A novel index of total oxygen demand for the comprehensive evaluation of energy consumption for urban wastewater treatment. (15th February 2019)

• Record Type: Journal Article
• Authors: Luo, Li; Dzakpasu, Mawuli; Yang, Baichuan; Zhang, Wushou; Yang, Yahong; Wang, Xiaochang C.
• Is Part Of: Applied energy. Volume 236(2019)
• Page Start: 253
• Page End: 261
• Publication Date: 2019-02-15
• Subjects: Wastewater treatment plants (WWTPs) -- Energy consumption -- Total oxygen demand (TOD) -- Treatment scale -- Loading rate; Power (Mechanics) -- Periodicals; Energy conservation -- Periodicals; Energy conversion -- Periodicals; 621.042
• DOI: 10.1016/j.apenergy.2018.11.101
• Languages: English
• ISSNs: 0306-2619

Abstract

Graphical abstract: Highlights: Total oxygen demand (TOD) is proposed for characterizing oxygen-consuming pollutants. TOD accounts for the mass of oxygen for both organics decomposition and nitrification. Energy consumption efficiency of WWTPs is evaluated by EO, the unit mass TOD removal. Operation data from 2022 WWTPs in China are analyzed using the novel EO index. Ammonia nitrification contributes almost half of the TOD based on WWTPs data in China. Abstract: In wastewater treatment plants (WWTPs), the majority of energy inputs is consumed by aeration systems to support both the biochemical oxidation of organics and transformation of ammonia-nitrogen into nitrate-nitrogen. Consequently, WWTPs energy efficiency evaluation only based on metrics derived from the organic constituents such as chemical oxygen demand (COD) or biological oxygen demand (BOD) may not reflect the true energy consumption of WWTPs with variable influent quality. Therefore, to overcome this limitation, total oxygen demand (TOD) is introduced in this article, and a novel index EO, namely the energy consumption for the removal of a unit mass of TOD is proposed for evaluating the energy efficiency in WWTPs. Furthermore, by considering the stoichiometric relations of oxygen consumption for the oxidation of both organics and ammonia-nitrogen, methods for calculating the EO are proposed. Using the novel EO index and the available annual operation data of 2022 WWTPs, the current status of energy consumption for wastewater treatment in China were analyzed. The findings show an average EO decrease from 5.2 kWh/kg to 1.2 kWh/kg as the WWTP loading rates increase from 20% to 100%. Also, EO decreased from 4.1 kWh/kg to 1.5 kWh/kg as the average TOD removal increased from 60% to over 90%. Moreover, EO decreased from 2.9 kWh/kg to 1.0 kWh/kg as the WWTP scale increased from less than 10, 000 m 3 /d to over 5, 00, 000 m 3 /d. Thus, the energy efficiency of WWTPs increases with increasing loading rates, TOD removal, and scale. Also, the wastewater treatment technology applied influences the EO significantly, especially for small- and medium-size WWTPs with capacities less than 50, 000 m 3 /d which account for circa 76% of all WWTPs in China. The WWTPs applying sequential batch tractors (SBR) tended to show lower average EO (<1.7 kWh/kg) than those applying anaerobic/oxic (A/O), anaerobic/anoxic/oxic (A 2 /O) and oxidation ditch (DO) (1.9 kWh/kg ≤ EO ≤ 2.0 kWh/kg). Thus, as an index of the energy consumption per unit mass of TOD removed, EO reflects the essence of wastewater treatment for pollutants removal in contrast to other existing energy indices based on the volume of treated wastewater. Moreover, due to the large variability of the WWTPs influent qualities, the TOD contributed by ammonia-nitrogen varied widely between 12.2% and 80.7% of the total TOD. Therefore, EO calculation based on TOD but not merely the organic component (COD or BOD) provides a more comprehensive index for evaluating and optimizing the energy efficiency of WWTPs.