Recovery of Ammonia as Struvite from Anaerobic Digester Effluents
Ipek Çelen1* and Mustafa Türker1,2
1
Department of Environmental Engineering, Gebze Institute of Technology, Gebze, Kocaeli Turkey
(e-mail: ipekcelen@usa.net)
2
Pak-Food Industries, PO Box 149, 41001, Izmit, Turkey (e-mail:mustafat@pakmaya.com.tr)
Abstract
The effects of environmental conditions on ammonia removal as struvite (Magnesium ammonium phosphate, MAP)
were studied in a laboratory scale batch reactor. MAP precipitation was carried out by adding phosphoric acid and
magnesium source either as MgCl2 or MgO. The effect of temperature, pH, M:N:P ratios were studied. Temperature
did not significantly affect ammonia removal between 25-40 0C and over 90% removal was obtained. The effect of pH,
however, was significant and highest removal was reached at pH 8.5-9.0. The various stoichiometric ratios of
ammonium to M and P have been tested and slight excess of M and P found to be beneficial for higher recovery of
ammonia as struvite. However further increase in M and P ratios did not result in further ammonia removal which is
also costly for the practical application of the process. When MgO was used as M source, the ammonia recovery was
60-70% whereas the use of MgCl2 has increased this figure up to 95%. In addition two steps purification process was
developed to recover MAP crystals from impurities of anaerobic digester. Firstly, precipitates were dissolved in acid
and impurities was removed by centrifugation. The clarified supernatant was re-precipitated by adjusting its pH with
caustic. It was shown that in two steps process white MAP crystals could be obtained with over 85% recovery to be
used for another applications. The economical analysis of the process has shown that ammonia in the digester effluents
can be recovered at the cost of $7.5-8.0/kg NH4+-N. The rate of reaction is very fast completed almost in minutes. This
simplifies the process design resulting in smaller reaction vessel.
Keywords:
Struvite, Magnesium Ammonium Phosphate, ammonia recovery, fertilizer, precipitation.
INTRODUCTION
Nitrogen compounds are present in some industrial as well as in domestic wastewaters in significant
quantities 1. Due to human impact on nitrogen cycle, the reactive nitrogen compounds are accumulating on earth
causing eutrophication in receving waters and deterioration of water quality [2]. Significant costs are associated
with extra treatment required to reduce discharge concentrations. There are number of physicochemical and
biological techniques available for the treatment of nitrogen containing waste streams. The techniques such as
biological nitrification/denitrification and breakpoint chlorination reduce nitrogen compounds to dinitrogen gas.
However, alternative technologies exist to convert ammonia into reusable and saleable useful product thus
contributing overall nitrogen cycle. One such technology is the recovery of ammonia as struvite which has a
potential as fertilizer.
PO4
-3
Struvite or magnesium ammonium phosphate (MAP) precipitates in the presence of Mg+2 (M), NH4+ (N) and
(P) according to following reaction when the thermodynamic solubility product, K s, is exceeded:
Mg 2  NH 4  PO43  6H 2 O  MgNH4 PO4 .6H 2 O



K s  Mg 2 NH 4 PO43

(i)
(ii)
However, it has been shown that in crystalization experiments, the precipitation of struvite reduces the pH which
suggests that HPO4-2 would precipitate in the reaction rather than PO4-3 according to following reaction
Mg 2  NH 4  HPO42  6H 2 O  MgNH4 PO4 .6H 2 O  H 
(iii)
In the literature, several papers addressed the recovery of ammonia or phosphate as struvite from industrial
and domestic wastewaters [3, 4, 5, 6, 7]. The precipitation of struvite may present some problems in wastewater
treatment plants causing deposits in pipe walls [8, 9]. However, struvite has a potential use as a fertilizer. It has
been shown to be a highly effective source of nitrogen, magnesium and phosphorus for plants and can be used as a
slow release fertilizer at high application rates without damaging plant roots [10, 11].
MATERIALS AND METHODS
Struvite precipitation experiments were carried out in a batch reactor with a volume of 200 ml mixed with
magnetic stirrer. The temperature of the reactor is controlled at desired value by thermostatic controller. The pH
measurements are made with pH meter E588 Metrohm Herisau and pH is adjusted either with HCl or NaOH
solutions. The chemicals used were commercial grade and their contents were determined before the experiments.
All analyses were carried out according to Standart Methods [12]. After each experiment, the supernatants and the
precipitates were analysed for M, N and P to check the consistency of experimental results and all balances closed
within acceptable limits.
RESULTS AND DISCUSSIONS
The composition of wastewater
The wastewater used in this work to recover ammonia is the effluent of anaerobic digester treating molasses
based industrial wastewater, the chemical composition of which is given in Table 1.
Table 1.
The approximate composition of wastewater.
Parameter
NH4+
Mg2+
PO43Ca2+
K+
COD
pH
Concentration in water (mg l-1)
1400
21.4
24
21.2
2150
3240
7.9
The effluent contains approximately 1400 mg l-1 ammonia and negligible amounts of phosphate and magnesium.
Therefore they are added in stoichiometric quantities in struvite precipitation studies. Calcium was also present in
negligible quantities. However, potasium concentration was not negligible around 2150 mg l-1.
The effect of time
NH4+(mgl -1)
The time course of the reaction between Mg+2 (M), NH4+ (N) and PO4-3 (P) to form struvite has been studied
in order to establish required equilibrium time. The time course of struvite precipitation has been followed by
analysing ammonia concentration in the supernatant when both MgCl2 and MgO were used as M source, as shown
in Figure 1. The ammonia concentration reached its equilibrium value immediately after the mixing with
phosphate and magnesium source either MgCl2 or MgO and remained constant during the rest of the test period.
Therefore, forty minutes reaction time has been accepted as safe time for the reaction to proceed to equilibrium.
1600
1400
1200
1000
800
600
400
200
0
MgCl2
MgO
0
5 10 15 20 25 30 35 40 45 50 55 60 65
Time (minutes)
Figure 1.
Time course of struvite precipitation
Effect of pH
Ammonia nitrogen is distributed between NH4+ and NH3 as a function of pH according to following
equilibrium reaction:
NH 4  H 2 O  NH 3  H 3O 
(iv)
where the equilibrium constant, Ka, is defined as [13].
Ka 
NH 3 H 3O    5.7 *10 10
NH 
(v)

4
The high pH values favor NH3 which is in equilibrium with air according to Henry Law and facilitates air
stripping of ammonia. The effluent containing NH4+ (1354 mg l-1) at 37 0C was kept at pH 7.9, 8.5 and 9.0 for 40
minutes. The results of the experiment is shown in Table 2 At pH 9.0 17.9% of ammonia is lost to air whereas at
the same condition no loss of ammonia is determined at pH 7.9.
pH
7.9
8.5
9.0
Table 2.
NH4+ (mg l-1)
1354
1205
1112
Ammonia lost to air (%)
11
17.9
The results of the experiment at pH 7.9, 8.5 and 9.0.
The effect of pH on ammonia recovery as struvite was studied at various stoichiometric ratios of ammonia to
M and P. The results were presented in Figure 2 when MgCl2 was used as magnesium source.
NH4+ Removal (%)
100
75
1:1:1
1:1:1.2
50
1.2:1:1
1.4:1:1
25
1.2:1:1.2
0
5.5
6.5
7.5
8.5
9.5
pH
Figure 2.
The effect of pH on ammonia removal at different M:N:P ratios.
As the pH of the medium increased, the percent of ammonia removed as struvite has increased as a function of
stoichiometric ratios of M:P relative to ammonia. Reasonably high removal ratios were obtained when M:N:P ratio
was 1.2:1:1.2 even at pH as low as 6.0.
Effect of M:N:P Ratios
According to reaction (i) magnesium, ammonium and phosphate are required in equimolar quantities to form
MAP. However, the experimentally obtained ratios may differ for optimum (or better) ammonia removal as
struvite due to the presence of some other species present in the effluent that may form by-products. The results
obtained with various M:P ratios relative to ammonia were presented in Figure 3.
NH4+ Removal (%)
100
90
80
70
60
50
0.8
1.0
1.2
1.4
1.6
1.8
2.0
Molar Ratio
Figure 3.
The effect of relative molar ratio of M:P to N on ammonia removal.
While slight excess of M and P were resulted in better removal of ammonia, the ratios above 1.2 did not yield
higher recovery of ammonia as struvite. Similarly, N and P were kept at equimolar concentrations and M
concentrations were increased to improve ammonia recovery. This resulted in improved recovery of ammonia as a
function of pH as shown in Figure 4.
NH4+ Removal (%)
100
90
80
pH 8.0
pH 8.5
70
pH 9.0
60
50
0.9
1.0
1.1
1.2
1.3
1.4
1.5
Molar Ratio
Figure 4.
The effect of relative molar ratio of M to N:P on ammonia removal.
Similarly, this time M and N were kept at same molar ratio and relative ratio of P was increased. However this
experimental protocol did not yield better ammonia recovery at or closer to optimum pH 9.0 as shown in Figure 5.
NH4+ Removal (%)
100
90
pH 8.0
80
pH 8.5
70
pH 9.0
60
50
0.9
1.0
1.1
1.2
1.3
Molar Ratio
Figure 5.
The effect of relative molar ratio of P to M:N on ammonia removal.
Effect of Magnesium Source
In addition to MgCl2, MgO has been tested as M source alternative to MgCl2. MgO is mentioned in the
literature as potential M source mainly due to its cheaper price and its basic character in pH adjustment [14].
NH4+ Removal (%)
100
90
80
pH 9
70
60
50
1.0
1.2
1.4
1.6
1.8
Molar Ratio
Figure 6.
The effect of molar ratio of MgO to fixed N:P at 1:1.2 on ammonia removal.
When MgO was used as M source, N:P ratio was kept at 1:1.2 and M ratio was changed from 1.2 to 1.6. As
the M ratio increased, relative removal ratio of ammonia increased from 60% to 70%. Both M sources were also
compared under identical conditions. In these experiments M:N:P ratios were maintained at 1.2:1:1.2 respectively
and results were presented in Figure 7. The ammonia recovery as struvite was realized over 95% when MgCl2 was
used whereas when MgO was used rather low values between 48% to 58% were obtained, similar to the results
presented in Figure 6.
NH4+ Removal (%)
100
90
80
MgO
70
MgCl2
60
50
40
7.5
8.0
8.5
9.0
9.5
pH
Figure 7.
The comparison of MgCl2 and MgO on ammonia removal at M:N:P ratios of 1.2:1:1.2.
In addition, the caustic consumptions were compared for both M sources for pH adjustment and the results are
shown in Table 3.
Table 3.
The effect of M source on caustic consumption for pH adjustment.
NaOH consumptions (30%)
(ml)
M source
pH 8
pH 8.5
pH 9
MgCl2.6H2O
MgO
5.5
2.25
5.75
2.35
6.0
3.5
Less caustic was consumed with MgO for pH adjustment indicating the more basic character of MgO in
comparison to MgCl2.
Effect of Temperature
NH4+ Removal (%)
The temperature of effluent may affect the equilibrium composition of struvite yielding different ammonia
recoveries. There are number of conradicting experimental results reported in the literature regarding the effect of
temperature [14]. Therefore the influence of temperature on ammonia removal as struvite has been studied
between 25-40 0C at M:N:P ratios 1.2:1:1.2. It was found that temperature has a negligible influence on ammonia
precipitation as struvite between the temperature range studied as shown in Figure 8. Almost over 95% ammonia
recovery as struvite was obtained over the range of temperatures studied.
100
90
80
70
60
50
20
25
30
35
Temperature (0C)
Figure 8.
The effect of temperature on ammonia removal.
40
45
Purification and Recovery of Struvite
In the struvite precipitation experiments, struvite precipitated together with impurities present in the effluent.
If the purified struvite is required for particular application, this can be achieved with two steps purification
process. Here we have studied clarification and mass balance of precipitated struvite obtained from coloured
effluent containing impurities. The flow diagram and results of mass balance are shown in Figure 9 and the
photograph of the steps is shown in plate-1
6.4 moles of NH4+
and 10.5 moles of Mg2+
in the supernatant
MgCl2.6H2O
(120 moles of Mg2+)
1.58 moles of NH4+ and
3.42 moles of Mg2+ in
the supernatant
Acid
pH
adjustment
100 moles
of NH4+
Caustic
Centrifuge
Supernatant
discarded
H3PO4
(120 moles of PO43-)
93.6 moles of NH4+
and 109.5 moles of Mg2+
in MAP
Figure 9.
Purification, and mass balances of struvite recovery in two steps process.
Plate 1.
The photograph of the two steps purification and recovery of struvite.
85.8 moles NH4+ and
107 moles of Mg2+
in MAP
The original effluent was coloured and white struvite crystals were precipitated with together impurities in the first
step. After discarding the supernatant and dissolving the precipitate in required volume of acid, the impurities were
removed from the suspension by centrifugation. Finally, clarified and dissolved struvite can quantitatively be
recovered by adjusting its pH by caustic solution. The mass balance of this purification process has shown that
85.8% of the ammonia present in the effluent could be recovered as white struvite crystals after two steps
clarification procedure. In fact the supernatant of the final step containing equilibrium concentrations of M, N and
P can be recycled back to the first step to increase the efficiency of the recovery process.
Economic Analysis of the Process
The cost of ammonia recovery as struvite was studied based on the experimental results presented here, in
order to assess the economical viability of the process in comparison to existing nitrogen removal technologies. In
this preliminary assessment, investment and utility cost such as electricity and water etc. is not taken into account
and only the cost of chemicals, M source, P source and caustic have been considered in the calculations as
examplified in figure 10. The commercial value of struvite was not considered, however, the market price of
struvite will determine the applicability of this process in practice. The market prices of the chemicals used in the
calculations are given in Table 4.
Table 4.
The market prices of the chemicals used in the experiments.
Chemical
H3PO4 (75%)
MgCl2.6H2O
MgO (85%)
NaOH (100%)
NH4+
Price ($/kg)
0.40
0.31
0.44
0.12
0.23
0.003 kg MgCl2.6H2O ($0.00093248)
Supernatant
1.23 ml H3PO4
($0.000584875)
Wastewater
200 ml
NH4+ = 1366 mgl-1
0.00339 kg MAP
($0.62/kg MAP)
($7.72/kg NH4+-N)
3.65 ml Caustic ($0.000128115)
Figure 10.
The simplified diagram for the calculation of cost of struvite from the experimental results.
The results of the economical analysis for the experimental results presented here are given in Table 5 and
Table 6 where the contribution of each chemicals to overall cost of struvite precipitated is calculated. The cost of
ammonia removal is the function of removal ratio and cost of the chemicals added per ammonia present in the
effluent. The relative cost of MgCl2 is highest compared to those of H3PO4 and caustic when it is used as M source,
amounting to approximately 55-65% of the overall cost. However the cost of the process per ammonia fixed as
struvite depends on removal ratio as well as stoichiometric ratio of chemicals used. When MgO is used as M
source, main cost factor is the cost of H3PO4 since the prices of M from MgO is relatively cheaper compared to that
of MgCl2. However, the cost of the process per ammonia removed as struvite does not change much since the
recovery of ammonia is relatively low in comparison to that MgCl 2 is used. In order to reduce the overall cost of
the process cheaper caustic source such as Ca(OH)2 can be used for pH adjustment. In this case there is a risk of
calcium phosphate precipitation with much higher pKs value than struvite which eventually reduces the availability
of phosphate for struvite thus increases the need for phosphate.
Table 5.
Economic analysis of the process when MgCl2.6H2O is used as M source
NH3 Removal
(%)
Cost
($/kg NH4+-N)
Cost
($/kg MAP)
Cost of the Chemicals (%)
H3PO4 : MgCl2 : NaOH
1:1:1 and 8.0
78.7
7.72
0.62
35.5 : 56.7 : 7.8
1:1:1 and 8.5
83.4
7.48
0.66
35.3 : 56.4 : 8.3
1:1:1 and 9.0
90.2
6.98
0.47
34.8 : 55.3 : 9.9
1.2:1:1 and 8.0
88
7.89
0.55
31.7 : 60.5 : 7.8
1.2:1:1 and 8.5
86
8.13
0.61
31.4 : 60.2 : 8.4
1.2:1:1 and 9.0
94.5
7.45
0.49
31.2 : 59.4 : 9.4
1.4:1:1 and 8.0
88.5
8.8
0.6
28.3 : 62.9 : 8.8
1.4:1:1 and 8.5
87.4
8.79
0.62
28.5 : 64 : 7.5
1.4:1:1 and 9.0
96.9
7.99
0.48
28.2 : 63 : 8.8
1.2:1:1.2 and 7.5
93.2
7.96
0.52
35.2 : 56 : 8.8
1.2:1:1.2 and 8.0
94.3
7.95
0.51
34.8 : 55.4 : 9.8
1.2:1:1.2 and 8.5
97
7.7
0.5
35 : 55.6 : 9.4
1.2:1:1.2 and 9.0
95.36
7.9
0.5
34.6 : 55.2 : 10.2
1:1:1.2 and 7.5
81.6
8.3
0.65
35.4 : 56.3 : 8.3
1:1:1.2 and 8.0
82
8.31
0.71
35.2 : 56.1 : 8.7
1:1:1.2 and 8.5
82.6
8.02
0.6
35.4 : 56.3 : 8.3
1:1:1.2 and 9.0
83.8
7.99
0.63
35.4 : 56.3 : 8.3
M:N:P Molar Ratio and pH
Table 6.
Economic analysis of the process when MgO is used as M source
NH3 Removal
(%)
Cost
($/kg NH4+-N)
Cost
($/kg MAP)
Cost of the Chemicals (%)
H3PO4 : MgO : NaOH
1.2:1:1.2 and 8.0
48.9
8.07
0.84
63.2 : 28.8 : 8
1.2:1:1.2 and 8.5
54.4
7.26
0.79
63.2 : 28.8 : 8
1.2:1:1.2 and 9.0
57.8
7.14
0.75
60.5 : 27.5 : 12
1.4:1:1.2 and 8.0
53.2
8.14
60.9 : 31.3 : 7.8
1.4:1:1.2 and 8.5
64.9
6.73
60.4 : 31.1 : 8.5
1.4:1:1.2 and 9.0
66.9
6.59
59.8 : 30.7 : 9.5
1.6:1:1.2 and 9.0
68.8
6.4
59.6 : 30.6 : 9.8
M:N:P Molar Ratio and pH
The economical analysis based on the experimental results presented in this work falls into the same range
carried out by other researchers. Siegrist et al [15] considered electricity and maintenance in addition to chemical
cost of the process and came up with $9.10-11.38/kg NH4+-N. Andrade and Schuiling [14] claimed that the cost is
between $4.55-9.92/kg NH4+-N at high ammonium concentrations. Siegrist [16] has calculated the cost of the
process $9.72/kg NH4+-N. In all these works, including ours, the commercial value of struvite is not taken into
account. Webb et al [17] has calculated the cost of chemicals as $14.9/kg NH 4+-N. When the market price of
struvite being $8.75/kg NH4+-N is deduced from the cost, the actual cost comes down to $6.15/kg NH4+-N. As a
result, local availability and price of chemicals and use of struvite determine whether this process can potentially
compete with existing nitrogen removal and recovery technologies currently available in the market.
CONCLUSIONS
Following conclusions can be drawn from the presented work:
1-The rate of struvite precipitation is very fast and completed in minutes.
2-The optimum pH for ammonia recovery is 8.5-9.0 over the range studied here. However, higher the pH higher
the loss of ammonia to air due to stripping since high pH favors NH3. Struvite precipitation has also been observed
as low as pH 6.0
3-For optimum recovery of ammonia slight excess of M and P are required.
4-MgCl2 is better M source than MgO. However less caustic is used for pH adjustment when MgO is used.
5-The reaction temperature did not have influence on the equilibrium concentration of ammonia between 25-40 oC.
6-In two steps purification process struvite can quantitatively be purified from impurities present in the effluent.
7-The cost of ammonia recovery is around $7.5-8.0/kgNH4+-N as a function of removal ratio and amount of
chemicals added. This process can be viable when the value of struvite is taken into account.
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