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Streamlining Treatment of Challenging Food Processing Wastewaters

Ken's Foods' upgraded wastewater treatment facility efficiently reduces chemical oxygen demand and total suspended solids in challenging food processing wastewaters, and supplies 200,000-plus cubic feet of biogas per day, providing 100 percent of the heat required for the treatment plant's operation.
By Jim McMahon
Although all food processors have to deal with wastewater generated in their operations, the characteristics of the effluent exiting their facilities can vary greatly, requiring different processing technologies for the most efficient handling of the wastewater. Ken's Foods of Marlborough, Mass., a large-volume food manufacturer of salad dressings and marinades, recently upgraded one of its three wastewater treatment facilities to more efficiently process its high-strength organic content wastewater, effluents which contain a high content of fat, oil and grease (FOG) and present serious challenges for waste treatment.

The upgrade incorporated a unique treatment process called anaerobic membrane bioreactor (ADI-AnMBR), a relatively new form of anaerobic treatment technology developed by ADI Systems Inc. in cooperation with Kubota Corp. of Japan, which utilizes submerged membranes for biomass retention and solids-liquid separation. The system maximizes biogas production, increases solids digestion and provides a means to easily handle wastewaters with high concentrations of organic matter. The treatment plant is the largest of its kind in the world, producing effluent that is virtually free of suspended solids, with a level of chemical oxygen demand (COD) removal of 99.4 percent, allowing its 100,000 gallons per day of wastewater to easily discharge into the municipal system. Considering the high-strength levels of organic content-COD, biochemical oxygen demand (BOD) and FOG-in the wastewater, this performance is exceptional by any industry standard.

The company's existing low-rate anaerobic reactor, previously built by ADI Systems (an ADI-BVF system), was converted to operate as the reactor portion of the new AnMBR. As a byproduct of the combined system, 200,000 to 300,000 cubic feet of biogas are being produced per day, which are being captured to provide not only 100 percent of the wastewater treatment plant's heating requirements, but enough residual biogas capable of powering more than 50 percent of the company's manufacturing facility.

Overloading the Original Wastewater System
The company's wastewater originates from wash-down of cleaning mixers, filling machines and other process equipment. It is then screened and pumped into an equalization tank to begin the treatment process.

After equalization, the wastewater is sent to the low-rate anaerobic reactor, which is capable of treating waste streams of moderate to very high organic strength. Before the upgrade, this was followed by a sequencing batch aerobic reactor (SBR) that was needed to polish the anaerobic effluent. These two stages of treatment achieved overall COD, FOG and total suspended solids (TSS) removals of 98 percent to 99 percent.

The plant was designed for a maximum flow of 550,000 gallons weekly and 100,000 gallons per day. Due to production increases, daily and weekly flows exceeded these design values causing excessive solids loading from the BVF reactor to the SBR.

"We had too many solids coming from our anaerobic digester," explains Dale Mills, treatment plant chief operator for Ken's Marlborough plant. "We were manually monitoring the SBR decant to the city and stopping it when the water quality was not good enough."

The city of Marlborough allows the release of 100,000 gallons of effluent per day and limits the concentration of suspended solids effluent to 600 milligrams per liter (mg/l). The manufacturing plant's production had become inhibited by the overload in the treatment plant.

Ken's brought in ADI Systems to engineer a solution to the problem. "The SBR aerobic system was never the bottleneck," says Dwain Wilson, director of process operations for ADI Systems. "The solution was to increase the capacity of the anaerobic reactor, and we suggested an anaerobic MBR application."

Existing BVF Anaerobic Reactor
Ken's BVF anaerobic system already in place has been removing more than 90 percent of the organic material from the wastewater. This system is capable of handling wastewaters that are high in FOG and variations in wastewater flow and characteristics. The BVF's organic loadings are low (typically 0.3 to 3.0 kilograms COD) and hydraulic retention times are relatively long (typically more than seven days), providing an inherent stability and robustness often not found in higher-rate anaerobic processes, and allowing for significant digestion of influent solids and waste activated sludge.

The large volume and inventory of biomass within the BVF reactor eliminates the need for extensive primary treatment of the waste stream (such as a primary clarifier).

The BVF is equipped with a floating, insulated geomembrane cover. Built by Geomembrane Technologies Inc., the cover collects biogas, minimizes heat loss and provides odor control.

New Anaerobic Membrane Bioreactor
The existing ADI-BVF system was converted to an ADI-AnMBR to expand its treatment capabilities. One of the key components to any anaerobic treatment system is effective separation of treated water from the biogas generated by the anaerobic digestion process, while ensuring the biomass is retained within the reactor. The ADI-AnMBR process, based on technology developed by Kubota, is a form of high-rate anaerobic contact process that uses a submerged membrane barrier to perform the gas/liquid/solids separation and biomass retention functions. This near-absolute barrier to solids ensures efficient system operation, even under high organic loading and intense mixing scenarios. Membrane treatment technologies are often employed when higher quality effluents are required, or when wastewater characteristics make conventional gravity settling technologies difficult or ineffective. The biogas generated in anaerobic digestion is utilized to continually clean the membranes during operation via a gas scour system.

Ken's ADI-AnMBR provides for higher organic loadings and mixing intensities compared with other anaerobic technologies, increasing organic removals, improving biogas production, and allowing for treatment of wastewaters with very high suspended solids and FOG. The system is most applicable to processing wastewaters with strong, concentrated wastes with poor settling characteristics.

The AnMBR system can operate at both thermophilic and mesophilic temperatures, yet it avoids common operating problems at thermophilic temperatures, such as biomass loss and unstable operation.

"The anaerobic MBR increases the solids retention time within the system," Wilson says. "The longer the solids retention time, the lower the biomass yield, reducing the amount of biomass that will require disposal. It also allows the development of specialized bacteria that can acclimate to unusual organics and break them down."

The AnMBR has four anaerobic basins, each equipped with seven submerged membrane units. A removable GTI geomembrane cover system on each AnMBR basin provides a gas-tight seal with biogas collection capabilities. These covers allow the biogas to be captured in the headspace above the cartridges and then returned to the gas scour system for reuse.

The TSS concentration coming out of the AnMBR averages less than 1 mg/l, BOD is typically less than 25 mg/l, and the COD removal in the AnMBR is greater than 99.4 percent

The ADI-AnMBR system at Ken's is the first installation of this technology in North America and the largest in the world.

Repurposing the Aerobic Sequencing Batch Reactor
The SBR's role is to polish the anaerobic reactor effluent to meet discharge standards, a function it was not able to do satisfactorily under the original system's higher-than-design loading conditions.

The now very clean effluent from the AnMBR goes into the SBR, which has been repurposed for use as a sulfide oxidation and nitrification tank. Ken's purposely adds and maintains biological solids in the tank as a suitable biomass population for the treatment process. Now that the solids loading to the SBR from the anaerobic process have been eliminated, the SBR is used to easily oxidize sulfide and ammonia.

Captured Biogas Powers Treatment Plant
The biogas generated by the BVF/AnMBR process is proving to be valuable for the replacement of conventional energy sources such as natural gas and electricity.

"We capture the biogas produced in the anaerobic digester and we heat the processing building and the reactor with it," Mills says. "So we do not have any fuel costs for heating the treatment building, or the reactor which is kept at 95 degrees Fahrenheit. We also have a considerable amount of extra biogas that we flare right now, between 200,000 and 300,000 cubic feet per day."

ADI provided a complete recovery and utilization system which included gas collection, treatment, storage, compression and delivery systems.

"We are planning on using the flared biogas for a waste-to-energy project for cogeneration of electricity in our manufacturing facility," says Mike Kolakowski, engineering manager for Ken's Foods. "It is a combined-heat-and-power project-the amount of biogas that is generated from the reactor will reduce our draw from the utility grid by well over 50 percent."

Improved Efficiencies
In addition to significantly lowered TSS, COD and BOD levels, and captured biogas as an energy resource, 36,000 gallons of fats, oils and grease are removed and rendered per year, as well as 500 tons of dewatered residual solids.

Also, the quality of the water in the SBR is consistently clean enough for automated, timed release, allowing the maximum 100,000 gallons per day to be decanted. This has eliminated the problem of manufacturing interruption previously caused by the challenges to the wastewater treatment system.

"The system is extremely cost effective," Kolakowski says. "The overall cost of operating the AnMBR processing facility represents at least a 50 percent reduction compared to other more traditional means of water treatment."

Jim McMahon of Zebra Communications writes about water and wastewater systems. Reach him at zebracom.net.
 

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