to read the full report - Ecolateral by Peter Jones

Evaluation of Opportunities for Converting Indigenous UK Wastes to Wastes and Energy
AEA/ED45551/Issue 1
4.4 Bio-solids
4.4.1 Definition
Sewage is the wastewater collected from homes, industrial premises, offices and commercial premises
and carried through the sewerage system to a wastewater treatment works. The residual component,
once the clean water has been discharged back into waterways, is the bio-solids, (also known as sewage
sludge).
4.4.2 Composition
Sewage is mostly water, including drainage waster from car parks, roads and other areas of concrete or
tarmac. Sewage contains about 3% solid waste, which reduces to under 1% once it has been screened
(i.e. all the grit, fats, soil, sanitary and contraceptive waste are removed) at the sewage treatment works.
Human waste forms a relatively small proportion of the total volume of sewage. 56
Sewage goes through several treatment processes at a waste-water works before it is returned to river or
the sea in a condition such that no significant pollution or environmental degradation occurs. The main
by-product of any sewage treatment works is bio-solids.
The treatment and disposal of bio-solids is subject to various regulations. The two most relevant pieces
are the Urban Waste Water Treatment Directive (UWWT Directive), and the Sludge (use in agriculture)
Regulations 1989. The Directive details controls for the treatment and disposal of bio-solids, and was the
legislation that banned disposal at sea, the option that was widely practiced, before 1998. The Sludge
Regulations control the spreading of this material on land. In addition there is the Safe Sludge Matrix
from 2001, a voluntary agreement that ensures bio-solids is applied only to certain crops and plants. 57
Sludge from secondary treatment and Anaerobic Digestion (AD) is still very high in water content (>90%
in most cases) and is therefore often subjected to further dewatering before final disposal. The
dewatering decreases the volume of sludge and hence the cost of transport considerably. Such
processes include centrifugation (which can increase the solid content to 24-30%, producing a sludge
cake); belt/filter pressing (which can decrease the volume to 10-20% of the un-pressed sludge); and
thermal drying (which increases the sludge solids to 80-95% and results in sludge in the form of dust or
pellets). These processes are used in particular in larger sewage treatment works where the cost of
disposing of large volumes of high liquid content sludge is prohibitive.
4.4.3 Current Arisings
Information on the quantity of UK bio-solids arising, and the methods of disposal are release by Defra, up
to 2005. 58 Since 1989/90 the level of arisings has increased to 1.4 million tonnes as a result of the
UWWT Directive and hence wastewater requiring more treatment. 59
The UK produces some 11 billion litres of waste water every day, which goes to around 9,000 sewage
treatment works around the country before the treated effluent is discharged to inland waters, estuaries
and the sea. 56
56
Sewage Treatment in the UK, Defra 2002, http://www.defra.gov.uk/environment/water/quality/uwwtd/report02/pdf/uwwtreport2.pdf
57
Sewage Sludge, Defra, http://www.defra.gov.uk/environment/statistics/waste/wrsewage.htm
58
e-Digest Statistics, Defra
59
Council Directive 91/271/EEC Urban Waste Water Treatment, 1991, http://eur-lex.europa.eu/LexUriServ/site/en/consleg/1991/L/01991L0271-
20031120-en.pdf
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