Methanogenesis in an Upflow Anaerobic Sludge Blanket Reactor at pH 6 on an Acetate-Propionate Mixture

High-rate anaerobic digestion can be applied in upflow anaerobic sludge blanket reactors for the treatment of various wastewaters. In upflow anaerobic sludge blanket reactors, sludge retention time is increased by a natural immobilization mechanism (viz. the formation of a granular type of sludge). When this sludge is cultivated on acid-containing wastewater, the granules mainly consist of an acetoclastic methanogen resembling Methanothrix soehngenii. This organism grows either in rods or in long filaments. Attempts to cultivate a stable sludge consisting predominantly of Methanosarcina sp. on an acetate-propionate mixture as substrate by lowering the pH from 7.5 during the start-up to approximately 6 failed. After 140 days of continuous operation of the reactor a filamentous organism resembling Methanothrix soehngenii prevailed in the sludge. The specific methanogenic activity of this sludge on acetate-propionate was optimal at pH 6.6 to 6.8 and 7.0 to 7.2, respectively.     

Feasibility of expanded granular sludge bed reactors for the anaerobic treatment of low-strength soluble wastewaters

The application of the expanded granular sludge bed (EGSB) reactor for the anaerobic treatment of low-strength soluble wastewaters using ethanol as a model substrate was investigated in laboratory-scale reactors at 30oC. Chemical oxygen demand (COD) removal efficiency was above 80% at organic loading rates up to12 g COD/L . d with influent concentrations as low as 100 to 200 mg COD/L. These results demonstrate the suitability of the EGBS reactor for the anaerobic treatment of low-strength wastewaters. The high treatment performance can be attributed to the intense mixing regime obtained by high hydraulic and organic loads. Good mixing of the bulk liquid phase for the substrate-biomass contact and adequate expansion of the substrate-biomass contact and adequate expansion of the sludge bed for the degassing were obtained when the liquid upflow velocity (V(up)) was greater than 2.5 m/h. Under such conditions, an extremely low apparent K(s) value for acetoclastic methanogenesis of 9.8 mg COD/L was observed. The presence of dissolved oxygen in the wastewater had no detrimental effect on the treatment performance. Sludge piston flotation from pockets of biogas accumulating under the sludge bed occurred at V(up) lower than 2.5 m/h due to poor bed expansion. This problem is expected only in small diameter laboratory-scale reactors. A. more important restriction of the EGSB reactor was the sludge washout occurring at V(up) higher than 5.5 m/h and which was intensified at organic loads higher than 7 g COD/L. d due to buoyancy forces from the gas production. To achieve an equilibrium between the mixing intensity and the sludge hold-up, the operation should be limited to an organic loading rate of 7 g COD/L d. and to a liquid up-flow velocity between 2.5 and 5.5 m/h (c) 1994 John Wiley & Sons, Inc.     

Anaerobic degradation of volatile fatty acids at different sulphate concentrations

The effect of sulfate on the anaerobic breakdown of mixtures of acetate, propionate and butyrate at three different sulfate to fatty acid ratios was studied in upflow anaerobic sludge blanket reactors. Sludge characteristics were followed with time by means of sludge activity tests and by enumeration of the different physiological bacterial groups. At each sulfate concentration acetate was completely converted into methane and CO₂, and acetotrophic sulfate-reducing bacteria were not detected. Hydrogenotrophic methanogenic bacteria and hydrogenotrophic sulfate-reducing bacteria were present in high numbers in the sludge of all reactors. However, a complete conversion of H₂ by sulfate reducers was found in the reactor operated with excess sulfate. At higher sulfate concentrations, oxidation of propionate by sulfate-reducing bacteria became more important. Only under sulfate-limiting conditions did syntrophic propionate oxidizers out-compete propionate-degrading sulfate reducers. Remarkably, syntrophic butyrate oxidizers were well able to compete with sulfate reducers for the available butyrate, even with an excess of sulfate. Correspondence to: A. Visser     

Anaerobic bioprocessing of organic wastes

Anaerobic digestion of dissolved, suspended and solid organics has rapidly evolved in the last decades but nevertheless still faces several scientific unknowns. In this review, some fundamentals of bacterial conversions and adhesion are addressed initially. It is argued in the light of G-values of reactions, and in view of the minimum energy quantum per mol, that anaerobic syntrophs must have special survival strategies in order to support their existence: redistributing the available energy between the partners, reduced end-product fermentation reactions and special cell-to-cell physiological interactions. In terms of kinetics, it appears that both reaction rates and residual substrate thresholds are strongly related to minimum G-values. These new fundamental insights open perspectives for efficient design and operation of anaerobic bioprocesses. Subsequently, an overview is given of the current anaerobic biotechnology. For treating wastewaters, a novel and high performance new system has been introduced during the last decade; the upflow anaerobic sludge blanket system (UASB). This reactor concept requires anaerobic consortia to grow in a dense and eco-physiologically well-organized way. The microbial principles of such granular sludge growth are presented. Using a thermodynamic approach, the formation of different types of aggregates is explained. The application of this bioprocess in worldwide wastewater treatment is indicated. Due to the long retention times of the active biomass, the UASB is also suitable for the development of bacterial consortia capable of degrading xenobiotics. Operating granular sludge reactors at high upflow velocities (5–6 m/h) in expanded granular sludge bed (EGSB) systems enlarges the application field to very low strength wastewaters (chemical oxygen demand < 1 g/l) and psychrophilic temperatures (10°C). For the treatment of organic suspensions, there is currently a tendency to evolve from the conventional mesophilic continuously stirred tank system to the thermophilic configuration, as the latter permits higher conversion rates and easier sanitation. Integration of ultrafiltration in anaerobic slurry digestion facilitates operation at higher volumetric loading rates and at shorter residence times. With respect to organic solids, the recent trend in society towards source separated collection of biowaste has opened a broad range of new application areas for solid state anaerobic fermentation.W. Verstraete and D. de Beer are with the Center for Environmental Sanitation, University of Gent, Coupure L 653, B-9000 Gent, Belgium; D. de Beer is also with the Max Plank Institut für Marine Mikrobiologie-Microzensor Group, Fahrenstrasse 1, 28359 Bremen, Germany. M. Pena is with the Groupo de Biotechnologia Ambiental, Departamento de Ingenieria Quimica, Universidad de Valladolid, Prado de la Magdalena, 47005 Valladolid, Spain. G. Lettinga is with the Department of Environmental Technology, Wageningen Agricultural University, Bomenweg 2, 6703 HD Wageningen, The Netherlands. P. Lens is with the Environmental Research Unit. Department of Microbiology, University College Galway, Galway, Ireland.     

Start-up of a thermophilic upflow anaerobic sludge bed (UASB) reactor with mesophilic granular sludge

Summary Fast start-up of thermophilic upflow anaerobic sludge bed (UASB) reactors was achieved at process temperatures of 46, 55 and 64° C, using mesophilic granular sludge as inoculum and fatty acid mixtures as feed. The start-up was brought about by increasing the temperature of mesophilic UASB reactors in a single step, which initially led to a sharp drop in the methane production rate. Thereafter, stable thermophilic methanogenesis was achieved within a period of 1 or 2 weeks depending on the temperature of operation. Mesophilic granules functioned initially as effective carrier material for thermophilic organisms. However, long-term operation led to disintegration of the granules, resulting in wash-out of thermophilic biomass. The temperature optima for acetotrophic methanogenic activity of the sludges cultivated at 46, 55 and 64° C, were similar, but differed significantly from the temperature optimum of the mesophilic inoculum. All the sludges examined were dominated by Methanothrix-like rods. These could be distinguished by antigenic fingerprinting into two subpopulations, one predominant at 36° C and the other predominant at 46° C and above. Offprint requests to: J. B. van Lier     

Continuous anaerobic treatment of autoxidized bark extracts in laboratory-scale columns

Debarking wastewaters of the forest industry contain high concentrations of tannins that are inhibitory to methane bacteria. The tannins can be polymerized to nontoxic colored compounds by the applications of an autoxidation pretreatment, enabling the anaerobic treatment of easily biodegradable components in the wastewater. The continuous anaerobic treatment of untreated and autoxidized pine bark extract was studied in laboratory-scale columns packed with a granular sludge bed. The autoxidation doubled the conversion efficiency of bark extract COD to methane (from 19 to 40%). After 5 months of operation, anaerobic treatment of the autoxidized extracts was feasible at high influent concentrations (14 g COD/L) and loading rates (26 g biodegradable COD/L . d) with 98% elimination of the biodegradable fraction. The detoxification pretreatment polymerized the toxic tannins to poorly biodegradable high molecular weight tannins and humic compounds which were not eliminated during anaerobic treatment. Although the original tannins of the untreated extract were eliminated by 60%, they were not biodegraded to volatile fatty acids and methane but instead were transformed to phenolic degradation intermediates (phenol, p-cresol, 3-phenyl-propionate, and carboxycyclohexane). Therefore, the autoxidation pretreatment did not decrease the content of readily biodegradable substrates which accounted for 53% of the extract COD. The recalcitrant COD expected in the effluents of reactors treating autoxidized debarking waste-water can be effectively separated by calcium precipitation prior to anaerobic treatment.     

Use of the upflow sludge blanket (USB) reactor concept for biological wastewater treatment,especially for anaerobic treatment

In recent years considerable effort has been made in the Netherlands toward the development of a more sophisticated anaerobic treatment process, suitable for treating low a strength wastes and for applications at liquid detention times of 3–4 hr. The efforts have resulted in new type of upflow anaerobic sludge blanket (UASB) process, which in recent 6 m³ pilot-plant experiments has shown to be capable of handling organic space loads of 15–40 kg chemical oxygen demand (COD)·m^?3/day at 3–8 hr liquid detention times. In the first 200 m³ full-scale plant of the UASB concept, organic space loadings of up to 16 kg COD·m^?3/day could be treated satisfactorily at a detention times of 4 hr, using sugar beet waste as feed. The main results obtained with the process in the laboratory as well as in 6 m³ pilot plant and 200 m³ full-scale experiments are presented and evaluated in this paper. Special attention is given to the main operating characteristics of the UASB reactor concept. Moreover, some preliminary results are presented of laboratory experiments concerning the use of the USB reactor concept for denitrification as well as for the acid formation step in anaerobic treatment. For both purposes the process looks feasible because very satisfactory results with respect to denitrification and acid formation can be achieved at very high hydraulic loads (12 day^?1) and high organic loading rates, i.e., 20 kg COD·m^?3/day in the denitrification and 60–80 kg COD·m^?3/day in the acid formation experiments.     

Kinetics and mass-transfer phenomena in anaerobic granular sludge

The kinetic properties of acetate-degrading methanogenic granular sludge of different mean diameters were assessed at different up-flow velocities (V(up)). Using this approach, the influence of internal and external mass transfer could be estimated. First, the apparent Monod constant (K(S)) for each data set was calculated by means of a curve-fitting procedure. The experimental results revealed that variations in the V(up) did not affect the apparent K(S)-value, indicating that external mass-transport resistance normally can be neglected. With regard to the granule size, a clear increase in K(S) was found at increasing granule diameters. The experimental data were further used to validate a dynamic mathematical biofilm model. The biofilm model was able to describe reaction-diffusion kinetics in anaerobic granules, using a single value for the effective diffusion coefficient in the granules. This suggests that biogas formation did not influence the diffusion-rates in the granular biomass.     

Cluster structure of anaerobic aggregates of an expanded granular sludge bed reactor

The metabolic properties and ultrastructure of mesophilic aggregates from a full-scale expanded granular sludge bed reactor treating brewery wastewater are described. The aggregates had a very high methanogenic activity on acetate (17.19 mmol of CH(4)/g of volatile suspended solids [VSS].day or 1.1 g of CH(4) chemical oxygen demand/g of VSS.day). Fluorescent in situ hybridization using 16S rRNA probes of crushed granules showed that 70 and 30% of the cells belonged to the archaebacterial and eubacterial domains, respectively. The spherical aggregates were black but contained numerous whitish spots on their surfaces. Cross-sectioning these aggregates revealed that the white spots appeared to be white clusters embedded in a black matrix. The white clusters were found to develop simultaneously with the increase in diameter. Energy-dispersed X-ray analysis and back-scattered electron microscopy showed that the whitish clusters contained mainly organic matter and no inorganic calcium precipitates. The white clusters had a higher density than the black matrix, as evidenced by the denser cell arrangement observed by high-magnification electron microscopy and the significantly higher effective diffusion coefficient determined by nuclear magnetic resonance imaging. High-magnification electron microscopy indicated a segregation of acetate-utilizing methanogens (Methanosaeta spp.) in the white clusters from syntrophic species and hydrogenotrophic methanogens (Methanobacterium-like and Methanospirillum-like organisms) in the black matrix. A number of physical and microbial ecology reasons for the observed structure are proposed, including the advantage of segregation for high-rate degradation of syntrophic substrates.     

Mesophilic and thermophilic activated sludge post treatment of anaerobic effluent. Sludge and wastewater characterisation using batch experiments

Anaerobic pretreated paper process water was characterized interms of readily biodegradable, slowly biodegradable, very slowly biodegradable and inert wastewaterfractions under mesophilic and thermophilic conditions. The anaerobic pretreated paper process water containeda relatively high amount of slowly biodegradable components and few easily biodegradable componentsas indicated by the ratio of short term BOD over the BOD5. Wastewater readily biodegradable COD, determinedas short term BOD, was almost similar when measured under both temperature conditions. Fractions ofslowly biodegradable COD and inert COD of the same wastewater were found to depend on the type of biomassinvolved in the test. Thermophilic aerobic biomass was not able to degrade the wastewater to the sameextent as the mesophilic biomass resulting in higher apparent inert COD levels. Furthermore, wastewater colloidalCOD did not flocculate under thermophilic conditions and was thus not removed from the liquid phase.