Vízügyi Közlemények, 2004 (86. évfolyam)
3-4. füzet - Gayer József: A települési csapadékvíz-elhelyezés aktuális problémáiról
A települési csapadékvíz-elhelyezés időszerű problémáiról 449 of earlier pollutant deposits of the sewers and the wet and dry deposition of pollutants from the atmosphere. All these result in heavily polluted runoff water in the first period of the runoff (called first flush), instead of the "clean precipitation water" that people like to believe, and its level of pollution might easily exceed that of the household sewage. This washoff load might cause a "stress-like" impact on the recipient stream, especially when the latter does not provide sufficient dilution and consequently the urban runoff water must be treated. A summary illustration of the impact of urbanisation is shown in Figure 2. Opinions on the need for treating urban precipitation runoff water have gone through significant changes in the second part of the 20 t h Century. After the Second World War the quantitative approach was dominating in the industrially developed countries, namely, the runoff water of paved areas was to be conveyed away in the possible fastest way. The result was that the groundwater table sank below the cities due to the lack of recharge from surface waters. The idea of creating dual (separated) sewer systems was born in the era of this quantitative approach (Figure 3). In the handling of urban runoff waters the notion of "getting rid of the staff as fast as possible" was changed in the past two decades and replaced by the "keeping the water as long as we can", eventually bearing in mind the avoidance of inundation problems. The methods of decreasing runoff quantities and delaying its occurrence can be split into two basic groups: - Handling the water right at the site of occurrence (source control); - Downstream (or in public facility) treatment at some point of the sewer system. The handling of precipitations waters at the site of their occurrence or within the process of surface runoff offers tools for the sustainable use of natural resources. These methods can be sorted into the category of best management practices (BMP). In achieving adequate groundwater recharge the following infiltration methods are known: "grassed channels or biological filter zones (Figure 5,) infiltration ponds, underground seepage storage zones (Dutch drains), porous permeable pavement (Figure 6) " With the adequate control of draining facilities and the related other structures runoff waters may be stored on impermeable surfaces where these pools of water will not cause inconveniences, such as the flat roofs of buildings and in car parking lots (Figure 7). Essentially stormwater storage ponds or basins are characterised by inflow channels of higher water carrying capacities than the outflow structures. These ponds are usually of multipurpose character, such as: - Control of quantities (decreasing the peak-discharge of the system) - Sediment catch basins; - Removal of pollutants via the uptake-screening function of pond vegetation Good examples of such ponds are found in Győr and Kecskemét (Figure 8) where the facilities were originally designed for multipurpose use. In-line or off-line storage facilities can also be constructed at the sewage treatment plants, an example of the latter is shown in Figure 9. A stormwater management scheme designed for quantitative control can also serve for quality control (within the reservoirs). Plants equally tolerant to inundation and dryness may well serve the joint control of quantity and quality of the runoff waters (Table Г). Different methods can serve the removal of various pollutants from the urban runoff waters and the optimum solution can be found after having the type of pollutants identified. The hierarchy of managing-treating urban storm-runoff waters is shown in Figure 10. In the first treatment stage the rough debris and street-litter shall be removed by screening or rapid sedimentation. (Figures 11 and 12). In the next stage finer sediments will be removed by sedimentation and filtration. This process will also remove certain portions of the pollutants that are attached to sediment (e.g. nutrients and metals). The dominating processes of the third treatment stage include intensified settling, filtration, biological uptake and adsorption. This would ensure the removal of further fractions of nutrients and metals (Figure 13). These processes correspond more-or-less to those of the sewage treatment. In most of the cases the combination of various methods will give the best results (Table II). This table shows the potential removal efficiencies of various pollutants by various methods.
