From an economic point of view, the specific biogas amounts and the biogas quality from anaerobic treatment of wastewater and sludge are important process parameters. Therefore, a basis for prediction of amount and composition of biogas was elaborated as early as the 1930s (Buswell and Neave, 1930; Buswell and Sollo 1948; Buswell and Mueller, 1952). If the elemental composition of a wastewater is known, the theoretical amount and composition of the biogas can be predicted with the Buswell equation (Eq. 11). The actual biogas amount is lower and can be calculated by including a correction factor for the degree of degradability, the pH (which influences CO2 absorption), and a 5%-10% discount for biomass formation.
CcHAO0NnSs+1/4(4c- h-2o + 3n + 2s)^O ^ 1/8(4c - h + 2o + 3n + 2s)CO2 (11)
According to the Buswell equation (Eq. 11), for the anaerobic treatment of a wastewater with carbohydrates as pollutants, the gas composition should theoretically be 50% methane and 50% CO2 (Eq. 12):
Since CO2 is increasingly soluble in water with decreasing temperature and increasing pH, CO2 reacts to form bicarbonate/carbonate, and the biogas may contain more than 80% methane. The total amount of gas is then diminished by the amount of CO2 that is absorbed and solubilized in the liquid. From a fat- and protein-containing wastewater, theoretically more than 50% methane can be generated (Table 1.5).
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