Keeping it Simple: Principles for Application of Traditional Mesophilic Digestion

As demands and performance requirements have continued to become more aggressive, many municipalities have considered or implemented high-rate forms of anaerobic digestion, including thermophilic reactors, thermal hydrolysis processes (THP), thermal-chemical hydrolysis process (TCHP), and temperature phased anaerobic digestion (TPAD) applications. These technologies come with advantages (higher biogas rates, greater solids destruction) as well as challenges for operation and maintenance (higher levels of instability, inhibition, odors, corrosion, and operational complexity). Despite the many available options, traditional mesophilic digestion (operational range of approximately 95-100°F) remains the workhorse across the majority of water resource and recovery facilities (WRRFs) over the last 50 years. While it lacks some of the kinetic advantages of high-rate technologies, it comes with a variety of benefits including simplicity, stability, safety and low cost of operation. This paper proposes a series of fundamental principles to evaluate and optimize mesophilic digestion operation and design. The principles can be applied to any form of digestion, but are critical to effectively evaluate mesophilic systems prior to considering high-rate modifications. -True Mass Balancing - accurate data and appropriate methods to calculate volatile solids reduction (VSR) performance. The calculation methods can vary significantly and can drastically change the perceived performance. -Feed Design and Control - the ability to feed consistently and slowly across a variety of sources as well as maximizing the kinetic advantages of mesophilic digester option (parallel and series control). -High Value Operational Features - incorporating simple, but effective means to address foam, mixing, overflow and chemical addition. -Keeping Options Available - flexibility in a design and approach that allows for changes (including to high-rate systems) in the future without losing the value of established infrastructure. These four principles will be reviewed using the case study of the Central Kitsap Treatment Plant (CKTP), operated by Kitsap County (Poulsbo, WA), as they developed the design of a complete digester replacement project. The County was looking at next steps to upgrade an aging mesophilic digestion system while having unique challenges with low volatile solids (VS) waste streams. A thoughtful application of these principles allowed them to move forward with a trusted technology while keeping a wide variety of options available for the future. Case Study CKTP presents a set of unique challenges that could be solved through either traditional or high-rate digestion technologies. The facility is a 6 million gallon per day (MGD) biological nutrient removal plant with two mesophilic digesters fed from four separate sources: co-thickened septage and primary sludge (PS), thickened waste activated sludge (WAS), scum, and FOG. This feed mixture has a large portion of low volatile solid (VS) waste (septage, WAS), combined with higher VS feed (PS and FOG). This has made VSR historically difficult to measure and confirm. Mechanical failures of the existing digesters (originally constructed in the 1970s) led to a project for replacement with two new 1.4 MG tanks. CKTP desired to maintain the simplicity of the mesophilic operation, but only if a clear pathway to address historic challenges could be achieved. Each of the principles below are discussed in context of their application to CKTP in determining if they should keep their mesophilic system. Principle 1 - True Mass Balancing Plants often lack instrumentation (flow meters, solids meters) or staffing (grab samples, monitoring) to obtain accurate data regarding the flow and volatile solids content of each portion of the feed stock. This makes it difficult to accurately correlate VSR to the hydraulic residence time (HRT). This is critical for CKTP, as the variety of feed stocks and proportions could drastically change the assumed/calculated VSR. Figure 1 provides a summary of the solids proportions feeding the CKTP digestion system, which emphasizes the large (27 percent) fraction of septage solids and high portions of WAS and biological solids. Historically, despite a relatively high solids retention time (SRT) within the digesters, CKTP was challenged in consistently achieving the necessary 38 percent VSR to meet the biosolids goal of Class B (Code of Federal Regulations [CFR] Title 40 Part 503). In 2021 a study of the digester feed estimated volatile solids fractions from each feed source (Table 1, Table 2) and compared VSR calculated by three methods: Van Kleeck (VK, Equation 1), the true mass balance (U, Equation 2), and the empirical method (Vd, Equation 3). As shown by Switzenbaum et al. (2003), VK calculations (traditionally used by CKTP), which estimate VSR using only volatile solids fractions, may underestimate VSR due to assumptions regarding inert solids. VSf = Volatile Solids Fraction (Feed) VSw = Volatile Solids Fraction (Digester Effluent) Vd = Volatile Solids Destroyed SRT = Solids Retention Time Van Kleeck (Switzenbaum et al., 2003): VSR=(VSf-VSw)/(VSf-(VSf*VSw))=(1-((VSw)/(VSf)))/(1-VSw) (1)True Mass Balance (Switzenbaum et al., 2003): U(%)=(VMass_in-VMass_out)/(VMass_in ) (2) Empirical Method (Metcalf & Eddy/AECOM, 2014): VSR (Vd)=13.7 lnâ¡(SRT)+18.9 (3) Figures 2 through 5 present the results of the comparison, with the empirical method using both the calculated SRT and the reported mean cell residence time (MCRT) from the original plant data. The analysis demonstrated, decisively, that the VK method underestimated the VSR and the mesophilic system was performing better than previously thought. This conclusion was corroborated when comparing the VSR to estimated gas production rates using a bracketed range of 12 to 18 cubic feet per pound of VS destroyed (Metcalf & Eddy/AECOM, 2014). The mass balance method predicted the actual biogas production as well as or better than the VK. Consequently, the mesophilic system was achieving the necessary VSR and a primary driver for moving to high-rate systems was less significant. Principle 2 - Feed Design and Control Operating digesters in parallel versus series will alter the kinetics. Series operation moves toward plug flow, allowing for increased VS removal of 3-5 percent over parallel operation but increases the loading rate to the first stage with less protection against instability (Zahller et al., 2007). Following this approach, CKTP designed their new digesters to allow for the effluent pumps of each digester to serve as feed pumps to the other tank. Simple series operation is efficient and flexible and can gain an extra edge on VSR, maximizing mesophilic performance. Principle 3 - High Value Operational Features Poorly mixed, poorly measured and irregular pulses in VS feeding create inconsistent gas production, poor temperature distribution, and reduce reactor stability. Mesophilic efficiency is partly a product of how a digester is fed, not only what it is fed. To address this, CKTP chose to integrate the following: Separate feed lines for control, metering and measuring individual feed stock (Figure 6). Flow meters and inline solids meters to improve data quality. Biogas flow meters for each digester to measure performance. Aggressive mixing and foam suppression designs for system stability. Principle 4 - Keeping Options Available While ultimately deciding to keep mesophilic digestion, CKTP maintained long-term options by designing for future process flexibility: Wall thickness and insulation to allow for higher thermal loads (thermophilic operation). Space for additional heat exchangers for conversion to high-rate systems. Centralized location of new digesters and control buildings, with available site space allowing for pre-treatment systems such as THP or THCP to easily be added. Summary While high-rate digester operation is often useful and necessary, particularly in situations where Class A biosolids products are needed, the advantages of mesophilic digestion should not be quickly overlooked, as the stability and simplicity of use is often the best choice for a given WRRF. The analysis and design principles presented in this paper provide a useful framework to allow municipalities to give traditional technology thoughtful consideration, ultimately providing more stable and efficient operation of a mesophilic systems, or providing better justification for a high-rate approach.