InfraStructures - June/July 2007 - Compact Settlement for Highly Seasonal Wastewater Plant

Compact Settlement for Highly Seasonal Wastewater Plant

Chris Taylor, Nordic Water Products AB
Special Collaboration

Every waste water treatment plant is different, so no 'one-size-fits-all' solution is available. Each individual plant must be custom designed to meet the needs of the area it serves - though, of course, the principal processes follow a similar pattern.

The key challenges at Anglian Water's plant at Caister include the dramatic variation in demand. Caister, and Great Yarmouth to which it is virtually joined, are relatively cold in winter - but hot and dry in summer. They are in the centre of the caravan/holiday park/tourist site area of the East Anglian coast, with the Norfolk Broads just inland, and the summer population peaks around 200 000, very much higher than the winter numbers.

The site is also below the level of the sea's natural flood plain - that is, left entirely to its own devices, Caister Waste Water Treatment Centre (WWTC) would be under water. The site has not flooded, but it could, so it has been constructed with some buildings raised a few feet from the ground, and others surrounded by a protective bund, designed to hold back flood waters.

It is a realistic threat. A short drive up to the North Norfolk coast brings you to Weybourne, where coastal erosion has already seen a sewage treatment plant lost to the sea, in an active and planned retreat of the protected tidal barrier.

But this low level in itself also creates other challenges. The site is largely reclaimed alluvial ground, and very soft; weighty structures need to be firmly anchored to prevent them from sinking.

Finally, the 8 ha site is close to residential areas, and has to allow for some possible future expansion. The land is there, as not all the site is currently used, but planning issues require Caister to be as compact a plant as can reasonably be achieved.

A little bit of history
Caister WWTC, on the south eastern tip of Norfolk's coastline, was built on the site of a former local sewage works which undertook preliminary screening then pumped residues first to the edges of the tidal estuary, then by increments further offshore.

"In 1986 the headworks were built for a better scheme," describes Anglian Water's Dave Steward, who is responsible for Caister from the company's nearby Stalham offices. "It was designed to take in all of the Caister catchment area, and part of Yarmouth, and pumped the treated water 4,5 km from the site, ending up 1,5 km out to sea. But this was still just preliminary treatment. Screening removed detritus and inorganic solids, and the rest was pumped away.

"The next substantial change took place over the mid to late 1990s, when the existing plant took shape. The catchment area increased, taking in the rest of Yarmouth, Gorleston, and the villages around. The goal at that time was to meet current and anticipated legislation, with a design life to 2015."

Rob Lewis is the process optimiser for the Caister Partnership which operates the site. "Wastewater flow comes from the headworks past rotating fine drum screens. Then we can dose it with ferric sulphate coagulant to help settlement in the Lamella tanks - we have three of them, each with six plate packs. Secondary treatment mixes the Lamella outflow with RAS (return activated sludge) at the ASP (activated sludge plant) which distributes it between one on four aeration lanes

"From here flow continues to the Final Settlement Tank, where we can remove any scum as well as scrape out any remaining sludge, before the treated water goes on to the outfall pumping station, and out to sea.

"In terms of outputs, during the summer we remove of 14 m3 of screenings per week, and much the same amount of grit; these go to landfill sites. We pump 27 000 m3 of treated water out to sea each day, and truck out up to 60 t/day of thickened dewatered sludge, which has been through pasteurisation and anaerobic digester plants - but only just over half that volume during the winter off-season months. This Biosolids product is recycled on local farmland under the 'Safe Sludge Matrix', current legislation and Maff guidance. The product is sold to farmers, as a soil conditioner and replacement for artificial fertilzer, containing a range of key nutrients and organic matter. It is a design configuration that meets Caister's unique challenges, described above.

The seasonality is addressed by flexibility in the process flow. Initially, flow to the works is governed to permit 850 l/s in summer, and 570 l/s in winter. Six pumps control this; summer settings provide for five working pumps, winter settings provide for up to four. There is also flexibility in the degree of chemical dosing prior to the Lamella separators, and in the degree of aeration employed in the secondary treatment.

The weight and compactness issues are addressed in part by the choice of the Lamella separators. "The main advantage of the Lamella separator is its space saving, offering up to 90% saving on the space taken by conventional settling tanks. They are also proven in this application - reliable, low maintenance, and with low operating cost due to having few moving parts," explains Roger Clark, process director of Cambridgeshire based Vexamus Water, UK agent for Nordic Water Products, who installed the system at Caister in the 1998.

"Caister is a conventional type of sewage works, although the Lamellas are housed in a building. There are 18 plate packs, each providing 119 m2 of protected plate area, with the pvc plates held in an epoxy coated mild steel frame supported on beams across the width of each tank."

Rob Lewis and Dave Steward confirm the Lamella's suitability for the task. "The Lamellas are very effective, but it is their footprint on the site which is the key factor," claims Dave Steward. "The reduction in the number of tanks required, compared to conventional methods of clarification, resulted in a substantial saving in construction costs. Even so, the two concrete tanks at Caister are very heavy, and on this ground it was a challenge to fasten them solidly in place. Each comprises thousands of tonnes of concrete, and each contains over 1000 m3 of water. We had to concrete pile down 27 m to provide a solid foundation, as the ground is soft for the first 24 m."

Whilst the Lamellas are effective, this creates different working practices on site, says Rob Lewis. "They easily outperform their design specification," he says, "removing up to 89% of solids. They are so simple, and so effective. It works so well we don't always need to add ferric sulphate coagulant to aid solids removal, though we treat more in the summer, when the flows are greater, and also that is the sea bathing season when we aim for the highest quality outflow.

The requirement to be able to expand the plant is met through, first, the availability of space. "The site is some 8 ha, but there is capacity within that for new plant," says Dave Steward. "Although the existing plant has a design horizon of 2015, and nothing immediate has been identified as necessary, it's likely we'll need at some stage to expand and enhance the site - perhaps new legislation will drive this. And it's possible we will be looking at new sludge mix facilities at some stage."

The compact nature of the site has driven its design parameters, enabling a powerful and flexible operation to sit comfortably on the edge of a busy tourist area. Effective odor control, alongside efficient running with plenty of spare capacity, makes Caister WWTC a good neighbour, as well as a good business.