Assessment of Chemical Oxygen Demand/Total Organic Carbon (COD/TOC) ratios in refinery effluents

15 Sep 2022

Chemical Oxygen Demand (COD) analysis is one of the most commonly used analytical methods to indirectly measure organic pollutants in effluent waters. The standard COD methods use potassium dichromate which is restricted under the REACH regulation. The European Commission (EC) and EU member states are considering whether to replace COD analysis with Total Organic Carbon (TOC) analysis for effluent discharge monitoring. An empirical relationship has been applied between TOC and COD in regulatory contexts, where a global COD/TOC ratio of 3 is most commonly suggested. In the Commission Implementing Decision establishing Best Available Techniques (BAT) conclusion for the refining of mineral oil and gas (REF BATc), it states that where on-site correlation is available, COD may be replaced by TOC and the correlation between COD and TOC should be elaborated on a case-by-case basis. Further, the REF BATc states that TOC monitoring would be the preferred option because it does not require the use of very toxic compounds, i.e., potassium dichromate, in laboratory testing.

In this report we assess if, and which, value of COD/TOC ratio would be applicable for treated wastewater effluents arising from oil refineries. To achieve this, we first performed a scientific literature study on reported COD/TOC ratios in effluents. As a second step, we investigated the COD/TOC ratio for eight refinery effluents.

In conclusion COD/TOC ratios varied, and a single global COD/TOC ratio could not be established for refinery effluents due to the specific conditions of each refinery. COD/TOC ratios specific to each site is challenging to establish. Nevertheless, a site-specific assessment allows for a more meaningful discussion on values that could be considered for discharge permits. In the case that it is not possible to establish a reliable COD/TOC ratio for a specific site, and thus the COD analysis cannot be omitted, we would recommend the use of the Sealed Tube (ST) COD method to achieve significant reduction in the laboratory use of hazardous chemicals compared to the traditional open reflux method.