SELECTION OF RECTANGULAR BASIN OVER VORTEX SEPARATORS: CSO TREATMENT ALTERNATIVES FOR WEST LAFAYETTE, INDIANA

The City of West Lafayette, Indiana (City), faced with the challenge of reducing combined sewer overflow (CSO) to the Wabash River, addressed future CSO control regulations by developing the West Lafayette Wastewater Treatment Plant Facilities Plan in 1993. In 1996, the City prepared Amendment No.1 to the facilities plan presenting various wastewater collection system improvements including the Wet Weather Treatment Facility. The facility, located adjacent to the wastewater treatment plant, would provide settleable solids removal and disinfection of flows in excess of the plant capacity. Partnering with the City of West Lafayette, Malcolm Pirnie, Inc. evaluated alternatives and designed the facility.The alternative initially recommended for CSO treatment was comprised of four vortex separators and chemical disinfection. During preliminary design of the Wet Weather Treatment Facility, performance was discussed with representatives of two major vortex separator manufacturers. Based upon the units installed to date, the performance of vortex separators has been much less than initially expected. In fact, the data indicate that a vortex separator can be expected to achieve no more than ten percent net solids removal. This low removal rate led to the evaluation of other alternatives.A rectangular basin was evaluated and compared to the vortex separators' solids removal, volume, disinfection contact time and cost. One basin of almost double the volume of four vortex separators could be constructed at approximately the same capital cost. The rectangular basin would provide 20 minutes of chlorine contact time at the design flow whereas four vortex separators provide 10 minutes. It also is anticipated that the rectangular basin would provide more solids removal than the vortex separators. Additionally, the rectangular basin would be capable of capturing a greater volume of overflow resulting from small storms, thereby providing treatment for a greater number of storm events. Based upon these benefits, the City proposed construction of a rectangular basin for treatment of excess wet weather flow.