Correcting a major error in assessing organic carbon pollution in natural waters

Abstract

Microbial degradation of dissolved organic carbon (DOC) in aquatic environments can cause oxygen depletion, water acidification, and CO2 emissions. These problems are caused by labile DOC (LDOC) and not refractory DOC (RDOC) that resists degradation and is thus a carbon sink. For nearly a century, chemical oxygen demand (COD) has been widely used for assessment of organic pollution in aquatic systems. Here, we show through a multicountry survey and experimental studies that COD is not an appropriate proxy of microbial degradability of organic matter because it oxidizes both LDOC and RDOC, and the latter contributes up to 90% of DOC in high-latitude forested areas. Hence, COD measurements do not provide appropriate scientific information on organic pollution in natural waters and can mislead environmental policies. We propose the replacement of the COD method with an optode-based biological oxygen demand method to accurately and efficiently assess organic pollution in natural aquatic environments.

Anthropogenic Inputs of Terrestrial Organic Matter Influence Carbon Loading and Methanogenesis in Coastal Baltic Sea Sediments

“…This estimate is based on an assumed Gaussian distribution of total COD emissions from Swedish forest industries over the period 1930-2000, fitted to the existing data from the Swedish Forest Industries Federation (Supplementary Figure S2) and corrected downwards assuming that only 5-15% of Swedish forest industry point source emissions enter the Baltic Sea, based on the locations of point sources in the maps of Norrlin and Josefsson (2017). The conversion from COD to total organic carbon (TOC) is based on the regression for highlatitude systems presented by Jiao et al (2021). This exercise yields a total carbon input of 3.2-9.5 million tonnes, up to an order of magnitude greater than the estimated carbon stock in the 29 mapped locations (Table 4).…”

“…b Total emissions in Sweden, modeled using data for COD from Swedish Forest Industries Federation from 1978 to 2000 and hindcasted to 1930 assuming a parallel Gaussian distribution to that observed in Finland (Supplementary FigureS2). c Estimated using the regressions for BOD and COD vs DOC in high-latitude systems presented inJiao et al (2021), assuming DOC TOC.…”

Excessive greenhouse gas emissions from wastewater treatment plants by using the chemical oxygen demand standard

“…Using the classical COD indicator has disadvantages, and in fact, a high COD value does not necessarily correspond to the poor water quality. There are many natural water environments with a richness of RDOM that are of high water quality, while the COD does not meet the standards (Aoki et al, 2004;Jiao et al, 2021;Räike et al, 2012). It has been reported that effluent wastewater from paper mills contains large amounts of lignin-based recalcitrant organic material that can contribute to COD but is not acutely toxic and is very similar to natural organic matter (Archibald et al, 1998).…”

“…It has been reported that a significant contributor to COD is recalcitrant dissolved organic matter (RDOM) (Jiao et al, 2021), which accounts for the majority of all dissolved organic matter and can act as an important carbon sink (Hansell, 2013;Jiao et al, 2010). RDOM exists in large quantities in influent wastewater, the wastewater treatment processes, and effluent wastewater (Archibald et al, 1998;Bockhorn et al, 2005;Jin et al, 2011;Lu et al, 2018).…”

“…BOD reflects the amount of oxygen consumed by biological respiration of organic matter in water bodies (Reynolds, 2002). Therefore, the difference between COD and BOD can be regarded as an approximation of nonbiodegradable RDOM in natural oxygenated waters (Jiao et al, 2021), which is here termed refractory COD (rCOD). The organic matter classified as rCOD can be degraded during anaerobic digestion processes that are usually carried out under severe conditions, such as extremely low pH, high temperature, and high buffering capacity (Ward et al, 2008).…”