Nutrient and Carbon Recovery from Household and Food Biowastes

Keywords: Anaerobic digestion; biogas; biosolids; biomethane; composting; food waste disposer; incineration; kerbside collection; landfill; phosphorus; sewage sludge; thermal hydrolysis.

Food waste (FW) is a problem and an opportunity. This paper will review the alternative routes for its management. In the UK, the majority of FW is still landfilled. Traditionally, FW has been fed to pigs but part of the reaction to the 2001 outbreak of foot and mouth disease in the UK was to ban this ‘swill feeding’ and introduce stringent requirements for treating FW; later, that was the basis of EU regulation. There is also an EU obligation to reduce landfilling of biodegradable municipal waste (domestic FW) to reduce the global warming potential (GWP — expressed as CO₂ equivalent) of greenhouse gas emissions, but not commercial biodegradable waste. Landfilling FW has a GWP of +832 kg CO₂e /t FW. Taking FW out of the waste stream makes separation of dry recyclables easier. Home composting is ideal in many ways, though it is difficult to determine its GWP because there is a wide range of skill in managing the process. However, many householders are unable or not inclined to undertake home composting. Kerbside collection involves storing FW, which has a number of disadvantages. Food waste is not ideal for centralised composting because of its high moisture content and rapid rate of biodegradability, the route has a GWP of -14 kg CO₂e /t FW. Composting conserves the phosphate but the readily available nitrogen is lost as ammonia or nitrous oxide. In contrast, centralised anaerobic digestion (AD) has a GWP of -162 or -215 kg CO₂e /t FW depending whether sanitisation is by pasteurisation or thermal hydrolysis (TH). Digestion conserves all of the nutrients in the digestate. Retrofitting TH to increase the capacity of existing AD is a good commercial opportunity for wastewater operators, as well as environmentally virtuous. However regulations might be a disincentive to this practicable and sensible development. Local authorities and others are becoming aware that domestic waste disposal units can divert FW at source and that they are convenient, and welcomed by householders. Field studies have demonstrated they do not adversely affect sewerage or wastewater treatment infrastructure. Where the sludge is treated by AD and the biogas used for electricity, the GWP of this route is -199 kg CO₂e /t FW. Phosphate is too precious to squander. Using digestate or compost as nutrient-rich soil improver completes nutrient cycles and replenishes soil organic matter.

Tim Evans, Tim Evans Environment, Stonecroft, Park Lane, Ashtead, KT21 1EU, UK.

23 pages, 2 figures, 7 tables, 35 references.