The economics of treatment versus discharge
Senior group engineer at W S Atkins Consultants, Martin Kimber, argues that if actual costs were applied, the treatment of wastewaters from water treatment plants would be seen to be more economical than discharge to a sewer.
The treatment and recycling of water treatment plant wastes offers several
benefits to Water Treatment Works (WTW) operators: an increase in deployable
output; control of the risk of Cryptosporidium in treated water; waste
minimisation; and, compliance with tighter Environment Agency standards for
discharges. It is evident that there are further benefits from additional treatment
What really stands out is that the works discharging to a sewer were reportedly
all operated by water and sewerage companies (WASCs) or authorities, responsible
for both services. Apparently the water treatment plants were not charged for
discharging their wastes to the sewer.
If standard trade effluent charges or even the economic costs were to be charged
there is no doubt that we would see dramatic changes in disposal routes, with
a sharp drop in the quantity of sludge disposed to sewers. The water treatment
plant’s wastewater discharged to sewers would, if they were not ‘internal’ to
the WASCs, normally be subject to trade effluent charges.
What are the costs of treatment of wastewater on site? Costs are site specific,
size dependent, and dependent also on the nature and quantity of the waste being
treated. For a large plant with no existing wastewater treatment facilities,
the capital cost would be of the order of £25,000 to £50,000 per
Mld (megalitres per day) of treated water (not volume of waste treated) and
the operating cost would be of the order of £1000 to £2500 per year
per Mld of treated water. If filtration is not required, as is the case for
most plants, then the costs will drop.
These costs need to be compared with the costs associated with discharging
to a sewer and sewage treatment. A reasonable way of assessing these costs is
to calculate the trade effluent charge that a discharge would attract.
Trade effluent charges depend on the strength of the waste and vary across
the country. The charge is made up of components relating to the volume of waste,
the chemical oxygen demand of the waste, and the suspended solids concentration.
For water treatment wastes, a reasonable assumption is a charge of £0.75/m³.
Based on this, if discharges were subject to trade effluent charges, the annual
charge across the country would be in the order of £60 million/yr. To
put this large sum into context, it represents a cost of around £0.04/m³
of water produced (equivalent to £14,600/Mld/yr), assuming all wastes
are discharged to sewer. If the wastes were pre-treated by settlement then the
volume charge would reduce greatly but this would be partially offset by a large
increase in the solids-based charge.
Discharging wastes to the sewer transfers treatment to a sewage treatment works.
Sewage treatment will not significantly reduce the quantity of water treatment
solids that ultimately needs to be disposed of, but does involve the diluting
of wastes with sewage, providing full sewage treatment, and then still having
to treat and dispose of the solids in the sewage sludge. It is therefore clear
that, in principle, discharge of water treatment wastes to sewers will incur
additional costs compared to on-site treatment at source.
Similarly, provided trade effluent charges equate to the actual costs of dealing
with trade wastes, which they should, then it will often be economic to provide
sludge treatment rather than discharge to a sewer. Of course smaller water treatment
works would have disproportionately high operating costs for waste treatment
and, for such plants, disposal to sewer may be more attractive particularly
for unmanned sites.
There is also an additional benefit arising from the water resource produced
by recycling, as the cost of buying new resources is generally significantly
higher than the cost of recycling. For example, the cost of developing and supplying
additional water resources is high, typically of the order of £1m /Mld
to £2m /Mld, which is similar to the capital cost of a treatment and recycling
plant. An efficient conventional water treatment plant using rapid gravity filtration
will produce a waste volume of two to three per cent of water put into supply.
For plants which draw water from large reservoirs the additional resource from
recycling can be discharged to the reservoir over the entire year. For plants
with direct abstraction recycling may only be of interest at times when throughput
is limited by the abstraction licence. However, in either case, recycling will
provide an additional water resource.
Clearly if trade effluent charges represent actual costs, there is an overwhelming
economic case for recycling which is further reinforced by comparison to the
cost of developing an equivalent new resource. The appreciation by companies
of the real costs of disposal of sludge to a sewer, together with the savings
to be made from recycling rather than developing new resources, may explain
why WS Atkins has recently been increasingly involved in projects involving
recycling of wastewater from water treatment works.
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