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The effectiveness of nitrogen removal in vertical-flow filters with double-layer filling composed of lightweight aggregate made of fly ashes from sewage sludge thermal treatment (FASSTT LWA) and gravel at a varying hydraulic load was determined. Synthetic wastewaters containing 47 mg/L of total nitrogen (TN) were fed into 15 filter columns filled with gravel and FASSTT LWA (gravel was the bed’s lower layer, whereas the top layer of the FASSTT LWA constituted 0, 12, 25, 50, and 100% of the depth of the column) at hydraulic loads of 3, 5, and 7 mm/day. The study demonstrated a significant TN removal by the filters, which was accompanied by a high concentration of NO3-N in the effluent. The results point to differentiated effectiveness of NH4-N removal by both the FASSTT LWA and gravel. The highest TN removal rate reaching up to 74.93% was achieved in the filter column with 100% FASSTT LWA content, irrespective of the hydraulic load applied. [ABSTRACT FROM AUTHOR]