News & Media > Environmental Services
29:04:2010
Food waste: best collected separately and weekly
The debate about dry recycling collection systems, either kerbside sort or commingled, entered a new phase last summer. WRAP published the evidence that for most local authorities the quality of materials and cost of collection favours kerbside sort, especially compared to single stream commingled collections.
This debate has its parallel in how local authorities are approaching their organic collections, in particular whether food waste should be collected separately or with garden waste. Where a local authority already supplies a garden waste service, especially where this is provided using a wheeled bin, it appears logical simply to ask households to add their food waste to the existing bin. At first sight, this only incurs the cost of the kitchen caddies, as the collection cost will be marginal.
There are several factors that influence the performance and potential cost of this approach. The frequency of the organic waste collection is usually every two weeks, often combined with an alternate weekly refuse service. In this case the amount of food waste will generally be between 10 and 15% of the organic bin. The garden waste will typically average about 10kg for each bi-weekly collection with no more than a kilo of food waste over the same period.
Garden waste separately collected can be composted using windrow techniques at relatively modest cost (£25 per tonne). However, food waste must be treated in an Animal By-Products Order (ABO) approved process where the gate fee will normally be double that of windrow. So, the cost of combined garden waste and food waste processing amounts to £14.30 per household per annum, calculated on 286kg (11kg per collection x 26 collections per year) at £50 per tonne. The cost of reprocessing the materials separately is £1.30 for food and £6.50 for windrow, giving an overall cost of £7.80 per household per annum - a saving of £6.50 per household.
In order to complete the picture the avoided cost of disposal also needs to be considered. Most garden waste is not accepted in residual refuse so in theory this is material that should not have been part of the waste system except as a contribution to the throughput of Household Waste Recycling Centres (Civic Amenity Sites).
However, the food waste is highly likely to be present in the residual and is therefore a diversion of waste. Looking at the next five to seven years, the mean cost of landfill is around £100 per tonne, with most alternative residual waste treatment solutions, e.g. MBT or incineration schemes, projecting very similar gate fees. We can therefore conclude that all the food waste is generating a gross saving of around £100 per tonne in disposal. The net costs of processing are therefore £11.70 per household per annum for the combined organics system or £5.20 where the food and garden waste is processed separately; the difference remains the same at £6.50 per household.
Collecting combined organics weekly improves the performance of the food waste capture by at least 100%, at the expense of doubling the collection cost. The impact on the capture of garden waste is marginal, so the additional collection cost of a weekly food waste service in this system is significant and the processing cost remains very high.
Looking at a separate collection of food waste, it is almost impossible to do this at a frequency of less than weekly while ensuring public acceptance and avoiding odours and fruit flies. The range of capture rates reported for separate weekly food waste is between 1.6-2.3Kg per household per week with 1.8Kg about the average, based on our own work and that reported by WRAP (“Evaluation of the WRAP Separate Food Waste Collection Trials” Final Report, updated June 2009).
This range of performance is consistent with an alternative weekly refuse regime; where refuse remains weekly the capture rate for separately collected food generally drops by half. A local authority collecting food waste separately also has more flexibility in offering a garden waste service, as this can be free to all and alternate weekly, or indeed a charged-for service. Once food waste is added to garden waste containers the option to charge for garden waste does not exist.
Returning to the cost of processing and the relative costs of collection, the reprocessing cost of the extra food waste collected in the weekly system is only half that of the extra saving in avoided disposal; the organics reprocessing cost of £11.18 is offset by a disposal saving of £9.36, producing a net cost of only £1.82. Basically, does a weekly food waste collection cost less than £9.88 per household per annum if we compare a combined organics service with an approach based on a separate collection? In our view, almost certainly.
Of course, a local authority collecting food waste and not providing a free to all garden waste service will generally have a lower total waste arising and a much lower cost base overall.
There is one other factor which enhances the benefit of separately collected food waste undertaken weekly. Where this type of service is offered and especially where it is combined with alternate weekly refuse, there is an observable decrease in the total amount of food waste available to be collected. Essentially we believe that this regime makes households realise they are wasting a lot of food they have shopped and paid for and they moderate their behaviour as a result.
We are currently researching this phenomenon with Exeter University, but we estimate the reduction is around 1kg per household per week, which is worth over £5 per year in disposal savings. In an alternate weekly combined organics collection we have failed to find a food waste reduction impact. Now the cost envelope for a separate collection compared to a combined system is £15.08 per household and a separate collection definitely costs much less than this.
Turning to the environmental benefits of the two approaches, the key driver for collecting garden waste has generally been its significant contribution to meeting recycling rate targets rather than any intrinsic environmental benefit. This is not true of food waste, where the environmental benefit is clearer. By choice our preference is an Anaerobic Digestion (AD) processing solution as this generates a better CO2 gain than In Vessel composting – 0.19 tonnes CO2 per tonne compared to 0.42 tonnes CO2 per tonne.
The benefit of the food waste reduction impact of separate collections is also very significant. WRAP states that we waste on average over £400 per household of food per year. 1kg a week represents about a quarter of this, so the average household is saving themselves about £100 per year. Every tonne of food waste avoided in this way is also worth 4.5 tonnes of avoided CO2 emissions, making this one of the most significant contributions of any element of a waste management system.
So in conclusion, taking into account capture rates, reprocessing and disposal costs and carbon savings, we believe that on both cost and environmental grounds food waste is best collected separately and weekly, with AD delivering the best processing outcome.
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