Radiocarbon analysis as a tool to understand the fate of fossil carbon in water recovery processes

The greenhouse gas (GHG) emission accounting procedures published by the Intergovernmental Panel on Climate Change (IPCC) assume that carbon dioxide (CO2) generated by water resource recovery facilities (WRRFs) in the treatment process is modern (or biogenic) thus is omitted. This assumption is applied to all types of industrial and municipal WRRFs. Using radiocarbon dating we confirmed that a combination of biogenic and fossil carbon enter municipal WRRFs. In addition, we found that the fossil carbon content varies with the type of industrial wastewater. The fossil content from the petrochemical refinery and the pulp/paper mill wastewater treatment plants set the boundary for the fossil carbon, with one containing almost exclusively fossil carbon and the other almost no fossil carbon, respectively. We also compared the fossil carbon content at different points of the WRRF with their GHG potential, calculated as CO2 equivalent from ideal combustion. The results showed that secondary sludge had the highest CO2 equivalent emission potential of up to 0.294 kg fossil CO2 equivalent m-3wastewater treated. Our calculations showed that sludge-generated biogas and biosolids had little potential to produce direct GHG emission, thus digesting secondary sludge instead of disposal may reduce the overall fossil GHG emission. Our study showed that a municipal WRRF can contribute a fractional fossil emission which may increase the current IPCC WRRF GHG inventory up to 23%.