Activated Sludge with Total Solids Retention: Modified-ASM1 Modelling and Simulation

Session G1: Using Simulation Models to Improve Understanding of Environmental Systems

There are many new strategies put forward for minimizing excess sludge generation in biological wastewater treatment. Some of them were developed within the context of dynamic activated sludge modelling (ASM). A number of means can be used to eliminate the traditional waste activated sludge flow and the build-up of inert solids in the aeration tanks. This includes the use of fine screens to remove the inert particulate organic fraction (X_I), hydrocyclones to lower the inorganic suspended solids (ISS), and different kinds of on-line digesters to further biodegrade the endogenous residues (X_P), 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). The model is an extended ASM1 assuming a small first order biodegradation constant for X_P (k_XP = 0.007 d^-1), and black boxes representing the XI and ISS inerts removal. The simulations depicted the way the different solids components 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 mixed liquor total suspended solids. With k_XP of about 0.007 d^-1 and for the studied case, at least 2% (and 6%) of the RAS flow must be sieved (and digested, respectively), to avoid the accumulation of X_I, ISS and X_P. Also, the size of the on-line digester will be about twice the volume of the aeration tank.