The COD delusion?

A Chinese study published in December aims to quantify the role of Chemical Oxygen Demand (COD) – and its predominance as a way of measuring organic pollution – in causing unnecessary GHG emissions.

COD is widely used as an indicator of organic pollution in wastewater. Large amounts of organic matter are removed during treatment processes to meet environmental standards, and as a result, significant amounts of GHGs such as methane are released into the atmosphere.

An early 2021 study proposed that the use of COD was responsible for this, and that it was therefore flawed as an indicator of organic pollution in wastewater. The microbial degradation that can arise in aquatic environments when wastewater is present results from labile DOC (LDOC) and not refractory (RDOC).

The reserchers noted that the COD measurement approach encompasses refractory organic matter, or RDOC, which needn’t be considered a pollutant and could be a potential carbon sink.

What quantity of emissions could be avoided if RDOC wasn’t removed? Published in a December edition of the journal Science China Earth Sciences, the more recent study collected and analysed COD data from 86 worldwide municipal wastewater treatment plants (WWTPs) and applied a model published by the IPCC to estimate CH4 emissions due to recalcitrant organic compound processing in China’s municipal wastewater treatment systems.

The study was led by Professor Nianzhi Jiao (Xiamen University), Professor Yao Zhang, and Dr Zongqing Lv.

The researchers collected COD and biochemical oxygen demand (BOD) data of influent and effluent wastewater at 86 wastewater treatment plants from five continents around the world, and using (COD-BOD) as an indicator of refractory organic matter, estimated the proportions of refractory organics in influent and effluent. They found that 55% of the organic matter removed during the wastewater treatment was contributed by refractory organic matter.


GHG estimates

From this, the team estimated the total annual removal of refractory organic matter from China’s domestic wastewater treatment plants, and further estimated the maximum amount of GHG emissions using a model of organic matter conversion into GHGs. They found that the amount of methane released from the treatment of recalcitrant organic matters in 2018 could have been as high as 38.22 million tons of carbon dioxide equivalent, which amounts to the annual carbon sequestered by China’s wetlands. This suggests that the use of COD as an indicator for organic pollution is undue and needs to be revised to reduce the emission of GHGs.

COD is a monitoring indicator that has been used worldwide for hundreds of years. However, its use may be to the detriment of the carbon sink capabilities of water bodies. The authors believe that leaving the nontoxic recalcitrant organic matter in the wastewater may create a significant carbon sink and will save energy during the treatment process, advancing progress towards carbon neutrality in the wastewater treatment industry.

Measurement methods compared

THERE is no definitive signifier of the presence of organic compounds in waterways and effluent streams. Parameters like biochemical oxygen demand (BOD), chemical oxygen demand (COD) and total organic carbon (TOC) each characterise different aspects of the multi-faceted concept of “organic loading”.

BOD’s history can be traced to the early 20th century, and the early days of sewage treatment. Measuring BOD generally requires several dilutions of a sample and a significant amount of laboratory expertise. And the results it produces can be highly inaccurate.

The search for a better alternative to BOD led to the development of measurement techniques for COD, which have the advantage of using cheap and readily available chemicals (albeit very toxic and unpleasant ones) and can produce a result in around three hours. Its also provides reproducibility in the region of 1 or 2%. But COD is less specific than BOD, since it measures pretty much anything in the sample that can be chemically oxidized rather than just the levels of biologically active organic matter, as with BOD.

In an April 2021-published study, Jiao et al propose the replacement of COD with an optode-based BOD measurement when looking to determine organic pollution in natural aquatic environments.