The results showed that the SMAmax increased at decreasing
the HRT from 16 to 6 h. This also was obtained by Leitao
(2004) in the domestic wastewater treatment in the UASB
reactor at tropical conditions. However, this phenomenon
cannot be generalised. In the domestic wastewater treatment
in a hybrid UASB reactor at low temperature, the SMAmax
decreased at lowering the HRT (Elmitwalli et al., 2002a, b).
Although the SMAmax increased at decreasing the HRT, the
CODt removal decreased. Therefore, it seemed that the
decrease in particulate removal and hydrolysis at lowering
the HRT might be the reason. The results of the SRT showed
that the SRTmin at HRT of 6 h is relatively low (27 days).
However, the system had a sufficient CODt removal and
methanogenesis (Table 6). Accordingly, the real SRT should be
between SRTmin and SRTmax.