Sludge reduction via biodegradation of the endogenous residue (X_E): experimental verification and modeling

The feasibility of sludge reduction via the X_E biodegradation process was explored both experimentally and through modeling, where the main focus was on determining the value of the b_E parameter (first order degradation of X_E) from a continuous process. Two activated sludge (AS) systems (30 L) were operated in parallel with synthetic wastewater during 16 months: a conventional activated sludge (CAS) system and a modified low-sludge production activated sludge (LSP-AS) process equipped with a side-stream digester unit (DU). First, the long term data of the CAS reactor (1 year) were used to calibrate the ASM model and to estimate the heterotrophic decay constant of the cultivated sludge (b_H = 0.29 d^-1, death-regeneration basis). Second, pre-simulations were performed to design the LSP-AS system and to estimate the DU volume required (40 L), to avoid X_E accumulation in the process. Third, the LSP-AS process was built, put in operation and monitored for more than 9 months. This allowed assessment of the actual behavior of the quasi-complete solids retention system. Once calibrated, the modified AS model estimated the value of the b_E parameter to be in the range of 0.003-0.006 d^-1, satisfactorily describing the overall sludge yield reduction of up to 49% observed in the experiments.