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Sewage
Treatment
The
outputs of sewage or wastewater treatment are water (effluent) which is
returned to rivers and the sea and solid material (biosolids or
sewage sludge) which is then further treated and disposed via a number
of possible routes (primarily land fill, incineration and
agriculture). In the UK sewage treatment is strictly legislated and
controlled to comply with UK codes of practice and EU directives.
Preliminary
treatment
Large
solids are removed such as rags and road grit. In some works grease and oil
is removed.
Primary
treatment
Heavy
solid material settles to the bottom of setting tanks, the biological oxygen
demand (BOD) is reduced and the total suspended solids must be reduced by
50%.
Secondary
treatment
The
processes used in this stage utilize the action of bacteria to breakdown any
remaining organic matter.
Biological
filtration: on a granular filter bed microorganisms
feed on the organic matter leaving a colourless liquid
Activated
sludge: in a concrete tank microorganisms breakdown the organic
matter, whilst paddles and jets of air accelerate the process.
Tertiary
treatment
Prior
to release the effluent is further treated to remove nutrients and suspended
matter. UV treatment (polishing or advanced treatment) may be
employed as a disinfection measure.
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A
number of disposal routes for biosolids are available. Past disposal in the UK
is shown below. Agriculture has always been a disposal method, however
with compliance with EU standards the removal of the sea disposal route in 1998
resulted in an increase in incineration. The proportion of biosolids going to land is expected to
increase, and in the UK according to the regulations all will be treated.
