High-Solids Anaerobic Digestion of the Organic Fraction of Municipal Solid Waste State of the Art, Outlook in Florida, and Enhancing Methane Yields from Lignocellulosic Wastes

Anaerobic digestion (AD) is a biotechnology that employs natural microbial metabolism under oxygen-free conditions to stabilize organic waste. AD has been shown to be the most environmentally sustainable technology for treating the organic fraction of municipal solid waste (OFMSW), as it allows for the recovery of energy and nutrients from the waste. AD of OFMSW also saves landfill space and reduces leachate generation and fugitive methane emissions from landfills. High-solids AD (HS-AD) technologies (those designed to process feedstocks with \u3e15% total solids content) have been shown to yield additional benefits when compared with liquid AD (L-AD) for treating OFMSW, including reduced parasitic energy demands, reactor volume requirements, water usage, and excess leachate generation. These factors paired with increasingly stringent environmentally-driven legislation have resulted in the steady development of HS-AD technologies in Europe since the 1990’s and the recent advancement of HS-AD in the United States. However, HS-AD implementation in the US is hindered by the low cost of landfilling and a general lack of regulatory drivers encouraging organics separation and recycling. The goal of this research was to contribute to accelerating the implementation and improving the efficiency of HS-AD technologies. The specific objectives were to: (i) assess the state of the art of HS-AD in Europe and the US and investigate trends in development; (ii) conduct a case study assessment of the outlook for implementation of HS-AD in the state of Florida; and (iii) investigate the potential to enhance methane (CH4) yields in HS-AD of lignocellulosic wastes through bioaugmentation with pulp and paper mill anaerobic sludge. Information sources for the assessment of the state of HS-AD in Europe and the US included “grey” and published literature and discussions with consultants and technology vendors. In Europe as of 2014 there were 244 full-scale AD facilities for processing OFMSW with a total capacity of almost 8 million tons per year (TPY), approximately 89% of capacity was “stand-alone” (systems treating only OFMSW), 62% was HS-AD, and 70% installed since 2009 was HS-AD. In the US, as many as 181 AD facilities are now processing OFMSW with an approximate total capacity of 780,000 TPY. Only 24% of the total capacity is currently stand-alone HS-AD with the remaining capacity being stand-alone L-AD (28%) or L-AD codigestion (48%) at wastewater treatment plants or on-farm systems. Development trends in the US are mirroring those in the EU, however, with stand-alone capacity steadily increasing and HS-AD capacity increasing particularly rapidly relative to L-AD for OFMSW processing. The number of full-scale HS-AD facilities in the US has increased from one in 2011 to eight in 2015 and another 19 systems are expected to be operational by 2017. There are at least nine vendors of HS-AD technologies in the US, including four with facilities currently in operation and another four with projects in the planning, permitting, or construction phases. Landfill bans and taxation, mandated source-separation of OFMSW, and policies incentivizing recycling and renewable energy generation are critical factors driving the development and implementation of HS-AD. The case study of HS-AD implementation in Florida incorporated information from industry and data from the Florida Department of Environmental Protection. There is high demand for organics recycling in Florida, with numerous counties generating several hundred thousand TPY of OFMSW and lacking organics recycling infrastructure. HS-AD implementation could increase the statewide recycling rate by as much as 13% and contribute significantly to the reaching the state’s recycling goal of 75% by 2020. Furthermore, up to 7,000 and 3,500 TPY of bioavailable nitrogen and phosphorus, respectively, and up to 500 MW of energy could be recovered through HS-AD of OFMSW in the state. Based on current energy conversion efficiencies, 500 MW of energy translates to either 175 MW of electricity (approximately 660,000 metric tons of CO2 equivalents offsets per year) and 200 MW of heat or nearly 80 million diesel gallon equivalents of vehicle fuel. However, because of the low cost of both landfilling and energy in the state and the lack of markets for compost and renewable energy certificates, legislative action is needed to improve the economic feasibility of HS-AD. Accordingly, a number of policy recommendations were formulated, including banning disposal of OFMSW to landfills and mandating source-separation of OFMSW by all generation sources. Two phases of side-by-side bench-scale batch HS-AD experiments were carried out to investigate the potential to enhance CH4 yield from lignocellulosic waste in HS-AD through bioaugmentation with pulp and paper mill anaerobic sludge. In the first phase, the average CH4 yield from yard waste inoculated with pulp and paper sludge reached 100.2 ± 2.4 L CH4/kg VS, a 73% enhancement compared with the average CH4 yield achieved through inoculation with domestic wastewater anaerobic sludge (58.1 ± 1.2 L CH4/kg VS). In the second phase, CH4 yield from yard waste inoculated with digestate from digesters originally inoculated with pulp and paper sludge was 68% greater than the CH4 yield achieved through inoculation of yard waste with digestate from digesters originally inoculated with domestic wastewater sludge (36.5 ± 0.2 L CH4/kg VS versus 21.7 ± 0.4 L CH4/kg VS). The enhancement in CH4 yield achieved in this study is comparable to enhancements achieved through lignocellulosic pretreatment methods. However, this strategy incurs significantly less additional environmental and economic costs when compared with pretreatment, suggesting that it could serve as an alternative to pretreatment and improve the overall sustainability of HS-AD processes