Understanding Food Waste Disposal Units
WRc Ref: UC6689/2 “Policy Document On Food Waste Disposers”
R. D. Davis, Adele Graham and Kirstie Hearn, February 2005
AMDEA and CESA, the trade associations of the manufacturers of food waste disposers, commissioned Dr Bob Davis of WRc to provide an independent review of the role that food waste disposers (FWDs) could play in waste management and any associated impacts of their use.
WRc is an internationally respected research-based group, providing consultancy in the water, waste and environment sectors. It has more than 200 staff with skills in engineering, science, environmental, social and financial fields and with an expertise built from 75 years of national and international work.
Having reviewed the world literature, WRc concluded that FWDs offer an opportunity for controlled source separation of kitchen food waste (KFW) and that there is scope to expand the use of FWDs in Europe with environmental benefits, especially in domestic premises where home composting is not possible.
FWDs are electro-mechanical devices that fit in the drain line from a kitchen sink. They are flushed with cold water and spin food waste onto an abrasive ring that reduces the waste to small sized particles (95% of particles are smaller than 2mm diameter). These fine particles join the wastewater collection and treatment system. FWDs grind rather than chop or smash so glass, stones and metal don’t splinter.
AMDEA and CESA stress that fat and oil should not be disposed down the sink; they should be collected, stored and disposed or recycled separately. If there is sufficient quantity specialists will recycle cooking oil and fats (e.g. http://www.recycle.mcmail.com/fats.htm).
The European Union has set targets for reducing the input of biodegradable waste to landfill sites in order to reduce the risk of methane leaking from them. Methane is a greenhouse gas with 21 times the climate change effect of carbon dioxide. Climate change is widely regarded as the greatest threat facing this planet.
The targets in the landfill directive are challenging. Home composting is ideal but where people are unable or unwilling to compost, FWDs enable separation of KFW at source without the risk of smell and disease associated with storing it in bins.
FWDs transfer the ground KFW to the wastewater collection and treatment system. WRc found a consensus from research reports that the ground KFW remained in suspension and did not settle out in the sewers. Researchers have also found that fat coalesces onto the food particles in the cold water used to flush the KFW. Because of this, video inspection of the sewers has been reported not to have seen fat accumulation on the walls so of sewers.
WRc concluded that the change in water use when a FWD is installed is effectively negligible. For domestic installations it is only about a 2% increase. Most studies have been unable to measure changes at the wastewater treatment works because there is naturally such a wide variation in the quantities treated because of rainfall, time of year, etc. However, in the few studies where FWDs have been fitted in a large proportion of households, the measurable changes were a significant increase in biogas, an increase in the biosolids produced and an improvement in the food to nutrients ratio in the wastewater.
Many wastewater treatment works treat the organic matter that they separate from wastewater so as to generate biogas, this is a source of renewable, non-fossil energy. Biogas is about two-thirds methane (natural gas) but, instead of it leaking into the atmosphere, in a biogas plant it can be used to generate heat and electricity. The energy balance is very favourable with the annual electricity usage per household being 3-4 kWh (which is less than the energy of collecting it in refuse trucks and treating it by centralised composting) whereas the biogas is equivalent to 85 kWh.
The biosolids left after biogas production can replace fertilisers and feeds soils with organic matter. At works that don’t have biogas generation the biosolids are treated with lime, composted or burnt.
The challenge for water companies at many wastewater treatment works in the future will be to remove even more nutrients from the water that they reclaim. There have been tremendous achievements, and large investments (which have inevitably resulted in higher water bills) but in future even higher environmental standards will be demanded, and improving the food to nutrients ratio (with ground KFW) helps to achieve this by feeding the microorganisms that are used to strip the nutrients out of the water.
The local authorities responsible for collecting solid waste can make considerable savings as a result of KFW being diverted to the wastewater system, but water companies’ operating costs increase, though by less than half the local authorities’ savings. AMDEA and CESA support WRc’s recommendation that there should be some equable sharing of the savings.
WRc concluded FWDs can contribute to meeting these [Landfill Directive] targets by diverting food waste residues away from landfill disposal and via urban wastewater into sewage sludge, most of which is recycled to land. The increase in load [on a WwTW] will be in proportion to the number of FWDs connected to the sewerage system. The impact of up to about 20% domestic usage of FWDs is likely to have a negligible impact on sewerage and STW provided that these are in good condition and have some spare capacity for wastewater and sludge treatment. The Italian Ministry of Environment has suggested to its water authorities that they provide free FWDs to the citizens in areas where there is not enough organic material arriving at WwTWs to ensure satisfactory secondary treatment of wastewater.
08/09/2008 11:52:31
