Polymer flocculation

Discharging turbid water with excessive levels of suspended solids is a serious problem for construction sites, particularly if it enters stormwater drains or a natural water body as it can seriously degrade the habitat of fish and other aquatic life, or cause silting and increase flood risk. As a result, to do so is an offense under UK legislation. However, whilst coarser particles can be readily removed by filtration or sedimentation, finer particles will often remain in suspension with the slightest water movement making them very difficult to remove without treatment.

Turbidity in water is measured using a nephelometer, which passes a light beam through a sample of water, and measuring the scattering at a detector set at 90 degrees to the source, reported as “Nephelometric Turbidity Units” or NTUs.

Flocculation is the process of using chemical agents (flocculants) to bind together small soil particles into larger ones (flocs) that are heavy enough to settle to the bottom of the liquid for removal. Introduction of positively charged (cationic) polymer macromolecules into turbid water attracts the naturally negatively charged clay particles, clumping into a floc having sufficient mass to sink to the bottom of the liquid. However, although cationic flocculants are highly effective in isolated systems, their positive charges make them toxic to aquatic organisms when dissolved in water, and they should NOT be used when runoff could enter stormwater drains or open-water bodies.

Anionic polymer flocHowever, anionic polymers, which carry a negative charge (like clay particles) are not toxic, and if added to stormwater together with positive calcium ions (Ca++) to form ionic bridges, anionic polymer flocculation will take place, reducing the turbidity of the treated water without harming aquatic life. Once the flocs have formed, provided the flow rate is sufficiently low, they should settle to the bottom of the water body ready for removal.

For polymer flocculation to be effective on site, three fundamental process must take place: chemical binding, settlement, and floc removal. To bind particles, polymers can be applied directly to soil surfaces, to water flowing in a collection channel, or into a settling pond, either through impregnated jute mats, hand or mechanical spreading of dry polymers, or direct application of liquid polymers. Following treatment, the objective is then to reduce the velocity and erosive force of the water by allowing it to spread out over a relatively level area, aided by perpendicular wattles, silt fences, and impregnated jute matting. Once settled, the sediment can be removed for reuse or disposal, and the now-clear water discharged to stormwater systems or open-water bodies.

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