Two Aldec G2 decanter centrifuges from Alfa Laval have helped Yorkshire Water clear a backlog of sludge in just six months, by outperforming humus belt presses and producing a cake with 25% DS.

How do you clear 50000 m3 of sludge in 6 months? Two Aldec G2 decanter centrifuges from Alfa Laval have helped Yorkshire Water avoid a potential environmental problem at its Esholt sewage treatment Works by doing just that: clearing a backlog of 50,000 cubic metres of sludge in just six months. Covering no less than 340 acres and serving the Bradford area, the Yorkshire Water Esholt Sewage Treatment Works is the largest filter site of its kind in Europe.

Its complex of mineral filters handles the sewage generated by a total equivalent population of 600,000 in West Yorkshire and produces a humus sludge that is treated, dewatered and then mixed with wood waste to produce a rich conditioned sludge - which is then used for either agriculture or land remediation.

Until last year, this job was performed by three traditional belt presses which produced a dry cake of 18% DS on new humus and slightly less than that from old humus.

Unfortunately, the belt presses never achieved the levels of throughput for which they were originally specified and, as a consequence, a backlog of humus was continuously building up.

Matters came to a head last year when a series of breakdowns exacerbated the situation and left Yorkshire Water with the problem of how to clear an accumulation of 50,000 m3 while still continuing to cope with an average daily output from the lagoons of around 1400 m3.

Yorkshire Water has an enviable reputation for environmental awareness and, in fact, earlier this year picked up the title of Utility Company of the Year at a prestigious awards ceremony in London.

They were, naturally, anxious to clear the accumulated sludge as quickly and efficiently as possible.

'We were facing a potentially major environmental problem, 'confirms Ben Roche, Yorkshire Water Catchment Manager: 'The Belt Presses simply couldn't provide the level of performance we needed to keep pace with the output of the lagoons and the store of treated humus was reaching crisis proportions'.

Having decided that the belt presses were unlikely to provide the answer either to the immediate problem of the accumulated humus or the predicted increase in demand, Yorkshire Water decided to use the situation as an opportunity to appraise the options open to them.

For this purpose, Alfa Laval provided a test rig consisting of a standard Aldec decanter centrifuge mounted in a trailer.

Tests were carried out at Esholt and two other Yorkshire Water sites to compare the rig-mounted decanter performance against the installed belt presses.

After reviewing the test results and confirming that the decanter centrifuge had outperformed the belts by a considerable margin, Yorkshire Water opted for two Aldec G2 decanter centrifuges which Alfa Laval had just recently introduced.

'We looked at a number of decanter options but the specification of the G2 looked tailor made for our situation,' says Ben Roche: 'As important, given the need for continuous operation to clear the backlog, was the decanter reliability and Alfa Laval's quality of service'.

More compact than its predecessors, the Alfa Laval Aldec G2 incorporates a new gearbox that provides more power for less energy consumption because it maintains high torque levels over a wide range of differential speeds.

These, in turn, are governed by the volume and composition of the solids in the sludge.

With a drive system that allows both the bowl speed and the conveyor speed to be controlled independently of each other, the Aldec G2 also has the lowest power consumption, under all conditions, of any sludge decanter centrifuge currently available.

Once installed and commissioned, the pair of Aldec G2 decanters were put to work clearing the 50,000 m3 of stored humus while still handling the daily output of the lagoons.

The machines run continuously, 24 hours per day, 365 days per year, each processing an average of 30 m3 of humus containing 4% DS every hour and producing a cake with 25% DS.

They have proved equal to the task not simply in terms of clearing the backlog, but also in the improvement in cake dryness and quality which makes it easier to handle and store.

The dewatered sludge is mixed with wood waste, stored in windrows at a rate of 2 tonnes per square metre and then left to compost.

Conditioned sludge is then spread across prepared areas on the site up to a foot deep where it is seeded with grass to remove more bacteria and also thicken the product prior to its final destination, after harvest as a valuable agricultural fertiliser.