The research investigated the implementation of suspended reactive filters to support the phosphorus (P) removal efficiency of constructed wetlands (CWs). The reactive material (RM) used in this study was autoclaved aerated concrete (AAC). The laboratory experiment consists of four plastic boxes filled with the volume of 10 L of artificial P solution with three variants of RM mass to volume ratio: 1:1 (g:L), 5:1 (g:L), 10:1 (g:L), and the blind probe 0:1 (g:L) as a reference. AAC of different weights (10, 50 and 100 g) was wrapped in a filter bag, put into boxes, and suspended. After 30 days of the laboratory experiment, AAC was able to reduce the P-PO4 concentration from 2.972 mg·L-1 to: 0.341 mgPO4-P·L-1, 0.006 mgPO4-P·L-1 and 0.004 mgPO4-P·L-1 for 10 g, 50 g and 100 g mass variant, respectively. This concentration reduction corresponds to unit sorption of: 2.53 mgP-PO4·g-1, 0.58 mgP-PO4·g-1 and 0.30 mgP-PO4·g-1 for 10 g, 50 g and 100 g, respectively. Based on the obtained data, the CW supporting filter was dimensioned to reduce the outflow P concentration to 0.01 mg·L-1. P removal efficiency prediction was calculated for Cetynia River, Poland.