Mathematical Modeling and Analysis of Wastewater Treatment Plant Using the Cannibal Process

The Combined Activated Sludge Anaerobic Digestion Model (CASADM) was configured to represent the operational results of a full-scale Cannibal wastewater treatment plant that operated over a 2-year period and had a net sludge yield that was low, approximately 0.21 g volatile suspended solids/g chemical oxygen demand (g VSS/g COD) removed based on the model. CASADM accurately described available information on effluent quality and mixed-liquor volatile suspended solids (MLVSS) concentrations in each tank. Modeling results led to important insights into what led to the low net sludge yield of this Cannibal plant. For example, widespread net decay or slow growth [negative or very large positive values of solids retention times (SRTs)] of biomass caused total active biomass to be only 13%-21% of the MLVSS, and about 28% of the total input COD was converted to CH4 in the system. Input of active biomass and a low sludge-wasting rate proved to be key factors causing net decay or slow growth of active biomass, which, when combined with significant CH4 production, led to low net sludge yield.