Voluflow overcomes the law of gravity in settling tanks

Having been tried and tested in other industry sectors, the Voluflow inlet converter from Fostech could be the answer to meeting increasing water quality standards and overcoming clarifier problems. WET News takes a look at the system.

Conventional gravity settlement tanks used in water and wastewater treatment appear to have reached a performance plateau that is not as high as theory would indicate. Consequently, when water quality standards increase, design footprint must be increased or throughput reduced.

There is, however, a better way forward - Voluflow, which has already been tried and tested successfully in a range of industries in England and Ireland.

The eight Voluflow units that have been installed so far, whether new build or retrofitted into existing tanks, have proved to be significantly more efficient all-round and more cost-effective.

The Voluflow system is being used for a range of sizes of primary and secondary treatment tanks, the biggest to date being 17.9m diameter. Tanks that benefit particularly well are those handling extremely high concentration suspensions, with high sludge volume index values.

The Voluflow system has been patented and pioneered through Fostech in Northern Ireland by managing director Dr Bill Foster.

Due to the manner in which liquors feed and spread into conventional settlement tanks, adverse flow patterns develop: these include short-circuiting, turbulence, large-scale recirculation, static zones and surface flow intermixing.

Also, notched weirs that are widely used to achieve uniform flow distribution tend to clog with scum, leading to further problems. Notches are also difficult to reach and therefore clean safely. Variable and surging inflows, and high winds across large tanks especially, also distort flow patterns.

All these conditions cause uneven treatment, persistently high concentrations of effluent suspended solids, scum contamination and, worst of all, sludge blanket carryover, which generate serious and costly problems downstream.

Oversizing helps, but it is an expensive route to take and fails to address the basic fault of conventional settlement tank technology - extreme sensitivity to changes in conditions.

Adverse flows
The Voluflow system eliminates adverse flows and their damaging effects. It provides uniform, slow, horizontal cross-flow conditions throughout the breadth and depth of the settlement zone of the tank, where particles descend and thicken.

The upper layers are undisturbed by intermixing currents, and the least concentrated fractions flow selectively to a short, flat-topped weir, which is self-cleaning. The entire system is very stable and tolerant to wide variations in process and other conditions.

There are no moving parts in the Voluflow system; the inlet flow converter is fixed at the centre of the tank and does not obstruct moving scrapers. It occupies less than 10% of the total volume.

Grand design
Designs have improved over the years, the most significant being a much-reduced central Voluflow inlet flow converter installed in a 17.9m diameter activated sludge clarifier at Muntons in Stowmarket, Suffolk. This flow converter sits under the existing sludge return well and occupies less than 2% of the tank volume.

The flat-topped weir is about one-tenth the length of the previously used notched peripheral weir.

Muntons' environment manager, Mick Cochrane, says: "The Voluflow retrofit to this clarifier has never stopped performing well from the day it was commissioned, which is just excellent."

Two more Voluflow units such as this were then installed at TMC Dairies near Strabane in Northern Ireland, resulting in scum reduction, improved weir overflow quality and increased throughput when required. Derek Hetherington, who runs that effluent treatment plant, is pleased with the Voluflow system.

A primary settlement tank at a Northern Ireland Water (NIW) treatment works now it has had a Voluflow retrofit. It is one of a pair of 6m square conical tanks, both of which suffered from periodical blockage of the sludge drain-off pipework. Now the sludge from the tank with the Voluflow retrofit never blocks, whilst the sludge blockage from the conventional tank is still as bad as it was.

Finally, at a wastewater treatment works that is being upgraded at Seahill near Belfast for NIW, one of the two 12.1m diameter final settlement tanks has just been fitted with the Voluflow system. After commissioning, the performance of the two tanks will be compared over several months.

www.fostech.co.ukc

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