Recovery of Phosphorus from Waste Activated Sludge
using a Selective Adsorbent
Tomotake Takai*, Takashi Murasawa**, Katsunori Nishida**, Yoshinori Iwashima*, Jyunji Ooshima*
* Takeda Chemical Industries,Ltd., Life-Environment Company, E-mail: Takai_Tomotake@takeda.co.jp
** Tsukishima Kikai Co., Ltd., E-mail: t_murasawa@tsk-g.co.jp
1. Introduction
Although a large number of studies have been made on a development of phosphorus removal from
sewage, so far the study of phosphate recovery process from waste activated sludge, has been superficial
in Japan. In recent years, however, there has been renewal of interest in phosphate recovery mainly for
necessities of phosphorus recycle from sludge, and an export control of a phosphorus ore from companies
in USA.
The purpose of this study is to develop a new system for decreasing waste activated sludge from sewage
treatment process by hydrolysis with sulfuric acid and heat treatment, and recovering phosphates from
supernatants of waste activated sludge using a newly developed selective adsorbent which has a large
adsorption capacity of a phosphate. In this study, some experiments were conducted to investigate the
effectiveness of phosphate recovery from supernatants of waste activated sludge using the adsorbent and
to evaluate recovered phosphates. The results of bench tests and a feasibility of phosphate recovery as a
sodium phosphate are presented in this paper.
2. Material and Methods
Phosphates extraction from waste activated sludge :
In this experiment, waste activated sludge in sewage treatment plant was first decomposed by hydrolysis
under pH 1~2 with sulfuric acid and heat treatment at 120 °c for 1 hr, which resulted in 30~70% reduction
of suspended solids. Supernatants containing very high strength of phosphorus about 2000 mg-P/l in a
solution, 90% of total phosphorus originally contained in waste activated sludge, were then obtained from
waste activated sludge using a dewatering centrifuge under 2000 G for 5 min. And then, adsorption
experiments were conducted with a selective adsorbent for a phosphate.
Selective adsorbent:
The selective adsorbent used in this study is mainly composed of zirconium hydroxide Zr(OH)4 and ferrite
compounds as Fe3O4, and round shaped about 0.7 mm in diameter, and very strongly and selectively
adsorb a phosphate through an exchange with a hydroxyl group of zirconium hydroxide especially under
acidic condition (pH 1~3). This adsorbent has a great advantage enable to be regenerated repeatedly
without deterioration using 7% sodium hydroxide solution, and to recover phosphorus as a sodium
phosphate from the solution in addition of an excess sodium hydroxide or by way of refrigeration.
Phosphorus recovery:
In this experiment, batch tests of adsorbing phosphates were conducted under the contact condition of 0.5
L extract solution per 1 L adsorbent per an hour up to the breaking point. The adsorbent was then soaked
into 3 times volume of 7% sodium hydroxide solution in order to separate a phosphate, and mixed by a
magnetic stirrer. Phosphate recovery was performed by way of concentrating the sodium hydroxide
solution up to a half volume under 90 °c and cooling down rapidly to a lower temperature. Subsequently,
the separated phosphate compounds were filtrated and dried under a room temperature.
The content of phosphorus and heavy metals in recovered phosphates was measured by using an ICP
photo-spectrometer.
3. Results and Discussion
Biological sewage treatment process generally makes sludge volumes generated larger, and thus needs to
reduce the amount of generated sludge taking into accounts the treatment cost and environmental impacts
of disposal and landfill and so on. From this experiment, it was found that suspended solids in waste
activated sludge could be decreased at least 30% in this process with sulfuric acid and heat treatment, and
a dewatering of the residue could be possible without a polymer in a dewatering centrifuge.
Fig.1 shows the time course of a phosphate adsorption of the adsorbent in an extract solution of waste
activated sludge, and also Fig.2 shows the accumulated adsorption of a phosphate. Phosphate adsorption
Phosphate removal ratio (%)
by the adsorbent was found to reach the breaking point after 100 hr. At the breaking point, a large quantity
of phosphate adsorption (34g-P/L-resine), which corresponded to 92% of total extracted phosphorus in an
extract solution from waste activated sludge, was observed. When synthesized water containing the same
phosphorus concentration used for adsorption tests, about 48 g-P/L of phosphate adsorption was observed
at a maximum. This result showed some components in an extract solution prevented a phosphate from
adsorbing to the adsorbent.
100
90
80
70
60
50
40
30
20
10
0
0
10
20
30
40
50
Bed volume of an extract solution (L/L-resine)
60
Fig.1. Time course of phosphate removal ratio from an extract solution
Adsorbed phosphate amout
(g-P/L-R)
40
30
20
10
0
0
10
20
30
40
50
60
Bed volume of an extract solution (L/L-resine)
Fig. 2. Time course of accumulated amount of phosphate adsorbed to the adsorbent
The concentration of phosphorus in a sodium hydroxide solution after soaking the adsorbent for 4 hr was
5390 mg-P/l, which corresponded to 75% of the phosphate release from the adsorbent. Table 1 shows the
concentration of a phosphate and heavy metals in an extract solution of waste activated sludge and a
sodium hydroxide solution after soaking the adsorbent. Almost of all heavy metals except arsenic could be
separated using the adsorbent. Although there was no doubt the adsorbent could slightly adsorb arsenic, it
was not contained in a recovered phosphate as indicated in Table 2. Moreover, a sodium phosphate of
about 90% purity was obtained with very few impurities of heavy metals. This separated sodium
phosphate could be reused mainly for detergents and so on.
Total phosphorus recovery ratio as a sodium phosphate was about 68% from waste activated sludge.
Improving the phosphate separation ratio from the adsorbent is quite important to elevate the effectiveness
of this phosphorus recovery process.
Table 1 Concentration of a phosphate and heavy metals
in an extract solution and a sodium hydroxide solution
measurement
pH
Total-P
PO4-P
Total-N
CODcr
SO4
Cl
Na
Ca
K
Zn
Cr
Pb
Al
Cd
Fe
Zr
As
Table 2
concentration (mg/l)
extract solution sodium hydroxide solution
1.4
13.2
1935
7820
1320
7180
2070
926
18750
6570
14750
4700
93.9
153
144
30100
647
1.4
520
90.3
41.9
N.D.
1.93
0.29
0.43
N.D.
255
127
0.09
N.D.
716
56.2
0.39
4.0
0.03
0.18
Measurements of phosphorus and heavy metals in recovered phosphate
measurement
P
Na3PO4
SO4
Na2SO4
Cl
Zn
Cr
Pb
Cd
Fe
Zr
As
* units: from P to Cl
contents (%, ppm)*
16.8
88.8
0.3
0.4
0.01
N.D.(less than 0.09)
7.5
N.D.(less than 1.3)
N.D.(less than 0.06)
6.8
13.5
N.D.(less than 1.6)
(%), heavy metals (ppm)
The running cost of this phosphate recovery process using the adsorbent was roughly estimated and
summarized in Table 3. Sludge disposal cost was not included in this estimation. As compared with a
usual sludge treatment process using a polymer for dewatering, the cost was higher due to expensive
chemicals like sodium hydroxide and the adsorbent, although the cost of a polymer for dewatering waste
activated sludge could be considerably decreased in this phosphate recovery process. The cost of this
phosphate recovery process was estimated to be almost the same with a sludge treatment process, in case
that recovered phosphates could be sold at a price of about 80 yen/kg (about 0.7dollars/kg).
Table 3 The running cost evaluation of the phosphate recovery process
and an usual sludge treatment process; dehydration using a polymer.
items
phosphate recovery process(yen) dehydration using a polymer(yen)
chemicals
12,087
7,411
adsorbent
1,551
recovered phosphate
-8,578
total
5,060
7,411
* assumptions:
(1)quantity of treated waste activated sludge ;
(2)total phosphorus load;
(3)adsorption ratio of phosphates in an extract solution;
(4)release ratio of phosphates from the adsorbent;
(5)total phosphate recovery rate;
5000 t/d
16 kg-P/d
90%
75%
54%
4. Conclusions
The results of this study can be summarized as follows:
1) Suspended solids in waste activated sludge were decreased into at least 30% by way of adding
sulfuric acid and heat treatment.
2) A large quantity of a phosphate adsorption, 34 g-P/L-R, which corresponded to 92% of total extracted
phosphorus in a solution from waste activated sludge, was observed.
3) The separation of phosphates from the adsorbent was about 75%, and it was possible to obtain a
sodium phosphate with very few impurities of heavy metals.
4) The cost of this process was estimated to be almost the same as a sludge treatment process using a
polymer for dewatering, in case that recovered phosphates could be sold at a price of about 80 yen/kg.
5) Total phosphorus recovery ratio as a sodium phosphate was about 68% from waste activated sludge.
References
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Sept., p1459-1461, 1967
2) Y.Inoue, H.yamazaki; Studies of the hydrous zirconium(IV) oxide ion exchanger. I. Ion-exchange
properties and effect of heat treatment, Bull. Chem. Soc. Jpn., 60, pp891-897,1987