Summary:

The problem of reducing the negative impact of rainfall sewage on ground and water environment has been the subject of research and scientific papers for many years. It becomes more and more frequently an inherent part of planning and projects of sewer systems in the anthropologically transformed areas. Non-point sources of pollution are one of the main causes of water quality degradation. Traffic and washing out the sediments deposited in sewer pipes have the most serious influence on pollutant contents in rainfall runoffs. Therefore, characterization of pollutants in rainfall sewage is necessary for preparing plans of water management in urbanized catchments. The article aims at an assessment of the sedimentation process effect on reduction of pollutants from rainfall sewage. Sewage samples for analyses were collected from the roadside ditches at Pułłanki street in Kraków-Bieżanów in April 2003 and February 2004 during surface runoff observations. After 120 minutes of sedimentation, the following pollution indices were determined in the collected raw sewage samples: BOD₅, COD_Cr and total suspended solids, sporadically phosphates, whereas after 15, 30 and 60 minutes of sedimentation only total suspended solids and COD_Cr were assessed. The analyses revealed the usefulness of sedimentation process for pollutant reduction from rainfall sewage. During the two-hour sedimentation process the reduction of total suspended solids ranged from 58.7 -76.2%, COD from 45.7 – 90.4% and the highest intensity was observed in the first 15 minutes. It was also detected that total suspended solids were correlated with organic substances expressed by COD but these are contained mainly in the fine fraction, the least susceptible to sedimentation. Therefore, an additional treatment of rainfall sewage is recommended to remove the pollutant adsorbed on the finest fraction of suspended solids.