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Climate Change Impacts of Food Waste Diversion to Anaerobic Digesters

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Treating food waste (FW) by anaerobic digestion to recover biogas and biofertiliser is a fantastic opportunity. The biogas is renewable, base-load, non-fossil energy. Using the digestate on land as biofertiliser completes nutrient cycles and conserves soil organic matter. The Part-503 rules protect health and the environment. Existing sludge digesters can be ‘turbo-charged‘ by retrofitting thermal hydrolysis, which allows the solids loading to be doubled or trebled, the specific biogas yield to be increased and the digestate (which will be Class-A) to be dewatered to cake that has approximately 50% greater dry solids content (34%DS compared with 24%).

FW amounts to about 20% of municipal waste. It has a high moisture content (˜70%), biodegrades rapidly and has weak structural strength. When it is stored it therefore tends to slump, ooze leachate, attract vermin and become smelly. When FW is mixed with other [dry] wastes it sticks to them and contaminates them, which compromises mechanical sorting and separating for recovering dry recyclables. Because of the high moisture content, FW reduces the calorific value and energy recovery from incineration; it also increases the volume of emissions that have to be treated. Curbside collections have to be frequent in warm weather if odor complaints are to be avoided. Centralized anaerobic digestion has a better global warming potential (GWP) than centralized composting. Home composting is a good solution but only a small proportion of households are willing or able to practice it. The GWP of food waste disposers (FWD) with sludge digestion is similar to centralized digestion. FWD are commonplace in the USA and New Zealand (where 50% and 34% of households have them respectively), whereas in the EU (European Union) fewer than 5% of households have them. Where they are unfamiliar, FWD are contentious because of concerns about water use, energy use, influence on FOG accumulation and effect on wastewater treatment and biosolids production. None of the research published on FWD substantiates these concerns. The additional carbonaceous load helps feed biological nutrient removal. The additional cost of wastewater management is less than the saving on solid waste management, but there is a question of fair reimbursement. This paper reviews the options for managing FW, their costs and their environmental footprints. It includes the benefits of recovering N and P fertilizer from dewatering liquor by physic-chemical stripping.
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Keywords: Ammonia stripping; FOG; anaerobic digestion; biofertilizer; biogas; biosolids; composting; digestate; food waste disposer; global warming potential; incineration; landfill; pasteurization; peak phosphorus; recycling; struvite; thermal hydrolysis

Document Type: Research Article

Publication date: 2009-01-01

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