Effect of mean cell residence time on the acid hydrolytic assist activated sludge process

Handling and disposal of residual solids from wastewater treatment plants is an expensive and difficult task. The acid hydrolytic assist activated sludge process is one method of minimizing sludge production from a biological wastewater treatment process.

Acid hydrolysis of waste sludge involves pH adjustment to 1.0 or less followed by heat treatment. The hydrolyzed sludge can then be adjusted to a neutral pH and recycled to the treatment process as soluble organic material. In effect, hydrolysis promotes cellular autodigestion by artificially inducing the normally difficult metabolic steps. The use of hydrolysis in the extended aeration process allows periodic sludge wasting for control purposes but avoids the problem of ultimate sludge disposal.

Previous research was limited to the use of hydrolysis in the extended aeration process. The effect of mean cell residence time, Θ_c, on an activated sludge process utilizing hydrolysis had not been evaluated. Also, only limited information is currently available regarding the kinetics of wastewater treatment with the hydrolytic assist activated sludge process. The purpose of this research was to gain additional insight into the hydrolytic assist activated sludge process with regard to mean cell residence time and the kinetics of wastewater treatment. Determination of the relative effect of hydrolysis on nitrification in the activated sludge process was a secondary objective of this study.

Mathematical and stoichiometric equations were used to predict process performance characteristics. A laboratory investigation was then conducted to obtain actual operational results for comparison. A description of the investigative procedures and results is included along with a review of the literature.