Ammonia removal from anaerobically digested dairy manure by struvite precipitation

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1 Process Biochemistry 40 (2005) Ammonia removal from anaerobically digested dairy manure by struvite precipitation S. Uludag-Demirer a, *,1, G.N. Demirer b,1, S. Chen c a Department of Industrial Engineering, Çankaya University, Ankara, Turkey b Department of Environmental Engineering, Middle East Technical University, Ankara, Turkey c Department of Biological Systems Engineering, Washington State University, Pullman, WA 99163, USA Received 9 June 2004; received in revised form 17 January 2005; accepted 21 February 2005 Abstract Ammonia is one of the most important contaminants impairing the quality of water resources. When this is considered along with the fact that the global demand for nitrogenous fertilizers is in constant rise, the need for recovery as well as removal of nitrogen is well justified. Crystallization of N and P in the form of struvite (MgNH 4 PO 4 6H 2 O), which is a slow releasing and valuable fertilizer, is one possible technique for this purpose. This study investigated the removal of NH 4 through struvite precipitation from the effluents of one- (R1) and twophase (R2) anaerobic reactors digesting dairy manure. To force the formation of struvite in the anaerobic reactor effluents, Mg 2 ion was added by using both Mg(OH) 2 and MgCl 2 6H 2 O. To prevent the effect of different total phosphorus (TP) concentration in the effluents of R1 and R2, as well as to not limit the formation of struvite, an excess amount of PO 4 (0.14 M) was added in the form of Na 2 HPO 4. Different stoichiometric Mg 2 :NH 4 :PO 4 ratios were tested to determine the required Mg 2 concentrations for maximum NH 4 removal by keeping NH 4 :PO 4 ratio constant for the effluents of reactors R1 and R2. The results revealed that very high NH 4 removal efficiencies (above 95%) were possible by adding Mg 2 ions higher than 0.06 M concentration in the effluents from reactors R1 and R2. It was also observed that the initial ph adjustment to 8.50 using NaOH did not result in any significant increase in the removal of NH 4 and the removal of NH 4 in the reactors treated with MgCl 2 6H 2 O was higher than those treated with Mg(OH) 2 for the same Mg 2 concentration. # 2005 Published by Elsevier Ltd. Keywords: Ammonia nitrogen; Struvite; Anaerobic digestion; Dairy manure 1. Introduction The most commonly encountered contaminants impairing the quality of the water resources in the U.S. and Europe are phosphorus (P) and nitrogen (N). For example, eutrophication problems have been identified in about half of the impaired lake areas and 60% of the impaired river reaches in the U.S. [1]. The inputs of P and N originate from both point (municipal and industrial wastewater, leachate and runoff from waste disposal sites, runoff and infiltration * Corresponding author. Tel.: ; fax: address: sibel@cankaya.edu.tr (S. Uludag-Demirer). 1 At the time of the study, the authors were affiliated with the Department of Biological Systems Engineering, Washington State University, Pullman, WA 99163, USA. from animal feed lots, construction sites, mines, etc.) and non-point (runoff from agriculture, pasture, atmospheric deposition, activities on land such as logging, development of land and waterways, etc.) sources. Point sources are usually continuous and can be monitored and controlled by adopting a treatment technology at the source, while nonpoint sources are usually intermittent and difficult to control. Recently, control over the point sources of N and P shifted from removal to recovery, with a particular emphasis on improving the sustainability of agricultural activities. This was mainly due to the increasing global demand for the nitrogenous fertilizer (from 10 Mt N in 1960 to 90 Mt N in 1998) [2] and the limited phosphorus rock reserves [3]. Therefore, the current attempts are not only to protect the water resources, but also to extract the maximum amounts of N and P from the recoverable sources, such as livestock /$ see front matter # 2005 Published by Elsevier Ltd. doi: /j.procbio

2 3668 S. Uludag-Demirer et al. / Process Biochemistry 40 (2005) manure. According to the USDA Economic Research Service [4], 1.7 Mt of recoverable N and 0.7 Mt of recoverable P were produced by the livestock and poultry in 1997 in U.S. One of the techniques to remove and recover N and P is crystallization of N and P in the form of struvite (magnesium ammonium phosphate or MgNH 4 PO 4 6H 2 O), which is a slow releasing (less soluble in water) valuable fertilizer. Moreover, the removal of P and N by struvite precipitation reduces the amount of sludge, which needs to be disposed from the treatment facilities. The struvite precipitation technique has been applied to various wastewaters, such as swine waste [5,6], agro-industrial effluents [7], landfill leachate [8,9], calf manure [10], coke manufacturing [11], leather tanning [12] and anaerobic digester sidestreams [13 16]. Depending on the composition of the wastewater, struvite precipitation can be used to remove ammonia (NH 4 ), phosphate (PO 4 ) or both. Regardless of the compound that is targeted for removal, all of the studies up to date except Battistoni et al. [14,15,17,18] have utilized the addition of Mg 2 ion, the usual limiting reactant in the formation of struvite, as the means for altering the solubility product equilibrium and initiating precipitation. Various sources of Mg 2 ion, such as Mg(OH) 2, MgO, MgCl 2 6H 2 O, etc., have been used. The chemicals used as Mg 2 ion source along with the molar ratios of Mg 2 :NH 4 :PO 4 adopted, the optimal ph values determined and the removal efficiencies achieved by struvite precipitation are summarized in Table 1. Since the phosphorus concentration was low in some of the wastewaters, it was added into the reactors in order not to limit the struvite formation (Table 1). Table 1 Data on the removal of NH 4 and PO 4 by struvite precipitation from different wastewaters (no field experiments or full scale operation) Type of the waste Combined wastewater from bovine and leather tanning factories Sidestreams from AD treating sludge from biological phosphorus removal in WWTP Chemicals added Amount of the chemicals Initial concentrations Removal (%) Optimum ph Reference NH 4 PO 4 NH 4 PO 4 MgCl 2 2H 2 O Mg:N:P = 1:1: NI 9.0 [12] Na 2 HPO 4 Mg(OH) 2 Slurry, 55% (w/w) Mg(OH) 2 (ortho-p) 6 94 (ortho-p) 8.5 (Mg(OH) 2 slurry) Mg:P = 1:1.3 Swine waste MgCl 2 Mg:TP = 1.6:1 572 (SP) 91 (SP) 9.0 (1 M NaOH) [5] Anaerobic supernatant from a centrifugation sludge section of a civil biological nutrient removal plant Effluent from the biologically (UASBR) treated opium alkaloid wastewater Effluent from the anaerobic treatment of the baker s yeast industry Effluent from the anaerobic treatment of domestic wastewater 2% leachate Effluent from the anaerobic treatment of landfill leachate Wastewater from the cochineal insects processing Supernatant from an anaerobic digestion of sludge from a sewage treatment plant No chemicals added NI (by aeration) [17] MgCl 2 6H 2 O Mg:N:P = 1:1: NI 9.2 [7] Na 2 HPO 4 MgCl 2 6H 2 O Mg:N:P = 1:1: NI 9.2 [7] Na 2 HPO 4 MgCl 2 6H 2 O Mg:N:P = 1:1: NI 9.2 [9] Na 2 HPO 4 2H 2 O MgCl 2 6H 2 O Mg:N:P = 1:1: NI 9.2 [9] Na 2 HPO 4 2H 2 O Low grade MgO 24 g/l (MgO) [31] MgSO 4 7H 2 O Mg:P = 1.1: NI [32] TP, total phosphorus; SP, soluble phosphorus; AD, anaerobic digester; WWTP, wastewater treatment plant; NI, not investigated. [16]

3 S. Uludag-Demirer et al. / Process Biochemistry 40 (2005) Struvite is a white crystalline solid forming according the general and simplified reaction of Mg 2þ þ NH 4 þ þ PO 4 þ 6H 2 O! MgNH 4 PO 4 6H 2 O: The precipitation of struvite is affected by several factors, namely the ph, the chemical composition of the wastewater (degree of saturation of the solution with respect to magnesium, ammonium and phosphate; presence of other ions, such as, calcium; ionic strength of the solution), and the temperature of the solution [19,20]. The precipitation of struvite in a solution is primarily controlled by ph because the concentrations of the ions forming the struvite are all ph dependent. For instance, the concentration of NH 4 decreases significantly from 99 to 64% when the ph increases from 7 to 9, while PO 4 concentration increases 250-fold in the same range of ph change [21]. Therefore, studies on the removal of N and P by struvite precipitation have considered ph as the parameter to be optimized or as a parameter to be adjusted through the use of a strong base or air stripping (Table 1) with the optimal ph values for struvite precipitation ranging from 9 to 10.7 [6]. Struvite formation, however, can be observed in a wider ph ranging from 7 to 11 [22]. Even though struvite precipitation and accompanied removal of P and N from the effluents of several biological treatment units has been studied previously by many researchers, the optimal conditions (ph, Mg 2 ion concentration) for struvite precipitation should be determined for different systems, such as dairy manure. This approach is particularly important for struvite research because the complex chemical principles, which govern its precipitation as described above are readily altered by slight changes in the chemical composition and temperature of the wastewater being studied. In this study, the effluents from two different anaerobic reactors for dairy manure (one- and two-phase) were subjected to struvite precipitation to remove ammonia nitrogen. The objective of this study was three-fold: (i) quantify the required Mg 2 concentration for struvite formation by using two different Mg 2 bearing chemicals, Mg(OH) 2 and MgCl 2 H 2 O, (ii) determine the effect of initial ph adjustment by NaOH on struvite precipitation and (iii) compare the ammonia nitrogen removal efficiencies in the effluents from one- and two-phase anaerobic reactors by struvite precipitation. 2. Materials and methods 2.1. Anaerobic reactors Fresh dairy manure was collected from the Dairy Center at Washington State University in Pullman, WA and stored at 4 8C prior to use. The composition of the raw dairy manure was analyzed and reported in Wen et al. [23]. The mixed anaerobic culture used as seed was obtained from the anaerobic lagoon of the same facility and stored at 4 8C prior to use. The mixed anaerobic culture was filtered through a screen of in. (1.19 mm) mesh size and concentrated by settling before being used as inoculum. The volatile suspended solids (VSS) concentration of the concentrated seed cultures used was mg/l. Two different anaerobic reactors (R1 and R2) digesting dairy manure were operated in the study. R1 represented the conventional one-phase anaerobic digester with a solids retention time/hydraulic retention time (SRT/HRT) of 20 days. The two-phase containing reactor configuration had R2-1 and R2-2 as the first (acidogenic) and second (methanogenic) phases. The SRT/HRT values of R2-1, R2-2 and the overall two-phase configuration were 2, 8 and 10 days, respectively. The effective volumes of R1, R2-1 and R2-2 were 2.0, 0.4 and 1.6 L, respectively. All of the reactors were fed daily. The gas production in R1 and R2-2 were monitored by a wet-tip gas-meter (Speece Tip, Nashville, TN). R1 and R2-2 were maintained at 36 8C (2 8C) in a temperature-controlled water bath and were shaken manually once a day. R2-1 was incubated in an incubator shaker (New Brunswick Scientific, Edison, NJ) at C and 180 rpm. R1, R2-1 and R2-2 were seeded with 500, 100 and 400 ml of mixed anaerobic seed culture, respectively. Before the onset of daily feeding from R2-1 to R2-2, R2-1 was operated for 26 days at an SRT/HRT of 2 days to achieve an active acidifying culture. Table 2 depicts the operating conditions and performance of R1 and R2 when their effluents were collected and subjected to struvite precipitation immediately after collection. Before the struvite experiments, the effluents of R1 and R2 were analyzed for ph, ammonia nitrogen (NH 4 N), total phosphorus (TP) and total dissolved solids (TDS) Struvite precipitation experiments The struvite precipitation experiments were conducted using the effluents collected from both R1 and R2. The effluents were collected in sealed plastic containers and used immediately after collection. Two different Mg 2 providing chemicals, namely Mg(OH) 2 (Fluka, Germany) and MgCl 2 6H 2 O (J.T. Baker, USA), were used in the experiments for comparison. The amounts of the chemicals were varied to obtain a Mg 2 concentration range of M so as to identify the required Mg 2 concentration for the ammonia removal in the form of struvite. In order to avoid the effect of variable PO 4 concentrations in the effluents of R1 and R2 and to not limit struvite precipitation, PO 4 was added to the reactors in excess (20 g/l or 0.14 mol PO 4 /L) in the form of Na 2 HPO 4 (J.T. Baker, USA). The ph adjustments were made using a 1.0N NaOH (J.T. Baker, USA) solution. Struvite precipitation experiments were conducted in continuously stirred batch reactors at room temperature ( C). The effluent solutions were dark in colour and high

4 3670 S. Uludag-Demirer et al. / Process Biochemistry 40 (2005) Table 2 The operating conditions and performance of R1 and R2 when their effluents were collected and subjected to struvite precipitation experiments COD loading rate (g/l day) VS in the feed (%) SRT/HRT (days) COD removal efficiency (%) Average daily gas production (L) R1 (one-phase) R2 (two-phase) in suspended solid concentration. A pretreatment of the effluents was adopted prior to struvite precipitation experimentation with the coarse solid material being separated from the effluents to obtain a more homogeneous solution by using a screen of in. (1.19 mm) mesh size. Each struvite reactor contained 50 ml of screened R1 or R2 effluent and was continuously mixed using a magnetic stirrer (Fisher Scientific, USA). Then Mg 2 containing chemical and excess PO 4 were added into the reactors. In the ph-adjusted set of experiments, the initial ph was fixed to 8.50 by using NaOH after the addition of other chemicals to test the effect of initial ph variation on the struvite precipitation. The non-ph-adjusted experiments had an initial ph in a range of The ph of the solution was monitored until equilibrium conditions prevailed in the reactor, which was determined by observing a fixed ph with a variation of 0.01 ph units. After the struvite formation reaction ceased, the samples were collected, centrifuged (DuPont Instruments, USA) for 20 min at 4000 rpm and stored at 10 8C until the analyses were conducted Analytical methods Chemical oxygen demand (COD), volatile solids (VS), NH 3 N, TP, TDS and ph analyses were performed at the WSU Water Quality Lab as described in Standard Methods [24]. 3. Results and discussion 3.1. Composition of the effluents of R1 and R2 The ph of the effluents both from R1 and R2 fluctuated in a range of (Table 3). This range might be considered a little high for anaerobic digestion of many substrates. However, these relatively high observed ph values can be explained by the alkalinity generated by the anaerobic biotransformation of nitrogenous organic compounds [25] contained in the dairy manure used in this study [23]. As depicted in Table 3, the ammonia nitrogen Table 3 Composition of the effluents from the anaerobic reactors Parameter R1 R2 ph NH 4 (mg/l) TP (mg/l) TDS (mg/l) concentration of R1 and R2 was also significantly different. The possible mechanisms which are thought to play roles in this observation can be listed as follows: different organic loading rates and influent VS concentrations for R1 and R2 (Table 2); anaerobic biotransformation of proteins contained in animal manure into amino acids and then ammonia; finally utilization of ammonia as a growth nutrient by the anaerobic bacteria [25 27]. Different organic loading rates and influent VS concentrations used for R1 and R2 (Table 2) also led to the difference in TP concentrations in the effluents of R1 and R2. As already explained above, this observed difference was the reason for adding excess PO 4 in the struvite precipitation experiments in order to avoid any possible effect of this fluctuation on the removal of NH 4. TDS concentration was high in the effluent of R1 as compared to that of R2; indicating a higher ionic strength in the effluent of R1 [21] The effect of Mg 2 concentration on the equilibrium ph Equilibrium conditions were achieved in less than 1 h in all of the experimental runs on the basis of fixed ph (equilibrium ph or ph eq ) with a variation of 0.01 ph unit. This fast struvite formation kinetics has also been observed by other researchers [6,28] and they have also reported equilibrium times less than or equal to 1 h. The change in the ph of the solutions during the experiments was similar regardless of the type of the reactor, Mg 2 bearing chemical used, and the initial ph adjustment by NaOH on the basis of the reaction time for reactors R1 and R2 as shown in Fig. 1a and b, respectively. There was a sudden decrease in the ph of the solution at the beginning of each experiment, which might correspond to the formation of struvite crystals followed up by the liberation of the H ion(s) from the dominant P containing ions, H 2 PO 4 and HPO 4 2, in the ph range of [29]. This decrease in the ph lasted during the first 5 10 min of the reaction time and then the ph started to increase until the equilibrium conditions established ((DpH/Dt) = 0.01) as a result of the chemistry of the solution after the Mg(OH) 2 and NaOH addition to the reactors (Fig. 1a and b). The reactors, which were dosed by Mg(OH) 2 had higher ph values as the concentration of the Mg 2 increased regardless of the initial ph adjustment by NaOH. This was because of the basic nature of Mg(OH) 2. Similarly the reactors fed by MgCl 2 6H 2 O and NaOH showed a continuous and gradual increase in ph until the equilibrium conditions prevailed, while the reactors fed only by MgCl 2 6H 2 O had slightly

5 S. Uludag-Demirer et al. / Process Biochemistry 40 (2005) Fig. 1. (a) The change in the ph of the solution during the struvite precipitation experiments for the effluent from reactor R1. The series in the figures correspond to the number of moles of Mg 2 added to the reactors for a constant NH 4 :PO 4 = 1.0:4.8 ratio. (b) The change in the ph of the solution during the struvite precipitation experiments for the effluent from reactor R2. The series in the figures correspond to the number of moles of Mg 2 added to the reactors for a constant NH 4 :PO 4 :1.0:10.0 ratio. fluctuating ph around 7.0 without an apparent increase (Fig. 1a and b). Observing no significant increase in the ph of these reactors was mainly due to the acidic effect of MgCl 2 6H 2 O in the solution. The change in the ph eq achieved versus the concentration of Mg 2 added is shown in Fig. 2 for the experiments performed using Mg(OH) 2 and MgCl 2 6H 2 O. The ph eq increased as the amount of Mg 2 ion increased when it was

6 3672 S. Uludag-Demirer et al. / Process Biochemistry 40 (2005) Fig. 2. The equilibrium ph in the reactors dosed with different Mg 2 ion concentrations using (a) Mg(OH) 2 and (b) MgCl 2 6H 2 O as the Mg 2 source. added in the form of Mg(OH) 2 for both the initial phadjusted and not adjusted experiments (Fig. 2a). This increase in ph eq was expected due to the basic characteristic of Mg(OH) 2. The ph eq was measured as 9.00 when the concentration of Mg 2 was about 0.03 M and continued to increase to the levels of as the concentration of Mg 2 increased (Fig. 2a). This is one of the disadvantages of using Mg(OH) 2 in addition to its low solubility. The ph eq does not only control the solubility of struvite but also the major removal pathway of NH 4. Since NH 4 presents itself in a volatile NH 3 form at higher ph values, the removal efficiency of any system operating at high ph and open to the atmosphere has to be considered to be a result of both struvite precipitation and air stripping. Thus, this fact may be an additional determining factor beyond cost that affects the type of chemical to be used as the Mg 2 source. There was, on the other hand, a decrease in ph eq as the Mg 2 ion concentration increased when it was added in the form of MgCl 2 6H 2 O regardless of the initial ph and this was due to added acidity originating from the Cl ions into the system (Fig. 2b). Therefore, there was no need to consider the removal of NH 4 in the form of NH 3 for the reactors dosed with MgCl 2 6H 2 O The effects of the Mg 2 concentration on the removal of NH 4 Struvite forms as a hard white crystalline solid when the molar ratio of Mg 2 :NH 4 :PO 4 is greater than 1:1:1 [30]. In this part of the study, a range of molar ratios of Mg 2 :NH 4 :PO 4 was tested by keeping the PO 4 concentration high in all of the experimental runs. The molar ratios of Mg 2 :NH 4 :PO 4 varied in 0.6:1.0: :1.0:4.8 and 1.1:1.0: :1.0:10.0 ranges for the Fig. 3. The remaining concentration of NH 4 in the effluent of (a) R1 with initial ph adjustment, (b) R1 without ph adjustment, (c) R2 with initial ph adjustment and (d) R2 without ph adjustment. reactors R1 and R2, respectively. The remaining concentrations of NH 4 at equilibrium for different doses of Mg 2 for R1 and R2 with and without initial ph adjustment are shown in Fig. 3. Regardless of the initial ph adjustment and the source of Mg 2 ion, the concentration of NH 4 decreased as the concentration of Mg 2 increased. In the reactors treated with Mg(OH) 2, the concentration of NH 4 at equilibrium decreased to the levels less than 10 mg/l corresponding to the Mg 2 concentrations above 0.03 M and the ph eq above Therefore the removal of NH 4 from the solutions could be due to the two mechanisms, struvite formation and NH 3 formation under the specified conditions. The concentration of NH 4 at equilibrium was lower than 10 mg/l in both of the reactors when MgCl 2 6H 2 O was added more than 3.0 mol for each mole of NH 4 (Mg 2 :NH 4 = 3.8:1.0 for R1 and 3.4:1.0 for R2), which is higher than the amount required stoichiometrically for the struvite formation. The use of Mg 2 at a concentration greater than the stoichiometric amount needed to achieve the desired effluent NH 4 concentration has also been observed in other studies mainly due to the composition of the wastewater investigated. The presence of complexing agents forming complexes with Mg 2, the equilibrium ph and the ionic strength are the major factors affecting the amount of Mg 2 required and the conditional solubility of struvite. Although the composition of the precipitate was not

7 S. Uludag-Demirer et al. / Process Biochemistry 40 (2005) Fig. 4. The removal efficiency of NH 4 from the effluents of R1 and R2 at different Mg 2 ion concentrations added using (a) Mg(OH) 2 and (b) MgCl 2 6H 2 O. analyzed in this study, the formation of other solids besides struvite may also be causing the use of Mg 2 in unpredictable amounts [5]. The initial ph adjustment to 8.50 by NaOH was performed to factor out the effect of the fluctuations of the initial ph on the removal of NH 4. As can be seen in Fig. 3, the remaining concentrations of NH 4 in the reactors with the initial ph of 8.50 did not differ significantly from that in the reactors without ph adjustment (ph ). This shows that the slight fluctuations in the initial ph of the effluents collected from R1 and R2 did not affect the performance of the NH 4 removal by struvite precipitation The comparison of NH 4 removal in the effluents of R1 and R2 The NH 4 removal efficiencies achieved were above 95% when the Mg 2 concentration was more than 0.06 M in both of the reactor effluents regardless of the type of chemical used as Mg 2 source as shown in Fig. 4a and b. In the reactors containing less than 0.06 M Mg 2, the NH 4 removal efficiency was lower in the R1 effluent compared to the R2 effluent due to the high initial concentration of NH 4. Since the effluent collected from the R1 had a higher initial NH 4 concentration than R2, the Mg 2 ion concentration was not sufficient to remove the NH 4 at a higher level. A better NH 4 removal was obtained when MgCl 2 6H 2 O was used and this could be due to the higher solubility of MgCl 2 6H 2 O compared to Mg(OH) 2, which makes the Mg 2 more available for the struvite formation. One of the interesting findings in this study was the difference in the concentration of Mg 2 added to the reactor effluents to achieve similar NH 4 removal efficiencies. For instance, to obtain 90% removal of NH 4 in the effluent of R1 and R2, M (Mg 2 :NH 4 = 2.2:1.0) and M (Mg 2 :NH 4 = 1.3:1.0) Mg 2 in the form of Mg(OH) 2 and M (Mg 2 :NH 4 = 1.3:1.0) and M (Mg 2 :NH 4 = 0.9:1.0) Mg 2 in the form of MgCl 2 6H 2 O had to be added into the reactors, respectively. The requirement for lesser Mg 2 concentration in the R2 reactor effluent for 90% removal efficiency was mainly due to the lower initial concentration of NH 4 and ionic strength in the effluent from reactor R2 than reactor R1. The ionic strength of the solution has been reported to be an important parameter controlling the formation and precipitation of the struvite [20]. However, the results also showed that similar and higher removal efficiencies could be obtained in the effluents of the reactors, R1 and R2, when the concentration of Mg 2 was higher than 0.06 M regardless of the type of the chemical used to add Mg 2 into the reactors testing the struvite precipitation. This could be due to the excess concentration of Mg 2 corresponding to the conditions, where the ionic strength difference is no longer effective on the formation and dissolution of the struvite and removal of NH 4 via the formation of NH 3 at high ph eq in the reactors dosed by Mg(OH) 2. However, there is a need to examine the effect of ionic strength on the formation and precipitation of the struvite under excess Mg 2 and PO 4 concentrations and the removal rate of NH 4 as a result of NH 3 formation under the specified conditions to obtain more conclusive results. 4. Conclusions The experimental results of this study indicated that the reduction of NH 4 concentrations below 10 mg/l is possible in anaerobically digested dairy manure by struvite precipitation technique. The main conclusions, which can be drawn from this study are as follows: The variation observed in the initial ph of the effluents from R1 and R2 did not affect the removal efficiency of NH 4 significantly. Therefore, there is no need to adjust the ph initially to favour struvite formation, which makes the process more cost-effective by omitting the use of NaOH. To achieve NH 4 removal efficiencies above 95%, 0.06 M Mg 2 was necessary for the effluents of the reactors R1 and R2. MgCl 2 6H 2 O proved to be more efficient Mg 2 source than Mg(OH) 2 in terms of final NH 4 concentrations obtained as well as the amount of Mg 2 required to obtain similar removal efficiencies observed in the reactors treated using Mg(OH) 2. Less Mg 2 concentration was required in the effluent of reactor R2 to achieve similar removal efficiencies obtained in the effluent of reactor R1 as a result of lower NH 4 concentration and lower ionic strength of the effluent of the reactor R2.

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