The walls and bottom of the digester are insulated with extruded polystyrene with a thickness of about 80 mm to reduce the loss of heat. The floor and walls of the upright digester are made of reinforced concrete. The digester has the constructed air sealed. For the purpose of integrated gas storage, the cover is made of a flexible, synthetic membrane roof. Biogas contains about 1% hydrogen sulphide (H2S). This has a corrosive effect on metals and will,
therefore, damage the engine and the piping. It is, therefore, important to remove the H2S. In this study, this has been done by adding some air (4%) into the upper part of the digester, close to where the biogas outlet to the biogas storage has been located. Three stirring devices in the AD plant of the case study are screw propellers. A screw propeller system consists of an electric motor with a load capacity of 15 kW, which drives a screw propeller. Within the digester, three stirring units have been operated at 20 min per hour. During the investigation period, the plants were
fed with dairy cow manure and sheep dung by-products. To transport the substrate, by means of a pump, substrates piping with a diameter of 160 and 200mmhave been used. At a longer distance, a diameter of at least 150 mm is required. The main parameters of the AD and digestate storage unit have been reported in Table 1. Approximately 82 m3 of fresh mixed (93.9% dairy cow manure, 6.1% sheep dung) was loaded daily into the slurry tank. The fresh feedstocks were loaded to the AD by means of the pumping unit of the slurry tank running at 5 min per hour. The biogas from the AD, produced by digesting dairy cow manure and sheep dung,was used for combined heat and power (CHP) generation. All of the produced biogas was burned in a CHP unit to generate electric power and heat. The biogas plant was supported with an installed electric capacity of 289 kWh producing the electrical energy equivalent to 8000 h load operating time per year. The produced electrical energy in a CHP unit is fed into the public grid at a Turkish market price of V0.1 kWh1. Thermal energy as a side product of the combustion process is used to heat the AD plant in order to maintain the anaerobic process. The market for heat is currently non-existent. Besides biogas, the farm-scale biogas plant produces digestate, which is separated into a solid and liquid fraction via the separator unit. First, the digested solid and liquid fraction are fed to the lagoon, which has a total volume of 300 m3 (height 3 m, length
10 m, and width 10 m) with installed mixing and pumping systems. The digested solid fraction is stored in a static heap on an uncovered concrete platform. The liquid fraction is stored within a 28,800 m3 pond (height 6 m, length 80 m, and width 60 m) without a gas-tight covering system.