Expanding the Capacity of a Controlled Discharge Lagoon System

Controlled Discharge Lagoon System: Woodward Resource Center

Aerial view of Woodward Resource Center with two primary and two secondary lagoons.

Operated by the Iowa Department of Human Services, the Woodward Resource Center in central Iowa supports individuals with disabilities and prepares them to live in the community of their choice. This sprawling campus is serviced by its own wastewater treatment facility. When the facility started experiencing incoming hydraulic loads that exceeded the capacity of its controlled discharge lagoon system, they reached out to the wastewater professionals at Snyder & Associates to develop a cost-effective and efficient solution.

Typically, controlled discharge lagoons are utilized by smaller, rural communities mainly in the Midwest. In these systems, wastewater is held in large storage ponds or lagoons for a minimum of six months as natural bacterial processes work to treat and clean the water over time. Once the natural treatment process reaches a safe level, it’s discharged in controlled amounts that comply with water quality standards to ensure stewardship of the natural environment. With the increased hydraulic loads, Woodward’s treatment system was struggling to achieve wastewater storage requirements of 180 days put in place by the Iowa Department of Natural Resources.

Our team began with a review of existing and future treatment requirements, an analysis of existing facilities, and an evaluation of lagoon alternatives. During the evaluation, it was determined that efforts to reduce inflow and infiltration, along with a significant expansion of the three-cell controlled discharge lagoon, would be necessary to meet treatment requirements long term.

Reducing Inflow & Infiltration During Conveyance

Pipe lining was added to reduce I/I in the conveyance system.

Excess water flowing into sanitary sewer pipes from groundwater and stormwater is referred to as inflow and infiltration (I/I). After our team’s site review, it was evident the removal of roof drains and downspouts on buildings in the vicinity of the project were key to reducing total I/I in the system. However, more focused revisions were also included.

The inflow aspect of I/I covers stormwater rapidly flowing into sewers through prohibited connections. To address these additional inflow sources, manholes within the project area underwent assessment and were either repaired or replaced depending on their condition. With that taken care of, our team shifted the focus to infiltration. This encompasses groundwater that seeps into sewer pipes through holes, cracks, joint failures, and faulty connections. With targeted sections of the collection system identified, our team incorporated pipe lining to improve performance and avoid additional infiltration. Together these individualized and broad I/I reduction strategies aided in reducing high flows and ultimately cost during wastewater conveyance. To continue this trend and remain in compliance, however, facility expansion was also necessary.

Controlled Discharge Lagoon Expansion

The existing treatment facility included a three-cell controlled discharge lagoon system with a total storage capacity of 18.39 million gallons and two 400 gallon-per-minute (GPM) influent pumps. Though fairly new, this system was not meeting capacities and had flows exceeding the plant’s NPDES permit limits for three consecutive years.

Although lagoon expansion required additional land, there is space readily available at the site and owned by the Woodward Resource Center.

Three alternatives were evaluated based on their practicability, economic efficiency, affordability, and degradation. Of those assessed choices, expansion of the controlled discharge lagoon through the addition of a second primary cell was found to be the most reasonable, cost-effective, and preferred alternative. The new cell was constructed directly adjacent to the existing primary cell and holds over 8.5 million gallons, adding roughly a 47 percent increase in storage volume.

Additional improvements included replacement pumps controlled by Variable Frequency Drives (VFD) that handle 800 GPM, and a larger diameter force main for efficiency purposes. These upgrades exponentially increased hydraulic capacity, ensuring the plant remains in compliance for many years to come.