Using WATS Sewer Process Model for Project Pre-Design

The City and County of San Francisco, SFPUC, Wastewater Enterprise (WWE) operates and maintains a combined sewer system which transports both dry weather (DW) and wet weather (WW) flows. This collection system consists of approximately 1000 miles of main line sewers and over 2000 miles of side sewers and catch basin culverts. The main line sewers include large transport/storage (T/S) boxes and large circular sewers to convey sanitary flows and to capture and convey storm runoff. WWE has decided to follow such an approach and apply a novel sewer processes model (the WATS model) to analyze the problems and develop solutions for odor management. In the past several years, multiple odor complaints were received by the Bay Area Air Quality Management District and the City around 3rd Street and Cargo Way all the way to the Custer Avenue. Recently, the WATS model was used for Cargo Way Odor Reduction Project during pre-design phase to determine the feasibilities of some of the alternatives in terms of liquid sulfide reduction, shear stress (also called tractive force, an indicator of solid settling), corrosion control, and chemical dosage. The purpose of the Cargo Way Odor Reduction project is to alleviate odors from the collection system along Cargo Way from Jennings Street to 3rd Street and on 3rd Street from Cargo Way to Custer Avenue.Five (5) initial alternatives were developed at a brainstorming session held with Operations, Engineering and Maintenance (OEM) representatives and the project team. They are as follows: 1. Manhole inserts; 2. Regulatory compliance and manhole inserts; 3. Hunters Point Dry-Weather flow diversion to Jennings St into Cargo Way and manhole inserts; 4. Flushing line; and 5. Dedicated Dry-Weather sewer pipe for Amador Street force main and manhole inserts. Sampling campaign was conducted to calibrate the WATS model for the project area before the WATS model was used for this project. Based on the modeling results for alternatives 3, 4, and 5, and careful consideration, only alternative 4 remained into the conceptual engineering report. Both alternative 3 and 5 will not be able to meet our project targets of liquid sulfide less than 0.5 mg/L (Target #1) and shear stress (tractive force) greater than 0.5 Pa (Target #2) after the completion of this project.After the alternative was chosen, WATS model was used to determine the project area and minimum flush rate to meet both project targets. WATS results have shown that upstream of Mendell St, liquid sulfide is below 0.5 mg/L, which indicates that the flushing line is not needed to start at Jennings St. Average flushing rate was determined based on the WATS simulation results with the target of liquid sulfide below 0.5 mg/L at the project area, and the minimum flushing rate was simulated based on the shear stress rate above 0.5 Pa.