Thermophilic anaerobic digestion for waste and wastewater treatment

This thesis deals with thermophilic anaerobic waste and wastewater treatment. A literature survey is presented, in which the thermophilic treatment processes are evaluated with respect to the loading rates and treatment efficiencies, and some relevant theoretical considerations concerning thermophilic anaerobic processes are discussed.Thermophilic anaerobic treatment of livestock wastes with a high total ammonia concentration cannot be recommended, due to the toxic effect of the ammonia. The volatile solids concentration turns out to be of minor importance in determining the efficiency of the thermophilic digestion of livestock wastes. The toxic effect of ammonia is exerted on the level of methanogenesis from H 2 /CO 2 , resulting in a buildup of the partial pressure of H 2 , which inhibits propionate degradation. The latter compound is shown to be toxic for the methanogenesis from acetate.The major part of this thesis deals with the processes in thermophilic upflow anaerobic sludge blanket (UASB) reactors. Solutions of sugars can be treated effectively in UASB reactors operated at 55 °C. With the granular sludge cultivated on sugars, other wastewaters can be treated effectively, with loading rates up to 103 kg COD/m 3 and treatment efficiencies exceeding 77 %. Vinasse, a high strength wastewater, could be treated also at high loading rates, but the efficiencies were rather low, due to the high concentrations of toxic compounds in the vinasse. The treatment efficiency appeared to be determined by the concentration of the vinasse applied, rather than by the loading rate, which was in the range of 17-86 kg COD/m 3 d.The decrease in the treatment efficiency at very high loading rates is mainly caused by a deterioration of the propionate degradation. The hydrogen concentration plays a very important role in the conversion of propionate. A two-step methanogenic UASB system was developed, in which the propionate degradation was delegated to the second step. The two-step system operated with appreciably higher efficiency than a one-step system with a similar total volume: at a loading rate of 52 kg COD/m 3 d. the treatment efficiencies were 92 and 82 % for the two and one stage system, respectively. As with mesophilic sludge, the unfed storage of thermophilic sludge at low temperatures results in a very slow decrease in its capacities. Food shortages at the operating temperature of 55 % however, result in a rapid decay of the sludge.The granulation of methanogenic sludge was studied with acetate as substrate. Granulation occurred only after approximately three months of operation when using mesophilic seed materials. This process could be speeded up by the use of adapted seed materials. The addition of inert particles to the seed material or the nature of the un-adapted seed materials did not have any influence on the ultimate granulation. By using different criteria for the operation of UASB reactors, different granules could be cultivated. Granules consisting of filamentous methanogenic bacteria are to be preferred above those consisting of sarcina-type methanogenic bacteria. With the granular sludge consisting of filamentous bacteria, loading rates of 162 kg COD/m 3 d. could be treated with over 89 % efficiency.The thermophilic UASB process is ready for application in practice.