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Report 5 (of 7): Stop the
Dump - England
Introduction
300 million tonnes of sewage are discharged into
the sea everyday via the outfalls around the British
coastline. This is the combined sewage of between
15-16 million people, almost a third of the population.
In addition to this, an estimated 2 million tonnes
of toxic waste are dumped in the sea every year.
Three major problems arise from the discharge of
sewage. The first is visible sewage debris - walk
along any beach near a raw sewage outfall and you
will see toilet paper, sanitary towels, condoms and
occasionally syringes. The second is the risk to
health that occurs when entering polluted waters.
Ear, nose and throat infections, diarrhoea, vomiting
and skin irritations are commonplace. The final problem
is the degradation to marine ecosystems and the consequent
impacts on marine wildlife.
The only solution to sewage disposal is sewage treatment.
Every year, sewage treatment in the UK produces over
30 million tonnes of sewage sludge - a product which
is bulky and expensive to treat.
Wessex Water
Wessex Water is responsible for the treatment of
sewage from a population of some 2.5 million, producing
1 million cubic metres of liqud sludge per annum.
Farmers have traditionally used two thirds of the
sludge produced by sewage treatment works as a soil
conditioner on their land because of its high nutrient
content. The remaining 30 per cent was disposed of
at sea in a licensed area.
In December 1998, an European Community directive
will stop the disposal of sewage sludge at sea. Wessex
Water, therefore, needed to find an alternative solution
to the disposal of sludge at sea. The focus was to
remain on the recycling of sewage for use in agriculture
but in a more practical, easy to handle and safe
form.
The Biodrier
The solution was the thermal drying of sewage using
a biodrier. The Wessex Water plant now uses a pioneering,
environmentally sound process to turn sewage sludge
into dry, odourless, germ-free granules which can
be used as a commercial fertiliser. The technology
was developed in Switzerland and produces between
30 to 40 tonnes of granules a day - Biogran - from
sewage sludge. Biogran is half organic matter and
rich in nutrients such as nitrogen, phosphorous and
potassium. It has none of the dangerous micro organisms,
for example, salmonella or E-coli, typically found
in sludge.
The biodrier generates very little noise and as
the machinery is totally enclosed, there are no smells
or dust. The whole operation can be controlled by
computer microchips and the plant can be run and
monitored by one person, using closed circuit television.
Energy Efficiency
The biodrier is energy efficient - up to 80 per
cent of the energy used can be recycled and it can
operate on "green energy" - methane gas produced
by the sewage treatment process. The sewage is treated
conventionally through a process called digestion
- the standard treatment for raw sewage. Digestion
produces the biogas which is used to heat the dryer.
One megawatt of energy is sufficient to evaporate
a tonne of water and the surplus biogas generated
during digestion is converted into electric power
which can be sold back to the national grid.
Operation of the Biodrier
Sewage from a population of 800,000 living in the
Bristol area enters the sewage treatment works. This
means that on average 165 million litres (36 million
gallons) of sewage passes through each day, a figure
which can increase to 260 million litres (57 million
gallons) during wet weather. Tankers regularly bring
waste from factories and abattoirs to be treated
at the plant. All the waste, which must be organic,
is delivered to the inlet where it joins the domestic
sewage.
Two massive screw pumps lift the sewage up 7.8 metres
(26 feet) so it can flow by gravity to the rest of
the works. Before it enters the central drying drum
which is at the heart of the plant, the digested
sludge, which is basically a liquid, is first passed
through a strain presser. The sewage is forced through
four large screens which are made up of fine mesh
cyclinders and act as automatic rakes, removing any
unwanted debris, such as plastics and rags. The waste
is compacted into a dry form and then taken away
for safe disposal.
The remaining liquid sludge is centrifuged to remove
water and by the time it has passed through this,
it is much drier and has thickened. The dewatered'
sludge is then mixed with fragments of recycled dried
sludge which thicken the mixture still further and
help to form the granules.
A large burner heats up gas and air to a temperature
of 450 degrees celsius. This passes through a heat
exchanger to heat the drying drum. In the drum, the
sludge mixture tumbles around in the hot gases until
the water in the material evaporates. The hot gases
are separated from the mixture and the granules are
produced. Some of the hot water generated by the
system is sold to the chemical industry and the rest
is used to warm untreated sludge for methane production
in the sewage treatment works site.
The air which has passed through the drum is recycled.
Any dust in the air is extracted and used again in
the process. The air containing any smells generated
by the process is passed back into the combustion
chamber and burnt off.
After leaving the drum, the dry granules, which
look like grey coffee granules, are separated - granules
which are too large or too small are used to mix
with the digested sludge at the beginning of the
process and those sized between 2 and 4 mm. pass
on a conveyor belt to a cooling silo.
To prevent any condensation building up, air is
passed over the granules to help the cooling process.
When they have cooled, the granules are bagged and
are then ready to be used. Wessex Water produces
8,000 tonnes of biogran a year, at US$40 a tonne.
The Use of Biograns
Through thermal drying, dewatered sewage sludge
becomes a dry and pasteurised granulate suitable
for a wide variety of uses. Drying reduces the volume
of the sludge by 95 per cent and the granular product
has a moisture content of less than 10 per cent -
making it light, easy to transport and handle.
Biogran is ideal for marketing in agriculture (trouble
free spreading with normal equipment and the slow
release of nitrogen throughout the growing season);
as lawn fertiliser for golf courses, parks etc.;
as coverage for sowing grass on landfills, quarries,
open cast coal mines etc.; and as additional fuel
in cement works, coal fired power stations and refuse
incineration plants etc. Only the biosolids drying
process enables the full potential utilisation of
this valuable resource.
In one German project, biograns are mixed with coal
for power generation and they are also used as a
fuel for cement manufacturing. Wessex Water produces
Biogran as a fertiliser and it has two main outlets
- agriculture and land reclamation. In South Wales,
the biograns are mixed with shale at a ratio of 100
tonnes per hectare. They are a perfect ingredient
in the restoration of old slag heaps because they
are made up of 50% organic matter which means that
they rapidly restore the lost nutrients in the soil
enabling trees to grow.
Benefits of the Biograns
- Ease of farm storage, transport and handling
- Minimal surface run off
- Controlled application
- Wet weather access
- Containment of smells and dust
- Cost effective
ITDG would like to thank Wessex Water for providing
the original information on the Swiss Combi Biodrier.
For further information, please contact:
Wessex Water
Swiss Combi Technology AG
Wessex House
Passage Street
Bristol
BS2 0JQ
ENGLAND
Tel: +44 (0) 117 975 7715
Fax: +44 (0) 117 975 7727
Surfers Against Sewage Limited
No.2 Rural Workshops
Wheal Kitty St.Agnes
Cornwall
TR5 0RD
ENGLAND
Tel: +44 (0) 1872 553001
Fax: +44 (0) 1872 552615
E-mail: info@sas.org.uk
Internet: http://www.sas.org.uk/
Further reading related to this subject from
ITDG Development Bookshop
Running a Biogas Programme: A handbook
David Fulford
Describes the designs and uses of biogas plants, with technical appendices,
for domestic and community plants. Likely economic and social effects of biogas
programmes are described from experience, and advice given in the problems
of management.
188 pp ISBN 0 946688 49 4 paperback 1988 (ITP) £14.95
Sewage Solutions: Answering the call of nature
Grant and Moodie
ISBN 1 89804 913 0 paperback (Centre for Alternative Technology) £8.95
Humanure Handbook: Guide to composting human
manure
Joe Jenkins
ISBN 0 964425 4 X paperback (Chelsea Green) £13.95
A Chinese Biogas Manual: Popularising technology
in the countryside
Edited by Ariane van Buren
Uses diagrams and pictures to show how the basic design of the biogas pit can
be adapted for construction in different soils, from sandstone to sheer rock,
which should encourage other developing countries to embark on their own biogas
programmes.
136 pp ISBN 0 903031 65 5 paperback 1979 (ITP) £8.95
To order any of these books from ITDG Development
Bookshop, send a Sterling Cheque (adding 15% for
postage and packing to European addresses, 25% elsewhere),
or credit card details (American Express, Visa or
MasterCard) to:
ITDG Development Bookshop
103-105 Southampton Row
London WC1B 4HH
United Kingdom
Tel +44 (0) 171 436 9761
Fax +44 (0) 171 436 2013
E-mail orders@itpubs.org.uk
or visit our website at http://www.developmentbookshop.com/
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