STUDY OF EFFLUENT DISINFECTION FOR URBAN RIVERS IN CHICAGO

Consoer Townsend Envirodyne Engineers, Inc. (CTE) was retained in 2004 by the Metropolitan Water Reclamation District of Greater Chicago (District) to provide engineering services to prepare a comprehensive Infrastructure and Process Needs Feasibility Study (Feasibility Study) for the North Side Water Reclamation Plant (WRP) and conduct studies of water quality management options for the urban rivers of Chicago.This paper documents the results of the CTE study of effluent disinfection alternatives for the District's North Side, Calumet and Stickney WRPs which have average design flows of 333 MGD, 354 MGD and 1200 MGD, respectively.The objectives of the study are:Provide a final recommendation for one or more disinfection technologies which is the best fit for the District's North Side, Stickney and Calumet WRPs.Prepare capital and operation and maintenance (O&M) cost estimates for the construction of the recommended technology or technologies for the North Side, Stickney and Calumet WRPs.In this planning level study, a long list of various effluent disinfection technologies were studied by a task force of national experts. Based upon a consensus decision by the task force and the District, some of the long list technologies were eliminated and the following list of disinfection technologies were subjected to a detailed matrix evaluation:Ozonegenerated from airgenerated from oxygenUltraviolet Disinfectionlow Intensityhigh IntensityCalcium hypochlorite plus sodium bisulfiteSodium hypochlorite plus sodium bisulfiteSodium hypochlorite (on-site generation) plus sodium bisulfiteBecause of District concerns with potential hazards to humans and the environment due to accidents or terrorism either at the treatment plant or during transportation, gas chlorination/gas dechlorination were not included in the above list. Also because of a lack of large scale wastewater treatment effluent disinfection experience, the task force and the District in a consensus decision eliminated the following long list technologies from further consideration:Chlorine DioxideBromine CompoundsSequential Disinfection ProcessesMembrane ProcessesUsing an evaluation matrix which was scored by the task force, it was a consensus decision by the task force and the District that both UV (high intensity-medium pressure) and ozone (oxygen generated) be carried forward for a detailed cost estimate. Based upon this cost estimate and depending upon the effluent disinfection technologies chosen, the combined capital cost for effluent disinfection for the three largest District Water Reclamation Plants could exceed 1.8 billion and annual operation and maintenance costs could exceed 43 million.The results of this study will be used by the state and federal environmental protection agencies in the Use Attainability Analysis (UAA) being conducted for the urban receiving rivers of the three District water reclamation plants. As part of the UAA, there is a possibility that the current secondary contact use for the urban rivers of Chicago may be changed which may require effluent disinfection at the District's North Side, Calumet and Stickney WRPs. The cost of effluent disinfection must be considered in the UAA process along with the potential public health impacts.