In treatment of waste waters, during periods of normal flow the waste water (11) is fed into a de-sanding vessel (2) where sand is allowed to settle out and is collected (22) and separated from the water stream (3), which is given further treatment. During periods of heavy flow the waste water stream is mixed with a coagulant agent (80) in a turbulent zone. It is then dosed with a coagulant agent (80) in a turbulent zone. It is then dosed with a flocculant (90) and ballast sand (64) and mixed (15) with sufficient turbulence to keep the sand in suspension while the colloidal particles aggregate around it. The mixture is then allowed to settle and separate in the decanting vessel (2). The recovered muds are separated (54 and 63) and a proportion of the ballast sand recycled.
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