Full-Scale Enhancement of Anaerobic Digestion with the Microbial Hydrolysis Process

INTRODUCTION
The Microbial Hydrolysis Process (MHP) using Caldicellulosiruptor bescii (CB) enhances anaerobic digestion (AD) by increasing volatile solids reduction (VSR) and biogas production. Unlike pre-digestion hydrolysis, MHP feeds digestate to a 75°C tank populated with CB (Parry et al. 2022). This process has a retention time of two days and hydrolyzes complex carbohydrates to volatile acids (VAs), which are then fed back into AD systems, where methanogens convert them into biogas. Figure 1 illustrates the microbial activity in AD with MHP.

MHP with AD has been studied at laboratory and pilot scale with solids from four different water resource recovery facilities (WRRFs) with consistent results: increased VSR, increased biogas production, and higher cake solids in the final product. Encouraged by these results, VandCenter Syd (VCS) in Denmark is now designing a full-scale MHP system at its Ejby Mølle WRRF, and preliminary designs have been developed for several WRRFs in the US.

Data from an ongoing pilot at North Davis Sewer District (NDSD) in Syracuse, Utah and prior studies are used to refine a predictive MHP process model (SUMO) to inform full-scale MHP designs based on sludge feed characteristics. The pilot study results, model outcomes, and analytical results for pathogens, microbial populations, dewaterability, and sludge characterization will be presented.

METHODOLOGY AD with MHP has been tested in laboratory- and pilot-scale AD systems with solids from several US WRRFs. Early studies integrated MHP on the sludge recirculation line for mesophilic AD systems to increase VSR and biogas production. The ongoing pilot study at NDSD, which began in May 2024 and reached steady state in July 2024, uses a two-stage mesophilic AD system with intermediate batch MHP to produce Class A biosolids in addition to increasing VSR and biogas production. Figure 2 shows pictures of the pilot trailer at NDSD.

Characteristics of the raw and digested sludge from the NDSD full-scale system and sludge from the pilot AD tanks and MHP reactor were measured using standard methods. Routine analysis to monitor performance and stability include VS, total solids (TS), total acids, alkalinity, and pH. Additional analyses included fecal coliform, microbial communities, rheology and dewaterability, carbohydrate composition, and VA fractionation. VSR was the key indicator of performance.

Using data from this pilot and previous studies, the Jacobs team is developing an improved SUMO process model to predict the effects of MHP. To refine assumptions and address unknown variables in the model, additional lab-scale experiments will provide pertinent data that has not yet been captured, such as total suspended solids, volatile suspended solids, soluble chemical oxygen demand, and MHP process gas production and quality.

RESULTS AND DISCUSSION The addition of MHP to AD systems significantly improved performance in all trials, increasing VSR from a baseline of 58 to 64% to over 75% in all studies (Parry et al. 2023, 2024). Figure 3 summarizes the results from four studies, and Figure 4 NDSD pilot VSR data.

The improvement is VSR primarily attributed to the hydrolysis of recalcitrant organics, such as cellulose and complex carbohydrates or polysaccharides, in the MHP reactor. This is supported by the rise in VA concentrations from 500 mg/L upstream of the MHP reactor to 1500 mg/L in the MHP reactor. VA concentration MHP during the NDSD study are presented in Figure 5.

Preliminary analysis of the VA profile revealed that acetic acid (50%) and propanoic acid (10%) were the dominant acids present, with remaining 40% of the total acids likely consisting of lactic acid. Pathogen reduction was significant, with non-detectable levels of fecal coliform following MHP treatment. Dewatering performance showed increased dewaterability, although increased polymer demand was required due to higher soluble COD concentrations. Further, Further analyses of microbial populations, sludge rheology, and other performance indicators are ongoing to better understand the long-term stability and effectiveness of the MHP process and will be presented in the final paper. The SUMO model will include the key physical and biological processes of AD with MHP, shown in Figure 6, including hydrolysis due to increased temperature and the enzymatic and metabolic pathways and products CB.

The model will also predict how different anaerobic organisms such as acidogens, acetogens, and methanogens grow in the anaerobic process because of CB's metabolic products such as acetate, lactate, and propionate. Results from the model development, as well as updates to previously presented designs for VCS and NDSD, will be shared during WEFTEC 2025.

SIGNIFICANCE
Jacobs' MHP technology has demonstrated significant improvements to anaerobic digestion, offering the highest achievable VSR for municipal WRRF AD systems to date while also reducing pathogen levels and improving dewaterability. The development of the SUMO process model is integral to further developing the technology and provides a valuable tool for predicting and optimizing MHP performance. This results in substantial cost savings on biosolids management — a major interest for WRRFs. Further, the predictive process model will inform the design and operation of MHP systems at other WRRFs, leading to cost savings in design and improved process performance.