Composting Grease Trap Wastes

BioCycle August 2006, Vol. 47, No. 8, p. 27

Fats and oils contain twice the energy of other organics such as sugars and starches – especially significant to rapid rise in composting temperatures.

Craig Coker

GREASE TRAPS are a form of wastewater pretreatment used by food service establishments to separate out fats, oils and greases from wastewater from dishwashers, sinks, floor drains and dumpster pads. Proper operation and maintenance of grease traps are an important means of preventing sanitary sewer overflows (SSOs), which can be caused by grease accumulation in sanitary sewers (much like cholesterol lines that narrow the body’s arteries). Grease traps are designed to slow the incoming wastewater so the grease congeals and floats to the top and solids settle to the bottom of the trap. If these accumulations of grease and solids are not routinely pumped out of the trap, the contaminants overflow the trap and contribute to SSOs.

Increasing public and regulatory concern over the potential public health and environmental impacts of SSOs has led to an increase in local and state regulatory requirements for proper maintenance of collection systems (including grease traps). For example, Cary, North Carolina (pop. 115,000) now requires all grease traps to be cleaned monthly. Similar local ordinances are in effect elsewhere.

Grease trap wastes (GTW) are normally siphoned out of traps by tanker trucks as a liquid with about five to six percent total solids content. Some haulers will “decant” GTW to raise the solids content (and thus reduce hauling costs) by heating GTW in an enclosed tank. This causes the GTW to stratify into three layers: sediment at the bottom, water in the middle and grease floating on the top. The water is discharged to sewer systems and the remainder is sent to disposal. Decanting can raise the solids content to 35-40 percent.

GTW contains grease, water, sediment (mostly food particles) and contaminants that are washed down the drain. Grease is essentially comprised of fats and oils. Fats and oils are made up of chemical compounds called triglycerides, which are comprised of one unit of a sugar alcohol called glycerol (C3H8O3) and three units of fatty acids. Typical fatty acids found in GTW are: lauric (C12H24O2), myristic (C14H28O2), palmitic (C16H32O2), stearic (C18H36O2), oleic (C18H34O2), linolic (C18H32O2) and linoleic (C18H30O2). It is the decomposition of these fatty acids that creates the odors associated with fats and oils that have spoiled, or turned rancid. Fats and oils (also known as lipids) contain twice the energy of other organic materials, like sugars and starches.

The total amount of GTW that has been collected for disposal has been rising steadily over the past several years. In North Carolina in 1999, a total of 8,000,000 gallons were collected for disposal; by 2005, that number had risen to 29,000,000 gallons (about 121,000 tons). With the rising interest in biodiesel production, recent articles have addressed the use of this material to produce biodiesel (see “New Companies are Entering the Biodiesel Industry,” BioCycle, December 2005).

Biodiesel production notwithstanding, the majority of GTW collected is disposed of via direct land application (if a liquid) or by landfilling (if solidified enough to pass the Paint Filter Test at MSW landfills). Wastewater treatment plants are increasingly reluctant to take in GTW due to operational difficulties. Spray application of GTW on fields where vegetation or crops are being grown can coat the plants with grease, suffocating the above-ground portions of the plants. The plants will recover (as the roots are not damaged), but reduced yields will occur. Spray application on land can also coat the surface of the soil with grease, making it water-repellent, and may clog soil pores. Due to this, land-applied GTW is often injected or incorporated into soils. Application rates are on the order of 4 dry tons/acre/year (about 16,000 gallons/acre/year at 6 percent total solids). North Carolina limits application rates to 25,000 gal/acre/year, but the GTW must be diluted 50 percent with water. The high energy content of the fats and oils encourages microbial decomposition in the soil. Cover crops are the same as those planted for land application of sewage, septage or biosolids.

Some of this GTW is being composted. This high energy content is advantageous to composting, where GTW is combined with other feedstocks and thermophilic temperatures are needed to ensure pathogen reduction. Carbon:Nitrogen ratios for GTW are variable and can be as low as 9:1 or as high as 15:1. GTW can be composted as a liquid (with adequate absorptive bulking agent) or as a decanted “semi solid.” (Decanting GTW is often done by haulers to raise the solids content to minimize transport costs.) The grease tends to coat the bulking agent(s) it is mixed with, which offers large surface areas for microbial decomposition in a compost pile. The high energy content encourages a more rapid rise in composting temperatures.

The amount of GTW that can be added to a compost recipe depends on several factors: Whether it is a liquid or decanted; the “freshness” of the GTW (rancid GTW will create very serious odor problems even if used in small quantities); the composting approach (aerated static pile seems to break down the fats and oils in GTW quicker than windrows); and the absorptive capacity of the bulking agent(s) used (dry sawdust being preferable to ground yard trimmings). GTW in a compost mix can range from less than 10 percent to over 40 percent, with variable odor problems, depending on the factors above.

McGill-Leprechaun operates a 100,000 ton/year aerated static pile composting facility in Chatham County, North Carolina, approximately 25 miles southwest of Raleigh. This facility composts biosolids, source separated organic solid wastes, food processing residuals, industrial nonsanitary sludges and GTW. They receive about 150 tons per week of undecanted GTW from six to eight local haulers.

According to Steve Cockman, Sales Manager for McGill-Leprechaun, GTW comes in and is off-loaded into a 130,000 gallon glass-lined steel storage tank. GTW is drawn from the tank with a Moyno progressive-cavity pump and metered into a custom-made pug mill to be blended with other feedstocks, sawdust and wood chips that have been bucket-blended on the mixing room floor. “We use GTW to adjust the moisture content of our mix before it gets loaded into one of the aerated composting bays,” said Cockman. A 2005 composite sample of GTW from the storage tank was analyzed for compostability parameters by a lab and was found to be 36 percent total carbon, 4 percent total nitrogen (for a C:N ratio of 9:1), and had a moisture content of 95 percent. Soluble salts were not analyzed.

GTW makes up only about 10 percent of the total incoming feedstocks to this composting facility. The process uses extended aerated static piles with blowers driven by variable-frequency drives and linked to the computer-based thermocouple control system. Curing is accomplished in turned windrows outdoors. Cockman notes that the final compost product contains no odors or textures that would indicate GTW was part of the mix. He added that the biggest operational challenge in composting GTW comes from ensuring adequate quantities of a very dry amendment be used in the initial mix to absorb the liquids. (McGill uses a kiln-dried sawdust and clean wood wastes ground on site.)

Crowell Dairy Farm, Inc. is a 78-acre farm located in Asheville, North Carolina, where Mike Crowell raises heifers. Crowell was introduced to composting in 1999, when a small contractor (Mountain Organic Materials) agreed to build and operate an aerated bin composting facility on the farm for the manure from the 850-head dairy farm Crowell operated at the time. Crowell has been composting – in open-air turned windrows – GTW with tree-trimming wastes (generated by utility line clearing contractors), heifer manure and bedding generated on-site, and some horse manure and bedding generated by a nearby Agricultural Center since 2003.

Crowell takes in about 50 tons/week of GTW delivered by a local hauler. “I got into composting as a way to handle the manure from my dairy operation,” explains Crowell. “Taking in grease wastes doesn’t hurt my composting process and it is a way to make a little extra money.” A 2003 analysis of a grab sample of the GTW indicated it was 47 percent total carbon, 3.2 percent total nitrogen (for a C:N ratio of 14.8:1) and had a moisture content of 91.8 percent. Soluble salts were 2.62 dS/m. Crowell uses two of the four larger bins on-site for processing GTW (each bin holds about 425 cy of compostable mix). GTW makes up about 40 percent of the mix of manures, tree-trimming wastes and GTW.

GTW is delivered in 5,000 gallon trucks (with an average load of 3,500 gallons). GTW is unloaded into one of the large solidifying bins onto a four-foot thick bed of ground tree-trimming wastes. It is unloaded at the far end of the bin, so that the woody waste throughout the bin absorbs the excess moisture. Once the initial four-foot layer becomes saturated, it is removed and stockpiled on the asphalt pad outside the bins for mixing with cattle and horse manure. Approximately one month’s worth of GTW can be absorbed by the woody wastes before its ability to absorb GTW becomes impaired. Some moisture is lost via evaporation from microbial action in the pile (due to meso-phillic composting).

The material removed is stockpiled on the asphalt pad outside the bins for 12 to 15 days before being mixed with the cattle/horse manure and bedding material. Once mixed, the compostables are moved into windrows. Residence time in the windrows average 60 days and then the material is moved to the curing area where it remains for several months. Following screening, the compost is used as a component of a manufactured topsoil product Crowell sells.

A qualitative examination of this topsoil material, even after more than one year in storage, indicated that there is still a slight greasy odor to the compost. This is believed to be due to several factors: over-saturation of the woody wastes with GTW, the initial high moisture content of the woody wastes, and the relatively lower temperatures Crowell reaches in his windrows. All of the windrows produced by Crowell meet the North Carolina requirement for Process to Further Reduce Pathogens (PFRP) and Vector Attraction Reduction (VAR).

Another North Carolina composter handling GTW is Brooks Contractor in Goldston. His operation was profiled in the January 2006 issue of BioCycle (see “Growing A Commercial Organics Company”).

Composting GTW can be a very viable means of managing this waste and a good source of tipping fees to a composter. To be successful with this feedstock, composters need to make sure of several issues: 1) They are properly permitted by the appropriate state regulatory authorities; 2) Haulers use dedicated pumper trucks (so as not to inadvertently take in septage); 3) Liquid storage facilities are available at the composting facility (as it is often difficult to add GTW to a mix as the truck arrives to discharge it); 4) There is an adequate amount of dry amendment available; 5) Adjust operations so that a combination of adequate temperatures (above 140°F – 145°F) and composting residence time is accomplished to allow the grease to completely decompose.

Craig Coker is a principal in the firm of Coker Composting & Consulting in Roanoke, Virginia, which specializes in providing technical support to the composting industry in the areas of planning, permitting, design, operations and compost sales and marketing.

COMPOSTING liquid wastes such as grease trap wastes (GTW) is difficult under the best of circumstances. Most composters prefer to deal with materials containing at least 15 to 20 percent total solids. Wastewater treatment plants commonly use dewatering equipment to separate out liquids and thicken the solids. For smaller processors, one option available is a rapid-gravity drainage box made from a modified, stainless steel 40-cubic yard roll-off container. The first step involves mixing polymers with the GTW in a separate dosing tank. The mix is then introduced into the drainage box. The solids are held in the container by a filter screen (400-700 micron size), while the free water drains out of the container and is discharged to the sewer system. The retained mix of sediment, food particles and grease averages 23 to 27 percent solids.

Atlantic Dewatering Systems in Clayton, North Carolina markets this dewatering unit known as the DeTainer. The company notes that equipment and operating costs vary from 2.5 to 4 cents/gallon. “We have three installations where GTW is the primary waste being dewatered by the DeTainer and several others where GTW is a portion of the wastes being handled,” says John deRham, formerly with Green Mountain Technologies and now a partner in Atlantic Dewatering, “The increase in focused fats, oils and grease (FOG) programs in municipalities and utility authorities is diverting more GTW to recycling and disposal and is a benefit to all aspects of the GTW business.”

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