NEW REGULATIONS FOR THE TREATMENT OF COMBINED SEWAGE IN AUSTRIA - BASED ON MINIMUM REQUIREMENTS AND WATER QUALITY CRITERIA

摘要

The minimum requirements for receiving water protection from combined sewer overflows laid down in the draft of the new regulations for Austria result in a significant reduction of the discharges of raw wastewater, suspended solids and heavy metals from urban sewer systems. For this reason it is not advisable to design CSOs on the basis of water quality criteria alone. However the minimum requirements for CSO design have to be characterised by a reasonable cost-benefit ratio. It was recommended to initially set low requirements (e.g. minimum efficiencies requiring specific volumes of storage tanks not exceeding 10-15 m~3/ha impervious area) for CSOs located in less sensitive ("normal") waters, and to make provision for potential upgrading. Development of minimum requirements in form of percentage load reduction requirements (instead of construction guidelines) for the entire catchment (instead of single CSO structures) makes planning significantly more flexible. A new version of the guideline of the Austrian professional association will define criteria (which can be calculated or measured with reasonable effort) in order to identify critical cases, in which the minimum requirements might not be sufficient to prevent the aquatic biota from being impaired significantly. However, combined sewer overflows are often located in urbanised rivers and investigations in several small rivers in Austria clearly showed that under these conditions aquatic organisms get impacted by other factors than CSOs. Thus in many cases it might be much more effective to improve the river morphology and to reduce permanent dry weather discharges than to invest in large stormwater tanks in order to abate acute water quality problems.