Description
Anaerobic digestion (AD) is an attractive technology because it can be used to generate energy from the organic fraction of municipal solid waste (OFMSW). AD can be used to convert solid waste material into biogas, or methane. AD is used broadly in the US in the wastewater industry to treat biosolids; however, the wet digestion technologies used for wastewater (complete mix and plug flow reactors) are not directly applicable to high-solids OFMSW. Alternatively, high-solids AD technologies are readily applicable to food waste or OFMSW, and high-solids AD is the technology used in recently built commercial-scale AD facilities (e.g., in San José, California). However, these currently available technologies are not economically viable in most regions. Thus, technology advances are needed.
Multi-stage AD technologies are of particular interest because they can produce more energy per mass of waste. Increased performance is due to use of separate reactors for the waste hydrolysis and methanogenic stages allowing each process to be optimized individually for maximum energy generation. However, multi-stage technologies typically incur higher capital as well as operation and maintenance costs; therefore, further process advancements are needed to reduce costs and increase performance (EPA Biosolids Technology Fact Sheet, 2006; Linville et al., 2015). Significant past work has focused on developing optimized technologies for the methanogenesis stage, but there has been a critical need to improve hydrolysis processes. Thus, the focus of this study was on improving hydrolysis processes for multi-stage AD applicable to OFMSW. The technology advanced in this work uses a high-solids leachate bed reactor operated in batch mode for waste hydrolysis, wherein liquids are percolated over the solids to promote microbially-mediated waste solubilization. The perchlorate, or leachate, is fed to a continuous high-rate methanogenic reactor to produce biogas.
A specific challenge for achieving maximum hydrolysis rates of OFMSW is the presence of inhibitors including ammonia and salinity, which limit microbial processes. Further, these inhibitors can lead to process instability and even process failures compromising economic viability. Elevated levels of ammonia and salinity (>1.7 g total ammonia nitrogen [TAN]/L and 3.5 g Na+/L) are often found in high-solids AD systems that recycle leachate, or percolate, because these inhibitors are often present in feedstocks and build up due to recirculation over long-term operation. However, our past research showed that optimal hydrolysis can be maintained by using specialized microbial inocula that are adapted to elevated salinity and ammonia levels (i.e., acclimated inocula). Thus, this past research suggested that advances in microbial community management (i.e., use of these acclimated inocula at startup and development of methods to maintain desired microbes within leachate beds) could improve process efficiency for high-solids waste hydrolysis processes. Economic analysis was conducted to evaluate the impact of technical solutions. The economic analysis included estimation of one-time capital investments, 2
revenues from biogas sales, and selected annual operating costs for the high-solids multi-stage technology. Comparisons with existing systems were made only with respect to capital costs due to the lack of availability of appropriate operation and maintenance costs data.
To guide development of microbial management strategies to avoid process upsets and failures due to inhibitors, laboratory-scale studies were conducted. Studies focused on development of suitable methods to maintain stable populations of optimal ammonia- and salt-tolerant microbial communities within reactors during long-term operation (months to years). Fresh batches of waste can be inoculated with pre-digested waste left behind from the previous batch, although robust inoculation methods had not previously been developed for high-solids hydrolysis processes. Such approaches must be able to support optimal performance over long-term operation even as suboptimal conditions, including elevated levels of salinity and/or ammonia, develop. Herein, performance was compared for leach bed reactors (LBRs) seeded with unacclimated or acclimated inoculum (0-60% by mass) at start-up and over long-term operation. Research showed that high quantities of inoculum (~60%) increase waste hydrolysis and are beneficial at start-up or when inhibitors start to be substantially elevated, which may occur after several months of operation. After start-up (~112 days) with high inoculum quantities, leachate recirculation leads to accumulation of specific inhibitor-tolerant hydrolyzing bacteria in leachate. Then, during long-term operation, low inoculum quantities (~10%) effectively increase waste hydrolysis relative to without solids-derived inoculum. Importantly, molecular analyses indicated that combining digested solids with leachate-based inoculum doubles (4.4 x 1010 vs. 2.1 x1010 bacteria/g fresh waste) the quantities of bacteria contacting waste over a batch. Additionally, digested solids inoculum provides different microbes than recirculated leachate. Critically, the known cellulose hydrolyzers Clostridia were only found at high levels in digestate. By contrast, other known hydrolyzers Bacteriodes were predominant in leachate. Thus, combining solids-based and leachate-based inoculation is expected to maximize hydrolysis rates.
To determine if findings regarding the benefits of providing solids-derived inoculum could be extended to large-scale AD processes, demonstration-scale studies were conducted. An improvement in hydrolysis rates as a function of inoculum percentage was not observed. However, at the demonstration-scale ammonia and salinity concentrations were much lower than at the laboratory scale. Conductivity values were typically below 1.0-1.1 mS/cm compared to 45 mS/cm in the laboratory-scale. Similarly, ammonia levels were less than 0.25 g TAN /L at the full-scale compared to 3.5 g TAN/L at the laboratory-scale. At the laboratory-scale, inhibitor levels were artificially elevated to allow us to study the benefits of inoculation when it is most critical, e.g., when conditions are not optimal. The laboratory-scale experiments were designed to be representative of full-scale systems after ammonia and salinity have built up (after months of operation). At the demonstration-scale, reactor start-up was conducted similarly to how it would be done at full-scale; however, over the operational period (6 months) inhibitors had not yet built up. Although ammonia and salinity concentrations were expected to increase over 3
time in the demonstration-scale system, concentrations may have remained low in this case because operation time was too short to observe significant buildup of these inhibitors, and dilution water was added to account for water losses caused by operating challenges. Operational challenges occurred due to extreme cold weather conditions, which caused some pipe damage and leaks during the course of the demonstration-scale system. These operational challenges due to cold weather are easily addressable in full-scale systems, but impacted results in our experimental set up. Thus, findings suggested that the benefits of inoculation may only be significant when high levels of inhibitors are present, which will depend on operation time and waste characteristics. However, additional large-scale studies with various waste sources would be needed before firm conclusions can be drawn. Such studies could be run for longer term (~over a year) or leachate inhibitor concentrations could be artificially altered.
Economic analysis indicated that the multi-stage technology investigated is competitive with existing technologies on the basis of capital costs for the same MMBtu/yr of biogas generated. Capital costs were comparable for both an existing full-scale low-solids multi-stage system and a high-solids single stage system. Payback periods for this investment were greater than 10 years across all scenarios with baseline energy prices due to low operating profit estimates compared to the capital investment. Payback period analysis considered revenue from power generation, select operating costs, and estimated maintenance costs. For increased energy selling prices ($15.30/ MMBtu sold), payback periods of 4-5 years are possible depending on the scale; this selling price approximately equates to a consumer price of $30.60 MMBtu. Further, analysis suggested that improving hydrolysis rates and therefore reducing solids residence times results in a reduction of capital costs (up to a ~12% reduction for a 14-day solids residence time).Therefore, the economic analysis indicates that advancing hydrolysis technologies will improve overall AD economics. Further advancements are desired to improve profitability when energy prices are low.
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