Scholarly record

ROLE OF THE SURFACE DEPOSIT LAYER IN THE FIRST STAGE OF FRENCH VERTICAL FLOW CONSTRUCTED WETLANDS CONNECTED TO A COMBINED SEWERAGE

Tereza Semradova, Ondřej Zedník, Michal Kriška Dunajský, Tereza Tumova, Pavel Skarpa

Abstract

The sludge layer formed during the first stage of French vertical flow constructed wetlands is a crucial component in achieving high removal efficiencies of water pollution, including suspended solids, chemical oxygen demand, nitrogen, and phosphorus. In the Czech Republic, persistent challenges with insufficient sludge layer development on the first stage have limited treatment performance due to the combined sewerage system. To address this, a pilot-scale experiment was conducted focusing exclusively on the formation of the sludge layer necessary for effective pollutant removal. This study determined and compared the saturated hydraulic conductivity (ks) of the sludge layer developing on the surface of the first stage of French system. Three measurement campaigns were performed: (i) on mineralized sludge of varying thicknesses, (ii) on primary sludge with and without an artificial grass layer, and (iii) on primary sludge with and without a geotextile layer. Saturated hydraulic conductivity was measured using a double-ring infiltrometer during an eight-day resting period. Results showed a strong dependence of ks on sludge thickness (8 cm: 1.7 10-4 m s-1; 5 cm: 8.6 10-4 m s-1; 3.8 cm: 1.3 10-3 m s-1). The grass layer on primary sludge initially reduced ks to 1.1 10-5 m s-1 compared to 5.7 10-5 m s-1 in the control, while the geotextile led to lower initial ks and slower conductivity increase over time. These findings confirm that sludge thickness and artificial modifications fundamentally influence the hydraulic behaviour of filter beds and can help optimize French system operation.

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