Biogas and biohydrogen production potential of high strength automobile industry wastewater during anaerobic degradation

Highlights

• Anaerobic treatment of auto industry wastewater was examined.

• High strength auto industry wastewater after pre-treatment was biodegradable.

• Co-substrates did not contribute to the biodegradability of this wastewater.

• No hydrogen could be produced using this wastewater as substrate.

Abstract

High strength automobile industry wastewater, collected from decanters (DECA) of the pre-treatment plant after oil, grease and sludge separation, was investigated for production of methane in the absence and presence of glucose or excess aerobic sludge (AS) from a lab scale suspension reactor as co-substrates. The highest methane production from DECA wastewater was 335.4 L CH₄/kg COD_soluble removal which decreased in the presence of the co-substrates to 232.5 (with 2 g/L glucose) and to 179 (with 40% AS) L CH₄/kg COD_soluble removal, respectively. Around 95% of total methane was produced within 5 days of incubation of DECA at 37 °C when no co-substrate was added. Addition of co-substrates did not improve biodegradation of DECA but overall methane production from DECA + co-substrates was increased due to co-substrate biodegradation. The anaerobic inoculum, capable of producing 2.4 mol of hydrogen/mol of glucose under zinc induced inhibitory conditions, was unable to produce hydrogen from DECA as substrate under the same conditions.

Introduction

Despite the high fuel prices, automotive industry is ever growing because of the demand for new models and also due to the rising Asian markets. In 2011 about 80.3 million passenger cars and commercial vehicles were manufactured worldwide (VDA, 2012). Because of the stringent environmental regulations, industries generating solid, liquid or gaseous wastes need to employ financially feasible and effective waste treatment before final discharge at permissible levels. For more effective treatment technologies, industries often employ source separation of pollutants resulting in a consequent reduction of pollutant loads in waste streams. In automobile industry, washing and paint shop sections are the main sources of wastewater discharges, which are rich in oil, grease, heavy metals and chemicals. Due to batch processing in washing and paint shops, waste streams are periodic in nature (CBI, 2005). An auto industry producing 400,000 vehicles per annum could generate about 410,000 m³ per annum of wastewater mainly discharged from emulsion and paint sections with an average COD of 11.4 kg/m³ (Herr et al., 2008). A low cost treatment like anaerobic digestion of high strength effluents is one of the popular techniques to minimize the release of organic pollution in the environment and at the same time it generates combustible biogas. Another advantage of anaerobic treatment is that it improves the efficiency of post treatment procedures that serve as polishing steps e.g. by biodegradation of poorly adsorbable or residual substances which cause overall improvement in the adsorption of suspended matter (Kim et al., 1994). Due to the current policy of CO₂ mitigation and finding alternatives for fossil fuels, biogas from anaerobic digestion of biomass and waste is one of the most dominant future renewable energy sources in Europe. In recent years, treatment of wastes by anaerobic digestion has been increased by an annual growth rate of about 25% (Appels et al., 2011, Buffiere et al., 2008). The challenges in biological treatment of industrial wastewater in comparison to treatment of sewage are to select and develop a stable microbial culture resistant to toxic organic and inorganic micro-pollutants, high chemical oxygen demand and shock loadings due to operational changes. During anaerobic fermentation acidogenic bacteria generate fatty acids, hydrogen and carbon dioxide. The fatty acids other than acetate are further degraded by syntrophic associations of acetogenic and methanogenic bacteria to acetate, CO₂ and hydrogen and these metabolites are then converted to CH₄ and CO₂ by methanogens (Gallert and Winter, 1999). Dysfunction of any of acetogenic and methanogenic groups of bacteria due to inhibition or wash-out leads to the accumulation of intermediates such as fatty acids, lowering of pH and to hydrogen production accompanied with an increased hydrogen partial pressure which could result in up-setting of the whole system. However, contriving anaerobic degradation toward hydrogen production would be desirable since hydrogen is a clean and CO₂ neutral renewable energy source with high energy content. In addition, the energy conversion efficiency of hydrogen is greater than that of methane as the former has a higher heating value (HHV). From one mole of glucose, 3.4 MJ of hydrogen or 2.7 MJ of CH₄ energy may be generated (Reith et al., 2003). In praxis, the major problem of biohydrogen production during dark fermentation is a low H₂ yield due to inefficient conversion of organic matter to H₂. Biogas production is already an established commercial process whereas commercial exploitation of hydrogen from waste still requires basic research and process development (Reith et al., 2003).

Raw industrial wastewater often contains inhibitory substances which could up-set the system in case of operational problems or process changes. Thus, it is necessary to pre-treat wastewater from metal and automobile industry by precipitation of its high content of heavy metals, in particular Cr, Ni, Cu and Zn before subjection to secondary biological treatments. Only scarce reports are available in literature about the possibilities of biological treatment of auto industry wastewater (EI-Gohary et al., 1989, Roberts et al., 2000, Oliveira et al., 2008). Thus, in the first part of this study, biodegradability and the methane production potential of pre-treated high strength automotive industrial wastewater was investigated in the absence and presence of two co-substrates using an anaerobic suspended inoculum under batch incubation conditions. In the second part of the study, the performance of the microbial inoculum for hydrogen production was investigated using glucose as a substrate in the presence of zinc as a known inhibitor of methanogenesis (Zayed and Winter, 2000). Further, the same inoculum was used to assess the suitability of a high COD concentration containing industrial wastewater for biohydrogen production.