Application of the Microbial Hydrolysis Process on an Existing Anaerobic Digestion System

Abstract Research and development of the microbial hydrolysis process (MHP) has progressed for application at existing anaerobic digestion facilities. MHP using the thermophilic, anaerobic bacterium: caldicellulosiruptor bescii (CB), has increased the volatile solids reduction (VSR) in pilot-scale anaerobic digestion systems. The increased VSR is attributed to the enzymatic activity of CB to hydrolyze cellulose into volatile fatty acids. Design of the first application of MHP is underway at VandCenter Syd's (VCS) Ejby Mølle water resource recovery facility (WRRF) in Odense, Denmark. A business case evaluation showed an attractive return on investment (ROI) based on capital expenses (CapEx) for the MHP system and savings on operating expenses (OpEx) from increased biogas production and reduced biosolids production. Introduction The Microbial Hydrolysis Process (MHP) using Caldicellulosiruptor bescii (CB), a hyper-thermophilic bacterium was developed to enhance the performance of any anaerobic digestion process. Instead of pre-digestion hydrolysis with CB (Hansen et al. 2021), post-digestion hydrolysis is used. MHP consists of feeding digestate from an anaerobic digester to a hydrolysis tank populated with CB for a hydraulic retention time of two days at 75 degrees C (Parry et al. 2022). The CB bacterium and associated enzymes hydrolyze cellulose and other volatile solids and produce volatile fatty acids (VFAs) in an anaerobic, neutral pH reactor. These VFAs are returned to the anaerobic digester where methanogens convert them into biogas. Figure 1.1 shows the microbial activity in anaerobic digesters before and after MHP. The performance of anaerobic digestion with MHP has been tested in pilot-scale digestion systems on solids from three different water resource recovery facilities (WRRFs). Results from the pilot tests demonstrated significant increases in volatile solids reduction (VSR) from around 50-60 percent without MHP to greater than 75 percent with the addition of MHP (Parry et al. 2023). MHP is a new technology that enhances the performance of any anaerobic digestion process for more biogas production and less biosolids production. Based on encouraging pilot tests at other WRRFs, VandCenter Syd (VCS) has started the design for application of MHP at their Ejby Mølle WRRF (EMWRRF) in Odense, Denmark (Nielsen et al. 2023). Material and Methods A calibrated process model was used to evaluate the mesophilic anaerobic digestion (MAD) performance at the EMWRRF with and without MHP. Since increased VSR is attributed to hydrolyzing residual cellulose, a pragmatic approach was used to predict VSR by measuring the cellulose fraction of the volatile solids. A process flow diagram of the existing MAD system is shown in Figure 1.2 with the addition of MHP. The four existing digesters are 2,800 m3 each and the new MHP tank is 1,000 m3. Results and Conclusions Based on the performance of the previous pilot tests, the expected performance of the EMWRRF with MHP is shown as compared to the current operation. Figure 1.3 shows the increased volatile solids (VS) conversion to biogas with MAD + MHP and the corresponding reduction in residual VS. A VSR of 50% for MAD and a VSR of 75% for MAD + MHP is shown. The residual VS after digestion is cut in half (from 38% to 19%) with MHP and corresponds to a 50% increase in biogas production. The additional biogas is attributed to the hydrolysis of cellulose by MHP for digestion. The results of the predicted mass and energy balance for the EMWRRF digestion facility is shown in Tables 1.1 and 1.2. Biogas will be used to fuel the existing combined heat and power (CHP) system. Increased biogas will increase the revenue to VCS from the power that is currently sold into the grid and the heat that is sold into the district heating system. A business case evaluation showed a positive return on investment (ROI) for the OpEx: revenue from the additional electricity and heat recovery, and the operating cost savings from the reduction in biosolids. The revenue and cost savings of the OpEx justified the CapEx of the MHP system for an attractive ROI. The presentation will show the business case evaluation results and overall expected performance impacts of the system.