Digesters on a Low-fat Diet

The maintenance manager at Landis Sewerage Authority was quite graphic in his description of the fibrous mess in his digester’s sampling port. Landis has been accepting FOG (fats, oil and grease) and liquid HSOW (high strength organic wastes) for about a year.  The operators believe rags are coming in with restaurant grease trap wastes. These are ground up at the liquid waste receiving station, and, once in the digester, the rag fibers have attracted grease and human hair. The maintenance team had seen these fibrous grease balls clogging the sampling port pipe, congealed into an indurated, hairy mess.  The pipe had to be cut out, and the manager intended to make a cross-sectional cut of the pipe for illustrative purposes. He hadn’t seen a mess quite like this before Landis had begun accepting the wonderful FOG and HSOW that had allowed its digester to nearly double biogas production.

Hmmm, this is a downside of co-digestion that I hadn’t heard articulated so graphically – raggy, hard grease balls.  I had for many years listened to operator stories of mysterious mop heads in digesters.  But the clog in the Landis digester pipes seemed a particularly nasty version of these.

I recalled a 2010 presentation by Virginia Tech professor, now emeritus, John Novak, about the complexity of oils and fats and their fate in mesophilic anaerobic digesters.  This was a presentation to the Mid Atlantic Biosolids Association specialty conference on co-digestion substrates in 2010. Dr. Novak made the point in his presentation, “Codigestion at WWTPs – Digester Operations,” that not all grease is good for biogas production in digesters. Specifically, certain heavy lipids (chains of 16 and 18 carbons of saturated and mono-unsaturated fats) resist digestion, are difficult to solubilize in digesters, and may exert toxicity. Meat-sourced oils (forming stearic acid) and favorite cooking oils such as palm and coconut (forming palmitic and myristic acids) are just such long-chain saturated fats. These oils degrade into fatty acids that agglomerate and form “micelles,” or what are more commonly known as grease balls. A fatty acid such as stearic acid can then saponify (form a soap) and, in the presence of calcium ions, become hard, greasy lumps, long before microbes can convert the fatty acids into biogas. 

The sources of saturated fats that clog our digesters are the same staples in the American diet that clog our arteries – pizza and burgers, garnished with cheese, bacon and sausage.  You can see this in a table of the sources of stearic acid.  

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.”

Clogging…. This is an essential issue for Landis. For all of the trouble that grease balls and mop heads present to wastewater operations, I am curious that they receive very little scientific and engineering attention.  And the role of rags in the formation of grease balls and mop heads has been barely noticed in the technical literature.  A word search of the WEF Manual of Operations on solids treatment came up with not a single reference to rags.

To my delight, the same Dr. de los Reyes who is researching the digestion of lipids has done some works with rags.  He practically stands alone in this field of research, testimony perhaps to how hard it must be to find engineering graduate students with senses of humor sufficient to study the physiochemical behavior or rags and grease in wastewater. But they do have a recent landmark study of sewer collectors, in which rags play a big part: Evidence for Fat, Oil, and Grease (FOG) Deposit Formation Mechanisms in Sewer Lines.

Our treatment plant operators are experts on what typically happens when rags get to the plant in the influent stream.  If the way treatment plant operators manage rags is to trap as many as possible on headworks bar screens, then to catch the rags that get through the screens within the scum collectors, or then to unwrap the rags loose from bars and chains in the treatment tanks, what happens when the rags ultimately get into the digesters?  Not much, except to presume they reside in the digester, gathering together with hair and grease to form mop heads, until the next digester cleaning.

If in accepting liquid wastes directly into the digesters your agency is thereby bypassing those several rag-trapping steps between the headworks and the digesters, how are you handling the rag challenge?  Aren’t you asking for a good deal more rags and fibers in the digesters than is typical in wastewater operations? Do we sufficiently understand how rags and fibers react with oils, grease and hair to know if we can take steps to avoid excessive grease ball formation and clogging?  

This is where real-world experience counts.  Derry Township, Pennsylvania, has been taken in high strength wastes for a decade. It’s not hard to get Executive Director Wayne Schutz to bottom-line his advice: “NEVER, EVER, EVER feed that FOG stuff directly to the digester!!!  Aerate, mix, chop, and bio-augment to break the long chains VFAs; do a pH adjust; screen, de-grit, settle, grind, macerate, chop and macerate again; then feed to digester!!”

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.” 

There you have it. If you take in an assortment of FOG and HSOW, be prepared for frequent digester cleaning.  For our anaerobic digesters, healthy co-digestion means Digesters on a Low Fiber Diet.