February 22, 2011 | General

Codigestion and Cogeneration in Des Moines

trucks deliver organic waste to Des Moine wastewater facilityBioCycle February 2011, Vol. 52, No. 2, p. 38
Servicing local industry and supplying biogas to a neighboring manufacturing plant – as well as moving toward zero net energy usage at the treatment plant – are winning solutions for the region’s wastewater authority.
Diane Greer

CODIGESTION of large volumes of high strength organic wastes is significantly increasing biogas production, reducing fossil fuel usage and generating revenue for the Des Moines Metro Wastewater Reclamation Authority (WRA) in Des Moines, Iowa. WRA serves a population of 500,000, treating 26-billion gallons/year of wastewater from 16-metro area municipalities, counties and sewer districts around Des Moines.
In 2009, WRA generated over 460-mcf of biogas by codigesting 26-million gallons of high-strength organic waste. This highly biodegradable waste is produced by regional businesses and hauled to the WRA’s digester complex, which includes six digesters, each with a capacity of 2.7 million gallons (16 million gallons total), operating at 100°F. Hauled waste, averaging 500,000-gallons/week, accounts for 42 percent of the total feed to the digesters.
Biogas produced at the WRA wastewater treatment plant (WWTP) fuels three 600 kW cogeneration units generating over 8-million kWh of electricity and fires dual-fuel boilers producing building and process heat. About 40 percent (211-mcf) of the biogas is sold to a neighboring industrial facility. This reduces or eliminates any flaring of excess gas that is not used internally or sold. Effluent (55,800-lbs per day) from the digesters is dewatered with three belt filter presses. Liquid is recycled to the head of the WWTP. Solids, totaling 55,000-wet tons in 2009, are land applied.

WRA started codigesting in 1991 when a local dairy needed an outlet for its whey waste, explains Larry Hare, WRA’s Regulatory Compliance Team Leader. Adding the whey to the digesters increased biogas production. “We became very interested in codigestion once we found out the benefits,” Hare says.
Originally the goal for codigestion was to produce enough biogas to run WRA’s three generators, explains Royce Hammitt, WRA’s Facilities Manager. “During the best of times we could run all three cogen units but usually it was two and sometimes it was just one.” Hammitt also saw the potential to operate the plant’s three dual-fuel (biogas or natural gas) boilers entirely on biogas.
Initially, WRA focused on local food processing industries for codigestion feedstocks. “We worked with soybean oil refiners and started accepting the float from packing plant pretreatment processes,” Hare says. The program saw a major increase in the volume of codigested waste with the explosion of biodiesel and ethanol plants in the area. These plants were built in small rural communities near their feedstocks, primarily corn and soybeans. In theory, little wastewater or sludge was expected to be generated from these biofuel plants. In reality, however, they produced a fair volume of wastes that were initially land applied on crop and pasture land.
“The Iowa Department of Natural Resources started cracking down on land application of those high strength organic wastes,” Hare says. Increasingly tighter environmental regulations called for full treatment of these sidestream wastes. As a result, the volume of hauled wastes from biofuel plants to WRA skyrocketed. As a rule, high strength wastes accepted for codigestion must have total solids of five percent or higher and a chemical oxygen demand of over 100,000 mg/l. “We make sure that hauled wastes are better quality than our plant wastes going into the digesters,” he adds.
Today WRA collects tipping fees from hauled organic waste produced by roughly 60 industrial producers, ranging from food processors to biofuel plants, and the FOG (fats, oil and grease) collected from grease interceptors at over 2,000 restaurants and food service establishments. The tip fees range between 1.5 to 6 cents/gallon when delivered to the digesters versus 12 cents to $1.00/gallon if tipped at the headworks. Annual revenues from tipping fees fluctuate and have declined more recently – from about $337,000 in 2008 to $200,000 in 2009.

WRA, as built, had no facilities for receiving hauled wastes. “Any waste received by truck was dumped right to the headworks of the facility with virtually no benefits to the digester,” Hammitt explains. WRA built a temporary pipeline system to experiment with hauled waste. The system conveyed the material from the tanker trucks into a blending tank, where it was mixed with primary sludge and thickened waste activated sludge and then pumped directly into the digesters. “That worked fairly well but there began to be some bottlenecks and dealing with [above ground] piping in the wintertime in Iowa is a challenge,” he adds.
To solve these problems, a $1.75 million hauled waste receiving and storage station was built. After unloading, material is stored in a 140,000 gallon in-ground holding tank before injection into the blending tank. The facility handles 30 to 70 tankers/day and reduces tanker unloading time by 50 percent. “We can gravity drain a 6,000-gallon tanker in about 15 minutes,” Hare says.
The holding tank permits the plant to achieve a steadier feed rate. “We found people like to bring in high strength organic wastes Monday through Friday, not Saturday or Sunday,” Hammitt explains. “Before the new facility, gas production would be quite high up until sometime on Saturday when the wastes would be mostly consumed. Sunday would be at our lowest gas production day of the week.”
Recently three boxes were added to remove debris, such as rocks, grit and silverware from the waste before it enters the storage tank. The debris was damaging the chopper pumps used to mix the materials in the tank, Hammitt says. Trucks now discharge into the three boxes. Heavy debris falls to the bottom of the boxes while the remainder of the material overflows into the holding tank.
“It is easier to clean out the smaller boxes when material accumulates in the bottom,” explains Dustin Craig, environmental engineer with Camp, Dresser and McKee (CDM), consultants to WRA. The boxes can also be taken offline one at a time for cleaning, minimizing disruptions to the operation. Due to the corrosive properties of the hauled wastes, the boxes are made of polymer precast concrete, which employs resin as a binder instead of Portland cement. “It proved to be a very cost-effective solution,” says Scott Carr of CDM.
Cargill plant adjacent to wastewater facility
Codigestion proved so successful that biogas production exceeded the levels required to fire the boilers and run the cogen units. Excess gas at that time, totaling 62.8-mcf in 2006, was flared since there was no outlet for it. And low electricity prices did not make it economically attractive for WRA to invest in additional generators for electricity production. Instead Hammitt focused on opportunities to produce revenue from the excess gas. “That is when the idea to install a pipeline over to our neighbor, Cargill, came into play.”
The Cargill facility, adjacent to the WWTP plant, uses natural gas and fuel oil to produce 452,000-MMlbs of steam per year. Cargill was receptive to the idea of using biogas to produce the steam. The company had recently retrofitted a boiler in South Dakota to run on landfill gas with good results.
WRA and Cargill agreed to biogas pricing and how to split project costs. WRA ultimately invested $1.1 million in the project and Cargill over $750,000. Hammitt estimates a 3.9-year payback for WRA on the project and a 1.5-year payback for Cargill.
A 600-ft. pipe was run from the WRA to the Cargill facility to transport the gas. Biogas is pressurized to 15 psi and chilled prior to shipment. Chilling is required to prevent the hot gas from damaging the underground plastic pipe. Chilling the biogas also reduces moisture levels, which provides benefits to WRA. “It turns out that drier gas reduced the maintenance impact on our generators and boilers,” Hammitt explains. Contaminant levels are also lower because siloxanes and hydrogen sulfide are captured in the condensate.
digester improvements
In July 2010, WRA started a $19 million project to upgrade its digester complex and gas distribution system, with completion of the upgrades slated for August 2012. Work on the digesters was spurred by the need to replace the digesters’ aging floating covers and mixing systems, which were no longer functional. WRA enlisted CDM to analyze options for replacing the covers and mixing systems along with identifying other cost-effective upgrades. As part of the project, CDM also analyzed options to improve biogas utilization.
Evaluations of covers and mixers took into consideration how the various alternatives handled foam. Digesting high strength wastes often increases foam production, which can result in spillage and impede gas collection. At the WRA, foam was seeping onto the top of the floating covers, Carr explains. “They were able to manage it but it was an operation and maintenance headache.”
After evaluating several different options, CDM helped the WRA select submerged fixed concrete covers (SFCC) for the five primary digesters. “The covers emulate the benefits of the top of an egg-shaped digester, reducing the surface area where you have an interface between the liquid and the gas,” Carr explains. “That allows for better control and easier removal of the foam.” SFCC are constructed with a small fixed dome that collects gas. Within the domes is a spray submission system, which sprays effluent circulated from the bottom of the tank through nozzles in the top of the dome to control the foam.
CDM also recommended five, 24-inch draft tube mechanical mixers for each digester. The mixers draw foam, scum and other materials from the top of the tank into a vertical tube that circulates the material to the bottom of the digester. This mixing technique better incorporates the foam/scum layer into the sludge and results in more through mixing.
All six digesters are currently used as primary units. As part of the upgrade, one digester will be converted to a secondary digester. Analysis determined that WRA had sufficient capacity with five primary digesters for future growth. The secondary digester will be outfitted with an inflatable dome. “We are more or less turning it into a gas storage unit as well as a secondary digester,” Hammitt says. Converting one of the digesters to a secondary system makes sense when using SFCC’s, Carr adds. “With SFCC, those tanks have to operate at a fixed level. If we had done all six that way there would not have been a wide spot in the line between their dewatering and digestion process.”
The secondary digester also can be operated as a primary tank, providing excess capacity and flexibility from an operations standpoint. “It was a great alternative,” Hammitt notes. “As a secondary digester it gives us a large amount of storage capacity and therefore a longer window for maintenance work on our presses if the need arises.”
One of the challenges with codigesting high strength wastes is that the material contains high levels of FOG that tends to congeal and clog pipes. To prevent clogging, the lines conveying hauled wastes will be outfitted with automated valves to enable in-situ line cleaning. The cleaning system uses hot sludge to flush the lines. “The temperature of the sludge coupled with a high scouring velocity helps to knock down the grease that congeals within the interior of the pipes,” Craig explains.

In anticipation of increased biogas production rates, the current moisture removal system will be replaced with a larger unit. The new system employs a shell and tube heat exchanger coupled with an air-cooled chiller. Gas transmission lines will be enlarged and two medium pressure boosters added. The boosters will increase the volume of gas that can be delivered to the Cargill facility from 700,000-cfd to 1.2 million-cfd and provide backup in the event a booster fails or requires maintenance.
WRA also is adding two 1.4 MW cogeneration units, which will expand power generation from the current 1.8 MW to 4.8 MW. The new cogen units, in conjunction with the ability to push more gas to Cargill, will provide the facility with “plenty of options” for capitalizing on increased biogas production, Hammitt says.
Additional generation capacity will also allow the WTTP to get closer to net zero energy use. This will become more important in 2014 when electricity rates, currently at $0.045 per kWh, are expected to increase significantly. “The utility and the Iowa Utility Board have an agreement to keep rates stable but that agreement expires in 2014,” Hammitt says. WRA is also considering expanding the use of biogas in its boilers. This would require converting additional existing natural gas boilers to dual fired boilers burning both biogas and natural gas.
As part of the project, CDM created a spreadsheet to help WRA evaluate the process economics of various biogas utilization options. The tool enables WRA to see the effect of varying digester operating parameters on biogas production rates, Carr explains. The economic viability of various biogas utilization scenarios can also be analyzed based on changes in the price of natural gas. “The more flexibility and usage you have for biogas, the less you are going to waste,” he says.

Diane Greer is a Contributing Editor to BioCycle.

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