A Deshpande and E Cross (Lutra Ltd.)
When analysing wastewater treatment plant performance and compliance of treated effluent discharge, two main parameters are tested: CBOD5 (carbonaceous biochemical oxygen demand), and total suspended solids (TSS).
CBOD5 testing is a time-consuming process that takes 5-6 days before the laboratory results are received, making it difficult to detect and react to any noncompliance issues in time. Further, the accuracy of this test has also been questioned since all the organic matter may not have degraded at the end of the 5-day test period. Moreover, it only measures biodegradable organic matter content.
Apart from the CBOD5, nutrients and pathogen parameters, discharge consents in Aotearoa typically specify a TSS limit. Generally, treated effluent from medium to large sized biological wastewater plants in Aotearoa have a TSS of around 10 mg/L or less. TSS testing is generally completed in line with the consent conditions, meaning that limited data is available through discrete data points, making it difficult to monitor and improve the performance of a plant.
So, what are the alternatives?
This paper will explore the feasibility of replacing CBOD5 tests with on-line Total Organic Carbon (TOC) measurements. There is a definite correlation between CBOD5 and TOC in typical municipal wastewaters. Internationally, various plants have successfully managed to revise their permits to allow them to use TOC as a surrogate for CBOD5 by establishing a ratio between the two parameters.
For TSS, we will explore the potential of replacing the tests with on-line measurements of turbidity to establish turbidity as a surrogate for TSS. It is noted that it may be feasible to establish a correlation between TSS and turbidity through an intense testing and analysis regime.
The key benefits of these alternative test regimes are as follows.
- Savings in time and resources,
- Availability of real-time data for performance monitoring, optimisation and troubleshooting,
- Availability of voluminous set of data during plant performance and upgrade studies.