Floating geomembrane cover improves biogas collection, heat retention, and odor control
Canadian corn products refiner, Casco Inc., upgraded its 4-million-gallon wastewater anaerobic digester to include a state-of-the-art insulated, floating geomembrane cover.
Geosynthetics | October 2009
By Jim McMahon
Casco Inc. is all about the processing of corn products. As one of Canada’s biggest, and oldest, manufacturers of corn-refined ingredients such as sweeteners, starches, oil, and animal feed, its products are used in more than 60 industries, from food and beverage to pharmaceuticals to paper manufacturing and animal nutrition.
Combined, its three manufacturing facilities located in Ontario process 4.5 million bushels of corn each month. One of its plants, located in Cardinal, on the St. Lawrence River 50 miles south of Ottawa, is among the most automated corn wet milling facilities in the industry. Opened in 1858, and today processing 70 million pounds of corn monthly, the facility produces high fructose corn syrup, glucose, specialty starches, and corn oil for Canadian and U.S. markets.
Along with the Cardinal facility’s high volume of corn processing production—it runs 24/7—is the plant’s need to process a continuous effluent of organic waste. An average of 792,000 gallons (106,000ft3) per day of wastewater enters its treatment facility for processing .
Casco’s Cardinal plant has used a geomembrane cover on its bioreactor since it became operational in 1988. In October 2008, an upgraded floating, insulated geomembrane cover with a streamlined capability to collect biogas was installed.
Floating geomembrane cover streamlines biogas collection
Anaerobic digestion is a process where microorganisms break down biodegradable material in the absence of oxygen. This process is used widely to treat wastewater sludges and organic waste because it provides volume and mass reduction of the input material.
At Casco, raw solids are added directly to its BVF (bulk volume fermenter) bioreactor, where they are digested, minimizing waste sludge handling.
The biological breakdown of organic matter in the absence of oxygen produces primarily methane, but also carbon dioxide and some traces of hydrogen sulfide, which altogether is labeled biogas. Although biogas-derived methane and carbon dioxide come from an organic source with a short carbon cycle, they do still contribute to increasing atmospheric greenhouse gas concentrations. This is diminished, however, when biogas is combusted. This energy release allows usage of biogas as a fuel to run any type of heat engine or to generate mechanical or electrical power. In essence, anaerobic digestion is a renewable energy source that converts wastewater to a methane- and carbon-dioxide-rich biogas suitable for energy production, replacing fossil fuels.
The Cardinal, Ontario, facility’s bioreactor has had a geomembrane cover on its BVF bioreactor since 1988. But after 20 years of use, Casco upgraded to an improved-design, floating, insulated geomembrane cover with a streamlined capability to collect biogas. The cover captures and reclaims all the biogas from the treatment process occurring inside the tank. Without a cover, the biogas would release into the atmosphere. This new geomembrane cover is collecting an average of 236,000ft3 of biogas per day, at a 65% methane concentration, from the BVF bioreactor.
“Over the past two years [2007–2008], Casco’s cover was getting to the point that it needed to be revamped or changed,” says Victor Cormier, engineer and project manager for GTI. “As the previous cover aged over the 20 years, it began to have issues inhibiting biogas collection. Our latest floating geomembrane cover system is significantly different from the 20-year-old cover. The [previous] cover fluctuated up and down with the level inside the tank. The new cover system is a trampoline type; it has no folds and the material is quite taut.”
Casco’s new floating and insulated geomembrane cover is manufactured with a 1-in. layer of polyethylene foam laminated to polyethylene sheeting on the bottom (wastewater facing) side. The top layer is a nonlaminated sheet of 40-mil specialty PVC (ethylene interpolymer alloy) that acts as a gas-tight barrier to keep the biogas from passing through.
It also incorporates a specialized weave design that provides maximum strength-to-weight ratio. Since this topsheet is exposed to the sun, it is also equipped with advanced UV inhibitors. The cover’s polyethylene sheeting and insulation is not designed to be gas tight, and it is specially perforated to allow the biogas to pass through and get trapped by the top layer.
The geomembrane cover lays on the surface of the bioreactor, which provides buoyancy for the cover system. The cover works under a vacuum, and it uses a blower system that keeps the gases withdrawn and suctioned underneath the cover.
It also incorporates an integrated floating beam, which not only assists in the initial deployment of the cover panels over large bioreactors, but it also creates a tent-like effect giving extra migration paths for the biogas to follow. The beams themselves are hollow molded plastic, but they are also biogas-tight. Aluminum angles are bolted down to all panel sides of the cover to make a gas-tight seal and a strong connection so the panels maintain a constant vacuum. This seal design also provides simple and efficient accessibility options for inspections and maintenance.
Once the biogas is collected, several options are available, including disposal of the gas in a flare, use as a fuel to provide process heat, or to generate electricity. Biogas must be very clean to reach pipeline quality and must be of the correct composition. Carbon dioxide, water, hydrogen sulfide, and particulates are removed before it can be used for heating or electrical generation. The plant is currently flaring the gas but is also examining options for utilizing the biogas in the plant.
Improving BVF heat retention
The efficiency of the BVF bioreactor—its ability to maintain digestion of the continuously incoming influent and its commensurate production of biogas—is critically dependent upon keeping the temperature of the BVF reactor at 25°–32°C (77°–90°F). This is particularly important in cooler, northern climates, such as Casco’s Cardinal location.
Heat loss in large volumes of wastewater translates to energy loss, and this lost heat must then be compensated for by adding heat. Casco has supplemented its BVF reactor with heat generated from refinery wastewater, which has been intentionally heated to maintain the bioreactor’s temperature.
The new geomembrane cover design provides a increased level of insulation material to better hold heat within the reactor. Its snug fit also reduces heat loss to a greater extent than the previous 20-year-old cover. Additionally, elimination of water evaporation and increased prevention of sunlight penetration improve maintenance of appropriate water temperatures. Minimizing heat loss, as well as preventing potential ice buildup in the BVF, decreases Casco’s energy consumption and reduces its operating costs.
Biogas odor control was a primary reason prompting Casco to move forward with the new upgraded geomembrane cover.
The biogas odor is generated mainly from hydrogen sulfide that, along with the methane and carbon dioxide, was burned off with a flare. But standards set by the Ontario Ministry of the Environment do not allow any methane to be released into the environment from Casco’s BVF wastewater treatment. From an operating perspective, the company needed to be certain that the new cover would meet these standards. Complicating the problem is that a residential neighborhood just 150ft from the bioreactor. A release of concentrated methane drifting into the neighborhood could present a serious safety hazard.
Challenging cover switch
Aside from a tight deadline required to replace the cover because of the potential for an unplanned, and potentially dangerous, biogas release—the design, manufacturing, and installation teams were required to complete the project in less than three weeks—a critical factor was the need to execute the cover switch while the plant remained in operation.
This meant that wastewater flow from manufacturing could not stop. The solution was to divert some of the plant effluent away from the BVF bioreactor to the aerobic lagoon while the work was in progress.
“We were concerned with the activity of the BVF unit while the cover was off,” said Gerald Morand, process engineer and environmental coordinator for Casco. “Because it does run anaerobically, when it is exposed to the air we felt we were going to get a decrease in activity, so we did not want to overload the system. If we could decrease the COD (chemical oxygen demand) going to the BVF it would not put too much of a strain on the system while it was exposed to the atmosphere, yet still allow it to have some nutrients so that the biological activity would remain active. We cut the wastewater volume to the BVF by 55%, and we overloaded the aerobic lagoon intentionally during the project to reduce the biogases in the digester while we had the cover off.”
Because the bioreactor is located directly adjacent to the St. Lawrence River, only 25ft of clearance was available on three sides of the system. The fourth side was bordered by the plant’s operating railroad line, again minimizing available space. This posed challenges in both removing the old cover and installing the new one.
To address this issue, the team manufactured and transported the 130-ft x 410-ft new cover in four large sections, which were folded and rolled. One at a time, the rolls were placed directly onto the BVF water, opened, and connected together using the floating-beam design.
“The floating beams allowed us to connect the large cover panels together without having to weld them,” said Cormier, the GTI project manager. “We minimized the use of heat because we did not want to ignite the biogas. The more we do mechanically to fasten the large floating panels together without the use of electrical tools or heat, the safer the installation.”
Cormier said a large crane, forklifts, and dump trucks helped to maneuver the cover sections. “While we were removing pieces of the old cover, we were simultaneously installing sections of the new cover to limit the reactor exposure to air and reduce the amount of odor coming off the wastewater. Usually, we remove the old cover and install the new one by pulling one off while we are pulling on the other. In this case, because there was limited space, we had to design, build, and install the new cover differently,” he said.