Water Environment Federation (WEF)

Maximazing 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.

City Development Planning
The LWWS developed a Wastewater Facilities Master Plan in conjunction with the 2002 Lincoln-Lancaster County Comprehensive Plan to proactively address wastewater issues and assure that the City is well prepared to meet wastewater service needs when they occur. These plans identified future utility needs for 25-year (Tier I) and 50-year(Tier II) planning periods using population projections provided by the Lincoln-Lancaster County Planning Department. Tier III represents planning beyond 50 years and was not a consideration of the project. The historical annual population growth rate of approximately 1.5 percent was determined to be applicable to predict the future growth of Lincoln and Lancaster County.

As population growth occurs within a service area, upgraded and expanded facilities will be required to meet service needs. The timing associated with future system improvements within each drainage basin depends on the rate of development within the basin. For the purposes of planning, development projections for “Tier-I” and “Tier-II” were utilized from the 2002 Lincoln-Lancaster County Comprehensive Plan. Wastewater collection system flow projections to meet future service requirements were identified for both Tier I and Tier II development scenarios.

Wastewater Collection
The existing Lincoln wastewater collection system service area covers 13 drainage basins encompassing 50 square miles. The collection system includes over 800 miles of wastewater collection pipelines ranging in size from 8-inch to 90-inch in diameter. Flow through the system is predominantly by gravity. However, there are 14 lift stations throughout the collection system to serve low elevation areas. The drainage basins of interest for this paper are the Little Salt Creek Basin and East Campus as shown on Figure 1. The Little Salt Creek basin is a growing service area which includes over 3,000 acres of developed land and 5,300 acres in all that could ultimately be developed.

Existing Little Salt Creek Wastewater Collection Facilities
LWWS desired to improve the outfall system from the North 31st Street Lift Station (C-11) to the Theresa Street Wastewater Treatment Facility (WWTF). The North 31st Street Lift Station serves the Little Salt Creek service area, located north and west of Salt Creek, and is shown on Figure 2. The lift station pumps wastewater across Salt Creek via a 24 inch diameter force main to a discharge manhole on the south side of Salt Creek. From there, the wastewater flows via a 24 inch gravity outfall sewer for a distance of 2,000 feet to where it enters a 30 inch diameter sewer before crossing under Cornhusker Highway, a six lane major arterial street. The 30 inch sanitary sewer continues for a distance of 2,450 feet south along the west side of State Fair Park Drive to a manhole on the east side of 27th Street and Kimco Court. At that point, the 30 inch outfall sewer intercepts a 42 inch diameter trunk sewer that crosses under 27th Street at Kimco Court. The 42 inch trunk sewer continues west into the Theresa Street WWTF.

The North 31st Street Lift Station was designed to accommodate a majority of the Little Salt Creek Basin service area growth north of Salt Creek through the addition and/or replacement of pumps. There are currently two pumps installed that provide a total firm capacity of 5.4 cfs (3.5 mgd), which is adequate for current dry weather flow conditions. The capacity of the lift station can be expanded to a total of 21.6 cfs (13 mgd) through the installation of 2 additional 3.5 mgd pumps.

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