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Capturing Atmospheric Carbon Dioxide by Depleting Inorganic Carbon in Municipal Wastewater
AuthorArabi, Seyed Mohammad Saeed
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CO2 removal from the atmosphere is likely necessary to limit global warming to the 2 °C goal of the Paris Agreement. This work aims to leverage the embedded conveyance energy within the existing wastewater infrastructure in the U.S. to remove inorganic carbon and develop a carbon negative CO2 removal technology. Although wastewater treatment plants are designed to remove organic carbon, a total of 588 Mt of inorganic carbon also enters the plants but is not removed. To demonstrate this, a bench-scale, membrane-based wastewater carbon-capture system was optimized. Commercially available, gas-permeable membranes (PFA) and hydrophobic, porous membranes (PVDF) fabricated in-house were evaluated in the system. The effects of multiple physiochemical parameters on inorganic carbon removal were investigated, with the best-case scenario removing 15% of the inorganic carbon from the feed stream. Deploying similar full-scale systems across US wastewater infrastructure without addition of acid for pH adjustment would remove up to 12.9 Mt-C/yr. The addition of hydrochloric acid (HCl) to bring the wastewater to 5.0 (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 HCl production, resulting in a net removal of 22.6 Mt-C/yr. When compared to direct air capture, a more mature technology, the new system was more sustainable at reduced feed stream pH (2.5) based on net CO2 removal.