July 14, 2008 | General

Using Treatment Plant Digesters To Process Fats, Oils And Grease

BioCycle July 2008, Vol. 49, No. 7, p. 47
Fats, oils and grease (FOG) in sewer pipes are a major cause of sanitary sewer overflows. The good news is that feeding FOG to wastewater treatment plant digesters significantly boosts biogas production.
Greg Kester, Perry Schafer and Bob Gillette

STATE and federal environmental agencies are pushing to eliminate sanitary sewer overflows, often caused by grease and fats moving through the sewer collection system. It builds up in wet wells and can completely block sewer pipes. Much of this material comes from commercial establishments and restaurants where grease traps are not serviced adequately or the fats, oils and greases (FOG) are flushed or washed directly down the drains. Called by various names, FOG can take the form of brown grease and grease balls, and is also referred to as scum. Cleaning FOG out of sewers is costly but, if the FOG causes problems, the material needs to be removed from the collection system.
Education can go a long way toward keeping FOG out of the sewer system. Developing flyers and information as part of an outreach and education program to food preparers and producers can be effective. This can include restaurant training and maintenance sheets, providing managers with best management practices and establishing a relationship with liquid waste haulers. Even when FOG is collected in grease traps, often times the grease winds up being taken to a landfill, which is not a beneficial option, and can contribute to greenhouse gas emissions. Introducing FOG directly into anaerobic digesters has been shown to increase gas production and accelerate the destruction of volatile solids.
Education is often not enough, however. In California, for example, statewide regulations are forcing municipalities to impose local ordinances and require restaurants and other FOG generators to meet design standards for grease traps. They require documentation of compliance and create a database to track when grease trap pumping occurred and record what happens to the material. Often, when grease traps are pumped, the material is taken to the wastewater treatment plants (WWTP) but introduced directly into the headworks. That creates additional challenges, including grease balls in the headworks and primary tanks, grease coatings of influent channels, tanks and equipment, increased maintenance costs and added odor emissions. Because of these operational challenges and difficulties, many treatment plant operators have started refusing to accept this material from haulers.
Increasingly, however, WWTPs are now recognizing that FOG can actually provide benefits to their operations. Some are working with the material to produce biodiesel. Plants with anaerobic digester systems are evaluating the addition of FOG, which has been proven to increase methane production and can help offset energy costs of WWTP operations.
The keys to utilizing this waste material are a well-designed receiving facility and adequate mixing in the digester. Complete mix systems are optimum to avoid having layers of grease at the top of the tank or digester, similar to what occurs in a wet well in the sewer collection systems.
Prior to loading FOG into a digester, it is best to send it through a grease holding tank with grinding and mixing (chopper pump) so that the FOG can be slowly metered into the digester. The metering allows a steady input rate and avoids a “slug load” to the digester. Heating may not be required in warmer climates and if the material is not too thick. However, some agencies report benefits in keeping the FOG material warmed into the range of 75° to 85°F.
Odor control for FOG holding tanks might have to be considered. There also should be a rinse system available for the haulers so they can clean their vehicles after discharge and an alarm to indicate overfill of the tank. Other possible receiving area considerations include liquid decanting (normally not necessary), an intake screen, or grinder in lieu of, or in addition to, the chopper pump. The holding tank should have two access manways or hatches to allow safe entry for cleaning.
FOG from grease traps and grease interceptors is normally contaminated with wastewater sewage. It is usually only a few percent solids, but ranges from 15 percent; temperature will be about the same as the wastewater, generally in the 50° to 80°F range. FOG material will stratify if it is not mixed, especially in the digester. The BOD is highly variable and can range from 10,000 to 130,000 mg/L. It has a slightly acidic pH (4 to 5). The percent volatile solids are very high – in the 90 to 97 percent range. FOG breaks down easily in a digester so that there can be very high volatile solids reduction. Assuming 80 to 90 percent volatile reduction, a lot of biogas will be produced. For FOG, the C:N ratio is high, also helping to generate more methane.
As noted earlier, the FOG needs to be metered into the digester at a consistent loading rate. Research has shown that digesters maintain stability when loaded with up to 30 percent FOG on a volatile solids basis. Loadings greater than 30 percent make the digester more susceptible to upsets. There is limited experience at FOG loading rates greater than about 30 percent.
Table 1 provides the theoretical methane content of biogas substrates. It illustrates that fat has the highest potential for methane, followed by protein and the carbohydrates. Table 2 provides a sample comparison of sewage sludge and FOG. The FOG has a much higher percent of volatile solids and volatile solids reduction, and thus a higher methane production capability.
Table 3 is an example with standard assumptions of adding an additional 25 percent FOG load to a wastewater treatment plant anaerobic digester. The increase in methane production is 65 percent, with a 47 percent increase in volatile solids destruction. In addition, there is increasing evidence that the FOG breakdown works in tandem with the sludge breakdown – in a symbiotic relationship – to produce a greater volume of methane and a higher percentage of volatile solids destruction. Table 4 uses the same loading as Table 3, but takes into account symbiotic assumptions.
In the example (Table 3) using the 25 percent increase in load, there is far higher volatile solids destroyed and methane produced than if the FOG or the sludge were separately digested. Adding the 25 percent more solids from FOG results in an 88 percent increase in methane production. The addition of FOG to an anaerobic digester provides a very beneficial and symbiotic relationship.
Possible explanations for this symbiotic behavior are: Higher biological activity within the digesting mass perhaps caused in part by a trace metal deficiency (without FOG) and trace metal stimulation (with FOG); and the increased C:N ratio helps with gas production.
The City of Watsonville, California installed a FOG receiving facility at its wastewater treatment plant in 2003 for the primary purpose of increasing gas production in the anaerobic digesters. There also was a real need in the area to accept the hauler loads that include FOG from restaurant grease traps and tallow processing facilities. The City also wanted to upgrade its digester mixing system, which was essential prior to accepting FOG. New mixing pumps and a FOG receiving facility were installed. Watsonville chose to use a basket strainer to take out any inorganic solids prior to the holding tank, in lieu of using a chopper pump or grinder. The FOG is pumped through the strainer to the receiving tank, and recirculated within the receiving tank via a chopper pump. From there it is metered into the suction side of an external high rate digester mixing chopper pump.
The plant received a total of $247,000 in tipping fees from 2003 to 2006 by accepting FOG, which by itself was enough to pay for the capital cost of the digester mixing and FOG system. The additional biogas has significantly reduced the purchase of natural gas that supplements their digested sludge biogas in a cogeneration engine. Table 4 shows the city’s cost savings since it began receiving grease trap waste. To date this has resulted in over half a million dollars in revenue over and above the digester mixing and FOG system installations costs.
In summary, for wastewater treatment plants with anaerobic digesters, there are clearly benefits to adding the capability to receive and process FOG. First, it limits FOG discharge into sewers and the introduction of grease at the headworks. It eliminates disposing FOG at the landfills, where it creates a lot of problems and is also a significant source of greenhouse gas emissions. In general, most wastewater treatment plants with anaerobic digesters can add the needed mixing and a FOG receiving station cost-effectively. Plant operators may be resistant because FOG is a messy, smelly substance – primarily if it is not handled properly. However, FOG is such a valuable feedstock for renewable energy and programs are being implemented that are economically viable and beneficial. Experience is showing that the production of the methane can be increased dramatically with the codigestion of FOG along with the sludge.
Greg Kester is the Biosolids Manager for the California Association of Sanitation Agencies. Perry Schafer is with Brown and Caldwell (Sacramento office) and Bob Gillette is with Carollo Engineers (Sacramento office). The authors would like to thank the Watsonville staff for use of their data and their willingness to share it for this article.

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