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The Wight stuff

ENGINEERING WATER Sewage treatment

Costs on the Isle of Wight's state of the art sewage treatment works at Sandown have rocketed by £30M.

Foot and mouth disease and a once in 350 year flood have been the least of the problems faced by the builders of an important new sewage treatment works at Sandown on the Isle of Wight.

The Sandown Joint Venture team of Skanska and Brown & Root (SJV) has had to deal with more than 350 changes in its five year involvement with the project. Some have had a major impact on the scope of the job.

After six months on site the JV had to regroup to cope with a 100% increase in the volume of sewage to treat and a 30% rise in the amount of sludge to handle.

Two years in and it was faced with a plan to introduce secondary treatment into the process.

'Our original contract price was £42M, ' says SJV site manager Phil Hosker. 'Now we are at £72M.'

Not surprisingly, the group working on the job is a tight knit unit, able to react to just about anything. More surprisingly, it still seems to be getting on well with the client, Southern Water.

'The changes have been phenomenal. I've never known a job like it, ' says SJV contract manager Len Small. 'We have really had to drive this on. We've been forcing decisions out in the end.'

Relations between all parties are on an even keel, he says, because the job has not operated under a partnering arrangement.

A partnering scheme would have imploded under pressure of all the changes, he believes.

The 60:40 Skanska/Brown & Root JV is working under a heavily modified Green Book target cost contract. The contractor is bearing all the risk but there is a very structured strategy for managing, and paying for variations, which has kept tempers reasonably sweet, and the work cracking on.

'We have ended up with one man fully employed running the change control process and there is a very quick turn around on variations, ' says Small. Early warning alerts have been raised and discussed as a priority with the client to decide if something is a change, and if it is, what the cost implication is. Decisions as to whether to go ahead or not have been taken quickly and there have been few disagreements and no outstanding variations to resolve, which is good going, given the project is due to complete next month.

The changes are not the result of whimsy from client Southern Water, but response to changing demands from the government and decisions to parcel up new work into the scheme now to save money in the long term.

The works at Sandown are part of the £200M Seaclean Wight scheme. This is collecting sewage from around the island that used to be discharged virtually untreated into the sea, and pumping it from eight transfer pumping stations via 60km of rising mains and gravity sewers to one state of the art treatment works. Once treated, the flow is discharged out into the English Channel through a new 3km outfall.

This discharge into the English Channel was crucial when the site for the works was chosen. At that time, the English Channel was designated a high natural dispersion area (HNDA) which meant only primary treatment would be needed to meet the demands of the Bathing Water and Urban Waste Water Treatment Directives.

But two years into the programme government redesignation meant full secondary treatment was required, adding an extra £10M to the scheme.

Prior to that - in fact nine months after detailed design was completed and six months into construction - the introduction of the Shellfish Waters Directive encouraged Southern Water to treat more of the island's effluent at Sandown than originally planned.

At the same time the local council lodged an objection to the layout of the plant, wanting the taller buildings relocated to in the middle of the plot rather than at one end. It made the site look more appealing, but says Hosker, 'it was not the ideal flow pattern - there are kilometers of extra pipe looping around underground.' Total cost to the project of all this was £7M.

In for a penny, in for a pound, Southern Water decided to add in sludge dryers to the facility to recycle the waste rather than dispose of it. Six months later, the dryer contractor was changed and the new dryer, from Andritz, required a different shaped building to that planned.

Skanska/Brown & Root had to butcher the prefabricated structure they had erected to house it.

That cost another £6M, money to be recouped from sales of sludge pellets to local farmers.

Land remediation work cost £5M and other changes account for the rest of the £30M project increase.

Construction has not been without its challenges either, though things started well.

The new treatment works is sited on the old town dump andsome 84,000m 3of category C and D waste was excavated along with 1,000m 3of Japanese Knotweed. All but 3,000m 3isdumped to the side of the site in a screening earth mound stabilised with clay retaining bunds, nicknamed Mount Etna.

The rest was taken off to landfill.

The big issues for the site management have been getting plant and equipment on to site, finding enough concrete locally to get the works built, and finding accommodation for the workforce.

It might not be a long trip across the Solent to the Isle of Wight but it is an island nevertheless. Cranes and piling rigs all came in by boat and their passage had to be booked well in advance as they took up the whole of the ferry.

'Likewise, to get the Lamella settlement tanks (see box) up and running we had to import 15,000m 3of sludge from the mainland, ' says Small. 'That took three tankers, eight drivers and 10 days.'

The Sandown project also exhausted the island's concrete supply when work was at its peak. 'Bringing in our own batching plant was not viable, but asking for any more than 80m 3here was unheard of, ' Small says. Pours had to be carefully planned.

'We bought extra trucks in from the mainland but they had to negotiate holiday traffic, especially caravans, and the island's 30mph speed limit, ' Small says. There were days when the site team were biting their nails to see if the ready mix would get from the plant at Newport to Sandown before it went off.

Foot and mouth, when it hit, actually created few difficulties at Sandown. The floods of 2000 did however. 'The site is below sea level and the floods here were the worst for 350 years, ' Hosker says.

'We thought the water would drop but after a month we realised it wouldn't and we were effectively working under water, ' Small says.

'We could pump the water from one area to another but we couldn't get rid of it. At some times the cable pullers were working with their hands under water.'

SJV reckon the floods cost it six weeks on the programme, which had to be pulled back by extended hours and weekend working, and extra labour. The labour force peaked at the height of the holiday season, which meant accommodation was a problem.

'We took over a holiday caravan site and in August, expecially Cowes Week, we had people staying in tents in rotation. Good tents, ' Small reports.

SJV is now on the last lap and expects to quit site next month.

Liquidated damages of £9,000/ day kick in at the end of the year 'so there is no question we are not going to hit our deadline', Hosker says.

And good golf courses not withstanding, everyone on the project is ready to return to the mainland.

Phased improvements The Sandown plant is being built and installed to treat effluent from a population equivalent of 125,000 but the kit can handle the 180,000 population predicted for the island by 2015.

Sailing events and a busy holiday season mean the number of people on the Isle of Wight fluctuates dramatically - between 100,000 and 140,000 - which has a noticeable impact on the flows going through the first phase of the Sandown plant, which opened in May this year.

'There was a dramatic fall in load after the Americas Cup for instance, ' says SJV contract manager Len Small.

The project has been built in three phases. The first was to build the sewage treatment works, the sludge recycling facility and outfall is up and running. Secondary treatment is being installed under phase two, and introduction of a sludge dryer is phase three. The works treat 180 litre/s of effluent and have the capacity to cope with 3,119 litre/s in storm conditions.

Primary treatment uses lamella plate settlement tanks. Solids removed from the flow are combined with imported sludge from rural treatment works and cess imports and retained in digesters for 12 to 14 days to biodegrade. Digested sludge is further dewatered and then enters the sludge dryer. Treated effluent is discharged through the long sea outfall.

When the secondary works kick in, effluent from the primary treatment will pass through biological aerated flood filters, using a DegrÚmont patented process to treat the effluent to more stringent levels.

Effluent from primary treatment will be mixed with air and flows upward through a natural expanded clay material. Aeration encourages biological growth, which reduces the solids, and bacteria content of the effluent.

The BAFF units will backwash daily to prevent build up of solids and any waste sludge dislodged is pumped to the primary settlement tanks where the solids are settled out and passed to the sludge stream.

Phase one has reduced the amount of solids entering the sea by 50%. When phase two comes on stream that will increase to 85%, making a dramatic difference to water quality around the island. Holidaymakers this year, however, were already commenting how much clearer the sea was, the JV says.

Who s who Client: Southern Water Contractor: Sandown Joint Venture (Skanska/Brown & Root) Mechanical installation: Amec Electrical works: EI-WHS Dryer: Andritz Outfall: Van Oord

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