THE SIGNIFICANT ROLE OF THE DETROIT WWTP IN CSO CONTROL

The Detroit Wastewater Treatment Plant (WWTP) plays a significant role in reducing Combined SewerOverflows (CSOs) and treating wet weather flows from the Detroit Water and Sewerage Department's collection system during wet weather events. The firm wet weather capacity of the primary treatment system - the world's largest - is currently 1,700 mgd and the firm secondary system is 930 mgd. In 1996, as part of the Department's Long Term CSO Control Plan, fullscale WWTP capacity testing was conducted to maximize flow to the WWTP, which is one of the nine minimum controls for CSOs specified by USEPA. This paper reviews the full-scale testing of the WWTP in 1995 and 1996 that served as the basis for significantly increasing the primary capacity from about 1,250 mgd to 1,520 mgd and increased secondary treatment from 859 mgd to 930 mgd.The Long Term CSO Control Plan in 1996 also recommended construction of two new primary clarifiers and an additional pump at one of the WWTP's influent pump stations to increase the firm wet weather primary capacity to 1,700 mgd. The primary clarifiers were completed in late 2004.With the anticipated completion of the new clarifiers, a "Wet Weather Operational Plan" for the WWTP was developed in 2003 and submitted to the Michigan Department of Environmental Quality (MDEQ). This document provides the general plan for WWTP operation 1) prior to wet weatherevents to maximize in-system wet weather storage volume in the collection system; 2) during wet weather events to maximize treatment of wet weather flows; and, 3) immediately after an event to treat flows and loads from CSO facilities in the collection system. The paper provides results from operation under this plan. Solids handling and disposal capabilities are a critical component of the Detroit WWTP for processing wet weather flows and loads. Solids loads can increase significantly during wet weather events due to higher flows and the scouring of solids from the collection system sewers (first-flush effect). Solids loadings to the WWTP also increase as CSO control facilities in the collection system dewater immediately after storm events. Due to significant solids loads received at the WWTPduring and immediately after wet weather events, the WWTP must have the ability to store a portion of the peak solids while processing the solids as quickly as possible to prevent the solids from going septic and becoming more difficult to treat. Furthermore, wet weather solids loads to theWWTP will continue to increase in the future with additional major CSO facilities that are currently under construction or design. This paper describes the approach used to estimate future solids loads from the collection system and the recent efforts taken by DWSD to ensure adequate solidshandling and disposal capabilities under peak wet weather conditions.