Majjie's Blog

I first wrote about waste diposal units (WDUs) in the kitchen back in 2007 after I'd been to a trade show and talked to the people at Waste Maid ...and I've been similarly prompted to think about the subject recently, after visiting the InSink Erator (ISE) stand at the Grand Designs Live show last week.

Representatives of both companies, on both occassions, assured me that WDUs are very environmentally friendly ... but then they would wouldn't they. Since 2007 many more councils have started to collect compostable waste and to provide separate little bins or caddies for food waste that can't go into the normal compost bin. So, I decided to do a bit of digging around (of the online digital variety) for more information about the pros and cons.

According to DEFRA, we produce about 12-20 million tons of food waste each year and it has been estimated that about 5.8 million tons is collected by local authorities. Most of that (around 70%) still goes into general waste and landfill (the rest is collected separately and treated biologically).

The habit most of us have, of putting some food waste into the general rubbish - which goes to landfill - has got to stop. The UK is already falling behind on targets to reduce biodegradable waste going into landfill, and the level needs to be reduced to zero by 2020. That's because the anaerobic breakdown of food waste, that occurs in landfill sites, produces methane (a climate change gas around twenty times more harmful than CO2).

There are two ways that this target (of no food waste going into general waste) can be achieved. The first is by composting, whether individually or by local councils.  Local councils provide a central composting facility which can safely convert food (like meat, fish, cheese, bread and dairy products) into compost/fertiliser, or some have anaerobic digesters which can produce biogas (using the methane as a renewable energy source) before the solids are treated in the usual way.

The alternative - is to have a kitchen waste disposal unit. After you've ground up your food waste, the solid material suspended in the waste water, goes to a waste water treatment plant (or sewage works). In 2007 around 66% of all sewage sludge treatment in the UK was by anaerobic digestion, so I'm guessing the level is even higher now (there don't seem to be any more up-to-date figures) and the biogas produced from the process is increasingly being used for more than just local electricity generation. In 2010 biogas from sewage works was cleaned up and used to fuel a car, aswell as being introduced directly into the gas grid for the first time.

There has been some heated debate about how environmentally friendly waste disposal units are. After all, they use drinking water and electricity whilst grinding the food and it has been suggested that there is an increased risk of blockage to drains ... which in turn could lead to increased flood risks ... and that more rodents and pests could be attracted into the sewers.

Most of those fears seem to be anecdotal, though. The Chartered Institute of Water and Environmental Management (CIWEM) has produced a recent policy statement about food waste disposers which looked at all the evidence and concluded that there was definitely a place for WDUs in environmentally sound waste management.

It seems that treatment of food waste in the sewage system compares very favourably with collecting it in waste trucks and composting it centrally and is about the same (in environmental tems) as collecting the waste and transporting it by road to an existing anaerobic digester. The fact that most sewage works already use anaerobic digestion and are developing biogas technologies, also means that putting food waste through domestic WDUs could be more favourable in the future, than building new anaerobic digesters (which requires a lot of capital).

Studies show that 95% of food particles from a WDU are smaller than 2mm in size and remain in suspension whilst travelling through the sewage system. This doesn't attract rats - who are more likely to be attracted by spilled food from collection bins - and it's only if there's a high proportion of fats and oils in the waste that blockages are likely. So, the moral of the story ... is not to put fats into your WDU (or generally down the sink come to that). You also need to be sure that your kitchen waste is connected to the sewer - and isn't just a surface water drain (which can be the case in older properties) - because the latter eventually runs into rivers and the sea, rather than a sewage works.

Actual studies of areas, where high numbers of domestic WDUs have been installed, seem to show that the consumption of drinking water increases by a small to negligible amount ... and the use of energy, in the form of electricity, is no greater than that required to transport waste to a central facility by road.

Perhaps the biggest argument for the use of a waste disposal unit in your kitchen, though, is that some of us just won't use the composting method, whether it's our own compost bins or those provided by the local council ... especially those of us who live in flats. We just don't have the time, the space or the motivation (or all three). Scraping your waste into the sink is very easy and - if it's environmentally friendly too - why not!