Enhancing Anaerobic Digestion Performance With The Microbial Hydrolysis Process

Introduction:
Anaerobic digestion is a proven technology to stabilize solids from water resource recovery facilities and produce biogas. The objective of this research is to enhance anaerobic digestion performance by increasing the volatile solids reduction (VSR) and biogas production, and consequently reducing the amount of biosolids produced. Typically, a well-performing anaerobic digester can achieve 55 percent VSR when fed a combination of primary and secondary sludge from a water resource recovery facility (WRRF). High-performing digestion systems can achieve 60 percent VSR or higher, but none (until now) have achieved over 70 percent VSR. With professors at Brigham Young University (Hansen, Jaron C., et al., Biofuel Research Journal Vol. 8 Issue 3 2021), Jacobs developed the Micro Hydrolysis Process (MHP) using Caldicellulosiruptor bescii (C. bescii), a hyper-thermophilic bacterium that enhances the performance of any anaerobic digestion process. The MHP flow diagram is shown in Figure 1. The C. bescii bacteria and associated enzymes hydrolyze cellulose and other recalcitrant volatile solids that are otherwise resistant to digestion into volatile acids. These hydrolyzed volatile solids are fed to an anaerobic digester where methanogens convert them into biogas. One configuration of MHP feeds digestate from an anaerobic digester to a hydrolysis tank populated with C. bescii for a hydraulic retention time of at least 2 days at 75 degrees C. The hydrolyzed digestate is then returned to the digester to complete the digestion process by converting volatile acids into methane and carbon dioxide. MHP is compatible with any anaerobic digestion process including mesophilic anaerobic digestion (MAD), thermophilic anaerobic digestion (TAD), and thermal hydrolysis process (THP). The increase in VSR results in corresponding increases in biogas production and reductions in residual biosolids. The value of the additional biogas and cost savings of producing less biosolids reduce the operating costs of a WRRF.
Methodology: The performance of anaerobic digestion with MHP was tested at lab-scale and pilot-scale systems on solids from three different WRRFs: City of Gresham Wastewater Treatment Plant in Gresham, OR with MAD and fats, oils, and grease (FOG) addition; Encina Water Pollution Control Facility (WPCF) in Carlsbad, CA with MAD; and Oakland County's Clinton River WRRF in Pontiac, MI with THP and MAD. The lab-scale system consisted of a test and control system: each with a 10 Liter (L) digester and a 5L hydrolysis tank. Figure 2 is a picture of the lab-scale system and Figure 3 is a simple process flow diagram of the system. The pilot-scale system consisted of a test system with a 1,200L digester and a 500L hydrolysis tank and a control system with a 1,200L digester. Figure 4 is a picture of the pilot-scale system and Figure 5 is a simple process flow diagram of the system. All tanks were mixed, heated, and automatically fed from digester feed tanks.
The test and control systems were operated to simulate the full-scale digester operation for feed, retention time, and transfer rate between the digester and hydrolysis tank. Characteristics of the feed and contents of the digesters and hydrolysis tanks were measured using standard methods. The stability of the digestion process was monitored measuring Volatile Fatty Acids, Alkalinity, and Ammonia. The VSR of the digesters was the key indicator of performance and was determined from the TS and VS fed to the digesters compared to the TS and VS withdrawn from the digesters.
Lab-scale tests for Gresham and Encina were conducted in the Gresham lab. Pilot-scale tests for Oakland County were conducted in the Clinton River WRRF lab. Labs at each WRRF were used to measure the characteristics of the feed and performance of the full-scale digesters.
Findings:
The digestion performance of the three WRRFs are indicated by the VSR. Results from lab-scale, and pilot-scale digestion systems have demonstrated significant increases in VSR compared to already high-performing digestion systems (e.g., THP and MAD). VSRs were observed to increase from 60 percent without MHP to greater than 75 percent with the addition of MHP. Figures 6-8 show the VSR results of the test and control systems compare to the full-scale systems. In all cases the MHP increased the VSR from current operation of 58-60 percent to over 75 percent VSR. Results were used to calibrate a process model (Sumo) to predict digestion performance for full scale facilities with different feed characteristics. Based on the results, conceptual designs were prepared for implementation at three WRRFs with three different technologies: MAD, TAD, and THP and MAD. The details of these designs are being reviewed by each utility and can be presented in the full paper.
Significance:
The significance of the research and development of MHP is the discovery of a new technology that enhances the performance of anaerobic digestion to the highest level achievable to date. Anaerobic digestion performance increased with the addition of MHP in tests on three different WRRFs. MHP increased the VSR from 58-60 percent to over 75 percent. The increase in VSR corresponded to a 25% increase in biogas production and a 25% decrease in biosolids production. Conceptual designs for implanting MHP have been developed for a MAD, TAD, and THP digestion system.