With the upgrading of the wastewater treatment plants in Berlin, the significance of stormwater as a loading factor for receiving waters increases. Therefore it is important to ensure an adequate treatment of the 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 cationic 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 destabilized 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 polyaluminium chloride because the pH value and tile acid combining capacity 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 separation by filtration. In this case, in particular phosphate, organic compounds, filterable solids and anthropogenic substances, e.g. lead (a high portion of which comes into receiving waters in Berlin by stormwater) and copper could be removed. These results indicate that, in general, this process works more efficiently than a stormwater lank where only particles can settle. The results were basically used for a draft concept for enhanced treatment of stormwater runoff from the motorway Avus in Berlin. The following treatment process should be implemented: coagulation and flocculation in pipes, sedimentation, filtration and percolation into the underground. Through the storage of stormwater before treatment a smaller treatment plant can be constructed and problems due to flow variations may be avoided. The storage should take place in a stormwater drainage system with a larger than normal volume. The cost for this treatment concept is approximately 10 % to 40 % higher than the cost of the treatment in a stormwater tank.
