Piloting Technologies for Increasing Wet Weather Biological Treatment Capacity at the Indianapolis Belmont AWT Plant

As part of Combined Sewage Overflow (CSO) control efforts, the City of Indianapolis must increase the peak wet weather treatment capacity at its two advanced wastewater treatment (AWT) plants. 1 To cost effectively meet this requirement, the City considered several technologies for treating the existing primary effluent (P.E.) wet weather bypass and the combined sewer overflow at the headworks of the Belmont AWT plant. Several alternative treatment strategies were considered, and are further described, in the report titled “Combined Sewer Overflow Long Term Control Plan and Water Quality Improvement Report”, dated April 30, 2001 (a.k.a., the draft CSO LTCP).The analysis presented in Appendix A of the draft CSO LTCP assessed the level of treatment necessary to meet water quality standards during CSO events and concluded that the appropriate degree of treatment falls somewhere between advanced primary treatment and full secondary treatment. Moreover, the assessment concluded that nitrification beyond that provided by the existing AWT facility would not be necessary for peak wet weather flows.To treat peak wet weather flows, the City of Indianapolis, Department of Public Works (DPW) is considering the installation of a bio-roughing solids clarification (BRSC) technology that would allow the existing two-stage nitrification system to be uncoupled at its Belmont AWT plant. The project could potentially double the plants wet weather secondary treatment capacity while saving the City tens of millions of dollars over the tertiary treatment option.This paper discusses a unique process for progressively uncoupling an existing two-stage secondary treatment system and piloting technologies for treating bio-roughing effluent. Based on the recommendations of the draft CSO LTCP, the City piloted several technologies (e.g., High Rate Clarification using the Kruger Actiflo. system; High Rate Clarification using the Infilco DensaDeg. system; and Conventional Clarification using a WesTech Solids Contact Clarifier). Bench scale testing for Conventional Clarification was also performed.The paper also describes the piloting efforts performed by the City to develop performance data with which to compare various clarification technologies, and further efforts to develop a recommended treatment process (e.g., the evaluation of a Trickling Filter / Solids Contact process).The technology based treatment and permitting strategies described in this paper and in the draft CSO LTCP are currently under negotiation between the City, the Indiana Department of Environmental Management and U.S. EPA - Region V, and may be applicable to other CSO communities faced with a need to control wet weather discharges.