aerobic thermophilic biodegradation of microbial cells

Appl Microbiol Biotechnol (1987) 25:568--576 Applied Microbiology

Biotechnology © Springer-Verlag 1987

Aerobic thermophilic biodegradation of microbial cells

Some effects of dissolved oxygen and temperature

C. Anthony Mason, Geoffrey Hamer, Thomas Fleischmann, and Candid Lang

Institute of Aquatic Sciences, Swiss Federal Institute of Technology Ziirich and Swiss Federal Institute for Water Resources and Water Pollution Control, Ueberlandstrasse 133, CH-8600 DObendorf, Switzerland

Summary. Results are presented comparing the extent of solubilization/biodegradation of whole yeast cells by mixed thermophilic bacterial cultures under conditions of oxygen excess and oxygen limitation. The process was most effective at a low dissolved oxygen concentration as suggested by solids removal data and by the production of often considerable quantities of carboxylic acids. The temperature optimum was also investigated and, under oxygen limited conditions, the most consistant results were obtained for operation at 65 °C, reflecting the true thermophilic nature of the process microbes. An operating temperature of 70°C probably exceeded the optimum for effective functioning of the thermophilic microbes and resulted in a less efficient process, whilst an operating temperature of 60°C was intermediate with respect to its effectiveness.

Introduction

Effective legislation by the governments of most West European countries with respect to pollution of the aquatic environment has resulted in the widespread installation of both municipal sewage and industrial wastewater treatment during the past 10--15 years (Hamer 1985). Conven- tional treatment technology comprises the me- chanical and physical separation of pollutants during preliminary and primary treatment and the bio-oxidation of pollutants in secondary aerobic biotreatment processes, most commonly of the ac- tivated sludge type. The principal by-product of treatment is waste sludge, comprising a complex mixture of suspended biodegradable, partially

Offprint requests to: G. Hamer

biodegradable and non-biodegradable solid matter and associated sorbed and dissolved pollutants. A significant fraction of waste sewage sludge comprizes microbial biomass, including some potentially pathogenic microbes present in the process feed and large quantities of process microbes from secondary treatment.

To solve the problem of sewage sludge hygienization, it has been suggested that anaerobic treatment be carried out at thermophilic temperatures, 55--60°C, in order to ensure the destruction of pathogenic organisms. However, pre- stressed concrete anaerobic digestors, designed for mesophilic operation, 30--37 ° C, and installed at many treatment plants, cannot be operated in the thermophilic temperature range. Hence, in Switzerland, West Germany and Austria, retros- pective installation of relatively short residence time, aerobic thermophilic pre-treatment/hygienization processes prior to anaerobic mesophilic digestion/stabilization processes is occurring (Hamer and Zwiefelhofer 1986). Such processes have a multiple role; the destruction (deactiva- tion) of pathogenic organisms, the solubilization of particulate biodegradable matter, particularly microbial biomass, and the bio-oxidation of sorbed pollutants, removed with solids during primary treatment, that are essentially recalcitrant to anaerobic biodegradation (McEvoy and Giger 1986).

In this contribution, the question of the effects of dissolved oxygen concentration and of temperature on the solubilization and biodegradation of microbial cells in aerobic thermophilic processes, when used for pretreatment is addressed. Solubili- zation of the biodegradable particulate fraction of waste sewage sludges is a prerequisite for com- plete treatment. Eastman and Ferguson (1981) re- cognized that the hydrolysis of particulate matter

C. A. Mason et al.: Aerobic thermophilic digestion: effects of DO,_ and temperature 569

to soluble substrates was the real rate limiting step in anaerobic digestion processes and recent ki- netic models for anaerobic digestion (Gujer and Zehnder 1983; Bryers et al. 1985) incorporate this concept. Because of the variability of real waste sewage sludges, it is necessary to select a source of microbial biomass that is of standardized and reproducible quality for experimental process studies. Here, pressed baker's yeast was used.

Materials and methods

Bioreactors and operatin 9 conditions. Laboratory scale bioreactors with operating volumes of 1.3 1, 6 1 and 9 1 each with full measurement instrumentation, i.e., pH, dissolved oxygen concentration, temperature and impeller speed monitoring were used for these experiments. The bioreactors were operated in the continuous flow mode and were continuously sparged with various air/nitrogen/oxygen mixtures at a volumetric flow rate of 0.005 s 1 so as to achieve either the oxygen excess or the oxygen limiting conditions defined subsequently. The impeller speeds used in the three bioreactors were respectively, 15, 7.5 and 10 s -1 for oxygen excess conditions and 15, 5 and 7.5 s -~ for oxygen limited conditions. The air flow rates to the spargers were varied to satisfy the oxygen availability regime required.

Aerobic thermophilic culture and feed. Aerobic thermophilic bacteria were obtained from an operating municipal waste sewage sludge aerobic thermophilic digestor operating at 60-- 62°C (UTB Umwelttechnik Buchs AG, Buchs/SG, Switzer- land). Inoculation was carried out by mixing equal volumes of aerobic thermophilic sludge with fresh bakers yeast preheated to thermophilic temperatures. Pressed bakers yeast as the sole biodegradable particulate carbon source was suspended in a nutrient solution containing NH4C1, 4 g - l - l ; KH2PO4, 8g.1 1;K,_HPO4,5.7g.I 1;ZnO, 3 .26mg. l -1 ;FeC13.6HzO, 43.2 mg. l -1 ; MnClz.4H20, 16mg. l l; CuCle-2H20, 1.36mg.1 i; COC12.6H20, 3 .Smg-l -~; H3BO4, 0 .5 rag- l - I ; MgC12, 0.2 g-I -~. The bioreactor feed was not sterilised and stored at 4°C for no longer than 5 days.

NH~--N. This was measured in the supernatants using an au- tomatic nitrogen analyzer (Skalar, Breda, The Netherlands).

Carboxylic acids. Low molecular weight carboxylic acids were analysed by acidifying the supernatants with formic acid and injecting into a Shimadzu GC-RIA gas chromatograph (Shi- madzu, Kyoto, Japan).

Scannin9 electron microscopy. Samples were freeze dried at - 8 0 ° C on mica, spatter coated with Au/Pd and viewed in a Hitachi S-700 (Nissai, Sangyo, Japan) scanning electron mi- croscope.

Chemicals. All chemicals were of analytical grade and sup- plied by either Fluka (Buchs/SG, Switzerland) or Merck (Darmstadt, FRG).

Results

The solubilization and subsequent biodegradation of whole yeast cells by a thermophilic aerobic mixed culture was investigated with respect to the effects of dissolved oxygen concentration and of temperature. Both thermophilic process microbes and substrate yeast cells can be seen in the scanning electron micrograph shown in Fig. 1. Some indication of morphological variation in the bacterial population together with partially lysed (ghost) yeast cells can be discerned in this electron micrograph. The results are shown in Figs. 2 to 9, and represent steady-state means of samples

Analytical

Dry weight. 5 ml samples were centrifuged at 30,000 9 and the supernatant removed. The solids were resuspended in water, poured into tared crucibles and heated overnight at 105°C. They were reweighed after cooling in a desiccator.

Ash/volatile suspended solids. After drying overnight, the crucibles used to determine dry weight were placed in an oven at 600°C for ca, 5 h, allowed to cool in a desiccator and then reweighed.

Dissolved oryanic carbon. Appropriate dilutions of the supernatant were made and the DOC assayed directly with a TO- COR 2 total organic carbon analyser (Maihak AG, Hamburg, FRG). Inorganic carbon was removed by acidification using concentrated HC1 and the excess CO2 removed by sparging with N2 for 12 min.

Fig. 1. Scanning electron micrograph showing the process microbes and the feed yeast cells during aerobic-thermophilic biotreatment. Several lysed (ghost) yeast cells are present composed only of cell wall debris

570 C.A. Mason et al.: Aerobic thermophilic digestion: effects of DO2 and temperature

80-

01 50"

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°l :~ 4o

~ o

~ 2o

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Fig. 2. Total suspended solids degraded as a percentage of the input total solids concentration for various residence times under a oxygen sufficient and b oxygen restricted conditions. Feed yeast cell concentrations between 30 and 40 g 1- ~ were used in these experiments. A lower concentration (20 g 1 -a) was used for 5 day residence times. Slight variations in the influent yeast cell concentration occurred as a result of the fact that the work reported is part of a process research programme that has been undertaken over the past 18 months, and some variation did occur. However, the conclusions drawn from the results should be independent of influent feed cell concentration. The data showing influent cell concentration has been omitted

taken daily over at least 7 days of steady state operation. It should be noted that slight variations in the influent yeast cell concentration occurred as a result of the fact that the work reported was part of a process research programme that was undertaken over some 18 months, and some changes were required. However, the conclusions drawn from the results are independent of the slight variations in influent feed cell concentrations that were used.

Figure 2 shows the extent of solids biodegraded under oxygen excess conditions and under oxygen limited conditions, where the former operating environment is defined as a dissolved oxygen level in excess of 30 percent of saturation with air at the temperature and pressure of operation and the latter environment defined as where the dissolved oxygen level was below 10 percent of saturation with air at the temperature and pressure of operation. The markedly heterogeneous nature of the process system dictates that at dissolved oxygen levels below 10 percent of saturation a distinct fraction of the process microbes in high intensity process environments will be subjected to oxygen limitation. Some fluctuation in the solubilisation/biodegradation efficiency can be seen by comparing the results for 1.2 and 1.5 day residence times under oxygen sufficient con-

ditions, but this might be explained by temporary malfunction of the pH control system during operation at 1.2 days. The data for 1.5 days were taken at a pH of 7.0 whilst those for 1.2 days were taken without pH measurement/control. Compar- ison between the extent of solubilization/biodegradation under oxygen sufficient conditions (Fig. 2a) with that achieved under oxygen limited conditions (Fig. 2b) shows that for most residence times studied, considerably greater solids removal is possible under the latter set of conditions.

Rationalising the variance in the amount of solubilization/biodegradation with different residence times provides the data presented in Fig. 3. Here, the total suspended solids removal rate is shown as a function of both dissolved oxygen concentration and residence time. Very low removal rates were found under oxygen excess conditions with the exception of that for the 1.2 day residence time. In contrast, the total suspended solids removal rate under oxygen limited conditions were, in general, much higher.

More distinct differences as a result of varying the dissolved oxygen concentration can be recog- nised in the dissolved organic carbon (DOC) con-

t en t (Fig. 4). Very little change in DOC occurs under oxygen excess conditions but significant increases in DOC occurred at all residence times

C. A. Mason et al.: Aerobic thermophilic digestion: effects of DOz and temperature 571

2O- A

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Fig. 3. Total suspended solids removal rate as a function of residence time under a oxygen sufficient and b oxygen restricted conditions

when oxygen was limited. The largest change in DOC occurred under oxygen limited conditions with a 1 day residence time. Figure 5 shows the carboxylic acid concentrations under oxygen limited conditions. No carboxylic acids were found under oxygen excess conditions. The amount of each acid produced varies with residence time, al-

though, clearly, acetate was produced in the high- est concentration.

The effects of temperature on process effectiveness were examined by operating at temperatures of 60°C, 65°C and 70°C. The reason for this selection was that optimised autothermal operation could be used to achieve such tempera-

I t ~

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Fig. 4. Dissolved organic carbon in feed yeast cells suspension (left hand bar of each pair) and in the bioreactor as a function of residence time under a oxygen sufficient and b oxygen restricted conditions

572 C.A. Mason et al.: Aerobic thermophilic digestion: effects of DOz and temperature

2 .5 -

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Fig. 5. Carboxylic acids measured in the bioreactor as a function of residence time under oxygen restricted conditions. No carboxylic acids could be detected under oxygen sufficient conditions

tures during technical scale operation and that such temperatures would allow rapid and effective hygienization. Bioreactor residence times were varied between 1.5 and 5 days at each temperature with the exception of 70 ° C, where only 1.5 and 3.0 day residence times were examined. The combined results are shown in Figs. 6 to 9.

More consistent results were achieved for operation at 65 °C compared with the other temperatures investigated (Fig. 6). A residence time of 1.5 days at 70°C resulted in very poor solubiliza-

t ion/biodegradation of the microbial cells, whilst extreme solubilization/biodegradation occurred with a 5 day residence time at 60 ° C. However, the solids removal rate under the latter conditions was low when compared to removal rates at shorter residence times (Fig. 7). The maximum substrate removal rate ( 5 . 4 g l - l d - a ) was achieved using 65°C and a 1.5 day residence time, a slightly lower substrate removal rate re- suited at the same residence time at 60°C (4.9 g 1-1 d-1). Operation at 3 days residence time and 65°C also resulted in a high rate of substrate removal (4.0 g 1-1 d-1).

N H + - - N accumulated under all conditions, although at short residence times this was hardly detectable (Fig. 8). Ammonia production is indicative of deamination of the nitrogen containing fraction of the biomass and thus an increase in NH ~ - - N content should reflect the extent of solubilization. This process probably occurs some- what slower than the overall solubilization of the organic matter, hence the low level of accumulation of NH ] - - N at short residence times. Some N H ~ - - N will also be lost by stripping in the ex- haust gases as a result of inefficient condenser operation and thus the measured concentrations can only be considered indicative of effective solubilization.

An increase in the level of dissolved organic carbon was found under all process operating conditions (Fig. 9). Accumulation of dissolved or-

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!i::i!i::i:!i!~iiii:iiiiiiii!ii 3.0 5.0

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Fig. 6. Amount of total suspended solids solubil ised/biodegraded as a percentage of the feed yeast cell concentration during aerobic thermophilic biotreatment at a 60 ° C, b 65°C and e 70°C

C. A. Mason et al.: Aerobic thermophilie digestion: effects of DOz and temperature 573

B- a

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Fig. 7. Total suspended soids removal rate as a function of both residence time and temperature during oxygen limited aerobic thermophilic biotreatment at a 60°C, b 65°C and e 70°C

ganic carbon occurs primarily as a result of carboxylic acid production as a consequence of fer- mentative mode of metabolism of the facultative anaerobic fractions of the thermophilic process culture at low dissolved oxygen concentrations

where oxygen limitation occurred. Whilst very low concentrations of carboxylic acids could be found when operating at 60°C and 3 day residence time, it is probable under this set of operating conditions oxygen was not limiting growth.

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Fig. 8. N H ~ - - N concentration in the feed (left hand bar of each pair) and after aerobic thermophilic biotreatment (right hand bar) at a 60 °C, b 65°C and e 70°C

574 C.A. Mason et al. : Aerobic thermophilic digestion: effects of DO2 and temperature

A T ~n 3 -

0 .o_

0

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1.5 3.0 5.0 Residence Time (d)

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Fig. 9. Dissolved organic carbon in feed yeast cells suspension (left hand bar of each pair) and after solubilisation/biodegradation (right hand bar) at a 60 ° C, b 65°C and c 70°C

Discussion

The results of the comparison of oxygen restricted conditions and oxygen sufficient conditions on the solubilisation/biodegradation of yeast cells under thermophilic conditions clearly shows that oxygen limited conditions are more effective, both in terms of the amount of solids removed and in the rate of removal. Indeed, at economi- cally feasable residence times for sludge in a pre- treatment process, 0.6--1 days, extremely high solids removal rates were demonstrated. The same complex mixed culture was used for both oxygen excess and oxygen limited conditions, as defined earlier, and whilst process conditions were changed at random without adverse effect on culture activity, it is likely that the culture was domi- nated by facultative anaerobic bacteria.

The growth of thermophilic bacteria on yeast cells requires the production of specific yeast cell wall degrading exo-enzymes by the bacteria. In the absence of these specific exo-enzymes no de- gradation can take place. The outermost layer of the cell wall of Saccharomyces cerevisiae (bakers yeast) is largely composed of polysaccharides, typically glucan-mannan polymers, together with small quantities of protein and lipids. Once this physical barrier has been disrupted, a pool of soluble organic compounds (proteins, amino acids, nucleic acids, etc.) are made available for utiliza-

tion by the thermophilic bacteria. These soluble compounds are, after further hydrolysis, the pre- ferred substrate for the bacteria and whilst they are available for assimilation, residual cell wall material remains largely intact (see Fig. 1).

The production of specific exo-enzymes de- pends upon the environment to which the process microbes are subjected. Temperature, pH, dissolved oxygen concentration and growth rate play a significant role in the control of production. Due to the diversity of soluble substrates released by cell breakage in aerobic thermophilic sludge treatment, a complex mixed microbial community must be expected to develop. This then implies that the process culture will have an extensive potential biodegradative capacity and also, the pos- sibility exists that positive interactions such as the removal of inhibitory by-products and the estab- lishment of nutrient gradients can occur. Detailed information concerning mixed substrate utilisa- tion in continuous flow bioreactors is largely restricted to the case of monospecies cultures and only few exceptions exist (Wilkinson and Hamer 1979). In a study on bacteria isolated from an aerobic thermophilic treatment process for waste sewage sludge, Sonnleitner and Fiechter (1983a) found a range of thermophilic strains of which most were identified as Bacillus spp. Of these, less than 10% were unable to degrade starch and less than 5% were unable to degrade casein. Extracel-

C. A. Mason et al.: Aerobic thermophilic digestion: effects of DO2 and temperature 575

lular enzyme production, particularly protease and polysaccharase production, are well known features of Bacillus spp. (Norris et al. 1981).

The caldoactive microbes tend to be more fas- tidious with respect to their nutritional require- ments for optimum growth (Sonnleitner and Fiechter 1983b) such that temperatures in excess of 70°C are unlikely to be suitable for this process. Additionally, Brock (1969) has pointed out that genuine thermophiles, as opposed to thermotolerant mesophiles are optimally adapted for growth at elevated temperatures and are not struggling for survival at such temperatures. Thus, it is centre range thermophilic microbes that should be considered for application in this type of process (Hamer and Bryers 1985).

Thermophilic bacteria are known to metabol- ise a wide range of substrates including common biopolymers, sugars, polypeptides, amino acids, carboxylic acids, aliphatic and aromatic hydro- carbons, etc. (Zeikus 1979). Thus it is not surpris- ing that very good solubilization and biodegradation can be achieved using mixed thermophilic cultures.

Given the importance of exo-enzyme production for the solubilization and biodegradation processes of microbial solids in waste sludge treatment, the factors regulating their production must be considered. Microbes have evolved to oc- cupy diverse environmental niches and a spec- trum of affinities by microbes for oxygen exists. Under anaerobic conditions, microbial biomass yield coefficients are typically lower than under aerobic conditions, a feature which obviously benefits anaerobic sludge treatment by minimis- ing biomass production. The results indicate that enhanced exo-cellular enzyme production occurs under oxygen limited conditions. This conclusion is supported by the observation that amylase production is induced in oxygen limited shake flask cultures of thermophilic Bacilli derived from thermophilic sludge (Grueninger et al. 1984).

During oxygen limited growth, organic compounds frequently serve as electron acceptors in the metabolic pathways of facultative anaerobic microbes and this results in considerable carboxylic acid and other product formation and a low biomass yield coefficient. Excess production of carboxylic acids is not an undesirable feature in aerobic thermophilic sludge treatment processes when such processes are coupled with an anaerobic treatment process as a second stage.

Unfortunately the degree of autolysis of bakers yeast that occurs under the physical process environments considered in this investigation can-

not be realistically determined. Bakers yeast is not an aseptic product and is always contaminated with bacteria. Sterilization of the yeast as a substrate by either heat or chemical means, would in- validated any study of live microbe biodegradation, the primary objective of this investigation. Hence, in any experiment to assess autolysis, the actions of contaminating bacteria cannot be elimi- nated.

The physical effects of increasing temperature in bioreactors are also important. Higher temperatures increase the vapour pressure, diffusivity and ionization of many compounds, and decrease viscosity and gas solubility (Zeikus 1979). Hamer and Bryers (1985) showed, on the basis of work by Scheibel (1954), that the oxygen diffusion coefficient in water increases markedly with increasing temperature and the increase largely compensates for the reduction in oxygen solubility with increasing temperatures. Thus, both the physiologi- cal properties of microbes and the variability of the physical properties of their environment dic- tate the optimum conditions for effective operation of heterogenous process systems.

Conclusions

The primary implication of the results is that aerobic thermophilic pre-treatment processes for waste sludges function best under oxygen limited conditions and at temperatures between 60 and 65 ° C. Some aerobic thermophilic sludge pretreatment/hygienisation plants already functioning have been reported to operate between 44°C and 55°C (Gould and Drnevich 1978; Jewell and Ka- brick 1980; Wolinski 1985) and temperatures between 60°C and 70°C are also used (Zwiefel- hofer 1985). At these latter temperatures, genuine thermophilic process microbes, as opposed to the thermotolerant mesophiles, are able to develop and assert their potential. Similarly, it is probable that many technical-scale plants already function at limiting dissolved oxygen concentrations, as suggested by high oxygen conversion data, even though process designers have sought to achieve oxygen excess conditions in aerobic thermophilic bioreactors (e.g. Match and Drnevich 1977) thereby ignoring the effective functioning of the mixed aerobic/facultative anaerobic nature of the process cultures that develop in such systems.

Acknowledgements. We would like to thank Dr. E. Webrli for the electron microscopy. This work was supported by a grant from the Swiss National Programme 7D.

576 C. A. Mason et al.: Aerobic thermophilic digestion: effects of DO2 and temperature

References

Brock TD (1969) Microbial growth under extreme conditions. In: Meadow P, Pirt SJ (eds) Microbial growth. Proceedings 19th Symp Soc Gen Microbiol. Cambridge University Press, Cambridge, p 15

Bryers JD, Berger J, Hamer G (1985) Interpretation of thermophilic anaerobic digestion experiments using a dynamic structured model. Proc IWTUS-3 Conserv and Recyc 8:251--266

Eastman JA, Ferguson JF (1981) Solubilization of particulate organic carbon during the acid phase of anaerobic digestion. J Wat Poll Cont Fed 53:352--366

Gould MS, Drnevich RF (1978) Autothermal thermophilic aerobic digestion. Env Engng Div Proc of Am Soc Civ Engnrs 104:259--270

Grueninger H, Sonnleitner B, Fiechter A (1984) Bacterial diversity in thermophilic aerobic sewage sludge. I l I A source of organisms producing heat-stable industrially useful enzymes, e.g., c~-amylases. Appl Microbiol Biotechnol 19:414--421

Gujer W, Zehnder AJB (1983) Conversion processes in anaerobic digestion. Wat Sci Tech 15:127--167

Hamer G (1985) The impact of government legislation on industrial effluent treatment. Proc IWTUS-3 Conserv and Recyc 8:25--43

Hamer G, Bryers JD (1985) Aerobic thermophilic sludge treatment -- some technological concepts, Proc IWTUS-3 Con- serv and Recyc 8:267--284

Hamer G, Zwiefelhofer HP (1986) Aerobic thermophilic hygienization -- a supplement to anaerobic mesophilic waste sludge digestion. IChemE Symposium series 96:163--180

Jewell WJ, Kabrik RM (1980) Autoheated aerobic thermophilic digestion with aeration. J Wat Poll Cont Fed 52:512--523

Matsch LC, Drnevich RF (1977) Autothermal aerobic digestion. J Wat Poll Cont Fed 49:296--310

McEvoy J, Giger W (1986) Determination of linear alkylben- zenesulfonates in sewage sludge by high-resolution gas chromatography/mass spectrometry. Environ Sci Technol 20:376--383

Norris JR, Berkely RCW, Logan NA, O'Donnell AG (1981) The Genera Bacillus and Sporolaetobacillus. In: Starr MP, Stolp H, TrOper HG, Balonos A, Schlegel HG (eds) The Prokaryotes -- a handbook on habitats, isolation and iden- tification of bacteria. Springer, Berlin, Heidelberg, New York, p 1711

Scheibel EG (1954) Liquid Diffusivities. Industr Engng Chem 46:2007--2008

Sonnleitner B, Fiechter A (1983a) Bacterial diversity in thermophilic aerobic sewage sludge -- II. Types of organism and their capacities. Appl Microbiol Biotechnol 18:174--180

Sonnleitner B, Fiechter A (1983b) Advantages of using thermophiles in biotechnological processes: expectations and reality. Trends in Biotechnol 1:74--80

Wilkinson TG, Hamer G (1979) The microbial oxidation of mixtures of methanol, phenol, acetone and isopropanol with reference to effluent purification. J Chem Tech Bio- technol 29:56--67

Wolinski WK (1985) Aerobic thermophilic sludge stabilisation using air. Wat Poll Cont 84:433--445

Zeikus JG (1979) Thermophilic bacteria: ecology, physiology and technology. Enzyme Microb Technol 1:243--252

Zwiefelhofer HP (1985) Aerobic-thermophilic/anaerobic mesophilic two stage sewage sludge treatment: practical ex- periences in Switzerland. Proc IWTUS-3. Conserv and Re- cyc 8:285--302

Received July 17, 1986/Revised November 27, 1986

aerobic thermophilic biodegradation of microbial cells

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