Flocculation on construction projects

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Flocculation is the process of causing small, suspended materials to stick to each other to form “flocs”. These flocs more readily settle out compared to the individual particles.

When soil is exposed during civil construction, stormwater runoff can to pick up the soil particles and carry them to the nearest water conveyance. The larger particles, such as pebbles and sand, will fall quickly to the bottom once the flow rate slows. However, clays and fine silts will tend to stay suspended because they are much lighter and slower to settle out. The resulting turbidity can travel many miles in streams or keep ponds and lakes muddy looking for a long time after a storm.

Turbidity reduces the biological productivity of the affected waters, decreases recreational value, and increases water treatment costs for industrial or drinking water plants. The purpose of flocculants is to treat the water so that suspended clays and fine silts will settle out of the water quickly. Without treatment, it may take weeks or years to settle out.

Construction sites over one acre are generally required to install measures to retain and stabilise the disturbed sediment with in the site. These measures include silt fence, sediment traps, storm drain inlet protection, and others. However, water discharged from these devices can be very turbid from the suspended clays and fine silts that are very difficult to settle out.

The most practical and least expensive option for most situations is flocculation. Gypsum can coagulate or bridge clay particles, which accelerates settling. Flocculants should be used to prevent damage to sensitive water resources such as ponds, lakes and trout streams or whenever turbidity control is required.

Application rates are dependent on the soil type and amount of turbidity, but typical rates of 10 – 100ppm. The best thing to do is a jar or bucket test using slightly varying rates of gypsum.

Regyp supplies gypsum suitable for manual applications in bulk and bulkbags and solution grade gypsum for faster flocculation results especially when used with the Soilution Master mechanical mixing system, pre-application. The best product for flocculation is SuperFlocc SFG which is available in 1.2 tonne bulkbags of pallets of 48x 25kg bags..

Dosage rates for Superflocc depend on the size of the water body/dam and the amount of turbidity. Performing jar tests using our gypsum ar different dosage rates with actual water samples is the fastest way to determine the best dosage rates. General broad dosage rates can vary between 30 – 130 kg per 100,000 lts.

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December 14th, 2009|