March 27, 2006 | General

Transforming Wastewater Into Electricity and Heat

BioCycle March 2006, Vol. 47, No. 3, p. 62
Anaerobic digester installed by food processing cooperative is operating beyond expectations, while wastewater strength is reduced 70 to 80 percent.
Molly Farrell Tucker

Wastewater from processing figs and raisins is being digested into renewable power in Fresno, California by Valley Fig Growers, a farmer-owned food processing cooperative. The firm has installed a covered lagoon anaerobic digester at its facility that will save approximately $128,000 in annual energy costs. All commercially-sold dried figs produced in the United States are grown near Fresno and central San Joaquin Valley. The region’s 100 growers produce 14,000 tons of figs annually.
Valley Fig processes 6,000 tons of figs and 2,500 tons of raisins each year along with smaller quantities of dates and prunes – selling its harvests worldwide since 1959 under the retail brands of Sun-Maid, Old Orchard and Blue Ribbon. The company has 100 employees and grossed $17.5 million in annual sales in 2005.
Valley Fig has two main production processes: cleaning and rehydration of dried figs, and custom production of raisin, fig, date and prune fruit concentrates. The cooperative receives dried figs from the growers and then washes and processes them. This plumps up the figs by increasing water content from less than 20 percent to up to 31 percent.
The fig processing and fruit concentrate production creates wastewater that is high in suspended solids and BODs (biochemical oxygen demand). Valley Fig generates 40,000 gallons of wastewater each day, seven days a week. Every gallon of its waste-water was being treated at the Fresno and Clovis Regional Wastewater Reclamation and Treatment Facility in southwest Fresno. The facility receives approximately 68 million gallons of wastewater a day, and food processing is the major source of high-strength wastewater. Valley Fig’s wastewater has a TSS content of 900 ppm and a BOD content of 4,500 ppm, two percent of the total industrial BOD processed by the Fresno facility.
Valley Fig was paying a sewer surcharge for its wastewater treatment, at a cost of $130,000 per year. It also uses natural gas fired boilers for the production of steam for processing and was facing increases in its natural gas bills, says Mike Emigh, president of Valley Fig Growers. “The price of natural gas in December 2005 was up 47 percent from the same time period last year, $10.18 per decatherm versus $6.94.”
Provost and Pritchard Engineering Group, Inc. began working with Valley Fig in 2000 to find a solution to its wastewater problems. “We went through a lot of options and the anaerobic digester became the most feasible,” notes Donald Ikemiya, managing engineer for the project. Other pretreatment options that were evaluated included facultative (anaerobic and aerobic) ponds, aeration systems, high rate digestion systems, and large filtration systems.
The goals of the anaerobic digester project were to reduce city sewer fees, capture and use methane to generate electricity, capture and use exhaust heat to reduce use of natural gas, and promote the project to other businesses.

How The System Works — Costs And Funding

The wastewater left over from washing the fruit flows into the anaerobic digester, which is a double-lined and fully covered pond. Microbes in the digester eat the sugar in the wastewater and produce biogas. The biogas consists of 50 to 60 percent methane. A microturbine then transforms the methane into electricity which is used on-site to run the equipment and machinery. The methane is also burned in boilers to heat water used in the plant.
The original budget for the project was $1.2 million, but it ended up costing approximately $1.6 million. “That’s because of compliance with permitting requirements, adding a storm water pond, landscaping and fully fencing off the facility,” notes Ikemiya. “Extra things were also added to enhance the facility, including extra heat exchangers.” These heat exchangers are plumbed to other heat sources (cooling towers) to provide additional heating to the digester. The $1.6 million total included permitting, design, engineering, a railroad under-crossing, storm water pond, earthwork, embankment construction, a concrete stem wall, an HDPE double liner and XR-5 cover for the digester, influent and effluent piping, pumps, a digester mixer, recirculation system, heat exchangers, gas collection piping, microturbine, backup boiler, fencing, landscaping, controls and electrical work.
Valley Fig Growers funded the majority of the project. It also received a $476,000 California Energy Commission PIER (Public Interest Energy Research) grant. This grant, funded by utility bills, was one of five PIER grants awarded in California in 2002. Valley Fig also received a $70,000 rebate from Pacific Gas & Electric (PG&E) for the microturbine through utility-sponsored rebate programs for renewable and efficient energy sources.

Permitting Agencies

Provost and Pritchard started designing the system in April 2003, but construction didn’t start until August 2004. “We had difficulty getting the permits,” explains Ikemiya.
A building permit was required from Fresno because Valley Fig Growers is located within the city limits. The California Regional Water Quality Control Board required a Groundwater Monitoring Network Plan and a Waste Discharge Permit. A permit from the San Joaquin Valley Air Pollution Control District was needed to operate the digester and microturbine. The California State Water Resources Control Board also required a permit to discharge storm water.
“We also had to obtain permits from the Fresno Metropolitan Flood Control District because of drainage concerns, and from the Union Pacific Railway for crossing under their tracks, because the facility is surrounded by the railroad,” adds Ikemiya. “We also needed approval from PG&E to connect the generated electricity to the PG&E meter.”

The Digester

Making the digester lagoon fit on the Valley Fig property took some work. “It was a challenging location and shape,” remembers Ikemiya. “The digester is almost trapezoidal in shape because it is surrounded by railroad tracks. It’s a triangle on one side and a rectangle on the other. There was a larger area that could have been used for the lagoon, but Valley Fig picked this site because it is closer to the facility where the wastewater is generated from washing the figs and washing down the equipment.”
The lagoon covers an area 260 feet by 157 feet (26,500 square feet or .6 acres.) It is 22 feet deep and has a capacity of 1.8 million gallons (45 days of process wastewater).
The digester has a bank-to-bank cover that makes it air tight. The lagoon is covered with a layer of XR5, a geosynthetic manufactured by Seaman Corporation. The lagoon is double-lined with 60 mil and 40 mil HDPE and has a leachate collection system. “If the primary liner were to fail or leak, there is a geo web mesh between the two layers that would collect the leachate,” explains Ikemiya. “The secondary liner would prevent the leachate from going into the soil beneath the lagoon.”
Valley Fig’s wastewater is fairly high in sugar, which is a good food source for the microbes. “As the microbes digest the sugar in the wastewater, BOD is reduced and the microbes produce biogas,” says Ikemiya. This biogas is stored under the XR5 cover in the lagoon. “It actually pillows up and balloons under the cover.”
Retention time in the lagoon can be as long as 45 days, but the digester was designed for a retention time as short as 15 days. The initial biogas average production is 2,000 to 2,500 cubic feet per hour. Biogas production varies with temperature and amount of the food source.
The gas is run through a moisture trap to remove any condensed moisture and is then pumped through stainless steel pipes to the microturbine or to the boiler. The boiler is located near the microturbine and provides hot water for use in the plant. Both the microturbine and the boiler can run on natural gas, methane, or a mixture of both.

Microturbine And Heat Exchanger

The MT70 microturbine, manufactured by Ingersoll-Rand Energy Systems, generates 60 kW of electricity for use on-site to run the processing equipment in the plant by burning the methane. All of the electricity and hot water produced is used on site. The microturbine has very low emissions and is rated to produce 70 kW of electricity. “We get a net gain of 60 kW, because the microturbine has certain internal power requirements,” notes Ikemiya. “That is probably near 90 percent of the minimum load of electricity used by the plant.”
The heat exchanger uses 50 to 100 percent of the microturbine’s exhaust heat. Waste heat is collected from the microturbine’s clean exhaust and is used to heat the water in the digester which also circulates the water. “The goal is to keep the temperature of the digester around 95°F,” says Ikemiya. “The microbes in the digester get the most efficient digestion at that heat range.”
Valley Fig started operating the digester in phases in July 2005. “We’ve been operating now for a number of months and the microbes are maturing,” points out Ikemiya. “We need to keep them fed, warm and happy. We’re still tinkering with the system to fine tune it.”

Benefits of Technology

Net annual benefit to Valley Fig will be roughly $128,000 per year, including $90,000 savings in Valley Fig’s annual sewer bill, $25,000 in electrical generation savings and $13,000 on capturing exhaust heat and/or using methane to heat the boiler instead of using natural gas.
“We estimated that $25,000 in electricity would be produced, but rising energy costs will increase this number over time,” says Mike Emigh. “We are expecting to reduce wastewater strength 70 to 80 percent with the digester. By reducing the flow of high strength wastewater to the Fresno sewer treatment plant, we’re freeing up capacity for other users and/or 2,500 homes.” The Fresno facility has the capacity to treat 80 million gallons a day of wastewater.
“This will be a huge benefit for the City of Fresno if other companies decide to use an anaerobic system,” says Ikemiya. “Fresno’s wastewater treatment system is aerobic. It requires more than 1,000 horsepower to aerate the wastewater in order to treat it. Anaerobic digestion requires no oxygen or air, so it is a low horsepower system. We’re letting the microbes do the work, while making electricity and heating water.”
“This was challenging but very rewarding, both as an engineering challenge and for Valley Fig to reap the benefits. It’s operating beyond their expectations with the amount of methane gas being produced.” Eventually, Valley Fig plans to reduce its offsite disposal of culled fruit solids from two truckloads per day to one per day, by pureeing the solids and adding them to the digester as a food source for the microbes, sums up Ikemiya.

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