Controlled environment agriculture (CEA), specifically advanced greenhouses, plant factories, and vertical farms, has a significant role to play in the urban agri-food landscape through provision of fresh and nutritious food for urban populations. With the push towards improving sustainability of these systems, a circular or closed-loop approach for managing resources is desirable. These crop production systems generate biowaste in the form of crop and growing substrate residues, the disposal of which not only impacts the immediate environment, but also represents a loss of valuable resources. Closing the resource loop through composting of crop residues and urban biowaste is presented. Composting allows for the recovery of carbon dioxide and plant nutrients that can be reused as inputs for crop production, while also providing a mechanism for managing and valorizing biowastes. A conceptual framework for integrating carbon dioxide and nutrient recovery through composting in a CEA system is described along with potential environmental benefits over conventional inputs. Challenges involved in the recovery and reuse of each component, as well as possible solutions, are discussed. Supplementary technologies such as biofiltration, bioponics, ozonation, and electrochemical oxidation are presented as means to overcome some operational challenges. Gaps in research are identified and future research directions are proposed.

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Carbon dioxide enrichment is a technique employed in controlled environment agriculture (CEA) systems (i.e., vertical farms) to improve crop yield. However, the CO 2 for enrichment is sourced from fossil fuels, increasing the carbon footprint of these operations by adding CO 2 to the atmosphere. Sourcing CO 2 from biowaste instead can be a more sustainable option. Particularly, composting crop residues generated in CEA systems can generate CO 2 while valorizing biowaste. This study assesses the possibility of meeting the CO 2 demand for enrichment in a CEA system by composting residues of baby lettuce - the most common crop grown in CEA systems. Using theoretical modelling, we estimated the CO 2 required to grow baby lettuce in a hypothetical CEA system at an enriched CO 2 concentration of 1000 ppm. The CO 2 derivable by composting lettuce residues generated by a CEA system with two different types of hydroponic system (mat- and plug-based) was determined through aerobic incubation and CO 2 respirometry. Depending on the hydroponic system, composting crop residues generated in the hypothetical CEA system could produce around 1.4-6% of the total CO 2 required for enrichment. Comparing the estimated demand versus supply of CO 2 on a mass basis showed that composting crop residues alone is likely insufficient to meet the enrichment demands of a CEA system growing baby lettuce at 1000 ppm. Additional biomass (e.g., source separated urban biowaste) might be required to meet the CO 2 demand for enrichment solely through composting.

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