Nitrates are a growing issue for water companies across many parts of the world. Naturally occurring in soils, nitrates are also used in fertilisers to increase crop yield, and because they are soluble, these are easily washed into rivers and ground water sources.
In the past the problem has been largely managed by blending water drawn from high nitrate sources with water that has a lower nitrate concentration, to achieve the current drinking water standard of less than 50 mg/l.
But warmer temperatures and changing rainfall patterns together with rising populations is fuelling the issue, and as nitrate levels continue to increase the option to blend different water sources is frequently failing to reduce daily nitrate levels sufficiently.
Where daily nitrate levels have exceeded the regulatory limit some water treatment works have faced temporary closure, which in turn has put additional stress on the water supply network. Clearly another solution is needed.
Nitrate removal solution
Nitrate levels at Yorkshire Water
’s Tophill Low water treatment works (WTW), which supplies half of the city of Hull’s daily drinking water, some 68 million litres, are now well within drinking water inspectorate water quality limits, thanks to a nitrate removal solution from ACWA Services, part of the ACWA Group, an international water and wastewater solutions provider.
The 24-month design and build contract, core to an overall £7.5 million drinking water improvement scheme lead by Amey, has involved the development of a complete water treatment solution centred around ACWA’s Nitreat process, now in use across 25 sites in the UK, 12 of these in the east of England.
Operational since July last year, the solution enables Yorkshire Water to combat rising nitrate levels from the river Hull, which had previously necessitated costly plant shut downs, providing a future proof solution for improved water quality in East Yorkshire and at the same time removing stress from water supply network.
The ACWA solution, an Ion Exchange nitrate removal plant, is designed to treat 22 million litres per day.
The ACWA solution, an Ion Exchange nitrate removal plant, is designed to treat 22 million litres per day in the process reducing nitrates from a maximum of 64 mg per litre to 5 mg per litre. The treated water is then blended with the remaining drinking water flow to ensure that 68 million litres of water per day are within the target nitrate concentration of 44 mg per litre or below.
The new plant is installed downstream of the plant’s existing rapid gravity filters (RGFs) in a side stream arrangement and consists of two 50% duty Ion Exchange skids to provide a robust and flexible solution able to cope with large variations in flow and nitrate levels, configured to provide the best possible treatment at Tophill Low.
ACWA’s Nitreat Ion Exchange solution was selected following an extensive evaluation process as the best ‘total expenditure’ solution and one that was already tried and tested at the UK’s largest nitrate removal plant (90 million litres per day) also operated by Yorkshire Water at the Keldgate WTW. The solution also offered the added benefit of the smallest plant footprint. This is important given the existing site constraints and the proximity of an adjoining SSSI, together with the environmental advantages of the lowest waste water flow of all the technologies considered.
Future proof water quality
Peter Brewer, ACWA Services’ UK General Manager says: “Our focus throughout has been to provide a robust and future proof water quality solution, collaborating with Amey and other contractors to overcome a number of technical challenges to satisfy the demands of both Yorkshire Water and the Environment Agency. We are delighted to be involved with this, our second Nitrate removal project for Yorkshire Water, the largest of their type in the UK.”
“The Nitreat continuous counter current Ion Exchange system is a landmark improvement on conventional ‘batch’ treatment systems, enabling exhausted resin to be continuously regenerated, rinsed and returned to work, with all phases of the cycle occurring simultaneously. The system offers a high degree of flexibility as it can cope with large variations in flow and nitrate levels and produces far less quantities of waste regenerants compared with conventional plants.”
Constructing the solution off-line made it possible to deal with many of the detailed design elements including process, mechanical, electrical and instrumentation works, and to minimise the impact on the nature reserve. It also meant that fewer shut downs and interruptions to the plant were required particularly those required when connecting to the existing RGF outlet pipework.