Capturing atmospheric carbon dioxide by depleting inorganic carbon in municipal wastewater

Abstract CO2 removal from the atmosphere will likely be necessary to limit global warming to 2 ℃. Existing wastewater infrastructure in the U.S. conveys a total of 588 Mt of inorganic carbon to wastewater treatment plants, which are designed to remove organic carbon, but do not remove the inorganic fraction. We believe that embedded energy used for wastewater conveyance may be leveraged to remove inorganic carbon and produce wastewater treatment plants that are net carbon negative. To demonstrate this, we optimized a bench-scale wastewater carbon-capture system composed of a gas permeable membrane and a pressurized feed. We investigated the effects of multiple physicochemical parameters on inorganic carbon removal. The best performance resulted in removal of 15% inorganic carbon from the feed stream. Deploying similar full-scale systems across U.S. wastewater infrastructure without addition of acid would remove up to 12.9 Mt-C/yr. Hydrochrloric acid addition to one pH unit below the bicarbonate pKa would increase removal to 30.5 Mt-C/yr, but this is partially offset by CO2 emissions from hydrochloric acid production, resulting in a net removal of 22.6 Mt-C/yr. Further research should focus on increasing removal efficiency, which, at 100% removal, would offset 11.2% of total U.S. CO2 gas emissions.