ROUTES and Sequential Digester Systems

Joanna Macy, 86-year-old Buddhist environmental activist, was featured in a podcast this week introducing her  “The Great Turning.”  By this she means a turning away from exploitation and extraction, and toward a uniting of people with their Earth.  I think her global vision can speak to us biosolids managers, where the nutrients we gather from people are reunited with Earth.

I have embraced “closing the circle” as a positive paradigm for biosolids resource recovery. Macy’s exhortation for a wholehearted commitment to humanity’s just and safe relationship to Earth reminded me of two issues that, with biosolids recycling to soil, niggle at me, just a little – microbial risks and persistent organic pollutants.

I am entirely confident that the scientific record exonerates biosolids today from meaningful risk of harm to humans and the environment and entirely supports my commitment to recovering biosolids for the purpose of soil improvement.  Yet, I also read of the performance of innovative technologies that enable us, tomorrow more so than yesterday, to eliminate microbial risks and organic pollutants. Perhaps these targets are not mandated by regulation, in particular not the 23-year-old Part 503 regulations. And, perhaps these new technologies are more costly and risky than those our industry have used for 100 years. But they are compelling in their high quality performance and biosolids product.

The promise of a leap forward in technology was the big message of Bill Barber’s presentation to the MABA Summer Technical Symposium in Baltimore. He had me “rethinking” the creative potential ahead of us to deploy high-performance processes as a turning toward a wholehearted commitment to highly stable biosolids.  There is not just one way to do this. In fact, Barber suggests that our fascination with thermal hydrolysis pretreatment for digesters is akin to putting decorative decals on a Model T Ford. You can see Barber’s MABA’s presentation on what may lie ahead for us in digester technology:  “RETHINKING ANAEROBIC DIGESTION TO COMPLEMENT 21ST CENTURY DRIVERS”. 

In diving deeper into Barber’s concepts, I came across the European-based “ROUTES” initiative, which has evolved as a tool to achieve Europe"s “safe sludge matrix.” The global conversation around ROUTES was sparked in October 2012 with a technical gathering in Rome, Italy. NOVEL PROCESSING ROUTES FOR EFFECTIVE SEWAGE SLUDGE MANAGEMENT.  I learned in the paper ROUTES: innovative solutions for municipal sludge treatment and management that the “project is addressed to discover new routes in wastewater and sludge treatment which allow: (a) to prepare sludge for agricultural utilization by transforming it in a very clean and stabilized product with respect to hygienic aspects and phytotoxicity;….” To my ear, this sounds all the world like the principal motivation for the WE&RF High Quality Biosolids research project.

The European champions of ROUTES have put out some extraordinary publications that take us toward “closed circle” goals in the stabilization of biosolids.

The report by prolific female researcher from Italy, Camille Braguglia, and her associates, Quality assessment of digested sludges produced by advanced stabilization processes, found that: “Removals of conventional and emerging organic pollutants were greatly enhanced by performing double-stage digestion (UMT and AA treatment) compared to a single-stage process as TT; the same trend was found as regards toxicity reduction.”

Double-stage digestion is neither new nor extremely risky. Reaffirming Bill Barber’s assertion in his paper from Baltimore, the ROUTES team concluded: “These results confirmed that temperature-phased anaerobic digestion systems could show better performances and higher process stability than single-stage mesophilic or thermophilic processes, carried out under the same operative conditions.”

The ROUTES team looked at a wide variety of other digestion approaches.  One approach was to attempt higher organic loadings to the digester, meaning smaller digesters and more room for co-digestion of sludges with trucked-in wastes.  In the article The impact of sludge pre-treatments on mesophilic and thermophilic anaerobic digestion efficiency: Role of the organic load the authors wrote that “at higher organic loads, the TAD [thermophilic anaerobic digestion] yields were significantly higher with respect to the MAD [mesophilic anaerobic digestion] ones, assuring the sustainable economic benefit of operating smaller anaerobic digesters to obtain higher methane production.

The ROUTE group looked at pre-treatment ahead of digesters, noting the different functionality of different processes: “Thermal hydrolysis enhanced the release of lipids and long chain fatty acids, while ultrasounds application resulted in proteins being the main component of the released matter.”

With the ROUTE group objective of meeting multiple goals for product stability, energy balance and dewaterability, but also importantly hygienization, they described in Enhanced Versus Conventional Sludge Anaerobic Processes: Performances and Techno-Economic Assessment the following findings: “The TPAD [two-phased anaerobic digestion] proved to improve the overall process by enhancing the individual steps (Francioso et al., 2010), and had the advantage to be operated at high loading rates (Azbar et al., 2001). At the same time the short thermophilic step did not guarantee total hygienization (Huyard et al., 2000). For this reason, in this work, an inverse TPAD (meso/thermo) is proposed, where the mesophilic acidogenic step is followed by an intense methanogenic step in thermophilic conditions aimed to achieve higher methane yields, obtaining a final hygienized product suitable for land application.”

In one experiment, WAS is treated with a physical disintegration process, sonication, and then put through temperature-phased anaerobic digestion (TPAD), but inversed, as described above, to have the feedstock treated with a mesophilic acid phase followed by a high temperature gas phase.  In a second experiment involving dual digestion, primary sludge is treated with wet oxidation, and the secondary sludge is treated with thermal hydrolysis, then combined for thermophilic digestion.

Wet oxidation?  I needed to look that up (Wet oxidation of sewage sludge: full-scale experience and process modeling).

One key driver for ROUTES is hygienization.  Standards of treatment go beyond indicator fecal coliform to include a higher test of virus inactivation.  Researchers concluded that viral inactivation requires higher temperatures in the thermophilic digester than those conventionally targetted: “…our study confirmed the higher resistance to thermal treatments of viral particles with respect to bacteria indicating the low reliability of bacteria as an indicator of virus fate at temperature greater than 55 °C…. In contrast to what was observed with bacterial indicators, the removal of viral indicators to below detection limit was observed only at high temperature in the TH+TAD [thermal hydrolysis followed by thermophilic anaerobic digestion] processes confirming, in agreement with other studies, that temperatures above 80 °C are necessary for an efficient inactivation of viruses.”

ROUTES also showed that biosolids quality made a difference to the evaluation of technologies.  The lowered cost of disposal of the high-quality biosolids and the greater recovery of energy for biogas released during digestion were two factors that justified the more costly equipment.  Measures of phytotoxicity and the soil-like quality of the residuals sold the biosolids for agricultural users. Even though new treatment practices required electricity input, the extra biogas production yielded electricity production above that additional need.

Yet, the ROUTES researchers worked through some troublesome areas.  Thermophilic systems can be harder to operate, harder to keep stable and difficult to avoid foaming and rapid rise.  The ROUTES paper Microbial diversity in innovative mesophilic/thermophilic temperature-phased anaerobic digestion of sludge explained that “thermophilic communities may be therefore more susceptible to sudden changes and less prompt to adapting to operative variations.” They also pointed to the release of difficult-to-dewater COD.  The impact of sludge pre-treatments on mesophilic and thermophilic anaerobic digestion efficiency: Role of the organic load  noted that “Nevertheless, the colloidal charge increase during thermophilic digestion impaired the sludge filterability much more rapidly than in mesophilic conditions.”

The mind boggles at all of the combinations and complex options for stabilization that have emerged recently for serious consideration by public agencies.  The most notable for its recent U.S. implementation is Cambi’s Thermal Hydrolysis Process at DC Water, but Trinity River Authority outside Dallas and Hampton Roads Sanitation Authority in Virginia Beach have announced plans to move ahead with Cambi. At the MABA meeting, GE Power described its Monsal biological hydrolysis system, and Ovivo provided case studies of six high-tech aerobic digesters. At last November’s MABA Annual Meeting, Veolia described European examples of advanced digestion, and we heard there also of Suez’s TPAD (temperature phased digestion) at the Hermitage (PA) Municipal Authority, where it is deployed for a very serious program of co-digestion.

This is just a start, I believe. I predict that within 20 years the common practices for biosolids stabilization will include a number of sequential digester systems that we see today as experimental.  This is inevitable because the early results in the lab, at pilot plants, at reference facilities, and in full scale in Europe, in terms of mass reduction, pathogen control, pollutant reduction, and product stability are very compelling, far above and beyond Part 503 standards requirements, but so much closer to our vision as environmental stewards toward “closing the circle” in our community.  I feel underway the Great Turning to High Quality Biosolids.