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Private treatment Scotland's first PFI water scheme has had a long gestation but should have a speedy construction.

Two inconspicuous sewage plants being built amid stunning Highland scenery around Inverness and Fort William represent the vanguard of Scotland's long awaited privately financed water industry. The pounds 600M plus package of European Union- demanded clean- up schemes is seen by many as Scottish construction's future lifeblood.

Of the nine PFI sewerage projects that must meet an EU deadline of December 2001, only the pounds 45M combined Inverness and Fort William scheme has so far made it to site (see table). But a further five projects, totalling pounds 310M, now have preferred bidders and rival contractors are keen to see if the winner of this first hard fought Highlands deal is likely to sink or swim.

The bottom line for the client, North of Scotland Water Authority, was simply how much its chosen concessionaire would charge for treating the waste. How the sewage was treated was largely the business of the winning team, which could scrap suggested designs so long as it provided the agreed standards.

In December 1996, after an 18 month tender period, the concession was given to Catchment - a consortium of North West Water owner United Utilities, plus contractors Bechtel and Morrison. The two contractors are building the facilities and the 25 year operating deal goes to NWW subsidiary Caledonian Water.

From day one, everyone pitched in to value engineer the entire construction and operating package, an exercise claimed to have cut pounds 16M off the bill.

'Unlike a PFI road contract with guaranteed shadow tolls, if our plant breaks down we get paid nothing,' stresses Catchment project director Mike Smith. 'So we brought the operator into the design team from the start to ensure no surprise interface problems later.'

Ductile iron pipes changed to plastic; planned new pump stations were either scrapped or incorporated into existing facilities; and precast concrete wall panels for treatment tanks were shipped in from Ireland to speed construction by 75%.

'We hope to commission the Inverness works two months ahead of schedule and Fort William's five months early,' claims JV project manager Neil Colman. 'This should allow us to use the income from Fort William, now due to come on stream a full year before Inverness, to help fund the latter bigger project.'

At Inverness, short outfalls currently discharge preliminary treated sewage direct into the Moray Firth. The new pounds 36M scheme will intercept these flows, feed them to a full treatment works along the shoreline at Allanfearn, and then discharge the cleaned effluent into the mouth of the Firth through a single 1200m outfall.

The value engineering exercise immediately homed in on the challenge of poor ground conditions, a near surface water table and tidal influences creating constantly saturated gravels. Out went proposed ductile iron pipes for much of the shoreline link to Allanfearn.

This had been planned as a deep laid gravity sewer to a new intermediary pump station and would have needed an extensive dewatering regime. Instead the gravity sewer was converted to a rising main and laid half as deep using more flexible plastic pipes. Installation time was halved and pounds 500,000 cut from the bill.

More time and cash was saved by eliminating the intermediary pump station and instead boosting capacity at the system's existing Longman facility.

At the new treatment plant, use of 6m high precast concrete planks to form the sides of sludge tanks has allowed seven to be erected in just over three weeks -nearly three times quicker than insitu construction. Similar time savings were achieved by using precast baffle walls for the 25m long aeration tank. These 9m tall slender walls would otherwise have been cast insitu in single full height pours with limited room for shuttering.

Focal point of the integral sludge facilities will be known as the Dutch barn: a large open sided portal frame building in which to store solid sludge cake. Inverness will treat not only its own 8t/day sludge output but everything produced at Fort William 100km away, and delivered daily by road tankers.

The arriving product can either be thickened to 6% dry solids and used for agricultural and forestry top dressing, or dewatered in a centrifuge to reduce it to sludge cake with 24% solids suitable mainly for landfill. Catchment decided it was more economic to treat all the sludge in one place, and Inverness offers better outlets for its end use.

Fort William's pounds 9M scheme will replace aged plants discharging primary treated sewage from 12,000 nearby inhabitants direct into Loch Linnhe. A new outfall is in place and its linked treatment works is already in the early stages of commissioning.

The treatment process at both sites is designed to be 'robust and tested' rather than state of the art. Any loss of production means zero income, so Catchment has more reason than most sewage plant owners to strive for reliability.

Its construction contract with Bechtel and Morrison carries an unusual sting. Instead of walking away from the project after the usual 12 month maintenance period, the contractor will share the pain and gain of the plant's first three years of operation.

A penalty and bonus system, set against pre-set treatment targets, ensures the contractor retains more than a passing interest in the future reliability of the equipment it is now installing.

Here the site team may find its concern is shared by a third party - Europe's largest school of dolphins. More than 50 of these sociable mammals regard the Moray Firth as home and scientists have long debated the degree of harm faced by these top tourist attractions swimming in its polluted waters.

Some say that any reduction in the Firth's biological load would affect the food chain topped by the dolphins. Any shortage of fish could lead to the dolphins moving away from the area.

The counter, and probably stronger, argument is that sores visible on the dolphins are aggravated, if not caused, by the pollution, so any water quality improvement would benefit their chosen habitat.

Either way, when the Inverness system comes on stream in December 1999, the tourist board may well be found taking an unusually healthy interest in its performance. And anxious civil engineers will barely be given time to recover from turn of the millennium hangovers before they start monitoring sewage flows both through the treatment plant and at its outfall.

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