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The feasibility of sludge reduction via the X biodegradation process was explored both experimentally and through modeling, where the main focus was on determining the value of the b parameter (first order degradation of X) 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 = 0.29 d, 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 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 parameter to be in the range of 0.003-0.006 d, satisfactorily describing the overall sludge yield reduction of up to 49% observed in the experiments.

Research on new ways for minimizing the residues generated in biological wastewater treatment has been encouraged within the context of activated sludge models (ASM). One of the methods used for minimizing sludge is through the elimination of traditional sludge wastage (WAS), while build-up of the inert solids in aeration tanks and is avoided by other means; the development of fine screens to remove the inert particulate organic fraction (XI), hydrocyclones for inorganic suspended solids (ISS) and different kinds of on-line digesters to further biodegrade the endogenous residues (XP) via the return activated sludge line (RAS). In this research, a model and a simulation program was developed, which was able to mimic the apparent behavior of such activated sludge variants with low-solids-production (LSP-AS). This model is an extended ASM1 assuming a small first order biodegradation constant for XP (kXP = 0.007 d-1) and black boxes representing the XI and ISS inerts-removal. The simulations depict the way that different components of solids build up in the aeration tanks when traditional activated sludge (C-AS) is operated at very high solids retention times (> 100 d). The simulations showed that the C-AS process could, hypothetically, be replaced by LSP-AS variants with similar levels of active biomass and of mixed-liquor suspended solids. The kXP was approximately 0.007 d-1 and, for this case, at least 2% (and 6%) of the RAS flow had to be sieved (and digested, respectively), to avoid the accumulation of XI, ISS and XP. Furthermore, the size of the on-line digester was about twice the volume of the aeration tank.