Thermophilic anaerobic wastewater treatment : temperature aspects and process stability

The main objective of this thesis was to assess the thermostability of thermophilic anaerobic wastewater treatment processes and the possibility to optimize the performance of thermophilic high-rate systems. Experiments were conducted to study the suitability of two types of seed material to start a thermophilic anaerobic process. Both mesophilic granular sludge and digested organic fraction of municipal solid waste were used as inoculum. The fate of mesophilic granular sludge under thermophilic conditions was studied in detail. Due to the temperature increase the mesophilic methanogens are replaced by thermophiles. In fact, the mesophilic granules appeared to serve mainly as carrier material for the thermophilic bacteria during the start-up. Since the thermophilic organisms attach quite well, the thermophilic specific methanogenic activity increased very rapidly in this period. Treatment of completely acidified wastewater leads to a deterioration of the 'mesophilic-thermophilic' granules. It therefore appeared extremely difficult to develop thermophilic granular sludge on this type of wastewater for both types of inocula. However, the thermophilic granulation process proceeded easily when sucrose was added to the influent. The temperature sensitivity of the various types of thermophilic anaerobic sludge depends strongly on the process conditions applied, such as temperature and reactor type. Thermophilic sludge cultivated in high-rate reactors with high solids retention shows a high thermostability. Therefore, thermophilic anaerobic treatment in high-rate reactors can be applied in a wide temperature range, even under mesophilic conditions. In contrast, sludge cultivated in batch reactors is very sensitive to relatively small temperature variations. Regarding the thermostability of the process, application of high-rate reactors is preferred over batch reactors or completely mixed reactors. The presence of granular sludge enhances the stability towards temperature fluctuations quite substantially. The maximum specific activity of the cultivated granules appeared to be limited by the mass transfer rate. Consequently, a 'biomass buffer' is created which can be drawn on if the specific activity drops as a result of a temperature decrease. A high process stability and high removal efficiencies were obtained in upflow staged sludge bed (USSB) reactors under extreme loading conditions. This USSB reactor consisted of 5 compartments along the reactor height. From each separate compartment of this reactor the produced biogas is withdrawn. The major effect of staging the thermophilic process is a very low concentration of intermediate products, such as hydrogen and acetate, in the last compartments of the system. A low concentration of these products enhances the anaerobic thermophilic degradation of all fatty acids. The properties of the sludge grown in the various compartments of the staged reactor depend on the environmental conditions prevailing in each compartment. Therefore, withdrawal of the produced excess sludge should be performed from each compartment or from the first compartment when an upflow reactor is used. Otherwise, a stable operation on the long term cannot be guaranteed because the voluminous acidifying sludge will eventually force out the extremely active acetogenic and methanogenic consortia in the subsequent compartments.