Good practice:

Sequential biofiltration and sedimentation systems for the purification of a small urban river.

The practice is situated in the city of Lodz, central Poland, whose hydrographic network consists of several small streams or rivers. One of these waterways is the Sokolowka river, flowing in the north-western part of the city. The Sokolowka river catchment area is 45.95 km2, and the total length of the watercourse is 12.8 km. The total length of the separated stormwater drain system connected to the river is three times that of the river itself, with over 50 outlets.
The SSBS is located of the left bank of the Sokolowka river, and connected by a side channel; the main channel of the river still operates as a “bypass channel” to prevent hydraulic overload and erosion damage, and to prevent flushing out pollutants stored in the SSBS during periods of high flow. During normal and high-flow periods, the SSBS system works as a free water surface wetland (FWS CW), while during winter, when ice covers the sys-tem, it acts as a horizontal sub-surface flow wetland (HSSF CW).
The SSBS consists of three zones: sedimentation zone with structures added to improve sedimentation, a geochemical barrier made of limestone deposit and biofiltration zone.
The main objectives (desired outcomes) of the application this technology is protecting of the cascade of downstream reservoirs against pollution from rainwater drainage, limitation of eutrophication of observation reservoirs, water retention for the improvement of microclimate.

Materials and methods: The SSBS is divided into three zones:
1)An intensified hydrodynamic zone,
2)An intensified biogeochemical processes zone ,
3) A post-treatment zone.
Costs: Hydrophytic treatment plant – cost of formation-220 000 dollars/cost of management per year – 300 dollars.

The results indicate that the sedimentation zone played a fundamental role in the removal of Total Suspended Solids and nutrients.
Monitoring conducted in the first two hydrological years after construction indicated that the SSBS removed 61.4% of TSS, 37.3% of TP, 30.4% of PO34 , 46.1% of TN, 2.8% of NH4þ, 44.8% of NO3 and 64.0% of Cl .

The use of constructed wetlands (CWs) is an example of a nature based-solution which has been successfully used for the retention and purification of stormwater runoff.
Simultaneously with urban applications, work was carried out on a sequential sedimentation and biofiltration system as a further stage of municipal sewage treatment in small sewage treatment plants. They can also be used in fish pond drains or on small watercourses and ditches in agricultural areas.
The system in Sokołówka is a prototype system - the first implementation. Experience from this solution has been transferred to solutions applied in Arturówek in Łódź, to solutions developed in Radom, and to the system developed in Gniezno.