Whitley Burchett

A Delicate Balance: Factors Effecting pH at a Central Valley Wastewater Treatment Plant

(Presented at the California Water Environment Association Annual Conference
Oakland, CA, April 1998)

Paul Friedlander: Whitley, Burchett & Associates
David Jenkins: University of California at Berkeley
David Michel: Selma-Kingsburg-Fowler County Sanitation District

ABSTRACT

pH is important in biological wastewater treatment, and is influenced by chemical parameters, such as alkalinity, carbon dioxide concentration, and nitrogen transformations. An understanding of these relationships is important in troubleshooting wastewater treatment plant operation problems. This paper presents an example of how a knowledge of wastewater chemistry was used to investigate the cause of low effluent pH (pH <6.5) at the Selma-Kingsburg-Fowler County Sanitation District wastewater treatment facility, an 8 mgd extended aeration plant with several large industrial users, located in Kingsburg, California.

Between 1994 and 1996 the plant periodically experienced low effluent pH values. An investigation revealed that the effluent pH was subject to long-term trends and short-term events. A long-term decrease in effluent pH was likely caused by a long-term decrease in effluent alkalinity caused by the biological nitrification of an increasing influent ammonia concentration. Dissolved carbon dioxide concentrations likely increased because of changes in plant operating conditions. Decreases in total alkalinity and increases in dissolved carbon dioxide both tend to reduce the pH. Long-term trends were exacerbated by short-term decreases in influent pH probably due to changes in the wastewater discharges from major industries.

Additional monitoring was conducted and methods for raising effluent pH were implemented. These included facilities for effluent reaeration and chemical addition.