P. Pullammanappallil

Protein degradation during anaerobic wastewater treatment: derivation of stoichiometry

The stoichiometry of reactions that describe protein degradation in anaerobic treatment systems were investigated. A methodology was developed to describe protein degradation to organic acids using a single reaction step. The reactions for individual amino acid fermentation and their mediating organisms were reviewed. The dominant fermentation pathways were selected based on a number of assumptions. Using the amino acid content of a model protein, it was then possible to determine stoichiometric coefficients for each major organic acid product in the overall degradation of the protein. The theoretical coefficients were then compared to those determined from two experimental runs on a continuously-fed, well-mixed, laboratory-scale anaerobic wastewater treatment system. In general, the coefficients compared well thus validating the use of a single reaction step for the overall catabolic reaction of protein degradation to organic acids. Furthermore, even when the protein concentration in feed or the feed flow rate was doubled, the amino acid fermentation pathways were found to occur predominantly by only one pathway. Although the choice of Stickland reactions over uncoupled degradation provided good comparisons, an electron balance showed that only about 40% of the amino acids could have proceeded coupled to other amino acid reactions. Uncoupled degradation of the remaining amino acids must have relied on the uptake of hydrogen produced from these reactions by hydrogen-consuming methane bacteria.

Degradation of unsorted municipal solid waste by a leach-bed process

In current landfills breakdown of municipal solid waste (MSW) occurs slowly and the landfill leaves a legacy of care, management, monitoring and potential catastrophic failure over several generations. Social concern over these long term issues, with their legislative and economic implementation, increasingly favour practices which promote short stabilisation times and minimise environmental impact. This paper describes experiments carried out on mixed and unsorted municipal solid waste (MSW) in which 75% of the rapidly biodegradable fraction was degraded in about 2 months with an average yield of 0.18 m(3) CH4/kg volatile solids at s.t.p. The experiments served to demonstrate that with proper leachate management very rapid decomposition of waste can be accomplished by taking the waste through a series of controlled degradation stages

Comparison of Thermophilic and Mesophilic One-Stage, Batch, High-Solids Anaerobic Digestion

The concept of starting up a batch, high-solids anaerobic digester by simply flooding the bed with a pH-buffer solution was tested using a mixture of vegetable waste and wood chips as feedstock at mesophilic (38° C) and thermophilic (55° C) conditions. At both temperatures stable and balanced methanogenesis was rapidly established within four days and was sustained until substrate was exhausted. Methanogenesis was more rapidly initiated in the thermophilic digester than in the mesophilic digester. Acetic, propionic and butyric acids accumulated in the leachate of both digesters during the start-up of digestion of uninoculated batch of waste. Thereafter all acids were degraded; which was quicker in the thermophilic digester. The accumulation and degradation of these acids was slower in the mesophilic digester. These studies showed that inoculum for carrying out thermophilic and mesophilic anaerobic digestion is readily available within the waste and its activity for complete mineralization of organic matter can be enhanced and sustained by providing adequate alkalinity. By employing a process in which anaerobic digestion of subsequent batches of waste was carried out by flooding with leachate drained from the digestion of a previous batch of waste, the volatile organic acid accumulation was maintained low and 95% of the methane yield potential of the waste was produced in 11 days under thermophilic conditions as opposed to 27 days under mesophilic conditions.

Evaluation of methanogenic activities during anaerobic digestion of municipal solid waste

Numerous researchers have demonstrated that the rate and extent of the degradation of municipal solid waste (MSW) can be enhanced beyond that observed in a conventional landfill by adding moisture, buffering agents and sources of microorganisms such as anaerobically digested sludge. One method of achieving the addition of these agents is by directing leachate that has trickled through a bed of anaerobically stabilised waste to beds of fresh MSW. Proper operational strategies need to be developed for successful implementation of this process on a large scale or in a landfill. Operational parameters of primary importance are the point of time at which a stabilised waste bed can be used for sequencing, the period of sequencing and the minimal amount of leachate that has to be recirculated to rapidly attain balanced microbial activity in a fresh waste bed. Assays that measure a substrate-specific methanogenic activity of an anaerobic microbial consortia have been previously developed by researchers. These assays were employed in this study to evaluate the microbial activity of the leachate for utilisation of substrates like cellulose, acetate and formate. Activity in leachate samples, taken from a batch of MSW at various times during the degradation process, was measured in terms of the amount of methane produced in 4 h after spiking the sample with one of the selected substrates. Activity resulting from the utilisation of formate and cellulose showed considerable promise as indicators for optimising operational strategies. It was observed that the formate degradation activity followed the methane production rate with both reaching a maximum at the same time and that this could be used as an indicator for determining the period of sequencing. Cellulase activity in fresh waste beds responded to flushes of mature leachate and peaked a few days after sequencing was terminated.

Cellulolytic activity in leachate during leach-bed anaerobic digestion of municipal solid waste

The degradation of municipal solid waste (MSW) under mesophilic conditions can be enhanced by exchanging leachate between fresh waste and stabilised waste. The optimum point in time when leachate from an anaerobically digesting waste bed can be used to initiate degradation of another waste bed might occur when the leachate of the digesting waste bed is highly active with cellulolytic and methanogenic bacteria. In this study, the cellulolytic activity of the leachate was measured using the cellulose-azure assay. As products of hydrolysis are soluble compounds, the rate of generation of these compounds was estimated based on a soluble chemical oxygen demand (SCOD) balance around the fresh waste bed. It was found that once the readily soluble material present in MSW was washed out there was very little generation of SCOD without the production of methane, indicating that flushing leachate from a stabilised waste bed resulted in a balanced inoculation of the fresh waste bed. With the onset of sustained methanogenesis, the rate of SCOD generation equalled the SCOD released from the digester as methane. The experimental findings also showed that cellulolytic activities of the leachate samples closely followed the trend of SCOD generation. reserved.

An operational strategy for improved biomethanation of cattle-manure slurry in an unmixed, single-stage digester

Abstract In the quiescent state, cattle-manure slurry stratifies into three discernible layers; a floating “scum” layer, a bottom “sludge” layer and a watery middle layer, with most of the biologically degradable component of the slurry being contributed by the particulate matter in these layers. The top layer has the highest biochemical methane potential and the middle layer the least. In a single-stage digester, an improved operational strategy to enhance methane production was determined. This strategy took advantage of the different rates of biomethanation and biochemical methane potentials of the different layers of the slurry. This was achieved by allowing the slurry to digest without mixing and discharging the digested material from the middle liquid layer rather than from the bottom or top layers.

A dynamic mathematical model for sequential leach bed anaerobic digestion of organic fraction of municipal solid waste

A mathematical model that describes the operation of a sequential leach bed process for anaerobic digestion of organic fraction of municipal solid waste (MSW) is developed and validated. This model assumes that ultimate mineralisation of the organic component of the waste occurs in three steps, namely solubilisation of particulate matter, fermentation to volatile organic acids (modelled as acetic acid) along with liberation of carbon dioxide and hydrogen, and methanogenesis from acetate and hydrogen. The model incorporates the ionic equilibrium equations arising due to dissolution of carbon dioxide, generation of alkalinity from breakdown of solids and dissociation of acetic acid. Rather than a charge balance, a mass balance on the hydronium and hydroxide ions is used to calculate pH. The flow of liquid through the bed is modelled as occurring through two zones-a permeable zone with high flushing rates and the other more stagnant. Some of the kinetic parameters for the biological processes were obtained from batch MSW digestion experiments. The parameters for flow model were obtained from residence time distribution studies conducted using tritium as a tracer. The model was validated using data from leach bed digestion experiments in which a leachate volume equal to 10% of the fresh waste bed volume was sequenced. The model was then tested, without altering any kinetic or flow parameters, by varying volume of leachate that is sequenced between the beds. Simulations for sequencing/recirculating 5 and 30% of the bed volume are presented and compared with experimental results

Effect of mixing on biomethanation of cattle-manure slurry.

The benefits and extent of mixing required during biomethanation of cattle-manure slurry was studied by investigating the effect of: 1) continuous and intermittent mixing; 2) agitator impeller speed and position; 3) not providing assisted mixing; 4) mixing on production of extracellular polymeric substances; and 5) mixing on the ultimate anaerobic biodegradability. Biomethanation was not adversely affected: during intermittent mixing; or when only sufficient mixing was provided to maintain off-bottom suspension of digester contents; or by doubling impeller speed. In fact continuous digestion of cattle-manure slurry without mechanical stirring was superior in terms of gas production. This can be attributed to increased loss of active volatile solids during stirring. Moreover, long-term batch digestion studies showed that the rate of biomethanation in a continuously stirred digester was inferior to that of a non-stirred one. Mixing was found to decrease production of extracellular polymeric substances (EPS). The presence of an increased level of EPS during the quiescent state could indicate increased attachment of cells to each other, resulting in larger agglomerates with better settling properties thus increasing biomass retention time.

Influence of co-substrates on structure of microbial aggregates in long-chain fatty acid-fed anaerobic digesters

Aims: The purpose of this study was to investigate the influence of co‐substrates, such as glucose and cysteine, on the structure of microbial aggregates in anaerobic digesters treating oleate, a long‐chain fatty acid (LCFA). u2028Methods and Results: Transmission electron microscopy (TEM) and confocal laser scanning microscopy (CLSM) were used to examine the structure of microbial aggregates. Fluorescence in situ hybridization (FISH) techniques were also used to characterize and localize the different trophic groups present in the aggregates. Oleate was found to inhibit the methanogenic activity and formation of granular biomass in digesters. The addition of co‐substrates, such as glucose and cysteine either singly or in combination, increased the methanogenic activity and formation of granular biomass. Glucose was more effective than cysteine in reducing the inhibition by oleate on the methanogenic bacteria and in enhancing the formation of granules. u2028Conclusions:u2002The addition of nutrient substrate, such as glucose and cysteine could decrease the toxicity of LCFA on anaerobic granulation.u2028Significance and Impact of the Study: The results suggest that the addition of other substrates might decrease the toxicity of LCFA on the granulation of biomass in anaerobic digesters and enhance methanogenic activity. A combination of TEM, CLSM and FISH techniques provides a better tool for visualizing microbial aggregates and for differentiating and localizing different microbial groups within these aggregates.

Enhanced degradation of waste grass clippings in one and two stage anaerobic systems.

The present work investigated the use of a simple rumen-fluid-inoculated anaerobic treatment system for the degradation of organic waste. Fresh rumen fluid collected from a fistulated sheep was used as the inoculum and fresh grass clippings were used as the waste material for treatment. Studies were carried out on both a one-stage system where the ligno-cellulosic fraction breaks down into a mixture of soluble products including volatile fatty acids and a two- stage system where these products are subsequently mineralised to biogas. In the one stage system about 70% of the organic waste was solubilized and in the two stage system about 60% waste material was solubilized in a week. About 50% of the degradation was achieved in a 4 day period, showing that a 4 day solids retention time would be a suitable operating regime. The maximum volatile fatty acid production rate was 327 mg COD l−1 h−1. A higher loading rate of 30 g l−1 d−1 was achieved in these systems compared to anaerobic digesters. Microbiological studies showed an increase in the number of fungal spores as well as a decrease in the number of protozoa in the treatment system. These numbers attained stable values over the duration of the experiments. The system developed is superior to conventional composting or anaerobic digestion and can be applied for the treatment of ligno-cellulosic agricultural residues.