Proceedings

To meet the targets of the Water Framework Directive, existing Environmental Quality Standards (EQS) for many substances, including iron, are under review.  The main purpose of iron dosing at wastewater treatment works is to reduce effluent phosphorus concentrations in order to meet other requirements of European legislation.  If the iron EQS value is reduced, a subsequent tightening of surface water discharge permits may result.  An UKWIR-funded project looked to assess the current impact of iron dosing on downstream water, sediment and ecological quality as well as the likely impact of tightening iron discharge permits on the Water Industry.  Analysis of pre- and post iron dosing water quality and ecology downstream of several treatment works shows that tightening of permits would provide no obvious environmental benefit at significant costs to the Water Industry and subsequently the consumer.  A series of permitting options were provided to ensure EQS compliance without incurring disproportionate costs to the water industry.

Keywords:

Iron discharge consenting, WFD, phosphorus reduction

Introduction:

Iron salts are currently used at approximately 700 Wastewater Treatment Works (WwTW) serving over 20 million customers (UKWIR, 2011) as a coagulant within wastewater treatment to precipitate phosphorus into sludge.  This process is used in order to meet European Directives for the control of phosphorus discharged to the aquatic environment, with the majority of works dosing to achieve phosphorus permits of either 1 or 2 mg/l. Where iron is added during the treatment process a permit to discharge is generally applied by the Environment Agency to ensure that the receiving water Environmental Quality Standard (EQS) is not exceeded, thus ensuring there is no negative impacts on downstream ecology. The current EQS set under the Dangerous Substances Directive (DSD) is 1 mg/l as dissolved iron.

The Water Framework Directive (WFD) is a relatively new European Directive which sets very stringent targets for water quality and ecology. As part of its implementation the DSD is being repealed and new EQS are being derived for a large range of potentially toxic substances. Iron is not a particularly toxic metal owing to its instability in water; however, excessively high concentrations can cause toxicity and so has meant an EQS was set under the DSD. Iron may now be included as a WFD Annex VIII Specific Pollutant and a revised EQS set by the Environment Agency, is likely to be published for consultation in 2011. Given that the WFD EQS setting methodology invariably leads to tighter EQS than those set under the DSD, there is risk to the Water Industry of a downward pressure on new and existing iron permits in the future.

Furthermore, in many cases phosphorus permits are also being tightened to meet WFD objectives and are likely to become more extensive in the future.  This in turn puts pressure on water companies to use more iron. Recent estimates (UKWIR, 2011) also suggest that to achieve the phosphorus EQS, many more wastewater treatment works (WwTW) may require some form of phosphorus removal (potentially at least an additional 1,000 WwTW) in an attempt to meet in-river phosphorus EQS.

Any decrease in iron permit values would put significant pressure on the Industry to install extensive tertiary treatment (sand filters) to reduce effluent concentrations or seek alternatives to its use. Concerns regarding cost and increasing carbon emissions have lead to the Water Industry via UK Water Industry Research (UKWIR) to review the current permitting policy, the fate and behaviour of iron in receiving waters; impacts on downstream ecology and water quality; costs of additional treatment and to offer options for possible future permitting regimes (UKWIR, 2011).  Decisions regarding the regulation of iron in the UK have yet to be made; this paper summarises the key outcomes of the industry’s research.