How to dose Polyaluminium chloride into ···

Filtration through a porous medium such as a sand bed is an efficient and cost effective means of reducing water turbidity. But sand filters are very limited in their ability to remove organic and some insoluble impurities.

Without coagulation removal rates in a single pass of a medium-rate filter were about 50%. This was dramatically improved, to over 90%, by dosing polyaluminium chloride.

- How to dose
For a pool with a capacity of 450m³ (100,000 gallons) and a turnover period of 2 hours the circulation rate is 225m³ per hour.

The PAC dosing rate for this pool would be 22.5ml per hour. This is a very small quantity (two and a quarter 10ml test tubes per hour) and is difficult to dose accurately.

Most chemical dosing pumps used for dosing the other chemicals used in swimming pools are not capable of adding this small quantity accurately and continuously. The most suitable type of dosing pump is a peristaltic pump.

- Dosing rate
PAC is directly and continuously dosed from the containers in which it is delivered at a rate of about 0.1ml/m³ of the circulation flow. (It is very important that it is not diluted with water as it will form a gel.)

This must be mixed as evenly as possible throughout the water flow.

- Where to dose
Following dosing, coagulation of pollutants happens quite quickly. Flocculation is a slower process, where the coagulated particles come together in flocs, so it is important that there is sufficient time before the water reaches the filter media.

At least 10sec is recommended, at a flow velocity not exceeding 1.5m/sec. This can normally be achieved in the space in the filter above the filter media.

But to ensure the best possible mixing, PAC should be injected as far before the filters as possible – but after the place where the sample for the chemical controller is taken.

PWTAG recommends a normal dosing rate of 0.1ml/m³ of the total flow rate, which it has also described as 0.005ml/m³ as aluminium.

- Summary
PAC should be dosed as far upstream of the filters as possible, but after the chemical controller’s sample point.
PAC should be dosed continuously using a peristaltic pump.
All grades of PAC should be dosed at a rate of 0.1ml/m³ of the total flow rate.

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