ANAEROBIC BIOPROCESSING OF ORGANIC WASTES

Anaerobic digestion of dissolved, suspended and solid organics has rap idly evolved in the last decades but nevertheless still faces several scientific unknowns. In this review, some fundamentals of bacterial co nversions and adhesion are addressed initially, It is argued in the li ght of Delta G-values of reactions, and in view of the minimum energy quantum per mel, that anaerobic syntrophs must have special survival s trategies in order to support their existence: redistributing the avai lable energy between the partners, reduced end-product fermentation re actions and special cell-to-cell physiological interactions. In terms of kinetics, it appears that both reaction rates and residual substrat e thresholds are strongly related to minimum Delta G-values. These new fundamental insights open perspectives for efficient design and opera tion of anaerobic bioprocesses. Subsequently, an overview is given of the current anaerobic biotechnology. For treating wastewaters, a novel and high performance new system has been introduced during the last d ecade: the upflow anaerobic sludge blanket system (UASB). This reactor concept requires anaerobic consortia to grow in a dense and eco-physi ologically well-organized way. The microbial principles of such granul ar sludge growth are presented, Using a thermodynamic approach, the fo rmation of different types of aggregates is explained. The application of this bioprocess in worldwide wastewater treatment is indicated. Du e to the long retention times of the active biomass, the UASB is also suitable for the development of bacterial consortia capable of degradi ng xenobiotics. Operating granular sludge reactors at high upflow velo cities (5-6 m/h) in expanded granular sludge bed (EGSB) systems enlarg es the application field to very low strength wastewaters (chemical ox ygen demand < 1 g/l) and psychrophilic temperatures (10 degrees C). Fo r the treatment of organic suspensions, there is currently a tendency to evolve from the conventional mesophilic continuously stirred tank s ystem to the thermophilic configuration, as the latter permits higher conversion rates and easier sanitation. Integration of ultrafiltration in anaerobic slurry digestion facilitates operation at higher volumet ric loading rates and at shorter residence times. With respect to orga nic solids, the recent trend in society towards source separated colle ction of biowaste has opened a broad range of new application areas fo r solid state anaerobic fermentation.