Enhancing Anaerobic Digestion with MHP

Introduction The Microbial Hydrolysis Process (MHP), developed to enhance anaerobic digestion (AD) performance, uses the hyper-thermophilic bacterium Caldicellulosiruptor bescii (C. bescii) in a post-digestion hydrolysis process. Unlike pre-digestion methods (Hansen et al, 2021), MHP feeds AD digestate to a 75°C hydrolysis tank populated with C. bescii for a hydraulic retention time of two days (Parry et al. 2022). Here, C. bescii and its enzymes hydrolyse cellulose and complex carbohydrates and convert them to volatile fatty acids (VFAs) in an anaerobic, neutral-pH reactor. The VFAs are then returned to the AD, where methanogens convert them into biogas. Figure 1 illustrates the microbial activity AD with MHP. The performance of AD with MHP has been tested at lab- and pilot-scale with consistent results; the addition of MHP increases volatile solids reduction (VSR) to greater than 75%. Figure 2 shows the results of MHP studies on solids from 4 different water resource recovery facilities (WRRFs) with well-performing existing AD systems. Encouraged by pilot results, VandCenter Syd (VCS) in Denmark is now designing MHP implementation at its Ejby Mølle WRRF, and conceptual and preliminary designs have been developed for several WRRFs in the US. An ongoing pilot at North Davis Sewer District (NDSD) in Syracuse, UT has operated at steady-state since July 2024. Data from this pilot and prior studies are used to refine a process model (Sumo) to predict AD performance with MHP based on feed characteristics. The results of the pilot study, the model and preliminary design, and analytical results for pathogens, microbial populations, sludge dewaterability, and sludge characterization will be presented. Material and Methods AD with MHP has been studied using solids from several facilities at both lab and pilot scales. Most recently, pilot-scale testing has been conducted at North Davis Sewer District in Syracuse, UT. The pilot-scale system consists two 325-gallon anaerobic digesters and one 140-gallon MHP reactor. Pictures of the pilot trailer located at NDSD and the pilot trailer process room are shown in Figure 3. The NDSD pilot system is operated to simulate proposed process changes to achieve Class A biosolids, boost VSR, and increase biogas production, using a staged AD with batch intermediate MHP and heat recovery. A simplified process flow diagram of the pilot MHP system is presented in Figure 4 and a process flow diagram of the proposed full-scale system with heat recovery is presented in Figure 5. Pilot Anaerobic Digester 1 (AD1) was fed digested sludge once per day from the full-scale digested sludge transfer pumps. Sludge from AD1 was automatically fed 4 times per day to the MHP tank, and hydrolysed sludge from the MHP tank was fed 4 times per day to Pilot Anaerobic Digester 2 (AD2). Characteristics of the full-scale system feed and digestate as well as the contents of AD1, the MHP tank, and AD2 were measured using standard methods. Routine analyses to monitor performance and stability include volatile and total solids, total acids, alkalinity, and pH, while additional sludge analysis assesses pathogen reduction, microbial populations, sludge dewaterability, carbohydrate composition, and VFA fractionation. The VSR of the digestion process with MHP, calculated using the volatile and total solids measurements, was the key indicator of performance. Results of this pilot study are used to calibrate a process (Sumo) model to predict digestion performance for facilities with different feed characteristics and will be discussed in the final paper. Findings Previous studies have proven significant improvement in digester performance with the addition of MHP, as indicated by the increase in VSR to 75 and higher. These findings were corroborated by the results of the pilot at NDSD, shown in Figure 6. The existing AD system at NDSD achieves 63.8 percent VSR on average, and the addition of MHP improved performance to 75 percent once steady state was achieved. Table 1 presents the comparison the proposed staged AD with batch intermediate MHP to baseline AD performance. The increased VSR and improved dewaterability result in a 38% decrease in wet tons of biosolids produced per year, and an estimated 18% increase in biogas production assuming the biogas yield is unchanged. Further, laboratory analyses confirm that batch MHP reduces the concentration of fecal coliform below detectable levels, meeting the EPA criteria for pathogen reduction in Class A biosolids. The operational data and results of routine analyses, microbial population analysis, and sludge characterization from the pilot study at NDSD have informed the continued development of a model to simulate the impact of MHP based on influent sludge characterization. With this model, performance of AD with MHP at other facilities can be predicted. Significance MHP represents a significant advancement in AD technology and offers the highest achievable VSR for municipal WRRF AD systems to date. By allowing the digestion of cellulose and other recalcitrant organics, MHP maximizes AD efficiency, thus increasing biogas yield and reducing biosolid production. These improvements result in substantial cost savings on biosolids management -- a major interest for WRRFs. These benefits position MHP as a scalable, innovative solution to enhance AD performance globally.