Nutrient Recovery from Piggy Wastewater by Enhancing Struvite Crystallization Process

Abstract

The problem of nutrient pollution from piggy wastewater has been considered as a serious problem. Enhancing struvite crystallization process is an efficient technology for ammonium recovery. In our study, the optimal reaction conditions of pH, Mg2+: NH4+: PO43- molar ratio, mixing reaction time, and different kinds of chemicals combinations for ammonium recovery were examined. The conclusions were drawn as following. The optimal pH for ammonium recovery was 9.0. The optimal Mg2+: NH4+: PO43- molar ratio for ammonium recovery was 1.3:1:1. The optimal mixing reaction time for ammonium recovery was 10 min. The chemicals combination of MgCl2 + Na2HPO4 was the most efficient for ammonium recovery. Scanning electron microscopy analysis indicated the unshaped struvite crystal was coarse and its size was irregular.

Full text

Nutrient recovery from piggy wastewater by enhancing struvite
crystallization process
Tao Zhang
a
, Ci Fang, Ping Li, Rongfeng Jiang
b
Key Laboratory of Plant-Soil Interactions of Ministry of Education, College of Resources and
Environmental Sciences, China Agricultural University, Beijing 100193, PR China
a
taozhang@cau.edu.cn,
b
rfjiang@cau.edu.cn
Keywords: nutrient; struvite crystallization; enhancing; piggy wastewater.
Abstract. The problem of nutrient pollution from piggy wastewater has been considered as a serious
problem. Enhancing struvite crystallization process is an efficient technology for ammonium
recovery. In our study, the optimal reaction conditions of pH, Mg
2+
: NH
4+
: PO
43-
molar ratio, mixing
reaction time, and different kinds of chemicals combinations for ammonium recovery were examined.
The conclusions were drawn as following. The optimal pH for ammonium recovery was 9.0. The
optimal Mg
2+
: NH
4+
: PO
43-
molar ratio for ammonium recovery was 1.3:1:1. The optimal mixing
reaction time for ammonium recovery was 10 min. The chemicals combination of MgCl
2
+ Na
2
HPO
4
was the most efficient for ammonium recovery. Scanning electron microscopy analysis indicated the
unshaped struvite crystal was coarse and its size was irregular.
Introduction
Nowadays, ammonium, a kind of nutrient for plant growth, contained in piggy wastewater has
become one of the main sources of pollution to the water environment. Therefore, the treatment of
ammonium from piggy wastewater is important for water environment. The traditional treatment
processes of ammonium removal are biological process and physico-chemical process. Biological
process is economical for ammonium removal, but it is not effective for pretreatment of ammonium
removal from wastewater. Physico-chemical processes can be applied in pretreatment of ammonium,
such as air stripping, ionic exchange, and struvite (MgNH
4
PO
4
·6H
2
O) precipitation
[1, 2]
.
Struvite crystallization of ammonium by forming magnesium ammonium phosphate has been
studied widely
[3, 4]
. The ammonium recovery from wastewater by enhancing struvite crystallization
process has been regard as a useful technology. Struvite is a white crystalline substance consisting of
magnesium, ammonium and phosphorus in equal molar concentrations. The basic chemical reaction
to form Struvite has been expressed in Eq. (1)
[5]
.
2 3
4 4 2 4 4 2
Mg NH PO 6H O MgNH PO 6H O
+ + −
+ + + → ⋅ ↓
(1)
In our study, the objective of this study was to investigate optimal pH, optimal molar ratio, optimal
mixing reaction time, and different kinds of chemicals combinations in laboratory scale experiments
for piggy wastewater.
Materials and Methods
Materials
The raw wastewater used in the experiments was synthetical piggy wastewater. Table 1 shows some
main parameters of the wastewater sample.
Methods
Experiments were performed as follows. Firstly, magnesium salt and phosphate were added to raw
wastewater at experimental condition (1.0:1:1-1.5:1:1). Secondly, the reaction solution pH was
adjusted at experimental condition (7.5-10.5). Thirdly, the reaction solution was agitated by magnetic
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stirrers at experimental mixing reaction time (5-60 min) and then settled for 10 min. Finally, the
reaction solution was filtered with 0.45µm membrane filter and the supernatant was collected to
measure NH
4+
.
Analysis
The experiments choose ammonium recovery ratio as the indicator for selecting optimal reaction
conditions. The concentration of NH
4+
was measured according to Standard Methods. The collected
precipitate was washed with deionized water for 3 times, and dried in an oven at 40
℃
for 48 hours,
and then analyzed by scanning electron microscopy (SEM, S-3400N, Hitachi, Japan).
Table 1 Characteristics of wastewater
Parameter Unit Concentrations
COD mg/L 600±30
NH
4
+
-N mg/L 200±10
PO
4
3
-
-P mg/L 30±3
Result and Discussion
Optimum pH
Solution pH is the most important factor for enhancing stuvite crystallization process. The experiment
was to study optimal pH of ammonium recovery from piggy wastewater by enhancing struvite
crystallization process. This experiment was tested by adding MgCl
2
and Na
2
HPO
4
into the samples,
and the molar ratio of Mg
2+
: NH
4+
: PO
43-
was 1.3:1:1. The pH range was 7.5-10.5. The mixing
reaction time was 10 min.
Fig.1 showed that the optimal ammonium recovery ratio for piggy wastewater was at pH 9.0. When
pH is lower than the optimal point, hydrogen ions in reaction solution inhibit struvite crystallization.
When pH is higher than the optimal point, Mg
3
(PO
4
)
2
is formed instead of struvite. In the literature,
there are some papers dealing with the optimal pH for ammonium recovery. Stratful et al.
[7]
reported
that ammonium recovery ratio was high than 97% at pH 9, 9.5 and 10. Zhang et al.
[8]
reported the
optimum pH for ammonium recovery from landfill leachate was 9.5.
Optimum Molar Ratio
Mg
2+
: NH
4+
: PO
43-
molar ratio is an important factor for enhancing stuvite crystallization process.
Overdosing either Mg
2+
or PO
43-
can lower the remaining ammonium concentration. In fact, although
overdosing PO
43-
could increase ammonium concentration, there are several problems for overdosing
PO
43-
. First of all, overdosing PO
43-
did not provide significant increasing in ammonium recovery. In
addition, overdosing PO
43-
will lead to the high concentration of PO
43-
in the effluent. As a result, the
experiment was to study the optimal Mg
2+
: NH
4+
: PO
43-
molar ratio of overdosing Mg
2+
for struvite
precipitation in piggy wastewater. This experiment was tested by adding MgCl
2
and Na
2
HPO
4
into the
samples. The pH was 9.0. The mixing reaction time was 10 min.
Fig.2 showed, the optimal Mg
2+
: NH
4+
: PO
43-
molar ratio for ammonium recovery from piggy
wastewater was 1.3:1:1. With an increase in Mg
2+
: NH
4+
: PO
43-
molar ratio from 1.0:1:1 to 1.3:1:1,
the recovery ratio of ammonium increased rapidly. With an increase in Mg
2+
: NH
4+
: PO
43-
molar ratio
from 1.3:1:1 to 1.5:1:1, the recovery ratio of ammonium did not increase significantly. Li et al.
[9]
were deal with NH
4+
recovery from landfill leachate by chemical precipitation and reported
overdosing 10% Mg
2+
could further increase ammonium efficiency.
580 Environmental Protection and Sustainable Development

Optimum Reaction Time
The optimal reaction time for struvite formation was investigated due to it is an important factor for
engineering application. This experiment was tested by adding MgCl
2
and Na
2
HPO
4
into the samples.
The molar ratio of Mg
2+
: NH
4+
: PO
43-
was 1.3: 1: 1. The pH range was 9.0.
Fig.3 showed, the optimal mixing reaction time for ammonium recovery from piggy wastewater
was 10 min. When the mixing reaction time was increasing from 5 min to 10 min, the ammonium
recovery ratio was increased significantly. When the mixing reaction time was increasing from 10
min to 60 min, the ammonium recovery ratio was increased slightly. Celen and Turker
[10]
have do
some research for struvite reaction time in anaerobic digester effluents and indicated the 40 min was
safe time for struvite mixing and constant rest.
Fig 1. NH
4+
recovery ratio at different pH
Fig 2. NH
4+
recovery ratio at different Mg
2+
:
NH
4+
: PO
43-
molar ratio
Optimum Chemicals Combinations
The factor of different chemicals combinations of magnesium and phosphate source is important for
enhancing struvite crystallization. The combinations of chemicals, MgCl
2
+ Na
2
HPO
4
, Mg(OH)
2
+
H
3
PO
4
, MgCO
3
+ H
3
PO
4
,
were employed for the experiments and carried out with Mg
2+
: NH
4+
: PO
43-
molar ratio of 1.3:1:1, pH of 9.0, and mixing reaction time of 10 min.
Fig.4 showed the relationship between chemicals combinations and ammonium recovery ratio. The
chemicals combination of MgCl
2
+ Na
2
HPO
4
was the most efficient for ammonium recovery. The
efficiency of chemicals combinations of Mg(OH)
2
+ H
3
PO
4
and MgCO
3
+ H
3
PO
4
for ammonium
recovery was lower than the chemicals combination of MgCl
2
+ Na
2
HPO
4
. But the chemicals
combination of MgCl
2
+ Na
2
HPO
4
was generated high salt concentration in the effluent. 1 mole of
ammonium recovery will lead to 2 mole of NaCl generated
[8]
. Adding the chemicals combinations of
Mg(OH)
2
+ H
3
PO
4
and MgCO
3
+ H
3
PO
4
as sources of magnesium salt and phosphate could
significantly minimize the salt concentration.
Surface characterization
The struvite precipitates was collected at the reaction condition of Mg
2+
: NH
4+
: PO
43-
molar ratio
1.3:1:1, pH 9.0, mixing reaction time 10 min, and the chemicals combinations of MgCl
2
and
Na
2
HPO
4.
SEM analysis (Fig.5) showed that the surface of the crystal was coarse, unshaped. The size
of struvite precipitates was irregular (20-200 um).
Applied Mechanics and Materials Vols. 522-524 581

Fig 3. NH
4+
recovery ratio at different
reaction time
Fig 4. NH
4+
recovery ratio at different
chemicals combinations
Fig 5. SEM analysis of struvite precipitates
Conclusions
As a method for ammonium recovery from piggy wastewater, the following conclusions could be
obtained for struvite precipitation.
(1) The optimal ammonium recovery ratio for piggy wastewater was at pH 9.0.
(2) The optimal Mg
2+
: NH
4+
: PO
43-
molar ratio for ammonium recovery from piggy wastewater was
1.3:1:1.
(3) The optimal mixing reaction time for ammonium recovery from piggy wastewater was 10 min.
(4) The chemicals combination of MgCl
2
+ Na
2
HPO
4
was the most efficient for ammonium
recovery.
(5) SEM analysis indicated the unshaped struvite crystal was coarse and its size was irregular.
Acknowledgements
The work was supported by a grant from the Specialized Research Fund for the Doctoral Program of
Higher Education (20120008120013), the Chinese Universities Scientific Fund of China Agricultural
University (2011JS169), and the TOTO Water Environment Fund of China Environmental Protection
Foundation. Thanks to the Analytical Instrumentation Center of Peking University.
582 Environmental Protection and Sustainable Development

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