The Fergus Falls WWTP operates three digesters in series, two primary and one secondary, each with a capacity of 471,000 gallons. The two primary mixed and heated digesters are maintained at 98°F. The first primary digester is equipped with a stationary roof, whereas the second primary and the secondary digesters have floating roofs and can hold 15,000 cubic-feet (cf) and 7,500 cf of biogas respectively, explains Stefan Nelson, WWTP manager.
The plant processes about 2.3 million gallons per day (mgpd) of wastewater. A few years ago, a steam boiler was converted to a hot water boiler capable of burning biogas, fuel oil and natural gas to heat the digesters and buildings at the facility in the winter. “From there we started thinking about ways to generate more biogas on site to better utilize the boiler,” says Patrick Rein of Rein and Associates in Moorhead, Minnesota, which conducted the feasibility study in 2007. The main boiler can process approximately 50,000 cf of biogas per day. Biogas production at the time of the study averaged 16,000 cf/day.
Sponsors of the feasibility study included the City of Fergus Falls; Otter Tail Power (OTP), the local power company; Otter Tail Ag Enterprises (OTAG), a local ethanol facility; and the Agricultural Utilization Research Institute (AURI). OTP and AURI provided funding for the study. OTAG had been developing plans for a 55-million-gallon-per-year (mgpy) corn ethanol plant in Fergus Falls. Rein and Associates heard about the potential for using thin stillage or thickened thin stillage (commonly referred to as syrup) from the ethanol process as a feedstock to enhance biogas production.
The material is derived from a slurry called whole stillage, a by-product of the fermentation process during ethanol production. The slurry, about 88 percent water, is centrifuged to separate the solids from the water. The solids, or distillers’ wet grains (DWG), are dried while the centrate, called thin stillage, is concentrated in an evaporator to make thick stillage or syrup. Typically the syrup is added back to the DWG and sent through a second drier to produce distillers’ dry grains with solubles (DDGS) that are used as livestock feed.
OTAG was not planning to use all of its syrup in the production of DDGS, Rein explains. Unused syrup was to be trucked out as liquid cattle feed. However, the digesters at Fergus Falls were operating below capacity. Codigesting syrup with biosolids would use the plant’s excess capacity and produce more biogas. OTAG also stood to benefit by finding a new market for its syrup, potentially increasing the value of its by-products.
Syrup was initially added to Fergus Falls digesters between January 29 and April 5, 2007. Since OTAG was still in development, syrup was donated by Chippewa Valley Ethanol Company in Benson, Minnesota, about 60 miles away. The syrup was stored in an indoor metal tank at at the WWTP and added to the digesters in batches. A total of 35,832 gallons of syrup was pumped into the digesters over a 64-day period, explains Rein. Maximum daily loading did not exceed 2,000 gallons.
During the same period, about 320,000 gallons of sludge were added to the digesters. The syrup accounted for only 10 percent of the hydraulic loading of the digesters. “What was amazing was the quantity of biogas produced per gallon of syrup fed,” Rein says. Adding the syrup doubled biogas production — an additional 1,075,000 cf of biogas. He calculates that biogas production from the syrup averaged 30 cf per gallon of syrup. Methane content of the biogas averaged 75 percent, yielding 23 cf of methane per gallon of syrup.
“When we fed the syrup into the digester, the methane content climbed by five percent immediately,” Rein adds. A possible explanation is that much of the starch and sugars had been removed from the syrup in the ethanol process, resulting in a feedstock with high levels of protein and fat. He explains: “It appears that the methane content is higher than any other feedstock we have tried.”
Sufficient biogas was produced during the study period to satisfy the maximum daily heating requirements in the winter. During the summer there would be enough excess biogas to operate a small generator. Fergus Falls investigated installing a small cogeneration system with the local power company, but decided not to pursue the option. “At this point it is not feasible because of the generator cost,” explains Anne Martens, Fergus Falls Public Works Director. “The payoff is not big enough.”
SYRUP CHARACTERISTICS, LOADING RATES
The syrup turned out to be beneficial in several regards, in addition to biogas production. “What they really like about this syrup is that it is very consistent coming out of the ethanol process,” Rein explains. “They know that when feeding ‘x’ gallons they are guaranteed to get so many cubic feet of biogas. With other feedstocks, it is hard to determine how much gas you are going to get from load to load.”
The syrup is highly concentrated with a chemical oxygen demand (COD) of 460,000-mg/L and total volatile solids (TVS) concentrations of 285,000-mg/L, which means there is less volume to digest. “I do not know of many feedstocks that have a COD of approximately 500,000-mg/L,” Rein says. During testing, the syrup more than doubled the organic loading rate of the digesters, adding approximately 149,420 lbs of COD.
The new feedstock was also easily digested into gaseous by-products. “Typical municipal sludge will continue to produce gas for several days,” Rein explains. “The syrup is essentially done producing biogas in 18 hours.”
At 2,000 gallons/day of syrup during testing, the maximum loading rate for the digesters was 0.05 pounds of volatile solids per cf per day (lbs VS/ft3/day). Syrup loading rates could have been doubled and still remained well within typical digester VS loading rates of 0.15 to 0.35 lbs VS/ft3/day, Reins says. Potential factors limiting loading rates include ammonia, which according to Rein can cause toxicity problems for anaerobic microorganisms at concentrations greater than 3,000 mg/L. Since the TKN (a precursor to ammonia) in the syrup was measured at 7,500 mg/L, feeding too much could cause the ammonia concentrations to exceed that level.
Rein calculates that the digesters could be loaded with syrup at 30 percent of the hydraulic load without exceeding ammonia toxicity levels of 3,000 mg/L. “We recommended that they limit the quantity fed to 1,680 gallons/day,” he says.
Struvite formation was another limiting factor. Magnesium, ammonium and phosphate released during the digestion of wastewater can form into a phosphate mineral called struvite. Anaerobic digester components are prone to fouling and encrusting with struvite, which can cause operational problems and increase maintenance costs. The syrup is rich in magnesium, ammonium and phosphate and therefore might cause struvite precipitation. At the syrup loading rates used during testing, there was a potential for modest increases in struvite precipitation that warranted ongoing monitoring. Recently the digesters were taken down for some repairs. “We looked in the digesters to see if there was struvite build up and so far we have not noticed any,” Nelson says.
Finally, the team wanted to make sure adding syrup did not affect compliance with the Minnesota Pollution Control Agency (MPCA) standards for land application of biosolids. The regulations address issues such as pathogen destruction, heavy metal limits and VS reductions. Feeding the syrup had very little effect on solids retention times and temperature. According to Rein, metal concentrations went down and VS destruction actually improved. “We calculated that the syrup is practically all destroyed during digestion,” Nelson says.
WINTER MONTHS ADDITIVE
The Fergus Falls WWTP continues to add syrup to its digesters during the winter months. “In the winter when it gets below 55°F, we start heating the buildings and the digesters require more heat,” Rein explains. “But in the summer time, demand can be met with the biogas from the municipal solids.”
Syrup is now obtained from OTAG, which began producing ethanol in April 2008. The city pays around $200 per 5,500 gallon truckload (including transportation) of syrup. “We haul it here ourselves and store it in two indoor tanks,” Nelson says. “Normally we feed 353 gallons of syrup a day depending on how cold it is, and we produce up to 30,000 cf of biogas a day.” Adding this amount of syrup equates to a 2.5 percent hydraulic loading rate with 770 lbs VS per day, providing 56 percent of the organic load.
Plant operators are getting better at matching the syrup feed rate with the demand for biogas. “When they decide to turn on the digester heating pump, they know that the demand [for biogas] is going to go up that day,” Rein explains. “They feed in the syrup early in the morning, which satisfies the daily heating demand at the plant.” “We continue to keep improving,” adds Nelson. For 2008, the facility produced about 2,100,000 cf of biogas. “This year  we produced 6,200,000 cf of biogas.” Natural gas bills that were running at $54,000 in 2008 are down to $11,000 in 2009.
“Economically this is a good choice for the City and a good environmental project,” Martens says. “Last year during the winter months we saved over $4,000 per month on natural gas cost. The net savings of adding syrup to the digester is $0.14 per gallon.” Each truckload of syrup is worth about $1,000, Rein explains. “If natural gas is about $7 per mmBTU, when it is all said and done we have netted $600 per load.” This includes the cost of transportation and land application of the biosolids.
Nelson’s advice for other facilities undertaking codigestion of syrup is to start slowly to avoid upsetting the digester: “We watch and monitor everything very closely and keep looking for anything out of the ordinary.”
Diane Greer is a Contributing Editor to BioCycle.