The aim of the study was to determine the impact of selected factors on the reduction of organic pollutants, expressed in BOD5 and CODCr, in wastewater treated in a laboratory scale model of moving bed bioﬁlm reactor (MBBR). The factors included in the experiment: the degree of ﬁlling the ﬂuidized bed with biomass carriers, hydraulic load, and aeration intensity. The tested model of the bioreactor consisted of ﬁve independent chambers with diameter D = 0.14 m and height H = 2.0 m, which were ﬁlled with biomass carriers at 0%, 20%, 40%, 60%, 70% of their active volume. During the test period, hydraulic loads at the level of Qh1 = 0.073 m3·m-2·h-1 and Qh2 = 0.036 m3·m-2·h-1 were applied, which ensured one-day and two-day sewage retention, respectively.The said reactors were subjected to constant aeration at P1 = 3.0 dm3·min-1 and P2 = 5.0 dm3·min-1. The highest efﬁciency of the reduction of the analysed indicators was demonstrated by reactors ﬁlled with carriers in the degree of 40–60%. Based on the statistical analyses (the analyses of the ANOVA variations and the Kruskal-Wallis test) carried out, it was found that the studied factors signiﬁcantly modiﬁed the mutual interaction in the process of reducing BOD5 in treated wastewater of the reactors tested. The signiﬁcance of the impact of the discussed factors on the values of the studied indicators in treated wastewater depends on mutual interactions between the investigated factors.