Bioplex Home Background Info Project Research Links Acknowledgements Contact Us	Operating Temperature Three different temperature schemes are used for digester operation; thermophilic, mesophilic, and ambient temperature. Thermophilic - 130-136 °F Mesophilic - 95-100 °F Ambient - Dependant upon ambient conditions Thermophilic and mesophilic digesters require heating units to maintain temperature, while ambient temperature digester temperature depends on the ambient temperature and solar energy. Each operating temperature has its advantages and disadvantages. Thermophilic The most efficient and rapid conversion of carbon into methane occurs at thermophilic temperatures. Typically, a digester operated at thermophilic temperatures will have an HRT (hydraulic retention time) of 10 days or less, with more volume of methane gas produced per weight of solid material fed than lower temperature digesters. This allows for reduced digester size compared to ambient temperature or mesophilic digesters for the same volume of material fed. At thermophilic temperature ranges, pathogens such as E. coli and Salmonella are destroyed at a higher rate than lower temperature digesters. Maintaining the digester at these temperatures requires external heat input. Therefore, thermophilic operation is best suited for larger digesters where the methane gas produced can be used to heat the digester. (On large-scale regional digesters heat recovered for electric generator operation can be used to heat the digester). Digesters operated at thermophilic temperatures can be more sensitive to changes in temperature and changes in feed input than lower temperature units, especially when the retention times are shorter. Larger, commercial digesters allow for close monitoring of the control parameters. Mesophilic Mesophilic digesters are suitable for both on-farm and large-scale digesters. While conversion of fed solids to methane is less than thermophilic, the performance is better than ambient temperature. Plug flow digesters are generally operated at mesophilic temperatures. Some pathogen reduction occurs, however the critical temperature for pathogen destruction seems to be 130 °F, therefore thermophilic digesters have higher rates of pathogen destruction. Typical hydraulic retention times for mesophilic digesters are 20 days. External heat input is required to maintain mesophilic temperatures. On smaller scale digesters, significantly less heat input is needed to maintain mesophilic temperatures compared to thermophilic, however on large scale units, the increased methane production of thermophilic units compensates for higher heat input. Ambient Temperature Ambient digesters are useful in small plug-flow or lagoon type systems where longer retention times are possible. The material from farms can be held in the digester for long periods with hydraulic retention times of up to 100 days and where the production of methane per cubic volume of digester is slower. The capital is low, but the land space required is high. The build up of solids not digested tends to be an operational problem and needs the digester to be closed for some time for cleaning out. The ambient temperature fixed film digester for treating separated raw sewage could be a future means of primary treatment to obviate the need for energy input for BOD (Biological Oxygen Demand) removal. These are being developed at experimental stage and could be available for treating wastes from small communities. Bioplex Home  |  Background Info |  Project Research Links  |  Acknowledgements  |  Contact Us Copyright 2005 Bioplex Project - Site Designed and Developed by WV Designs