Abstract
This paper proposes a simple and easy-to-use methodology for forecasting the impact of changes in influent chemical oxygen demand (COD) and in the emission limit values (ELVs) of COD and total nitrogen on average energy requirements for aeration and sludge production by activated sludge wastewater treatment plants (WWTPs). The methodology is based on mass balances of sludge production and oxygen requirements for carbonaceous material biodegradation and/or nitrification, oxygen transfer and aeration equipment efficiency. Using average values of historical data of regular monitoring (water quality and operating conditions) WWTP-specific equations of oxygen requirements, energy consumption and sludge production are derived as a function of influent COD and influent N-total, which may be used to quantify the impact of influent and ELV changes. The methodology was tested in five extended aeration WWTPs for three scenarios established by the utility. The results show that increasing influent COD, from 900 to 1300 mg/L, for example, significantly increases the energy consumption by 49% and sludge production by 53%. For influent 54–68 mg/L N-total, imposing 15 mgN/L ELV results in a 9–26% increase in energy consumption. The COD ELV change studied (season-specific, from 150 mg/L 12 months/year to 125 mg/L 8 months/year to 100 mg/L 4 months/year) increases the energy consumption by 1.8–2.6% and the sludge production by 4.3–5.4%.
Highlights
The study of water resources and water pollution control has a long tradition
After characterizing the influent wastewater of the five extended aeration wastewater treatment plants (WWTPs) studied (Section 3.1), the assumptions needed for computing the oxygen and energy requirements as well as sludge wasting in each WWTP were comprehensively presented (Section 3.2); the calculations were conducted based on 5-year data and the impact of each change was quantified as a percentage of the baseline situation (Section 3.3)
The results show a Chemical Oxygen Demand (COD) influent increase from, for example, 900 to 1300 mg/L corresponds to results show a COD influent increase from, for example, 900 to 1300 mg/L corresponds to an oxygen consumption increase of 49% (Figure 3), an energy consumption increase of an oxygen consumption increase of 49% (Figure 3), an energy consumption increase of
Summary
The study of water resources and water pollution control has a long tradition. TheWater Framework Directive (WFD), Directive 2000/60/EC [1], has led to the reduction of Biochemical Oxygen Demand (BOD), Chemical Oxygen Demand (COD), nitrates (N)and phosphorus (P) in surface water and coastal waters. The. Water Framework Directive (WFD), Directive 2000/60/EC [1], has led to the reduction of Biochemical Oxygen Demand (BOD), Chemical Oxygen Demand (COD), nitrates (N). The results from the JRC’s modelling show that BOD, N and P loads in treated wastewater have decreased by 61%, 32% and 44%, respectively, since the adoption of the Urban Waste Water Treatment Directive (UWWTD) [2]. While for some (coliforms, certain chemicals and, to a large extent, BOD). This has been decisive in the improvement of the condition of the water bodies (including bathing waters), for BOD and nutrients the effect on water bodies is less apparent due to the importance of other sources of pollution [3]. While the UWWTD does not call for specific action on energy efficiency of wastewater collection or treatment activity, common economic sense has led operators of wastewater treatment plants (WWTPs) to invest in research and development and to upgrade some plants towards more energy efficient treatment processes [2].
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