In this season of ghosts and goblins, slime seems always to be an appropriate prop. In my very first professional presentation to a biosolids forum, an IWA conference in Los Angeles in 1989, I used a literary device, as I am wont to do, for my speech. I used a silly children’s book I had for my boys that described the many uses of Slugs. I, of course, transmogrified this conversation to one about “sludge,” in a sort of Gilda Radner’s Emily Litella “never mind” riff popular several decades ago on Saturday Night Live (see my favorite onViolins on TV). But, really, the connection between slugs and sludge is more than onomatopoetic and a “d.”
One thing slugs and sludge have in common is “slime.” And slime is a lot more complicated and engaging upon second reflection than upon first, and certainly deserving of deep study.
In the case of slugs, I learned in The Biochemistry and Mechanics of Gastropod Adhesive Gels just how complex is the trail mucus of a terrestrial slug. If you have ever inadvertently stepped on one, “slime” takes on real meaning. But to malacologists, it is a gel with a dilute polymer mixture that contains a specific glue protein that crosslinks with other polymers, providing the adhesive properties necessary for snail locomotion. This “trail mucus” slime may very well may hold the secrets of future strong, flexible adhesives. Who knew?
So, what do we NOT know about slime in biosolids? A whole lot less, I think, than we need to know. Slime may be the key to digestibility, dewaterability, energy recovery and odors, a sort of “four horsemen of the biosolids apocolyse” (another Halloween allusion.)
The slime I am referring to is a part of the emerging science of EPS, or extracellular polymeric substances. (This is not a mystical, seasonal nod to ESP, or extrasensory perception.) How EPS in biosolidschanges with treatment before, during and after sludge digestion is key to the quality of the final product. By quality I mean, at the end of the treatment process, does the product look and smell like a lump of p__p, or does it look and smell like garden soil?
Scientists are doing some amazing work on the EPS in sludge. EPS is released by microbial cells during either anaerobic or aerobic treatment processes. EPSs come in several forms. In the paper Effect of proteins, polysaccharides, and particle sizes on sludge dewaterability, EPS is separated into “four fractions: (1) slime, (2) loosely bound extracellular polymeric substances (LB-EPS), (3) tightly bound EPS (TB-EPS), and (4) pellet.” EPSs are very complex and their role in creating stable flocs, their influence on dewaterability, and the impacts of different treatment on their characteristics have been difficult for scientist to tease apart. Extracellular polymeric substances (EPS) of microbial aggregates in biological wastewater treatment systems: A review concluded: “the knowledge regarding EPS is far from complete and much work is still required to fully understand their precise roles in the biological treatment process.”“During hydrolysis and acidification, PN [protein] was transferred from the pellet and TB-EPS [tightly bound-EPS] fractions to the slime fraction… Further investigation suggested that CST [capillary suction time] was affected by soluble PN…, “ which is not a positive attribute.
Slime is in many ways a positive component of EPS. The journal article Extracellular polymeric substances (EPS) producing bacterial strains of municipal wastewater sludge: Isolation, molecular identification, EPS characterization and performance for sludge settling and dewatering concluded: “The slime EPS was better for bioflocculation….” Against this flocculation role, slime is a negative for dewatering, and a research focus is on how treatments alter the slime. In the Effect of proteins, polysaccharides, and particle sizes on sludge dewaterability
Some creative research is in full swing to modify EPS. A bioengineer, S Kavitha, at Anna University in India, entered full force in 2014 the science literature with studies examining an array of biological and chemical approaches to altering the EPS of sludges to achieve improved processes. Her focus has been primarily on the digestibility and dewaterability of waste activated sludge (WAS). She has examined different additives for breaking up EPS-controlled flocs in WAS to expose them to biological decomposition:
- NaCl (Enhancing the functional and economical efficiency of a novel combined thermo chemical disperser disintegration of waste activated sludge for biogas production)
- Thermo-chemical (Enhancing the functional and economical efficiency of a novel combined thermo chemical disperser disintegration of waste activated sludge for biogas production)
- MgCl2 (Achieving profitable biological sludge disintegration through phase separation and predicting its anaerobic biodegradability by non linear regression model)
- Citric acid (Improving the amenability of municipal waste activated sludge for biological pretreatment by phase-separated sludge disintegration method)
- Surfactants (Upgrading the hydrolytic potential of immobilized bacterial pretreatment to boost biogas production)
But Kavitha is far from alone in studying ways of altering EPS. Researchers from other groups have recently given us these papers:
- Effects of short-time aerobic digestion [STAD] on extracellular polymeric substances and sludge features of waste activated sludgefound that “The sludge after STAD exhibited better flocculability and dewaterability than that after the prolonged aerobic digestion.
- Variations in distribution and composition of extracellular polymeric substances (EPS) of biological sludge under potassium ferrate conditioning: Effects of pH and ferrate dosagefound that “The results indicated that sludge dewaterability was improved by decreasing solution pH [with potassium ferrate] in terms of filtration rate and cake solids content.”
We even have two new terms of art describing biological approaches to biosolids processing -- bioleaching and biodrying.
The term “bioleaching” is applied to a biological approach to conditioning biosolids for dewatering. Bioleaching involves inoculating WAS with specialized bacterium, along with an effusion of iron, to chew up the EPS, and hence improving dewaterability. Researchers are looking at Acidithiobacillus ferrooxidans -- Fate of extracellular polymeric substances of anaerobically digested sewage sludge during pre-dewatering conditioning with Acidithiobacillus ferrooxidans culture. This paper had four highlights: “Rapid flocculation of sewage sludge was achieved using iron-oxidizing bacteria; A. ferrooxidans biogenic flocculant significantly improved the sludge dewatering; a rapid reduction of EPS content was achieved during sludge flocculation; and, a positive correlation between EPS reduction and sludge dewaterability was observed.” This all sounds great.
Other researchers report on “biodrying.” In the paper Structure modification and extracellular polymeric substances conversion during sewage sludge biodrying process the authors claim “62% of total water removal[in a ] thermophilic phase ... transforming bound water to free water and modify(ing) the sludge structure and improves dewaterability.” This approach has already leaped to the commercial side. It was presented as the BioDryer reactor at the WEF Intensification of Resource Recovery conference in August 2015 by BioForceTech Corporation.
Uniting these approaches to altering biosolids properties is the deployment of biological, in contrast to mechanical, processes to reduce EPS in sludges and drive physical properties in favorable directions of digestibility and dewaterability.
This brings me back to one of the mysteries of my early days in biosolids, back to my “never mind” word play with slugs and sludge, and back to the day before “biosolids.” I wondered then why was the Chicago air-lagooned biosolids so pleasant and why was Tacoma’s Tagro such a great product? Perhaps what we didn’t know then, and what we are learning today, is that those processes chewed up the EPS effectively, with a big payback. I asked back then why was Philadelphia’s biosolids compost so gummy and I ask today why is the thermally hydrolyzed biosolids surprisingly pungent? Perhaps what we still don’t know today is how to chew up the EPS effectively.
We are still very early on the learning curve with EPS, so when we do learn what we need to learn, I am betting that we will then be close to describing a “high quality biosolids” product. Then our EPS will be transmogrified into ESP, an Extra Special Product.