Water Research Pergamon Press 1971. Vol. 5, pp. 681-687. Printed in Great Britain

ANAEROBIC DIGESTION OF RAW SEWAGE

W. A. PR~TORIUS National Institute for Water Research, Council for Scientific and Industrial Research, P.O. Box 395,

Pretoria, South Africa

(Received 20 April 1971)

Abslract--The use of the anaerobic contact process for the pre-treatment of raw sewage is described. By using a rather simple settling system combined with a stone packed bio-physical filter, up to 90 per cent COD reduction of raw sewage is obtained at residence times of 24 h at a temperature of 20"C.

I N T R O D U C T I O N

WITH the recent emphasis on the reclamation of potable water from sewage water in South Africa (VAN VUUREN et al., 1967; STANDER and VAN Vtrtr~N, 1969) the quality of the water to be renovated by chemical treatment has become important. While it is relatively easy to remove the normal sewage constituents such as COD, ammonia, phosphates, detergents and tastes and odours, the nitrogen in the nitrate form can as yet not be removed economically.

Since it is known that anaerobic biological processes convert nitrogenous compounds to either ammonia, organic nitrogen compounds, or both, effluents f rom digesters would be very well suited for chemical treatment. The problem, however, is that thus far anaerobic digestion has been applied primarily to the treatment of concentrated wastes, and that very little information exists as to the application of anaerobic digestion to dilute wastes such as raw sewage water.

Depending on the relative "strength" of the waste water, anaerobic treatment systems have been designed and operated as the conventional process for strong wastes and the anaerobic contact process to handle the more dilute wastes (MCCARTr, 1964).

As biological processes are fairly temperature dependent, the anaerobic contact process has found its application primarily in the field of industrial effluents. This is because the organic load of the latter is usually of a soluble nature while the effuent itself is at a sufficient temperature to maintain high biological activity without addi- tional heating. In this regard MeCarty calculated that organic wastes with COD concentrations of 2000-5000 mg 1-1 were too low in nutrients to produce sufficient methane to increase the temperature of the waste significantly.

The application of anaerobic digestion directly to raw sewage is rather limited, possibly because of the relatively low temperatures of raw sewage as well as the relatively low COD concentration. The experience gained with the treatment of raw domestic sewage by anaerobic ponds (AeBoa-r, 1962; VAN ECK and co-workers, 1969) has led to a pilot plant study on the use of the anaerobic contact process for treating raw domestic sewage (SIrOSON, 1971). While the hydraulic residence time was varied between 10 and 22 h, total COD reductions were in the order of 70 per cent and more. Although temperature was important , one of the major problems was the loss of the sludge and active cells in the effluent. In the application of the contact process to industrial wastes this problem was overcome by degasification (ScI-rROEP~R et aL, 1955; Sca-mo~r~R and ZmMI~, 1959; DmTZ et aL, 1966). SIMPSON

681

682 W . A . PRETORIUS

(1971) used two setthng tanks in series, whtle COULTER et al (1957) used a rock bed after sedimentation for the treatment of raw sewage

Based on the work of ZOBELL (1943) and subsequent articles by RENN (1964) on the microbtology of sohd surfaces, the anaerobic contact system used by COULTER et al.

(1957) was preferred to the system used by SIMPSON (1971) In this paper some of the observations of the treatment of raw sewage on a laboratory size anaerobtc contact unit are given.

MATERIALS AND METHODS

Laboratory anaerobic contact umt

Based on the fact that active fermenting sludge settles poorly, a &gester was built along the lines of COULTER et al. (1957). The digester system consisted of a contact or digester chamber combined with a settling zone and a b]ophysical filtration system. The different units are diagrammatically shown in FIG. 1.

To gas meier Feed inlet l t

i I I

Sludge blanket at top

15cm

Sludge conloct o - - ~ chamber

~,~,og bor . . . . , , .

Maqnetlc mixer- 4 0 ] Sornplmg " port

O v e r f l o w weir

Setl lmg zone-- Sludge re turn

To gas meier

ilj nal e f f luen t

o-,,.%'.-" ,.:-.,:;.-.. *i: .:.0 3~'cm "ston~*~:

*~. " .~ -. :~_.: .. • . . . . o ~ 0"65 c~m° s i~n~es~ o ~ o ~ o o ~-%~

I~o~1 ZScm slone~ 0 O00QO0 0 000' O0 O0 O0

t ,~ "-0-~° ' ~ °-:J-'~Q° "Q ] -Folse bottom --~ t=== .~ - Sludge dram

FIG 1 Schemattc outlay of anaerobtc contract untt

The digester consists of a chamber of 8 1. provided with a mtxmg bar and a feed inlet to the bottom. Since most digesters which receive effluents containing suspended solids form a scum layer on top, an inverted contcal type of settling and gas collecting device has been used. As there was about 1 cm opening between the cone and digester wall, a low upward flow velocity was maintamed.

The bio-physlcal filhatlon system consisted of a unit slmdar m volume to the digester, packed with stones vary]ng from 2 cm dia. at the bottom to filter sand (16-30 mesh) at the top, giving a void ratio of 0.6

Feed

Twice a week 401. of raw screened sewage from domestic ong]n were collected at the Pretoria sewage works at Daspoort. For umformlty these batches of sewage were

Anaerobic Digestion of Raw Sewage 683

diluted with tap-water to give a strength of 500 mg 1-1 COD. The sewage was stored at 2°C.

Digester operatwn

The digester was charged with 2"5 1. active digesting sludge and filled up with raw screened sewage. Feeding was commenced to give a hydraulic residence time of 72 h. After 1 month of operation the digester was then operated at constant overall hydraulic residence times of 45, 33 and 24 h respectively. The digester and its contents were kept in a constant temperature room at 20°C.

Gas readings were taken dally, while COD, volatde fatty actds (V.F.A.), ammonia, orthophosphate, suspended solids (S.S.) and volatile suspended solids (V.S.S.) were done twice weekly.

All the chemical analyses were done on an autoanalyser (Technicon, Tarry Town, New York). COD was done by the method of ADELMAN (1968). V.F.A. by the method of HARWOOD and HUYSER (1970a). Ammonia by the method of HARWOOD and HtrYSER (1970b) and orthophosphate by the method of MURPHY and RILEY (1962). Suspended sohds, V.S S. and gas analysis were done manually.

RESULTS

A summary of the results obtained over a period of 4 months is given in TABLES 1-3.

DISCUSSION

The combined digester-biophysical filter performed very well with raw sewage in reducing the COD load and converting mtrogen-containing compounds to ammonia (see TABLES 1 and 2). In general the system functioned on two basic lines. The digester part concentrated the suspended solids and at the same time hydrolysis of the complex molecules occurred. Sixty to sixty-five per cent of the incoming suspended sohds accumulated in the digester and had to be removed periodically. During this study only small quantities of sludge were withdrawn to do studies on drying. The drying pro- perties of the sludge were very good. The difference in S.S. has been transformed to soluble COD. This liberated COD as well as the soluble COD present in the mfluent are mainly broken down in the biophysical filter. This is evident from the relative amounts of gas produced by the digester (28 per cent) to that produced by the biofilter (72 per cent).

At the different residence times employed, excess fatty acid production (the common indication of digester failure) was virtually absent. The low values of fatty acid con- centraUon were probably due to the high dilution rates employed.

Due to the sohds retaining capacity of the combined system, it is possible to treat highly diluted wastes (such as raw sewage) at very short residence times. Although the 24 h hydraulic residence time run employed in this study corresponded to a loading rate of approximately 0.03 lb COD ft -3 day -1, it seems at this stage as if the hydraulic loading would be a better design parameter to be used than loading based on con- centration.

After the encouraging results obtained with the laboratory unit, a small pilot plant wtth a volume of about 2000 1. was constructed at the Pretoria sewage works and

TAB

LE I

, A

VE

RA

GE

OP

ER

AT

ION

DA

TA

OF

A D

IGE

ST

ER

OPE

RA

TE

D

AT

20

°C

ON

RA

W S

EW

AG

E W

AT

ER

CO

NT

AIN

ING

50

0 m

g

1-1

TO

TA

L C

OD

Day

s at

ste

ady

stat

e H

ydra

ulic

(1

) (2

) re

side

nce

Sol

uble

CO

D (

mg

1-1)

G

as p

rodu

ctio

n (d

ay-J

) ti

me

(h)

Inll

uent

D

iges

ter

Eff

luen

t y,

D

iges

ter

Bio

-fil

ter

redu

ctio

n T

otal

>

21

20

28

45

172

165

110

36 (

78)*

0-

07 (

12)t

0'

52 (

88)

33

111

121

53

52 (

89)

0 42

5 (4

1)

0 61

0 (4

9)

24

148

212

75

49 (

90)

0'31

8 (2

8)

0'83

4 (7

2)

0 59

1

035

1'15

2

• B

rack

ets

indi

cati

ng a

ppro

xtm

ate

figu

res

of o

vera

ll C

OD

red

ucti

on.

t B

rack

ets

indi

cati

ng p

erce

ntag

e of

tot

al g

as p

rodu

ctio

n.

TAB

LE 2.

A

VE

RA

GE

C

HE

MIC

AL

A

NA

LY

SIS

OF

W

AT

ER

A

T V

AR

IOU

S

PL

AC

ES

T

HR

OU

GH

T

HE

S

YS

TE

M

SS (

g 1-

1)

CO

D (

mg

1-1)

V

SS (

g 1 -

t)

VF

A (

mg

I -t)

N

Ha-

N (

rag

1 -~)

O

rtho

P (

mg

1 -~)

T

otal

S

olub

le

go

O

t~ v.

Fee

d D

iges

ter

cont

ent

Bio

phys

ical

fi

lter

eff

luen

t

0"25

2

1.28

-24.

4

0-03

0.21

7 50

0 17

8 11

-4

25"4

1-14

-17.

4 ?

200

59

30'2

0 02

7 11

0 96

12

"9

34.8

6'4

7-8

7"5

1:3

O

la

t.A

686 W A. PRETORIUS

TABLE 3. AVERAGE GAS COMPOSITION CORRECTED FOR

C O 2 AND CH4 ONLY

Digester (~) Biophysical filter (~g)

CO2 25 8 CH4 75 92

operated for nearly a year without any noticeable headloss m the b~ofilter. Apar t from periodic mixing to break up sludge layer in inverted cone, better results were obtained without mixing. The results obtained were virtually the same as found w~th the laboratory digester. Fur thermore , none of the adverse effects of high sulfide product ion found by COULTER e t al. (1957) were observed.

The pretreatment of raw sewage by this means could offer great advantages to the subsequent t reatment of the water, either by aerobic biological or chemical purifying systems

CONCLUSION

With a rather simple anaerobic contact process, with no recycle, it has been found that up to 90 per cent of the COD m raw sewage water can be removed in 24 h at a

temperature of 20°C. Thirty five to forty per cent of the suspended solids are hydrolized in the digester- -

the difference accumulated and has to be wi thdrawn periodically, usually when a S.S. concentra t ion of 25 g 1 -~ or higher is reached_

The digester part is more or less responsible for solids concentra t ion and hydrolysis, while the bio-physical filter par t was responsible for the gasification. In this respect the solid surface concentra t ion effect described by ZOBELL (1943) is extremely effective in removing low concentrat ions of nutr ients under anaerobic conditions.

For dilute wastes i e. 1000 mg 1-1 COD or less, the hydrauhc loading rate would be a better parameter to use for designing purposes, than waste concentrat ion.

Acknowledgement - -The technical help of Mr J. H NELL and Miss P. I PALMER are gratefully acknowledged.

REFERENCES AEBOTr A L (1962) The Wynberg-Mmzenberg sewage treatment scheme J Inst. Sen'. Put,tic 3, 224 ADELMAN M. H. (1968) Automated measurement of COD Water Wastes Engng 5, 52-55. COULTER J. B, SONEDA S and ETTmGER M B (1957) Anerobic contact process for sewage disposal_

Sew lnd Wastes 29, 468 DIETZ J_ C , CLINEBELL P. W. and STRUn A. L. (1966) Design considerations for anaerobic contact

systems J. Wat. Pollut Control Fed. 38, 517. HARWOOD J E. and HUYSER D J. (1970a) Letter to the editor: Note on Simplified automatedvolatde

fatty acids analysis Water Research 4, 383-385 HARWOOD J E. and HUVSER D J (1970b) Automated analys~s of ammonia in water. Water Research

4, 695-704 MCCARTY P. L 0964) Anaerobic Waste Treatment Fundamentals Parts I-IV_ Pubhc Works. MURPHY J and RILEY J P. (1962) A modified single solution method for the determination of phos-

phate in natural waters Anal ch:m. Acta 27, 31-36. RENN C. E (1964) The bacteriology of interfaces. Principles and Apphcatmns in Aquatic [tt~croblology

(Edited by HEUKELEKIAN H. and DONDERO N C.) pp. 193-201, Wiley, New York. SCHROEPFER G J., FULLEN W. J , JOHNSON A S, ZtEMKE N. R. and ANDERSON J. J 0955) The

anaerobic contact process as applied to packinghouse wastes Sew. Ind. Wastes 27, 460-86

Anaerobic Digestion of Raw Sewage 687

SCHROEP~R G. J. and ZIEMKE N. R (1959) Development of the anaerobic contact process. 1 Pilot plant investigations and economics. Sew. Ind. Wastes 31, 164.

SIMPSON D. E. (1971) InvestigaUons on a pilot plant contact dtgester for the treatment of a dilute urban waste. Water Research 5, 523-532.

STANDER G. J. and VAN VUUREN L. R. J (1969) The reclamation of potable water from wastewater. ,I. War. Pollut. Control Fed. 41 (3), 355-367.

VAN EC• H. and co-workers (1969) The Chatsworth Umlazi and Lamontville sewage stabihzatlou ponds in the Durban area. CSIR Contract Report C. ;Vat. 20.

VAN VUUREN L. R. J., STANDER G. J., HENZeN M. R., M£1RXNG P. G. J. and VAN BLERK S. H. (1967) Advanced purification of sewage works effluent using a combined system of hme softemng and flotation. Water Research 1, 463-474.

ZOBELL C E (1943) The effect of solid surfaces upon bacterial activity. J Bact. 46 (2), 39-56.