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generation; a 0.9 factor to account for consumptive use. This effective concentration has been <br /> approved in consultation with MassDEP. <br /> Most Ground Water Discharge Permits(GWDPs)granted by MassDEP for private WWTFs are <br /> assigned an effluent TN concentration of 10 mg/L_ For larger municipal WWTFs(typically with <br /> flows greater than 1 million gallons per day),MassDEP has approved effluent TN concentrations <br /> as low as 3 mg/L. Average effluent concentrations for the existing WWTFs in Mashpee range <br /> between 2.4 and 9 mg/L; these concentrations are used in the MEP modeling of existing <br /> conditions in the Popponesset and East Waquoit Technical.Reports_ <br /> Twelve alternative on-site septic systems currently have provisional use approval from MassDEP <br /> for nitrogen reduction.8 The provisional use permits limit the installation of these technologies <br /> to 50 units with the thought that monitoring results will be used to guide regulatory standards for <br /> a general use installation permit. Generally, these use permits have nitrogen effluent limits of 19 <br /> mg/l for residential uses and 25 mg/l for commercial uses. In the scenarios discussed in this <br /> memo, S&W used 10 mg/1 TN for alternative septic systems, while LAI used 3.75 mg/1 TN. <br /> Since the assigned wastewater effluent concentrations are the key to determining the wastewater <br /> nitrogen loads, care should be taken to ensure that these concentrations are appropriate based on <br /> the performance of these technologies and how they are likely to be permitted in regulatory <br /> settings. This step will allow the Sewer Commission to have confidence that remediation goals <br /> in Popponesset and East Waquoit Bays will be attained. <br /> i <br />