PII: SO273-1223(99)00101-S

Waf. Sci. Tech. Vol. 39, No. 5, PP. 187-194, 1999 0 1999 IAWQ

Published by Elsevier Science Ltd Printed in Great Britain. All rights reserved

0273-1223199 $19.00 + 0.00

THE ROLE OF ANAEROBIC DIGESTION OF DOMESTIC SEWAGE IN CLOSING THE WATER AND NUTRIENT CYCLE AT COMMUNITY LEVEL

G. Zeeman and G. Lettinga

Department ofAgricultural, Environmental and Systems Technology, Sub-department of Environmental Technology. Agricultural University, Wageningen, Post Box 8129, 6700 EV Wageningen.

ABSTRACT

Decentralised sewage treatment is more and more considered to be a sustainable way of waste(water) treatment. Three different options for the on-site treatment of sewage in combination with VFY (Vegetable, Fruit and Yard waste) with anaerobic treatment as a central technique are presented. The collection system and therewith the composition and concentration of the sewage will determine the type of anaerobic technology and the process conditions to be applied. Model calculations are made for determining the HRT to be applied in a UASB system at an obliged SRT at a certain temperature. Q 1999 IAWQ published by Elsevier Science Ltd. All rights reserved

KEYWORDS

Anaerobic; black water; closed cycle; community; decentralised treatment; domestic sewage; grey water; HRT; night soil; UASB; UASB-septic-tank, VFY; SRT.

INTRODUCTION

The most common method of domestic wastewater management is its discharge, in extended sewer systems in order to be transported over a large distance, mainly together with rainwater and subsequently treated in centralised treatment systems. The domestic wastewater is generally treated aerobically, both in developed and developing countries. Aerobic treatment consumes a considerable amount of energy and produces large quantities of secondary sludge, which are too heavily polluted with for example heavy metals to be used in agriculture. Combining water of different qualities and varying quantities in the sewer is mostly not the most cost effective and environmental friendly option. In fact less polluted water is polluted and strongly polluted wastewater is diluted, thereby creating expensive transport of large amounts of water which eventually have to be purified and discharged (v.d.Bergen, 1997). Decentralised sewage treatment is more and more considered to be a more sustainable way of wastewater treatment, both in developing and developed countries as it:

. creates possibilities to reuse treated wastewater for example for irrigation and fertilisation.

. offers the possibility to separately collect and treat the different wastewater streams. Thus less diluted streams can be reused in for example the household itself (Terpstra, 1996) and more concentrated streams can be treated with a more appropriate technique, such as anaerobic digestion, and subsequently reused in agriculture (Otterpohl et al, 1997). Figure 1 distinguishes three different options for the on-site treatment of sewage with anaerobic treatment as a central technique, viz.

187

~ Pergamon

PH: S0273-1223(99)OO101-8

War. Sci. Tech. Vol. 39, No.5, pp. 187-194,1999

© 19991AWQPublished by Elsevier Science Ltd

Printed in Great Brilain. All rights reserved0273-1223/99 $19.00 + 0.00

THE ROLE OF ANAEROBIC DIGESTIONOF DOMESTIC SEWAGE IN CLOSINGTHE WATER AND NUTRIENT CYCLE ATCOMMUNITY LEVEL

G. Zeeman and G. Lettinga

Depar~ment ofAgricultural, Environmental and Systems Technology, Sub-departmentofEnVironmental Technology, Agricultural University, Wageningen, Post Box 8129,6700 EV Wageningen.

ABSTRACT

Decentralised sewage treatment is more and more considered to be a sustainable way of waste(water)treatment. Three different options for the on-site treatment of sewage in combination with VFY (Vegetable,Fruit and Yard waste) with anaerobic treatment as a central technique are presented. The collection systemand therewith the composition and concentration of the sewage will determine the type of anaerobictechnology and the process conditions to be applied. Model calculations are made for determining the HRTto be applied in a UASB system at an obliged SRT at a certain temperature. © 1999 lAWQ Published byElsevier Science Ltd. All rights reserved

KEYWORDS

Anaerobic; black water; closed cycle; community; decentralised treatment; domestic sewage; grey water;HRT; night soil; UASB; UASB-septic-tank; VFY; SRT.

INTRODUCTION

The most common method of domestic wastewater management is its discharge, in extended sewer systemsin order to be transported over a large distance, mainly together with rainwater and subsequently treated incentralised treatment systems. The domestic wastewater is generally treated aerobically, both in developedand developing countries. Aerobic treatment consumes a considerable amount of energy and produces largequantities of secondary sludge, which are too heavily polluted with for example heavy metals to be used inagriculture. Combining water ofdifferent qualities and varying quantities in the sewer is mostly not the mostcost effective and environmental friendly option. In fact less polluted water is polluted and strongly pollutedwastewater is diluted, thereby creating expensive transport of large amounts of water which eventually haveto be purified and discharged (v.d.Bergen, 1997). D(jcentralised sewage treatment is more and moreconsidered to be a more sustainable way of wastewater treatment, both in developing and developedcountries as it:

• creates possibilities to reuse treated wastewater for example for irrigation and fertilisation.• offers the possibility to separately collect and treat the different wastewater streams. Thus less diluted

streams can be reused in for example the household itself (Terpstra, 1996) and more concentratedstreams can be treated with a more appropriate technique, such as anaerobic digestion, and subsequentlyreused in agriculture (Otterpohl et ai, 1997). Figure 1 distinguishes three different options for the on-sitetreatment of sewage with anaerobic treatment as a central technique, viz.

187

188 G. ZEEMAN and G. LETTINGA

The collection and treatment of total sewage (black + grey water). The domestic wastewater is treated with UASB technology. When a one-step UASB is applied, the produced excess sludge will be rather well stabilised. When a two-step UASB (high loaded UASB followed by a methanogenic UASB) is applied, the excess sludge is to be post-digested possibly in combination with VF(Y) (Vegetable, Fruit and eventually Yard) waste in a separate CSTR. The effluent is to be post-treated to remove pathogens and can subsequently be used for irrigation and fertilisation.

The separate collection and treatment of grey- and blackwater. Most of the COD, nutrients and pahgens are concentrated in the black water. The black water, eventually in combination with the kitchen water, is treated separately in an UASB. The liquid effluent will still contain pathogens and nutrients, the first should be removed in a second step, the latter can be used for fertilisation. When a one step UASB is applied, the produced excess sludge will be rather well stabilised. When a two step UASB is applied, the excess sludge is to be post-digested possibly in combination with VFY in a separate CSTR. Grey water is less heavily polluted and the pollution is mainly limited to COD. After application in for example a (sand) filtration system (v. Buuren et al., 1998), the water can or be re-used in the household or used for irrigation.

The separate production and treatment of grey water and ‘night soil ‘. When ‘night soil’ is produced instead of black water, Ee. by application of vacuum toilets (Otterpohl, 1998), this slurry is to be treated in a conventional CSTR or fed-batch (= Accumulation) system together with VFY. Grey water can be treated as indicated above.

The collection and treatment of total sewage (black + grey water) or the separate collection and treatment of grey- and blackwater.

,VFY-_ ~+-@=--lp soil conditioning/ fertilization

* when blackwater is treated in the UASB

The separate production and treatment of grey water and ‘night soil’.

night soil soil conditioning/

grey water .-b Reuse (household)/

irrigation

Figure 1, Possibilities for on-site treatment of domestic sewage with anaerobic digestion as a central technique.

188 G. ZEEMAN and G. LEITINGA

The collection and treatment oftotal sewage (black + grey water). The domestic wastewater is treated withUASB technology. When a one-step UASB is applied, the produced excess sludge will be rather wellstabilised. When a two-step UASB (high loaded UASB followed by a methanogenic UASB) is applied, theexcess sludge is to be post-digested possibly in combination with VF(Y) (Vegetable, Fruit and eventuallyYard) waste in a separate CSTR. The effluent is to be post-treated to remove pathogens and can .subsequently be used for irrigation and fertilisation.

The separate collection and treatment ofgrey- and blackwater. Most of the COD, nutrients and pathogensare concentrated in the black water. The black water, eventually in combination with the kitchen water, istreated separately in an UASB. The liquid effluent will still contain pathogens and nutrients, the first shouldbe removed in a second step, the latter can be used for fertilisation. When a one step UASB is applied, theproduced excess sludge will be rather well stabilised. When a two step UASB is applied, the excess sludge isto be post-digested possibly in combination with VFY in a separate CSTR. Grey water is less heavilypolluted and the pollution is mainly limited to COD. After application in for example a (sand) filtrationsystem (v. Buuren et al., 1998), the water can or be re-used in the household or used for irrigation.

The separate production and treatment ofgrey water and 'night soil '. When 'night soil' is produced insteadof black water, f.e. by application of vacuum toilets (Otterpohl, 1998), this slurry is to be treated in aconventional CSTR or fed-batch (= Accumulation) system together with VFY. Grey water can be treated asindicated above.

The collection and treatment oftotal sewage (black +grey water) or the separate collection and treatmentofgrey- and blackwater.

! sewage '>F81 "'" VASB Post-treatment for

or pathogens

black water: I d I~- s u geblOgas

, J CSTRlAC- 1_ soil condltioningli VFY . 1~ system . fertilization

irrigatlonl

-------­fertilization

:*grey waterRe-use

--... (household)irrigation

* when blackwater is treated in the UASB

The separate production and treatment ofgrey water and 'night soil'.

night soil

VFY

soil conditioning!fertilization

grey waterfiltration

system

Re-use (household)1irrigation

Figure 1. Possibilities for on-site treatment of domestic sewage with anaerobic digestion as a central technique.

Role of anaerobic digestion of domestic sewage

CONCENTRATION AND COMPOSITION OF THE SEWAGE 189

At decentralised treatment of domestic sewage both the total COD concentration as well as the fraction of suspended solids is expected to increase in comparison with central treatment, as a result of the absence of diluted wastewater and/or rainwater, and the shorter retention in the sewer. Table 1, illustrates the expected concentration of the different COD fractions of on-site produced sewage. The table distinguishes between 4 different options. viz. blackwater produced at conventional toilet flushing or at low toilet flushing and grey + black water with ‘normal’ or low water use. The presented figures are based on Bogte et al. (1993) and Lettinga et al. (1993) and show for each type of sewage high concentrations of CODS% (>4.4.pm), viz. between 65- 72%. Additionally 15-17% colloidal ( >0.45pm, <4.44pm) COD is to be expected.

Table 1. Expected concentrations of grey+black- and black-water as produced on-site, without the collection of rainwater (based on Lettinga et al., 1993 and Bogte et al., 1993).

wastewater Total Suspended Colloidal Dissolved COD COD COD COD

grey + black

(g/l) (g/l) 0.98 0.64 (0.13) 100%

(g/l) n.m.

(g/l) n.m.

comments The Netherlands, total sewage, without rainwater (Bogte et al., 1993)

black

black

1.72 1.20 n.m. n.m. The Netherlands, (0.26) blackwater 100% with flushing toilets

(Bogte et al., 1993) 5.99 3.73 1.0 1.26 Indonesia, black water (1.5) with pour-flush toilets 100% (Lettinga et al., 1993)

grey + 1.85 1.33 0.27 0.24 Indonesia, total sewage black (0.95) without rainwater, low

100% Standard deviation is given in brackets

water use.

APPLICATION OF UASB TECHNOLOGY

The UASB was developed in The Netherlands, in the seventies (Lettinga et al., 1980). Many UASB systems have been put into practice since, firstly mainly for the treatment of (agro-) industrial wastewaters but nowadays also more and more for the treatment of sewage at tropical conditions. For treatment of sewage at low and moderate temperature conditions no full scale application is so far realised, but results of laboratory and pilot scale research open up new perspectives (Wang, 1994, Elmitwally et al., 1998). Biomass retention, accomplished by the formation of good settling sludge aggregates and the application of an internal gas/sludge/liquid separation system, is an important characteristic of the UASB system, with the result that long SRT’s can be achieved at very short HRT’s.

MODEL FOR CALCULATION OF THE HRT FROM THE REQUIRED SRT

The SRT is determined by the amount of sludge that can be retained in the reactor and the daily excess sludge production. The daily excess sludge production is determined by the biomass yield and the removal and conversion of suspended solids. At a certain temperature the SRT will determine whether methanogenesis will occur or not. So when the required SRT is known, the corresponding HRT can be calculated providing that the sludge concentration in the reactor (X), the fraction of the influent SS that is removed (R) and the fraction of the removed SS that is hydrolysed (H) is known.

Role of anaerobic digestion of domestic sewage

CONCENTRATION AND COMPOSITION OF THE SEWAGE

189

At decentralised treatment of domestic sewage both the total COD concentration as well as the fraction ofs~spended solids is expected to increase in comparison with central treatment, as a result of the absence ofdiluted wll;Stewater and/or rainwater, and the shorter retention in the sewer. Table I, illustrates the expected~ncentratIo~ of th~ different COD fractions of on-site produced sewage. The table distinguishes between 4different options. VIZ. blackwater produced at conventional toilet flushing or at low toilet flushing and grey +blac~ water with 'normal' or low water use. The presented figures are based on Bogte et ai. (1993) andLettmga et ai. (1993) and show for each type of sewage high concentrations ofCODss (>4.4.~m), viz. between 65­72%. Additionally 15-17% colloidal (>0,45Ilm, <4.44llm) COD is to be expected.

Table 1. Expected concentrations ofgrey+black- and black-water as produced on-site, without the collection ofrainwater (based on Lettinga et ai., 1993 and Bogte et ai., 1993).

wastewater Total Suspended Colloidal DissolvedCOD COD COD COD(gil) (gil) (gil) (gil) Comments

grey + 0.98 0.64 n.m. n.m. The Netherlands, totalblack (0.13) sewage, without

100% rainwater(Bogte et ai., 1993)

black 1.72 1.20 n.m. n.m. The Netherlands,(0.26) blackwater100% with flushing toilets

(Bogte et ai., 1993)black 5.99 3.73 1.0 1.26 Indonesia, black water

(1.5) with pour-flush toilets100% (Lettinga et al., 1993)

grey + 1.85 1.33 0.27 0.24 Indonesia, total sewageblack (0.95) without rainwater, low

100% water use.Standard deviation is given in brackets

APPLICATION OF UASB TECHNOLOGY

The UASB was developed in The Netherlands, in the seventies (Lettinga et ai., 1980). Many UASB systemshave been put into practice since, firstly mainly for the treatment of (agro-) industrial wastewaters butnowadays also more and more for the treatment of sewage at tropical conditions. For treatment of sewage atlow and moderate temperature conditions no full scale application is so far realised, but results of laboratoryand pilot scale research open up new perspectives (Wang, 1994, Elmitwally et ai., 1998). Biomass retention,accomplished by the formation of good settling sludge aggregates and the application of an internalgas/sludge/liquid separation system, is an important characteristic of the UASB system, with the result thatlong SRT's can be achieved at very short HRT's.

MODEL FOR CALCULATION OF THE HRT FROM THE REQUIRED SRT

The SRT is determined by the amount of sludge that can be retained in the reactor and the daily excesssludge production. The daily excess sludge production is determined by the biomass yield and the removaland conversion of suspended solids. At a certain temperature the SRT will determine whethermethanogenesis will occur or not. So when the required SRT is known, the corresponding HRT can becalculated providing that the sludge concentration in the reactor (X), the fraction of the influent SS that isremoved (R) and the fraction of the removed SS that is hydrolysed (H) is known.

190 G. ZEEMAN and G. LETTINGA

The HRT can be calculated with the following formulas.

SRT= X/ Xp X=sludge concentration in the reactor (g COD/l); 1 g VSS= 1.4 g COD; Xp= sludge production (gCOD/l.d)

Xp=O*SS*R* (1-H) O=organic loading rate (kg C0DW.d); SS= CODss/ CODinfhtent;

HRT= C/ 0 (days) C= COD concentration in the influent (gCOD/I)

(1)

(2)

(3)

* HRT= (C*SS/X)*R*(l-H)*SRT; SRT=sludge retention time (days); R=fraction of the CODss removed; H=fraction of removed solids which is hydrolysed. No distinction is made between the fraction of CODss that is removed but not hydrolysed and the biomass yield.

(4)

Table 2. HRT* to be applied, to achieve the indicated SRT assuming 50% or 75% CODss removal at the treatment of domestic wastewater with a concentration of 1 g COD/l of which 65% is suspended, at different % hydrolysis and a sludge concentration in the reactor of 15 g VSS/l

% % hydrolysis of SRT=25 days SRT=50 days SRT=75 CODss removed SS &YS removal

50% 25% 0.28 0.56 0.84 75% 25% 0.42 0.84 1.26 50% 50% 0.19 0.38 0.57 75% 50% 0.28 0.56 0.84 50% 75% 0.09 0.18 0.27 75% 75% 0.14 0.28 0.42

* calculated with formula (4 )

SRT=lOO SRT=lSO days &YS

1.12 1.68 1.68 2.52 0.76 1.14 1.12 1.68 0.36 0.54 0.56 0.84

ONE OR TWO PHASE UASB-SYSTEMS

The choice between a one and a two phase system (high loaded LJASB with mainly removal of suspended solids followed by a methanogenic UASB) is in essence based on the required SRT and therefore on the ambient temperature and the fluctuation in temperature over the year and moreover the concentration, removal and hydrolysis of suspended solids. Since the late eighties full scale UASB-installations have been put in operation for domestic sewage treatment in a number of tropical countries (Schellinkhout ef al., 1985; Vieira and Souza, 1986; Letting? and Hulshof Poll , 1991, Lettinga et al., 1993; Draaijer et al., 1994; Schellinkhout and Osorio, 1994; Lettinga, 1996). The experiences with these conventional UASB-plants for sewage treatment are quite satisfactory, and as a result the method gradually is becoming increasingly popular. At the application of one step UASB systems at low temperature climates (5-20°C) however the hydrolysis of entrapped COD becomes limited in winter resulting in a decrease of the methanogenic activity of the sludge and therefore deterioration of the treatment process, unless long HRT’s are applied (de Man, 1990). For temperatures below 15”C, a SRT >I 00 days is necessary to retain sufficient methanogenic activity in the reactor. Zeeman (1991) shows that at 15°C methanogenesis can be achieved at digestion of manure at a SRT of 100 days, while it could not at 50 days. O’Rourke (1968) showed that no methanogenesis was provided at the digestion of sewage sludge at a SRT of 60 days. Even at this long SRT no complete hydrolysis is to be expected (O’Rourke, 1968; Zeeman, 1991). Zeeman (1991) shows that the % hydrolysis at 125 days batch digestion of cow mamtre is 12, 14, 18,27 and 45% at process temperatures of respectively 5, 10, 15,25 and 3O“C.

Table 2 illustrates that, when wastewater with a concentration of 1 g COD/l of which 65% is suspended, is treated in a UASB and 50% of the CODSS is removed of which half is hydrolysed, an HRT 14 and 20 hours is needed in order to achieve a SRT of respectively 100 and 150 days. The table shows that when only 25% of the retained SS is hydrolysed, a HRT of 40 hours is necessary to achieve a SRT of 150 days. At a removal efficiency of 75% of the CODss, the HRT to be applied will increase up to 61 hours. For the more concentrated wastewater’s even higher HRT’s are needed in order to achieve the same SRT, assuming that

190 G. ZEEMAN and G. LETTINGA

The HRT can be calculated with the following formulas.

SRT=x/XpX=sludge concentration in the reactor (g COD/I); I g YSS= 1.4 g COD; Xp= sludge production (gCOD/l.d)

Xp=O*SS*R* (I-H)O=organic loading rate (kg COD/m3 .d); SS= CODss/ CODinfluent;

HRT= C/ 0 (days)C= COD concentration in the influent (gCOD/I)

* HRT= (C*SS/X)*R*(l-H)*SRT;SRT=sludge retention time (days); R=fraction of the CODss removed; H=fraction ofremoved solids which is hydrolysed.No distinction is made between the fraction of CODss that is removed but not hydrolysed and the biomass yield.

(1)

(2)

(3)

(4)

Table 2. HRT* to be applied, to achieve the indicated SRT assuming 50% or 75% CODss removal at thetreatment of domestic wastewater with a concentration of I g CODn of which 65% is suspended, at different% hydrolysis and a sludge concentration in the reactor of 15 g VSS/l

% % hydrolysis of SRT~25 days SRT~50days SRT~75 SRT=IOO SRT=150CODss removed SS days days daysremoval50% 25% 0.28 0.56 0.84 1.12 1.6875% 25% 0.42 0.84 1.26 1.68 2.5250% 50% 0.19 0.38 0.57 0.76 1.1475% 50% 0.28 0.56 0.84 1.12 1.6850% 75% 0.09 0.18 0.27 0.36 0.5475% 75% 0.14 0.28 0.42 0.56 0.84* calculated with formula (4)

ONE OR TWO PHASE UASB-SYSTEMS

The choice between a one and a two phase system (high loaded UASB with mainly removal of suspendedsolids followed by a methanogenic UASB) is in essence based on the required SRT and therefore on theambient temperature and the fluctuation in temperature over the year and moreover the concentration,removal and hydrolysis of suspended solids. Since the late eighties full scale UASB-installations have beenput in operation for domestic sewage treatment in a number of tropical countries (Schellinkhout et al., 1985;Vieira and Souza, 1986; Lertinga and Hulshof Poll, 1991, Lertinga et aI., 1993; Draaijer et ai., 1994;Schellinkhout and Osorio, 1994; Lertinga, 1996). The experiences with these conventional UASB-plants forsewage treatment are quite satisfactory, and as a result the method gradually is becoming increasinglypopular. At the application of one step UASB systems at low temperature climates (5-20°C) however thehydrolysis of entrapped COD becomes limited in winter resulting in a decrease of the methanogenic activityof the sludge and therefore deterioration of the treatment process, unless long HRT's are applied (de Man,1990). For temperatures below 15°C, a SRT >100 days is necessary to retain sufficient methanogenic activityin the reactor. Zeeman (1991) shows that at 15°C, methanogenesis can be achieved at digestion of manure at aSRT of 100 days, while it could not at 50 days. O'Rourke (1968) showed that no methanogenesis wasprovided at the digestion of sewage sludge at a SRT of60 days. Even at this long SRT no complete hydrolysisis to be expected (O'Rourke, 1968; Zeeman, 1991). Zeeman (1991) shows that the % hydrolysis at 125 daysbatch digestion of cow manure is 12, 14, 18,27 and 45% at process temperatures of respectively 5, 10, 15,25and 30°C.

Table 2 illustrates that, when wastewater with a concentration of I g CODII of which 65% is suspended, istreated in a UASB and 50% ofthe COD" is removed of which half is hydrolysed, an HRT 14 and 20 hours isneeded in order to achieve a SRT of respectively 100 and 150 days. The table shows that when only 25% ofthe retained SS is hydrolysed, a HRT of 40 hours is necessary to achieve a SRT of 150 days. At a removalefficiency of 75% of the CODss, the HRT to be applied will increase up to 61 hours. For the moreconcentrated wastewater's even higher HRT's are needed in order to achieve the same SRT, assuming that

Role of anaerobic digestion of domestic sewage 191

similar removal and conversion efficiencies are achieved. Wang (1994) developed a two stage DASR process for the anaerobic treatment of domestic sewage at low temperature conditions. The first reactor is a high loaded UASR system, where entrapment and hydrolysis of suspended COD occurs. The second stage is a methanogenic reactor where mainly dissolved COD is converted to methane gas. These two step systems are eSPeCiallY appropriate for the treatment of wastewater with a high concentration of suspended solids at low temperatures (Zeeman et al., 1997). A disadvantage of this two-step system is the development of a non or less stabilised sludge in the first step of the system. The sludge can however be further stabilised in a sludge digester, possibly together with VF(Y)-waste as also proposed by Otterpohl (1997). Considering the long HRT’s, it is obvious that a two step system can be profitable for the treatment of sewage at low temperature conditions, as much lower total HRT’s can be applied. Moreover, Wang (1994) indicates that the application of a two step system will increase the removal of SS in the first step as a result of the limited gas production. At a total HRT of 5 hours (3+2) removal efficiencies for CODt, CODSS, CODCOI and CoDdis were respectively 5 l-69,67-79,24-40 and 41-5 1% at temperatures of 12-17’C. Ehnitwally et al. (1998) shows that the removal efficiencies, especially for colloids could even be improved by the application of a two phase hybrid reactor.

For sewage varying in temperature in the range of 15-25’C, it still has to be proven whether a two step system is better as compared to a one step system. For example in the Middle East, the summer sewage temperature is indeed high, cu. 25”C, but in winter the temperature can go down to cu. 15°C. When a one step reactor is designed on the lowest temperature, a minimum SRT of 75 days is required, and therefore a HRT of at least 19 hours and 27 hours, considering a CODss removal efficiency of respectively 50 and 75% at 50% hydrolysis. With a two phase system a much lower total HRT, can be applied. In Jordan, the first two-step UASB system for the treatment of domestic sewage is installed on a pilot scale of 100 m3. Results of laboratory research with the first step of a two step UASB reactor of 85 litres, treating sewage of the village of Bemrekom, concentrated by addition of primary sludge, at a HRT of 4 hours and temperatures of 15 and 25°C are shown in Figure 2 (Miron, 1997). The results show an increase in removal efficiency of CODSS with increasing loading rate. The loading rate is increased by increasing the influent concentration from 0.7 g COD/l to 2.8. g COD/l with a CCDu of 53 to 86%. High COD removal efficiencies were achieved, but as a result of the short SRT ((c 8 days for 15OC and (~4 days for 25Y) hardly any hydrolysis and methanogenesis was occurring. At the Operation of a two phase system the sludge produced in the first step will be poorly stabilised, especially at low temperature conditions. Miron (1997) shows that ca. 40 % of the retained CODa can be hydrolysed at an SRT of 15 days at 25OC, but at decreasing the temperature to 15°C at the same SRT the hydrolysis decreases towards cu. 5 %. Considering the characteristics of the domestic wastewater as proposed in Table 2 and 50% SS removal and 40% hydrolysis, a HRT of 4 hours is sufficient to achieve a SRT of 15 days at 25’C. Miron (1997) found lower SRT’s as a result of the lower amount of sludge retained in the reactor (ca. log COD/l).

1

* 45 y = 1.9704x y = 1.6665X

+ 54.61 40 + 49.406

0 2 4 6 6 10 12 1416 16 Rz = 0.9959

0 2 4 6 6 10 12 14 16 Rz = 0.6797

Loading [gCOWUdl Loading [gCCWdl

--

Figure 2, Removal efficiencies for CODss as a function of the loading rate at the treatment of concentrated sewage in the fust step of a two step UASB system at 4 hours HRT and 15 and 25’C.

Role of anaerobic digestion of domestic sewage 191

similar removal and conversion efficiencies are achieved. Wang (1994) developed a two stage UASB processfor the anaerobic treatment of domestic sewage at low temperature conditions. The first reactor is a highloaded UASH system, where entrapment and hydrolysis of suspended COD occurs. The second stage is amethanogenic reactor where mainly dissolved COD is converted to methane gas. These two step systems areespecially appropriate for the treatment of wastewater with a high concentration of suspended solids at lowtemperatures (Zeeman et al., 1997). A disadvantage of this two-step system is the development of a non orless stabilised sludge in the first step of the system. The sludge can however be further stabilised in a sludgedigester, possibly together with VF(Y)-waste as also proposed by Otterpohl (1997). Considering the longHRT's, it is obvious that a two step system can be profitable for the treatment of sewage at low temperatureconditions, as much lower total HRT's can be applied. Moreover, Wang (1994) indicates that the applicationof a two step system will increase the removal of SS in the first step as a result of the limited gas production.At a total HRT of 5 hours (3+2) removal efficiencies for CODt. CODss, CODco) and CODdis were respectively51-69, 67-79, 24-40 and 41-51% at temperatures of 12-17°C. Elmitwallyet aI. (1998) shows that the removalefficiencies, especially for colloids could even be improved by the application of a two phase hybrid reactor.

For sewage varying in temperature in the range of 15-25°C, it still has to be proven whether a two step systemis better as compared to a one step system. For example in the Middle East, the summer sewage temperature isindeed high, ca. 25°C, but in winter the temperature can go down to ca. 15°C. When a one step reactor isdesigned on the lowest temperature, a minimum SRT of75 days is required, and therefore aHRT of at least 19hours and 27 hours, considering a COD" removal efficiency of respectively 50 and 75% at 50% hydrolysis.With a two phase system a much lower total HRT, can be applied. In Jordan, the first two-step UASB systemfor the treatment ofdomestic sewage is installed on a pilot scale of 100 m3• Results of laboratory research withthe first step ofa two step UASB reactor of85litres, treating sewage of the village ofBennekom, concentratedby addition of primary sludge, at a HRT of 4 hours and temperatures of 15 and 25°C are shown in Figure 2(Miron, 1997). The results show an increase in removal efficiency of CODss with increasing loading rate. Theloading rate is increased by increasing the influent concentration from 0.7 g CODII to 2.8. g CODII with aCOD" of 53 to 86%. High COD removal efficiencies were achieved, but as a result of the short SRT (<< 8 daysfor 15°C and «4 days for 25°C) hardly any hydrolysis and methanogenesis was occurring. At the operation of atwo phase system the sludge produced in the first step will be poorly stabilised, especially at low temperatureconditions. Miron (1997) shows that ca. 40 % of the retained CODsscan be hydrolysed at an SRT of 15 days at25°C, but at decreasing the temperature to 15°C at the same SRT the hydrolysis decreases towards ca. 5 %.Considering the characteristics of the domestic wastewater as proposed in Table 2 and 50% SS removal and40% hydrolysis, a HRT of 4 hours is sufficient to achieve a SRT of 15 days at 25°C. Miron (1997) foundlower SRT's as a result of the lower amount of sludge retained in the reactor (ca. 109 COD/I).

IIT~

./11 ;"II

J--

_ : f- ~ t-+-+-+-+=+--T+'~H

I 1Et~----1-I-_-+:T--+-,,--1.o"'ll""''----t-';'-,+-+l-----II j ;~ \--Ll-~

.,. 50 i --1--+--+-+-+---++-----1

:~ Tti-=-tj-=t~+:::+___4o 2 4 6 8 10 12 1416 18

Loading [gCotVUdl

• UASR15

__Linear

(UASR15)

y = 1.6665x+54.61

R' =0.9959

6580

iii 757065

! 60

8 :.,. 45

40o 2 4 6 8 10 12 14 16

Lolldlng [gcotVUdl

-11-. UASR25

1,__ Linearl (UASR25)

Y = 1.9704x+49.406

R' =0.6797

___~ __L _

Figure 2, Removal efficiencies for CODss as a function of the loading rate at the treatment of concentrated sewagein the flfSt step of a two step UASB system at 4 hours HRT and 15 and 25°C.

192 G. ZEEMAN and G. LETTINGA

UASB-SEPTIC TANK SYSTEMS

For house-on-site treatment of domestic wastewater the UASB-septic-tank system is a promising alternative

for the conventional septic-tank (Bogte et al., 1993; Lettinga et al, 1993). The most important difference with the traditional UASB system is that the UASB-septic-tank system is also designed for the accumulation and stabilisation of sludge. It differs from the conventional septic tank system by the upflow mode in which

the system is operated, resulting in both improved physical removal of suspended solids and improved biological conversion. So an UASB-septic-tank system is a continuous system with respect to the liquid, but a fed-batch or accumulation system, with respect to the solids. Bogte et al. (1993) and Lettinga et al. (1993) researched the use of an UASB-septic system for the on-site treatment of ‘black’ wastewater and total domestic sewage (black + grey) at Dutch and Indonesian ambient conditions. At high temperature conditions very high removal efficiencies could be achieved. Below 12% (Dutch winter conditions) the conversion of produced VFA to methane gas was too low, although the research period was too short to draw definite conclusions. For low temperature conditions the application of a two step UASB-septic-tank system could be profitable. The first reactor will in winter mainly retain solids, while just a limited amount of hydrolysis, acidification and methanogenesis will occur. In the second reactor mainly methanogenesis will occur at the low temperature conditions. In summer hydrolysis and acidification of both fresh and accumulated solids will take place in the first reactor together with methanogenesis, while the second reactor acts as a polishing step for removing and converting remaining VFA and suspended COD, washed from the first reactor as a result of the increased gas production. Expected removal efficiencies at treatment of black water and grey + black water at low and tropical temperature conditions are shown in Table 3.

Table 3, Removal efftciencies (%) for CODt and CODss at the treatment of ‘black’ and ‘grey + black’ wastewater in the first step of a two-step UASB-septic-tank system (based on Bogte et al., 1993; Lettinga et al, 1993).

removal temperature black water grey + black water efficiency (“C) W) CODt 5-20°C * 54-52 * 58 CODt >20°c **go-93 **67-77 CODss 5-20” * 71-86 * 62 TSS >2O”C **93-97 **74-81

* normal’ water use * * low water use

NIGHT SOIL, SLUDGE AND VFY DIGESTION

On site treatment offers the possibility to combine the digestion of VFY with that of the produced sludge and reuse the digested fraction as a fertiliser and soil conditioner in agriculture. When ‘night soil’ is produced instead of black water it directly can be digested together with VFY. Two possible systems are considered, viz. the CSTR and the fed-batch or accumulation system (AC). The CSTR is the most generally applied system for sludge digestion mainly at mesophilic conditions and retention times of 15-20 days. Table 4 indicates the digestion volumes necessary to treat the wastewater in a two phase UASB and the reactor volume necessary for sludge digestion considering low temperature climates.

Results of VFY digestion at laboratory, pilot scale and full scale have been reported (ten Brummeler, 1993; Fruteau de Laclos et al., 1997). Digestion of the high concentrated VFY is to be sure possible but requires a more complex reactor design or handling of the substrate as compared to the conventional CSTR type digesters for slurries. The combined treatment of VFY with sludge or night soil will benefit both, as VFY is diluted with the consequent improvement of handling and sludge is increased in concentration resulting in an improved use of the reactor volume. Co-digestion but for different substrates, viz. manure and industrial wastes, is reported to increase the volumetric gas production (m)/m’.day) as compared to that at manure digestion (Ahring et al., 1992)

192 G. ZEEMAN and G. LETTINGA

UASB-SEPTIC TANK SYSTEMS

For house-on-site treatment of domestic wastewater the UASB-septic-tank system i~ a promising alternativefor the conventional septic-tank (Bogte et al., 1993; Lettinga et ai, 1993). The most important differencewith the traditional UASB system is that the UASB-septic-tank system is also designed for the accumulationand stabilisation of sludge. It differs from the conventional septic tank system by the upflow mode in whichthe system is operated, resulting in both improved physical removal of suspended solids and improvedbiological conversion. So an UASB-septic-tank system is a continuous system with respect to the liquid, but afed-batch or accumulation system, with respect to the solids. Bogte et al. (1993) and Lettinga et al. (1993)researched the use of an UASB-septic system for the on-site treatment of 'black' wastewater and total domesticsewage (black + grey) at Dutch and Indonesian ambient conditions. At high temperature conditions very highremoval efficiencies could be achieved. Below 12°C (Dutch winter conditions) the conversion of producedVFA to methane gas was too low, although the research period was too short to draw definite conclusions. Forlow temperature conditions the application of a two step UASB-septic-tank system could be profitable. Thefirst reactor will in winter mainly retain solids, while just a limited amount of hydrolysis, acidification andmethanogenesis will occur. In the second reactor mainly methanogenesis will occur at the low temperatureconditions. In summer hydrolysis and acidification of both fresh and accumulated solids will take place in thefirst reactor together with methanogenesis, while the second reactor acts as a polishing step for removing andconverting remaining VFA and suspended COD, washed from the first reactor as a result of the increased gasproduction. Expected removal efficiencies at treatment of black water and grey + black water at low andtropical temperature conditions are shown in Table 3.

Table 3, Removal efficiencies (%) for CODt and CODss at the treatment of 'black' and 'grey + black'wastewater in the first step of a two-step UASB-septic-tank system (based on Bogte et al., 1993; Lettinga et ai,1993).

CODt 5-20°CCODt >20°CCODss 5-20°TSS >20°C

removalefficiency(%)

temperature(0C)

black water

* 54-52**90-93* 71-86**93-97

grey + black water

* 58**67-77* 62**74-81

*normal' water use** low water use

NIGHT SOIL, SLUDGE AND VFY DIGESTION

On site treatment offers the possibility to combine the digestion of VFY with that of the produced sludgeand reuse the digested fraction as a fertiliser and soil conditioner in agriculture. When 'night soil' isproduced instead of black water it directly can be digested together with VFY. Two possible systems areconsidered, viz. the CSTR and the fed-batch or accumulation system (AC). The CSTR is the most generallyapplied system for sludge digestion mainly at mesophilic conditions and retention times of 15-20 days.Table 4 indicates the digestion volumes necessary to treat the wastewater in a two phase UASB and thereactor volume necessary for sludge digestion considering low temperature climates.

Results ofVFY digestion at laboratory, pilot scale and full scale have been reported (ten Brumme1er, 1993;Fruteau de Laclos et al., 1997). Digestion of the high concentrated VFY is to be sure possible but requires amore complex reactor design or handling of the substrate as compared to the conventional CSTR typedigesters for slurries. The combined treatment of VFY with sludge or night soil will benefit both, as VFY isdiluted with the consequent improvement of handling and sludge is increased in concentration resulting inan improved use of the reactor volume. Co-digestion but for different substrates, viz. manure and industrialwastes, is reported to increase the volumetric gas production (m3/m3.day) as compared to that at manuredigestion (Ahring et al., 1992)

Role of anaerobic digestion of domestic sewage 193

In low temperature climates, like the Netherlands, it is forbidden to apply fertilisers on the field during the winter period. The latter means that a storage of at least 5 months is necessary. Combined storage and digestion in a fed-batch or accumulation system (AC), is then a feasible alternative (Zeeman, 1991) for a CSTR system.

Table 4. Calculated reactor volumes to be installed (two-phase UASB and sludge CSTR-digester) when applying total sewage, black water or night soil digestion at low temperature conditions

Concentration volume CODss CODss volume two-phase produced

Volume #sludge (%I accumulation UASB (l/p.e.) /night soil CSTR

per p.e. per (%*) HRT 0.33days digester (l/p.e) day (litres) HRT Isdays

my 1 .O g COD/l 125 65 75 41 30 water black 1.7 g COD/l 29 65 75 9.6 15 water nnight 33 g COD/l 1.5 n.a.t.. n.a.t. n.a.t. 22.5 soil mat. = not applicable to; * % of the CODssinfluent: #sludge concentration of 3Og COD/I; CI night soilproduction : 5Og COD/p.e./day (IBA richtlijn) in 1.5 litres (Bell et al., 1968); p.e.= population equivalent

FINAL DISCUSSION AND CONCLUSIONS

The choice for a one or two phase UASB system mainly depends on the temperature, temperature fluctuations and concentration of the sewage. For treatment of total sewage at low temperature conditions a two phase system is surely profitable in comparison with a one-step-system, at high temperatures a one-step- system is more attractive. When temperatures fluctuate between 15-25’C a two-step UASB system also seems an attractive option, certainly when sewage with a high concentration is to be treated. Both laboratory and pilot-scale research have to prove the feasibility of the system. When the more concentrated black water is to be treated, a two- phase system can, also under higher temperature conditions, be an attractive alternative. When a sludge digester is to be installed also the profits of adding VFY, and recovering the methane, have to be considered. When night soil can be produced instead of black water or combined sewage, for example by the introduction of vacuum toilets, a CSTR-system can be used for digestion resulting in a smaller total reactor volume to be installed. The application of an AC-system is especially interesting at low temperature climates where slurries can only be applied on the field during the summer period, so that a large storage capacity is necessary anyway (Zeeman, 1991).

The choice between a UASB or UASB-septic-tank system will mainly be based on the scale. A UASB- septic-tank system needs less attention and is therefore an attractive alternative for the conventional septic- tank system for house on-site application.

Anaerobic effluent still contains most of the nutrients, present in the influent. Closing the water and nutrient cycle can be achieved by the use of these effluents in for example agriculture, providing that an appropriate post-treatment system for the removal of pathogens is included. Van Buuren (1998) illustrates the possibilities of using a (sand) filtration system. On separate collection and treatment of black water or night soil, the produced grey water could also be treated by (sand) filtration systems and f.e. used as irrigation water or reused in the household itself.

LITERATURE

Ahring, B. K., Angelidaki, I. and Johansen, K. (1992). Anaerobic treatment of manure together with organic industrial waste. Wet. Sci. Tech., Z(7), 311-318

Bell, G. H., Davidson, J. N. and Scarborough, H. (1968). Text Book of Physiology and Biochemistry, 7th edition. Livingstone Limited.

Bergen, V. W. J. v.d. (1997). Challenges in urban water management. Proceedings of the Third Japan-Netherlands Workshop on Integrated Water Quality Management.

Bogte, J. J., Breure, A. M., Andel J. G. v. and Lettinga, G. (1993). Anaerobic treatment of domestic wastewater in small scale UASB reactors. Wat. Sci. Tech., 27(9), 75-82.

Role of anaerobic digestion of domestic sewage 193

In. low te~perature climates, like the Netherlands, it is forbidden to apply fertilisers on the field during the~Inte~ pe~od. The latter means that a storage of at least 5 months is necessary. Combined storage anddIgestion In a fed-batch or accumulation system (AC), is then a feasible alternative (Zeeman, 1991) for aCSTR system.

Table. 4. Calculated reactor volumes to be installed (two-phase UASB and sludge CSTR-digester) whenapplymg total sewage, black water or night soil digestion at low temperature conditions

1.0 gCOD/1

1.7 g CODII

Concentration

greywaterblackwatercnight 33 g CODIIsoil

volumeproducedperp.e. perday (litres)125

29

1.5

CODss(%)

65

65

n.a.t..

CODssaccumulation(%*)

75

75

n.a.t.

volume two-phaseUASB (lIp.e.)HRT O. 33days

41

9.6

n.a.t.

Volume #sludge/night soil CSTRdigester (l/p.e)HRT 15days30

15

22.5

n.a.t. not applicable to; *% ofthe CODssinfluent; #sludge concentration ofJOg CODII; c night soil production: 50gCOD/p.e./day ( IBA richtlijn) in 1.5 Ii/res (Bell et al.. 1968); p.e. ~ population equivalent

FINAL DISCUSSION AND CONCLUSIONS

The choice for a one or two phase UASB system mainly depends on the temperature, temperaturefluctuations and concentration of the sewage. For treatment of total sewage at low temperature conditions atwo phase system is surely profitable in comparison with a one-step-system, at high temperatures a one-step­system is more attractive. When temperatures fluctuate between 15-25°C a two-step UASB system alsoseems an attractive option, certainly when sewage with a high concentration is to be treated. Both laboratoryand pilot-scale research have to prove the feasibility of the system. When the more concentrated black wateris to be treated, a two- phase system can, also under higher temperature conditions, be an attractivealternative. When a sludge digester is to be installed also the profits of adding VFY, and recovering themethane, have to be considered. When night soil can be produced instead of black water or combinedsewage, for example by the introduction of vacuum toilets, a CSTR-system can be used for digestionresulting in a smaller total reactor volume to be installed. The application of an AC-system is especiallyinteresting at low temperature climates where slurries can only be applied on the field during the summerperiod, so that a large storage capacity is necessary anyway (Zeeman, 1991).

The choice between a UASB or UASB-septic-tank system will mainly be based on the scale. A UASB­septic-tank system needs less attention and is therefore an attractive alternative for the conventional septic­tank system for house on-site application.

Anaerobic effluent still contains most of the nutrients, present in the influent. Closing the water and nutrientcycle can be achieved by the use of these effluents in for example agriculture, providing that an appropriatepost-treatment system for the removal of pathogens is included. Van Buuren (1998) illustrates thepossibilities of using a (sand) filtration system. On separate collection and treatment of black water or nightsoil, the produced grey water could also be treated by (sand) filtration systems and f.e. used as irrigationwater or reused in the household itself.

LITERATURE

Ahring, B. K., Angelidaki, I. and Johansen, K. (1992). Anaerobic treatment of manure together with organic industrial waste. Wat.Sci. Tech., 25(7), 311-318

Bell, G. H., Davidson, J. N. and Scarborough, H. (1968). Text Book of Physiology and Biochemistry, 7th edition. LivingstoneLimited.

Bergen, V. W. J. v.d. (1997). Challenges in urban water management. Proceedings of the Third Japan.Netherlands Workshop onIntegrated Water Quality Management.

Bogte, J. 1., Breure, A. M., Andel J. G. v. and Lettinga, G. (1993). Anaerobic treatment of domestic wastewater in small scaleUASB reactors. Wat. Sci. Tech., 27(9), 75-82.

194 G. ZEEhbUi and G. LElTINGA

Bmmmeler, E. ten (1993). Dry anaerobic digestion of the organic fraction of municipal solid waste. POD thesis, Agricultural University, Wageningen, The Netherlands.

Buuren, J. C. L., Abusam, A., Zeeman, G. and Lettinga, G. (1999). Primary effluent filtration in small-scale installations. war. sci, Tech., 39(S), (this issue).

Draaijer H., Pereboom, J. H. F. and Sontakke, V. N. (1994). Four years experience with the s MLDJJASB reactor for sewage treatment at Kanpur, India. Paper reprints Seventh Int. Symp. on Anaerobic Digestion, January 23-37, Cape Tom, South Africa.

Elmitwally A. T., Zandvoort, M. H., Zeeman, G., Bnming, H. and Lettinga, G. (1998). Low temperature treatment of domestic sewage in upflow anaerobic sludge blanket and anaerobic hybrid reactors. War. Sci. Tech., 39(5) (this issue).

Fmteau de Laclos, H., Desbois, S. and Saint-Joly: Anaerobic digestion of municipal solid organic waste: Valorga full-scale plant in Tilburg, The Netherlands. War. Sci. Tech., 36(6-7), 457-463.

IBA guideline (1991). Individual treatment of wastewater for scattered Buildings; Research Phase 4B: IBA g~i&$ne (in Dutch), Den Haag, Ministerie van Volkshuisvesting, Ruimtelijk Ordening en Milieubeheer, publikatiereeks milieubeheer nr.1991/7

Lettinga, G., Velsen, A. F. M. v., Hobma, S. W., Zeeuw, W. J. de and Klapwijk, A. (1980). Use of the Upflow Sludge Blank& (USB) reactor concept for biological waste water treatment. Biotechnology ond Bioengineering, 22,699-734.

Lettinga, G. and Hulshoff Pol, L. W. (1991). UASB-process design for various types of wastewater. War. Sci. Tech., 24(8), 87-107.

Lettinga, G., Man, A. d, Last, A. R. M. v. d., Wiegant, W., Knippenberg, K., Frijns, J. and Buuren, J. C. L. v. (1993). Anaerobic treatment of domestic sewage and wastewater. War. Sci. Tech., 27(9), 67-73.

Lettinga, G. (1996). Treatment of raw sewage under tropical conditions. In Design of anaerobic processes for the heufmen? of indusbial and municipal wastes, J. F. Makina and F. G. Pohland (eds), Water Quality Management Library, Volume VII, 147-166. Tecbnomic Publishing Co., 1992.

de Man, A. W. A. (1990). Anaerobic treatment of raw domestic sewage with granular sludge UASB-systems (in Dutch). Department Environmental Technology, Agricultural University, Wageningen.

Miron, Y. (1997). Anaerobic treatment of concentrated sewage in a two step UASR-UASB system. The effect of SRT, sludge characteristics and upflow velocity on the performance of the UASR. MSc. thesis Agricultural University, Wageningen, The Netherlands.

Otterpohl, R., Grottker, M. and Lange, J. (1997). Sustainable water and waste management in urban areas. War. Sci. Tech., 35(9), 121-133.

G’Rourke, J. T. (1968). Kinetics of anaerobic treatment at reduced temperatures. Dissertation Stanford University, Department of Civil Engineering.

Schellinkhout, A., Lettinga, G., Velsen, A. F. M. van, Louwe Kooijmans, J. and Rodriguez, G. (1985). The application of the UASB-reactor for the direct anaerobic treatment of domestic waste water under tropical conditions. In: Proc. ofseminar Anaerobic treatment of Sewage, M. S. Switzenbaum (ed), June 27-28, Amherst USA, pp. 259-276.

Schellinkhout, A. and Gsorio, E. (1994). Long-term experience with the UASB technology for sewage treatment on large scale. Paper reprints Seventh Int. Symp. on Anaerobic Digestion, January 23-37, Cape Town, South Africa.

Terpstra, P. M. J. (1996). Sustainable water using systems. Models for a sustainable use of domestic wastewater in an urban area (in Dutch). Capita lectures. Department Environmental Technology, Agricultural University, Wagenmgen.

Wang, K. (1994). Integrated anaerobic and aerobic treatment of sewage. PhD thesis, Department of Environmental Technology, Agricultural University, Wageningen.

Zeeman, G., Sanders, W. T. M., Wang, K. Y. and Lettinga, G. (1997). Anaerobic treatment of complex wastewater and waste activated sludge. Application of an Upflow Anaerobic Solid Removal (UASR) reactor for the removal and pre-hydrolysis of suspended COD. War. Sci. Tech., 35(10), 121-128.

Zeeman, G. (1991). Mesophilic and psychrophilic digestion of liquid manure. PhD thesis, Department of Environmental Technology, Agricultural University, Wageningen.

194 G. ZEEMAN and G. LETTINGA

Bl1lmmeler, E. ten (1993). Dry anaerobic digestion of the organic fraction of municipal solid waste. PhD thesis, AgriculturalUniversity, Wageningen, The Netherlands.

Buuren,1. C. L., Abusam, A., Zeeman, G. and Lettinga, G. (1999). Primary effluent filtration in small-scale installations. Wat. Sci.Tech., 39(5), (this issue).

Draaijer H., Pereboom, J. H. F. and Sontakke, V. N. (1994). Four years experience with the 5 MLD-UASB reactor for sewagetreatment at Kanpur, India. Paper reprints Seventh Int. Symp. on Anaerobic Digestion, January 23-37, Cape Town, SouthAfrica.

Elmitwally A. T., Zandvoort, M. H., Zeeman, G., Bruning, H. and Lettinga, G. (1998). Low temperature treatment of domesticsewage in upflow anaerobic sludge blanket and anaerobic hybrid reactors. Wat. Sci. Tech., 39(5) (this issue).

Fl1lteau de Laclos, H., Desbois, S. and Saint-Joly: Anaerobic digestion of municipal solid organic waste: Valorga full-scale plantin Tilburg, The Netherlands. Wat. Sci. Tech., 36(6-7), 457-463.

IBA guideline (1991). Individual treatment of wastewater for scattered Buildings; Research Phase 4B: IBA guideline (in Dutch).Den Haag, Ministerie van Volkshuisvesting, Ruimtelijk Ordening en Milieubeheer, publikatiereeks milieubeheernr.1991/7

Lettinga, G., Velsen, A. F. M. v., Hobma, S. W., Zeeuw, W. J. de and Klapwijk, A. (1980). Use of the Upflow Sludge Blanket(USB) reactor concept for biological waste water treatment. Biotechnology and Bioengineering, 22, 699-734.

Lettinga, G. and Hulshoff Pol, L. W. (1991). UASB-process design for various types of wastewater. Wat. Sci. Tech., 24(8),87-107.

Lettinga, G., Man, A. d, Last, A. R. M. v. d., Wiegant, W., Knippenberg, K., Frijns, J. and Buuren, J. C. L. v. (1993). Anaerobictreatment of domestic sewage and wastewater. Wat. Sci. Tech., 27(9), 67-73.

Lettinga, G. (1996). Treatment of raw sewage under tropical conditions. In Design of anaerobic processes for the treatment ofindustrial and municipal wastes, 1. F. Makina and F. G. Pohland (eds), Water Quality Management Library, Volume VII,147-166. Technomic Publishing Co., 1992.

de Man, A. W. A. (1990). Anaerobic treatment of raw domestic sewage with granular sludge UASB-systems (in Dutch).Department Environmental Technology, Agricultural University, Wageningen.

Miron, Y. (1997). Anaerobic treatment of concentrated sewage in a two step UASR-UASB system. The effect of SRT, sludgecharacteristics and upflow velocity on the performance of the UASR. MSc. thesis Agricultural University, Wageningen,The Netherlands.

Otterpohl, R., Grottker, M. and Lange, J. (1997). Sustainable water and waste management in urban areas. Wat. Sci. Tech., 35(9),121-133.

O'Rourke, J. T. (1968). Kinetics of anaerobic treatment at reduced temperatures. Dissertation Stanford University, Department ofCivil Engineering.

Schellinkhout, A., Lettinga, G., Velsen, A. F. M. van, Louwe Kooijmans, J. and Rodriguez, G. (1985). The application of theUASB-reactor for the direct anaerobic treatment of domestic waste water under tropical conditions. In: Proc. ofseminarAnaerobic treatment ofSewage, M. s. Switzenbaum (ed), June 27-28, Amherst USA, pp. 259-276.

Schellinkhout, A. and Osorio, E. (1994). Long-term experience with the UASB technology for sewage treatment on large scale.Paper reprints Seventh Int. Symp. on Anaerobic Digestion, January 23-37, Cape Town, South Africa.

Terpstra, P. M. 1. (1996). Sustainable water using systems. Models for a sustainable use of domestic wastewater in an urban area(in Dutch). Capita lectures. Department Environmental Technology, Agricultural University, Wageningen.

Wang, K. (1994). Integrated anaerobic and aerobic treatment of sewage. PhD thesis, Department of Environmental Technology,Agricultural University, Wageningen.

Zeeman, G., Sanders, W. T. M., Wang, K. Y. and Lettinga, G. (1997). Anaerobic treatment of complex wastewater and wasteactivated sludge. Application of an Upflow Anaerobic Solid Removal (UASR) reactor for the removal and pre-hydrolysisof suspended COD. Wat. Sci. Tech., 35(10),121-128.

Zeeman, G. (1991). Mesophilic and psychrophilic digestion of liquid manure. PhD thesis, Department of EnvironmentalTechnology, Agricultural University, Wageningen.