Articles & Case Studies

Tertiary treatment of Kendal wastewater

Posted: Monday 8th February 2010

United Utilities’ Kendal works in Cumbria is the latest wastewater treatment plant to use Veolia Water Solutions & Technologies’ Hydrotech® Discfilter for tertiary sewage filtration. The Kendal works serves a population equivalent of 83,000, including contributions from local industries, and has to handle significant storm flows. Final effluent is discharged to the River Kent, a Special Area of Conservation, so the Environment Agency require that phosphate levels are reduced to 1.0mg/l P (80 percentile).

Kendal operates a Covered Oxygenated Activated Sludge Plant (COAST) and this was reducing the phosphate concentration biologically to 1.6mg/l P in accordance with the Urban Wastewater Treatment Directive, although additional facilities were required to meet the new consent. The challenge was taken up by Integrated Alliance North, a consortium of United Utilities, MWH and KMI (Keir Murphy Interserve), who engineered the £10.6m modifications to the works. Their solution was to provide phosphate precipitation by ferric chloride dosing upstream of the COAST plant with a secondary dose applied automatically before final settlement if the phosphate level downstream of the COAST plant is too high.

Phosphate is bound in the biological flocs as well as being present as precipitated ferric phosphate. Tertiary treatment was required to remove these suspended solids to ensure compliance not only with the phosphate consent but also the 4mg/l consent for iron. United Utilities had monitored the performance of the Hydrotech® Discfilter at their Windermere works where the technology showed significant capital and operating cost savings compared to alternative processes when treating final effluent to better than 15mg/l TSS. In addition the footprint was smaller. As a result the company installed Hydrotech® Discfilters at their Holme, Leigh, Glazebury and Coniston works, making it a strong contender for the Kendal project.

The technology was developed and patented by Hydrotech, part of the Veolia Group, who manufacture the units in Vellinge, Sweden. Their first full scale plant was installed in 1995, and there are now over 500 installations in operation globally. In the UK, the first Hydrotech® Discfilter pilot tests were carried out early in 2003, followed by the first full scale tertiary treatment installation in the same year. With over fifteen years of operational experience it is not surprising that Hydrotech is the world leader in the application of this process.

The Hydrotech® Discfilter (Fig 1) consists of a series of hollow segmented discs, which support a woven polyester or stainless steel mesh. The discs are mounted on a central shaft. Influent liquor enters along the centre line of the unit and is distributed radially on the inside of the discs. The flow passes through the mesh leaving suspended solids retained on the inner face of the filter elements. Filtered water is collected in the chamber around the Discfilter and an outlet weir, that is part of the structure, maintains the filtered water level so that the discs are approximately 65% submerged. This retained volume provides backwash water for filter cleaning. Filtered water flows over the weir and out of the unit.

As the retained solids start to blind the filter mesh, the head loss across the Discfilter increases up to a maximum of about 250mm, and the influent liquor level rises. At a predetermined level a backwash cycle is initiated which rotates the Discfilter and presents clean mesh to the influent liquor. At the same time filtered water from the integral reservoir is pumped at high pressure to jet wash the dirty mesh.

The dirty backwash water is collected in a trough within the centre of the Discfilter and flows away by gravity to a pump sump for return to the head of the works. The backwash water volume is typically between 1% and 3% of the maximum design flow. Filtration is continuous throughout the backwash cycle and, once the cycle is completed, the Discfilter stops rotating and the cyle restarts.

The Kendal project uses three Hydrotech® Discfilters with all three normally operating in parallel, although any two can handle the full flow to treatment. To fit in with overall plant operating schedules, the Discfilters are set to backwash automatically, but will backwash on a default timer at times of low flow condition. Dirty backwash water is discharged to a sump formed from a segment of the interstage pumping station caisson, from which it is pumped to the inlet of the COAST plant. Filtrate from the Discfilters is discharged into another interstage pumping station caisson housing the clean washwater pumps that feed the works washwater main, and then overflows to the outfall.

'During the detailed design stage, Veolia Water Solutions & Technologies proposed an alternative version of the Hydrotech Discfilter with an internal overflow which reduced the overall footprint of the tertiary filtration plant. The revised layout created sufficient space for planting a tree screen between the tertiary treatment plant and the site boundary, reducing the environmental impact of the project.

'Hydrotech® Discfilter modules, in four ranges with individual unit flows up to 1,000 l/s allows any range of flows to be catered for using multiple units. The Hydrotech Discfilter can be supplied on stainless steel frames for building into concrete chambers or as stand-alone stainless steel tank units. The modular design makes filter selection simple and allows for accommodation of future expansion. The modular design allows flexibility in choice of materials - ABS plastic or stainless steel for the discs, 304 or 316 stainless steel for frames and tanks, with special alloys also available and a choice of 10 - 500 micron mesh depending on the application.'

Small footprint, low headloss, low backwash water volumes, low power consumption and minimum maintenance by comparison with competitive technologies make the Hydrotech® Discfilter not only cost effective but a more sustainable option.

It is not just the municipal wastewater treatment sector that has found applications for the technology. Hydrotech® Discfilters have also been used for industrial effluent treatment, washwater recovery, algae removal for drinking water treatment, water recovery and re-use and storm water treatment. Units have also been supplied for aquaculture applications in both freshwater and saltwater, and even tropical saltwater in a special alloy version. In fact this robust technology can be used for almost any filtration job where space and cost are key factors.

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