Abstract
In the last decade, China has sharply tightened the monitoring values for wastewater treatment plants (WWTPs). In some regions with sensitive discharge water bodies, the values (24 h composite sample) must be 1.5 mg/L for NH4-N and 10 mg/L for total nitrogen since 2021. Even with the previously less strict monitoring values, around 50% of the wastewater treatment plants in China were permanently unable to comply with the nitrogen monitoring values. Due to the rapid changes on-site to meet the threshold values and the strong relation to energy-intensive aeration strategies to sufficiently remove nitrogen, WWTPs do not always work energy-efficiently. A Chinese WWTP (450,000 Population equivalents or PE) with upstream denitrification, a tertiary treatment stage for phosphorus removal and disinfection, and aerobic sludge stabilisation was modelled in order to test various concepts for operation optimisation to lower energy consumption while meeting and undercutting effluent requirements. Following a comprehensive analysis of operating data, the WWTP was modelled and calibrated. Based on the calibrated model, various approaches for optimising nitrogen elimination were tested, including operational and automation strategies for aeration control. After several tests, a combination of strategies (i.e., partial by-pass of primary clarifiers, NH4-N based control, increase in the denitrification capacity, intermittent denitrification) reduced the air demand by up to 24% and at the same time significantly improved compliance with the monitoring values (up to 80% less norm non-compliances). By incorporating the impact of the strategies on related processes, like the bypass of primary settling tanks, energy consumption could be reduced by almost 25%. Many of the elaborated strategies can be transferred to WWTPs with similar boundary conditions and strict effluent values worldwide.
Highlights
A Chinese WWTP (450,000 Population equivalents or PE) with upstream denitrification, a tertiary treatment stage for phosphorus removal and disinfection, and aerobic sludge stabilisation was modelled in order to test various concepts for operation optimisation to lower energy consumption while meeting and undercutting effluent requirements
Even though intermittent aeration (S5 and S6) is a strategy that would require m significant changes in the WWTP equipment, and the current tanks configuration and geometry (PFR) is not for this type of nitrogen removal configuration—round tanks are the 1u9soufa2l3configura [17] —a flexible operation shows to be an effective way to deal with the somet unfavourable conditions for denitrification that are common in China
In the pre-treatment and biological treatment stages, sometimes small changes in operating and automation strategies can contribute to large savings in energy and resource consumption; for example, the reduction of the dissolved oxygen (DO) set point, the bypass of primary clarifiers or the reduction of the aerated volume or the incorporation of ammonium sensors to the aeration control loop
Summary
WWTPs are a significant energy consumer, with electricity representing one of the major operational costs in most WWTPs around the world [1]. Another study in Brazil estimates that the energy represents 11–31% of the total operational costs in WWTP with activated sludge and nutrient removal [3]. It is estimated that WWTPs represent about 1–3% of the overall energy use of a country [5]; in EuroIpt ei,stehsetiomvaetreadlltehlaetctWriWcitTyPussreeopfreWseWntTaPbsou>t 210–030%PoEf, itsheaboovuetra0l.l8e%neorfgy use of the total electriccoituyntcroyn[s5u];minpEtiuornopine, the oEvUer-2al8l e[6le]c. The rise due to the modernisation aInndreacdeanpttyaetaiorsn, oinftperrnoaclesesneesrgtoy mcoenestutmheptiinocnreinasWinWglTyPsstrsienegmensttoefbfleuoenntthe rise d discharge standtoartdhse tmhaotdaeprnpilsyatiinona agnrdowaidnagptnautimonbeorf opfrcooceusnstersietso [1m].eeTt htihseisinhcaresabsienegnly stringe observed in Cheinffalu, ewnht edriescthhaergefeflsutaenndt adridsschthaargt eapsptalyndinaradgsrhoawviengbeneunmtbigehr toefnceodunintrrieesce[1n]t. This is h years [8]. The power reduction for mixing in anoxic and anaerobic tanks can offer advantages for the energy efficiency, and to the process efficiency, as reduced surface turbulence minimises the transfer of oxygen to the sludge liquor [5]
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