A retractable, structurally-supported geomembrane cover system provides odor control and ease of maintenance access for the Vallejo Sanitation and Flood Control District.
By Jim McMahon
The initial goal was to contain odors from its wastewater treatment plant.
What the Vallejo (Calif.) Sanitation and Flood Control District (VSFCD) eventually realized is a fully retractable, structurally-supported geomembrane cover system that provides odor control plus ease of access for maintenance of its wastewater treatment basins.
This plant, located near the northeastern stretches of San Pablo Bay north of San Francisco, was engaged in a program to scrub off-gas odors from all aspects of its wastewater treatment plant. Early in the project, the district covered the facilities in its headworks and primary treatment steps to control off-gas.
Later, it developed a process for the management and disposal of its biosolids, including designing a specialized hopper for storage of the plant’s dewatered solids and an automated truck-filling for transportation of this material to VSFCD-owned land at nearby Tubbs Island. The plant disposes of 20,000yd3 of biosolids per year, where it is used as a soil additive to improve farmland at the Tubbs Island location. The VSFCD treatment plant also differs from others in that it uses no digesters in this process.
The wastewater plant then focused on scrubbing off-gas odors from its secondary treatment processes and, specifically, its two open aeration basins. To contain these odors, the district eventually opted for a retractable, structurally-supported geomembrane cover system, which has not only proven effective for the collection of off-gas, but has also provided an efficient flexibility and ease-of-access for tank monitoring, maintenance, and repairs.
VSFCD’s wastewater treatment process
Passing through Vallejo’s primary water treatment units—its headworks, grit chamber, and primary clarifiers—where the solids are separated out, the liquid part of the waste stream flows to the plant’s secondary treatment for biological processing.
After biofiltration, the wastewater is pumped into two aeration basins. The aeration tanks condition the solids particles discharged from the biotowers so they settle more readily in the downstream secondary clarifiers. Blowers and fine-bubble diffusers mounted on the floor of the basins introduce air that is necessary for the flocculation of particles, converting the organic solids into heavier clumps that settle and are removed by sedimentation in the secondary clarifiers.
Streamlined basin covers
The Vallejo plant’s two secondary wastewater processing aeration basins were installed in 1988. They are each 15ft deep, 15ft wide, and 110ft long.
Every few weeks, the plant’s operatorsconduct visual inspections into the aeration tanks from the top. The tanks are drained annually and workers go down inside to conduct a physical inspection of the blowers and diffusers at the bottom, and to hose down the sides of the basins.
For almost 20 years the basins remained uncovered. But as part of the plant’s odor-control upgrade, the district looked into options for covering them. Carollo Engineers, an environmental engineering firm specializing in the planning, design, and construction of water and wastewater facilities, was retained by the VSFCD to handle the design and construction management for the plant odor control upgrade, and began reviewing different cover options for enclosing the basins.
“We wanted the covers first for odor control, so they needed to be corrosion resistant,” said Tim Tekippe, Carollo’s project manager handling the Vallejo project. “But we also needed the covers to be easy to open and close 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 access they provide. 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.”
“Along with Carollo, our engineering firm, we looked at a number of other wastewater plants and what they were using to cover their aeration tanks,” said Barry Pomeroy, director of Operations and Maintenance at VSFCD. “We went to a water treatment plant in Colorado that was using retractable, structurally-supported covers made with a geomembrane fabric. They looked like they would be very easy to remove for maintenance, and [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.”
Installation and operation
Vallejo’s new retractable, structurally-supported geomembrane cover system consists of a composite sheet of high-strength, UV-protected, coated fabric tensioned across a series of low-profile aluminum arches that span the tank’s opening. Intermediate aluminum walkways spanning the tank are used to divide the fabric cover sections into appropriate lengths for easy retractability.
The geomembrane cover fabric is a laminated sheet of 40-mil specialty PVC (Ethylene Interpolymer Alloy or EIA) that acts as a gastight barrier to keep the off-gas from passing through. It incorporates a specialized weave design that provides maximum strength-to-weight ratios.
Since this topsheet is exposed to the sun, it is also equipped with advanced UV inhibitors. The material can withstand temperatures to minus 30°F. This cover’s attributes include: seam strength, 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, making it ideal for wastewater cover applications.
The covers for the Vallejo site’s basins are gastight, operating under negative air pressure. A ventilation system draws air through the tank and underneath the covers, and pulls along with it the off-gas from the aeration process. Off-gas removal piping is connected directly to the cover system and out to a soil filter for odor scrubbing.
Although the membrane covers are gastight, they can be detached and rolled up along the frame. This gives operators access to inspect and maintain internal components of the two basins. The membrane covers then reattach in a time-efficient and safe process. Additional hatches in the intermediate aluminum walkways allow access by plant operators without retracting the entire cover.
Attractive option for municipal wastewater and drinking water plants
“The expected life of these retractable covers is about 15 years,” said Tekippe. “And the cost is very attractive. If a cover [had] to be replaced, it would be easy to change out and could be done in minimal time.
“These retractable covers are well-suited for both municipal wastewater and drinking water plants. We have since specified them for use in other public water and wastewater projects,” Tekippe added.
Today, many municipalities are looking for efficient tank cover systems to contain off-gases, reduce algae growth, simplify maintenance and repairs, and cut expenses. Geomembrane covers have become an increasingly attractive option for streamlining wastewater plant operations.