Rising Receiving Water Level Leads to 70 MGD Emergency Pump Station

PURPOSE
The Kenosha Water Utility (KWU) provides wastewater service to more than 100,000. Rising Lake Michigan water level put the wet weather capacity of the KWU Water Reclamation Facility (WRF) and collection system at risk. The collection system includes multiple wet weather control elements. Rapidly rising water level in 2019 rendered one critical control element in the collection system unusable. This abstract details the response to protect the property and citizens of Kenosha which resulted in an operational 70 MGD pump station just 7 months after the initial concept.

COLLECTION SYSTEM
KWU's collection system is a separated sewer system with significant infiltration and inflow during storm events. The WRF operates up to 100 MGD peak day flow. However, the collection system can experience flow in excess of 200 MGD during extreme storm events. The collection system has two significant flow control elements when unusually high peak flows are experienced: 1. Equalization (EQ) Basin rated for an influent flow of 136 MGD 2. 3rd Avenue overflow structure for conveying excess system flows to Lake Michigan via a 99-inch storm sewer, after the Equalization Basin is full During short term storm events, water in the EQ basin is drained back into the collection system to the WRF headworks. In an extended storm event, the EQ basin fills and water overflows to the WRF chlorine contact tanks. The EQ basin overflow is limited to around 30 MGD. If the storm event continues, the collection system continues to back up and water begins overflowing at the 3rd Avenue structure. The structure has a gravity overflow weir; overflow is combined with storm water and flows by gravity to Lake Michigan. The overflow is typically used once every few years; however, the frequency has increased in the past decade. Flow is highly variable by storm event. During a major storm in July 2017, the estimated peak overflow was 80 MGD. The July 2017 event resulted in multiple basement sanitary sewer backups.

LAKE LEVEL
Lake Michigan water level has risen from around 576.00 ft in 2013 to 580.50 ft in 2018. Lake Michigan water level varies during any given year; see Figure 1. Due to rising water level, the 3rd Avenue overflow weir was raised to prevent lake water from back-flowing into the collection system. This change reduced the overflow capacity and increased the overall hydraulic profile in the collection system. As a result, the risk of basement backups has increased.

PLANNING
KWU recognized that it was time for planning and action in late 2018. At that time, a system wide study was completed to evaluate wet weather protection alternatives for the WRF and collection system. The alternatives included variations of these improvements: two WRF outfalls, WRF effluent pump station, WRF hydraulic modifications, 3rd avenue excess flow pump station, excess flow treatment, and separate EQ basin flow treatment and discharge. The alternatives provided varying degrees of wet weather treatment, but all achieved management of 220 MGD collection system flow at the 100-year flood elevation. The projects ranged in cost from $10M for the excess flow pump station and two WRF outfalls to $25M for full flow disinfection and WRF effluent pumping.

DESIGN
By the time planning was completed, Lake Michigan was near record water level. Excess flow could not be conveyed by gravity through the overflow structure; see Figure 2 for a hydraulic elevation diagram. KWU needed a fast response; however, the projects proposed in planning would take 2 to 3 years to design, permit and construct. Emergency pumping options were explored. Diesel driven centrifugal pumps were considered; however, six large pumps are needed for 50 MGD capacity and space was limited. The vendor offered diesel powered, hydraulically driven submersible axial flow pumps. Two axial flow pumps could achieve 70 MGD. The cost of the axial flow pumps was about $1M compared to $2M for the centrifugal pumps. Axial flow pumps were preferred because they provided more flow, could be placed in the existing storm water outfall and were lower cost. Due to the configuration of the outfall, one axial flow pump was located in the 99-inch storm sewer and one located in the outfall structure. Passive overflow at the 3rd Ave structure from the sanitary sewer to the 99-inch storm sewer continues via a flow control weir. Pumps transfer excess flow from the storm sewer to Lake Michigan.

CONSTRUCTION
KWU purchased the axial flow pumps and power packs in August 2019. The Contractor began construction of the pump station in September 2019 and completed the work in December 2019. In January 2020, the pump station was tested using water from Lake Michigan. See photos of the submersible pumps and pump station attached. OPERATION On May 17, 2020, the emergency pumps got their first real test. Kenosha received 3-1/2 inches of rain just days after previous rainfalls. The collection system flow was estimated at 200 MGD. The Lake Michigan water level near 583.00 ft, a record high elevation. The WRF was operating at 100 MGD with the influent wet well 10 ft above normal operating level. The EQ basin was full, 2 EQ basin pumps were operating, and EQ flow was traveling to the WRF. The 2 emergency pumps at 3rd Avenue were operating during the event. In the beginning, at a low overflow rate, the pumps operated intermittently as needed. As the overflow rate reached up to 70 MGD, the pumps operated more frequently. The entire system was able to handle the 200 MGD event.

CONCLUSIONS
The submersible, hydraulically driven pumps in an existing structure provided a unique and much needed solution to an emergency situation caused by rising Lake Michigan water levels. The pumps successfully protected properties and the citizens of Kenosha. Without this unique solution, wet weather flows would not have been managed in May 2020 leading to widespread basement flooding. KWU continues to pursue wet weather management work to maintain or increase the collection system and WRF capacity. Ongoing work includes chlorine contact tank hydraulic modifications, WRF outfall modifications and disinfection expansion.