Removing phosphorus from wastewater has been an issue for UK water companies since the introduction of the Urban Wastewater Treatment Directive in 1991. This challenge grew with the introduction of the Water Framework Directive. Now the sixth asset management plan period, AMP6, heralds even tighter phosphorus consent limits, with consent values of less than 0.5 milligrams of phosphorus per litre (mgP/l) under discussion, with the added sting of tightening iron limits. It will be possible to achieve these consents, but expertise will be needed to intertwine technical solutions with other factors such as reducing operating costs, minimising carbon emissions and water footprint, increasing energy production and overall resource recovery.
Twenty years ago, Australia and the US saw first-hand the environmental impact of phosphorus in receiving waters. It depleted oxygen in water, encouraging the growth and decomposition of oxygen-depleting plant life, harming other organisms (eutrophication). This in turn reduced the recreational value of water and drove the need for tighter phosphorus discharge permits.
MWH was involved in the design of three plants in Sydney to meet very low phosphorus targets, treating 1.3 to 4 megalitres of wastewater per day. A range of processes were deployed at the plants, including biological phosphorus removal methods. Biological nutrient removal with chemical back-up and tertiary filtration was also deployed by MWH at Iowa Hill and Pinery in Colorado to achieve 0.5 and 0.05 mg/l phosphorus permit conditions.
Biological nutrient removal has not been widely adopted in the UK. Chemical dosing is the standard method employed by most water companies here. This is partially due to the higher number of filter works, but co-precipitation in activated sludge plants or oxidation ditches is also practised.
By 2015 more than 650 UK wastewater plants will have phosphorus-removing technology – the vast majority chemical dosing. AMP5 will see 360,000 to 572,000 tonnes of iron products used each year, at a chemical cost of £50 million a year. This is likely to increase during AMP6 as more sites get phosphorus consents and the iron dose is increased to achieve the tighter phosphorus-consent limits. But is this sustainable?
The Water Framework Directive and Ofwat’s move to an put the emphasis on outcomes rather than outputs has provided water companies with an opportunity to review the total phosphorus load to a watercourse. By revisiting the cause and effect of multiple phosphate sources within a catchment it is possible to deliver improved river quality and reduce the cost and carbon footprint of the end-of-pipe solution.
MWH has drawn on its experience of phosphorus trading in the US to offer a similar service to UK water clients. For example, MWH has helped a leading water company to reduce its potential AMP6 phosphorus drivers from 77 to 18 sites. Tight phosphorus consents will still be required but they will be delivered where they deliver the most benefit to the environment.
The higher iron dose rates required to meet these tighter phosphorus consents increases the risk of breaching iron discharge limits, which are themselves tightening. The commonly held view is that to achieve consent standards below 1 mg/l requires dual point dosing of chemical followed by tertiary treatment to meet the associated iron consent standards. The second dosing
point, prior to secondary clarification, increases the risk of iron carryover in the final effluent – hence the need for tertiary solids removal.
Work by MWH has shown that single point dosing into the primary settlement tanks with good site operation and sludge management can eliminate the need for additional tertiary treatment where excessively stringent consents are not in place. The final settlement tank design as well as the quality of the dosing arrangement is key to achieving good quality effluent without tertiary treatment.
Even with the excellent design and operational practices in place, there are situations where tertiary filtration either with or without dosing will be increasingly necessary, especially where extremely tight phosphorus (from about 0.8 mgP/l) or iron standards are applied.
While this may seem daunting because tight phosphorus consents are new territory in the UK, what is needed is the sound application of basic chemical engineering principles to deliver the desired outcome and not a new coverall technology. Levels of 0.1 mgP/l are achievable using technologies and techniques that have been available for many years; the proper application on a site by site basis is what is needed.
The key to meeting the new, tighter phosphorus challenges of AMP6 lies not in simply bolting new technologies on to existing plant but starting at source to rationalise the investment required to deliver maximum environmental benefit. From there it is a case of making best use of the existing assets on each site to meet the required performance level. Once this performance has been secured, additional processes should be considered and their design balanced accordingly. Taking this approach will deliver the overall environmental benefits the industry and legislator strive to achieve. And by being smart through a holistic view, the costs can be partially offset if integrated into an energy production facility.
Julie Jeavons is the process engineering technical discipline leader, Ian Cranshaw is technical specialist and Ajay Nair technical director – all at MWH. They are experts in water, wastewater treatment and resource recovery