Chaos in the Activated Sludge Process

Abstract

The activated sludge process is used to remove biodegradable organic impurities in wastewater using bacteria and other microorganisms to decompose the organic impurities into water, carbon dioxide, and additional microbial cells. One of the main problems in the operation of the activated sludge process is the occurrence of sudden and often unexplained process instability or upsets. A number of hypotheses have been developed to explain the nature and cause of process instability. The process kinetics involve a number of non-linear and linear differential equations to define the predator-prey relationship among various microbial species, the dominance and predominance of specific species in the presence of poisons, biochemical catalysts, and types of substrate. The kinetic models of the activated sludge process are very complex.

Using the basic kinetic model for the completely mixed activated sludge process, the chaotic behavior of the system was studied. The study also identified the process and design variables creating chaos and estimated the ranges of those design and operating variables within which chaos takes place. The study confirmed that the activated sludge process is unstable when operated under (i) a very short detention time of less than 2 hours; (ii) very stringent effluent quality, i.e. less than 2 mg/l; and (iii) very high influent COD concentration with detention time set at 4 hours or less. Increasing the detention time increases the system’s stability.