Sustainable solutions for scattered settlements in Kaszuby Lake district
Solution to which problem
In Polish countryside, the difficulties with wastewater management are mainly caused by dispersed settlements, located far away from the conventional sewage system, which results in high investment costs for new sewerage lines. Another problem is a construction of sewage systems without any wastewater treatment plants (WWTPs). Although, the number of WWTPs serving for villages increased from 433 to 2 341, from 1995 to 2010 respectively, still only 25.7% of rural population was connected to sewer system and municipal WWTPs in 2010. Due to the above facts, there has been an immense interest in developing and establishing decentralized wastewater treatment systems (DWWTSs).
Primary treatment of sewage takes place in septic tanks. Then the wastewater is discharged to treatment wetlands working with three configurations. The sludge accumulated in septic tanks is periodically removed and processed in reed draying beds, where intensified natural processes take place. The reject water generated in the dewatering process is reticulated to treatment wetlands and treated together with wastewater. The dewatered and stabilized sewage sludge becomes a valuable humus substance, that can be used as soil fertilizer at the farm lands.
Three different configurations of subsurface (SSF) bed were applied to exam whether to use vertical or horizontal flow beds in Polish countryside conditions. In all configurations septic tank was applied as a mechanical treatment and last stage of treatment was performed in a pond. In configuration I: single vertical subsurface flow (VSSF) bed with the unit area of 4 m2/PE was applied while in configuration II sequentially working VSSF beds were applied with total unit area of 2.5 m2/PE. Moreover, the configurations with singe VSSF bed and two VSSF beds working in series were tested to investigate how double contact time affect the pollutants efficiency removal. The configuration III was designed in accordance to the Norwegian principles: pre-filter followed by horizontal subsurface flow bed. Instead of LECA domestic material like Pollytag was used.
The proposal of an innovative sanitary system is based on an idea of a closed cycle of matter in the environment. Sewage from a single household is treated at individual treatment plants based on treatment wetland technology. The benefit of applying the treatment wetlands is the lack of excessive sludge generation during the wastewater treatment.
Within the project, nine individual farms were selected in the analyzed catchment area. The farms selected to the project were already equipped with the septic tank. The investors (farmers) accepted the project conditions. Within the project, the formal questions were set up at the local administration level and the necessary materials for treatment wetlands' construction were purchased. The construction of treatment wetlands was carried out by the farmers themselves. Three configurations were proposed: two VSSF beds and the third one with a horizontal subsurface flow (HSSF) bed preceded by a prefilter.
The sampling period was divided into two phases: 1st – in years 2010 and 2011 with seven sampling events and 2nd – from October 2012 till May 2013 (with three sampling events). The 2nd period should be regarded as a post-vegetation.
Innovation in applied solutions was the connection to an effluent polishing pond and in this way increasing the retention of the water in the small catchment as well as reed sludge draining beds for primary sludge produces in septic tank.
The treatment wetlands operated in Kaszuby Lake district receive much higher concentration of pollutants in comparison to the treatment wetlands operated in Europe and USA. Despite such high inflow pollutants concentrations, quite effective removal of pollutants was observed at most single-family treatment wetlands. In first two years of operation good treatment effectiveness BOD 64-92 %, TN 44-77 %, TP 24-66 % was observed.
After three years the efficiency removal of organic and TN increased for 12-20 % except TP. The best quality of effluent was achieved in single vertical subsurface flow (VSSF) with bigger unit area and thus smaller loading in comparison to sequentially working VSSF beds.
Comparing the achieved efficiency removal in three applied configuration shows:
- importance of TSS removal in prefilter before application of treatment wetlands
- double contact time in sequentially working VSSF beds improves the efficiency removal up to 20% in first two years of operations.