Actiflo triumphs in the Scottish Highlands
In the decade since Veolia introduced its Actiflo process, it has proved invaluable for a range of applications worldwide. And when Scottish Water needed to uprate a rural treatment works, it proved a winner once again.Scottish Water provides water and wastewater services to around five million customers spread over an area of 79,000km2 - less than a tenth of the UK's population in a third of its area.
One of the company's many rural works is Newmore Water Treatment Works (WTW) in the Scottish Highlands near the cruise port of Invergordon. It supplies 9Mld of drinking water to a population of about 45,000.
The raw water source is a typical Upland river characterised by high colour as a result of the peat / moorland in the catchment area. It is a "flashy" source with very rapid increases in natural organic matter, particularly following rainfall after a prolonged dry period.
Coagulation and flocculation can remove about 70% of the organic matter, but because the turbidity is relatively low the resulting flocs are small and low in density, so settling velocities are slow. Sudden increases in floc loading resulting from these flashy sources mean that conventional sedimentation processes are prone to carryover, and dissolved air flotation has historically been the preferred clarification process.
Actiflo is a high rate clarification process that increases the particle density by incorporating high density sand into the floc. This ballast increases the settling velocity by at least an order of magnitude of 40 times, thus making Actiflo one of the most compact clarifiers on the market.
Mixing stages Veolia Water Solutions & Technologies (Veolia WS&T) first introduced Actiflo ten years ago, and now has 500 installations worldwide in applications ranging from drinking water clarification to sustainable urban drainage systems (SUDS) and from phosphate removal to power stations.
The Actiflo process consists of three mechanical mixing stages before the settlement. The first stage provides rapid mixing of the coagulant, just as in conventional clarification, but the second stage is where the recirculated microsand is added and mixed along with polyelectrolyte.
The third stage provides slow stirring to grow large flocs around the microsand particles. The result, even with moorland waters, is a dense floc with a high settling velocity.
The high rate settling stage uses lamella plates to increase the available settling area and minimise the footprint. The mixture of settled sludge and microsand is collected by a circular scraper and pumped continuously to a hydrocyclone, which separates the sludge from the sand.
The lower density sludge is discharged in the overflow from the top of the hydrocyclone whilst the underflow containing the sand is recovered and re-used. The overall effect is a clarifier operating at surface loadings of up to 120m/h with a footprint something like 5% of that of a conventional sedimentation clarifier for the same duty. A further advantage of the ballasted floc process is a short flocculation time, which means rapid start-up.
Scottish Water had previous experience of Actiflo technology when it hired a mobile plant from Veolia WS&T at the 1Mld Dornoch Water Treatment Works while essential maintenance was carried out on the single stream dissolved air flotation plant.
During that period the plant worked well and demonstrated the ability of Actiflo to cope effectively with upland waters in comparison with dissolved air flotation. On the basis of the reduced footprint giving cost savings, and increasing process security, the Actiflo process was specified for Newmore WTW.
Colour events The Newmore WTW needed to be uprated to meet increasing demand and have the ability to maintain high production levels during colour events. Actiflo had proven capable of meeting this requirement and its ability to start up and achieve treatment requirements within minutes made it more versatile than the original clarifiers.
Veolia WS&T provided the process design, construction, installation, testing and commissioning of two packaged Actiflo high rate clarifiers. The project included the microsand storage and transfer system, polymer make up and dosing system, coagulation and pH correction system, MCC and software for the plant, which was commissioned and taken over in December last year.