Effect of temperature on anaerobic treatment of black water in UASB-septic tank systems

Bioresource Technology 98 (2007) 980–986

EVect of temperature on anaerobic treatment of black water in UASB-septic tank systems Sari Luostarinen a

a,¤

, Wendy Sanders b, Katarzyna Kujawa-Roeleveld b, Grietje Zeeman

b

Department of Biological and Environmental Science, University of Jyväskylä, P.O. Box 35, FIN-40014 University of Jyväskylä, Finland b Sub-department of Environmental Technology, Wageningen University, P.O. Box 8129, 6700 EV Wageningen, The Netherlands Received 16 February 2006; received in revised form 16 March 2006; accepted 21 April 2006 Available online 12 June 2006

Abstract The eVect of northern European seasonal temperature changes and low temperature on the performance of upXow anaerobic sludge blanket (UASB)-septic tanks treating black water was studied. Three UASB-septic tanks were monitored with diVerent operational parameters and at diVerent temperatures. The results indicated the feasibility of the UASB-septic tank for (pre)treatment of black water at low temperatures with respect to removal of suspended solids and dissolved organic material. Inoculum sludge had little eVect on CODss removal, though in the start-up phase some poorly adapted inoculum disintegrated and washed out, thus requiring consideration when designing the process. Removal of CODdis was at Wrst negative, but improved as the sludge adapted to low temperature. The UASBseptic tank alone did not comply with Finnish or Dutch treatment requirements and should therefore be considered mainly as a pre-treatment method. However, measuring the requirements as mgCOD l¡1 may not always be the best method, as the volume of the eZuent discharged is also an important factor in the Wnal amount of COD entering the receiving water bodies. © 2006 Elsevier Ltd. All rights reserved. Keywords: Anaerobic wastewater treatment; Black water; Domestic wastewater; Low temperature; UASB-septic tank

1. Introduction Domestic wastewater can be divided into diVerent streams according to their origin. Generally two streams are distinguished: concentrated – black water from toilets (faeces, urine and Xushing water) and diluted – grey water from bath, wash and kitchen (Henze and Ledin, 2001). Most of the organic material, nutrients and pathogens in domestic wastewater are in black water (51% of COD, 91% of nitrogen, 78% of phosphorus; Terpstra, 1999), making its treatment of the greatest importance. Moreover, treatment of more concentrated wastewater decreases the reactor size needed, therefore reducing manufacturing costs and space requirements (Lettinga et al., 2001). Recovery and reuse of the eZuent may also be promoted if black water is sepa-

*

Corresponding author. Tel.: +358 14 260 1211; fax: +358 14 260 2321. E-mail address: [email protected] (S. Luostarinen).

0960-8524/$ - see front matter © 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2006.04.018

rated from the more diluted streams (Otterpohl, 2001) and house- or community-on-site treatment systems are used. On-site treatment systems are needed in rural areas due to special circumstances such as low population density, and especially in developing countries, due to their costeYciency and ease of use. Average characteristics of domestic wastewater, black water and grey water are presented in Table 1. Anaerobic wastewater treatment is considered sustainable (Lettinga, 1996; Hammes et al., 2000) and suitable for on-site treatment (Zeeman and Lettinga, 1999) due to its low energy consumption, small space requirement and relatively simple reactor design. In the northern European climate, wastewater temperatures change with the seasons: in the summer the temperature can increase up to 20 °C, but in the winter, temperatures as low as 4 °C are possible. Temperature is an important factor in anaerobic treatment of domestic wastewater: the higher the temperature, the higher the conversion rates.

S. Luostarinen et al. / Bioresource Technology 98 (2007) 980–986 Table 1 Average characteristics of domestic wastewater, black water and grey water from conventional Xush toilets (Henze and Ledin, 2001) Parameter (mg l¡1)

Domestic wastewater

Black water

Grey water

BOD COD Total N Total P

115–400 210–740 20–80 6–23

300–600 900–1500 100–300 40–90

100–400 200–700 8–30 2–7

981

seasonal temperatures was compared to the present results, obtained after 13 years of operation. In addition, results from a UASBST at a constant 15 °C inoculated with sludge from the above-mentioned system and from a UASBST at a constant 25 °C with no inoculation were used as comparisons. 2. Methods 2.1. Reactor

Slow hydrolysis and accumulation of suspended solids present in domestic wastewater may decrease the methanogenic activity of sludge at low temperatures, which in turn may deteriorate the process. Suspended solids can also cause formation of scum layers and sudden washout of sludge, if they are only accumulated and not stabilised within the reactor. Long hydraulic and sludge retention times (HRT, SRT) and relatively low organic loading rates (OLR) are therefore needed (Zeeman and Lettinga, 1999). The UASB-septic tank (UASBST) is a promising alternative for house-on-site treatment of domestic wastewater. Unlike the conventional septic tank (Polprasert et al., 1982; Philippi et al., 1999), it is used in an upXow mode, improving the contact between wastewater and sludge, and thus resulting in better physical removal of suspended solids and biological removal of dissolved compounds (Zeeman et al., 2001). As most of the organic material in the wastewater is already removed in the UASBST, possible post-treatment (nutrient and/or pathogen removal) becomes simpler and the operating life of the post-treatment system (e.g. sand Wlter) may be prolonged. Moreover, the aim of the UASBST is not only to accumulate and to stabilise the sludge within the reactor, but also to convert dissolved solids (Zeeman et al., 2001). UASBST have been studied previously in tropical climate conditions (Lettinga et al., 1993), but only a few results have been published for northern European climate conditions (Bogte et al., 1993; Luostarinen and Rintala, 2005). At low temperatures, biogas production in upXow reactors is low and does not provide enough mixing. Poor mixing can cause channelling of wastewater through the sludge bed, thus decreasing removal eYciency, and formation of gas pockets, which in turn may lead to incidental lifting of large sludge aggregates and pulse-like eruption of gas from these areas (Mahmoud et al., 2003). A suYcient upXow velocity is, therefore, needed to mix the reactor contents and to provide good contact between the wastewater and the sludge (van Lier et al., 1997). If upXow velocity cannot be increased, mechanical mixing may be needed. On the other hand, high biogas production as well as excessively high upXow velocity may lead to detachment of already captured solids (Mahmoud et al., 2003). In this study, the eVect of northern European seasonal temperature changes and low temperature on the performance of UASBST treating black water was studied. For this, the 1st year operation of a UASBST system operated at

Experiments were conducted at the Experimental Hall of the Sub-Department of Environmental Technology at Wageningen University, the Netherlands, using three UASBST treating black water (Figs. 1 and 2). The 1.2 m3 UASBST monitored in the 1st and 13th years of operation (Fig. 1) was located in an underground concrete cellar outside the Experimental Hall and was made of steel plate with internal structures made of PVC. The Xow to the system varied somewhat, since it received black water from 1 to 2 persons during the 1st year of operation and from 3–4 persons during the 13th year (one quantum per person per day including one portion of faeces and Wve portions of urine with six portions of Xush). The system was originally inoculated (1st year) with 100 l of granular methanogenic sludge from a paper mill and was nearly full of sludge at the beginning of the 13th year of operation. Black water from three conventional Xush toilets was chopped with a shredding pump before feeding to the reactor through an interceptor tank with a volume of 18 l. When the tank was full, 12 l of black water was pumped into the reactor. EZuent was collected into a tank from which it was pumped further to a local wastewater treatment plant. The second UASBST (volume 0.2 m3; Fig. 2) was inoculated with 80 l of sludge from the 1.2 m3 system. The 1st year operation of the 0.2 m3 system was performed at a constant temperature of 15 °C. The third UASBST was similar in construction but received no inoculation and was started-up at a constant temperature of 20 °C. The temperature was increased to 25 °C after 17 weeks from the start-up (henceforth the system will be referred to with constant 25 °C). The Xow to the two 0.2 m3 systems was exactly one quantum of black water per person per day, Xush volume of the vacuum toilet being approximately 1 l. The black water was Wrst collected into a 10 l equalisation tank, from which it was pumped with a shredding pump to a pressure release vessel on top and transported by gravity to the bottom of the system. 2.2. Analyses The performance of the 1.2 m3 UASBST was monitored for 52 weeks during the 1st year and for 13 weeks during the 13th year of operation, while the two 0.2 m3 UASBST were monitored for 51 (15 °C) and 47 (25 °C) weeks from the start-up. Grab samples of inXuent and eZuent were analysed for total COD, suspended solids COD and dis-

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S. Luostarinen et al. / Bioresource Technology 98 (2007) 980–986

gas influent

6

2

A

1

4

water B

5

3

effluent

C slud ge

wwtp

Fig. 1. The 1.2 m3 UASB-septic tank. 1: automatic sampler for inXuent; 2: interceptor tank; 3: taps (A, B, C); 4: thermometer; 5: eZuent bucket; 6: gas meter.

CODss as a fraction retained by the Wlter. Temperature of the 1.2 m3 UASBST was measured with a thermometer placed on the side of the reactor. CO2 was removed from the biogas with 3% NaOH and methane production was monitored with gas meters. In addition to regular analyses, the composition of the black water treated was controlled by asking people to mark their contributions to lists as they used the toilets. 3. Results

Fig. 2. The 0.2 m3 UASB-septic tank. VT: vacuum toilet; VP: vacuum pump; ET: equalisation tank; WP: shredding pump; GSL: gas–liquid separator.

solved COD (CODt, CODss, CODdis; Jirka and Carter, 1975), with CODdis deWned as a fraction that passed through a 0.45 m Wlter (Schleicher & Schuell ME25) and

The 1.2 m3 UASBST was started-up in January 1988. During the 1st year of operation, the temperature of the black water ranged from 5 to 17 °C. Overall results were presented in Bogte et al. (1993) but for the present study, the detailed results (Bogte et al., 1989) were used and divided into warm (wastewater temperature >14 °C) and cold (<14 °C) seasons (Table 2). The results from the present monitoring period in the 13th year of operation were compared to the 1st year, though they were only from the warm season (14–19 °C). Additional comparisons were made using results from two 0.2 m3 UASBST during their 1st year of operation at constant 15 and 25 °C.

Table 2 Temperature range, average hydraulic retention time and organic loading rate in the 1.2 m3 UASB-septic tank (UASBST) in the 1st and 13th year of operation and in two 0.2 m3 UASB-septic tanks at constant 15 and 25 °C Reactor

Study

Temp (°C)

HRT (d)

OLR (kgCOD m¡3 d¡1)

Inoculum

1.2 m3 UASBST 1.2 m3 UASBST 0.2 m3 UASBST 0.2 m3 UASBST

1st year 13th year 15 °C 25 °C

5–17 14–19 15 20–25

4.3 4.1 29 29

0.43 0.89 0.33 0.42

100 l of granular sludge from a paper mill – 80 l of sludge from the 1.2 m3 UASBST No inoculum

S. Luostarinen et al. / Bioresource Technology 98 (2007) 980–986

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Table 3 Comparison between the results from the 1st and the 13th year of 1.2 m3 UASB-septic tank (UASBST) operation and the two 0.2 m3 UASB-septic tanks at constant 15 and 25 °C 1.2 m3 UASBST

0.2 m3 UASBST

1st year 5–13 °C

CODt CODss CODdis

1st year 14–17 °C

13th year 14–19 °C

Inf

EV

R

EV

R

Inf

EV

R

Inf

EV

R

Inf

EV

R

1716 (261) 1201 (222) 515 (64)

602 (185) 204 (62) 465 (89)

65

1155 (332) 460 (151) 588 (124)

33

2897 (1199) 2428 (1220) 269 (107)

865 (417) 718 (388) 127 (33.5)

70

9503 (6460) 8070

3699 (366) 1621

61

12311 (7782) 10311

2733 (491) 2147

78

1433 (479)

2086 (449)

¡31

2001 (1209)

980 (495)

51

83 10

59 ¡24

15 °C

71 53

25 °C

80

79

The unit for COD values in inXuents (inf) and eZuents (eV) is mg l¡1 and for removals (R) %. Standard deviations are in parenthesis.

During the 1st year of operation, HRT of the 1.2 m3 UASBST was 4.3 d and OLR 0.426 kgCOD m¡3 d¡1 (Table 2). However, in the 13th year of operation, OLR was nearly double at the same HRT due to the fact that more people were using the toilets and the toilets had lower Xush volume (10 l in the 1st year and 6 l in the 13th), thus resulting in a more concentrated inXuent (Table 3). In the 1st year, both CODss and CODdis removal were better in the cold than in the warm period. However, in the warm period of the 13th year, CODss removal had improved to the level of the 1st year cold period and CODdis removal was also constantly higher at approximately 53% (Table 3). The two 0.2 m3 UASBST at constant 15 and 25 °C had similar HRTs of 29 d and OLRs of 0.33 and 0.42 kgCODm¡3 d¡1, respectively (Table 2), due to slightly diVering inXuent COD compositions (Table 3). Since low Xush volumes were used, the inXuents were considerably more concentrated than those of the 1.2 m3 UASBST. The COD removals at constant 15 °C were lower than at 25 °C and comparable to the results of the 1st year of the 1.2 m3 system, while at 25 °C the removals were higher and similar to the results from the 13th year of the 1.2 m3 system (Table 3). In Fig. 3, the temperature proWle, CODss and CODdis removals, and biogas production are presented for all the reactors and study periods (the 1st and 13th year of the 1.2 m3 UASBST and the two 0.2 m3 UASBST). The 1st year operations of the 1.2 m3 and the two 0.2 m3 UASBST were fairly similar to each other. With respect to CODdis removal, especially, the 1st year of the 1.2 m3 and the 0.2 m3, constant 15 °C systems were similar, taking nearly 40 weeks to reach positive removal. In the 0.2 m3, constant 25 °C system, CODdis removal increased earlier and then resembled that of the 13th year of the 1.2 m3 UASBST. Also, the increase in biogas production took some time during all three 1st year operations, while in the 13th year of the 1.2 m3 system it was high throughout the warm period. CODss removal was the lowest in the warm period of the 1st year of 1.2 m3 UASBST operation, decreasing to as low as 40%, while in the 13th year and in the two 0.2 m3 systems the removals were comparable to each other (approximately 75–80%). The mass balance, as percentage of the average CODt of inXuent, of the 1st year of the 1.2 m3 UASBST further clari-

Wes the diVerences between cold and warm periods (Fig. 4). During the cold period, most of the COD removed was accumulated as solids (little CODss wash out) and only very little COD was converted to methane, also indicated by the signiWcant amount of CODdis washed out of the reactor with eZuent. In the warm period, however, conversion of COD was enhanced and methane production increased, accompanied by decreased CODss removal. The 13th year warm period showed similar methane production and amount of washed out CODss as the 1st year warm period. However, washed out CODdis was decreased and a higher amount of COD was accumulated. Mass balance of the 0.2 m3 UASBST at constant 15 °C (Fig. 4) resembled those of the 1.2 m3 system warm periods with similar methane production and some wash-out of solids. CODdis removal was lower than in the 13th year of the 1.2 m3 system, but similar to its 1st year. The 0.2 m3 UASBST at constant 25 °C showed enhanced methane production and somewhat lower washout of COD with high accumulation. 4. Discussion The results indicate the feasibility (eYcient removal of suspended solids and dissolved organic matter) of the UASBST for (pre)treatment of black water at the low temperatures of the northern European climate. During colder periods (<14 °C), there was indication of the system working more as a settler accumulating CODss without further conversion. Even then, the UASBST was considered more eYcient than a conventional septic tank due to the upXow mode Wltering the wastewater through the sludge bed instead of mere settling in horizontal Xow tanks. During the warmer periods (>14 °C), CODdis removal, biogas production and conversion of the accumulated solids were also improved. Laboratory experiments at 10 °C have, however, shown that a two-phase UASBST treating synthetic black water can remove above 90% of CODt and BOD7, and 70% of CODdis (Luostarinen and Rintala, 2005), indicating that high biological conversions can also be achieved during the cold periods. During the cold period of the 1st year of operation, the 1.2 m3 UASBST worked mainly as a settler with no biogas production or CODdis removal. However, in the warm period, hydrolysis of accumulated solids apparently started

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S. Luostarinen et al. / Bioresource Technology 98 (2007) 980–986 30 25

15

Temp (°C)

Temp (°C)

20

10 5

20 15 10 5

0

0 0

10

20

30

40

50

60

CODss removal (%)

CODss removal (%)

100 80 60 40 20 0

10

20

30

40

50

20

30

40

50

60

0

10

20

30

40

50

60

0

10

20

30

40

50

60

0

10

20

30

40

50

60

100 80 60 40 20

60

100 80 60 40 20 0 -20 -40 -60

CODdis removal (%)

CODdis removal (%)

10

0

0

0

10

20

30

40

50

0.32 0.28 0.24 0.2 0.16 0.12 0.08 0.04 0 0

10

20

30

40

100 80 60 40 20 0 -20 -40 -60

60

0.32 0.28 0.24 0.2 0.16 0.12 0.08 0.04 0

BG (gCOD gCOD-1)

BG (gCOD gCOD-1)

0

50

60

Time (weeks)

Time (weeks)

Fig. 3. Temperature proWles, CODss and CODdis removals, and biogas production (BG) in the 1.2 m3 UASB-septic tanks during the 1st (䊐) and the 13th (䉬) year of operation and in the two 0.2 m3 UASB-septic tanks at constant 15 °C (£) and 25 °C (䉭). 100% CH4 80% 60%

Total CODss in effluent

40%

Total CODdis in effluent

20%

Total accumulated/wasted

0% 1

2

3

4

5

3

Fig. 4. Mass balances, as percentage of the average CODt of inXuent, of the 1.2 m UASBST during the cold (1) and warm (2) period of the 1st year operation, and the warm period of the 13th year operation (3), and the 0.2 m3 UASBST at constant 15 °C (4) and 25 °C (5).

and dissolved compounds were formed with no or partial removal (negative CODdis removal). The beginning of biogas production was accompanied by a decline in CODss

removal, which may have been due to disintegration and washout of granular sludge or incomplete hydrolysis and washout of the resulting small, possibly colloidal particles.

S. Luostarinen et al. / Bioresource Technology 98 (2007) 980–986

Moreover, the CODss washout may have been caused by a high scum-forming potential of the sludge (Halalsheh, 2002). When biogas production exceeded hydrolysis, CODdis removal became positive and eventually the accumulated small particles were also hydrolysed or the scumforming potential decreased with sludge stabilisation, restoring CODss removal to its earlier level. No such washout of CODss occurred in the 0.2 m3, constant 15 °C system, though it was inoculated with sludge from the 1.2 m3 UASBST, taken in a cold period (January). Moreover, its biogas production was comparable to that of the 1st year warm period of the 1.2 m3 system, achieving simultaneous high removal of CODss. Apparently, accumulated and unstabilised solids and scum-forming potential had nothing to do with the CODss removal of the 1.2 m3 system, but the decline was due to the granular inoculum used. As the granules, originally adapted to high OLR, were subjected to low OLR and low temperature, their inner core apparently no longer received VFA and therefore died and eventually disintegrated (Aiyuk and Verstraete, 2004). The resulting particulate material was initially retained inside the system but later washed out as the biogas production started in the warm period, thus causing the decline in CODss removal. As the 0.2 m3, constant 15 °C UASBST was inoculated with sludge from the 1.2 m3 system, already adapted to black water and low temperature, no decline in CODss removal was observed during biogas production. When comparing the two warm periods of the 1.2 m3 UASBST, biogas production started earlier in the 13th year than in the 1st year of operation, indicating the development of a more temperature-adapted sludge in the reactor over the years. In addition, CODss removal did not decline to under 60%, further indicating that the washout in the 1st year was due to the inoculum. Moreover, neither temperature nor inoculum sludge seem to have had a signiWcant eVect on CODss removal. Removal of CODdis also improved to an average of 53% throughout the 13th year warm period as compared to reaching positive removal only by the end of it in the 1st year. Similar improvement of sludge adaptation to low temperature has also been shown in a two-phase UASBST treating synthetic black water, as CODdis removal increased from 54% at 20 °C to 70% at 15 and 10 °C (Luostarinen and Rintala, 2005). The main reason for treatment of domestic wastewater is to reduce the concentration of harmful compounds in the eZuent. In the Netherlands, the requirements for eZuent quality in decentralised wastewater treatment (house- or community-on-site) are divided into four diVerent classes (IBA Manual, 2001; Table 4) and in Finland into two categories (Lehtovuori, 2003; Table 4) according to diVerent vulnerabilities of the eZuent discharge areas. In the Netherlands, some areas are very vulnerable, e.g. due to a nature reserve or an important groundwater resource area, and require eZuent quality of class 3a or 3 b. In Finland, all areas are considered vulnerable or very vulnerable due to the many waterways and groundwater reserves and can be compared to classes 3a and 3b of the Dutch requirements.

985

Table 4 Minimum requirements for eZuent quality in diVerent areas in the Netherlands (IBA Manual, 2001) and in Finland (Lehtovuori, 2003) Country

Parameter (mg l¡1)

Class 1

Class 2

Class 3a

Class 3b

The Netherlands

COD N P

750

300

200 60

200 60 4

Finland

BOD7 N P

83 82 5.5

42 70 2.8

Class 3b is required in the most vulnerable areas.

EZuent quality of the 1.2 m3 UASBST complied with the class 1 Dutch requirements, meant for combined domestic wastewater, during its 1st year of operation but not in the 13th year of operation. Moreover, the eZuents of the 0.2 m3 UASBST were too concentrated to meet any of the requirement classes. However, if eZuent volume is considered, the 0.2 m3 systems produced the smallest volume and the 1st year 1.2 m3 system the largest. Therefore, the amount of COD discharged per Xush would be 10 times the eZuent COD (10 l Xush) for the 1.2 m3 system in the 1st year, 6 times the eZuent COD (6 l Xush) for the 13th year, and the eZuent COD as such (approximately 1 l Xush) for the two 0.2 m3 systems, resulting in 6020, 11550, 5190, 3699, and 2733 mgCODt Xush¡1, respectively, showing that the impact of the 0.2 m3 systems on the receiving water bodies was actually the lowest. Therefore, measuring the treatment requirements as mgCOD l¡1 may not be the best possible way to ensure low environmental impact. To achieve the Dutch class 1 or 2 requirements with the UASBST eZuents, CODss removal needed the most improvement. Elmitwalli et al. (1999) treated raw domestic wastewater in UASB systems at 13 °C with HRT of 8 h and achieved 79% (§12) removal eYciency for suspended solids. In anaerobic hybrid reactors with vertical sheets of reticulated polyurethane foam in the upper part of the reactor, the removal eYciency reached 89% (§10). The conclusion was that the use of sheets in the anaerobic hybrid reactor signiWcantly increased CODss removal compared to the UASB reactor. Similarly, an option for improved CODss removal is the introduction of vertical sheets to UASBST, turning them into upXow-hybrid septic tanks (Elmitwalli et al., 2003). On the other hand, most eZuents from UASBST will not be discharged directly, but will be post-treated to remove residual COD and to remove or to recover nutrients. In the latter case, the eZuent should be judged on its suitability for nutrient recovery: the more concentrated the eZuent, the more suitable it is (Hammes et al., 2000). As to the Finnish requirements, no anaerobic process would suYce alone due to the strict nutrient removal criteria (Lehtovuori, 2003). 5. Conclusions • Temperature has no signiWcant eVect on CODss removal in UASBST treating black water.

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• Inoculum sludge has little eVect on CODss removal, though in the start-up phase some poorly adapted inoculum may disintegrate and wash out, thus requiring consideration when designing the process. • Removal of CODdis may at Wrst be negative, but will improve as the sludge is adapted to low temperature. • UASBST alone may not meet the treatment requirements for wastewaters in rural areas without post-treatment to remove residual COD and to remove or to recover nutrients. • Measuring treatment requirements as mgCOD l¡1 may not always be the best method, as the volume of the eZuent discharged is also an important factor in the Wnal amount of COD entering the receiving water bodies. Acknowledgements We gratefully acknowledge RTD project CORETECH for Wnancing the study. We would also like to thank the Finnish Foundation of Soil and Water Technology (MVTT) and NUFFIC (Netherlands Organisation for International Cooperation in Higher Education) for supporting Ms Luostarinen’s stay in the Netherlands. References Aiyuk, S., Verstraete, W., 2004. Sedimentological evolution in an UASB treating SYNTHES, a new representative synthetic sewage, at low loading rates. Biores. Technol. 93, 269–278. Bogte, J.J., Breure, A.M., van Andel, J.G., Lettinga, G., 1989. Kleinschalige anaerobe zuivering van huishoudelijk afwalwater – Praktijkproef met drie UASB-reaktoren. Eindrapport. RIVM report number 738518005 (in Dutch). Bogte, J.J., Breure, A.M., van Andel, J.G., Lettinga, G., 1993. Anaerobic treatment of domestic wastewater in small scale UASB reactors. Water Sci. Technol. 27 (9), 75–82. Elmitwalli, T.A., Zaandvoort, M., Zeeman, G., Bruning, H., Lettinga, G., 1999. Low temperature treatment of domestic sewage in upXow anaerobic sludge blanket and anaerobic hybrid reactors. Water Sci. Technol. 39 (5), 177–185. Elmitwalli, T., Sayed, S., Groendijk, L., van Lier, J., Zeeman, G., Lettinga, G., 2003. Decentralised treatment of concentrated sewage at low temperature in a two-step anaerobic system: two upXow-hybrid septic tanks. Water Sci. Technol. 48 (6), 219–226.

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