COAGULATION AND FLOCCULATION OF STORMWATER FROM A SEPARATE SEWER SYSTEM - A NEW POSSIBILITY FOR ENHANCED TREATMENT

With the upgrading of the wastewater treatment plants in Berlin, the s ignificance of stormwater as a loading factor for receiving waters inc reases. Therefore it is important to ensure an adequate treatment of t he stormwater from a separate sewer system. The preliminary tests have shown that for a coagulation and flocculation of the weakly buffered and very soft stormwater polyaluminium chloride (with a share of catio nic flocculant aid), which reacts less acidic, is especially suitable. The optimum dose is 0.06 mmol/l Al3+, whereby the pH value has never fallen below 6. The colloids (as a rule a negative charge) have been d estabilized by the mechanism of adsorption coagulation. Sometimes the mechanism of sweep coagulation came into operation. In the pilot plant investigations it was possible to work with a constant dosage of poly aluminium chloride because the pH value and tile acid combining capaci ty during the stormwater flow remained relatively unchanged. The best enhanced stormwater treatment process is coagulation and flocculation in a pipe designed for both microfloc and macrofloc formation and sepa ration by filtration. In this case, in particular phosphate, organic c ompounds, filterable solids and anthropogenic substances, e.g. lead (a high portion of which comes into receiving waters in Berlin by stormw ater) and copper could be removed. These results indicate that, in gen eral, this process works more efficiently than a stormwater lank where only particles can settle. The results were basically used for a draf t concept for enhanced treatment of stormwater runoff from the motorwa y Avus in Berlin. The following treatment process should be implemente d: coagulation and flocculation in pipes, sedimentation, filtration an d percolation into the underground. Through the storage of stormwater before treatment a smaller treatment plant can be constructed and prob lems due to flow variations may be avoided. The storage should take pl ace in a stormwater drainage system with a larger than normal volume. The cost for this treatment concept is approximately 10 % to 40 % high er than the cost of the treatment in a stormwater tank.