Langsett WTW gets THM levels reduced

Langsett Water Treatment Works lies near the town of Stocksbridge, north of Sheffield and close to the Peak District National Park, and supplies water to a 200,000 population in Sheffield and Barnsley.

The Yorkshire Water-owned works recently underwent a £6.2M project led by framework contractor Earth Tech Morrison (ETM) to reduce levels of Trihalomethane (THM) precursors and improve iron and manganese removal.

ETM was appointed by Yorkshire Water in August 2005 as preferred contractor, designer and commissioning engineer.

Arups became technical consultant and Turner & Townsend awarded the role

of commercial consultants.

Since the water treatment works was commissioned in 1985, raw water quality in Langsett and Midhope impounding reservoirs that feed it had deteriorated as iron and manganese deposits from the soil and minerals in the Pennine catchment increased.

As a result, the water was becoming increasingly discoloured and periodic high turbidity was a problem. Rising levels of manganese and iron deposits in the water were causing build-ups in the distribution system that were affecting the efficiency of the water treatment works.

As is typical of most Pennine raw water sources, Langsett reservoir also contains high levels of naturally occurring organic material that was reacting with chlorine in the disinfection process to form Trihalomethanes (THMs). The existing water treatment system was unable to remove these efficiently and needed upgrading to meet with new Drinking Water Inspectorate (DWI) requirements which came into force on December 31, 2006.

These stated that drinking water had to meet prescribed concentration of value (PCV) regulatory standards for THMs of 100 micro grams per litre.

Throughout the 12-month project, the clean water supply from the existing works had to be maintained which meant that shut down times to carry out major work were restricted. Each new stage of process had to be built and tested independently before cross connections could be opened up to divert flows into supply.

The scope of the project included improving treatment efficiency and performance of the iron and manganese removal system by building a new second-stage filtration plant on the existing site.

The new plant was fitted with six manganese contactors and built downstream of the existing Rapid Gravity Filters (RGFs) to capture manganese and any remaining iron contained in the filtered water as it flows from the primary filters. The role of the new contactors would be to complement the work of the RGFs which removes solids in suspension.

To improve levels of THM precursor removal, chlorine dosing points were relocated so dosing would take place after the first stage of filtration by the RGFs but before the second by the manganese contactors.

The specific aim of this was to remove THM precursors from the water before they had the opportunity to react with the chlorine and form THMs.

Work began in September 2005 and, thanks in part to a mild winter, civil construction activities were completed three weeks ahead of schedule in mid-April 2006.

The mechanical installation, which was due to start in early April, subsequently began in mid-March, enabling commissioning to start in July.

As a result, the completed works went into supply in mid-August 2006, giving Yorkshire Water compliance ten weeks ahead of its deadline with the regulator.

Great care was taken at the design stage to minimise the environmental impact of the work in an area that attracts thousands of recreational walkers and visitors annually.

The new treatment facility was built as close as possible to the face of a disused quarry, where it would be less visible.

A bank of semi-mature trees and hedgerow was planted as natural screening to obscure the new building from view and green cladding was used to help it blend into its environment.

Treatment cycle

The water treatment process starts when raw water from Langsett and Midhope reservoirs is fed into a series of pre-treatments beginning with screening and pH correction in an inlet flash mixer. This is followed by flocculation with the aid of ferric sulphate and polyelectrolyte and clarification in four superpulsator streams.

Water which would previously have been dosed with chlorine at this point is instead dosed only in lime before being fed into the RGF filters where the THM precursors are removed along with any residual solids.

By repositioning the chlorine dosing point after the RGF filters, THM precursors are removed before they can react with the chlorine and form THMs.

Following chlorine and lime dosing through which manganese is oxidised, water enters the new manganese filtration tanks where the contactors enhance the manganese and colour removal process which began in the RGF filters.

The final treatment stages involve chlorine dosing before the water is fed into a new contact tank with a capacity of 1,400m3 and on to a clear water reservoir.

Critical to the speed and success of the project has been the close working relationship of the core delivery team, including the client, Yorkshire Water Services, and the framework contractors and consultants, ETM and Arup.

From the early stages of the feasibility study, through design, construction and commissioning, all parties collaborated to ensure the delivery of a solution that was good value for money, completed ahead of schedule and posed minimal risk to water supply.

Programme constraints by the DWI’s compliance deadline of last December prompted the delivery team to adopt an efficient way of working that was underpinned by the concept of ‘Critical Chain Project Management’.

Instead of creating a works schedule that allocates extra time according to the risk involved in each task, this programme removes risk allocations to a ‘project buffer’ which is continually monitored and updated to ensure a suitable but not excessive time buffer is maintained to the end of the project.

The result is that the works programme is far more flexible and responsive to work moving ahead or indeed behind schedule without disrupting the successful delivery of the project.

In this case, the flexibility of the programme and cooperation of all those involved meant the project was able to move ahead of schedule by nearly three months.