Optimization of Anaerobic Digestion of Sewage Sludge Using Thermophilic Anaerobic Pre-Treatment

In this study, we investigated by running continuous stirred tank reactor the effects of different thermophilic temperatures (55, 60, 65, 70, 75 and 80C) for different hydraulic retention times (0.5, 1.0, 1.5, 2.0, 2.5 and 3.0 days, respectively) on the pretreatment of sewage sludge. We found that hydrolysis was mainly determined by temperature instead of hydraulic retention time, i.e. the higher the temperature was, the more dissolved COD produced, while extension of hydraulic retention time did not significantly increase dissolved COD products. Acidogenisis had the highest products when temperature was at 70C or 75C and for a hydraulic retention time around 2 days, under the conditions of which higher acidogenic activity was maintained and methogenic activity was inhibited. We also determined the minimum guaranteed retention time for each temperature when faecal streptococci were totally inactivated. Introduction Anaerobic digestion (AD) is a matured and widely used technology to treat sewage sludge. By anaerobic digestion, organic compounds such as carbohydrates, proteins, lipids and etc, are converted to methane, carbon dioxide, water and etc. through the sequential four steps of hydrolysis, acidogenisis, acetogenisis and methanogenisis. At the same time, nutrients such as N and P are kept in the residue of the effluent. So, anaerobic digestion is a way of solving environmental problem with simultaneous production of energy in the form of biogas and nutrients enriched effluent that can be recycled back to the farmland. Studies have shown that hydrolysis is the rate-limiting step during the whole AD process in treating sewage sludge, because organic matters in it are in the form of particulates (Tiehm, et al., 2001; Wang, et al., 1999; Li & Nioke, 1992). One way to enhance hydrolysis is to add a pretreatment step before AD. Different pretreatment methods, such as thermal (heat), mechanical (mixing, ultra-sonic), chemical (acid or base, ozone) and biological methods (aerobic composting, methophilic or thermophilic anaerobic) have been studied at laboratory scale and some of which have already been implemented for industry production. It will be of advantage if the acidogenisis process can be enhanced by the pretreatment so that more substrates available for acetogens and methogens in the subsequent second step reactor can be produced. Also, it will be very interesting if pasteurization can be incorporated into this pretreatment step, so that the hygienic requirement on AD effluent for reuse on farmland can be satisfied. Based on these considerations, it will be very interesting to focus pretreatment on thermophilic anaerobic method. The reasons for this are (1) it is reported that high temperature above 60C can improve hydrolysis (Li & Noike, 1992), (2) thermophilic bacteria that can hydrolyze and/or degrade organic compounds are widely exist and they have optimal temperature in the range from 60 to 75C (Kristjansson, 1992), (3) when AD is run at thermophilic temperatures, pasteurization can be improved (Nielsen & Petersen, 2000). Both temperature and hydraulic retention time (HRT) are important parameter for AD. The main advantages of increased temperature are generally higher reaction rates, higher solubility of most chemicals, and increased fluidity and diffusion rates (Kristjansson, 1992). For continuous stirred tank reactor (CSTR), HRT is not only a good operational parameter that is easy to control, but also a macro-conceptual time for the organic material to stay in the reactor. In bio-reaction engineering studies, the reverse of HRT is defined as dilution rate, for which if it is bigger than the growth rate of microbial cells in the reactor, the microbe will be washed out, and otherwise the microbe will be accumulated in the reactor. Either of these situations may result in the breakdown of the biological process happening in the reactor. Literatures that can be referred to regarding to the study on anaerobic