Sewer Main Line Extension Agreement - St Joseph County

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Ms. Jessica Clark DRAFT SJEC Cost Opinion & Sewer Analysis Page 4 1 September 21, 2015 flow rate; however, as the District builds out, additional warehousing and/or low water demand manufacturing will likely develop eventually and lower the actual average rate. Additional analyses of the flow rates within SJCEDD2 are recommended as the district develops, but are not currently required for the purpose of this analysis. The flows identified from tributary areas and the 2,200 gpd/Ac flow rate, were summed together for each leg of the sewer system as shown in the Sewer Capacity Calculations (Exhibit E, attached). The original calculations did not include a peaking factor since industrial flows typically do not have extreme peaks like residential or commercial flaws. If a particular industry does have extensive peaking factors, the local pretreatment ordinance will typically require that flows be regulated at the discharge to be no more than a certain rate or peaking factor. Very often the peak rate is identified as 1.5 or 2 times the average or normal rate. Another way to consider the peaking factor is to use a typical peaking factor like one based on the Ten States Standards methodology. Since it is unclear which peaking factor would be appropriate, all three methods (no peaking factor, 2.0 peaking, and Ten States Standards peaking factor) were calculated and shown in the Sewer Capacity Calculations. Results For this analysis, it was apparent that using the Ten States peaking factor would produce extremely high flow rates, which together with other flows from the District, would likely exceed the downstream sewer capacity at the eastern boundary of SJCEDD2. The use of the Ten States peaking factor would also exceed the currently planned sewer sizes significantly for the segments along US 20. Since this method produced extreme results, which could be regulated by other means such as pretreatment ordinances, the need to provide sewer capacity to convey the results with Ten States peaking factors was considered to be excessive. Similarly, the lack of any peaking factor was felt to be somewhat risky since there is no safety factor built into the design for accidental releases or discharges somewhat in excess of planned rates. Therefore, the recommended sewer size was established as the size that could convey the flow with a 2.0 peaking factor in all cases. It should be noted that the sizes and slopes recommended originally easily conveyed the design flow rates and could be considered as valid design approaches if the SCJEDD2 would restrict development activities to a peak discharge rate of 2,200 gpd/Ac for all areas, or reduce the allowable flows in some areas if previous development activity generated higher peak flows. If policies and procedures to track capacity utilization and limit peak discharge rates are not in place already to track development within the SJCEDD2 boundaries, it is strongly recommended that such systems be immediately developed and implemented to limit peak discharge rates to no more than 2 times the design rate of 2,200 gpd/Ac or some lesser rate. The sewer capacity calculations were generally done assuming that the minimum slope for the pipe size was used. Sewer sizing analysis for conveyance of the calculated flows with a peaking factor of 2.0 did not result in any changes to the sewer sizing as originally planned. It is also recommended that the 24-inch sewer planned for the Walnut Road section north of the railroad tracks be extended under the RR to the south side of the tracks, instead of the 16-inch sewer that was planned for the crossing. This extension is recommended to assure that adequate capacity is