Maximizing Operational Capacity for Outfall Sewers Through Innovative Design

The City of Lincoln, Nebraska has undertaken a strategy to accelerate development of infrastructure systems for critical areas of the community, with the goal of nearly doubling the development rate for those areas. The Little Salt Creek drainage basin is a growing service area which includes over 3,000 acres of developed land and ultimately 5,300 acres in all. The majority of the existing and future service area drains to a lift station that pumps under Salt Creek through an inverted 24 inch force main and outfalls into a 24 inch gravity sewer. The lift station is configured to expand to match future increased flows through installation of additional pumps.Several capacity-related problems exist with the current collection system downstream from the lift station;The 24 inch sewer that receives flow from the force main is undersized for receiving future (and present peak) flows.To avoid surcharging and overflowing of the existing gravity sewer, only one lift station pump can be operated at a time.Because only one pump can be operated, the system capacity is limited to approximately 7.8 cfs (5.0 mgd). Furthermore, sewage velocities within the 24 inch inverted force main are less than 2 feet per second making grit and solids deposition a concern.For the Lincoln Wastewater System (LWWS), the most critical need is to upgrade the lift station outfall sewer system to prevent overflows now and into the immediate future. A secondary need is to provide a system concept that maximizes the outfall's near-term capacity and allows development to full build-out capacity without major additional investment in existing trunk sewer infrastructure. This secondary goal requires a development strategy that accommodates a wide range of flows, from the current effective capacity of 7.8 cfs (5.0 mgd) to a future build-out of 32.4 cfs (20.0 mgd). Original concepts had assumed that this project would need to be followed by a similar project later once flows further increased.An innovative design for the project was proposed to LWWS to construct what has been termed a “gravi-force” main to achieve the full range of capacity with a single cost-effective project. The “gravi-force” concept optimizes construction costs by constructing a gravity sewer system sized for near-term capacity needs that can be pressurized in the future to achieve build-out capacity when operated in “force main” mode. Furthermore, this concept integrates with the existing 24 inch diameter sewer for optimum flexibility and responsible utilization of the city's existing infrastructure assets for the long-term. Another element of the project is the proposed construction of a second force main under Salt Creek to facilitate cleaning of the twin main when necessary (initially only one main would be in service at a given time), and to ultimately be in service in parallel, and simultaneously with the 24 inch force main at build-out high flow rates.In summary, instead of constructing a parallel gravity sewer or force main now, followed by a third in the future (within an increasingly congested area and at inflated cost), the “gravi-force” main project solution accomplishes the same effective result with only a single project constructed now and ultimately at a substantially lower life cycle cost.