While the biochemistry of the different fats in wastewater operations is not well understood, the subject is now gaining research interest, as in “Anaerobic co-digestion of fat, oil, and grease (FOG): A review of gas production and process limitations.” This work was done at a laboratory of Dr. Francis L. de los Reyes III, at North Carolina State University, whose paper observed: “anaerobic digestion of high lipid wastes has been reported to cause inhibition of acetoclastic and methanogenic bacteria, substrate, and product transport limitation, sludge flotation, digester foaming, blockages of pipes and pumps, and clogging of gas collection and handling systems.”
Dr. Novak offered in his 2010 presentation several ideas about the grease balls. He suggested that those agencies looking at trucking in HSWO containing long-chain oils consider deploying advanced digestion, such as acid-phase digestion and/or thermophilic digestion. He recommended vigorous digester mixing, to help break up the agglomeration of grease balls. He believes that microbial communities will, over time, acclimate to the character of the feedstock to digesters, so plant managers ought to work toward lining up steady, consistent sources of HSOW. But, as Landis Sewerage Authority has discovered, this can be a difficult task in the dog-eat-dog world of waste haulers, for whom a fraction of a cent per gallon lower disposal price down the street has them drive past the reliable neighborhood treatment plant.
If your digesters have gotten filled with grease balls and mop heads, what can you do? I spoke to Dr. de los Reyes about the challenges of rags and oils in digesters. He noted the difficulty of projecting from lab-scale digesters the behavior of the full range of FOG in full-scale sludge digesters: “Once the structure is there, it is difficult for the bugs to get to the grease.” He has been examining approaches that include increased digester mixing, microbial community acclimation, bio-augmentation and biodegradable detergent supplementation to see if such tailored strategies might deal with the challenge of grease balls and mop heads. But his most effective tool is old-school: “What I see is digester shut downs and companies coming in to pump it all out.”