The economics of treatment versus discharge

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 of wastewater.

Waste discharge
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.

Sludge treatment
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.