Laboratory Development of Site-specific Equations for BOD to TOC Conversion and Application in NPDES and Treatment Processes Control

The need for reliable and rapid process control options in wastewater treatment for ocean discharging facilities is perhaps equal only in importance to the perceived need to maintain compliance with their NPDES effluent discharge limits. Thus the laboratory at the ocean discharging facility of the City of Santa Cruz's WWTF, in cooperation with the California Regional Water Quality Control Board at San Luis Obispo, developed equations that express the long-term relationship between TOC and BOD measurements in the Influent and Effluent of the facility. The 2-year study culminated in the development of the following changes to the facility's NPDES limits:The 30 day limit of 30 mg/L of BOD in the effluent with its site-specific equivalent of 15.5 mg/L of TOC.Replacing the 7 day average limit of 45 mg/L of BOD in the effluent with its specific equivalent of 25 mg/L of TOC. AndReplacing the 30 day minimum BOD removal of 85% with its site-specific equivalent of 70% TOC Removal.This study covered the typical wasteload (BOD/TOC) characteristics and the operational practices at the City of Santa Cruz WWTF from May 2005 through November 2006 to meet a standard definition of long-term requirement for the conversion exercise specified at 40CFR133.104(b).This presentation will discuss data and the equations derived for establishing the numeric equivalence between individual BOD control limits to their TOC control limits. In addition, a graph was developed for the use of operators to convert the legacyBOD values to TOC for process control activities.The effective realization of TOC equivalence for BOD at a secondary wastewater treatment plant improves the options for rapid process control, and cost effective compliance measurement. Thefollowing are tables of data and graphs developed for the transformation of the facility's NPDES effluent limits and process control options.