AbstractVermifilter is a cost-effective and environmentally friendly technology for domestic and industrial wastewater treatment. However, filter bed clogging is an operational challenge of vermifilter. The present study was conducted to investigate the impact of different hydraulic loading rates (HLRs) on treatment performance and filter bed clogging of horizontal subsurface flow macrophyte-assisted vermifilters (HSSF-MAVF). Three laboratory-scale horizontal subsurface flow vermifilter reactors planted with Canna indica were used in this study. Each reactor was fed synthetic dairy wastewater with HLRs of 0.66, 1.5, and 2.34 m/day, respectively. Based upon the results of this study, the treatment performance and filter bed clogging of the system were observed to be closely dependent on the applied HLRs. The highest treatment performance (89 ± 2%, 94 ± 2%, 95 ± 4%, and 76 ± 5%, respectively, for COD, TSS, ammonium-N, and phosphate-P) with minimum filter bed clogging (10 ± 2% hydraulic conductivity reduction and 0.19 ± 0.05-cm head loss) was observed in the reactor fed with the lowest HLR of 0.66 m/day. Similarly, the lowest treatment performance (67 ± 3%, 61 ± 4%, 87 ± 2%, and 63 ± 4%, respectively, for COD, TSS, ammonium-N, and phosphate-P) with severe clogging of the filter bed (33 ± 2% hydraulic conductivity reduction and 2.1 ± 0.5-cm head loss) was observed in the reactor fed with the highest HLR of 2.34 m/day. The clog matter (CM) from each filter bed is composed of both organic and inorganic materials, and the quantity of CM varied from 6.46 ± 0.28 to 24.44 ± 1-mg/mL, with protein being the major organic clog matter component.