Carbon dioxide emission reduction from anaerobic digesters by calcium carbonate precipitation with waste cementitious materials.

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Canada's densely populated cities treat their sewage in municipal wastewater treatment plants (WWTPs). Some municipalities use anaerobic digestion (AD) to stabilize sewer sludge. AD off-gas is approximately 65 % CH4 and 35 % CO2 by volume. Both CH4 ("biomethane" or "biogas") and CO2 are greenhouse gases (GHG). Biogas can be burned to produce electric power with a negative carbon footprint, or to produce thermal energy and/or steam, or flared because of the higher global warming potential (GWP) of CH4 (GWP: 28-36). AD is a trade-off; biosolids are consumed while GHGs are generated. AD is a smaller GHG producer than electrical power generation or transportation. WWTPs are, however, "(a)n overlooked source of carbon emissions". One recent study measured the impact of dissolved inorganic carbon from large Chinese municipal WWTP effluents on seawater. Others have demonstrated lab- and pilot-scale ADs with reduced CO2. ADs at the City of Ottawa municipal wastewater plant (ROPEC) produce ~14 million m3/a of biogas.The proposal is to asses if waste cement precipitates calcium carbonate in the AD and reduces the total carbon dioxide gas released. Waste cement recovered from demolition projects is an inexpensive source of calcium ions and alkalinity. The effect of mixing waste cement powder on AD inorganic carbonate chemistry has not been explored, and is the subject of this smaller, two year proposal for two PDFs, one masters, six co-op and two summer students. Solids/liquid ratio, cement surface area, residence time, and mixing dynamics effects would be studied with a 1 L gas-tight benchtop reactor. The inorganic carbon solid/liquid gas equilibria would be difficult to model exactly with the complex AD solution composition, however a simplified understanding of inorganic carbonate chemistry model n AD may be useful for targeting future CO2 release from municipal AD units. Logistical, economical, and geographical aspects of this project will also be modeled and assessed.The wastewater treatment experience of USP Technologies, and the carbon capture skills of Fortera USA would assist with investigating the precipitation of calcium carbonate in ROPECs ADs with waste cement.