Techno-economic characteristics of wastewater treatment plants retrofitted from the conventional activated sludge process to the membrane bioreactor process

While a growing number of wastewater treatment plants (WWTPs) are being retrofitted from the conventional activated sludge (CAS) process to the membrane bioreactor (MBR) process, the debate on the techno-economy of MBR vs. CAS has continued and calls for a thorough assessment based on techno-economic valuation. In this study, we analyzed the operating data of 20 large-scale WWTPs (capacity 10000 m(3)/d) and compared their techno-economy before and after the retrofitting from CAS to MBR. Through cost-benefit analysis, we evaluated the net profit by subtracting the operating cost from the environmental benefit (estimated by the shadow price of pollutant removal and water reclamation). After the retrofitting, the removal rate of pollutants increased (e.g., from 89.0% to 93.3% on average for NH3-N), the average energy consumption increased from 0.40 to 0.57 kWh/m(3), but the operating cost did not increase significantly. The average marginal environmental benefit increased remarkably (from 0.47 to 0.66 CNY/g for NH3-N removal), leading to an increase in the average net profit from 19.4 to 24.4 CNY/m(3). We further scored the technical efficiencies via data envelopment analysis based on non-radial directional distance functions. After the retrofitting, the relative cost efficiency increased from 0.70 to 0.73 (the theoretical maximum is 1), while the relative energy efficiency did not change significantly. The techno-economy is closely related to the effluent standard adopted, particularly when truncating the extra benefit of pollutant removal beyond the standard in economic modeling. The modeling results suggested that MBR is more profitable than CAS given stricter effluent standards.

Automated summary

This study compares the techno-economy of the conventional activated sludge (CAS) process and the membrane bioreactor (MBR) process by analyzing the operating data of 20 large-scale wastewater treatment plants. The results show that after retrofitting to MBR, pollutant removal efficiency increases and energy consumption also increases, but operating costs do not significantly increase. Additionally, the environmental benefit and net profit increase. Technological efficiency, particularly cost efficiency, improves after the retrofitting. The study emphasizes the importance of effluent standards in determining the techno-economic viability of different processes.