BioCycle May 2011, Vol. 52, No. 5, p. 43
Wastewater treatment plant’s ability to service manufacturers of cheese and yogurt with its excess digester capacity has led to a thriving biogas production facility.
Gloversville and Johnstown, New York, once known as the glove and leather capital of the world, suffered significant job loss over the past two decades as manufacturers moved operations overseas. The industrial exodus caused hardships for the local economy and created challenges for the Gloversville-Johnstown Joint Wastewater Treatment Facility (GJJWTF) in Johnstown.
Completed in 1972, the 13.8 million gallon per day (mgd) GJJWTF treated wastewater generated by area industry, primarily leather-related manufacturers, and 25,000 residential customers in Gloversville and Johnstown. By March 1972, industrial users accounted for 80 percent of the facility’s 11-mgd load.
Plant upgrades in the early 1990s included installation of a two-stage anaerobic digestion system, with a 1.5-million gallon primary digester and 1.3-million gallon secondary digester, designed to accept 100,000 gallons per day (gpd) of sludge from the wastewater treatment plant. Biogas from the digesters fueled two 150-kW Caterpillar engines driving generators producing electricity for the facility. Heat recovered from the engines warmed the digesters.
“Right after the plant was upgraded some of the manufacturing jobs started to get exported overseas,” explains George Bevington, the facility’s manager. By 2004, industrial loading at the plant had dropped almost 90 percent and wastewater inflow decreased to under 6 mgd.
To offset the declining industrial loads, Bevington looked for ways to cut costs and increase revenues by improving operating efficiency. In 2001, GJJWTF enrolled in the New York State Energy Research and Development Authority’s (NYSERDA) FlexTech program to study the energy efficiency of the plant’s aeration system. FlexTech pays half the cost of studies identifying and encouraging the implementation of cost-effective energy efficiency measures, combined heat and power (CHP) systems and renewable generation projects, explains Kathleen O’Connor, NYSERDA project manager. GJJWTF enlisted Malcolm Pirnie, the Water Division of ARCADIS, to conduct the study.
Based on study recommendations the facility spent $1.5 million upgrading the aeration system. Approximately $1 million of this project was attributed to the installation of fine bubble diffusers and an energy efficient blower. Improvements saved over $100,000 a year.
Realizing that cost-effective energy efficiency measures could only go so far at a wastewater treatment plant, Bevington turned his attention to the anaerobic digestion system. “If you can’t eliminate kilowatts why not generate kilowatts,” he says. “That is when we started focusing on CHP.” A second FlexTech study conducted by Malcolm Pirnie identified improvements to the anaerobic digestion and CHP systems. The evaluation found numerous issues and suggested a range of remedies.
An inoperable floating cover on the secondary digester limited biogas storage at the plant. Typically a floating cover rises as gas is produced and acts as a storage chamber. Without sufficient gas storage the cogeneration facility ran 8 to12 hours a day on biogas and then switched to natural gas, or the engines ran significantly below capacity. “They wanted to keep the cogeneration (CHP) facilities running since they were using waste heat to heat the digesters,” explains Robert Ostapczuk, senior project engineer with Malcolm Pirnie. “But it’s not typically cost-effective to purchase natural gas to make electricity and use the waste heat.”
The system’s carbon steel gas piping needed to be replaced or cleaned to eliminate sediment and scale that was restricting, and in some cases virtually blocking, gas flows. Other recommendations included an overhaul for the engines that were operating below rated capacity, improving the primary digester’s mixing system, adding a mixing system to the secondary digester and replacing the gas safety equipment.
Performance statistics gathered during the FlexTech study established a baseline for measuring improvements. Standard retention time (SRT) in 2000-2002 was 34 days with volatile solid (VS) loading rates of 0.06 lbs per cubic foot (lb/cf) and VS reduction rates of about 40 percent. The digester produced 13 cf of biogas per pound of VS (cf/lb VS) with a carbon dioxide content of 32 percent. Daily biogas generation averaged 83,000 cf. Electricity production averaged 816,000-kWh annually, satisfying 9 to 12 percent of the facility’s requirements. “We found the digester, from an industry standard, to be underloaded,” Ostapczuk says.
The first phase of digester improvements, completed in 2003, overhauled the engines and flushed sediment and scale from the piping. “Right off the bat we saw an increase in digester gas utilization,” he adds. Electricity generation rose to 1-million-kWh annually. A lack of gas storage facilities limited further increases.
While upgrades were underway to cut costs, Bevington looked for ways to increase revenues. The idea was to market the underutilized digester to attract new sources of high strength waste and earn tipping fees. Additional electricity generated by increased biogas production would decrease electricity outlays.
In 2003, the facility began accepting 20,000 to 30,000 gallons/week of cheese whey from a local cheese manufacturer. The high strength waste was hauled to the plant in tanker trucks and fed directly into the digester. “Biogas generation started to increase rather sharply,” Bevington says.
Meanwhile, GJJWTF worked with the Fulton County Economic Development Corporation to market the underutilized digester’s high strength waste treatment capabilities. The outreach helped attract two companies, feta cheese manufacturer Euphrates Cheese and Greek yogurt manufacturer Fage USA, to the industrial park adjacent to the treatment plant.
“Having that excess capacity really allowed us to attract those companies, particularly Fage,” says Michael J. Reese, president and CEO of the Fulton County Economic Development Corporation in Johnstown. “Because of the amount of whey they projected producing from the yogurt they needed a site that had a great deal of capacity in the sewage treatment facility. George Bevington was a great partner and instrumental in our efforts to bring these companies to Johnstown. He was able to talk to their engineers and assure the companies that GJJWTF would be able to handle the waste they were producing.”
In 2005, GJJWTF started a second round of upgrades, targeting capital projects outlined in the FlexTech assessment, to improve the generation, capture and use of biogas. Funding was not available to replace the floating cover. Instead the cover was converted to a fixed cover with a separate 43,000-cf dual membrane gas holder.
The mixing system in the primary digester was replaced with a confined gas mixer. A similar system was added to the secondary digester. Much of the digester piping was converted from carbon steel to stainless steel, which is easier to flush and more resistant to scale buildup. Gas safety equipment, including the flare and pressure relief valves, was replaced.
A 2-inch forcemain was installed between the Fage plant, which was under construction, and the GJJWTF to eliminate trucking the waste. A 90,000-gallon whey storage tank and whey feeding system were built at the treatment facility. The system controls the whey feed rate, enabling a steady flow from the storage tank to the digesters 24 hours a day. The upgrades increased whey volumes to 5-million gallons annually in 2006, and boosted digester utilization and biogas production. (This did not include whey from the Fage plant, which was not online at the time.)
VS loading almost doubled to 0.11 lb/cf and the SRT dropped to 25 days. VS reduction rates increased 47 percent and digester gas production increased to 14.6 cf/lb VS. Biogas generation increased by 60 percent to 137,000 cf/day, enough gas to run the engines 24 hours a day and produce 1,881,000 kWh of electricity. Excess biogas was flared. Waste heat captured from the engines reduced natural gas purchases from 20,000-therms to 1,000-therms; gas was only purchased when engine maintenance occurred.
Designs for the second round of upgrades assumed Fage would generate 35,000 gpd of whey. But by the time Fage broke ground, the Greek yogurt craze had hit the U.S. Fage ended up signing a contract with GJJWTF for 66,000 gpd.
To accommodate Fage’s growth and increased whey shipments from the two cheese companies, GJJWTF embarked on an $11 million upgrade in 2008. Malcolm Pirnie designed the upgrade based on projected 2011 VS loading rates, expected to climb to 0.21 lbs/cf. Calculations showed the digesters, once underloaded, were going to be overloaded. “We went from having excess capacity to insufficient capacity,” Ostapczuk says.
Options to increase digester capacity included a pretreatment digester, a third digester or, the option selected, adding a recuperative thickening loop (RTL), which was less capital intensive. An RTL takes partially digested solids from the primary digester to a thickening process and then back to the primary digester. A gravity belt thickener removes liquids from the solids.
“With high strength wastes, the limiting factor is the organic loading,” Ostapczuk explains. “The more you thicken [the more water removed], the less reactor [digester] volume you need.” The process increases the SRT without increasing the hydraulic load or incurring the expense of building an additional digester.
The RTL qualified for a $400,000 NYSERDA grant under a program to fund innovative technologies for waste and wastewater projects. The project also used a $1 million grant from NYSERDA’s Customer-Sited Tier Anaerobic Digester Gas to Electricity Program to replace its engines with two 350-kW engines and install a new heat recovery system. The program procures renewable energy sources to meet New York State’s Renewable Portfolio Standard goals, O’Connor explains.
The generator’s electrical system was modified to feed power to the on-site 13.2 kilovolt electrical substation. In the event of a power outage the engines, which are synchronous generators and do not require power from the grid to operate, serve as the facility’s emergency backup. Since the upgrade, GJJWTF has lost power a few times and did not even know it, Ostapczuk says.
The project built a second whey equalization tank capable of storing 200,000-gallons and connected it to the forcemain running to Fage. The forcemain was increased in size to 4-inches and a new whey pumping station installed. The existing 90,000-gallon storage tank is now dedicated to hauled waste from the cheese manufacturers. Pumping stations on each tank feed whey into a common forcemain connected to the digester.
Two larger belt filter presses replaced the old presses for dewatering solids. Dewatering can now be completed in 10 to 12 hours compared to 16 hours with the old system. Upgrades to the sludge thickening process eliminated the rotary drum thickener and gravity thickeners; new gravity belt thickeners were installed in their place. The new system thickens sludge to about 6 percent compared to 3 percent with the old system. The digested biosolids are landfilled.
Typically manufacturers like Euphrates and Fage build pretreatment facilities for process wastewater. Instead, GJJWTF constructed a new dissolved air flotation (DAF) facility connected to the industrial park via a dedicated dairy sewer. “The DAF system was part of the upgrade project,” explains Bevington. “Fortunately, a lot of grant money was received to reduce the local share of the costs. There was no need to increase sewer rates to pay for the upgrade.” Adds Ostapczuk: “The advantage is there are economies of scale for the two manufacturers and the float (solids skimmed from the DAF) is directed to the digester.” The process decreases the load to GJJWTF’s aeration basins (which is an energy intensive operation) while increasing the load to the anaerobic process for energy recovery.
Additional funding for the project, completed in late 2010, included a $6 million stimulus award from the New York State Environmental Facilities Corporation Green Innovation Grants Program and a $2.2 million grant from the Economic Development Administration targeting job growth. The digesters are processing 85,000-gpd of whey and 70,000-gpd of sludge to produce 10-million cubic feet (mcf) of biogas a month. “The underutilized digester of 2005 is back to capacity,” Bevington says.
The engines are producing at a rate of 5,100-megawatt hours annually. With the engines running full time, Bevington expects to produce between 90 to 100 percent of the power required by the facility. Expenditures on electricity will be reduced by about $480,000/year.
Sufficient waste heat is generated by the engines to eliminate the need to purchase natural gas for the digester process. Excess biogas, currently about 3-mcf/month, is flared. GJJWTF is exploring possible uses for it. One option under review is selling some of the biogas to Fage for use in its boilers.
Revenue generated from tipping fees to accept high strength waste average about $58,000/month. “Fees for high strength whey range from 2.2 to 4 cents/gallon, depending upon organic strength,” Bevington explains.
The local economy is also benefiting. Euphrates Cheese and Fage USA are contributing over $975,000 in property taxes for the town, county and school district and employing about 160 people. “Wages and benefits are extremely competitive for our area,” Reese says. “Those are the jobs that help an area like ours because of the huge economic impact.” Increased purchases of milk by the cheese plants are helping dairy farmers. Locally the dairy sector has added about 200 to 250 employees.
The key to codigesting high strength waste is feeding small amounts around the clock, Bevington says. “We like a slow and even pump rate and equalization of the high strength waste.” Feed rates are controlled to insure the digester receives the same amount of whey on a weekend day as a week day.
Understanding the organic strength of the waste going into the digester is also critical. The whey has a BOD (biological oxygen demand) between 40,000 to 100,000 mg/l depending on the source. “The cheese whey tends to be at the high end of the spectrum and the yogurt whey at the lower,” he explains.
Overfeeding can result in digester upsets, which occurred in fall 2010. Very high strength cheese whey was typically pumped into the digester about 4 hours a day and then the yogurt whey for 18 hours. One day the cheese whey was pumped for the full 24 hours, overloading the digester and causing an upset. No one is certain what biological factors caused the upset. The best guess is that the acid formers in the digester were extremely well fed and very active, causing the formation of high levels of volatile fatty acids, Ostapczuk explains. The pH levels in the digester, which normally range from 6.9 to 7.1, dropped to 5. “The volatile acid to alkalinity ratio went out of sync,” he adds. “That is what we believe happened, but it is anybody’s guess.”
To get the digesters back online, the team used the recuperative loop to dewater solids from the primary digester, thereby removing water-soluble volatile fatty acids. The solids were then returned to the primary digester. “Over the course of two weeks it was producing gas again,” Ostapczuk says. “Without the recuperative loop we were probably looking at draining the digester and starting over. The operations staff worked tirelessly around the clock to regain control of the digester. In the end, learning the boundaries of the system has been some of the best data obtained to date.”
Diane Greer is a Contributing Editor to BioCycle.
May 17, 2011 | General
Municipal And Industry Synergies Boost Biogas Production
BioCycle May 2011, Vol. 52, No. 5, p. 43