A study of the role of the depth in the performance of laboratory-scale down-flow anaerobic fixed-bed reactors (DFAFBR) was carried out at different nominal hydraulic retention times (HRT(N)) using piggery waste as substrate at different influent concentrations (2, 4, 6 and 8 g COD/L). The profiles of soluble chemical oxygen demand (COD) (SCOD), organic nitrogen (O.N.), ammonia nitrogen (A.N.), pH and electrical conductivity (E.C.) through the reactor depths showed an initial highly active zone, which was located around the first half of the reactor depth, and a second zone with a lower biological activity. It was found that the depth of the active zone decreased as the HRT(N) increased and that the slopes of the profiles obtained increased with the rise in the influent concentration. A hydraulic test showed an increase in the dispersion number when the HRT(N) increased. The reactors showed a hydraulic pattern between plug-flow and back-mix. The real values of HRT (Theta) also defined as real contact times were determined to be 0.7, 2.1, 3.4, 4.7, 6.4 and 8 days for values of HRT(N) of 1, 2, 3, 4, 5 and 6 days, respectively. It was found that the concentration of SCOD within the reactor decreased exponentially with the increase in the value of theta. Additionally, the influent concentration had a strong influence on the SCOD variation concentration, mainly at values of theta under 1.5 days, which corresponded to the first part of the reactors.