Addressing BOD5 Limitations Through TOC Correlations: A Five Facility International Investigation

Wastewater treatment plants (WWTPs) are challenged with the objective of treating wastewater to a level that is acceptable for discharge into receiving waters (lakes, streams, rivers, estuaries, bays, etc.). Regulatory agencies require that WWTPs routinely measure and report by permit, the biochemical oxygen demand (BOD) of raw and treated wastewater to determine the strength and wastewater loadings to and from WWTPs (USEPA, 2000).

BOD5 Limitations
Although the BOD5 test is still a widely accepted parameter by regulators for characterizing water and wastewater quality, it is inaccurate, unreliable, not effective at low concentrations, and it cannot be used for process control or real-time monitoring due to the time it takes to receive test results, at least five days (Constable, 1979). Wastewater treatment plants can discharge treated effluent that may not meet permit limitations for up to five days before a compliance issue can be identified by the BOD5 test.

TOC: A New Approach
The wastewater industry has expressed a need for an analytical test to replace BOD5 that can provide quantifiable, precise and timely measurements of receiving stream wastewater loadings and plant removal efficiencies, in addition to providing monitoring and process control capabilities. TOC and/or COD have been commonly accepted and used in industrial wastewater treatment as key parameters for process monitoring and control. Global regulations allow chemical oxygen demand (COD) or total organic carbon (TOC) to be substituted for BOD5 when a long-term BOD5:COD or BOD5:TOC correlation can be demonstrated.

Wastewater treatment professionals are very eager to make a change from the BOD5 regulatory reporting requirements. TOC as an alternative to the BOD5 analysis will save WWTPs in analytical costs and potentially could reduce chemical and energy costs if used for process control purposes. TOC as a BOD5 alternative will also increase plant operations efficiency and response time to waste loading upsets.

Although there have been many studies that examine the correlation of BOD5 to TOC; research is needed to demonstrate that TOC can be reasonably used to monitor plant performance as an alternative to BOD5 measurements for permitting purposes. This research report presents the correlation of BOD5:TOC for five facilities of various flows, treatment levels and climates and present site-specific equations for each site. The correlations and the statistical significance of each are also summarized.

TOC Research Findings
A statistically significant BOD5:TOC correlation was demonstrated when all BOD5 data sets throughout the treatment process including raw sewage, primary effluent, and final effluent were plotted against their respective TOC data sets. Using all BOD5 data sets throughout the treatment process (from influent to effluent), is what is required to demonstrate a site-specific BOD5:TOC correlation to allow a TOC limit to be substituted for a BOD5 limit in permits.

When considering the final effluent data set independently (where low BOD5 concentrations are present), a statistically significant BOD5:TOC correlation was difficult to be demonstrated due to the inherent inaccuracies of and ineffectiveness the BOD5 measurement at low BOD5 concentrations.

In addition, to guide wastewater treatment plants in developing a site-specific TOC:BOD5 correlation that could be used to allow a TOC limit to be substituted for a BOD5 limit in permits, an implementation protocol is presented. 

Correlation Study

Now Available for Immediate Download

Please see ITA's newest research report regarding BOD5:TOC correlations for more information.

Use ITA's report to learn how to correlate BOD5 to TOC for permit modification using the report's Implementation Protocol for WWTPs to develop their own site-specific BOD5:TOC correlation study.

This report also presents comparative analysis and summary for 5 WWTPs of various flows, treatment levels and climates and identifies long-term practices of using TOC as a process control parameter-specifically to monitor the removal of organics in a wastewater treatment process and the utilization of carbon balances.