Quantitative determination of the population biological activity of biological sludges used in the activated sludge waste water treatment process

The population biological activity of biological sludges used in the activated sludge waste water treatment process is defined as that minimum amount of biological sludge, still able to reduce in a 21 h contact period, the soluble COD of a waste water by one half the value equivalent to the BOD_5d. This A_mg^BOD50% 21 h value was determined by incubating hundred fold serial dilutions of a biological sludge with a previously sterilized waste water and by plotting the residual soluble COD remaining after a 21 h incubation period vs the sludge dilution. The method allows any given biological sludge to be readily assayed on any given waste water. Biological sludges were found stable for periods up to three months in the frozen state or when stored at 4°C. Lyophilization of biological sludges resulted in a severe loss of population biological activity. Biological sludges of trade waste water treatment plants were followed for periods up to 70 days. A correlation was found between the population biological activity and changes in environmental factors such as reduced N and P nutrient feeding, reduced aeration or changes in pH. Minor changes resulted in a decrease in population biological activity. Drastic changes resulted in a modified shape of the soluble COD vs biological sludge dilution curves, presumably reflecting deep modifications of the waste water-treating biological community.