Tank Covers Help California District Streamline Plant Operations

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Tank Covers Help California District Streamline Plant Operations
GTI designed, engineered and built a new retractable, structurally supported geomembrane cover system for Vallejo Sanitation and Flood Control District.

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The Customer:

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 by treating wastewater generated by more than 115,000 residents of Vallejo and the surrounding area. 

The district is known for its innovative use of technology in wastewater handling. It has been recognized by the Environmental Protection Agency and awarded the National First Place Award for the most outstanding project involving land application of biosolids in the United States. The plant disposes of 20,000 cubic yards of biosolids a year, where it is used as a soil amendment to improve farmland. The plant differs from most in that it uses no digesters in its process. 

The Customer’s Needs:

The Vallejo facility is permitted to treat 15.5 mgd, but has the capacity to provide full secondary treatment of 35 mgd. During wet weather, the plant is capable of processing 25 mgd primary treatment, combined with the 35 mgd secondary treatment for a total of 60 mgd. 

Vallejo’s wastewater treatment process involves first separating solids from the rest of the wastewater; the liquid part of the waste stream then flows to the plant’s secondary treatment for biological processing. The wastewater is sprayed down over plastic media in a filter, providing a surface for bacteria to grow and consume the organic material in the effluent. 

After biofiltration, the wastewater is pumped into two secondary process aeration basins. These tanks were originally built in 1988 and are each 15 feet deep, 15 feet wide, and 110 feet 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, venture inside to conduct a physical inspection of the blowers and diffusers at the bottom, and 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 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, so they needed to be corrosion resistant,” says Tim Tekippe, project manager at Carollo Engineers.

“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 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.” 

Depending on their size, aluminum panels can be heavy and, since both aluminum and fiberglass covers are rigid, they can be bulky and difficult to maneuver for workers perched above an open tank. This tricky maneuver requires a fair amount of labor — both to take the aluminum panels off, place the covers aside, and later put the covers back on — but also poses a significant safety hazard for the workers. 

“Along with our engineering firm, Carollo, 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. They looked like they would be very easy to remove for maintenance, and 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.” 

The Solution:

GTI designed, engineered and built a new retractable, structurally supported geomembrane cover system for Vallejo. It consists 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 are used to divide the fabric cover sections into appropriate lengths for easy retractability. 

The geomembrane cover fabric used by GTI is made up of a laminated sheet of 40 mil specialty PVC (Ethylene Interpolymer Alloy) that acts as a gastight barrier to keep the off-gas from passing through. It incorporates a highly specialized weave design that provides maximum strength-to-weight ratios. Since this top sheet is exposed to the sun, it is also equipped with advanced UV inhibitors. The material can withstand temperatures to -30 degrees Fahrenheit. 

This type of cover is ideal for wastewater cover 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. 

However, not all geomembrane cover designs work this efficiently. Polyethylene top sheets, 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. GTI’s cover system has overcome these deficiencies. 

Vallejo’s covers are gastight and operate under negative air pressure. A ventilation system draws air through the tank and underneath the cover, pulling along with it the off-gas from the aeration process. Off-gas removal piping is connected directly to the cover system and out a soil filter for odor scrubbing. 

The Results:

Although the membrane covers are gastight, they can be quickly detached and easily rolled up along the frame. This gives operators access to inspect and maintain internal components of the basins. Reattaching the membrane covers is quick and easy, making for a time-efficient and safe process. 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 15 years,” explains Tekippe. “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.” 

Municipalities are looking for more efficient tank cover systems to contain off-gases, reduce algae growth, simplify maintenance and repairs, and cut expenses. Retractable, structurally supported geomembrane covers have become an increasingly attractive option for streamlining wastewater plant operations. 

For more information, visit www.gticovers.com or contact Brent Howe, GTI product line manager at brent.m.howe@gticovers.com or 506/449-0993.


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