Anaerobic digestion of olive oil mill wastewaters

Agricultural Wastes 10 (1984) 161-175

Anaerobic Digestion of Olive Oil Mill Wastewaters

Gianfranco Boari,* Alberto Brunetti,* Roberto Passinot & Alberto Rozzi*

* IRSA, Via De Blasio 5, 70123 Bari, Italy. t IRSA, Via Reno 1, 00184 Roma, Italy.

A B S T R A C T

Anaerobic treatment of olive oil mill wastewaters (COD up to 220 kg/m 3) is feasible, and the most promising results were obtained on UASB reactors, both at laboratory and pilot scale (tank capacity 15 litres and 5 m3), fed on diluted waste (COD = 13-18 kg/m3). Volumetric loading rates ranging from 16-21"5 kgCOD/m3day and 70 % removal efficiencies were obtained with these digesters.

Start-up of UASB reactors fed on olive oil mill waste is a delicate step which still has to be fully controlled and optimized. The best results were obtained by starting with very diluted waste (COD = 5 kg/m3).

Granulation of the sludge, as achieved in Dutch UASB digesters fed on sugar beet wastewaters, was not obtained, but, even so, the settleability o f the sludge was very good.

N O M E N C L A T U R E

S y m b o l Unit BA kgCaCO3/m 3 BOD 5 kg/m 3 COD kg/m 3 EE kg/m 3 HRT h L kgTOC/maday

Bicarbonate alkalinity Biochemical oxygen demand after 5 days Chemical oxygen demand Ether extract Hydraulic retention time Loading rate

16l Agricultural Wastes 0141-4607/84/$03'00 © Elsevier Applied Science Publishers Ltd, England, 1984. Printed in Great Britain

162 Gianfranco Boari, Alberto Brunetti, Roberto Passino, Alberto Rozzi

Symbol Unit Pgas m3/m3day Gas production RSu kg/m 3 Reducing sugars Sal kg/m 3 Salinity SS kg/m 3 Suspended solids SVI ml/g Sludge volume index t days Time TA kgCaCO3/m 3 Total alkalinity TC kg/m 3 Total carbon TKN kg/m 3 Total Kjeldahl nitrogen TOC kg/m 3 Total organic carbon TS kg/m 3 Total solids TSS kg/m 3 Total suspended solids TVA kgHAc/m 3 Total volatile acids TVS kg/m 3 Total volatile solids Y - - Yield coefficient VS kg/m 3 Volatile solids VSS kg/m 3 Volatile suspended solids r/ - - Removal efficiency

Subscripts EFF Effluent IN Influent R Reactor

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

Olive oil processing is an important business in the Mediterranean area where 1.4-1.8 million tons of this product are produced each year (Fedeli, 1974; Istituto geografico de Agostini, 1979).

Olive oil processing is mainly carried out by means of the traditional discontinuous press or by the more recent continuous solid/liquid centrifuge system. Both processes produce two by-product streams: the residual solids (husk), which contain oil to be recovered by means of solvent extraction, and the vegetation waters. The concentration of these wastewaters varies widely because variable amounts of washing waters are added to the liquid waste stream in both processes and because dilution water is added to the paste (pomace) to improve the separation in the solid/liquid centrifuge.

Anaerobic digestion of olive oil wastewater 163

In general, more diluted wastewaters are discharged by the continuous process, but the organic polluting load per unit weight of processed olives is practically independent of the processing method and amounts to 45-55 kg BOD 5 per ton of olives (Balice et al., 1982). The corresponding volume of raw vegetation water produced by the traditional press process is 0.4-0.5 m 3.

The polluting load of the olive oil mills located in the Mediterranean area is considerable (2800-3600 tons BOD 5 per day assuming a milling season lasting 100 days) and in some Italian and Spanish countries the equivalent polluting load of the oil industry during the milling season is 5-10 times higher than the domestic sewage load. Pollution abatement for olive oil vegetation waters is difficult because they are seasonal, highly concentrated and very polluting.

Most of the treatment processes used for industrial and domestic wastewaters have been tested on olive oil effluents. In Table 1 an approximated evaluation of the costs and the energy consumptions of the processes tested on vegetation waters is given. Incineration and concentration by distillation are reliable but quite expensive and energy consuming.

Aerobic biological processes are not advisable because of: high consumption of mechanical energy; high consumption of nutrients (to reach a ratio BOD5: N: P = 100: 5:1 from B OD5: N: P = 100: 1 : 0.5); very high production of secondary sludge which has to be disposed of; high capital cost.

Anaerobic biological processes are advisable because of their well known advantages related to energy and chemicals saving and to low sludge production.

Seasonal operation of olive oil mills is not a disadvantage for anaerobic processes because the observed decay rates for methanogens are very low and a digester can be easily restarted after several months of shut-down (Lettinga, 1980).

Most of the polluting load in olive vegetation waters is soluble and colloidal and this feature suggests the anaerobic treatment (of this waste) by high-load reactors such as the Upflow Anaerobic Sludge Blanket (UASB) (Lettinga, 1980; Rozzi & Verstraete, 1981).

Taking into account experimental results previously obtained in the IRSA laboratories on aerobic treatment (activated sludge and trickling filters) of vegetation waters, and considering the potential advantages of anaerobic digestion, a research programme on the anaerobic treatment of olive oil mill wastewaters was started by IRSA in 1978.

TA

BL

E 1

P

roce

ss E

valu

atio

n fo

r O

live

Oil

Was

tew

ater

Tre

atm

ent

Trea

tmen

t C

apita

l E

nerg

y C

once

ntra

te

cost

a as

hes~

slud

ge

( US$

/m3 d

ay)

Elec

tric

Th

erm

al

(kg/

m 3)

(k

Wh/

m 3)

(k

Wh/

m 3)

Dra

wba

cks

Inci

nera

tion

5

× 10

3 8

670

2 T

S

Sin

gle

effe

ct d

isti

llat

ion

1.1

x 10

4 20

b 73

0 90

TS

Act

ivat

ed s

ludg

e 2

x 10

4 30

--

30

VS

S

Tri

ckli

ng f

ilte

rs

1 x

104

15

--

20 V

SS

Ana

erob

ic d

iges

tion

4

x 10

3 <

1 <

20

0 c

10 V

SS

24

0 a

Des

truc

tion

of

reco

vera

ble

orga

nics

.

Pos

t-tr

eatm

ent

of d

isti

llat

e (2

-3 k

gC

OD

/m3

day

).

Low

eco

nom

ic v

alue

for

con

cent

rate

.

Dil

utio

n w

ater

. N

utri

ents

add

itio

n. S

ludg

e di

spos

al.

As

acti

vate

d sl

udge

.

Dil

utio

n w

ater

. L

on

g s

tart

-up.

Per

m 3

was

te p

rodu

ced

per

day

assu

min

g B

OD

5 =

50

kg

/m 3

. b

Ene

rgy

requ

ired

by

cool

ing

tow

er f

an.

c H

eati

ng e

nerg

y, t

akin

g in

to a

ccou

nt d

ilut

ion

wat

er,

wit

hout

hea

t ex

chan

ger.

E

nerg

y re

cove

red

from

met

hane

.


Laboratory batch reactors (for preliminary biodegradability tests) and anaerobic contact digesters at laboratory and pilot scale (reactor volumes 5 litres and 2.6 m 3 respectively) were used.

Starting up the reactors fed with high strength waste (COD = 60kg/m 3) proved to be a problem, and operating conditions were difficult to control because of build up of volatile acids. Relatively low loadings (lower than 2.5kgCOD/m3day) and efficiencies (70~ removal on COD and on TOC) were obtained both with the laboratory and the pilot digesters (Antonacci et al., 1981).

In 1980 experiments were begun on laboratory scale UASB digesters fed on concentrated waste and the same start up problems were encountered.

MATERIALS AND METHODS

Characteristics of the vegetation waters

In olive oil wastewaters produced by the traditional mill and press processes the average concentration of volatile solids (VS) is 15 ~o and the average concentration of inorganic matter is 2 ~ . The organic fraction is made up of sugars, polyphenols, polyalcohols, proteinaceous and lipid compounds, pectins, etc. (Fiestas Ros de Ursinos, 1977). Most of the VS ('~90~o) are soluble or colloidal. BOD 5 and COD maximum concentrations reach 100 and 220kg/m a respectively (Balice et al., 1982) (Fiestas Ros de Ursinos, 1958). The main constituent of the inorganic fraction is potassium. Its concentration can exceed 10kg/m 3.

In Table 2 some analytical data related to olive oil vegetation waters sampled in Apulia (Southern Italy) are given (Balice et al., 1982). The data are mean values from several samples. Because of the seasonal production and because of the variability of the waste, 80m a of vegetation waters were stored in a refrigerated tank (at 5- l0 °C) and used (raw or diluted) to prepare the feed for the experimental digesters. The average concentration of these wastewaters was approximately 30 kgTOC/m a, corresponding to 77 kgCOD/m 3 or 34 kgBODs/m 3.

The experimental reactors

Laboratory and pilot scale UASB digesters were tested (reactor volumes 15 litres and 5 m 3 respectively). The laboratory digester had two gas/solid separation baffles and no mechanical stirrer.

TA

BL

E 2

C

ompo

siti

on o

f V

eget

atio

n W

aste

wat

er

Sam

ple

pH

Ca 2

+

Mg

2 +

(eq/

m 3)

(e

q/m

3)

Na

+

K +

C

I-

SO 4

TA

P T

KN

Sa

l (e

q/m

3)

(eq/

m 3)

(e

q/m

3)

(eq/

m 3)

(e

q/m

3)

(eq/

m 3)

(e

q/m

3)

(eq/

m 3)

a 5.

4 17

.7

38.6

b

5.1

18.5

29

-8

c 5.

5 11

.7

8.9

5.63

29

6.0

60.0

7.

8 65

-5

14.6

66

-0

373-

0 7.

40

139-

3 32

-6

5-0

66.5

5.

7 33

.2

197.

0 3-

8 70

.3

37.2

3.

6 33

.7

3-2

29"6

94

.7

Sal,

Sal

init

y.

TA

, T

otal

alk

alin

ity.

Sam

ple

CO

D

TC

BO

D 5

TS

TV

S SS

E

E

RSu

(k

g/m

3)

(kg/

m 3)

(k

g/m

3)

(kg/

m 3

) (k

g/m

3)

(kg/

m 3

) (k

g/m

3)

(kg/

m 3)

B

OD

s: N

: P

a 20

8.0

81.0

90

.2

165-

0 12

9.5

23.0

5-

8 20

.4

b 98

.5

38.4

48

.4

68.6

58

.1

10-9

0.

3 5.

0 c

49.5

21

.9

28-7

44

-6

38.5

9.

6 3.

6 2.

5

100:

1-02

:0.5

10

0:0.

96:0

.4

100:

1.20

:0.3

EE

, E

ther

ext

ract

. R

Su,

Red

ucin

g su

gar.

a,

Sam

ples

fro

m t

he c

entr

ifug

e ou

tlet

(di

scon

tinu

ous

pres

s pr

oces

s).

b, S

ampl

es f

rom

the

was

tew

ater

tan

k (d

isco

ntin

uous

pre

ss p

roce

ss).

c,

Sam

ples

fro

m t

he c

entr

ifug

e ou

tlet

(co

ntin

uous

pro

cess

).


In Figs l(a) and (b) the outlines of the laboratory and of the pilot UASB reactors are drawn. Design of the pilot plant was mainly based on information from the available literature (Lettinga et al., 1980; Van den Meer, 1979). The settler is conceptually the one with the tubes for sludge recirculation patent by Centrale Suiker Maatschappij (1981) but the height of the dome for gas collection is lower than in Dutch digesters. Laboratory and pilot reactors were run at 35-37°C.

~G

\" S B z )4 . . / . '

k'~ SB:

. . . - . .

: ' . . , . . ~ . >,,:-'; ~ •

t,

G

)° ' , ,~ ,:

1 5 0 0 r n m

I, t t, b

Fig. 1. (a) Outline of the laboratory UASB reactor (volume = 15 litres); (b) outline of the pilot UASB reactor (volume = 5 m3). I, influent; E, effluent; G, gas; SB, sludge bed.

Analytical methods

Total suspended solids (TSS), volatile suspended solids (VSS), total alkalinity (TA), COD and BOD 5 were determined according to Anon. (1975). Total organic carbon (TOC) was determined on a 915 TOC Analyzer (Beckman, Fullertown). Total volatile acids (TVA) were determined by the chromatographic H4SiO * column method described in Anon. (1975). Individual aliphatic acids (C2-C5)were determined by GC


on a Sigma 1 Analyzer (Perkin Elmer, Norwalk) following the method described by Abbaticchio et al. (1983).

The concentration of pollutants in the influent and in the effluent was routinely measured as TOC, and consequently loadings, concentrations and efficiencies plotted in the figures refer to TOC. Nevertheless, for readers' convenience the values of the above parameters are given in the text as COD which is a unit more generally used in anaerobic digestion studies (1 kgTOC _~ 2.6 kgCOD).

Methanogenic (acetoclastic) activity measurements were performed according to the method described by Valcke & Verstraete (1981).

8 TOC ,N

0 30-

HRT (hi

10-

8-

L /k@ TOC~ \ rn3 d/ 4-

egas [ t j -

,°° t -.801 0 50 100 150 t (d)

Fig. 2. Experimental results for the laboratory UASB reactor (volume = 151itres): (a) operational parameters related to the feed and to the biogas; (b) mixed-liquor (and

effluent) characteristics.


RESULTS

Start-up problems were solved by diluting the waste and increasing its concentration of available nitrogen by addition of urea. The COD concentration of the feed was lowered to 5 kg/m 3 and COD:N ratio was decreased from 256:1 to the value 170:1 as suggested by Pette (1980).

Experimental data related to the start-up and operation of the 15 litre volume laboratory scale USAB reactor are plotted in Fig. 2(a) and (b). The digester was inoculated with approximately 105 g VSS (7 kgVSS/mR a) of anaerobic sludge from an activated sludge digester. The waste concentration was increased by steps at constant HRT until a final load of 21.5 kgCOD/m3day was reached at a removal efficiency of r/= 70 % on COD and on TOC. The alkalinity concentration in the reactor mixed- liquor was controlled by adding NaHCO3, NaOH and Ca(OH)2.

In Fig. 3 the experimental data related to the start-up of the 5 m 3 pilot UASB digester are plotted. The reactor was inoculated with approximately 40 kgVSS (8 kgVSS/mR 3) of anaerobic sludge from an activated sludge digester. The loading rate was increased up to 16 kgCOD/m3day

b

-70 TOCEFF

-50 f~)

TVA ~kcj HAc'~ \ n,3 /

BA /kg Ca CO-~ 4-

2 8

pH .

7 0

4-

2-

0 ° ' ° " 6"

J

, 3 0

Pgas __m.L

0

50 100 150 t (d}

Fig. 2--contd.

0 J

8-

t

\ m

3 R

d /

4-

0

'w

t \

.~3

/ 0 80

roc 6

0 t (~)

40 20 0

F

Fig

. 3.

~0

' 40

l

610

I 80

r

100

t (a

) E

xper

imen

tal

resu

lts

for

the

pilo

t U

AS

B r

eact

or (

volu

me

= 5

m3)

.

5"

e~

j-..

e~

e~ 2"

Iq

Anaerobic" digestion of olive oil wastewater 171

and the feed concentration was increased up to 13 kgCOD/m 3. Hydraulic retention time (HRT) was maintained in the range 20-24h. Removal efficiency both on COD and on TOC was of the order of 70 ~.

The methanogenic activity of the inocula used for the laboratory and pilot digesters was low (5-10 mlCH4/gVSSday ). Sludge activity at the end of the experiments was of the order of 100-120mlCHJgVSSday.

Granulation of the sludge as described by Lettinga et al. (1979) was not obtained, although the settleability of the sludge particles was good (SVI < 50 ml/g). Sludge concentration in the bed of the pilot plant was about 144kgSS/m 3 (53kgVSS/m~). The concentration of suspended solids in the effluent was about 0.3-0.5 kg/m 3 except during overloads (20-26kgCOD/m3day) when severe foaming occurred. No attempt to control the latter by antifoaming agents was made.

DISCUSSION

Build-up of volatile acids during the digestion start-ups using concentrated feed waste can be easily explained if one considers that unadapted inocula from activated sludge digesters contain low concentrations both of methanogens and of microorganisms which metabolize complex and potentially inhibiting aromatic compounds. These organics are found in olive oil wastes in appreciable concentrations (1-10 kg/m3). During start-up, acidifying microorganisms grow easily on the carbohydrates dissolved in the waste while the methanogenesis, which represents the limiting step in anaerobic digestion of soluble compounds, is severely hindered by the combined inhibition caused by the high concentration of aromatic compounds and by the build-up of volatile acids. Neutralization by base addition (e.g. NaOH, NH4OH ) is not very effective in overcoming these problems because it can lead to cation toxicity. It follows that it is difficult to start a digester on concentrated olive oil wastewater using unacclimated inocula because the environmental conditions are particularly unfavourable to the growth of methanogens. This suggestion was supported by the very long start-up period, lasting about one year (Fiestas Ros de Ursinos, 1982), required by a 70 m a volume anaerobic contact pilot plant fed with undiluted olive oil waste waters (COD = 40-60 kg/m 3) (Fiestas Ros de Ursinos et al . , 1982) and by the difficulties encountered by the authors and by others (Aveni, 1982) in attempting to grow unadapted inocula on olive oil mill wastewaters at the same concentrations.


Performance of IRSA anaerobic contact digesters tested in a previous study (Antonacci et al., 1981) was lower than the performance obtained in similar experiments by Fiestas Ros de Ursinos et al. (1982) as shown in Table 3. The unsatisfactory results and especially the very poor settleability of the suspended solids in the mixed-liquor were most probably caused by excessive mechanical mixing of the digester contents.

TABLE 3 Performance of Anaerobic Contact Digesters Fed with Olive Oil Mill Wastewaters

Volume Load Efficiency Reference (m 3) (kgCOD/m3~lay) (%COD)

0'005 2.5 70 2-6 1.25 70

70 2'5 80

Antonacci et al. (1981) Antonacci et al. (1981) Fiestas Ros de Ursinos et al. (1982)

Indeed, the mixed-liquor was both stirred by a mixing propeller and recirculated by a Mono pump through a heat exchanger for temperature control. The resistance to shear stresses of anaerobic flocs seems to be low and, if mechanical mixing is kept to a minimum, the settleability of the sludge is enhanced (Lettinga et al., 1980; Pette & Versprille, 1981). Lack of available methods for microbial activity determinations at the time of the experiments (1979) also made the interpretation of the results very difficult.

As the polluting load in olive oil mill wastewaters is mainly soluble, the ensuing experiments were carried out on the UASB reactor which has been developed in the Netherlands (Lettinga et al., 1979). Full scale UASB plants are presently operating on sugar beet wastewaters and on other wastes principally made of soluble carbohydrates at loading rates up to 20 kgCOD/m3day and treatment efficiencies of 80-95 ~o.

In the first tests carried out on UASB laboratory reactors (15 litre volume) during this study, relatively concentrated olive oil mill wastewaters ( C O D = 15kg/m 3) were used as feed during start-up. Loadings of 2.5 kgCOD/m~day and 80 ~ COD and TOC reduction were reached and maintained for 6 weeks.

However, because of the instability and the poor reproducibility of the start-ups, a more rational approach to acclimation of the sludge inoculum was undertaken. In order to rule out the possibility that the amount of available nitrogen could be rate limiting for biomass synthesis, especially


during start-ups when the growth yield (Y) increases because of the high organic loadings on the active sludge (Henze & Harremoes, 1982), urea was added as indicated above. In anaerobic systems the acclimation time can be minimized by providing the methanogens and other rate-limiting bacteria with the most favourable conditions for maximum growth rates. The simplest way to obtain inhibition-free environmental conditions during start-ups with concentrated soluble substrates is by dilution. During start-up, the concentrations of the microorganisms which convert volatile acids to methane and of those microorganisms which detoxify the wastewater by metabolizing inhibiting compounds (e.g. to the methanogens) are low, so dilution is very effective in keeping the concentration of volatile acids and of potentially inhibiting compounds below a safe level until the proper microbial populations build up. Dilution is also beneficial because, by increasing the hydraulic loadings, it exerts a selective pressure on the best settling aggregates of microorganisms and promotes the formation of a well-settling sludge (Lettinga et al., 1980).

In most cases of anaerobic digestion, dilution is not required either because methanogenesis is not the rate-limiting reaction (e.g. for slurries made of suspended solids) or because the waste is already very diluted, as is the case with the majority of the wastewaters treated up to now by the UASB process (COD < 10kg/m3), although Pipyn & Verstraete (1979) proposed waste dilution during start-up of UASB digesters.

Figs 2(a) and 3 show that, by using the start-up dilution technique, very high volumetric loadings could be reached in relatively short times (4-6 weeks) and maintained steadily. Assuming that the concentration of methanogens increases proportionally to methane production if the substrate is not limiting (as during the time interval 18-28 days for the pilot UASB, Fig. 3, when gas production increased and removal efficiency decreased) rates of the order of 0" 16/day were calculated, which compare very favourably with values equal to 0.02/day determined by Fiestas Ros de Ursinos et al. (1982) in anaerobic digestion of more concentrated olive oil mill wastewaters.

A major problem during start-up was foam formation following accidental organic overloadings. In these conditions the efficiency of the settler decreases, and the enhanced loss of suspended solids in the effluent can reduce the biomass concentration in the reactor and increase still more the organic overloading. Consequently, the loading has to be temporarily reduced to prevent 'souring' of the digester by volatile acids. The transient described above is shown in Fig. 3, days 28-40.


Sludge fluidification by the gas is likely to be the controlling factor which limits the maximum volumetric loading in quasi-steady state conditions, as increasing flow rates of organics which are converted to CH 4 and CO 2 enhances gas production and sludge fluidification. As a consequence the mass flow of sludge lost in the effluent can increase to a point where it overreaches the production of active sludge. Then the biomass held in the reactor decreases, gas production decreases because conditions of limited overloading are set up, but loss of sludge through the effluent decreases and the process conditions fluctuate around an equilibrium point. This behaviour is apparent in Fig. 2(a), which refers to the laboratory scale digester, f rom days 100-160.

REFERENCES

Abbaticchio, P., Balice, V. & Cera, O. (1983). Determination of volatile fatty acids by gas chromatography after a rapid steam distillation of wastes. Environ. Tech. Lett., 4(4), 179-82.

Anon. (1975). Standard methods of the examination of water and wastewater. APHA- WPCF (14th edn).

Antonacci, R., Brunetti, A., Rozzi, A. & Santori, M. (1981). Trattamento anaerobico di acque di vegetazione di frantoio. I. Risultati preliminari. Ingegneria Sanitaria, 6, 357--62.

Aveni, A. (1982). Personal communication. Istituto di Ricerche Breda, Bari. Balice, V., Boari, G., Cera, O. & Abbaticchio, P. (1982). Indagine analitica sulle

acque di vegetazione. Nota 1. Inguinamento, no. 7-8, 49-53. Centrale Suiker Maatschappij, N. V. (1981). Dutch Patent Request 76.06.904

(awarded on 16-5-81). Fedeli, E. (1974). Lipids of olives. Prog. Chem. Fats Lipids, 15, 57-74. Fiestas Ros de Ursinos, J. A. (1958). Alpechines. Grasas y Aceites, 9, 126-35. Fiestas Ros de Ursinos, J. A. (1977). Depuracion de aguas residuales en las

industrias de aceitunas y aceites de oliva. Grasas y Aceites, 28, 113-21. Fiestas Ros de Ursinos, J. A. (1982). Personal communication. Istituto de la

Grasa y sus Derivados, Sevilla. Fiestas Ros de Ursinos, J. A., Navarro Gamero, R., Leon Cabello, R., Garcia

Buendia, A. J. & Maestrojuan Saez de Jauregui, G. M. (1982). Depuracion anaerobia del alpechin como fuente de energia. Grasas y Aceites, 33, 265-70.

Henze, M. & Harremoes, P. (1982). Literature review, anaerobic treatment of wastewater in fixed film reactors. Proc. IA WPR Specialized Seminar: Anaerobic Treatment of Wastewater in Fixed Film Reactor, Copenhagen, 16-18 June, p. 21.

Istituto Geografico de Agostini (1979). Calendario Atlante de Agostini. Novara.


Lettinga, G. (1980). Anaerobic treatment of low strength wastes. Inter University Course on Anaerobic Digestion, Dijon.

Lettinga, G., van Velsen, L., de Zeeuw, W. & Hobma, S. W. (1979). The application of anaerobic digestion to industrial pollution treatment. Proc. 1st Int. Syrup. on Anaerobic Digestion, Cardiff. In: Anaerobic Digestion (D. A. Stafford, B. I. Wheatley & D. E. Hughes (Eds)). Applied Science Publishers, London (1980).

Lettinga, G., Van Velsen, A. F. M., Hobma, S. W., de Zeeuw, W. & Klapwijk, A. (1980). Use of the upflow sludge blanket (USB) reactor concept for biological wastewater treatment, especially for anaerobic treatment. Biotech. & Bioeng., 222, 699-734.

Pette, K. E. (1980). Anaerobic wastewater treatment at CSM sugar factories. Sucr. Beige Sugar Ind. Abstr., 99(12), 473-9.

Pette, K. E. & Versprille, A. I. (1981). Application of the U.A.S.B.--conceptfor wastewater treatment (D. E. Hughes et al. (Eds)). Elsevier Biochemical Press, Amsterdam.

Pipyn, P. & Verstraete, W. (1979). A pilot scale anaerobic upflow reactor treating distillery wastewaters. Biotech. Lett., 1,495-500.

Rozzi, A. & Verstraete, W. (1981). Calculation of active biomass and sludge production vs waste composition in anaerobic contact process. Trib. Cebedeau, 34, 421-7.

Valcke, D. & Verstraete, W. (1981). A practical method to estimate the acetoclastic methanogenic biomass in anaerobic sludges. Poster Paper 2nd Int. Syrup. on Anaerobic Digestion, Travemunde, W. Germany, 6-11 September.

Van den Meer, R. R. (1979). Anaerobic treatment of wastewater containing fatty acids in upflow reactors. PhD Thesis, Delft University Press, Delft.

Documents

Anaerobic digestion of olive oil mill wastewaters