This study investigates the feasibility of using an organic substrate in the form of acetic acid to enable wastewater denitrification in a Sequencing Batch Biofilm Reactor (SBBR). The impact of nitrates presence on the yield of biological dephosphatation were determined as well. The experiment included 296 cycles and was divided into 4 series differing in the load of nitrates. Ratio N:(C and P) was: 7:(140 and 7); 35:(140 and 7); 70:(140 and 7) and 140:(140 and 7). Hydraulic retention time in the reactor was 12h (6h mixing – dissolved oxygen concentration below 0,1 and 6h aeration – concentration of dissolved oxygen 3.0±0.8 mgO2•dm-3). Study demonstrated that the 30-day adaptation period (60 cycles) was sufficient for the development of a stable biofilm. The C:N ratio of 2 ensured total nitrogen concentration in the effluent below 1 mgN•dm-3. Mean efficiency of biological dephosphatation reached 7.0, 17.4, 18.7, and 30.3% in series 1-4, respectively. In the case of series 2 and 3, no significant differences were demonstrated in total phosphorus concentration in the effluent. In the other series, the differences turned out to be significant.