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Environmental sciences, Hydrology, Water resources, Civil engineering, Environmental and geological engineering
Wastewater arising from the Coachford agglomeration is treated at the existing Coachford Waste Water Treatment Plant (WWTP). This wastewater treatment facility has a primary treatment only with a design capacity of 450 population equivalent (PE) and it discharges the effluent into the Inniscarra Reservoir. The discharge from Coachford WWTP has been identified by the Lower Lee - Owenboy Water Management Unit Action Plan (WMUAP) as a point source pressure on the waters of the Lower Lee - Owenboy catchment and also as a cause of the strongly eutrophic status of Inniscarra Reservoir water quality by the EPA in 2009. Therefore, new emission limit values (ELVs) for the main water quality parameters in the effluent from Coachford WWTP have been proposed by the EPA to ensure compliance with the relevant water quality standards downstream of effluent discharging point. For lakes, the most important water parameters are phosphorus and nitrogen and the relevant environmental quality standards define permissible concentrations for total phosphorus and total ammonia as 0.025 mg/l and 0.14 mg/l respectively. The new ELVs will be implemented by the end of 2018 after the construction of a new WWTP in Coachford. The impact of the new ELVs on water quality of receiving water has been examined by a previous assimilative capacity model developed by NUI Galway. This model used effluent discharge loading reflecting the current number of population in Coachford. However, an increase in the PE of the agglomeration is expected in the future and there is a need to predict the impact of the increased effluent loading from Coachford WWTP on the receiving water. An analogous assimilative capacity model to the NUI Galway (NUIG) model has been developed in this study based on simple mass balance computation. The newly developed model was used to predict concentrations of main water quality parameters downstream of effluent discharging point due to: (1) increased effluent discharge loadings representing the projected increase in number of population in Coachford agglomeration while maintaining the proposed ELVs; (2) a range of ELVs to test the effect of increasing the emission limit values on the water quality of the downstream reach of the lake. The calibrated MBAC model was then used to estimate the concentration (Cout) of TP and TAmmonia at the complete mixing zone downstream of the discharge point of Coachford under: (i) a range of Qeff scenarios representing the projected increase in number of population in Coachford agglomeration; and (ii) a range of ELVs to test the effect of increasing the emission limit values on the water quality of the downstream reach of the lake.
The MBAC model predictions indicate that the concentrations of TP in the downstream complete mixing zone are unlikely to exceed the standard value of 0.025 mg/l under the proposed ELV of 1.2
mg/l and also under a higher value of 2.0 mg/l for TP. However, if the ELV is increased to 5 or 10 mg/l the concentrations of TP in the downstream complete mixing zone is likely to exceed the standard value of 0.025 mg/l under low flow conditions. The MBAC model predictions for TAmmonia indicate that under the tested ELVs (5, 6.5, 10, and 15 mg/l), the concentrations of TAmmonia in the downstream complete mixing zone is unlikely to exceed the standard value of 0.14 mg/l.
Bedri, Z., Nasr, A., Crowley, S. (2017) Mass-balance assimilative capacity model of Inniscarra Reservoir. National Hydrology Conference, Athlone, Ireland, 21 November, 2017.