A comprehensive substance flow analysis of a municipal wastewater and sludge treatment plant.

The fate of total organic carbon, 32 elements (Al, Ag, As, Ba, Be, Br, Ca, Cd, Cl, Co, Cr, Cu, Fe, Hg, K, Li, Mg, Mn, Mo, N, Na, Ni, P, Pb, S, Sb, Se, Sn, Sr, Ti, V, and Zn) and 4 groups of organic pollutants (linear alkylbenzene sulfonates, bis(2-ethylhexyl)phthalate, polychlorinated biphenyl and polycyclic aromatic hydrocarbons) in a conventional wastewater treatment plant were assessed. Mass balances showed reasonable closures for most of the elements. However, gaseous emissions were accompanied by large uncertainties and show the limitation of mass balance based substance flow analysis. Based on the assessment, it is evident that both inorganic and organic elements accumulated in the sewage sludge, with the exception of elements that are highly soluble or degradable by wastewater and sludge treatment processes. The majority of metals and metalloids were further accumulated in the incineration ash, while the organic pollutants were effectively destroyed by both biological and thermal processes. Side streams from the sludge treatment process (dewatering and incineration) back to the wastewater treatment represented less than 1% of the total volume entering the wastewater treatment processes, but represented significant substance flows. In contrast, the contribution by spent water from the flue gas treatment process was almost negligible. Screening of human and eco-toxicity by applying the consensus-based environmental impact assessment method USEtox addressing 15 inorganic constituents showed that removal of inorganic constituents by the wastewater treatment plant reduced the toxic impact potential by 87-92%.

Aeration tank settling and real time control as a tool to improve the hydraulic capacity and treatment efficiency during wet weather: results from 7 years' full-scale operational data.

This paper investigates the aeration tank settling (ATS) operation in combination with real time control (RTC) as a tool for increasing the hydraulic capacity and improving the treatment efficiency of a wastewater treatment plant (WWTP) during wet weather flows. Results from 7 years' full-scale operational data at the Avedøre WWTP, Denmark, show that ATS operation in combination with RTC increases the hydraulic capacity of the treatment plant with up to 150 and 67% of the design capacity during winter and summer respectively. Compared to the conventional wet weather operation, the ATS in combination with RTC operation resulted in lower effluent concentrations for total phosphate (40-50%), suspended solids (30-60%) and chemical oxygen demand (30-50%), whereas no significant effect was observed on total nitrogen. Apart from the reduced effluent concentrations, the RTC resulted in economic savings in the form of reduced costs for electricity and green taxes. However, in very few cases the ATS operation in combination with RTC was not able to handle design capacity, and some overflows occurred at flows below the design capacity. The frequency of these overflows may increase in the future due to increased rain intensity resulting in shorter prediction time available for ATS.

Energy savings by reduced mixing in aeration tanks: results from a full scale investigation and long term implementation at Avedoere wastewater treatment plant.

The aim of this project was to investigate the potential of reducing number of mixers in the biological treatment process and thereby achieve energy and economical savings and contribute to cleaner environment. The project was carried out at Avedoere wastewater treatment plant and a full scale investigation was conducted to study the effect of reduced mixing on flow velocity, suspended solid sedimentation, concentration gradients of oxygen and SS with depth and treatment efficiency. The only negative effect observed was on flow velocity; however the velocity was above the critical velocity. The plant has been operating with 50% of its designed number of mixers since September 2007 and long term results also confirm that reduced mixing did not have any negative effect on treatment efficiency. The estimated yearly electricity saving is 0.75 GWh/year.

Modelling and control strategy testing of biological and chemical phosphorus removal at Avedøre WWTP.

The biological phosphorus removal process is often implemented at plants by the construction of an anaerobic bio-p tank in front of the traditional N removing plant configuration. However, biological phosphorus removal is also observed in plant configurations constructed only for nitrogen removal and simultaneous or post-precipitation. The operational experience with this "accidental" biological phosphorus removal is often mixed with quite a lot of frustration, as the process seems to come and go and hence behaves quite uncontrollably. The aim of this work is to develop ways of intentionally exploiting the biological phosphorus process by the use of instrumentation, control and automation to reduce the consumption of precipitants. Means to this end are first to calibrate a modified ASM2d model to a full-scale wastewater treatment plant (WWTP), including both biological and chemical phosphorus removal and a model of the sedimentation process. Second, based on the calibrated model a benchmark model is developed and various control strategies for biological phosphorus removal are tested. Experiences and knowledge gained from the strategies presented and discussed in this paper are vital inputs for the full-scale implementation of a control strategy for biological phosphorus removal at Avedøre WWTP, which is described in another paper. The two papers hence show a way to bridge the gap from model to full implementation.

Introducing biological phosphorus removal in an alternating plant by means of control: a full scale study.

In this paper, a control strategy for introducing enhanced biological phosphorus removal (EBPR) in an alternating plant designed for enhanced biological nitrogen removal (EBNR) is presented. Alternating aerobic and anaerobic conditions to promote EBPR are provided by controlling the phases of the operational cycle, instead of a separate anaerobic volume. By utilising the control schemes already built in the STAR control system for nitrogen removal, the control strategy is fully integrated in the system. The control system relies on on-line measurements of nitrogen (ammonia and/or nitrate) and orthophosphate. The control strategy has been implemented in full-scale operation at the Avedøre wastewater treatment plant in Denmark and the results show clear indications of success. The control strategy has operated robustly for several months with a 60% decrease in use of precipitation chemicals.

Economic and environmental optimization of phosphorus removal.

Do operating costs conform to environmental impact after introduction of charges on discharge of wastewater? A study on optimization of phosphorus removal at two Danish wastewater removal plants shows that this is actually the case. By measurement of inlet and outlet concentrations and of chemicals added it was possible to determine the relationships between chemical dosing and phosphorus discharge and thus calculate the operational cost and environmental impact of different dosing/discharge levels.

Environmental accounting--a decision support tool in WWTP operation and management.

The various emissions to water, air and soil from the municipal wastewater treatment plant of Avedore Wastewater Service Company are accounted for and quantified in terms of the environmental impacts to which they contribute: global warming, acidification, eutrophication, space demand for controlled deposition of residues, as well as persistent toxicity, human toxicity and eco-toxicity. The impacts are expressed on the same scale, namely as fraction of the total per capita loads in a national scenario 1990, also called the person equivalent or PE1990. This provides a compact and informative overview of the environmental impacts and allows for a holistic prioritisation in the operation and management of the plant. The accounting shows that the resulting emissions per person in the catchment area of the plant correspond to 0.5-5.0% of the average Danish PE1990 for the impacts in question.