/𝑓 NH = 1). At L/G ratios of 4.5–26.8 L/m , it is predicted that a greater volume of air will will be required more than 1600 and 300 L/L liquid, respectively. be required more than 1600 and 300 L/L liquid, respectively. From Figures 6 and 7, it was found that as L/G ratio increased, the mass transfer rate From Figures 6 and 7, it was found that as L/G ratio increased, the mass transfer rate improved, and the more air supply was required to ensure the same ammonia removal improved, and the more air supply was required to ensure the same ammonia removal rate. To increase the air supply, the size and power costs of the blower need to be increased. rate. To increase the air supply, the size and power costs of the blower need to be in- Therefore, it is necessary to determine the optimal L/G ratio considering removal efficiency, creased. Therefore, it is necessary to determine the optimal L/G ratio considering removal air supply, and time. In this study, the most efficient L/G condition was 26.8 L/m , and efficiency, air supply, and time. In this study, the most efficient L/G condition was 26.8 53.6 L/m also seemed to be appropriate when trying to lower the total air supply. In 3 3 L/m , and 53.6 L/m also seemed to be appropriate when trying to lower the total air sup- consequence, a liquid-gas ratio of 26.8–53.6 L/m was selected as an optimal condition for ply. In consequence, a liquid-gas ratio of 26.8–53.6 L/m was selected as an optimal condi- packed tower operating at 70 C, pH 8.5. tion for packed tower operating at 70 °C, pH 8.5. 4.5 L/m 1400 3 26.8 L/m 53.6 L/m 107.2 L/m 600 70 C, pH 8.5 Liquid flow rate = 117.0 L/h 0 20 406080 100 NH removal (%) Figure 7. Ammonia removal rate according to the volume of air supplied at 70 °C, pH 8.5, and different L/G ratios. NH removal (%) Total air supplied (L-air/L-liquid) T T -ln( [NH ] / [NH ] ) 3,L t 3,L 0 Appl. Sci. 2021, 11, x FOR PEER REVIEW 10 of 13 100 2.0 o o 70 C, pH 8.5 70 C, pH 8.5 Liquid flow rate = 117.0 L/h Liquid flow rate = 117.0 L/h 1.5 1.0 L/G slope K a LNH3 3 3 4.5 L/m (L/m ) (/h) (/h) 4.5 0.143 0.194 26.8 L/m 0.5 20 26.8 0.159 0.222 53.6 L/m 53.6 0.122 0.174 107.2 L/m 107.2 0.068 0.099 0 0.0 0 2468 10 12 14 16 18 20 22 24 0 2468 10 12 14 16 18 20 22 24 Time (h) Time (h) (a) (b) Figure 6. Ammonia stripping performance according to the liquid-gas ratio at 70 °C and pH 8.5: (a) Ammonia removal rate; (b) logarithm of ammonia concentration ratio. Figure 7 shows the total air supplied against the ammonia removal rate at different L/G ratios. Here, the total air supplied refers to the total cumulative air for 1 L of artificial wastewater. To compare the total air supplied according to L/G ratio, it was revealed that the lower L/G ratio, the greater the total air consumed at the same removal rate. The total 3 3 volume of air required was 240 L/L liquid at 53.6 L/m and 212 L/L water at 107.2 L/m to achieve 76.1%, where the ammonia removal rate was equal to the ammonia fraction (𝜂/𝑓 = 1). At L/G ratios of 4.5–26.8 L/m , it is predicted that a greater volume of air will be required more than 1600 and 300 L/L liquid, respectively. From Figures 6 and 7, it was found that as L/G ratio increased, the mass transfer rate improved, and the more air supply was required to ensure the same ammonia removal rate. To increase the air supply, the size and power costs of the blower need to be in- creased. Therefore, it is necessary to determine the optimal L/G ratio considering removal efficiency, air supply, and time. In this study, the most efficient L/G condition was 26.8 3 3 L/m , and 53.6 L/m also seemed to be appropriate when trying to lower the total air sup- ply. In consequence, a liquid-gas ratio of 26.8–53.6 L/m was selected as an optimal condi- Appl. Sci. 2021, 11, 441 10 of 13 tion for packed tower operating at 70 °C, pH 8.5. 4.5 L/m 26.8 L/m 53.6 L/m 107.2 L/m 600 70 C, pH 8.5 Liquid flow rate = 117.0 L/h 0 20 406080 100 NH removal (%) Figure 7. Ammonia removal rate according to the volume of air supplied at 70 C, pH 8.5, and Figure 7. Ammonia removal rate according to the volume of air supplied at 70 °C, pH 8.5, and different L/G ratios. different L/G ratios. 4.3. The Comparison of Results with Literature Table 4 shows the comparison of results for ammonia stripping system with the literature. First, the stripping column results according to pH were compared. In this study, the ammonia removal efficiency increased in proportion to ammonia fraction as the pH increased. Zhu et al. [15] reported that an increase in pH increase leads to improve ammonia stripping rate and mass transfer rate. When ammonia fraction and air supply are similar, the removal rate and mass transfer coefficients are also similar to those derived from literature [15]. Secondly, packed tower results were compared. It is difficult to compare the mass transfer coefficient arithmetically because the interfacial area for the packed tower varies depending on the design of the tower and the packed material. However, it is possible to compare the approximate mass transfer rates by considering the ammonia fraction, air supply, and liquid ratio. Liu et al. [14] derived the mass transfer coefficient according to air flow rate. Although there is no mention of the L/G ratio, it can be interpreted that the lower L/G, the higher K a, since the L/G decreases as the air flow rate increases. When compared with this L, NH study, it was confirmed that K a was almost same under similar air supply conditions. L, NH It is considered that the calculated K a values are within a reasonable range when L, NH compared with the reports by Liu et al. [14] and Ferraz et al. [12]. Li et al. [16] reported the highest value of K a values under very low L/G ratio when compared to other L, NH literatures. In addition, ammonia removal rate was also compared for the jet loop reactor and the water-sparged aerocyclone. In these two reactors, as in the column and packed tower, the mass transfer coefficient increased as the liquid to gas ratio decreased. NH removal (%) Total air supplied (L-air/L-liquid) T T -ln( [NH ] / [NH ] ) t 0 3,L 3,L Appl. Sci. 2021, 11, 441 11 of 13 Table 4. The comparison of removal rate and overall volumetric mass transfer coefficient based on the liquid phase of ammonia. T Time Air Supplied L/G h K a L, NH Equipment pH f Ref. NH 3 ( C) (h) (L-Air/L-Liquid) (L-Liquid/m -Air) (%) (/h) 8.9 0.260 13.9 0.219 9.4 0.527 38.0 0.340 20 2.5 2250 This work 10.2 0.875 62.4 0.462 Stripping 10.8 0.965 72.6 0.574 column without circulation 10 0.864 1125 24.8 0.084 12 0.998 1125 - 55.4 0.24 25 3 [15] 12 0.998 2250 76.1 0.44 6 1572 4.5 57.1 0.194 6 262 26.8 63.0 0.222 70 8.5 0.761 This work 12 262 53.6 77.3 0.174 24 262 107.2 81.5 0.099 720 63.6 0.086 Packed tower 1440 83.4 0.166 [14] with circulation 50 10 0.970 12 2880 98.7 0.368 25 11 0.985 24 4500 6.67 99 0.18 [12] b b 15 10.8 0.937 3.5 3000 0.332 75 0.42 [16] 7.8 930 2500 45.6 0.081 Jet loop reactor 20 11 0.978 7.8 1400 1667 87.2 0.299 [24] 6.8 2030 1000 96.3 0.629 Water-sparged 1540 0.0032 98.9 0.78 [22] 25 0.995 3.5 11.5 aerocyclone 2660 0.0018 93.7 1.32 a b Liquid to gas ratio, L/G. Average value. Appl. Sci. 2021, 11, 441 12 of 13 5. Conclusions Stripping experiments were performed using an air stripping column and a packed tower on an ammonium hydroxide solution to calculate the major operating factors for each device. For ammonia stripping with a column, an increase in pH resulted in higher recovery efficiency and a larger mass transfer coefficient, and a proportional relationship was observed between ammonia removal efficiency and ammonia fraction. However, ammonia concentration did not affect ammonia removal rate or the mass transfer coefficient. For the packed tower, the operating factors were analyzed at 70 C, pH 8.5. As the liquid-gas ratio decreased in the range of 26.8–107.2 L/m , the mass transfer rate and air consumption increased. The 26.8 L/m condition achieved the largest mass transfer coefficient and 63.0% of ammonia removal rate by only 6 h operation with 262 L-air/L-liquid. And at L/G of 53.6 L/m , ammonia removal was 77.3% for 12 h with same air consumption. Therefore, the optimal liquid-gas ratio was chosen as 26.8–53.6 L/m , by considering the optimal mass transfer coefficient and air supply. In addition, through high-temperature stripping, excellent ammonia removal rate was achieved with low NaOH consumption. Finally, overall operating conditions such as temperature, pH, and air consumption were optimized to ensure both performance and economics for ammonia stripping devices design. Author Contributions: Conceptualization, E.J.K.; writing—original draft preparation, E.J.K.; funding acquisition, H.K.; project administration, H.K.; writing—review and editing, E.L. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Korea Agency for Infrastructure Technology Advancement (KAIA) grant funded by the Ministry of Land, Infrastructure and Transport (Grant 21UGCP-B157945-02). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Data available in a publicly accessible repository. Acknowledgments: This subject was supported by the Korea Agency for Infrastructure Technology Advancement (KAIA) grant funded by the Ministry of Land, Infrastructure and Transport (Grant 21UGCP-B157945-02). Conflicts of Interest: There are no conflict of interest to declare. References 1. Bousek, J.; Scroccaro, D.; Sima, J.; Weissenbacher, N.; Fuchs, W. Influence of the gas composition on the efficiency of ammonia stripping. Bioresour. Technol. 2016, 203, 259–266. [CrossRef] 2. Wang, W.; Ding, Y.; Wang, Y.; Song, X.; Ambrose, R.F.; Ullman, J.L.; Winfrey, B.K.; Wang, J.; Gong, J. Treatment of rich ammonia nitrogen wastewater with polyvinyl alcohol immobilized nitrifier biofortified constructed wetlands. Ecol. Eng. 2016, 94, 7–11. [CrossRef] 3. Sun, Y.; Yang, Q. Research on the transformation of nitrogen during hydrothermal carbonization of sludge. MATEC Web Conf. 2018, 175, 1–3. [CrossRef] 4. Panequea, M.; Rosa, D.L.J.M.; Kern, J.; Reza, M.T.; Knicker, H. Hydrothermal carbonization and pyrolysis of sewage sludges: What happen to carbon and nitrogen? J. Anal. Appl. Pyrolysis 2017, 128, 314–323. [CrossRef] 5. Vecino, X.; Reig, M.; Bhushan, B.; Gibert, O.; Valderrama, C.; Cortina, J.L. Liquid fertilizer production by ammonia recovery from treated ammonia-rich regenerated streams using liquid-liquid membrane contactors. Chem. Eng. J. 2019, 360, 890–899. [CrossRef] 6. Bonmatí, A.; Flotats, X. Air stripping of ammonia from pig slurry: Characterisation and feasibility as a pre- or post-treatment to mesophilic anaerobic digestion. Waste Manag. 2003, 23, 261–272. [CrossRef] 7. Vaddella, V.K.; Ndegwa, P.M.; Ullman, J.L.; Jiang, A. Mass transfer coefficients of ammonia for liquid dairy manure. Atmos. Environ. 2013, 66, 107–113. [CrossRef] 8. Jia, D.; Lua, W.; Zhang, Y. Research on mechanism of air stripping enabled ammonia removal from Industrial wastewater and Its application. Chem. 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Effect of pH, temperature and air flow rate on the continuous ammonia stripping of the anaerobic digestion effluent. Process Saf. Environ. Prot. 2011, 89, 61–65. [CrossRef] 14. Liu, B.; Giannis, A.; Zhang, J.; Chang, V.W.C.; Wang, J.Y. Air stripping process for ammonia recovery from source-separated urine: Modeling and optimization. J. Chem. Technol. Biotechnol. 2015, 90, 2208–2217. [CrossRef] 15. Zhu, L.; Dong, D.M.; Hua, X.Y.; Xu, Y.; Guo, Z.Y. Ammonia nitrogen removal and recovery from acetylene purification wastewater by air stripping. Water Sci. Technol. 2017, 75, 2538–2542. [CrossRef] 16. Li, L.; Wang, H.W.; Lu, J.H. Nitrogen removal using air stripping tower in urban wastewater treatment plant. China Water Wastewater 2006, 22, 92–95. 17. Jiang, A.; Zhang, T.; Zhao, Q.B.; Li, X.; Chen, S.; Frear, C.S. Evaluation of an integrated ammonia stripping, recovery, and biogas scrubbing system for use with anaerobically digested dairy manure. Biosyst. Eng. 2014, 119, 117–126. [CrossRef] 18. Escala, M.; Zumbuhl, T.; Koller, C.H.; Junge, R.; Krebs, R. Hydrothermal Carbonization as an Energy-Efficient Alternative to Established Drying Technologies for Sewage Sludge: A Feasibility Study on a Laboratory Scale. Energy Fuels 2013, 27, 454–460. [CrossRef] 19. Wang, T.F.; Zhai, Y.; Zhu, Y.; Peng, C.; Xu, B.; Wang, T.; Li, C.; Zeng, G. Influence of temperature on nitrogen fate during hydrothermal carbonization of food waste. Bioresour. Technol. 2018, 247, 182–189. [CrossRef] 20. Fritz, U.; Matthias, B. Ullmann’s Encyclopedia of Industrial Chemistry, 7th ed.; Willey VCH: Weinheim, Germany, 2011. 21. Matter-Müller, C.; Gujer, W.; Giger, W. Transfer of volatile substances from water to the atmosphere. Water Res. 1981, 15, 1271–1279. [CrossRef] 22. Quan, X.; Wang, F.; Zhao, Q.; Zhao, T.; Xiang, J. Air stripping of ammonia in a water-sparged aerocyclone reactor. J. Hazard. Mater. 2009, 170, 983–988. [CrossRef] 23. Agnieszka, U.; Małgorzata, K.K.; Mateusz, W.; Przemysław, S.; Marcin, B.; Halina, P.K.; Monika, S.T.; Krystian, K.; Lukasz, N. Treatment of liquid by-products of hydrothermal carbonization (HTC) of agricultural digestate using membrane separation. Energies 2020, 13, 262. [CrossRef] 24. Degermenci, N.; Ata, O.N.; Yildız, E. Ammonia removal by air stripping in a semi-batch jet loop reactor. J. Ind. Eng. Chem. 2012, 18, 399–404. [CrossRef] http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Applied Sciences Multidisciplinary Digital Publishing Institute

Influence of Ammonia Stripping Parameters on the Efficiency and Mass Transfer Rate of Ammonia Removal

Kim, Eun Ju; Kim, Ho; Lee, Eunsil
Applied Sciences , Volume 11 (1) – Jan 5, 2021
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applied sciences Article Influence of Ammonia Stripping Parameters on the Efficiency and Mass Transfer Rate of Ammonia Removal Eun Ju Kim , Ho Kim and Eunsil Lee * Center for Plant Engineering, Institute for Advanced Engineering, Yongin-si 17180, Korea; ejkim@iae.re.kr (E.J.K.); kimh0505@iae.re.kr (H.K.) * Correspondence: les0302@iae.re.kr Abstract: This study analyzed the influence of different ammonia stripping parameters on ammonia removal efficiency and mass transfer rate. Ammonia stripping was performed on two devices, a column and a packed tower, with artificial ammonium hydroxide wastewater. First, ammonia concentration and pH were varied in a column without liquid circulation. At the same pH, the removal efficiency and mass transfer rate were constant, irrespective of initial ammonia concentration. When pH was increased, the ammonia fraction also increased, resulting in higher removal efficiency and mass transfer rate. Second, the effects of stripping were assessed using a packed tower with fluid circulation. The ammonium hydroxide concentration did not affect the removal efficiency or mass transfer rate. Furthermore, at apparatus liquid-gas ratios of 26.8–107.2 L/m , a lower liquid-gas ratio led to increased ammonia removal efficiency and mass transfer rate. Conversely, the lower the liquid-gas ratio, the greater the air consumption. In conclusion, considering the removal rate and volume of air supply, the range of optimal liquid-gas ratio was determined as 26.8–53.6 L/m . In particular, the 26.8 L/m condition achieved the best ammonia removal rate of 63.0% through only 6 h of stripping at 70 C and pH 8.5. Keywords: wastewater treatment; nitrogen removal; ammonia stripping; air stripping; stripping tower Citation: Kim, E.J.; Kim, H.; Lee, E. Influence of Ammonia Stripping Parameters on the Efficiency and 1. Introduction Mass Transfer Rate of Ammonia With industrial development and population growth, there has been a rapid increase Removal. Appl. Sci. 2021, 11, 441. in the amount of wastewater containing ammonia from various sources, including sewage https://doi.org/ sludge, cattle excrement, food waste, and biomass [1]. Ammonia containing wastewater is 10.3390/app11010441 also generated in the energizing process of waste resources [2,3]. Ammonia in wastewater should be removed because it causes eutrophication and ecotoxicity when it leaks into Received: 10 November 2020 ecosystems [4,5]. Accepted: 31 December 2020 There are various methods for removing ammoniacal nitrogen from solution, including Published: 5 January 2021 physical, electronic, and biological methods. Ammonia stripping is a particularly efficient Publisher’s Note: MDPI stays neu- method for ammonia recovery from high concentration wastewater [6,7], In ammonia tral with regard to jurisdictional clai- stripping system, ammoniacal nitrogen is removed as a gas by supplying gas such as air ms in published maps and institutio- or steam [6–9]. The reactor is usually a fixed-bed column or packed bed tower [6–9]. In nal affiliations. tower-type reactors, ammonia mass transfer is achieved by gas-liquid contact within the packing material [10]. Therefore, the liquid-gas ratio should be carefully considered to optimize the operating conditions of packed towers. In this regard, substantial research and development have focused on the operating Copyright: © 2021 by the authors. Li- conditions and other factors affecting stripping towers for different types of wastew- censee MDPI, Basel, Switzerland. ater [11–16]. For example, Ferraz et al. [12] performed ammonia stripping from land- This article is an open access article fill leachate using a packed tower at room temperature and pH 11 and achieved a re- distributed under the terms and con- moval efficiency of 98% after 24 h of stripping at a liquid-gas ratio (L/G) of 6.7–20 L/m . ditions of the Creative Commons At- Guštin et al. [13] used a continuous flow packed tower to strip ammoniacal nitrogen from tribution (CC BY) license (https:// anaerobic digestate at a liquid-gas ratio of 2.5 L/m , achieving a removal efficiency of 55% creativecommons.org/licenses/by/ at 50 C and pH 10. Liu et al. [14] evaluated the ammonia removal efficiency and rate 4.0/). Appl. Sci. 2021, 11, 441. https://doi.org/10.3390/app11010441 https://www.mdpi.com/journal/applsci Appl. Sci. 2021, 11, 441 2 of 13 for the urine with temperature, pH, concentration, and liquid-gas ratio. Zhu et al. [15] and Li et al. [16] also analyzed ammonia removal rate by temperature, pH and air supply condition. Conventional ammonia stripping processes are typically performed at a temperature between room temperature and 50 C and in a pH range of 10–12 [11–16]. These process require a large amount of reagents to adjust the pH [13,17]. In the ammonia stripping process, a basic reagent is used to increase the pH. After ammonia removal, the pH must be adjusted to neutral for wastewater treatment and discharge. One method for reducing the use of these reagents is high-temperature stripping; at a high temperature, the same removal efficiency can be achieved with a smaller quantity of reagent [17]. Although there is a problem that a heat is required for high-temperature stripping, the heat energy consumption can be minimized by using waste heat. For example, the hydrothermal thermal carbonization (HTC) reaction is carried out at 180–260 C [18,19], and it is possible to raise the temperature of the HTC wastewater up to 70–80 C by using HTC heat. In addition, the high-temperature process can be operated more economically by reducing the air supply [8]. The purpose of this study is to design a high-temperature stripping tower and derive the optimal operating factors that will reduce the use of reagents and air supply while maintaining the ammonia fraction at an operating temperature of 70 C. To this end, the ammonia removal efficiency and mass transfer coefficient are assessed at different temperatures, pH, ammonium hydroxide concentration, and airflow volume. Specifically, by deriving the optimal liquid-gas ratio and air supply volume for efficient liquid-to-gas contact in the packing layer, this study provides useful data on the key factors in the design of high-temperature ammonia stripping devices. 2. Theoretical Considerations 2.1. Ammonia–Water System In aqueous solution, total ammonia exists as both free ammonia (NH ) and ammonium ions (NH ), as shown in Equation (1), and the water is dissociated into ions, as shown in Equation (2) [10]. NH + H O $ NH + OH , (1) 3 2 H O $ H + OH , (2) The total ammonia concentration in the solution is expressed by Equation (3). [NH ] = [NH ] + NH , (3) 3,L 3,L where [NH ] , [NH ], and NH mean the molar concentration (mol/L) of total 3,L 3,L ammonia, free ammonia, and ammonium ion, respectively. The ionization constants for water (K ) and ammonia (K ) are known in Equations (4) H O NH 2 3 and (5), respectively. K = H OH , (4) H O NH [OH ] K = , (5) NH [NH ] 3,L where H and OH are the molar concentration (mol/L) of hydronium ion and hydrox- [ ] [ ] ide ion, respectively. In addition, the ionization constants are expressed as a function of absolute tempera- ture (K), as shown in Equations (6) and (7) [6,10,11,20]. ln K = 140.932 13445.9/T 22.4773 ln T, (6) H O ln K = 97.976 5930.7/T 15.063 ln T 0.01127T, (7) NH 3 Appl. Sci. 2021, 11, 441 3 of 13 2.2. Ammonia Stripping In aqueous solution, the free ammonia can be stripped through transfer from liquid to gas. Thus, the concentration ratio of the free ammonia to total ammonia is a major factor in stripping and is defined as the ammonia fraction ( f ) in Equation (8) [13]. NH [NH ] 3,L f =   , (8) NH [NH ] + NH 3,L The ammonia fraction is also expressed by Equation (9) from Equations (4) and (5). H O f = =  , (9) NH 3 + pH K + K [H ] 1 + K /K 10 H O NH NH H O 2 3 3 2 Since the ionization constants is described as a function of absolute temperature in Equation (6), the ammonia fraction is dependent on both temperature and pH. The percentage of ammonia removal (h) is calculated as the amount of ammonia removed compared to the initial ammonia and is expressed as Equation (10): T T [NH ] [NH ] 3,L 3,L 0 t h = , (10) [NH ] 3,L 2.3. Mass Transfer of Ammonia In the process of ammonia stripping, the mass transfer rate of ammonia (F ) from NH liquid to gas phase is given by Equation (11) [14,21,22]. [NH ] 3,L F = V , (11) NH L dt where t is time and V is the total volume of liquid. In a batch reactor, the mass transfer rate for liquid ammonia to gas is defined by Matter-Mueller, as shown in Equation (12) [14,22]: K aV L, NH L F = Q H [NH ] 1 exp , (12) NH G NH 3,L 3 3 Q H NH where Q is the gas flow rate and H is the dimensionless Henry’s law constant. K G NH L, NH 3 3 is the mass transfer coefficient of liquid ammonia. The interfacial area per unit volume between liquid and gas is indicated by a. Combining Equations (8), (10) and (12) and integrating the total ammonia concentra- tion over time gives Equation (13): [NH ] Q H f K aV 3,L G NH NH L, NH L t 3 3 3 ln = 1 exp t, (13) V Q H L G NH [NH ] 3,L 3 The slope for the logarithm of ammonia concentration ratio over time is then given by Equation (14): Q H f K aV G NH NH L, NH L 3 3 3 slope = 1 exp , (14) V Q H L G NH From Equations (13) and (14), the overall mass transfer coefficient for liquid ammonia to gas is calculated as follows: ( !) Q H [NH ] G NH 3,L 3 t K a = ln 1 + ln , (15) L, NH V Q H f t L G NH NH [NH ] 3 3 3,L 0 Appl. Sci. 2021, 11, 441 4 of 13 Q H G NH K a = ln 1  (slope) , (16) L, NH V Q H f L G NH NH Appl. Sci. 2021, 11, x FOR PEER REVIEW 4 of 13 3 3 The main variables in ammonia stripping are the ammonia fraction, the rate of change in the ammonia concentration, the volume of gas supplied, and the overall mass transfer coefficient according to the type of reactor as shown in Equations (12) and (14). Hence, transfer coefficient according to the type of reactor as shown in Equations (12) and (14). in this study, we analyzed the effect of the major stripping factors—ammonia concentra- Hence, in this study, we analyzed the effect of the major stripping factors—ammonia con- tion, fraction and gas flow rate—on the ammonia removal efficiency and mass transfer centration, fraction and gas flow rate—on the ammonia removal efficiency and mass coefficient. transfer coefficient. 3. Materials and Methods 3. Materials and Methods 3.1. Preparation 3.1. Preparation In this study, air stripping experiments were conducted and stripping factors were In this study, air stripping experiments were conducted and stripping factors were evaluated for high ammonia concentration wastewater such as HTC or anaerobic liq- evaluated for high ammonia concentration wastewater such as HTC or anaerobic liquids uids [1–4,18,19,23]. These wastewaters contain ammonia up to 4000 mg/L [1,3,4]. To [1–4,18,19,23]. These wastewaters contain ammonia up to 4000 mg/L [1,3,4]. To simulate simulate artificial wastewater, 1000–3700 mg/L ammonia solution was prepared by dilut- artificial wastewater, 1000–3700 mg/L ammonia solution was prepared by diluting a 28– ing a 28–30% ammonia solution (SAMCHUN) with tap water. To evaluate the physical 30% ammonia solution (SAMCHUN) with tap water. To evaluate the physical degassing degassing properties of ammonia, the effects on trace amounts of nitrate, nitrite were properties of ammonia, the effects on trace amounts of nitrate, nitrite were excluded. excluded. 3.2. 3.2. Stri Stripping ppingColumn Column A column-type ammonia stripping device was configured as shown in Figure 1. The A column-type ammonia stripping device was configured as shown in Figure 1. The capacity capacity of of the the column was 2 L, a column was 2 L, and nd a di a disk-type sk-type air air dif difuser ffuser was was in installed stalled at at the the bottom, bottom, thr through wh ough which ich extern external al air could be air could be supplied. suppliThe ed. The air r equir air required ed for stripping for stripping was contr was con- olled at a pressure of 30 Pa using a blower and regulator, and a flow meter was used to control trolled at a pressure of ≤30 Pa using a blower and regulator, and a flow meter was used to the volume. To control the pH of the solution, NaOH solution was supplied using an auto control the volume. To control the pH of the solution, NaOH solution was supplied using titration system (905 Titrando, Metrohm AG, Switzerland). Stripping was performed on an auto titration system (905 Titrando, Metrohm AG, Switzerland). Stripping was per- 1 L of artificial wastewater, and the air flow rate was set to 900 L/h. The stripping factors formed on 1 L of artificial wastewater, and the air flow rate was set to 900 L/h. The strip- were assessed while varying the ammonia concentration and the pH of the wastewater, ping factors were assessed while varying the ammonia concentration and the pH of the under the conditions shown in Table 1. wastewater, under the conditions shown in Table 1. Figure 1. Schematic view of the ammonia stripping column. Figure 1. Schematic view of the ammonia stripping column. Table 1. Experimental conditions of NH3 stripping using a stripping column. Temperature pH Initial NH3 Concentration (°C) (-) (mg/L) 8.9 2900 9.4 1500–2900 10.2 2900 10.8 2900 Appl. Sci. 2021, 11, 441 5 of 13 Table 1. Experimental conditions of NH stripping using a stripping column. Temperature pH Initial NH Concentration ( C) (-) (mg/L) 8.9 2900 9.4 1500–2900 Appl. Sci. 2021, 11, x FOR PEER REVIEW 5 of 13 10.2 2900 10.8 2900 3.3. Packed Tower 3.3. Packed Tower Figure 2 shows the packed tower system constructed to determine the effect of the Figure 2 shows the packed tower system constructed to determine the effect of the liquid-gas contact ratio on the stripping factors. The tower was designed with a diameter liquid-gas contact ratio on the stripping factors. The tower was designed with a diameter of 0.2 m and a height of 1.5 m to enable observation of the physical characteristics of NH3 of 0.2 m and a height of 1.5 m to enable observation of the physical characteristics of air stripping at the bench scale. The precise specifications are shown in Table 2. The inter- NH air stripping at the bench scale. The precise specifications are shown in Table 2. The nal packing material consisted of 18 mm PP pall rings, and the height of the packing was internal packing material consisted of 18 mm PP pall rings, and the height of the packing 0.48 m. The maximum capacity of the tank was 147 L; the effective capacity for the strip- was 0.48 m. The maximum capacity of the tank was 147 L; the effective capacity for the ping experiment was set to 100 L. The stripping temperature was set to 70 °C, and a heater stripping experiment was set to 100 L. The stripping temperature was set to 70 C, and a in the tank and a heat gun were used to control the stripping temperature. Air was sup- heater in the tank and a heat gun were used to control the stripping temperature. Air was plied using the heat gun into the bottom of the tower and was emitted from the top of the supplied using the heat gun into the bottom of the tower and was emitted from the top tower. The flow rate of air was controlled using a valve and measured through a flow of the tower. The flow rate of air was controlled using a valve and measured through a meter. The liquid was supplied into the top of the tower from the tank using a pump; after flow meter. The liquid was supplied into the top of the tower from the tank using a pump; spraying through a nozzle, the liquid passes through the packing layer, then cycled back after spraying through a nozzle, the liquid passes through the packing layer, then cycled back into the ta into the nk. tank. The cycli Then cycling g rate of the rate of liq the uid liquid and the flo and the w rate flow of the rate of gas the are gas specified are specified along along with twith he otthe her operat other operating ing condit conditions ions in Tabl ine Table 3. A te 3. mperature A temperatur -adjusted pH e-adjusted se pH nsor was sensor was instal ins letalled d inside the t inside the ank to moni tank to monitor tor the pH the an pH d t and emp temperatur erature, and e, t and he N the aOH NaOH solusolution tion was was suppli supplied ed into the ta into thenk using a tank using pump t a pumpoto co contr ntrol the pH. Ammonia ol the pH. Ammonia stripping stripping using using the the packed tower was performed at 70 C and pH 8.5 (NH fraction 76.1%). To ascertain the packed tower was performed at 70 °C and pH 8.5 (NH3 fraction 76.1%). To ascertain the optimal operating conditions for the tower, the liquid flow rate was fixed at 117.0 L/h, and optimal operating conditions for the tower, the liquid flow rate was fixed at 117.0 L/h, and the stripping efficiency was assessed according to the liquid-gas ratio by varying the gas the stripping efficiency was assessed according to the liquid-gas ratio by varying the gas flow rate. flow rate. Figure 2. Schematic view of the ammonia stripping packed tower system. Figure 2. Schematic view of the ammonia stripping packed tower system. Table 2. Design parameters of the stripping tower. Design Parameter Value Unit Tower diameter 0.2 m Tower height 1.5 m Packed layer height 0.48 m Packed materials PP pall ring - Liquid tank volume 147 L Appl. Sci. 2021, 11, 441 6 of 13 Table 2. Design parameters of the stripping tower. Design Parameter Value Unit Tower diameter 0.2 m Tower height 1.5 m Packed layer height 0.48 m Packed materials PP pall ring - Liquid tank volume 147 L Table 3. Experimental conditions of NH stripping using a packed tower. Initial NH Conc. Liquid Flow Rate Gas Flow Rate Liquid-Gas Ratio, L/G (mg/L) (L/h) (L/h) (L-Liquid/m -Gas) 1100–3700 117.0 26,200 4.5 1100 117.0 4367 26.8 1100 117.0 2183 53.6 1100 117.0 1092 107.2 3.4. Analysis The ammonia concentration in the aqueous solution was analyzed to assess the stripping efficiency. Solution samples of 10 mL were taken from stripping column and packed tower tank at regular time intervals. All samples were treated with sulfonic acid to pH 2.5, and the concentrations of ammonium ions were quantified using ion chromatography (930 Compact IC Flex, Metrohm AG, Herisau, Switzerland) equipped with a cation separation column (Metrosep C 4 150/4.0, Metrohm AG, Herisau, Switzerland). 4. Results 4.1. Air Stripping without Liquid Circulation Ammonia stripping was performed using the stripping column in Figure 1. In the column reactor, the contact surface between liquid and gas is the interface of the air bubble [14]. The mass transfer interfacial area is related to the size and number of air bubbles. Therefore, it is possible to compare the mass transfer coefficient in the constant conditions: total liquid volume; air flow rate; temperature [14]. As an experimental condition, the airflow rate was constant of 900 L/h on 1 L of artificial wastewater. The stripping efficiency and mass transfer rate of ammonia were analyzed according to initial concentration and pH. Ammonia removal efficiency and mass transfer coefficient were calculated by Equations (10), (15), and (16). Figure 3 shows the results of stripping at different ammonium hydroxide concentra- tions at 20 C and pH 9.4. Figure 3a shows the ammonia removal rate according to the concentration. The removal rate increased with increasing time. After 3 h, similar results of 38.4%, 38.5%, and 38.3% were observed for concentrations of 1500, 2200, and 2900 mg/L, respectively. Figure 3b shows the natural logarithm of the relative change in total ammonia con- centration over time. A linear regression analysis was conducted against time; the mean slope was 0.16  20.011. The R values were 0.980, 0.973, and 0.996 for each respective concentration, indicating that the results were reliable. The overall volumetric mass transfer coefficients on the liquid phase (K a) were calculated from the slope of Figure 3a and L, NH Equation (16). The mean K a was 0.319 h , which remained constant regardless of L, NH the initial ammonia concentration in the solution. Therefore, the ammonium hydroxide concentration had little effect on the removal efficiency or rate. The change of pH in the ammonia solution causes a change of ammonia fraction, which means that the higher pH, the more ammonia can be removed. pH is an important parameter affecting the stripping efficiency and mass transfer coefficient. Appl. Sci. 2021, 11, x FOR PEER REVIEW 7 of 13 Appl. Sci. 2021, 11, 441 7 of 13 100 1.6 o o 2 290 900 mg/L 0 mg/L 20 C, pH 9.4 20 20 C, pH C, pH 9.4 9.4 1.4 2 220 200 mg/L 0 mg/L Air flow rate = 900 L/h 80 A Ai ir f r fl lo ow w rate = 90 rate = 900 0 L L/ /h h 1 150 500 mg/L 0 mg/L 1.2 Initial NH slope K a 3 LNH3 1.0 (mg/L) (/h) (/h) 0.8 2900 0.173 0.309 2200 0.156 0.307 0.6 1500 0.158 0.340 0.4 0.2 0 0.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 0.00.5 1.01.5 2.02.5 3.0 Tim Time e (h (h) ) Time (h) (b) (a) Figure 3. Ammonia stripping performance according to initial concentration at 20 °C and pH 9.4: (a) Ammonia removal Figure 3. Ammonia stripping performance according to initial concentration at 20 C and pH 9.4: (a) Ammonia removal rate; (b) logarithm of the ammonia concentration ratio. rate; (b) logarithm of the ammonia concentration ratio. Figur The chang e 4 repre esents of pH the in t results he am of mo ammonia nia soluti stripping on causes accor a change ding to ofpH amat mo 2900 nia fr mg/L action, ammonia concentration and 20 C. The ammonia removal rate was shown as a linear which means that the higher pH, the more ammonia can be removed. pH is an important increase over time; after 2.5 h of stripping, the removal rate increased from 13.9% to 72.6% parameter affecting the stripping efficiency and mass transfer coefficient. when pH Figu was re 4 incr repr eased esents the r from 8.9 esults o to 10.8. f ammo To analyze nia stripping a the influence ccording of to pH pH onat ammonia 2900 mg/L removal, the ratio of removal rate to ammonia fraction (h/ f ) was compared. h/ f ammonia concentration and 20 °C. The ammonia removal rate was shown as a linear in- NH NH 3 3 was approximately constant for pH 9.4–10.8; The ratios were 72.0%, 71.2%, and 75.2% at crease over time; after 2.5 h of stripping, the removal rate increased from 13.9% to 72.6% pH 9.4, 10.2 and 10.8, respectively. From these results, it was confirmed that the ammonia when pH was increased from 8.9 to 10.8. To analyze the influence of pH on ammonia removal rate was linearly affected by the ammonia fraction according to the pH change. removal, the ratio of removal rate to ammonia fraction (𝜂/𝑓 ) was compared. 𝜂/𝑓 Figure 4b shows the natural logarithm of the relative change in total ammonia con- was approximately constant for pH 9.4–10.8; The ratios were 72.0%, 71.2%, and 75.2% at centration against time; the slopes for each pH value were obtained by linear regression pH 9.4, 10.2 and 10.8, respectively. From these results, it was confirmed that the ammonia (R = 0.997, 0.980, 0.993, 0.989). As a result, the slope and mass transfer coefficient increased removal rate was linearly affected by the ammonia fraction according to the pH change. proportionally with increasing pH. Figure 4b shows the natural logarithm of the relative change in total ammonia con- The amount of NaOH which was used to adjust the pH was determined. NaOH centration against time; the slopes for each pH value were obtained by linear regression Appl. Sci. 2021, 11, x FOR PEER REVIEW 8 of 13 consumption was 0.74, 1.44, 1.84, and 1.92 g of 100% NaOH/L liquid for pH 8.9, 9.4, 10.2, (R = 0.997, 0.980, 0.993, 0.989). As a result, the slope and mass transfer coefficient increased and 10.8, respectively, to maintain pH at 20 C. When pH increased, the ammonia removal proportionally with increasing pH. rate and mass transfer rate were increased, but the amount of chemical required also The amount of NaOH which was used to adjust the pH was determined. NaOH con- increased. sumption was 0.74, 1.44, 1.84, and 1.92 g of 100% NaOH/L liquid for pH 8.9, 9.4, 10.2, and 10.8, respectively, to maintain pH at 20 °C. When pH increased, the ammonia removal 100 1.6 rate and mass transfer rate were increased, but the amount of chemical required also in- pH slope K a pH8.9 20 C LNH3 1.4 (/h) (/h) pH9.4 creased. Air flow rate = 900 L/h 8.9 0.056 0.219 pH10.2 1.2 9.4 0.173 0.340 pH10.8 10.2 0.384 0.462 1.0 10.8 0.521 0.574 0.8 0.6 0.4 20 C Air flow rate = 900 L/h 0.2 0 0.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 Time (h) Time (h) (a) (b) Figure 4. Ammonia stripping performance according to pH at 20 °C: (a) Ammonia removal rate; (b) logarithm of ammonia Figure 4. Ammonia stripping performance according to pH at 20 C: (a) Ammonia removal rate; (b) logarithm of ammonia concentration ratio. concentration ratio. 4.2. Packed Tower Air Stripping with Liquid Circulation To analyze the effect of liquid-to-gas contact efficiency on the stripping tower oper- ating factors, the stripping performance was assessed using the packed tower in Figure 2. In the packed tower, the mass transfer interfacial area is a water film on the surface of packed materials. In this study, tower design, packed layer height, and liquid flow rate were constant as shown in Tables 2 and 3. In consequence, a relative comparison of mass transfer coefficients is possible using overall volumetric mass transfer coefficients. Stripping was performed on 100 L of artificial wastewater at ammonia concentrations of 1100 and 3700 mg/L with the packed tower. The liquid flow rate was 117.0 L/h and the gas flow rate was 26,200 L/h. The stripping results for ammonia concentration in the packed tower were shown in Figure 5. Figure 5a shows the ammonia removal rate over time. After 1 h, the removal rate reached 30.1% and 18.4%, in 1100 and 3700 mg/L solutions, respectively. The reason for the difference in ammonia removal rate is that pH of the 1100 mg/L solution decrease to 8. Thereafter, NaOH was used to maintain the pH at approximately 8.5, and the ammonia removal rate showed similar trends over total stripping time. Figure 5b shows the natural logarithm of the change in the ammonia concentration ratio. As a result of linear regression analysis against time, the R values were relatively low (0.923–0.957), but reliable results were obtained. The slopes were similar between the two solution concentrations and the mass transfer coefficients of liquid ammonia were also similar. At 70 °C, pH 8.5, and 4.5 L/m L/G ratio, the mean mass transfer coefficient −1 was 0.192 ± 0.003 h . Stripping using the packed tower exhibited similar patterns regard- less of the solution concentration as shown in Figure 3. Finally, the ammonia concentra- tion of solution had little effect on the removal rate or mass transfer rate in both the strip- ping column and packed tower. NH NH r re em mova oval ( l (% %) ) 3 3 NH removal (%) T T -ln( [NH ] / [NH ] ) 3,L t 3,L 0 T T -ln( [NH ] / [NH ] ) 3,L t 3,L 0 Appl. Sci. 2021, 11, 441 8 of 13 4.2. Packed Tower Air Stripping with Liquid Circulation To analyze the effect of liquid-to-gas contact efficiency on the stripping tower operating factors, the stripping performance was assessed using the packed tower in Figure 2. In the packed tower, the mass transfer interfacial area is a water film on the surface of packed materials. In this study, tower design, packed layer height, and liquid flow rate were constant as shown in Tables 2 and 3. In consequence, a relative comparison of mass transfer coefficients is possible using overall volumetric mass transfer coefficients. Stripping was performed on 100 L of artificial wastewater at ammonia concentrations of 1100 and 3700 mg/L with the packed tower. The liquid flow rate was 117.0 L/h and the gas flow rate was 26,200 L/h. The stripping results for ammonia concentration in the packed tower were shown in Figure 5. Figure 5a shows the ammonia removal rate over time. After 1 h, the removal rate reached 30.1% and 18.4%, in 1100 and 3700 mg/L solutions, respectively. The reason for the difference in ammonia removal rate is that pH of the 1100 mg/L solution decrease to 8. Thereafter, NaOH was used to maintain the pH at approximately 8.5, and the ammonia removal rate showed similar trends over total stripping time. Figure 5b shows the natural logarithm of the change in the ammonia concentration ratio. As a result of linear regression analysis against time, the R values were relatively low (0.923–0.957), but reliable results were obtained. The slopes were similar between the two solution concentrations and the mass transfer coefficients of liquid ammonia were also similar. At 70 C, pH 8.5, and 4.5 L/m L/G ratio, the mean mass transfer coefficient was Appl. Sci. 2021, 11, x FOR PEER REVIEW 9 of 13 0.192  0.003 h . Stripping using the packed tower exhibited similar patterns regardless of the solution concentration as shown in Figure 3. Finally, the ammonia concentration of solution had little effect on the removal rate or mass transfer rate in both the stripping column and packed tower. 1.2 1100 mg/L Initial NH slope K a 3 LNH3 3700 mg/L (mg/L) (/h) (/h) 1.0 1100 0.147 0.189 3700 0.143 0.194 0.8 0.6 0.4 o o 70 C, pH 8.5 70 C, pH 8.5 Air flow rate = 26,200 L/h 0.2 Air flow rate = 26,200 L/h Liquid flow rate = 117.0 L/h Liquid flow rate = 117.0 L/h 0.0 0 12345 6 01 23456 Time (h) Time (h) (b) (a) Figure 5. Ammonia stripping performance according to ammonia concentration for the packed tower at 70 °C and pH 8.5: Figure 5. Ammonia stripping performance according to ammonia concentration for the packed tower at 70 C and pH 8.5: (a) Ammonia removal rate; (b) logarithm of ammonia concentration ratio. (a) Ammonia removal rate; (b) logarithm of ammonia concentration ratio. ToTo i identify dentify the opti the optimal mliquid-to-gas al liquid-to-gcontact as conta range ct range for the tower, for the tower, the stripping the strippi perfor ng per - - mance was compared at different gas flow rates in the range of 4.5–107.2 L/m . Figure 6a formance was compared at different gas flow rates in the range of 4.5–107.2 L/m . Figure shows 6a shows the the removal remova rate l ra forte 1100 for mg/L 1100 mg/ ammonium L ammonium hydr hydrox oxide solution ide solution at dif fer at different L/G ent L/G ra- tios. rati The os. The ammonia ammoni removal a remova rateltended rate tended to to increase incr asease as the L/G the L/G ratio decr ratio eased. decre Aadecr sed. A ease de- in the L/G ratio corresponds to an increase in the volume of air supplied per unit effective crease in the L/G ratio corresponds to an increase in the volume of air supplied per unit liquid capacity, which improves the ammonia removal efficiency [14,15]. However, when effective liquid capacity, which improves the ammonia removal efficiency [14,15]. How- the L/G ratio was excessively reduced (4.5 L/m level here), airflow rate was too much, ever, when the L/G ratio was excessively reduced (4.5 L/m level here), airflow rate was resulting in a reduction in ammonia removal efficiency. In Figure 6a, the removal rates too much, resulting in a reduction in ammonia removal efficiency. In Figure 6a, the re- moval rates after 6 h were 57.1%, 63.0%, and 57.1% for L/G of 4.5, 26.8, 53.6 L/m , respec- tively. The most effective removal performance was shown at L/G 26.8 L/m . Figure 6b shows the natural logarithms of the relative concentration according to L/G ratios. Figure 6b indicates that the slope and mass transfer coefficients increased with de- creasing L/G ratio, except for 4.5 L/m . The reason the mass transfer coefficient increases with reducing L/G is that contact efficiency improves at the liquid-to-gas interface due to increased airflow rate. Base on the L/G ratio of 107.2 L/m , when it decreases by 1/2 or 1/4, the gas flow rate increases by two or four times, and the mass transfer coefficient increased 1.4 or 1.6 times, respectively. However, when L/G reduced to 4.5 L/m , the gas flow rate increases significantly by 24 times, whereas the mass transfer coefficient increases by 1.5 times. As a result, it was derived that the mass transfer coefficient increased according to L/G decrease but decreases below a certain level. The gas flow rate and the mass transfer coefficient are not necessarily proportional, and the optimal point should be derived. The amount of NaOH consumed was 1.09 g of 100% NaOH/L liquid to maintain pH 8.5 at L/G 107.2 L/m for 24 h. When compared to the results by pH at 20 °C, the ammonia removal efficiency is excellent despite less air supply and NaOH consumption for the unit liquid. NH removal (%) T T -ln( [NH ] / [NH ] ) 3,L t 3,L 0 Appl. Sci. 2021, 11, 441 9 of 13 after 6 h were 57.1%, 63.0%, and 57.1% for L/G of 4.5, 26.8, 53.6 L/m , respectively. The most effective removal performance was shown at L/G 26.8 L/m . Figure 6b shows the natural logarithms of the relative concentration according to L/G ratios. Figure 6b indicates that the slope and mass transfer coefficients increased with decreasing L/G ratio, except for 4.5 L/m . The reason the mass transfer coefficient increases with reducing L/G is that contact efficiency improves at the liquid-to-gas interface due to increased airflow rate. Base on the L/G ratio of 107.2 L/m , when it decreases by 1/2 or 1/4, the gas flow rate increases by two or four times, and the mass transfer coefficient increased 1.4 or 1.6 times, respectively. However, when L/G reduced to 4.5 L/m , the gas flow rate increases significantly by 24 times, whereas the mass transfer coefficient increases by 1.5 times. As a result, it was derived that the mass transfer coefficient increased according to L/G decrease but decreases below a certain level. The gas flow rate and the mass transfer coefficient are not necessarily proportional, and the optimal point should be derived. Appl. Sci. 2021, 11, x FOR PEER REVIEW 10 of 13 The amount of NaOH consumed was 1.09 g of 100% NaOH/L liquid to maintain pH 8.5 at L/G 107.2 L/m for 24 h. When compared to the results by pH at 20 C, the ammonia removal efficiency is excellent despite less air supply and NaOH consumption for the unit liquid. 100 2.0 o o 70 C, pH 8.5 70 C, pH 8.5 Liquid flow rate = 117.0 L/h Liquid flow rate = 117.0 L/h 1.5 1.0 L/G slope K a LNH3 3 3 4.5 L/m (L/m ) (/h) (/h) 4.5 0.143 0.194 26.8 L/m 0.5 20 3 26.8 0.159 0.222 53.6 L/m 53.6 0.122 0.174 107.2 L/m 107.2 0.068 0.099 0 0.0 0 2468 10 12 14 16 18 20 22 24 0 2468 10 12 14 16 18 20 22 24 Time (h) Time (h) (a) (b) Figure 6. Ammonia stripping performance according to the liquid-gas ratio at 70 °C and pH 8.5: (a) Ammonia removal Figure 6. Ammonia stripping performance according to the liquid-gas ratio at 70 C and pH 8.5: (a) Ammonia removal rate; rate; (b) logarithm of ammonia concentration ratio. (b) logarithm of ammonia concentration ratio. Figur Figure e 77 shows shows the tota the total air l air supp supplied lied aga against insthe t the a ammonia mmonia r remov emovalarate l rate a at dif t di fer fferent ent L/G ratios. Here, the total air supplied refers to the total cumulative air for 1 L of artificial L/G ratios. Here, the total air supplied refers to the total cumulative air for 1 L of artificial wastewater. To compare the total air supplied according to L/G ratio, it was revealed that wastewater. To compare the total air supplied according to L/G ratio, it was revealed that the lower L/G ratio, the greater the total air consumed at the same removal rate. The total the lower L/G ratio, the greater the total air consumed at the same removal rate. The total 3 3 3 3 volume of air required was 240 L/L liquid at 53.6 L/m and 212 L/L water at 107.2 L/m volume of air required was 240 L/L liquid at 53.6 L/m and 212 L/L water at 107.2 L/m to to achieve 76.1%, where the ammonia removal rate was equal to the ammonia fraction achieve 76.1%, where the ammonia removal rate was equal to the ammonia fraction (h/ f = 1). At L/G ratios of 4.5–26.8 L/m , it is predicted that a greater volume of air (𝜂/𝑓 NH = 1). At L/G ratios of 4.5–26.8 L/m , it is predicted that a greater volume of air will will be required more than 1600 and 300 L/L liquid, respectively. be required more than 1600 and 300 L/L liquid, respectively. From Figures 6 and 7, it was found that as L/G ratio increased, the mass transfer rate From Figures 6 and 7, it was found that as L/G ratio increased, the mass transfer rate improved, and the more air supply was required to ensure the same ammonia removal improved, and the more air supply was required to ensure the same ammonia removal rate. To increase the air supply, the size and power costs of the blower need to be increased. rate. To increase the air supply, the size and power costs of the blower need to be in- Therefore, it is necessary to determine the optimal L/G ratio considering removal efficiency, creased. Therefore, it is necessary to determine the optimal L/G ratio considering removal air supply, and time. In this study, the most efficient L/G condition was 26.8 L/m , and efficiency, air supply, and time. In this study, the most efficient L/G condition was 26.8 53.6 L/m also seemed to be appropriate when trying to lower the total air supply. In 3 3 L/m , and 53.6 L/m also seemed to be appropriate when trying to lower the total air sup- consequence, a liquid-gas ratio of 26.8–53.6 L/m was selected as an optimal condition for ply. In consequence, a liquid-gas ratio of 26.8–53.6 L/m was selected as an optimal condi- packed tower operating at 70 C, pH 8.5. tion for packed tower operating at 70 °C, pH 8.5. 4.5 L/m 1400 3 26.8 L/m 53.6 L/m 107.2 L/m 600 70 C, pH 8.5 Liquid flow rate = 117.0 L/h 0 20 406080 100 NH removal (%) Figure 7. Ammonia removal rate according to the volume of air supplied at 70 °C, pH 8.5, and different L/G ratios. NH removal (%) Total air supplied (L-air/L-liquid) T T -ln( [NH ] / [NH ] ) 3,L t 3,L 0 Appl. Sci. 2021, 11, x FOR PEER REVIEW 10 of 13 100 2.0 o o 70 C, pH 8.5 70 C, pH 8.5 Liquid flow rate = 117.0 L/h Liquid flow rate = 117.0 L/h 1.5 1.0 L/G slope K a LNH3 3 3 4.5 L/m (L/m ) (/h) (/h) 4.5 0.143 0.194 26.8 L/m 0.5 20 26.8 0.159 0.222 53.6 L/m 53.6 0.122 0.174 107.2 L/m 107.2 0.068 0.099 0 0.0 0 2468 10 12 14 16 18 20 22 24 0 2468 10 12 14 16 18 20 22 24 Time (h) Time (h) (a) (b) Figure 6. Ammonia stripping performance according to the liquid-gas ratio at 70 °C and pH 8.5: (a) Ammonia removal rate; (b) logarithm of ammonia concentration ratio. Figure 7 shows the total air supplied against the ammonia removal rate at different L/G ratios. Here, the total air supplied refers to the total cumulative air for 1 L of artificial wastewater. To compare the total air supplied according to L/G ratio, it was revealed that the lower L/G ratio, the greater the total air consumed at the same removal rate. The total 3 3 volume of air required was 240 L/L liquid at 53.6 L/m and 212 L/L water at 107.2 L/m to achieve 76.1%, where the ammonia removal rate was equal to the ammonia fraction (𝜂/𝑓 = 1). At L/G ratios of 4.5–26.8 L/m , it is predicted that a greater volume of air will be required more than 1600 and 300 L/L liquid, respectively. From Figures 6 and 7, it was found that as L/G ratio increased, the mass transfer rate improved, and the more air supply was required to ensure the same ammonia removal rate. To increase the air supply, the size and power costs of the blower need to be in- creased. Therefore, it is necessary to determine the optimal L/G ratio considering removal efficiency, air supply, and time. In this study, the most efficient L/G condition was 26.8 3 3 L/m , and 53.6 L/m also seemed to be appropriate when trying to lower the total air sup- ply. In consequence, a liquid-gas ratio of 26.8–53.6 L/m was selected as an optimal condi- Appl. Sci. 2021, 11, 441 10 of 13 tion for packed tower operating at 70 °C, pH 8.5. 4.5 L/m 26.8 L/m 53.6 L/m 107.2 L/m 600 70 C, pH 8.5 Liquid flow rate = 117.0 L/h 0 20 406080 100 NH removal (%) Figure 7. Ammonia removal rate according to the volume of air supplied at 70 C, pH 8.5, and Figure 7. Ammonia removal rate according to the volume of air supplied at 70 °C, pH 8.5, and different L/G ratios. different L/G ratios. 4.3. The Comparison of Results with Literature Table 4 shows the comparison of results for ammonia stripping system with the literature. First, the stripping column results according to pH were compared. In this study, the ammonia removal efficiency increased in proportion to ammonia fraction as the pH increased. Zhu et al. [15] reported that an increase in pH increase leads to improve ammonia stripping rate and mass transfer rate. When ammonia fraction and air supply are similar, the removal rate and mass transfer coefficients are also similar to those derived from literature [15]. Secondly, packed tower results were compared. It is difficult to compare the mass transfer coefficient arithmetically because the interfacial area for the packed tower varies depending on the design of the tower and the packed material. However, it is possible to compare the approximate mass transfer rates by considering the ammonia fraction, air supply, and liquid ratio. Liu et al. [14] derived the mass transfer coefficient according to air flow rate. Although there is no mention of the L/G ratio, it can be interpreted that the lower L/G, the higher K a, since the L/G decreases as the air flow rate increases. When compared with this L, NH study, it was confirmed that K a was almost same under similar air supply conditions. L, NH It is considered that the calculated K a values are within a reasonable range when L, NH compared with the reports by Liu et al. [14] and Ferraz et al. [12]. Li et al. [16] reported the highest value of K a values under very low L/G ratio when compared to other L, NH literatures. In addition, ammonia removal rate was also compared for the jet loop reactor and the water-sparged aerocyclone. In these two reactors, as in the column and packed tower, the mass transfer coefficient increased as the liquid to gas ratio decreased. NH removal (%) Total air supplied (L-air/L-liquid) T T -ln( [NH ] / [NH ] ) t 0 3,L 3,L Appl. Sci. 2021, 11, 441 11 of 13 Table 4. The comparison of removal rate and overall volumetric mass transfer coefficient based on the liquid phase of ammonia. T Time Air Supplied L/G h K a L, NH Equipment pH f Ref. NH 3 ( C) (h) (L-Air/L-Liquid) (L-Liquid/m -Air) (%) (/h) 8.9 0.260 13.9 0.219 9.4 0.527 38.0 0.340 20 2.5 2250 This work 10.2 0.875 62.4 0.462 Stripping 10.8 0.965 72.6 0.574 column without circulation 10 0.864 1125 24.8 0.084 12 0.998 1125 - 55.4 0.24 25 3 [15] 12 0.998 2250 76.1 0.44 6 1572 4.5 57.1 0.194 6 262 26.8 63.0 0.222 70 8.5 0.761 This work 12 262 53.6 77.3 0.174 24 262 107.2 81.5 0.099 720 63.6 0.086 Packed tower 1440 83.4 0.166 [14] with circulation 50 10 0.970 12 2880 98.7 0.368 25 11 0.985 24 4500 6.67 99 0.18 [12] b b 15 10.8 0.937 3.5 3000 0.332 75 0.42 [16] 7.8 930 2500 45.6 0.081 Jet loop reactor 20 11 0.978 7.8 1400 1667 87.2 0.299 [24] 6.8 2030 1000 96.3 0.629 Water-sparged 1540 0.0032 98.9 0.78 [22] 25 0.995 3.5 11.5 aerocyclone 2660 0.0018 93.7 1.32 a b Liquid to gas ratio, L/G. Average value. Appl. Sci. 2021, 11, 441 12 of 13 5. Conclusions Stripping experiments were performed using an air stripping column and a packed tower on an ammonium hydroxide solution to calculate the major operating factors for each device. For ammonia stripping with a column, an increase in pH resulted in higher recovery efficiency and a larger mass transfer coefficient, and a proportional relationship was observed between ammonia removal efficiency and ammonia fraction. However, ammonia concentration did not affect ammonia removal rate or the mass transfer coefficient. For the packed tower, the operating factors were analyzed at 70 C, pH 8.5. As the liquid-gas ratio decreased in the range of 26.8–107.2 L/m , the mass transfer rate and air consumption increased. The 26.8 L/m condition achieved the largest mass transfer coefficient and 63.0% of ammonia removal rate by only 6 h operation with 262 L-air/L-liquid. And at L/G of 53.6 L/m , ammonia removal was 77.3% for 12 h with same air consumption. Therefore, the optimal liquid-gas ratio was chosen as 26.8–53.6 L/m , by considering the optimal mass transfer coefficient and air supply. In addition, through high-temperature stripping, excellent ammonia removal rate was achieved with low NaOH consumption. Finally, overall operating conditions such as temperature, pH, and air consumption were optimized to ensure both performance and economics for ammonia stripping devices design. Author Contributions: Conceptualization, E.J.K.; writing—original draft preparation, E.J.K.; funding acquisition, H.K.; project administration, H.K.; writing—review and editing, E.L. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Korea Agency for Infrastructure Technology Advancement (KAIA) grant funded by the Ministry of Land, Infrastructure and Transport (Grant 21UGCP-B157945-02). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. 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Published: Jan 5, 2021

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