LTCP Development and Results Summary
In an average year, a continuous simulation of the sanitary sewer collection system model for the City of Terre Haute predicts that 284 million gallons of combined sewage is discharged from CSOs. Terre Haute is fortunate in that the receiving stream has a large average flow rate relative to the volume of CSO overflow, which can provide significant assimilative capacity or dilution. Despite this fact, simulations indicate that in an average year, the Wabash River exceeds the water quality standards for E. coli approximately 30% of the time during the recreation season (April-October) when bacteria loads from all pollutant sources are considered and less than 5% of the time if non-CSO sources effects are eliminated.
Flow, water quality and rainfall data were collected and both the combined sewer system and the Wabash River in the CSO areas were modeled to assist in the City's CSO LTCP planning process. No areas were qualified as “sensitive areas” but the outfalls around Fairbanks Park were designated as a priority. The LTCP recommended a plan to reduce the number of CSO events per year (average year) from 37 to 6 times per year, which will reduce the number of hours when bacteria loadings from the CSO’s exceed recommended levels in the river by 75%, from 174 hours to 45 hours.
Both Federal and State CSO policies are divided into two phases. Phase I (CSO Operational Plan) was submitted to IDEM and approved by IDEM in 1996 with the most recent update in 2006. Phase II includes the LTCP and the submittal of the document completed in 2011. All of the regulatory requirements are intended to reduce the in-stream impact from CSO discharges during wet conditions and ultimately make the Wabash River more compliant with its official classification of “fishable and swimmable”.
The final revised project approach used to evaluate CSO long-term control alternatives was as follows:
- Identify feasible CSO control technologies.
- Based on new collection system flow and rainfall data monitoring collected in 2006 - 2008, calibrate and develop a SWMM model for evaluation of CSO system reaction to storm events and control alternatives
- Develop integrated system-wide CSO control alternatives to capture and treat typical year rainfall at various levels of control (measured in number of overflows per year). Each alternative included feasible CSO control technologies specific to each CSO or combinations of CSOs, and other technologies identified by the City and the Citizen Advisory Committee (CAC) that would reduce CSO impacts on the Wabash River.
- Estimate the cost for each feasible CSO system alternative and also for complete combined sewer separation.
- Evaluate each CSO alternative’s performance using a “typical year” rainfall approach and calculate the associated costs for various levels of control.
- Perform an economic affordability evaluation and determine if a Use Attainability Analysis (UAA) must be performed if the design storm flow requirements as prescribed by IDEM cannot be economically achieved.