Potentials for microalgae sequestration of carbon dioxide (CO2) from composting off-gas; a review

Food and other bio-waste management is an integral part of urban development and living. Composting is a common practice in most developed countries, while open burning and landfilling is widely used in waste management in low and middle income countries. The outputs of composting include compost, heat, leachate, and off-gas. The off-gas consists of carbon dioxide (CO2), methane, nitrous oxide, water vapor, ammonia and volatile organic compounds. The CO₂, although biogenic could contribute to climate change mitigation if the emissions are controlled. CO₂ sequestration using microalgae has been widely reported has been widely reported as a viable alternative to geological storage. CO₂ sources in microalgal cultivation include ambient air, composting off-gas, combustion flue gas, wastewater aeration gas, syngas, and biogas. Carbon dioxide from composting can be used in controlled environment agriculture instead of commercially produced alternative, or from ambient air. This review examines the available information on composting off-gas dynamics, particularly CO₂ evolution, and the challenges and prospects of CO₂ use in microalgal cultivation, ensuring circularity in the composting process. This review recommends the utilization of CO₂ from composting as alternative to direct air extraction. However, achieving higher CO₂ concentration relative to oxygen is challenging. While efforts are made towards reduction in greenhouse gas emissions during composting, near zero oxygen concentration in the off-gas is essential to enhancing CO₂ utilization in microalgae cultivation. This should be achieved without compromising compost quality such as germination index and chemical oxygen demand/heavy metals reduction efficiency.