Anaerobic Digestion FOG
Wastewater treatment plants that collect and utilize the methane produced from anaerobic digestion of sludge can enhance performance and methane production from the process by feeding fats, oils, and grease (FOG) into the digester.
FOG is produced in large part by commercial establishments and restaurants. The buildup of grease in sewer systems can cause major problems as the grease can block pipes and cause sewer overflows. To mitigate these issues, there are industrial discharge limits on FOG that necessitate the installation of grease traps or interceptors to prevent discharge of grease into the sewer system. Interceptors must be cleaned out on a regular basis to prevent overflow into the sewer and for odor control. Yellow grease (cooking oil) can be processed and sold for beneficial reuse by industry. Brown grease (animal fat, waste oil, and grease trap grease) can also be resold; however, it is not as valuable as yellow grease and is generally removed at a cost to the owner and sent to a landfill.
Grease Interceptor Schematic
Disposal of the grease at landfills can be problematic and contribute to greenhouse gas emissions. Introducing FOG at the headworks of a WWTP can create operational issues including odor emissions, grease balls and grease coatings in channels, tanks and equipment. Facilities utilizing methane produced from the anaerobic digestion of primary and secondary sludge can boost methane production by feeding FOG directly to the anaerobic digesters instead of the headworks of a plant. The additional methane can be used to increase on site power generation and reduce the plant’s energy costs or can be sold to a third party as methane or as power.
Methane production is accomplished by a host of microorganisms that convert organic materials into a gas that is primarily a mixture of methane and carbon dioxide. FOG is first converted by certain bacteria referred to as fermenters and acetogens into short chain acids, alcohols, carbon dioxide and hydrogen gas. Other bacteria (methanogens) then produce methane from acetic acid or carbon dioxide. Control of pH is an important consideration as methanogen activity slows below a pH of 6.5. Grease trap loads frequently have a pH of 4 to 5 and FOG accepting facilities may require neutralization of FOG by the hauler if the pH of the delivery is not in the range of 5.5 to 9.5.
After delivery, FOG is mixed and heated prior to being fed into a digester at a constant loading rate. Adequate mixing within the digester is required to ensure proper contact between grease and microorganisms. FOG can be added to make up 30 percent of the volatile solids (VS) loading in the digester, although some facilities were successful beyond the 30 percent limit. Pilot testing is recommended prior to FOG loading beyond this limit. Adequate mixing within the digester is also important.
Onsite FOG Processing Schematic
Cities throughout the US that utilize anaerobic digestion have successfully implemented FOG digestion to boost methane production. FOG digestion provides a more sustainable alternative for FOG treatment than disposal in a landfill. Tipping fees from accepting FOG loads help recover the capital cost of equipment required for FOG processing and feeding into digesters.
The City of Las Vegas Water Pollution Control Facility currently uses collected methane from the digestion of primary and secondary sludge to power two engines and four boilers, although both engines and the boilers cannot be operated simultaneously due to insufficient methane production. The City is currently planning to seek Council approval to move forward with a FOG treatment pilot to evaluate biogas augmentation available for use onsite.