Impact of low loading on digestion of the mechanically-separated organic fraction of municipal solid waste

Exploring Farm Anaerobic Digester Economic Viability in a Time of Policy Change in the UK

The combination of a post-Brexit agricultural policy, the Global Methane Pledge announced during the last United Nations Climate Change Conference in Glasgow (COP26), and urgency of meeting climate goals means the UK has a unique opportunity to create an exemplar through recognition of the benefits of small-scale farm anaerobic digesters that valorise on-site wastes for renewable electricity and heat, cushioning agri-businesses against energy perturbations. To explore economic viability of farm-based biogas production, combinations of support levels, energy prices, capital cost, internal rate of return (IRR), and digestate value were analysed, employing a 550-cow dairy farm with access to other agricultural wastes. A 145 kWe system utilising 100% of CHP electricity (grid value: £0.1361 per kWh) and 70% of the heat (heating oil value: £0.055 per kWh) could achieve an IRR above 15.5% with a median electricity tariff of £0.1104 per kWh at a heat tariff from £0.0309 to £0.0873 per kWh thermal. Under a subsidy-free regime, the same system could achieve a 10% IRR with electricity prices in the range £0.149 to £0.261 per kWh. High fertiliser prices could increase digestate value, further improving viability. With late-2021 high energy prices, the technology approaches subsidy-free viability, but uptake is unlikely unless wider environmental and societal benefits of on-farm systems can be explicitly valued.

Dry anaerobic digestion of the fine particle fraction of mechanically-sorted organic fraction of municipal solid waste in laboratory and pilot reactor

High-solid anaerobic digestion of the very small particle fraction of mechanically-sorted organic fraction of municipal solid waste (OFMSW) was examined in mesophilic digestion tests in a conventional laboratory (0.013 m³) and a pilot (0.300 m³) reactor. The non-biodegradable and recalcitrant molecules together with the low protein and starch contents of the small-particles of OFMSW limited the methane generation potential of substrate. In the conventional AD system, methane yields remained low at 0.139 m³kg_VS^-1 due to formation of a non-reacting layer on digestate surface, which restricted utilization of the available in OFMSW digestible organics. The absence of surface solid crust in the pilot unit favoured consumption of a greater proportion of volatile solids of the OFMSW. Dry AD was remarkably stable over the entire period and negligibly effected by the toxic H₂S yields. Methane generation (0.167 m³kg_VS^-1) was increased 1.2-fold compared to the conventional system due to a better mixing of substrate and microorganisms achieved inside the pilot reactor, which led to an increase of the digested volatile organics. Digestate presented low stability and high heavy metal content, both of which restrain its implementation as soil conditioner or fertilizer in agriculture. A secondary co-digestion treatment may be required for the neutralization of digestate.

Assessment of high-solid mesophilic and thermophilic anaerobic digestion of mechanically-separated municipal solid waste

Mechanically-sorted organic fraction of municipal solid waste (OFMSW) was tested to determine its biogas and biomethane generation efficiency. Methane production capability of OFMSW was examined in biochemical methane potential (BMP) tests. The factors affecting the high-solid anaerobic digestion (AD) of feedstock were investigated in a series of long-term semi-continuous digestion tests performed at dry mesophilic and thermophilic conditions in a continuously rotating drum reactor with working volume of 0.013 m³. OFMSW presented low biogas and methane generation capacity due to its contained non-biodegradable components and the low proteins and starch proportions. Dry mesophilic AD allowed only a relatively limited fraction of OFMSW volatile solids to be consumed for biogas and methane production. Reducing particle size favoured utilization of higher proportions of the available digestible organic substances, and concurrently promoted biogas and biomethane generation rate. Stability of methane generation was also significantly improved by particle downsizing. Small particles compensated the limited mass transfer and restricted distribution of methane production intermediate metabolites caused by water absence in the dry AD system. Dry thermophilic AD converted sufficient quantity of OFMSWs biodegradable content. The average methane released from dry thermophilic AD (0.176 m³kg_VS^-1) was higher than that of dry mesophilic AD of fine particles (0.148 m³kg_VS^-1) and much higher than that of dry mesophilic AD of same grain size (0.114 m³kg_VS^-1). High temperature proved more suitable for anaerobically digesting mechanically-sorted OFMSW.