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Pascal Marlin, product manager at Veolia Water Solutions and Technologies, has introduced the Actiflo high-rate clarifier for the treatment of storm water from combined sewer overflows (CSOs).

Changes in weather patterns, whether a result of global climate change or natural variation, have focused the attention of engineers on rainfall and particularly, how to handle storm water.

In most European countries, storm water from urban drainage systems is generally handled by CSOs, resulting in large quantities of rainwater that are potentially contaminated by domestic sewage and industrial wastewater.

Where space constraints allow, this water may be held in storm-water tanks and then pumped to the sewage treatment works after the storm event.

Where there is insufficient storage, the contaminated rainwater is discharged untreated to surface water courses and sometimes even to bathing beaches.

Even where drainage systems have been designed for rainwater harvesting, the water requires treatment to reduce the organics and mineral pollutants diluted with surface run-off.

Typically, treatment systems are physico-chemical or biological and in storm conditions they are likely to be overwhelmed by high flows, albeit for a short duration.

Treatment of these intermittent, short-term high flow rates presents a problem for engineers.

Because the flow rates are potentially very high, conventional treatment plant such as grit channels and clarifiers, designed for a 50- or a 100-year storm condition, need to be over-designed, expensive and are likely to have a relatively low utilisation.

This means that they are difficult to justify economically.

Furthermore, if they are to be effective, they have to start up very quickly and with little or no warning; not a mode of operation conducive to good performance in these types of plant.

Over the last few years, Veolia Water Solutions and Technologies has been working with utilities in the UK and worldwide to assess the suitability of its Actiflo high-rate clarifier for the treatment of storm water from CSOs.

A ballasted settlement process such as Actiflo fits the profile because the addition of a ballast material – microsand in the case of Actiflo – reduces the time required for flocculation and provides settlement velocities that are about 10 times faster than those possible in conventional lamella settlement systems.

This combination accelerates start-up, bringing the Actiflo up to full performance within a few minutes.

The Actiflo process uses the same principles as conventional gravity settlers.

It is used mainly as a clarifier for drinking water and process water production or as primary and tertiary settler for municipal and industrial wastewater.

To date there are 32 Actiflo plants treating storm overflow.

Actiflo is a physical-chemical treatment process employing coagulation and flocculation in conjunction with a conserved dense particulate phase (or ballast) to enhance solid-liquid separation.

The principal benefits of ballasted flocculation are related to two physical effects induced by the presence of the microsand.

The high specific gravity ballast (normally fine sand with a specific gravity of 2.65) weights flocculated particles leading to solids-settling velocities more than 10 times faster than conventional physical-chemical flocculation processes.

The high collision frequencies between the ballast and flocculated particles increase flocculation kinetics, resulting in short mixing times.

These two effects combine to yield a small-footprint solution with total hydraulic retention times of less than 10 minutes and solids removal efficiencies exceeding conventional physical-chemical processes.

The high collision frequencies associated with differential sedimentation result in a reduction in mixing times compared with conventional flocculation processes.

Whereas conventional physical-chemical processes flocculate within approximately 15-20 minutes prior to sedimentation, the ballasted flocculation process flocculates within 2.5 minutes.

A coagulant – typically a ferric or aluminium salt – is added and this is followed by three stages of mechanical flocculation.

The first and second chambers are equipped with rapid mixers to disperse the coagulant and mix the recirculated microsand with the first addition of polymer.

Pin flocs are developed in the flocculation chamber where a slow mixer provides the energy required to bind the microsand to the flocs via second polymer addition.

The result is a dense floc, ballasted by very fine silica sand, which has a settling velocity of circa 120m/h for CSO applications, making the plant footprint extremely attractive in comparison with conventional clarifiers.

Clarification is further enhanced by the use of lamella plates, which is a series of plates set at an angle of 60deg in the sedimentation chamber.

The plates effectively increase the available settling area in the chamber.

The mixture of settled sludge and microsand is collected by a circular scraper at the invert of the sedimentation chamber 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 while the underflow containing the sand is recovered and re-used.

The overall effect of these improvements to the settling process is a small footprint, high-capacity plant that has a very short start-up time and meets the criteria required for treating storm water as it is produced.

In the UK, a 200m3/h Actiflo pilot plant was operated for 12 months at United Utilities’ Millom works on the Cumbrian coast in the Lake District National Park.

Some 90 trials were carried out on the pilot plant over the 12-month period; 50 under storm conditions and the remainder taking stored storm water from the works’ storm-water tank.

In every case, the effluent UV transmissivity was consistently high enough (greater than 50 per cent) for an ultraviolet disinfection unit downstream of the clarifier to meet the microbiological consent for discharge to a bathing beach or shellfish water.

Both ferric and aluminium coagulants were trialled, and both gave suspended solids reduction in excess of 90 per cent, producing effluent suspended solids below 25mg/l; a figure that was reached within 10 minutes of start-up.

COD reduction was generally 60-70 per cent and total phosphorus removal was in excess of 80 per cent.

A shorter-term trial was carried out, this time over 52 days, in Germany starting in mid-January 2008.

A mobile pilot plant was located in the car park of the Berliner WasserBetriebe office building in BellermannstraBe, Berlin.

It was situated approximately 5m away from, and running parallel to the CSO chamber installed at this location.

A temporary submersible pump was installed to take water from the chamber and deliver it to the inlet of the Actiflo unit.

Clarified water from the Actiflo unit was discharged under gravity to the adjacent sewer, together with the sludge from the hydrocyclone.

During the trial period, 10 runs were carried out: the first with simulated raw water produced by diluting raw sewage with mains water in the CSO chamber, the rest of the runs were conducted during actual storm-overflow conditions.

Mirror velocities were up to 90m/h and were limited by the maximum available flow of 100m3/h.

In each run, ferric chloride was used as the primary coagulant without any pH adjustment, the dose being increased in the range 10-70mg/l as Fe as residual colour increased.

Various polyelectrolytes were tested: one cationic, one non-ionic and three anionic.

Neither the cationic polymer nor the non-ionic were effective but the anionic products, dosed in the range 1-2mg/l increasing as the effluent suspended solids increased, proved effective in reducing effluent turbidity.

The pilot plant results from the Berlin trial confirmed the results from the Millom trial, namely a better than 60 per cent removal of COD, more than 80 per cent reduction in total suspended solids and more than 80 per cent reduction in total phosphorous.

They also confirmed that this performance can be achieved within 10 minutes of start-up.

The philosophy behind sustainable urban drainage systems is to mimic as closely as possible the natural drainage from a site before development and to treat run-off to remove pollutants.

The pilot trials carried out by Veolia Water Solutions and Technologies have demonstrated that the small footprint, rapid start-up and high removal efficiencies achieved Actiflo make it a major contributor to the implementation of sustainable water management by treating CSOs to standards high enough for discharge to surface water courses or for the pre-treatment of harvested urban storm-water run-off for further treatment and re-use.

In combination with downstream ultraviolet disinfection, Actiflo is capable of meeting the microbiological standards for discharges to bathing and shellfish waters and for grey-water-recycling applications.

Veolia Water Solutions & Technologies

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