Crystal Project, Douglas Water Treatment Plant (WTP), Isle of Man

Fifteen months after its commencement, work on the new Douglas WTP has progressed apace. With over a third of the civil work completed, the plant is on target to be finished early in 2008, ahead of its original scheduled date, and is expected to be supplying water later that year. The new facility is the main element of the Crystal Project, which itself forms part of the Isle of Man's ongoing major infrastructure refurbishment programme and will meet the bulk of the needs of the island's projected population to the year 2021.

The IoM Water Authority awarded the contract for the new 37 million l/day works – their largest capital project to date – to Earth Tech.

In addition to the new treatment facility, the project has two other components, under separate contracts. Both of these involve the construction of new pipelines; one to take raw water from West Baldwin reservoir and the other – a joint initiative with Manx Gas – being a new treated water mains leading to Cronk ny Mona reservoir.

The total figure approved by Tynwald – the island's parliament – to fund essential infrastructure improvements is £131m. The cost of the new Douglas WTP is £25m; the smaller Sulby plant – also part of the wider improvement scheme and which was completed in 2005 – was £12.3m.

BACKGROUND

The Isle of Man Water Authority is responsible for collecting, treating and distributing some 10,000 million litres of water – around 26 million litres of water per day in the winter and approaching 35 million in the summer – to 31,000 connections over an area of 250 square miles. The system consists of ten upland reservoirs, five treatment works, 33 underground treated water service reservoirs, over 40 miles of large diameter raw water mains and 750 miles of treated water mains.

Prior to the formation of the Authority in 1972, much of the island's water supply system had been run independently by a number of small regional water boards. As a result, the new authority inherited a complex system of mains, service reservoirs and pumping stations, much of which was antiquated. Many of the reservoirs and pumping stations dated from the 40s and 50s, while some of the mains were a century older.

The increased demands of a growing population have inevitably placed significant strain on the island's ageing systems. In addition, the inability of these facilities to guarantee the removal of mineral deposits from the raw water led to an escalating number of complaints about discoloured water.

Driven by increasingly stringent EU regulation, in June 1999 the Tynwald formally adopted a strategic plan for the replacement and refurbishment of the infrastructure. At the time, the target date was set at 2016 and the corresponding population projection taken at 81,000. However, later studies subsequently led to a revision – the design horizon was reset to 2021 and the projection population accordingly adjusted to 90,000, with parallel modification of the daily per capita usage from 150l in 2016, to an anticipated 185l by 2021.

With something in the region of 60% of the IoM's drinking water coming from a single ageing WTP – Douglas' Glencrutchery Works – replacing this plant was of clear national strategic importance. In October 2004, 70 years after the plant it is to supplant was constructed, planning approval was obtained for this new facility at Homefield Road.

The contract for the WTP was awarded in May 2005 and work began the following month. Construction began on the raw water and treated water lines in January and June 2006, respectively; the former was completed in August and the latter will be finished in December.

PLANT DESIGN

The existing Glencrutchery plant, like the island’s other four plants, uses single-stage treatment. This, coupled with the failures caused by age and embrittlement of the work’s pipes and filters has been implicated in the rising discolouration complaints. In addition, single-stage systems cannot deal effectively with Cryptosporidium and also have inherent difficulty controlling trihalomethane formation.

To address these issues in line with the need to meet the new compliance standards, the new plant uses three-stage Dissolved Air Flotation (DAF). This approach has a number of merits. Firstly, it has a proven efficiency in removing the sort of small colouring particulates from raw water which pose a problem in this instance. Secondly, since it is faster than traditional alternatives such as sedimentation and filtration, it requires smaller tanks and vessels to clarify a given volume of water, which can lead to a significant reduction in construction costs.

In addition, chemical costs are also typically lower as flotation generally demands less pre-treatment to form a light, small floc. The sludge solids produced from this process will subsequently be distributed to local farmers for agricultural purposes.

The site is located in a largely residential area, near to the Isle of Man College. Accordingly, both the design and construction regime reflect the need to minimise disruption to neighbouring residents. Three new buildings will be constructed partially below ground level in order to reduce the overall height and provide additional acoustic shielding. Approximately 16,000m³ of soil and rock will need to be removed for the process building and the sludge and chemical buildings will also require significant excavation.

Operational efficiency and sustainability have been important design criteria. The plant will be integrated into the Central Telemetry System (CTS) being installed throughout the island to monitor the entire water supply network. A variety of sustainable design features are also set to be incorporated into the final facility. These include sun pipes and grey water recycling, together with the widespread use of recycled and/or sustainable products throughout the site, wherever practicable.

Once completed, it will supply Douglas, Onchan, Braddan and the South of the Island.

KEY PLAYERS

Earth Tech Engineering is the main contractor, working for the Isle of Man Water Authority, with Charles Brand (a Lagan company) as the civil engineering sub-contractor. and site excavation work being done by JCK. Island Drainage & Groundwork is the contractor for the new raw water main. Mann Construction is responsible for the treated water pipeline – a part of the project being implemented in partnership with Manx Gas. Mitsubishi Electric are providing the telemetry and PLCs throughout the authority’s refurbishment projects, to install an integrated CTS for the island.

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