A comparison of different scenarios for on-site reuse of blackwater and kitchen waste using the life cycle assessment methodology

摘要

Decentralized systems for treatment of domestic effluents have been suggested as a sustainable alternative for situations where conventional sanitation has not yet been established. This study compares three possible scenarios for the destination of separated blackwater (BW) and kitchen waste (KW), being: co-digestion of both (BW &KW) in loco with subsequent reuse of sub-products like nutrients and water (scenario 1); discharge of BW into a septic-tank and composting of the KW (scenario 2) and discharge of the BW into a septic-tank and of the KW at a solid waste disposal (scenario 3, actually the most common situation in large part of Brazil). In order to compare the scenarios, the use of equivalent amounts of water and fertilizer was considered in scenarios 2 (septic-tank and composting) and 3 (septic-tank and landfill), in order to maintain functional equivalence with that of scenario 1 (co-digestion and reuse). In scenario 1, treatment of the organic waste in an Upflow Anaerobic Sludge Blanket (UASB) reactor, for a period of 6 (scenario 1A) or 12 days (scenario 1B), with effluent recycling, was considered. In order to verify the effect of the use of energy in scenario 1 and to verify the effect of the degree of treatment realized in scenarios 2 and 3, a sensitivity analysis was realized for the parameters energy use and COD removal. The management of the sub-products in scenario 1 resulted in emission gains, when compared to scenarios 2 and 3, where the need to include amounts of water and fertilizer, to obtain functional equivalence, increased the environmental impacts associated to these scenarios. However the energy consumption increased the impact levels in scenario 1 and, overall, scenario 2 (septic-tank and composting), obtained the lowest impact scores, presenting the most advantageous scenario, among the three scenarios studied, from an environmental point of view, considering the restrictions as defined here, followed by scenarios 3 (septic-tank and landfill), 1A (6-day co-digestion and reuse) and 1B (12-day co-digestion and reuse). After the sensitivity analysis the influence of energy consumption on the impact categories for scenario 1 became evident, whereas, on the other hand, the degree of pollution removal from the blackwater did not change the overall results, making thus minimizing energy consumption during treatment more relevant than improving pollutant removal, in order to minimize the environmental impacts.