Controlled environment agriculture (CEA) is a nascent technology for sustainable vegetable production. However, mismanagement of biowaste produced stymies CEA's sustainability. Closed- loop CEA systems follow a circular resource-loop, wherein the biowaste is valorized to recover resources. This work investigated the viability of composting CEA biowaste to close the CO2 loop for CEA systems. First, a theoretical framework was delineated for a compost-based closed-loop CEA system. Next, a respirometric system was developed to characterize the CO2 recovery of CEA biowaste. Theoretical analyses indicated a substantial deficit of CO2 recoverable by composting CEA biowaste of baby lettuce needed for atmospheric enrichment. Therefore, spent coffee grounds (SCG) as an additive to meet the deficit was tested using respirometric methods. Composting baby lettuce biowaste and SCG at a 1:1 ratio could still be insufficient to meet the CO2 deficit. Large-scale composting of external waste could be required to establish a viable closed-loop CEA system.