The sewage sludge disposal systems involving anaerobic digestion process were investigated from points of economics and energy. Four optional systems with eight alternatives were analysed, setting the plant at three fixed scale, and the separate and combined sewer collection systems were chosen as the model. Profit of the additional investment to the conventional one and net profit were calculated and compared with each other. Net energy consumption, production and self-sufficiency of electricity were discussed. The optimal type of these systems depends on the scales and sewer systems of plants, and the digestion with heat treatment and dewatering without chemicals were the most effective systems.
The thermal processing has become one of indispensable processes of sewage sludge treatment in Japan. However, it is the most energy consuming process of sewage sludge treatment processes. Therefore, energy saving is now the most important subject in this field. As the equipment of sludge thermal processing, Multiple Hearth Furnace (M.H.F.) is mostly used in Japan. M.H.F. has hitherto been operated at high air ratio of 2.0-3.0. The main operating factor which influences energy consumption of M.H.F, is air ratio for combustibles. So, low air ratio operation is the most effective method for energy saving of M.H.F. Low air ratio operation is defined as the operation at lower air ratio of 1.2-1.5. Due to reduction of waste gas volume followed by reduction of sensible heat carried from M.H.F., auxiliary fuel for the furnace is reduced. A study on relationships between air ratio and furnace performance in M. H. F Was conducted. Material balance model of M.H.F.with some assumptions was chosen and the above relationships was simulated. This simulation verified how furnace performance is influenced by air ratio.
Practical operating conditions of the rotating biological contactor (RBC) wastewater treatment facilities were investigated by the questionary method. The results showed that the exaggerated design of the treatment facilities caused from the overestimated prediction of BOD load gives excess energy requirement in operation, while the BOD concentration in the effluent from RBC units are considerably lower than the design level. It was also noted that the electrical power consumption for the aeration in the reservoir tank is usualy greater than the power consumed by the rotating discs. In the design stage of the treatment facilities, the need of an exact estimation of BOD load is the most important. The modification of the reservoir tank system should be suggested for energy saving in RBC units.