Using Predictive Modeling Tools and Temporary Treatment Methods to Meet Effluent Limits During Construction Outages: Pulling Out all of the Stops

The Morris Forman Wastewater Treatment Plant (MFWTP) is the largest wastewater plant in the Louisville & Jefferson County Metropolitan Sewer District (MSD) system. The original plant, a primary treatment facility, was constructed in the 1950s and the secondary system was added in the 1970s. Recent improvements have included the addition of bio-roughing towers, conversion to a sodium hypochlorite disinfection system, and installation of a dechlorination system.In 1996, the MSD entered into a capital improvement program to replace aging facilities at the MFWTP and several of the projects had the potential of reducing treatment capacity during construction. Most notably, replacement of the aeration equipment in the high purity oxygen (HPO) reactors reduced the secondary treatment capacity of the plant by 40% for over a year. In order to minimize the impact to the receiving stream, the Ohio River, a plan of operations was developed and presented to the Kentucky Division of Water (KDOW). The plan predicted the impact of construction outages on effluent quality and recommended short term improvements that could be implemented, reducing the impact of construction. In order to quantify the impacts, a treatment process model, PRO2D, was used to develop an overall plant mass balance and predict the performance of all unit processes and plant effluent based on the influent characteristics, tank sizes, and equipment sizes.Several approaches were used to minimize construction impacts. Since replacement of the aeration equipment in the HPO reactors would have the greatest impact on treatment capacity, particular attention was paid to construction sequencing. The initial construction could begin with only 20% of the secondary treatment system taken out of service. Operating the plant with reduced disruption allowed the contractor to develop a “learning curve” for performance of the work and also allowed the MSD to calibrate the PRO2D model for the plant and develop a comfort level for using the model as an operational tool.The model was useful for developing process control criteria and was also able to predict conditions where we expected non-compliance with established effluent limits. When it was predicted that effluent limits could not be met under existing conditions, that the Kentucky DOW was notified and additional temporary treatment technologies were employed to minimize the excursions.