Steady-state analysis and design of activated sludge processes including compressive settling

A simplified activated sludge process (ASP) with one substrate and one particulate biomasscomponent has been analyzed with respect to its steady states. The ASP consists of a completelystirred biological reactor and a settler. The biomass growths according to the Monod function and the decay rate is constant. In the considered one-dimensional settler model, the biomass undergoes hindered settling and compression according to constitutive functions that can be chosen by the investigator. The dynamic settler model is a nonlinear partial differential equation (PDE), so the steady-state equation is an ordinary differential equation. This is a difficulty in the steady-state analysis of an ASP. The key findings are the following:• By analyzing the model PDE, a simple algebraic equation has been obtained that captures the necessary information related to the limiting flux of a settler in steady state in normal operation with a sludge blanket in the thickening zone. The equation relates the recycle concentration to the bulk velocity in the thickening zone with the sludge blanket kept at a specified level.• All steady states of an ASP working in normal operation are given as a one-parameter family of solutions where the parameter is the recycle ratio 푟. Numerical solutions show that the effluent substrate concentration 푆∗ is reduced significantly already for small 푟 and is a decreasing function for all 푟 > 0. The sludge age 휃 can be expressed as an explicit decreasing function of푆∗.• A control curve 푤 = 푤(푟), relating the wastage ratio 푤 to the recycle ratio 푟, gives the possible values that yield the desired steady state with the sludge blanket kept at a specified level.• For given inputs (substrate concentration 푆 in and volumetric flow 푄) to the ASP, a procedure is suggested for the design of an ASP that is constrained to work in normal operation. The procedure gives support in the decision on how to balance the building or start-up costs (which increase with the total area) and operational costs (which increase with 푟 – energy for pumping water).