Eliminating SS

This is a priority problem in petrochemical plants where settled water can still contain 80 to 150 mg-l of SS or more after biological treatment, often in conjunction with high salinity. 

The most cost-effective process from the standpoint of investment consists in distributing a nonionic or cationic organic flocculant at the inlet of the existing secondary settler. 

As a result, the floe thickens and this brings about a clear-cut improvement in its settleability. Some 30 to 40% of the SS which previously escaped secondary clarification can be retained. Accordingly, a SS content in settled water of 80 to 90 mg l is cut down to 35 to 50 mg l in gasification or petrochemical facilities. Only joint use of an inorganic coagulant, Al or Fe, would make it possible to reach the threshold of 30, but the doses are high and it is not always advisable to enrich sludge in hydroxides when it is recycled to aeration. 

A second process is direct filtration of water after the secondary settler with some possible help provided by an organic coagulant (1 to 2.5 mg l of active matter). Wlien water containing 60 to 70 mg l of SS is to be treated, a filtrate with 30 mg l of SS (efficiency 50%) can be anticipated without coagulant or 20 with a coagulant. If water contains more than 80 to 100 ppm SS, a complete tertiary clarification with a DAF unit is advisable. 

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