A mathematical model for the interactive behavior of sulfate-reducing bacteria and methanogens during anaerobic digestion

Anaerobic degradation of waste involves different classes of microorganisms, and there are different types of interactions among them for substrates, terminal electron acceptors, and so on. A mathematical model is developed based on the mass balance of different substrates, products, and microbes present in the system to study the interaction between methanogens and sulfate-reducing bacteria (SRB). The performance of major microbial consortia present in the system, such as propionate-utilizing acetogens, butyrate-utilizing acetogens, acetoclastic methanogens, hydrogen-utilizing methanogens, and SRB were considered and analyzed in the model. Different substrates consumed and products formed during the process also were considered in the model. The experimental observations and model predictions showed very good prediction capabilities of the model. Model prediction was validated statistically. It was observed that the model-predicted values matched the experimental data very closely, with an average error of 3.9%.     

State of municipal solid waste management in Delhi, the capital of India

Delhi is the most densely populated and urbanized city of India. The annual growth rate in population during the last decade (1991-2001) was 3.85%, almost double the national average. Delhi is also a commercial hub, providing employment opportunities and accelerating the pace of urbanization, resulting in a corresponding increase in municipal solid waste (MSW) generation. Presently the inhabitants of Delhi generate about 7000tonnes/day of MSW, which is projected to rise to 17,000-25,000tonnes/day by the year 2021. MSW management has remained one of the most neglected areas of the municipal system in Delhi. About 70-80% of generated MSW is collected and the rest remains unattended on streets or in small open dumps. Only 9% of the collected MSW is treated through composting, the only treatment option, and rest is disposed in uncontrolled open landfills at the outskirts of the city. The existing composting plants are unable to operate to their intended treatment capacity due to several operational problems. Therefore, along with residue from the composting process, the majority of MSW is disposed in landfills. In absence of leachate and landfill gas collection systems, these landfills are a major source of groundwater contamination and air pollution (including generation of greenhouse gases). This study describes and evaluates the present state of municipal solid waste management in Delhi. The paper also summarizes the proposed policies and initiatives of the Government of Delhi and the Municipal Corporation of Delhi to improve the existing MSW management system.     

Bioleaching of heavy metals from sewage sludge: A review

During the treatment of sewage, a huge volume of sludge is generated, which is disposed of on land as soil fertilizer/conditioner due to the presence of nitrogen, phosphorus, potassium and other nutrients. However, the presence of toxic heavy metals and other toxic compounds in the sludge restricts its use as a fertilizer. Over the years, bioleaching has been developed as an environmentally friendly and cost-effective technology for the removal of heavy metals from the sludge. The present paper gives an overview of the various bioleaching studies carried out in different modes of operation. The various important aspects such as pathogen destruction, odor reduction and metal recovery from acidic leachate also have been discussed. Further, a detailed discussion was made on the various technical problems associated with the bioleaching process, which need to be addressed while developing the process on a larger scale.     

Shift to continuous operation of an air-cathode microbial fuel cell long-running in fed-batch mode boosts power generation

Microbial fuel cells (MFCs) which are operated in continuous mode are more suitable for practical applications than fed batch ones. The aim of the present study was to characterize an air-cathode MFC operating in continuous mode and to determine the intrinsic properties for suitable performance and scalability. Air-cathode MFCs were constructed from plexiglass with a total working volume of 220 mL. Zirfon® separator used in this MFC had cross section area of 100 cm². The air cathode MFCs were operated in fed-batch mode and then shifted to the continuous mode. To determine the behavior of anode and cathode in long term operation (274 days), their contribution in MFC performance was evaluated over time. Once the active biofilm was formed, power production and substrate consumption rate were significantly higher. The internal resistance increased with the passage of time. After stabilization of biofilm when the MFC was placed in close circuit by connecting an external resistance, the anode-reference and cathode-reference electrode behavior showed that anode potential is near to the bacterial cell inside potential. The maximum open circuit voltage achieved was 623 mV and the highest power and volumetric power density were 38.03 mW/m² and 1296 mW/m³, respectively.     

Autoheated thermophilic aerobic sludge digestion and metal bioleaching in a two-stage reactor system

A two-stage process has been developed for stabilization of sludge and removal of heavy metals from the secondary activated sludge with high rate of energy and time conservation.The first stage of the process involves autoheated thermophilic aerobic digestion at 55-60°C inoculated with less-acidophilic thermophilic sulfur-oxidizing microorganisms(ATAD).The results show that it is possible to maintain the autoheated conditions(55-60°C) in the ATAD reactor up to 24 hr,leading to reduction of 21% total solids(TS),27% volatile solids(VS),27% suspended solids(SS) and 33% volatile suspended solids(VSS) from the sludge.The sludge pH also decreased from 7 to 4.6 due to the activity of less-acidophilic thermophilic microorganisms.In the second stage operation,the digested sludge(pH 4.6,TS 31.6 g/L) from stage one was subjected to bioleaching in a continuous stirred tank reactor,operated at mean hydraulic retention times(HRTs) of 12,24 and 36 hr at 30°C.An HRT of 24 hr was found to be sufficient for removal of 70% Cu,70% Mn,75% Ni,and 80% Zn from the sludge.In all,39% VSS,76% Cu,78.2% Mn,79.5% Ni and 84.2% Zn were removed from the sludge in both the stages.     

A mathematical model for a hybrid anaerobic reactor

A mathematical model for a hybrid anaerobic reactor (HAR), which uses self-immobilized anaerobic bacterial granules under completely fluidized condition, has been developed. Stoichiometry of glucose fermentation into methane has been considered in this model. The model includes: (1) a biofilm model which describes substrate conversion kinetics within a single granule; (2) a bed fluidization model which describes the distribution of biogranules within the fluidized bed and (3) a reactor model which links the above two to predict the substrate and products concentration profile along the reactor height. Product and pH inhibition for each group of bacteria has been considered in the kinetic model. The spatial distribution of each group of anaerobic bacteria within granules has been found to play a vital role in bringing about the conversion. Experiments were conducted in the reactor using a synthetic effluent containing glucose as the carbon source to study the treatment efficiency. The model was simulated first assuming a 3-layered distribution [MacLeod, F.A., Guiot, S.R., Costerton, J.W., 1990. Layered structure of bacterial aggregates produced in an upflow anaerobic sludge bed and filter reactor. Applied and Environmental Microbiology 56, 1598-1607.] of anaerobic bacteria within granules and then homogeneous distribution [Grotenhuis, J.T.C., Smit, M., Plugge, C.M., Yuansheng, X., van Lammeren, A.A.M., Stams, A.J.M., Zehnder, A.J.B., 1991. Bacterial composition and structure of granular sludge adapted to different substrates. Applied and Environmental Microbiology 57, 1942-1949.] of anaerobic bacteria. The predictions of model simulation with the assumption of layered structure closely represented the experimental data.     

Modelling anaerobic biofilm reactors--a review

Anaerobic treatment has become a technically as well as economically feasible option for treatment of liquid effluents after the development of reactors such as the upflow anaerobic sludge blanket (UASB) reactor, expanded granular sludge bed (EGSB) reactor, anaerobic biofilter and anaerobic fluidized bed reactor (AFBR). Considerable effort has gone into developing mathematical models for these reactors in order to optimize their design, design the process control systems used in their operation and enhance their operational efficiency. This article presents a critical review of the different mathematical models available for these reactors. The unified anaerobic digestion model (ADM1) and its application to anaerobic biofilm reactors are also outlined.     

Effect of depth of landfill on the characteristics of soil-like material of aged waste: a case study of Bhalswa dumpsite,India

Journal of Material Cycles and Waste Management - This study characterizes the municipal solid waste (MSW) accumulated for more than 25 years at Bhalswa dumpsite, Delhi, India. 50...