Maintaining optimum settling rate of activated sludge

Abstract

A completely mixed system for controlling the return rate of activated sludge to the aeration tank in activated sludge sewage treatment system plants is disclosed wherein the return rate of activated sludge is controlled to maintain the density of the activated sludge in the aeration tank at an optimum value, which value will result in the optimum settling rate of the activated sludge in the secondary clarifier. The control system is continuous and requires the use of a computer, which continuously receives various measured quantities necessary to compute the control signal for the returned activated sludge pump. The carbon dioxide respired by the cells in the aeration tank is collected along with the aeration air and a non-dispersive infrared ray analyzer determines the percent of carbon dioxide in the collected gases. This determination is one of the factors entering into the computation of the rate of substrate consumption by the activated sludge. Other measured quantities entering the computer are the rate of air flow entering the diffuser, the density of the activated sludge in the aeration tank, the rate of flow of effluent from the aeration tank and the density of returned activated sludge collected in an activated sludge storage tank. Various physical and biological constants are manually set into the computer, the principal output of which is a signal for controlling the rate of return of activated sludge in the sludge storage tank to the aeration tank. The density of the stored activated sludge is also determined and enters the computer. The computer also compares this density with the actual measured density of the activated sludge in the aeration tank and if the stored sludge density does not exceed the measured density, a warning is given indicating that it is futile to return the stored activated sludge to the aeration tank.