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The Vallejo Sanitation and Flood Control District is an independent special district created by the State of California to collect and treat wastewater and protect the Vallejo community from flooding. Since 1952, the district has protected public health and the San Francisco Bay Area by treating wastewater generated by the more than 115,000 residents of Vallejo and surrounding area. The district is known for its innovative use of technology in wastewater handling and differs from most in that it uses no digesters in its process.
Vallejo’s wastewater is pumped into two secondary process aeration basins that are each 15 ft (4.6 m) deep, 15 ft (4.6 m) wide, and 110 ft (33.5 m) long. Every few weeks the Vallejo operators conduct a visual inspection into the aeration tanks from the top. Once a year they drain the tanks in order to venture inside to hose down the sides of the basins and conduct a physical inspection of the blowers and diffusers.
For almost 20 years the basins remained uncovered, but as part of the plant’s odor control upgrade, the district investigated cover options. Vallejo retained Carollo Engineers, an environmental engineering firm specializing in wastewater facilities, to handle the design and construction management of the plant’s odor control upgrade.
“We wanted the covers first for odor control,” says Tim Tekippe, project manager at Carollo Engineers, “but we also needed the covers to be easy to open for access to the tanks for sampling, scheduled maintenance, and repairs. We felt structurally supported covers would be the best system for the plant’s needs because of the better access they provide over other systems, such as floating covers. We first looked at rigid type covers such as aluminum and fiberglass, but both of these proved more labor-intensive for operators to gain access to the basins.”
“We looked at a number of other wastewater plants and what they were using to cover their aeration tanks,” says Barry Pomeroy, director of operations and maintenance at the Vallejo Sanitation and Flood Control District. “We went to a water treatment plant in Colorado that was using retractable, structurally supported covers made with a geomembrane fabric. We watched how easy they were to open and close. We even walked on them while they were in place over the tank to see how strong and durable they were. Based on that trip, we decided to design these retractable covers into our aeration basins.”
Evoqua’s Geomembrane Technologies was selected to design and build a new retractable, structurally supported cover system for Vallejo. This cover system consisted of a composite sheet of high-strength, coated fabric tensioned across a series of low-profile aluminum arches which span the tank opening. Intermediate aluminum walkways spanning the tank were used to divide the fabric cover sections into appropriate lengths for easy retractability.
Vallejo’s covers are now gastight and operate under negative air pressure. The geomembrane cover fabric used is made up of a laminated sheet of 40 mil modified PVC that acts as a gastight barrier to keep the offgas from passing through. A ventilation system draws air through the tank and underneath the cover, pulling the offgas from the aeration process with it. Offgas removal piping is connected directly to the cover system and through a soil filter for odor scrubbing.
Not all geomembrane cover designs work this efficiently. Polyethylene topsheets, for example, typically have a poor coefficient of expansion and contraction. The material expands in warm temperatures and contracts as it cools down. Over time, this growing and shrinking effect contorts the shape of the cover, creating a series of hills and valleys that retain rainwater. Evoqua’s Geomembrane Technologies™ brand cover system has overcome these deficiencies. The material is equipped with advanced UV inhibitors and can withstand temperatures up to -30°F (-34.4°C).
Evoqua’s Geomembrane Technologies™ brand retractable, structurally supported covers have become an increasingly attractive option for streamlining wastewater plant operations. This type of cover is ideal for wastewater applications because it has exceptional seam strength, extreme puncture and tear resistance, low thermal expansion and contraction properties, a wide range of chemical resistance, high flexibility, and dimensional stability under high loads and temperature fluctuations.
Although the membrane covers are gastight, they can be quickly detached and easily rolled up along the frame. This gives operators at Vallejo easy access to inspect and maintain internal components of the basins. Reattaching the membrane covers is quick and easy, making the process both safe and time-efficient. Additional hatches in the intermediate aluminum walkways allow access by plant operators without retracting the covers.
" The expected life of these retractable covers is about fifteen years, and the cost is very attractive compared to other cover systems. If a cover did have to be replaced, it would be easy to change out and could be done in minimal time. These retractable covers are very well-suited for both municipal wastewater and drinking water plants. We have since specified them for use in other public water and wastewater projects. "
