Fear Grease No More

Historically, grease waste often has been treated with lime and disposed of by means of land application. However, this option is becoming a thing of the past, as regulatory agencies have begun restricting the practice in certain states. Florida is one such example. In 2016, the Florida Department of Environmental Protection severely restricted the land application of treated grease wastes.

The term grease in wastewater primarily refers to lipid and lipophilic wastes produced by lard and various cooking oils from restaurants, fast food franchises, and, in small part, domestic waste from septic tanks. Pumping companies empty the grease traps at these establishments and deliver their wastes to treatment facilities, which then face the task of having to dispose of these...

Grease chemical forms vary from saturated fatty acids such as stearic acid to unsaturated fatty acids, such as oleic acid and linoleic acid. Key components of grease are triglycerides, which consist of a glycerol molecule bonded to three fatty acids (see Figure 1, below). Triglycerides are responsible for the stickiness that causes grease to foul belt filter presses.

The reaction involving the grease, septage, and the surfactant within the receiving vessel is known as ester hydrolysis, meaning that the ester compound within the grease literally is split by water. The surfactant is able to do this because it has a hydrophilic end and a lipophilic end. On average, 5 kg/d (11 lb/d) of surfactant is added to the receiving vessel.

As the sand particles decrease in size, the water film surrounding them has more negative bars of adhesional force (see Figure 2, above). In some cases, this adhesional force can be as high as 1,000 bars. The thinner the water film around the sand particles, the larger the negative and positive covalent regions of that meniscal film become.

For the treatment process to work effectively, septage must contain significant quantities of sand or silt. Fortunately for the SCRWTF, septage haulers deposit a large volume of sand in its receiving vessel as part of their normal operations. For example, during one period of 101 days, from March 14 to June 23, 2017, the SCRWTF received 3,970 L/d (1,049 gal/d) of sand.

When septage haulers arrive at the SCRWTF, they attach their trucks by means of camlocks to a 100-mm-diameter (4-in.-diameter) hose and slightly pressurize the truck’s tank. The tank’s contents then enter a screening unit to remove such items as rocks, rags, bolts, and other objects that would damage the downstream pumps. A screw augur on the screening unit conveys the screened...

After undergoing mixing in the receiving vessel, the mixture of septage, grease, sand, and surfactant is sent to a 3-m-tall (10-ft tall), 15-m-diameter (50-ft-diameter) sludge holding tank that has a volume of 570 m3 (150,000 gal). The sludge holding tank also has a 6.2-kW (8.3-hp) propeller mixer. Aeration within the tank ensures that the microorganisms in the WAS consume the organic...

The SCRWTF has achieved significant savings as a result of this method for processing grease. In 2010, the last year during which the facility used land application to dispose of grease waste, Brevard County spent slightly more than $23,000 for the lime used to treat the SCRWTF’s grease waste, which totaled 3,981 m3 (1,051,860 gal). The county also spent approximately $75,000 to...

The method described in this article can help solve a vexing problem for many WRRFs faced with having to treat septage and grease trap wastes. By incorporating grease within wet cake and sending it to a landfill, this process also can benefit landfill owners that recover methane from their facility. Many landfills use this recovered methane as a fuel source for their vehicles.