HANDLING FATS, OILS & GREASES (FOG) AT WASTEWATER ...

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sedimentation basins with the primary scum. A scum concentrator thickens the mixture where itcollected in a hopper and heated to approximately 130 o F. The FOG rich float is decanted andpumped to the midpoint of the dewatered solids conveyor for blending prior to entering theincinerator. The aqueous subnatant from the scum concentrator is recycled back to the head ofthe scum concentrator.Pinellas County, FLThe Pinellas County, FL South-Cross Bayou Water Reclamation Facility (SCBWRF) utilizesanaerobic digestion and sludge drying for solids disposal. FOG is accepted at an offsitereceiving facility where dewatering operations take place. Generally, the dewatering operationsare as follows:• The daily receipts are accumulated, mixed, and dosed at 1.5% polymer on a volumetric basis(15 gallons of polymer solution per 1,000 gallons of FOG waste).• The polymer-conditioned waste is stored overnight in a fine-screen dewatering tank, resultingin about 80% reduction in the weight of the FOG due to dewatering. The dewatered FOGconcentration is typically about 20-30% solids.• The dewatered FOG is then transferred from the dewatering tank to a roll-on/roll-offcontainer for transfer to the WWTP.• The roll-on/roll-off container is equipped with a bottom drain and coupling such that whenthe container is received at the WWTP, the container is connected by hose to a rotary lobepump for transfer to a storage tank.Once at the facility, the FOG is injected into one of the egg-shape anaerobic digesters. Thebiogas produced from the facility currently used by the sludge pelletizer. Planned upgradesinclude heating the stored FOG and feeding it to new acid-phase reactors, along with receiving,storing and feeding lime-dewatered FOG. It is anticipated in the future, that that biogasproduction will exceed sludge drying fuel requirements wherein a new co-generation facility willbe constructed to produce energy.Essex Junction, VTThe Village of Essex Junction, Vermont operates a 3.3 mgd permitted wastewater treatmentfacility (EJWWTF) that is currently treating 2.0 mgd on an annual average daily basis. Thefacility provides service to 30,000 residents within the Towns of Essex and Williston, VT as wellas the Village. Waste primary sludge and GBT thickened WAS (TWAS) from the treatmentprocess is stabilized within mesophilic anaerobic digesters. In late 2003, the EJWWTF installeda combined heat and power (CHP) system utilizing two microturbines. The 30 kWmicroturbines utilized the digester gas of which nearly 50% was previously flared.Influent conditions at the facility indicated that FOG is an increasing problem. Without localresources for disposal of FOG, staff felt that it was not prudent to enforce existing ordinanceswithout providing a disposal solution. Through discussions of how to potentially handle the FOGas a separate waste, it was decided that direct addition of FOG to the anaerobic digesters wouldbe most beneficial as the facility had surplus hydraulic capacity in the digesters. As FOG is acarbon source and directly digestible, direct digestion was considered as it would increasemethane generation and power generation by the CHP system. In early 2005, the facility beganexperimenting with FOG addition and by the June was accepting FOG on a regular basis. This
experiment was a precursor to the acceptance of brown grease, yellow grease and other suitablematerials. Other materials include:• Bottoms from grease that has been separated out as part of biodiesel generation• Emulsified oils from a local natural beauty product manufacturer• Settlement waste from cider manufacturer• High Strength BOD/COD waste from a local recycling companyFOG is accepted from trucks by gravity directly into the existing primary scum pumping pit.The existing primary scum pump (a double disc positive displacement pump), conveys the scumand FOG mixture directly to the anaerobic digesters. The pumping rate is based on staffexperience to supplement the sludge (TWAS/PS) flows. This system currently has noautomation.At the initial startup, FOG was fed at a weekly average rate between approaching 15% byvolume s compared to total volume of sludge applied to the digester. The rate varies based onsurplus waste material availability. Overall, the FOG addition has enhanced digester volatilesolids (VS) reduction which previously was already performing well with VS reductionconsistently greater than 65%. EJWWTF staff noted increased methane gas production andpower generation. Currently, plant staff continues to study the effects of the direct digestion ofthese waste materials.DISCUSSIONIf a semi-solid FOG can be obtained from various sources, then this material could potential beadded directly to a thermal oxidation process with little handling. This method produces the extraenergy benefits without the need for biogas clean up, provided a reliable high concentrationsource of semi-solid FOG can be obtained. However, the thermal oxidation process will need tobe operated under sustainable loading limits to avoid air emission excursions, which means thatonly a limited amount of FOG can be accepted. If not enough FOG is available to maximizeenergy production, then a concentrator would be necessary to dewater the liquefied FOG.An advantage could come from operating a separate FOG receiving facility with anaerobicdigestion. This liquid FOG waste can be blended with sludge prior to entering an anaerobicdigestion process or directly fed to the digesters, again with minimal handling. However, thisfacility requires screening, mixing, and heating facilities to insure the FOG maintains a liquidform for easy handling. An important consideration when adding FOG waste to anaerobicdigesters is to monitor the performance of the digesters to ensure the first stage is not overloadedor produces excessive foam. Shock loading of the digesters must be avoided. The digester gashandling systems must be adequately sized to handle the additional biogas production and thebiogas typically requires cleaning prior to beneficial reuse in cogeneration units.A major disadvantage can be seen from using dewatered FOG waste with anaerobic digestion.This would require re-slurrying of the FOG (i.e. add water) after dewatering (i.e. removingwater) which may not make sense unless trucking costs are significantly reduced. Similarly,using a liquefied FOG in conjunction with thermal oxidation may not be advantageous unlessscum is added as well given that a scum concentrator is required.
Page 2 and 3: INTRODUCTIONFats, Oils, and Grease
Page 4 and 5: REFERENCE FACILITIESTable 2 describ
Page 8 and 9: Accepting FOG at a separate facilit

HANDLING FATS, OILS & GREASES (FOG) AT WASTEWATER ...

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