- Email: [email protected]
High-rate anaerobic wastewater treatment under psychrophilic and thermophilic conditions
~
Pergamon
Wal. Sci. Tech. Vol. 35, No. 10, pp. 199-206, 1997. © 1997 IA WO. Published by Elsevier ScIence Lid
Pnnled in Great BrilOln.
Pll: S0273-1223(97)00202-3
0273-1223/97 $17'00 + 0'00
HIGH-RATE ANAEROBIC WASTEWATER TREATMENT UNDER PSYCHROPHILIC AND THERMOPHILIC CONDITIONS Jules B. van Lier, Salih Rebac and Gatze Lettinga Department of Environmental Technology, Wageningen Agricultural University, Bomenweg 2. 6703 HD Wageningen. The Netherlands
ABSTRACf Anaerobic wastewater treatment is an attractive and generally accepted technology for the treatment of various types of medium- and hIgh-strength wastewaters. So far. this treatment technology is mostly applied at the mesophilic temperature range between 25 and 40°C. However, results of recent research conducted under both psychrophilic (< 20°e) and thermophilic (> 45°C) conditions, reveal Ihat temperature is not a limiting factor in applying anaerobic treatment, provided the appropriate process design is chosen. Temperature has a considerable Impact on various biological and physical factors of the anaerobic conversion process. For instance, the biogas production rate IS reduced to a minimum at low temperatures, while it can reach extreme values under thermophilic condiuons. In sludge bed systems, the biogas production rate determines the degree of mIxing between the biomass and the wastewater and should, therefore, be conSIdered in the process design. Other impacts of temperature are related to inhibitIon effects under thermophilic conditions and to a non-desirable accumulation of non- or partly degradable organic matler under psychrophilic conditions. ObVIOusly, these effects may hamper the utility of the commonly applied slRgle stage reactor systems. However, by adapting the process design to the expected prevailing condItions IRside the reactor, the loading potentials and overall stability of the anaerobic high-rate process may be dlsunctly improved. @ 1997 IAWQ. Published by Elsevier Science Ltd
KEYWORDS Anaerobic; biogas production rate; inhibition; psychrophilic; staging; thermophilic; volatile fatty acids,
INTRODUCTION High-rate anaerobic wastewater treatment systems are characterized by high concentrations of bacterial mass, which is generally present as biofllms andlor granular aggregates (Lettinga, 1995; Speece, 1996). Generally, these systems are operated at low hydraulic retention times (HRTs) at temperatures between 2540°C. However, the possibilities of the anaerobic high-rate technology could be increased if the process could also be applied under 'sub-optimal' temperature conditions, i.e. at temperatures higher than 45°C andlor lower than 20°C. Since the seventies, the feasibility of high'rate thermophilic treatment, and to a lesser extent, psychrophilic treatment is studied by various researchers as reviewed recently (Van Lier, 1996; Rebac et al., 1995). However, due to the sometimes disappointing results, this has not yet resulted in the construction of large scale reactor systems. Because temperature strongly affects the anaerobic conversion process, changes to the conventional design are likely required if high-rate reactor systems are going to be applied at 'sub-optimal' temperatures. 199
200
J. B. VAN LIER dal
Most important is the effect of temperature on the growth rate and activity of the methanogenic bacteria. The anaerobic bacteria are generally divided into three thermal groups: psychrophiles, mesophiles and thermophiles. with optimum temperatures at < 20°C. 25-40°C. and > 45°C. respectively (Figure I). So far, anaerobic digestion is reported at temperatures as high as 75°C. while methanogenesis from HtCOz proceeds at temperatures exceeding 100°C. as reviewed by Van Lier (1996). Within the temperature range of one species the growth rate increases exponentially with temperature. This is rapidly followed by an exponential decline if the ambient temperature exceeds the optimum temperature. The same IS true for the methanogenic activity. being the sum of the most important catabolic reactions of the methanogens. Generally. psychrophiles perform the conversion reactions at a low rate while thermophiles are characterized by a high metabolic activity.
g
100
III
c: (\)
01 0
80
III
60
c:
~
:E
~
a:
~
e
(!)
mesophiles
40 20 0
0
20
40
temp.
rC)
60
80
Figure 1. Relative growth rate of psychrophilic, mesophlhc and thermophilic methanogens. PSYCHROPHILIC CONDmONS Under moderate climate conditions. many wastewaters are discharged at low temperatures. e.g. those from bottling. maiting. brewery. and wastewaters diluted with washing waters. The concentrations chemical oxygen demand (COD) of these wastewaters are generally relatively low « 1500 mgol· I ). So far. these types of wastewaters are considered not to be suitable for anaerobic treatment. However. due to the presence of large amounts of biocatalyst (anaerobic sludge). the COD removal capacity of anaerobic high-rate reactors. in principle. is adequate. despite the eventually low operational temperature of the treatment plant. A major concern in anaerobic low-temperature reactors is the very low biogas production rate which may result in a low mixing intensity and a poor substrate-biomass Contact. If. for such type of wastewaters. conventional upflow anaerobic sludge bed (UASB)-type high-rate reactors are applied. then process failure may result from channelling of the wastewater through the sludge bed. This. however. can be prevented by increasing the superficial liquid upward velocities (V upw) which results in an expansion of the sludge bed. The required V upw and the degree of expansion is dependent on the wastewater characteristics and the settling properties of the sludge granules. So far. lab-scale psychrophilic expanded granular sludge bed (EGSB) reactors are operated with V ul'w between 4 to 10 moh· l . The. increased V upw is brought about by applying effluent recycle andlor by Increasing the height/diameter ratIo of the reactor. The EGSB reactors were firstly applied for pre-settled domestic sewage and were inoculated with mesophilic methanogenic granular sludge (De Man et al.• 1988; Van der Last and Lettinga. 1992). Results indeed showed an improved soluble COD removal efficiency at a high V upw. despite the low temperatures applied (De Man et al.• 1988; see also Table
I).
Table 1 presents some of the recent results regarding psychrophilic anaerobic treatment of low strength wastewater in EGSB reactors. The influent consisted either of mixtures of volatile fatty acids (VFA) or barley wastewater coming from a maltery. both at concentrations of 0.5-1.5 g COOl!. The table includes also some previous results obtained with settled domestic sewage.
High-rate anaerobic wastewater treatment
201
Table J. Results psychrophilic EGSB reactor studies Substrate
Volume I
Temperature ·C
Loading'rate g COD.!" I .d- I
COD removal eft. %
HRT h
4
10-12
10-12
90
1.6-2.5
VFA
Ref.·
VFA
4
5
5
90
4
2
VFA
4
2
5
75
4
2
225
16
4.4-8.8
56
2.4
3
Malting Malting
225
20
8.8-14.6
66-72
1.5-2.4
3
Barley! Malting
140 (2 x 70)
10-15
5-15
65-80
3-5
2
Sewage
120-205
6!9 - 15/20
5-6
50-95
1-3
4
• References: 1. Rebac et al.• 1995; 2, Rebac et aI., unpublished; 3, Rebac et aI., 1996; 4, Van der Last and Lettinga (1992) Despite the low substrate levels inside the reactor !he obtained results are very satisfactory. Apparently, the EGSB reactor provides optimum mixing conditions while the concentration of active biomass inside the reactor is high enough to compensate !he low specific sludge activity. The results under psychrophilic conditions are in accordance to results obtained by Kato et al. (1994) who could efficiently treat extremely dilute wastewaters « 100 mgel- I ) under mesophilic conditions at an organic loading rate (OLR) of more than 15 kg CODem- 3eday-1 and removal efficiencies of 85%. In fact, effluent COD concentrations were lower than the intrinsic half-saturation-constant, Km, of the predominant methanogenic bacteria in the sludge particles. This phenomenon was explained by the occurrence of non-conventional, convective type of mass transfer from the bulk of the liquid into !he granular sludge, which may result from the formation and release of gas bubbles. Convective mass transport in !he biofilm (or in !he pores of the film laggregate) may be enhanced by intensifying the hydraulic turbulence in the bulk of the liquid by increasing the superficial liquid velocity in the reactor.
..-.-
~
"0
0
0
()
.9
40
10
"iii > 0
20
0
0
()
0 0
75
;;;
50
!!!. (1) a; 0 iii'
25
.Q
3 0
30
E 2!
()
50
$
...
100
60
Removal rate c Removal efficiency 11
10 0
0
25
50
75
100
125
150
175
<
::J
:.e
~
~
0
Time [days] Figure 2. Start-up of a 4.0 I EGSB reactor fed with low strength VFA mixtures at IO-Ire (adapted from Rcbac el al.• 1995).
Rebac et al. (see Table 1) operated !he EGSB reactors for prolonged periods of time under psychrophilic conditions (2-20·C). The reactors were fed with either a VFA-mixture (Figure 2) or maltery wastewater (Figure 3). which consists of different sugars, lactic acid, ethanol, VFA, and proteins. The EGSB reactors
J. B. VAN LIER etal.
202
were inoculated with mesophilic granular sludge, originating from a 760 m 3 VASB reactor (20-24°C) of th Bavaria brewery at Lieshout, The Netherlands. After a start-up period of 2-3 month, the performances of th reactors were remarkably stable, despite the variations in load and temperature which sometimes occurred ~ a result of disturbance in feed supply and differences in ambient temperature. Due to the low temperatur and the low COD concentration, the observed biogas-methane yield was very low, which can be attributed t the high solubility of methane in the effluent at low temperatures. Despite the imposed high upflow velocit the effluent biomass concentration, measured as the fraction of suspended solids (SS) in the effluent, w~ low in all studies (Rebac et al., 1995, 1996). OLR (g COD dm-3 day-1)
COD removal & conversion (%)
20r-----------~r-------~-,
80~------------ir---------~
15
60
10
40
5
20
O~----~----~~h----~----~
o
25
50
150
Time (days)
175
200
8
O~--------
o
25
__50
-COOsol
____
o COOmeth
~,~----
150
175
~
200
Time (days)
Figure 3. Start-up of a 225 I EGSB reactor. fed with low strength maltery wastewater at IS-20°C A. Organic loading rate based on CODtotal ' B. (--) CODsoluble removal efficiency. (0) COD convened to methane (adapted from Rebac et al.• 1996).
Psychrophjljc or psychrotolerant The increase in the methane production rate during the course of the start-up of psychrophilic EGSB reactor reveals the net growth of acetogenic and methanogenic bacteria at the low temperatures applied (Figures : and 3). Whether this should be attributed to growth of the original mesophilic methanogenic populatio: andlor to growth of psychrophilic organisms is not clear yet. The sludge from the 10-12°e EGSB reacto was sampled at various time intervals and its activity was measured at different temperatures by usin; substrate depletion activity tests (Rebac et al., 1995). The results clearly illustrate that the temperaturl curves from the sludge exposed to long term psychrophilic conditions were very similar to those of th mesophilic inoculum. The optima were between 30-400 e, indicating that no sPecialized psychrophile developed in the sludge or cannot be seen due to large amounts of mesophiles. However, during the coursl of the start-up the increase in activity measured at lOoe was significantly higher than the activity assessed ~ 30°C. For acetate, propionate and butyrate, the maximum conversion rates at lOoe increased by a facto 3.65, 1.52 and 4.34, respectively, while this factor was 2.25, l.01 and 2.82. respectively, at 30°C (Table 2J The relatively high increase in specific activity at lOoe might indicate growth of psychrophilic organism which don't contribute to the activity assessed at 30°C. On the other hand, the activity measured at 30°C might have been limited by diffusion limitation of the substrate into the granule, particularly with respect te acetate. During the triplicate activity tests, the substrate concentrations were never below 500 mg eODol• (Rebac et al., 1995). From the results it was concluded that the dominant populations were still mesophilic after 8 months 0 operation under psychrophilic conditions. Apparently, if the temperature is shifted from the mesophilic te the psychrophilic range, the large number of mesophiles remain active despite the low metabolic rates at lo\! temperatures. Under such conditions, growth of real psychrophiles is very difficult, if possible at all. So far the existence of psychrophilic homologues in the anaerobic digestion process remains unclear. However cultivation of psychrophilic organisms is apparently not a prerequisite for successful anaerobic treatmen
High-rate anaerobic wastewater treatment
203
under low temperature conditions. In all probability. the sufficient sludge-water contact is a much more important criterion. Table 2. Maximum specific substrate conversion rates at various temperatures of the mesophilic inoculum and the I O-12°C-EGSB-sludge after 235 days of continuous operation. Standard deviation is given between parentheses, methods described by Rebac et al. (1995) Propionate •
Acetate·
Butyrate·
inoculum
t = 235 d
inoculum
t = 235 d
inoculum
t = 235 d
JOoC
0.09 (0.00)
0.33 (0.01)
0.05 (0.00)
0.07 (0.01)
0.05 (0.01)
0.23 (0.00)
20°C
0.38 (0.01)
1.06 (0.01)
0.30 (0.02)
0.29 (0.01)
0.17 (0.01)
0.53 (0.00)
2.20 (0.01)
0.55 (0.03)
0.56 (0.02)
0.33 (0.04)
0.92 (0.03)
Temperature
30°C
0.98 (0.12)
• Substrate conversion rates are expressed in g COD· g-I VSS· day-I Limitations of psychrophilic EGSB reactors EGSB reactor systems are very effective in removing soluble COD. With regard to the non-soluble COD fraction, De Man et al. (1988) and Van der Last and Lettinga (.1992) showed that, due to the high V u W' colloidal COD in pre-settled domestic sewage is hardly removed In EGSB systems. Wang (1994) concluded that, for treating raw domestic sewage at ambient Dutch temperatures, the anaerobic system should consist of at least two stages in which the first reactor is dedicated to the entrapment and hydrolysis of the incoming SS (Wang, 1994). The second stage may consist of an EGSB-type reactor. Because of the low hydrolytic activity under psychrophilic conditions, a similar system might also be feasible for low temperature industrial wastewater which is characterized by high SS concentrations. Recent pilot-research performed in a mattery shows that accumulating suspended solids deteriorated the granular sludge bed of an EGSB reactor (manuscript in preparation). As a result of the low metabolic rates. psychrophilic sludge stabilization proceeds at a very low rate. This means that. in addition to the SS concentration. the fraction non-acidified organic matter is a very important design criterion, since over-growth of acidogenic biomass should be prevented in a single stage reactor. Results of psychrophilic EGSB experiments performed with either a sucrose-VFA mixture or non-acidified maltery wastewater, show that the specific sludge load of non• acidified COD should not exceed 0.05-0.JO kg COD·kg VSS-I·day-I. Otherwise. the bulking acidifying sludge deteriorates the granular sludge bed (unpublished reSUlts). This value is about 5-10 times lower than the value found by Alphenaar (1994) for mesophilic conditions. Therefore, high rate psychrophilic anaerobic treatment of complex wastewater requires a biological process with more stages, or an integrated physical pretreatment combined with a methanogenic polishing stage. Further research is required to elucidate the most optimal process design for psychrophilic wastewater treatment in relation to the wastewater characteristics. The obtained results so far, show that an EGSB reactor profitably can be used for removing the soluble and merely acidified organic fraction from the wastewater, even at temperatures as low as 2°C. THERMOPHILIC CONDITIONS From Figure I it is clear that the other boundary of the mesophilic temperature range, i.e., 45°C. is much more critical for the existing methanogenic subpopulations than the border around 20°C. Generally. a temperature increase from the mesophilic to the thermophilic range immediately results in a shift in the methanogenic subpopulations as shown previously (Van Lier et al.• 1992). The latter easily can be explained by a rapid die-off of mesophilic organisms at temperatures exceeding the maximum growth temperature of these bacteria. Under thermophilic conditions there is no competition between mesophiles and thermophiles. Since reaction rates increase with temperatures (Figure I), significantly higher loading potentials and/or considerably shorter retention times are expected if anaerobic treatment is applied under thermophilic JWST 35: IO·H
J. B. VAN LIER et al.
204
conditions (Zinder, 1986). Therefore, thermophilic anaerobic treatment could be an attractive alternative particularly when the wastewater is discharged at high temperatures. In the past decades much research hru been conducted to study the feasibility of thermophilic treatment for various types of hot wastewaters mainly coming from food industries and pulp & paper industries, as reviewed recently (Van Lier, 1996). II most cases either UASB reactors or anaerobic filters (AF) were used, operated at high temperatures. The impacts of the high operation temperature on the anaerobic biology are generally not taken into account i~ the used process design. This will be discussed in more detail below. Limitations of thermophilic sin~le
sta~e hi~h-rate
reactors
One of the main reasons for applying thermophilic conditions in high-rate wastewater treatment reactors i! the expected much higher loading potential. However, conventionally designed thermophilic treatmenl systems can become limited by various biological and physical factors (Van Lier, 1996). For example, biomass retention might become critical in single stage sludge bed reactors if COD removal rates exceed 5060 kg-m- 3 reactor-day- I. Sludge separation might even be more critical under high temperature conditiom because of the lower liquid viscosity and the occasional occurrence of less stable thermophilic aggregates, The latter is principally attributed to the high mineralisation rate of thermophilic methanogenic sludge (see also Van Lier, 1996). In general, two approaches are followed to achieve a high biomass hold-up: i) auto· immobilization of anaerobic bacteria which leads to the formation of granular sludge; ii) adherence of the microorganisms to inert support media. The latter mechanism is essential in fixed film reactors such as anaerobic filters and fluidized bed reactors, while sludge granulation is characteristic for upflow (hybrid) sludge bed reactors. As mentioned above, AF and UASB received most interest in the various studies 011 thermophilic high-rate reactors. However, if immobilization of thermophilic biomass is difficult to achieve, the use of reactors with inert support material (fixed film reactors) should be reconsidered. Moreover, also the use of single stage sludge bed reactors is questionable, because high loading conditions ma~ subsequently result in a high wash-out of dispersed, and even granular, sludge. In such cases, the use 01 compartmentalized reactor systems in which the produced biogas is evenly withdrawn from the reactor is apparently more appropriate (Van Lier, 1996). Various authors pursue a similar approach even for the mesophilic temperature range, particularly if the formation of granular sludge does not proceed satisfactoril~ (Bachman et al., 1985; EI-Mamouni etal., 1992; Mark! and Reinhold, 1994). Another drawback of thermophilic digestion is the often found high effluent VFA concentrations (Zinder, 1986). However, the occurrence of bad-quality effluents might be attributed to the applied process technology rather than to the thermophilic digestion process itself. Due to the relatively high sensitivity of thermophilic methanogenic sludge for relatively low concentrations of intermediate compounds such as hydrogen and acetate, VFA concentrations easily rise in high loaded single stage reactor systems. Furthermore, the VFA build-up is enhanced by the high apparent Km of thermophilic granular sludge (Van Lier, 1996). Therefore, VFA concentrations will rise even more when the reactor is fed with very concentrated wastewater. It will be clear that thermophilic single stage reactor systems must be operated at relatively low loading rates in order to meet the conditions for a high VFA reduction. This, however, is contrasting one of the principal advantages of thermophilic treatment. Application of staged thermophilic reactor systems resulted in significant improvements with respect to the applicable loading capacity and the overall process stability. Interestingly, effluent VFA concentrations were found to be in the similar range as mesophilic UASB reactors (Van Lier, 1996). The compartmentalized upflow reactor, denominated as the upflow staged sludge bed (USSB) reactor, showed stable long-term reactor performance under extreme loading conditions. Within 3 months the organic loading rate of a reactor treating a sucrose-VFA mixture could be increased up to 100 kg COD-m-3-day··. The COD removal rate exceeded 90%. No significant wash-out of the thermophilic biomass was observed despite the extreme biogas loading rate of about 50 m3-m· 3 reactor-day' •. The main achievements of the 'plug·flow' reactor are: i)
very low effluent VFA concentrations under extreme loading conditions, i.e. 10·120 mg acetate COD-I'. and 200-500 mg propionate COD-I'.,
ii)
high degree of sludge retention due to very low turbulence in the final reactor compartment, and
High-rate anaerobic wastewater treatment
iii)
205
stable reactor performance due to the segregated development of methanogenic sludge along the reactor height. To maintain stable long-term performance, sludge segregation should be kept by specific sluicing of excess sludge (Van Lier et al., 1996)
The above findings are based on experiments performed in 5 I lab-scale reactors which were fed with sucrose-VFA mixtures. Presently, the feasibility of a thermophilic (55°C) staged reactor system is further investigated in the research laboratory of Paques B.V., The Netherlands, by using a liS I pilot reactor fed with beet-sugar distillery wastewater. Although the reactor is still under start-up conditions, the achieved results are very promising. At the moment of writing this paper, the reactor is treating organic loading rates between 20-30 kg COD om- 3oday-1 at HRTs of about 10 hours and COD removal efficiencies of 70-75%. The latter values agree with the maximum anaerobic biodegradability of the treated wastewater (results not shown). So far, effluent acetate concentrations can be kept below 200 mgol- I, while residual propionate is more or less in the same range. The biogas production rate is currently exceeding 10 m 3o m- 3 reactoroday-I. CONCLUSIONS Anaerobic wastewater treatment systems are generally developed for application in the mesophilic temperature range, i.e. between 25 and 40°C. However, literature results reveal that anaerobic conversion of organic matter effectively proceeds from below 5°C to temperatures as high as 75°C, which implies that, in principle, anaerobic wastewater treatment is feasible in the entire temperature range. In order to apply temperatures highr-r and/or lower than the mesophilic range, the process design of conventional anaerobic reactor systems requires various adjustments. The biogas production rate is considered to be very important with respect to the mixing characteristics as well as the biomass retention in anaerobic reactors. Therefore, the absence or excessive presence of biogas turbulence determines the most optimal reactor configuration. Under psychrophilic conditions, a sufficient sludge-water contact is a prerequisite for successful application of anaerobic treatment. Appropriate mixing conditions can be achieved by increasing the liquid upward velocity which results in an expansion of the sludge bed. In contrast, under thermophilic conditions, the extr.eme turbulence caused by the high biogas production, may lead to an excessive carry over of methanogemc sludge. This, however, can be prevented by the application of a staged process design in which the produced biogas is evenly withdrawn from the system. In addition, staging the thermophilic digestion process enhances distinctly the biodegradation of soluble compounds, due the reduction of possible inhibition effects. Gaseous toxic compounds are removed early by the first gas-liquid separator, while biodegradable soluble toxic compounds (e.g. intermediates) are hardly present in the final stage(s) of the system, reSUlting in a high degree of effluent polishing. It should be noted that also under psychrophilic conditions, a staged system can be advantageous, particularly whenever the wastewater is characterized by a high concentration of suspended solids or a high concentration of soluble non-acidified organic matter. Due to the extremely low hydrolysis rate at low temperatures, the methanogenic stage should be characterized by a very long solids retention time. ACKNOWLEDGEMENT This work was financially supported by the Ministry ofVROM, Novem-grant 51120/1510, The Netherlands, Paques B.V. Balk, The Netherlands and Bavaria B.V., Lieshout, The Netherlands. REFERENCES Alphenaar. P. A. (1994). Anaerobic granular sludge: characteri~atio~ and fac~ors affecting its functioning. Ph.D. thesis. Department of Environmental Technology. Agncultural Untvemty. Wagemngen. The Netherlands. Bachmann. A.• Beard. V. L. and McCarty. P. L. (1985). Performance and characteristics of the anaerobic baffled reactor. Wat. Res.• 19.99-106. De Man. A. W. A .• Van der Last. A. R. M. and Lettinga. O. (1988). The use of EGSB and UASB anaerobic systems for low strength soluble and complex wastewaters at temperatures ranging from 8 to 30°C. In: Anaerobic Digestion /988 (Adv. Wat. Pollut Control no. 5). E. R. Hall and P. N. Hobson (eds). Pergamon Press. pp. 197-209.
J. B. VAN LIER etal.
206
EI-Mamouni. R.• Rouleau. D .• Mayer. R.• Guiot. S. R. and Samson. R. (1992). Comparison of the novel multiplate anaerobic reactor with the upflow anaerobic sludge blanket reactor. In: Proc. of 46th Industrial Waste Conference. C.S. Dalton aO( R.F. Wukasch (Eds). Lewis Publishers. Chelsea Michigan. USA. pp. 681-687. Kato. T. M .• Field. J. A .• Versteeg. P. and Lettinga. G. (1994). Feasibility of Expanded granular sludge bed reactors for the anaerobic treatment oflow strength soluble wastewaters. Biotechnol. Bioeng.• 44.469-479. Lettinga. G. (I 99S). Anaerobic digestion and wastewater treatment systems. Antonie van Leeuwenhoek. 67. 3-28. Markl. H. and Reinhold. G. (1994). Der Biogas-Turmreaktors. ein neues Reaktorkonzept fUr die anaeroben Abwasserreinigung Chem.-Ing.-Tech.• 66(4). S34-S36 (in German). Rebac. S.• Ruskova. 1.. Gerbens. S .• Van Lier. J. B .• Starns. A. J. M. and Lettinga. G. (I99S). High-rate anaerobic treatment 0 wastewater under psychrophilic conditions. J. Ferment. and Biotechnol.• 80. 499-S06. Rebac. S.• Van Lier. 1. B.• 1anssen. M. G. 1.• Dekkers. F .• Swinkels K. T. M. and Lettinga. G. (1996). High-rate anaerobic treatment of malting wastewater in a pilot-scale EGSB system under psychrophilic conditions. J. Chem. Technol.
Biotechnol. (Accepted).
Speece. R. E. (1996). Anaerobic Biotechnology for Industrial Wastewaters. Archae Press. Nashville Tennessee. ISBN 0-96S02260-9. Van der Last, A. R. M. and Lettinga. G. (1992). Anaerobic treatment of domestic sewage under moderate climate (Dutch: conditions using upflow reactors at increased superficial velocities. Wat. Sci. Tech .• 25(7).167-178. Van Lier. J. B. (1996). Limitations of thermophilic anaerobic wastewater treatment and the consequences for process design Antonie van Leeuwenhoek, 69. 1-14. Van Lier. J. B .• Grolle. K. C. F .• Starns. A. 1. M.. Conway de Macario. E. and Lettinga. G. (1992). Start-up of a thermophilic Upflow Anaerobic Sludge Bed (UASB) reactor with mesophilic granular sludge. Appl. Microbiol. Biotechnol.. 37. J3().
J3S.
Van Lier. 1. B.• Groeneveld. N. and Lettinga. G. (1996). Development of thermophilic methanogenic sludge in compartmentalized upflow reactors. Biotechnol. Bioeng.. 50. II S-124. Wang. K. (1994). Integrated anaerobic and aerobic treatment of sewage. Ph.D. Thesis. Department of Environmental Technology. Agricultural University. Wageningen. The Netherlands. Zinder. S. H. (1986). Thermophilic waste treatment systems. In: Thermophiles: General. Molecular and Applied Biology. T. D Brock (Ed). Wiley-Interscience. New York. pp. 257-277.
Pergamon
Wal. Sci. Tech. Vol. 35, No. 10, pp. 199-206, 1997. © 1997 IA WO. Published by Elsevier ScIence Lid
Pnnled in Great BrilOln.
Pll: S0273-1223(97)00202-3
0273-1223/97 $17'00 + 0'00
HIGH-RATE ANAEROBIC WASTEWATER TREATMENT UNDER PSYCHROPHILIC AND THERMOPHILIC CONDITIONS Jules B. van Lier, Salih Rebac and Gatze Lettinga Department of Environmental Technology, Wageningen Agricultural University, Bomenweg 2. 6703 HD Wageningen. The Netherlands
ABSTRACf Anaerobic wastewater treatment is an attractive and generally accepted technology for the treatment of various types of medium- and hIgh-strength wastewaters. So far. this treatment technology is mostly applied at the mesophilic temperature range between 25 and 40°C. However, results of recent research conducted under both psychrophilic (< 20°e) and thermophilic (> 45°C) conditions, reveal Ihat temperature is not a limiting factor in applying anaerobic treatment, provided the appropriate process design is chosen. Temperature has a considerable Impact on various biological and physical factors of the anaerobic conversion process. For instance, the biogas production rate IS reduced to a minimum at low temperatures, while it can reach extreme values under thermophilic condiuons. In sludge bed systems, the biogas production rate determines the degree of mIxing between the biomass and the wastewater and should, therefore, be conSIdered in the process design. Other impacts of temperature are related to inhibitIon effects under thermophilic conditions and to a non-desirable accumulation of non- or partly degradable organic matler under psychrophilic conditions. ObVIOusly, these effects may hamper the utility of the commonly applied slRgle stage reactor systems. However, by adapting the process design to the expected prevailing condItions IRside the reactor, the loading potentials and overall stability of the anaerobic high-rate process may be dlsunctly improved. @ 1997 IAWQ. Published by Elsevier Science Ltd
KEYWORDS Anaerobic; biogas production rate; inhibition; psychrophilic; staging; thermophilic; volatile fatty acids,
INTRODUCTION High-rate anaerobic wastewater treatment systems are characterized by high concentrations of bacterial mass, which is generally present as biofllms andlor granular aggregates (Lettinga, 1995; Speece, 1996). Generally, these systems are operated at low hydraulic retention times (HRTs) at temperatures between 2540°C. However, the possibilities of the anaerobic high-rate technology could be increased if the process could also be applied under 'sub-optimal' temperature conditions, i.e. at temperatures higher than 45°C andlor lower than 20°C. Since the seventies, the feasibility of high'rate thermophilic treatment, and to a lesser extent, psychrophilic treatment is studied by various researchers as reviewed recently (Van Lier, 1996; Rebac et al., 1995). However, due to the sometimes disappointing results, this has not yet resulted in the construction of large scale reactor systems. Because temperature strongly affects the anaerobic conversion process, changes to the conventional design are likely required if high-rate reactor systems are going to be applied at 'sub-optimal' temperatures. 199
200
J. B. VAN LIER dal
Most important is the effect of temperature on the growth rate and activity of the methanogenic bacteria. The anaerobic bacteria are generally divided into three thermal groups: psychrophiles, mesophiles and thermophiles. with optimum temperatures at < 20°C. 25-40°C. and > 45°C. respectively (Figure I). So far, anaerobic digestion is reported at temperatures as high as 75°C. while methanogenesis from HtCOz proceeds at temperatures exceeding 100°C. as reviewed by Van Lier (1996). Within the temperature range of one species the growth rate increases exponentially with temperature. This is rapidly followed by an exponential decline if the ambient temperature exceeds the optimum temperature. The same IS true for the methanogenic activity. being the sum of the most important catabolic reactions of the methanogens. Generally. psychrophiles perform the conversion reactions at a low rate while thermophiles are characterized by a high metabolic activity.
g
100
III
c: (\)
01 0
80
III
60
c:
~
:E
~
a:
~
e
(!)
mesophiles
40 20 0
0
20
40
temp.
rC)
60
80
Figure 1. Relative growth rate of psychrophilic, mesophlhc and thermophilic methanogens. PSYCHROPHILIC CONDmONS Under moderate climate conditions. many wastewaters are discharged at low temperatures. e.g. those from bottling. maiting. brewery. and wastewaters diluted with washing waters. The concentrations chemical oxygen demand (COD) of these wastewaters are generally relatively low « 1500 mgol· I ). So far. these types of wastewaters are considered not to be suitable for anaerobic treatment. However. due to the presence of large amounts of biocatalyst (anaerobic sludge). the COD removal capacity of anaerobic high-rate reactors. in principle. is adequate. despite the eventually low operational temperature of the treatment plant. A major concern in anaerobic low-temperature reactors is the very low biogas production rate which may result in a low mixing intensity and a poor substrate-biomass Contact. If. for such type of wastewaters. conventional upflow anaerobic sludge bed (UASB)-type high-rate reactors are applied. then process failure may result from channelling of the wastewater through the sludge bed. This. however. can be prevented by increasing the superficial liquid upward velocities (V upw) which results in an expansion of the sludge bed. The required V upw and the degree of expansion is dependent on the wastewater characteristics and the settling properties of the sludge granules. So far. lab-scale psychrophilic expanded granular sludge bed (EGSB) reactors are operated with V ul'w between 4 to 10 moh· l . The. increased V upw is brought about by applying effluent recycle andlor by Increasing the height/diameter ratIo of the reactor. The EGSB reactors were firstly applied for pre-settled domestic sewage and were inoculated with mesophilic methanogenic granular sludge (De Man et al.• 1988; Van der Last and Lettinga. 1992). Results indeed showed an improved soluble COD removal efficiency at a high V upw. despite the low temperatures applied (De Man et al.• 1988; see also Table
I).
Table 1 presents some of the recent results regarding psychrophilic anaerobic treatment of low strength wastewater in EGSB reactors. The influent consisted either of mixtures of volatile fatty acids (VFA) or barley wastewater coming from a maltery. both at concentrations of 0.5-1.5 g COOl!. The table includes also some previous results obtained with settled domestic sewage.
High-rate anaerobic wastewater treatment
201
Table J. Results psychrophilic EGSB reactor studies Substrate
Volume I
Temperature ·C
Loading'rate g COD.!" I .d- I
COD removal eft. %
HRT h
4
10-12
10-12
90
1.6-2.5
VFA
Ref.·
VFA
4
5
5
90
4
2
VFA
4
2
5
75
4
2
225
16
4.4-8.8
56
2.4
3
Malting Malting
225
20
8.8-14.6
66-72
1.5-2.4
3
Barley! Malting
140 (2 x 70)
10-15
5-15
65-80
3-5
2
Sewage
120-205
6!9 - 15/20
5-6
50-95
1-3
4
• References: 1. Rebac et al.• 1995; 2, Rebac et aI., unpublished; 3, Rebac et aI., 1996; 4, Van der Last and Lettinga (1992) Despite the low substrate levels inside the reactor !he obtained results are very satisfactory. Apparently, the EGSB reactor provides optimum mixing conditions while the concentration of active biomass inside the reactor is high enough to compensate !he low specific sludge activity. The results under psychrophilic conditions are in accordance to results obtained by Kato et al. (1994) who could efficiently treat extremely dilute wastewaters « 100 mgel- I ) under mesophilic conditions at an organic loading rate (OLR) of more than 15 kg CODem- 3eday-1 and removal efficiencies of 85%. In fact, effluent COD concentrations were lower than the intrinsic half-saturation-constant, Km, of the predominant methanogenic bacteria in the sludge particles. This phenomenon was explained by the occurrence of non-conventional, convective type of mass transfer from the bulk of the liquid into !he granular sludge, which may result from the formation and release of gas bubbles. Convective mass transport in !he biofilm (or in !he pores of the film laggregate) may be enhanced by intensifying the hydraulic turbulence in the bulk of the liquid by increasing the superficial liquid velocity in the reactor.
..-.-
~
"0
0
0
()
.9
40
10
"iii > 0
20
0
0
()
0 0
75
;;;
50
!!!. (1) a; 0 iii'
25
.Q
3 0
30
E 2!
()
50
$
...
100
60
Removal rate c Removal efficiency 11
10 0
0
25
50
75
100
125
150
175
<
::J
:.e
~
~
0
Time [days] Figure 2. Start-up of a 4.0 I EGSB reactor fed with low strength VFA mixtures at IO-Ire (adapted from Rcbac el al.• 1995).
Rebac et al. (see Table 1) operated !he EGSB reactors for prolonged periods of time under psychrophilic conditions (2-20·C). The reactors were fed with either a VFA-mixture (Figure 2) or maltery wastewater (Figure 3). which consists of different sugars, lactic acid, ethanol, VFA, and proteins. The EGSB reactors
J. B. VAN LIER etal.
202
were inoculated with mesophilic granular sludge, originating from a 760 m 3 VASB reactor (20-24°C) of th Bavaria brewery at Lieshout, The Netherlands. After a start-up period of 2-3 month, the performances of th reactors were remarkably stable, despite the variations in load and temperature which sometimes occurred ~ a result of disturbance in feed supply and differences in ambient temperature. Due to the low temperatur and the low COD concentration, the observed biogas-methane yield was very low, which can be attributed t the high solubility of methane in the effluent at low temperatures. Despite the imposed high upflow velocit the effluent biomass concentration, measured as the fraction of suspended solids (SS) in the effluent, w~ low in all studies (Rebac et al., 1995, 1996). OLR (g COD dm-3 day-1)
COD removal & conversion (%)
20r-----------~r-------~-,
80~------------ir---------~
15
60
10
40
5
20
O~----~----~~h----~----~
o
25
50
150
Time (days)
175
200
8
O~--------
o
25
__50
-COOsol
____
o COOmeth
~,~----
150
175
~
200
Time (days)
Figure 3. Start-up of a 225 I EGSB reactor. fed with low strength maltery wastewater at IS-20°C A. Organic loading rate based on CODtotal ' B. (--) CODsoluble removal efficiency. (0) COD convened to methane (adapted from Rebac et al.• 1996).
Psychrophjljc or psychrotolerant The increase in the methane production rate during the course of the start-up of psychrophilic EGSB reactor reveals the net growth of acetogenic and methanogenic bacteria at the low temperatures applied (Figures : and 3). Whether this should be attributed to growth of the original mesophilic methanogenic populatio: andlor to growth of psychrophilic organisms is not clear yet. The sludge from the 10-12°e EGSB reacto was sampled at various time intervals and its activity was measured at different temperatures by usin; substrate depletion activity tests (Rebac et al., 1995). The results clearly illustrate that the temperaturl curves from the sludge exposed to long term psychrophilic conditions were very similar to those of th mesophilic inoculum. The optima were between 30-400 e, indicating that no sPecialized psychrophile developed in the sludge or cannot be seen due to large amounts of mesophiles. However, during the coursl of the start-up the increase in activity measured at lOoe was significantly higher than the activity assessed ~ 30°C. For acetate, propionate and butyrate, the maximum conversion rates at lOoe increased by a facto 3.65, 1.52 and 4.34, respectively, while this factor was 2.25, l.01 and 2.82. respectively, at 30°C (Table 2J The relatively high increase in specific activity at lOoe might indicate growth of psychrophilic organism which don't contribute to the activity assessed at 30°C. On the other hand, the activity measured at 30°C might have been limited by diffusion limitation of the substrate into the granule, particularly with respect te acetate. During the triplicate activity tests, the substrate concentrations were never below 500 mg eODol• (Rebac et al., 1995). From the results it was concluded that the dominant populations were still mesophilic after 8 months 0 operation under psychrophilic conditions. Apparently, if the temperature is shifted from the mesophilic te the psychrophilic range, the large number of mesophiles remain active despite the low metabolic rates at lo\! temperatures. Under such conditions, growth of real psychrophiles is very difficult, if possible at all. So far the existence of psychrophilic homologues in the anaerobic digestion process remains unclear. However cultivation of psychrophilic organisms is apparently not a prerequisite for successful anaerobic treatmen
High-rate anaerobic wastewater treatment
203
under low temperature conditions. In all probability. the sufficient sludge-water contact is a much more important criterion. Table 2. Maximum specific substrate conversion rates at various temperatures of the mesophilic inoculum and the I O-12°C-EGSB-sludge after 235 days of continuous operation. Standard deviation is given between parentheses, methods described by Rebac et al. (1995) Propionate •
Acetate·
Butyrate·
inoculum
t = 235 d
inoculum
t = 235 d
inoculum
t = 235 d
JOoC
0.09 (0.00)
0.33 (0.01)
0.05 (0.00)
0.07 (0.01)
0.05 (0.01)
0.23 (0.00)
20°C
0.38 (0.01)
1.06 (0.01)
0.30 (0.02)
0.29 (0.01)
0.17 (0.01)
0.53 (0.00)
2.20 (0.01)
0.55 (0.03)
0.56 (0.02)
0.33 (0.04)
0.92 (0.03)
Temperature
30°C
0.98 (0.12)
• Substrate conversion rates are expressed in g COD· g-I VSS· day-I Limitations of psychrophilic EGSB reactors EGSB reactor systems are very effective in removing soluble COD. With regard to the non-soluble COD fraction, De Man et al. (1988) and Van der Last and Lettinga (.1992) showed that, due to the high V u W' colloidal COD in pre-settled domestic sewage is hardly removed In EGSB systems. Wang (1994) concluded that, for treating raw domestic sewage at ambient Dutch temperatures, the anaerobic system should consist of at least two stages in which the first reactor is dedicated to the entrapment and hydrolysis of the incoming SS (Wang, 1994). The second stage may consist of an EGSB-type reactor. Because of the low hydrolytic activity under psychrophilic conditions, a similar system might also be feasible for low temperature industrial wastewater which is characterized by high SS concentrations. Recent pilot-research performed in a mattery shows that accumulating suspended solids deteriorated the granular sludge bed of an EGSB reactor (manuscript in preparation). As a result of the low metabolic rates. psychrophilic sludge stabilization proceeds at a very low rate. This means that. in addition to the SS concentration. the fraction non-acidified organic matter is a very important design criterion, since over-growth of acidogenic biomass should be prevented in a single stage reactor. Results of psychrophilic EGSB experiments performed with either a sucrose-VFA mixture or non-acidified maltery wastewater, show that the specific sludge load of non• acidified COD should not exceed 0.05-0.JO kg COD·kg VSS-I·day-I. Otherwise. the bulking acidifying sludge deteriorates the granular sludge bed (unpublished reSUlts). This value is about 5-10 times lower than the value found by Alphenaar (1994) for mesophilic conditions. Therefore, high rate psychrophilic anaerobic treatment of complex wastewater requires a biological process with more stages, or an integrated physical pretreatment combined with a methanogenic polishing stage. Further research is required to elucidate the most optimal process design for psychrophilic wastewater treatment in relation to the wastewater characteristics. The obtained results so far, show that an EGSB reactor profitably can be used for removing the soluble and merely acidified organic fraction from the wastewater, even at temperatures as low as 2°C. THERMOPHILIC CONDITIONS From Figure I it is clear that the other boundary of the mesophilic temperature range, i.e., 45°C. is much more critical for the existing methanogenic subpopulations than the border around 20°C. Generally. a temperature increase from the mesophilic to the thermophilic range immediately results in a shift in the methanogenic subpopulations as shown previously (Van Lier et al.• 1992). The latter easily can be explained by a rapid die-off of mesophilic organisms at temperatures exceeding the maximum growth temperature of these bacteria. Under thermophilic conditions there is no competition between mesophiles and thermophiles. Since reaction rates increase with temperatures (Figure I), significantly higher loading potentials and/or considerably shorter retention times are expected if anaerobic treatment is applied under thermophilic JWST 35: IO·H
J. B. VAN LIER et al.
204
conditions (Zinder, 1986). Therefore, thermophilic anaerobic treatment could be an attractive alternative particularly when the wastewater is discharged at high temperatures. In the past decades much research hru been conducted to study the feasibility of thermophilic treatment for various types of hot wastewaters mainly coming from food industries and pulp & paper industries, as reviewed recently (Van Lier, 1996). II most cases either UASB reactors or anaerobic filters (AF) were used, operated at high temperatures. The impacts of the high operation temperature on the anaerobic biology are generally not taken into account i~ the used process design. This will be discussed in more detail below. Limitations of thermophilic sin~le
sta~e hi~h-rate
reactors
One of the main reasons for applying thermophilic conditions in high-rate wastewater treatment reactors i! the expected much higher loading potential. However, conventionally designed thermophilic treatmenl systems can become limited by various biological and physical factors (Van Lier, 1996). For example, biomass retention might become critical in single stage sludge bed reactors if COD removal rates exceed 5060 kg-m- 3 reactor-day- I. Sludge separation might even be more critical under high temperature conditiom because of the lower liquid viscosity and the occasional occurrence of less stable thermophilic aggregates, The latter is principally attributed to the high mineralisation rate of thermophilic methanogenic sludge (see also Van Lier, 1996). In general, two approaches are followed to achieve a high biomass hold-up: i) auto· immobilization of anaerobic bacteria which leads to the formation of granular sludge; ii) adherence of the microorganisms to inert support media. The latter mechanism is essential in fixed film reactors such as anaerobic filters and fluidized bed reactors, while sludge granulation is characteristic for upflow (hybrid) sludge bed reactors. As mentioned above, AF and UASB received most interest in the various studies 011 thermophilic high-rate reactors. However, if immobilization of thermophilic biomass is difficult to achieve, the use of reactors with inert support material (fixed film reactors) should be reconsidered. Moreover, also the use of single stage sludge bed reactors is questionable, because high loading conditions ma~ subsequently result in a high wash-out of dispersed, and even granular, sludge. In such cases, the use 01 compartmentalized reactor systems in which the produced biogas is evenly withdrawn from the reactor is apparently more appropriate (Van Lier, 1996). Various authors pursue a similar approach even for the mesophilic temperature range, particularly if the formation of granular sludge does not proceed satisfactoril~ (Bachman et al., 1985; EI-Mamouni etal., 1992; Mark! and Reinhold, 1994). Another drawback of thermophilic digestion is the often found high effluent VFA concentrations (Zinder, 1986). However, the occurrence of bad-quality effluents might be attributed to the applied process technology rather than to the thermophilic digestion process itself. Due to the relatively high sensitivity of thermophilic methanogenic sludge for relatively low concentrations of intermediate compounds such as hydrogen and acetate, VFA concentrations easily rise in high loaded single stage reactor systems. Furthermore, the VFA build-up is enhanced by the high apparent Km of thermophilic granular sludge (Van Lier, 1996). Therefore, VFA concentrations will rise even more when the reactor is fed with very concentrated wastewater. It will be clear that thermophilic single stage reactor systems must be operated at relatively low loading rates in order to meet the conditions for a high VFA reduction. This, however, is contrasting one of the principal advantages of thermophilic treatment. Application of staged thermophilic reactor systems resulted in significant improvements with respect to the applicable loading capacity and the overall process stability. Interestingly, effluent VFA concentrations were found to be in the similar range as mesophilic UASB reactors (Van Lier, 1996). The compartmentalized upflow reactor, denominated as the upflow staged sludge bed (USSB) reactor, showed stable long-term reactor performance under extreme loading conditions. Within 3 months the organic loading rate of a reactor treating a sucrose-VFA mixture could be increased up to 100 kg COD-m-3-day··. The COD removal rate exceeded 90%. No significant wash-out of the thermophilic biomass was observed despite the extreme biogas loading rate of about 50 m3-m· 3 reactor-day' •. The main achievements of the 'plug·flow' reactor are: i)
very low effluent VFA concentrations under extreme loading conditions, i.e. 10·120 mg acetate COD-I'. and 200-500 mg propionate COD-I'.,
ii)
high degree of sludge retention due to very low turbulence in the final reactor compartment, and
High-rate anaerobic wastewater treatment
iii)
205
stable reactor performance due to the segregated development of methanogenic sludge along the reactor height. To maintain stable long-term performance, sludge segregation should be kept by specific sluicing of excess sludge (Van Lier et al., 1996)
The above findings are based on experiments performed in 5 I lab-scale reactors which were fed with sucrose-VFA mixtures. Presently, the feasibility of a thermophilic (55°C) staged reactor system is further investigated in the research laboratory of Paques B.V., The Netherlands, by using a liS I pilot reactor fed with beet-sugar distillery wastewater. Although the reactor is still under start-up conditions, the achieved results are very promising. At the moment of writing this paper, the reactor is treating organic loading rates between 20-30 kg COD om- 3oday-1 at HRTs of about 10 hours and COD removal efficiencies of 70-75%. The latter values agree with the maximum anaerobic biodegradability of the treated wastewater (results not shown). So far, effluent acetate concentrations can be kept below 200 mgol- I, while residual propionate is more or less in the same range. The biogas production rate is currently exceeding 10 m 3o m- 3 reactoroday-I. CONCLUSIONS Anaerobic wastewater treatment systems are generally developed for application in the mesophilic temperature range, i.e. between 25 and 40°C. However, literature results reveal that anaerobic conversion of organic matter effectively proceeds from below 5°C to temperatures as high as 75°C, which implies that, in principle, anaerobic wastewater treatment is feasible in the entire temperature range. In order to apply temperatures highr-r and/or lower than the mesophilic range, the process design of conventional anaerobic reactor systems requires various adjustments. The biogas production rate is considered to be very important with respect to the mixing characteristics as well as the biomass retention in anaerobic reactors. Therefore, the absence or excessive presence of biogas turbulence determines the most optimal reactor configuration. Under psychrophilic conditions, a sufficient sludge-water contact is a prerequisite for successful application of anaerobic treatment. Appropriate mixing conditions can be achieved by increasing the liquid upward velocity which results in an expansion of the sludge bed. In contrast, under thermophilic conditions, the extr.eme turbulence caused by the high biogas production, may lead to an excessive carry over of methanogemc sludge. This, however, can be prevented by the application of a staged process design in which the produced biogas is evenly withdrawn from the system. In addition, staging the thermophilic digestion process enhances distinctly the biodegradation of soluble compounds, due the reduction of possible inhibition effects. Gaseous toxic compounds are removed early by the first gas-liquid separator, while biodegradable soluble toxic compounds (e.g. intermediates) are hardly present in the final stage(s) of the system, reSUlting in a high degree of effluent polishing. It should be noted that also under psychrophilic conditions, a staged system can be advantageous, particularly whenever the wastewater is characterized by a high concentration of suspended solids or a high concentration of soluble non-acidified organic matter. Due to the extremely low hydrolysis rate at low temperatures, the methanogenic stage should be characterized by a very long solids retention time. ACKNOWLEDGEMENT This work was financially supported by the Ministry ofVROM, Novem-grant 51120/1510, The Netherlands, Paques B.V. Balk, The Netherlands and Bavaria B.V., Lieshout, The Netherlands. REFERENCES Alphenaar. P. A. (1994). Anaerobic granular sludge: characteri~atio~ and fac~ors affecting its functioning. Ph.D. thesis. Department of Environmental Technology. Agncultural Untvemty. Wagemngen. The Netherlands. Bachmann. A.• Beard. V. L. and McCarty. P. L. (1985). Performance and characteristics of the anaerobic baffled reactor. Wat. Res.• 19.99-106. De Man. A. W. A .• Van der Last. A. R. M. and Lettinga. O. (1988). The use of EGSB and UASB anaerobic systems for low strength soluble and complex wastewaters at temperatures ranging from 8 to 30°C. In: Anaerobic Digestion /988 (Adv. Wat. Pollut Control no. 5). E. R. Hall and P. N. Hobson (eds). Pergamon Press. pp. 197-209.
J. B. VAN LIER etal.
206
EI-Mamouni. R.• Rouleau. D .• Mayer. R.• Guiot. S. R. and Samson. R. (1992). Comparison of the novel multiplate anaerobic reactor with the upflow anaerobic sludge blanket reactor. In: Proc. of 46th Industrial Waste Conference. C.S. Dalton aO( R.F. Wukasch (Eds). Lewis Publishers. Chelsea Michigan. USA. pp. 681-687. Kato. T. M .• Field. J. A .• Versteeg. P. and Lettinga. G. (1994). Feasibility of Expanded granular sludge bed reactors for the anaerobic treatment oflow strength soluble wastewaters. Biotechnol. Bioeng.• 44.469-479. Lettinga. G. (I 99S). Anaerobic digestion and wastewater treatment systems. Antonie van Leeuwenhoek. 67. 3-28. Markl. H. and Reinhold. G. (1994). Der Biogas-Turmreaktors. ein neues Reaktorkonzept fUr die anaeroben Abwasserreinigung Chem.-Ing.-Tech.• 66(4). S34-S36 (in German). Rebac. S.• Ruskova. 1.. Gerbens. S .• Van Lier. J. B .• Starns. A. J. M. and Lettinga. G. (I99S). High-rate anaerobic treatment 0 wastewater under psychrophilic conditions. J. Ferment. and Biotechnol.• 80. 499-S06. Rebac. S.• Van Lier. 1. B.• 1anssen. M. G. 1.• Dekkers. F .• Swinkels K. T. M. and Lettinga. G. (1996). High-rate anaerobic treatment of malting wastewater in a pilot-scale EGSB system under psychrophilic conditions. J. Chem. Technol.
Biotechnol. (Accepted).
Speece. R. E. (1996). Anaerobic Biotechnology for Industrial Wastewaters. Archae Press. Nashville Tennessee. ISBN 0-96S02260-9. Van der Last, A. R. M. and Lettinga. G. (1992). Anaerobic treatment of domestic sewage under moderate climate (Dutch: conditions using upflow reactors at increased superficial velocities. Wat. Sci. Tech .• 25(7).167-178. Van Lier. J. B. (1996). Limitations of thermophilic anaerobic wastewater treatment and the consequences for process design Antonie van Leeuwenhoek, 69. 1-14. Van Lier. J. B .• Grolle. K. C. F .• Starns. A. 1. M.. Conway de Macario. E. and Lettinga. G. (1992). Start-up of a thermophilic Upflow Anaerobic Sludge Bed (UASB) reactor with mesophilic granular sludge. Appl. Microbiol. Biotechnol.. 37. J3().
J3S.
Van Lier. 1. B.• Groeneveld. N. and Lettinga. G. (1996). Development of thermophilic methanogenic sludge in compartmentalized upflow reactors. Biotechnol. Bioeng.. 50. II S-124. Wang. K. (1994). Integrated anaerobic and aerobic treatment of sewage. Ph.D. Thesis. Department of Environmental Technology. Agricultural University. Wageningen. The Netherlands. Zinder. S. H. (1986). Thermophilic waste treatment systems. In: Thermophiles: General. Molecular and Applied Biology. T. D Brock (Ed). Wiley-Interscience. New York. pp. 257-277.