Analysis of an activated sludge model in which dead biomass is recycled into slowly biodegradable particulate substrate

Abstract

A significant drawback of the activated sludge process is the production of excess 'sludge'; the disposal of which can account for 50-60% of the running costs of a wastewater treatment plant. We investigate a model for the activated sludge process in which the influent contains a mixture of soluble and slowly biodegradable particulate substrate. Within the bioreactor, the particulate substrate is hydrolyzed to form soluble substrate. In turn, these are used for growth by the biomass. Biomass decay produces slowly biodegradable substrate and non-biodegradable particulates. Steady-state analysis is used to investigate how the amount of sludge formed depends upon the residence time and the use of a settling unit. We show that when the steady-state sludge content is plotted as a function of the residence time that there are three generic response diagrams. The value of the effective recycle parameter determines which type of response diagram is observed. If this parameter is greater than a critical value, then the sludge content is guaranteed to be greater than a target value. The dependence of this critical value upon the chemical oxygen demand in the feed and how the chemical oxygen demand is partitioned between its constitutive components is investigated. (2) School of Physical, Environmental and Mathematical Science, UNSW at ADFA, Canberra, ACT 2600, Australia Disciplines Engineering | Science and Technology StudiesNovember 15, 2014 AbstractThe activated sludge process is one of the major processes used in the biological treatment of wastewater.A significant drawback of this process is the production of excess sludge, the disposal of which can account for 50-60% of the running costs of a plant.We investigate a model for the activated sludge process in which the influent contains a mixture of soluble and slowly biodegradable particulate substrate. Within the bioreactor the biodegradable particulate substrate is hydrolyzed to form soluble substrate. In turn these are used for growth by the biomass. Biomass decay produces slowly biodegradable substrate in addition to non-biodegradable particulates.We use steady-state analysis to investigate how the amount of sludge formed depends upon the residence time and the use of a settling unit. We show that when the steady-state sludge content is plotted as a function of the residence time that there are three generic response diagrams, depending upon the value of the effective recycle parameter. We find that if the effective recycle parameter is higher than a critical value then sludge content is guaranteed to be greater than a target value. We investigate how this critical value depends upon the chemical oxygen demand in the feed and how the chemical oxygen demand is partitioned between its constitutive components.

The Biological Treatment of Wastewater: Mathematical Models

“…Unlike the model investigated in Chapter 2, we find that in practice excessive sludge formation is inevitable. The results of the successive stages of this research have been published in [1][2][3].…”