A case study of the performance of pilot scale light weight aggregates (LWA) based hybrid soil filters in Estonia

Two pilot scale experimental hybrid soil filters (SF) filled with light weight aggregates were established during the summer of 2005 for the treatment of different types of wastewater, with the aim of developing compact hybrid constructed wetlands (CW) for use in cold climates. Both SFs are designed on the same principle: a vertical subsurface flow filter followed by a horizontal subsurface flow filter. Six different operational regimes were tested during the experiments. With a decrease in wastewater and pollutant load and the larger re-circulation of wastewater, purification efficiency increased in terms of most water quality indicators. The highest purification efficiencies in the Nõo SF, 99% and 82% for BOD7 and Ntot removal respectively, were achieved when the re-circulation rate of 300% was applied and in the Rämsi SF, 99% and 87% for BOD7 and Ntot removal respectively, were achieved, when the re-circulation rate of 300 to 600% was applied. In order to achieve effective organic matter removal, nitrification/denitrification and TSS removal, the re-circulation rate must be from 100 to 300% of the inflowing wastewater. The re-circulation of wastewater in overloaded systems is a good solution for the improvement of the aeration and overall purification efficiency of CWs.