A PILOT PLANT STUDY ON USING SEAWATER AS A MAGNESIUM
SOURCE FOR STRUVITE PRECIPITATION
K. Kumashiro1, H. Ishiwatari2, Y. Nawamura3*
1Water
Quality Control Section, Construction Bureau, 96-3 Nishiminato-Mati, Kokurakita-Ku, Kitakyushu, 803-0801,
Japan.2Japan
Institute of Wastewater Engineering Technology (JIWET), Ikebukuro Chitose Blg. 7F 1-22-8
Nishi-ikebukuro, Toshima-ku, Tokyo, 171-0021, Japan. 3Nishihara Environmental Sanitation Research Corporation Ltd.,
3-6-18 Shibaura, Minato-Ku, Tokyo, 108-0023, Japan email : yoshiharu_nawamura@nishihara.co.jp
Abstract
In recent years, a number of biological treatment processes have been developed for phosphorus (P) as well as
combined nutrient (N&P) removal. In anaerobic/oxic (A/O) and anaerobic/anoxic/oxic (A2/O) processes, it is necessary to
lower the concentration of
P in the sidestream from the sludge treatment facilities. To accomplish this, the
magnesium-ammonium-phosphate (MAP) system (P removal from the sidestream by MAP crystallization) has been
developed and applied to full scale plants. However, because the earlier systems need chemicals such as magnesium
chloride as the source of magnesium (Mg) as well as sodium hydroxide (NaOH) for pH control, they were found to be
economically less viable. To lower the operational cost of the MAP system, seawater was substituted for chemicals and
succeeded in developing a new MAP system. In this paper, we focus on the relationship between P removal efficiency and
operational conditions. By feeding 50.5 to 110 (mg/l) in dissolved phosphorus (D-P) having pH above 7.77 from
sidestream, together with a 9.0 to 10 % seawater inflow into the reactor, holding a 4.3 to 13 % volume of MAP (MV), and
with 29 minutes HRT, over 70 % D-P removal efficiency was achieved without pH control. Although the use of seawater
requires the power cost for pumping, substantial cost reduction can be achieved when compared with chemicals. Also, the
method using seawater is superior, as it can simplify the facilities and operation, and does not affect the existing treatment
facilities.
Keywords:
Crystallization, MAP, Phosphorus Removal, Seawater, Sidestream
INTRODUCTION
In last two decades, a number of advanced biological treatment processes have been developed for P as well as
combined N & P removal. These processes have considerable appeal to designers and operators because the use of
chemicals has been eliminated or reduced substantially. In 1994, the design manual for anaerobic/oxic (A/O) and
anaerobic/anoxic/oxic (A2/O) processes was issued. It made a major contribution toward encouraging municipalities to
adopt both processes for their plant [1].
In A/O and A2/O processes, it is necessary to lower the concentration of P in sidestreams from the sludge
disposal processes [2]. For this purpose, the magnesium-ammonium-phosphate (MAP) system (technology for P removal
from the sidestream by crystallization of MAP) has been developed [3] and applied to full-scale plants [4]. MAP is a raw
Kumashiro et al, page1
material for fertilizer, and due to the rich content of phosphorus and its refractory nature, it has a characteristic for excellent
slow fertilizer. It is also an environmentally friendly, as it does not increase the electrical conductivity of soil, due to less
excess salt contents.
The MAP system can be recognized not only as one of the advanced wastewater treatment systems but also as a
P recycling system. However, because previous MAP systems need chemicals such as magnesium chloride as the source of
magnesium (Mg) as well as sodium hydroxide (NaOH) for pH control [4,5,6], they are economically less viable. As Japan
is surrounded by seas and many of wastewater treatment plants are located near the sea, so it is easy to keep the seawater as
a Mg source for MAP system. By substituting seawater for chemicals, operational cost of the MAP system can be
substantially reduced. In this paper, we focus on the relationship between P removal efficiency and operational conditions
in the newly developed MAP process using seawater as Mg source. The target P removal efficiency was 70% in terms of
D-P [7]. A study was also conducted to increase the value of the product as fertilizer, and to promote the beneficial use of
the product with higher marketability.
Kumashiro et al, page2
OPERATIONAL FACTORS OF MAP SYSTEM
MAP crystallizes by the reaction of magnesium ion (Mg2+), ammonium ion (NH4+), and orthophosphate ion
(HPO42-) on an equal mole basis as follows [4].
Mg2++NH4++HPO42-+OH-+6H2O
MgNH4PO4・6H2O+H2O
(i)
The solubility of MAP is dependent on pH. It increases with the lowering of pH and vice versa. The P removal
efficiency is affected by the molar ratio of dissolved magnesium (D-Mg) to dissolved phosphorus (D-P) in the mixture of
influent and the chemicals added as the source of Mg (Mg/P). The nucleus of the MAP system is considered to be
circulating MAP particles in the reactor. The concentration of MAP particles is relevant to the reaction rate [4]. The longer
the hydraulic retention time of the reactor (HRT), the higher is the P removal efficiency [4]. Therefore, pH, Mg/P, MAP
concentration in the reactor, and HRT are important operational factors of the MAP system.
METHODOLOGY
Pilot plant
The MAP pilot plant was constructed at Hiagari Sewage Treatment Plant (HSTP) in Kitakyushu City. The
process flowsheet is shown in Figure 1. This objective is to treat wastewater from sidestreams, such as dewatering effluent
containing high concentration of ammonium (NH4+) and orthophosphate ions (HPO42-). By converting phosphorus as MAP
particles, the concentration of phosphorus in mainstream is reduced. Furthermore, another aim was to recycle MAP
particles as fertilizer. The reactor developed for this study is shown in Figure 2. The reactor is concentric cylinder type. The
upper part of reactor has a larger diameter, and the separation of treated water and MAP particles taking place in that
settling space. Air is blown into the inner cylinder, and that causes upward flow in inner cylinder and downward flow
between inner and outer cylinder. Influent, seawater and MAP particles are mixed and circulated, and causes MAP particles
to grow larger by crystallization. The mixed liquid flows to settling space and MAP particles settles there, and treated water
overflows from the top of settling space. The treated water is returned to mainstream. There are baffles in settling space to
guide downward flow. The volume of the inner cylinder is 99 L and the outer is 218 L. The volume of the settling space is
792 L. The produced MAP particles are drawn off from the bottom of reactor alongwith water by opening the solenoid
valve for about a half minute. The produced MAP particles are white color, about 1 mm in diameter, and hard type granular
material. The water is easily separated by fine screen with 0.2 to 0.3 mm bar spacing. The seawater was pumped up from
the sea near the treatment plant. It contains around 1250 (mg/l) of Mg2+.
Kumashiro et al, page3
Experimental conditions and Analysis
The experiment was carried out from January 23rd 1996 to January 30th 1997. Three different influents with varying P
concentration fed to the reactor. The experimental conditions are summarized in Table 1.
Table 1. Experimental Conditions
Exp.
1
2
3
Date
Influent
Flow (m3/d)
14.4-43.2
14.4-21.6
7.2-57.6
23.1.1996 to. 29.5.1996
30.5.1996 to. 15.7.1996
16.7.1996 to. 5 .1. 1997
Source
Belt Press and Centrifuge
Diluted filtrate from Belt Press
Filtrate from Belt Press
RSF
%
2-12
10
10-15
HRT
Min
9.5-29
19-29
7-58
MV
%
3.3-15
10-11
2.3-16
RSF-The ratio of seawater to the influent flow
MV-The ratio of MAP to the reactor in volume
Influent
Grit
Primary
chamber
settling
tank
Aeration
tank
Final
Disinfect
settling
Effluent
-ion tank
tank
Return sludge
Gravity
overflow
Gravity
Flotation
thickening
thickening
Excess sludge
Flotation
effluent
Digestion tank
Dewatering machine
Dewatered cake
Dewatered effluent
Seawater
MAPParticles
MAP reactor
Dryer etc
Rycycle as fertilizer
Figure 1 Flow of MAP pilot system
During the initial period, pH was controlled from 7.66 to 8.4 using NaOH to study the relation between pH and P removal
efficiency. During the latter period, pH was controlled from 7.65 to 7.96 using NaOH due to the significant decrease of
influent pH caused by the malfunction of sludge digestion. There is no data directly measured the velocity of circulation
flow but the linear velocity (LV) of inner cylinder is 9.5 to 76.4 m3/m2↵ h, and the air supply of airlift for circulation is 130
m3/ m2↵ h.. The analysis of temperature, pH, T-P, D-P, NH4-N, D-Mg, and SS of the influent and effluent was conducted
according to Sewage Analysis Methods [8].
Kumashiro et al, page4
F 1200 mm
effluent
Settling space
outer cylinder
f 350 mm
inner cylinder
f 200 mm
sampling mouth
sampling mouth
seawater
air
influent
MAP collection mouth
Figure 2 Schematic of Pilot Scale Reactor
Kumashiro et al, page5
3700 mm
baffle
Crystallization space
RESULTS & DISCUSSIONS
Influent and effluent
The average influent water quality, effluent quality, P removal efficiency, and Mg/P are shown in Table 2.
Table2 Influent & Effluent quality, P Removal Efficiency, and Mg/P
Temp pH
－
℃
T-P D-P NH4-N D-Mg SS Mg/P
(mg/l) (mg/l) (mg/l) (mg/l) (mg/l) －
influent
19.4 7.75
Experiment 1 effluent
21.1 7.86
removal eff. －
－
73.1 69.6
32.1 19.1
56% 73%
influent
Experiment 2 effluent
removal eff.
influent
Experiment 3 effluent
removal eff.
66.5
20.2
70%
121
32.2
73%
26.3
28.6
－
29.5
31.1
－
7.75
7.88
－
7.71
7.76
－
423
383
9%
9.4
88
－
64.3 －
－
15.9 －
－
75% －
－
110 623 14.3
26.4 517
67
76% 17% －
136
188
－
2.2
－
－
38
45
－
49
63
－
2.4
－
－
1.6
－
－
The T-P removal efficiency of Experiment 1, 2, and 3 were 56, 70, and 73%. The D-P removal efficiency of Experiments 1,
2, and 3 were 73, 75, and 76%, respectively. The T-P removal efficiency of Experiment 1 was low compared with other
experiments’ although the D-P removal efficiencies are almost same. The reason is the high effluent SS concentration. The
average effluent SS concentration of Experiment 1 was 188 (mg/l) which was around 3.0 to 4.2 times higher than the other
effluents.
Removal efficiency and operational conditions
The relation between influent pH and D-P removal efficiency for the different HRTs is shown in figure 3. All
data was taken without pH control.
Kumashiro et al, page6
100
90
80
D-P Removal (%)
70
60
50
40
HRT=10～12min
30
HRT=14～19min
20
HRT=29min
10
0
7.2
7.3
7.4
7.5
7.6
7.7
Ｉnfluent pH
7.8
7.9
8
8.1
8.2
Figure 3. Relation between Influent pH and D-P Removal Efficiency
When the HRT is 10 to 12 minutes and the influent pH is higher than 8.03, the D-P removal efficiency is above 70%.
However, the efficiency is below 70% when the influent pH is 7.82 to 7.98. It is considered difficult to meet the D-P
removal efficiency of 70% under this condition. When the HRT varies from 14 to 19 minutes and the influent pH is higher
than 7.8, most results shows efficiency less than 70%. Again, it is considered difficult to meet the D-P removal efficiency
of 70% due to this condition. When the HRT is 29 minutes and the influent pH is higher than 7.77, the D-P removal
efficiency is above 70%. In this case, the influent D-P concentration was from 50.5 to 110 (mg/l), the Mg/P ratio was 1.4 to
3.1, the RSF was 9.0 to 10%, and the MV was 4.3 to 12.6%.
Kumashiro et al, page7
Relation between influent flow and D-P removal efficiency
100
90
y = -0.0476x + 83.752
D-P Removal　％
80
70
y = -0.5411x + 87.131
2
R = 0.8812
60
50
y = -0.673x + 85.038
R2 = 0.7792
40
Low Concentration Influent pH Unadjusted
30
High Concentration Influent pH Unadjusted
20
High Concentration Influent pH7.8
Adjusuted
High Concentration Influent pH8.0 Adjusted
10
0
0
10
(13.3)
20
30
(26.5)
(39.8)
3
Influent Flow m /d
40
(53.1)
50
(66.3)
60
(79.6)
Figure 4. Relation between influent flow and D-P removal
In the low concentration influent (EXPERIMENT 1), D-P removal efficiency is more than 70% at influent flow of 22m3/d
(LV29.2m/h, 4.1 kg/m3/d loading). In the high concentration influent without adding alkali (EXPERIMENT 3), D-P
removal efficiency became more than 70% at treated water flow of 30 m3/d (LV39.8m/h, 9.5 kg/m3/d loading).If the pH is
adjusted above 7.8, decrease in phosphorus removal was not observed thus showing its high dependency on pH.
Relation between influent phosphorus loading and effluent phosphorus concentration
Effluent P Concentration mg/l
50
40
30
Ｄ－Ｐ　Low Concentration Influent
20
Ｔ－Ｐ　〃
Ｄ－Ｐ　High Concentration Influent
10
Ｔ－Ｐ　〃
(3.2)
(9.5)
(15.8)
0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Influent P Loading　kg/d　（ kg/m3・d）
4.0
4.5
5.0
Figure 5 Relation between influent phosphorus loading and effluent phosphorus concentration
In both low and high concentration influent, it was observed that there was a tendency that effluent phosphorus
concentration becomes higher as the loading increases, although there was a limit in the effluent phosphorus concentration
(Fig.5). In other words, in case of low concentration influent, effluent phosphorus
concentration reaches to reaction equilibrium concentration by operational conditions such as pH and brings the
phosphorus removal low, while in the case of high concentration influent, operation at higher loading condition was
Kumashiro et al, page8
possible to achieve phosphorus removal at 70% or more.
Kumashiro et al, page9
Relation between effluent pH and P removal efficiency
Experimental result on varying the effluent pH in high concentration influent from unadjusted (7.65) to 8.6, while
maintaining the influent flow in constant condition at 43.2 m3/d (LV57.3m/h) is shown in Fig. 6.
100
y = 210.24Ln(x) - 356.29
2
R = 0.8979
90
y = 210.41Ln(x) - 359.65
R2 = 0.9683
80
P Removal　%
70
60
50
40
D-P Removal
30
T-P Removal
High Concentration Influent
Treated Water Flow 43.2m3/d
20
10
0
7.4
7.6
7.8
8
8.2
Effluent pH
8.4
8.6
8.8
Figure 6. Relation between effluent pH and P removal
The pH without adding alkali was 7.65, and the removal efficiency was approximately 70% for both D-P and T-P. It was
observed that higher the operational pH, better removal efficiency could be obtained, thus showing the high dependency of
phosphorus removal on pH.
Ratio of seawater to the influent flow (Mg/P)
The figure 7 shows the relation between influent Phosphorus (P) ratio and seawater (Mg) injection ratio (Mg/P), and D-P
removal efficiency. There was no clear relation between them at the time of pilot test but beaker test shows that when Mg/P
ratio is high, removal ratio of D-P is high. To achieve 70% removal ratio of P, Mg/P ratio was required over 1.5. Therefore,
Mg/P ratio for operating at pilot plant was set higher than 1.5 to react with variable P concentration influent.
100
90
y = 20.011Ln(x) + 59.991
2
R = 0.7174
D-P Removal　％
80
70
60
50
40
30
20
10
0
0
0.5
1
1.5
Mg/P Ratio
2
2.5
3
Figure 7 Relation of Mg/P and D-P removal efficiency
The influence of seawater injection was studied based on the calculation at 200,000 m3/d HSTP influent, digested sludge
Kumashiro et al, page10
dewatering effluent was about 760 m3/d, and seawater injection was about 76 m3/d (10% of dewatering effluent). Therefore,
the ratio of influent and seawater injected was 76/200,000 = 0.038%, the increase in concentrations of Na, Cl, and SO4 (as
S) were 3.99 mg/l, 7.22 mg/l, and 0.32mg/l, respectively. From these values, we can say that seawater injection does not
influence the treatment of mainstream.
Kumashiro et al, page11
Relation between MV and D-P removal efficiency
Relation between MV (MAP volume) and D-P removal efficiency was studied, as it was observed that phosphorus removal
efficiency of MAP forming reaction becomes higher when the surface area of MAP in the reactor increases. The result of
100
90
(50)
D-P Removal　％
80
(83)
(117
(151)
70
60
(114)
(189)
(236)
(284)
(65)
50
40
3
Influent Flow 28.8m /d
30
Low Concentration Influent pH Unadjusted
20
10
High Concentration Influent　pH7.8 Adjusted
0
0.0
2.0
4.0
6.0
8.0
10.0
12.0
2
ＭＶ　％
(Surface Area m )
14.0
16.0
18.0
the study is shown in Fig. 8.
Figure 8. Relation between MV and D-P removal
In the study using low concentration influent, decrease in D-P removal efficiency was observed when MV was lowered
than 3.3%, however, in high concentration influent, D-P removal efficiency remained constant while MV was varied in the
range of 4 to 12%.
Relation of effluent SS Concentration and Phosphorus removal efficiency
The T-P removal efficiency goes down when effluent SS concentration is high because of fine MAP particles were washout
with SS (Fig.9). There are no relation between D-P removal and SS concentration of treated water.
Produced MAP and Operation Cost
The figure 10 shows produced MAP particles. The produced MAP particles are white color, about 1 mm in diameter, and
100
90
80
Ｐ Removal ％
70
60
50
40
30
T-P Removal
20
D-P Removal
10
0
0
50
100
150
200
250
Effluent　ＳＳ　Concentration　mg/l
300
350
hard type granular material. The table 3 shows constitution and component of produced MAP particles.
Figure 9 Relation of effluent SS concentration and P removal
Kumashiro et al, page12
Table 3 Constitution of produced MAP particles
Item
N (%)
P (%)
Mg (%)
K (mg/kg)
Water content (%)
As (mg/kg)
Hg (mg/kg)
Cd (mg/kg)
Theoretical values
5.7
12.6
9.9
44.0
Measurement values (average)
5.5
12.5
9.6
565
41.8
0.7
<0.003
Less than quantitative limit
Figure 10 MAP particles
The constitution of N, P, Mg and water content in MAP pellets are almost same as theoretical values. The constitution of As,
Hg and Cd shows that MAP pellets are no problems for using as fertilizer. Operational cost for producing 1 kg of MAP was
56 yen (Table 4). The electric power consumption per 1 kg of produced MAP particles was 1.41 kWh and the heavy oil
consumption were 57.5 ml.The personal expense and expense of expendable supplies are also required.
Table-4 Operational Cost (Scale：Treated flow 760m3/d、MAP products 505kg/d)
Item
Operational Cost(yen/kgMAP)
Electric power(8 yen/kwh)
11.3
Heavy oil (40 yen/l)
2.3
Labor(2.8 million yen/year)
21.6
Repair(2.7 million yen/year)
20.8
Total
56.0
Remarks
1.4kwh/kgMAP
57ml/kgMAP
0.7 person/d
CONCLUSIONS
Operational condition of MAP production
Pilot testing was conducted in order to develop a MAP system using seawater. By feeding the sidestream having 50.5 to
110 (mg/l) dissolved phosphorus (D-P), pH above 7.77, together with a 9.0 to 10 % seawater inflow into the reactor,
holding a 4.3 to 13 % volume of MAP (MV), and with 29 minutes HRT, over 70 % D-P removal efficiency was achieved
without pH control. It was observed that higher the operational pH, better removal efficiency could be obtained.
Evaluation on using sea water as Mg source
In the operation using seawater as Mg source, when Mg/P ratio was maintained above 1.5 (about 10% of influent), a
stabilized result was obtained for phosphorus removal efficiency (70% of D-P), and the operation was easy in spite of the
variation in influent phosphorus concentration. Chemical cost for using magnesium chloride and magnesium hydroxide for
Kumashiro et al, page13
Mg source are quite high and also associated with handling problems. Therefore, it can be said that the method using
seawater is able to reduce the production cost substantially, and is advantageous from the point of facilities and operation.
Also, it was confirmed that seawater had no influence to the mainstream operation. Furthermore, the produced MAP can be
used as fertilizer and it has equal or better quality as a fertilizer, and is an environmentally friendly product.
Kumashiro et al, page14
ACKNOWLEDGEMENTS
This study was conducted under the cooperative project of Kitakyushu City and JIWET with the help of the technical
advisory committee. The authors appreciate the contribution of committee members and financial support from the
Ministry of Construction.
REFERENCES
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of Sewerage, Monthly, .20, No.5, (1997), (in Japanese).
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SEWERAGE, 134-138, (1994), (in Japanese).
3) Murrel, L. S., Martia G. D., Kenneth M. R., and Joseph J. S., “Ultimate Disposal of Phosphate from Waste Water by
Recovery as Fertilizer”, Effluent and Water Treatment Journal, Oct., 509-519, (1972).
4) S., Abe, “Phosphate removal from dewatering filtrate by MAP process in full scale experiment at Seibu Wastewater
Treatment Plant”, Proceedings 4th CIWEM & JSWA Technology Exchange Workshop, 260-277, (1995)
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6) H., Tsuno, I., Somiya, M., Yoshino, “Study on struvite recovery from digester supernatant”, Journal of Japan Sewage
Works Association, Research Journal, 28, No. 324, 58-77, (1991), (in Japanese).
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(in Japanese).
8) Wastewater Management Department Ministry of Construction and Water Management Department Ministry of Health,
SEWAGE ANALYSIS METHODS, Japan Sewage Works Association, (1997), (in Japanese).
Kumashiro et al, page15