3. Results and discussion
3.1. COD and color removals in anaerobic UASB reactor
The effects of HRT and organic loading on the color and
COD removal efficiencies are shown in Fig. 5a and b. At a
HRT of 15.7 h; 59% COD and 85% color removal efficiencies
were obtained. Increases in HRT caused increases in
COD removal while no variation was observed in color removal.
This result can be explained by the complete cleavage
of azo bond of Reactive Black 5 dye in very short HRTs
(2 h). In other words, the azo dyes decolorized under reductive
anaerobic conditions in very short HRT but the breakdown
products such as aromatic amines were not ultimately
metabolized and caused residual COD. Increases in HRT
provide enough time for partial mineralization of COD and
intermetabolites.