TERU Focus Report - Sewage to Renewable Energy

Enhance Anaerobic Digestion with Pre- and Post-Treatment of Biomass
June 3, 2011 -- Michael Theroux

Introduction: A detailed description of pre- and post-treatment of biomass enhancing anaerobic digestion was provided at "Cambi Academy 2011", held to coincide with the Residuals and Biosolids Conference and Exhibition in Sacramento, California. The workshop on May 25, 2011 brought together an international team of specialists from Cambi, Brown and Caldwell, and a number of university researchers. A brief case study of integrated advanced processing now being implemented at one of the world's largest wastewater treatment plants (WWTP) was also presented.  

The District of Columbia Water and Sewer Authority (DC Water) operates the Blue Plains advanced WWTP with a design capacity of 370 million gallons per day (mgd) of municipal sewage and peak flows of around one billion mgd. Blue Plains already meets nitrogen removal standards of 7.5 micrograms per liter (mg/l), but now must reduce that to about 4 mg/l. The massive plant was designed and built with a 6.5 acre area reserved adjacent to the anaerobic digestion (AD) vessels for whatever biosolids (biomass) pre- and post-treatment processing might eventually be needed. Starting in 2001, DC Water entered into meticulous global assessment of available technology vendors. After a decade of vetting and collaborative engineering, Cambi pre-treatment and AirPrex post-treatment systems were selected.  

Cambi's core Thermal Hydrolysis Process (THP) pre-treatment approach was described step by step, with many case study examples. Also introduced was the AirPrex post-treatment nutrient removal and recovery procedure, which when placed between a digester and the sludge dewatering systems, can separate almost all of the available phosphorus from the effluent and facilitate "de-ammonification" for nitrogen removal. The combined Cambi / AirPrex methodology is marketed as the "LysoGest" process. 

The decade-long conversion technology due diligence and selection process, the existing and planned plant design and the overall cost/benefit analysis were outlined by the Brown and Caldwell engineers tasked with project oversight. DC Water recently started construction on the 6.5 acre Blue Plains plot, which will include the Cambi THP and AirPrex modules to optimize sludge treatment for biogas production and nutrient recovery at the WWTP. 

Thermal Hydrolysis Process: The THP pre-treats sewage sludge, bio-waste or other biomass to improve anaerobic digestion and increase production of biomethane. The first step of the approach pre-heats the incoming slurry. The second step increases pressure and injects steam. The critical third step hydrolyzes the biomass via an explosive pressure drop. By breaking cell walls and "decoupling" long-chain polymers and hydrolyzing proteins, the viscosity of the slurry lessens, becoming more free-flowing with smaller particle size. Greater flow rates with less bulking means better mixing with less energy use in the digester; smaller particle size means faster and more thorough microbial activity. 

Biomass feedstock tends to naturally resist microbial decomposition. As a result of widely varying degrees of natural cellular resilience and inaccessibility to digestive action, much of the biomass entering an anaerobic digester passes right through to exit as a fibrous sludge. Plant and animal wastes inherently present structural barriers to release of fluids from solids, and can recombine when mixed to form long-chain polymers that greatly increase the thickness, the viscosity, of the material being processed. High viscosity slows flow rates and thus decreases the volume that may be processed by a vessel per unit time. Tight polymer structures and large particulate sizes keep microbial activity to the "outside", reducing biogas production per unit feedstock and thus increasing overall processing cost. Pre-treatment processes are designed to increase biomass structural decomposition and to break down polymer chains; this allows "fine tuning" biomass and biosolids processing, preparing biomass for faster and more thorough anaerobic digestion. 

Hydro-treated biomass slurry is more rapidly digested, which can about double a digester vessel's capacity through time. Better, faster digestion results in greater percentage solids reduction, as up to 60% of the available volatile organic compounds (VOCs) are broken down and converted to biomethane. The combined temperature, pressure, and explosive decompression also ensures the final residual will exhibit Class A pathogen control, crucial for beneficial use in the soil amendment marketplace. Water bound in naturally occurring cell polymers is released upon decomposition; smaller particle size and greatly reduced viscosity improves dewatering of the final sludge cake whether destined for use in agricultural or as a biofuel. 

With over 20 installations world-wide, the THP has proven effective for treatment of most materials destined for anaerobic digestion. The same methods that effect viscosity reduction also prepare food waste and other high-fiber feedstock (including straw) for rapid decomposition. The explosion of cell wall structure as a result of a rapid drop in pressure is a method that originated in wood pellet making operations. This was adapted to similarly treat biosolids and other bio-feedstock. Whatever procedure is to come next, an explosive pressure drop effectively liquefies most forms of solid biomass similar to acid hydrolysis, but without the problems of toxic chemical use.  

DC Water's groundbreaking announcement mentioned potential inclusion of food waste, fats, oils and greases in the Blue Plains AD system once the new installation is operating. Cambi's Ola Berntsen, Project Director for the Blue Plains project, explained that with the THP allowing recirculation of hot digestate, fats, oils and greases (FOG) never gel while undergoing depolymerization, and the resilient fibrous elements common to food waste are decomposed prior to AD vessel entry. A combined food waste/FOG quantity of 10% to 15% of the total digester loading may then be introduced to the tanks, dramatically improving biogas production over digestion of biosolids alone. Cambi's recent contract with the City of Oslo, Norway, will convert 50,000 tons per year of the City's food waste to biogas. 

AirPrex Procedure: Soluble nutrients are retained in the effluent and sludge cake exiting anaerobic digestion, especially phosphorus and nitrogen. These are valuable nutrients, if the wastewater and sludge residuals can be safely returned to the soil for agriculture. Yet along with valuable nutrients, sewage carries a varying load of contaminants that often can preclude agricultural use. Heavy metals and other toxins from industrial processes are irregularly released to the municipal sewage system to enter the WWTP, and can make use of the sludge residuals too risky to allow. In many urban regions, especially in Europe, there simply is too much sludge generated to find beneficial usage. In most cases, sludge cake is sent to incineration for energy recovery, where toxins can be sequestered for effective management in the ash.  

Of the many methods for phosphorus removal and recovery from liquid and solid wastes, the AirPrex procedure is designed for the effective treatment of sewage sludge. In Germany, phosphorous must be greatly reduced in sludge cake prior to incineration; this regulatory driver helped prompt advanced research into phosphorus removal and recovery at the Technische Universität Darmstadt, Institut IWAR. The German firm Berliner Wasserbetriebe (BWB) [Berlin Water Supply & Wastewater Disposal Company] developed and patented the AirPrex procedure. BWB's licensee Dr. Wolfgang Ewert, Pollution Control Service (PCS) GmbH and professor at the Institute, spoke during the Academy and provided a description of the AirPrex procedure and its intended integration at Blue Plains. 

During AD, Dr. Ewert indicated that the flux between chemical and biologically-bound phosphorous is critical to the breakdown of acetate following acetogenesis; the presence of undissociated acetic acid was needed to "crack" the volatile fatty acids for methanogenesis. Yet phosphorous then precipitates as a crystalline deposit called "struvite" that can completely plug piping. Increasing phosphorous concentration is also correlated with increased polymerization, making dewatering more difficult and costly. The AirPrex procedure intercepts and sequesters the phosphorous for removal after leaving the digester but prior to dewatering.  

Exiting the AD vessel, the sludge is fed to a multi-stage reactor system where it is air-stripped; pH increases with CO2 off-gassing. The simultaneous addition of magnesium salts causes the formation and precipitation of magnesium ammonium phosphate (struvite), a beneficial soils amendment and fertilizer component when controlled, but the culprit in pipe clogging when left in flow. The targeted integration of a “phosphate lowering mechanism” in the sludge treatment process compensates for potential disadvantages of the biological phosphorus process and provides the following advantages: (1) Significant improvement in the sludge dewatering rate, (2) Reduction of the recycle phosphorus load by 80 - 90%, and (3) No crystallization problems due to struvite. 

DC Water Application: DC Water announced groundbreaking on the new pre- and post-treatment facility on May 17th, just before the Academy. Perry Schafer, Brown and Caldwell's Biosolids Planning and Design Manager for the Blue Plains project, presented DC Water's need, and the timeline leading up to the event.  

Currently, 65 truck-loads per day totaling 1,200 wet tons of dewatered sewage sludge are transported out-of-state for land application. Concerns with dwindling state, federal and local agricultural acceptance of the practice coupled with continually-rising fuel costs dictated that a solution be found, a condition that DC Water anticipated and began to research a decade ago. In 2003, pilot tests of Cambi's THP were run at the Virginia Polytechnic Institute and State University (Virginia Tech), proving efficacy.  

The tests also indicated that the processing helped address a troubling phenomenon observed in already-processed sludge labeled "ROSI": bacterial Regrowth, increased Odors, and Sudden Increase in pathogenicity. The implications of ROSI are that federal pathogen control standards are insufficient to ensure against regrowth of E. coli bacteria, which can "re-activate" during dewatering. When THP was used for AD sludge, the pre-treatment provided a far more complete pathogen kill as the pressurized, steam-injected slurry was subjected to 165º C for 20 minutes. The THP also increases AD conversion of total VOCs to biogas, dramatically reducing tonnage exported while increasing produced biogas volume. 

Site preparation began last month and the new facilities are expected to become operational in mid-2014. In addition to the Cambi and AirPrex modules, the project includes more than 40 million gallons of additional new anoxic reactor capacity for nitrogen removal, new post-aeration facilities, an 890 mgd lift station, new channels and conveyance structures, demolition of existing buildings, addition of a protective sea wall and modifications to the existing facilities to enhance performance. Combined heat and power (CHP) will be an added benefit of the anticipated increase in biogas production, and new CHP systems will be selected from currently short-listed suppliers, by mid summer this year.  

Conclusions: In all areas of waste conversion, the resilient nature of biomass presents challenges that sometimes appear insurmountable. To find such thorough assessment, and see the resulting selection move now toward full implementation at one of the planet's largest and most advance wastewater treatment facilities is indeed heartening. 

Reduction in disposal and recovery of resources must extend to extraction of nutrients and beneficial biomass from sewage while both pathogens and toxins are controlled. DC Water is accomplishing this at a world-class scale. The lessons learned there can be applied to almost every bio-recovery pathway.  

Disruption of cellulose integrity and depolymerization of hydrolyzed biomass slurry is perhaps more commonly effected in our industrial sector using acid hydrolysis, in spite of the difficulties inherent in managing any industrial acid process. Enzymatic hydrolysis has advanced significantly in precision, availability and effectiveness just in the last few years. Non-standard microbial species selection and / or genetic engineering certainly have also been proven to disassemble complex cellulose and ligno-cellulose into fermentable sugars. To this short-list of tools we can add Cambi's THP, with the assurance provided by DC Water's decade of diligence.