- Category: Blog
A properly designed wetlands lagoon can be a low-maintenance solution to ever-increasing permit requirements. At least 120 days detention in a conventional three-cell system, or at least 150 days in a two-cell system are required before effluent can be discharged to a wetlands. Wetlands become more economical when annual evaporation is high (western Kansas and Colorado) or when built in sandy, well-drained soil. Many things need to be considered during the design phase:
- Seepage: Estimate the seepage rate by performing extensive boring of soils and testing the permeability on the least permeable layer from each bore. Clay liners are normally not required, unless the lagoon will be located in an especially sensitive groundwater area. Be extra-conservative, because the seepage rate may decrease over time as sludge builds up.
- Surface or Submerged Flow: With their thick gravel beds, submerged flow wetlands are excellent at removing pollutants. Submerged flow wetlands are typically constructed for higher loading than wastewater lagoon effluent since the gravel bed provides a higher surface area and more efficient use of HRT (Hydraulic Retention Time). However, since we are dealing with already treated wastewater and our main aim is to go non-discharging, a surface flow wetlands is typically sufficient – and much less expensive!
- Holding Capacity: Design so that wetlands will be non-discharging during an average year, but will have enough capacity to hold water during unusually wet years. Compare normal annual evaporation, normal precipitation, and monthly pan evaporation records. Consider that if the sewer system has a lot of possibly broken clay pipe installed in the 1950’s, or if there are foundation drains tied into the sanitary sewer, the infiltration and inflow during wet years can be a lot higher than expected.
- Plants: Vegetation removes nitrogen and phosphorus, provides shelter for insect-eating fish, increases bottom roughness, and promotes efficient use of HRT. Root growth promotes slope stability and reduces erosion. Even though wetlands plants should eventually be established by wind-blown seed and birds, it’s a good idea to plant seeds shortly after pond construction is complete. Pick plants based on climate and expected water level. Alkali and hardstem bulrushes can tolerate 2 to 3 feet of water when fully grown, cattails thrive in saturated soil and can tolerate about a foot of submergence, and grasses prefer well drained soil. A varied mix of wetlands plants will provide slope protection as the water level fluctuates throughout the year. Some plants, like Phragmites (Common Reed) can be invasive, so check with NRCS or your local Wildlife & Parks office.
- Water Depth: Varying pond depths are crucial! Natural wetlands aren’t perfectly flat and square. Design for a good ecosystem, and nature will work with you! An area of water 3 to 4 feet deep year-round will provide a habitat for mosquito-eating fish and dragonflies. Ponds at least 3 feet deep give fish a place to survive icy Kansas winters. Designing a perfectly flat lagoon may result in a large area with only a few inches of water during dry seasons – perfect for mosquito larvae. They are also tenacious. Larvae can survive completely dry years, so it’s important to provide a habitat for their predators.
- Slopes: Design inside slopes to be gradual, and cover with vegetation to prevent erosion. Even with a good stand of vegetation, and inside slopes of 4:1 or 5:1 (or even gentler), non-cohesive silt and sand should not be used to construct the berms.
EBH has worked with several municipalities recently to add a non-discharging wetlands cell to their systems. Most of this work was done because the Cities had received schedules of compliance from KDHE, whether for total suspended solids (TSS), biochemical oxygen demand (BOD), ammonia, or E. Coli violations. We helped the City of Lakin finish construction last year at relatively low cost, and the project was featured in the March 2018 issue of The Kansas Lifeline.
If you are curious about whether a wetlands could work for you, please give us a call!
Richard Ammel, PE