[Assessing environmental and economical benefits of integrated sewage treatment systems]

Sewage treatment, treated water treatment and sludge treatment are three basic units of an integrated sewage treatment system. This work assessed the influence of reusing or discharge of treated water and sludge landfill or compost on the sustainability of an integrated sewage treatment system using emergy analysis and newly proposed emergy indicators. This system's value included its environmental benefits and the products. Environmental benefits were the differences of the environmental service values before and after sewage treatment. Due to unavailability of data of the exchanged substance and energy in the internal system, products' values were attained by newly proposed substitution values. The results showed that the combination of sewage treatment, treated water reuse and sludge landfill had the strongest competitiveness, while the combination of sewage treatment, treated water reuse and earthworm compost was the most sustainable. Moreover, treated water reuse and earthworm compost were helpful for improving the sustainability of the integrated sewage treatment system. The quality of treated water and local conditions should be also considered when implementing the treated water reuse or discharge. The resources efficiency of earthworm compost unit needed to be further improved. Improved emergy indices were more suitable for integrated sewage treatment systems.