Fluidized-Bed Process for Phosphate Removal by Calcium Phosphate

Chapter 26

Fluidized-Bed Process for Phosphate Removal by Calcium Phosphate Crystallization 1

IzumiHirasawa and Yasunori Toya

Downloaded by UNIV LAVAL on July 14, 2016 | http://pubs.acs.org Publication Date: September 21, 1990 | doi: 10.1021/bk-1990-0438.ch026

Ebara Research Company, Ltd., 4-2-1 Honfujisawa Fujisawa, Kanagawa 251, Japan

We have proposed a fluidized bed type process, which can be applied to phosphate removal from wastewater containing phosphate 223 mg/l as P.By the results of experiments using equipment of capacity 1-4m /day, factors such as supersaturation, recirculation ratio and space velocity were recognized to affect crystallization rate or phosphate removal efficiency. By mathematical analysis, we could obtain the characteristic equation for fluidized bed process, to agree well with experimental results. 3

Phosphate removal processes from wastewater have been studied by many workers, in order to protect stagnant water area, such as lakes and coastal region from eutrophication. Among conventional phosphate removal processes, the representative one was flocculation and sedimentation process, which was based on precipitation of insoluble metal phosphate or hydroxide. However, the main problem with this process, is to produce large amounts of sludge, which is difficult to dehydrate. To cope with these problems, we have developed phosphate removal process using crystallization, which can minimize the amount of sludge and recover phosphate. Mechanism of this process is crystallization of calcium phosphate on the surface of phosphate rocks by contacting supersaturated solution with them. In case of application to wastewater containing 1-3 mg/l phosphate as P, we proposed fixed bed type process, which has demonstrated excellent performance in the sewage treatment.* Table 1 shows the performance of fixed bed type process, in application to various wastewaters. The merit of this process is stability in ability of phosphate removal and low sludge production. Sludge production of this process is from 1/5 to 1/10 lower than that of the conventional flocculation and sedimentation process. We have now proposed fluidized bed type process, which can be applied to wastewater, containing from 2 to 23 mg/l phosphate as P. This report reveals fundamental studies on factors affecting phosphate removal and crystallization rate in the fluidized bed process. 1

1

Current address: Waseda University, 3-4-1, Ohkubo, Shinjuku-ku, Tokyo 169, Japan 0097-6156/90/0438-0355$06.00/0 © 1990 American Chemical Society

Myerson and Toyokura; Crystallization as a Separations Process ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

CRYSTALLIZATION AS A SEPARATIONS PROCESS

356

Mechanism Figure 1 shows the schematic i l l u s t r a t i o n of phosphate removal mechanism. Phosphate i n wastewater contacts with seeds made of phosphate rock, a f t e r chemical conditioning such as supply of calcium and hydroxide ion. Then calcium phosphate, mainly hydroxyapatite, c r y s t a l l i z e s , according to e q . ( l ) , on the surface of phosphate rocks. 10Ca2+ + 6HP0 2- + 80H- -> C a ( P 0 ) ( O H ) 4

10

4

e

2

+ 6H 0

eq.(l)

2


Table 1 The r e s u l t s applied to various wastewater

Secondary eff Secondary eff luent of sewaqe luent of sewage (m /day)

100

12,000

Concentration of phosphate as P (mg/1) in the influent

2-4

1-2

28—123

50-100

5

1-6

0.26—0.35

0.30-0.40

6.0

5.0-7.0 SV 2.5 (1/h) LV 2.5 (1/h) Up flow

Capacity

3

M- alkalinity in the influent (mg/1) Test periods (year) Concentration of phosphate as P (mg/1) in treated water 3

Sludge production (g/m ) Operational conditions

SV 2.5 (1/h) LV 2.5 (1/h) Down flow

C h a r a c t e r i s t i c s of F l u i d i z e d Bed Process Figure 2 shows the o u t l i n e of f l u i d i z e d bed process. Seeds are fluidized by the upflow of influent and r e c i r c u l a t e d water. Recirculation i s necessary to maintain water q u a l i t y of the influent average, lower phosphate concentration i n the influent, and also to obtain flow rate f o r f l u i d i z a t i o n . Mathematical Analysis By considering material balance of phosphate i n the f l u i d i z e d bed as shown i n Figure 3, eq.(2) can be introduced. By integration of eq.(2), eq.(3) and eq.(4) are obtained. And also we can obtain eq.(6) by introducing r e c i r c u l a t i o n factors such as eq.(5). Therefore we can make eq.(8) as c h a r a c t e r i s t i c equation, applied to t h i s f l u i d i z e d bed process.



26. EDRASAWA & TOYA

Figol

357

Phosphate Removal Process

Schematic i l l u s t r a t i o n

of phosphate removal

Pretreatmentr»|Fluidized bed type reactor

Posttieatment

* Seeds are fluidized by up-flow of influent and recirculated water * Alkali soln. is dosed into the bottom of fluidzed bed reactor, and calcium soln. is dosed into the recirculation pipe. * Seeds are made of phosphate rock, which has efficient size 0.31 mm, and has density 2.4g/cm*. * Calcium phosphate crystallizes on the surface of the seeds by the following equation. ,+

10Ca + 6 HPOr + 80H"-Ca,„(P0 ),(OH) +6H O 4

Fig„2 C h a r a c t e r i s t i c s

J

of the f l u i d i z e d

l

system


358

CRYSTALLIZATION AS A SEPARATIONS PROCESS

Q.C - Q(C+dC) + Sdz.K .a.C

eq.(2)

n

0

Input P

Output P

P fixed on the surface of the seeds eq.(3)

-K .a0» - e o

n

1

[( Q-) - -1] n * 1

0' =

K -a-C n-i 0

C* Downloaded by UNIV LAVAL on July 14, 2016 | http://pubs.acs.org Publication Date: September 21, 1990 | doi: 10.1021/bk-1990-0438.ch026

0

0

eq.(4)

(n-1)

Cg+rC

eq.(5)

1 + r

eq.(6)

,

(l+r)expK -a-^ -r 0

C

"C7

1

Fluidized-Bed Process for Phosphate Removal by Calcium Phosphate

Recommend Documents