Date of visit: 22
September 2008.
Tour guide: Colin Marriage, Unit Manager,
Viridor.
Tour organiser: Daniel Wareing.
Background
Foxhall household waste recycling centre is one of the 18 sites
in Suffolk that take household garden waste including grass
cuttings, tree and bush cuttings, weeds and plants. Once garden
waste is put in the green containers staff remove any obvious
contamination that can be reached and then the waste is compacted
to increase the bulk collected in each load. The waste is then
taken to a composting area at the adjoining landfill site,
processed and turned into soil improver for agricultural use or
compost for gardens.
Composting at Foxhall
The visit focused on the composting
area before the heavens opened and we waded back through ever
increasing puddles. It was an interesting if brief
introduction to the world of composting, due to weather, but
we left with some tips on how to improve our own compost
including regular turning to improve aeration.
The first stop was at the windrows which are formed from green
waste shredded to a suitable size to aid the composting process and
to ensure that biodegradation is achieved.
Temperature readings are taken
initially on a daily basis and then three times a week to
ensure the windrows are warm enough to kill weed seeds and
harmful bacteria (between 60 and 80 degrees Celsius in the
centre of the windrows), but not so hot that the good bacteria
die off. The material is turned when necessary to increase
oxygen to aid bacteria and adjust temperatures. When
conditions are too dry, pumped groundwater is added using an
agricultural sprayer (the process uses on average 100,000
cubic metres of water a year)
The warmth was easy to feel even at
the edges and in the windrow created from material received
that day.
This composting process takes about 13 weeks and the finished
material is screened through a 10mm sieve for compost and a 25mm
sieve for soil improvement. This prevents contaminants such as
pieces of plastic bags from entering the final product.
Any larger organic material left after screening, such as larger
roots and sticks, is added to the centre of a new windrow to
continue composting and improve aeration. These larger items aid
oxygenation at the bottom of the windrow via the air spaces that
they trap.
It was emphasised that the composting carried out at Foxhall
takes advantage of a wholly natural process, the windrows working
in the same way as a garden compost heap (though on a much larger
scale, with each windrow consisting of around 750 tonnes of
material), in contrast to the in-vessel composting system seen at
Parham. As such, this facility is only licensed to process the
lowest-risk organic waste, being plant matter originating from the
garden only.
Foxhall produces about 5 to 6,000
tonnes of soil improver per year from the 15,000 tonnes of
green waste received on the site.
Just as the heavens opened we saw the shredder and the pile of
new material awaiting shredding.