Enhancing the biomethane production from tannery wastewater by thermal pretreatment

Abstract

Anaerobic Digestion (AD) is often chosen as a suitable treatment for tannery wastewater (TWW) since this waste stream is rich in organic matter. Nevertheless, the high organic load and the presence of compounds commonly used in the leather production process and responsible for inhibiting the biological activity (e.g. chromium and sulphide) limit the full capitalization of the whole potential in producing biomethane from this low-cost and interesting source of renewable energy. Pretreatment of substrates is considered by most as the solution to overcome this negative aspect as a higher biomethane production in a shorter time is expected after that. Actually in this study three different temperatures have been tested, in detail, 90, 80, and 70 °C, to pretreat a real TWW prior to a mesophilic AD process conducted in a series of batch biomethane potential (BMP) tests aimed at evaluating the optimum temperature that can result in increasing the soluble COD content and consequently the AD performance . After the pretreatment, the soluble COD concentration increased from a minimum of 11% to a maximum of 80%: which resulted in an increase of the biomethane production compared with the raw substrate. The highest temperature (90°C) investigated gave the highest increasing in soluble COD but had a negative effect on AD process performance, as the final production of biomethane was 178 ml CH4/g TVS, more than 4 folds less than the maximum yield of 891 ml CH4/g TVS obtained from the sample pre-treated at 80°C where the increase in soluble COD was by 57%. These results highlight the critical aspect of the thermal pretreatment: the increase of temperature can enhance the biomethane production as well as reduce it, therefore the choice of the most suitable temperature depends strongly on the characteristics of the substrate and it can be made only through experimental tests.