KEY POINTS:
Food waste disposal units could be a more environmentally friendly way of getting rid of food scraps than roadside organic collections.
Research commissioned by Parex Industries, which markets InSinkErators, has found potential benefits in putting food scraps down the sink into wastewater systems.
Currently many people dispose of their food scraps with their general household rubbish.
But 23 per cent of all waste to landfills is organic and releases greenhouse gases as it decays.
John Cocks, an environmental engineer, said his research showed that putting food waste down sinks into the wastewater and sewerage systems provided benefits beyond reducing landfill and methane emissions.
For instance, the food waste can improve the ease by which nitrogen and phosphorous can be removed from wastewater.
Also, where councils had wastewater treatment systems to collect biogas from the residual solid waste, like sludge, the extra bulk of food products increased the biogas supply.
Mr Cocks said sewerage systems where biogas was currently harvested included those at Mangere, Hamilton and Christchurch.
Other city sewerage schemes which had potential for further use of biogas include those in the North Shore, Tauranga, Rotorua and Palmerston North.
He accepted the building of biogenerators was expensive but so were other options such as kerbside organic collections with centralised composting, as some councils were considering.
Mr Cocks said it was hard to manage such large-scale composting effectively and it was also expensive given the numbers of trucks needed to collect the waste.
There can also be issues around odour and vermin in having organic refuse stored on properties and put out for separate collection, he said.
Mr Cocks said that although good home composting remained the ideal way to get rid of food scraps, that was not always possible, like for apartment dwellers. The alternative of using food waste disposal units was hygienic and did not use the huge amounts of water as some might imagine, he said.
Mr Cocks estimated an average household used about 18 litres of water a day on flushing away food wastes compared with 450 litres of water used for flushing toilets, cooking, cleaning and washing.
Food waste disposal units also did not put a strain on sewerage systems, he said. "People should avoid a simplistic approach and not give in to preconceptions about food waste disposal units."
Andrew Higgs, general manager of Parex Industries, said a key finding of the report was that food waste disposers had a role to play in a 'closed loop' waste management process where bio-solids were recycled for land applications or as a renewable energy source.
Mr Higgs said around 30 per cent of homes had a food waste disposer.
"More New Zealanders need to recognise that these appliances offer a cost-effective, convenient and hygienic means of separating domestic kitchen food waste from other domestic waste going to landfills."
Mr Higgs said many households with septic tanks could also safely use food waste disposers.
Home composting had its limitations, he said.
"Home composting done poorly, without adequate quantities of woody garden waste, can produce a stinking, wet, slimy mess that emits methane, a greenhouse gas 23 times more damaging than carbon dioxide."
Mr Higgs said given the number of local authorities practising separate kerbside collection and others considering introducing similar schemes, the research findings suggested the need for a re-think.
In a supplementary report, UK expert Dr Tim Evans addressed misperceptions around the use of food waste disposers and found they did not cause blockages in waste water systems and could enhance the biological processes used in wastewater treatment plants.
A LIGHTER FOOTPRINT
Food waste makes up an estimated 53 per cent of New Zealand's organic waste stream, says the Ministry for the Environment.
Most of the sewage sludge in New Zealand is currently sent to landfills so the output from food waste disposers would still be sent there rather than to composting initiatives, it said.
The greenhouse gas emission from landfills is much higher than the emission from composting facilities.
Food waste disposal units also increase the pressure on existing wastewater treatment plants.
Although the facilities were designed with future growth in mind, they have finite capacities and life spans, the ministry says.
In New Zealand, a significant number of wastewater treatment plants use pond-based treatment and are unable to satisfactorily treat waste if they are subjected to an increase in organic loading.
Increasing the amount of waste would shorten their functional life spans and could potentially overburden the treatment processes, the ministry says.
Increased maintenance and upgrades to the facilities would ultimately be borne by ratepayers.
To overcome such problems, the ministry is working with local government and researchers to develop and implement innovative and culturally acceptable options for recycling and reusing sewage sludge.
When designing new wastewater treatment plants, the ministry says, local councils should consider the role of food waste disposers in decreasing their community's carbon footprint and increasing sustainability.
