Table 6 shows the percentage of methanogenesis and
hydrolysis in the UASB reactor. The Mt, Ht and Hr decreased
at lowering the HRT, while the Mr was almost constant.
Therefore, lower CODt removal and the higher SMAmax at
shorter HRT is due to the decrease in the particulate
removal and hydrolysis and the increase of the organic
loading rate at lower HRT. The difference between the
removed CODt and Mt is limited (20–24% of the removed
CODt) and is due to the growth of anaerobic bacteria and the
accumulation of slowly biodegradable and non-biodegradable
particulate in the reactor. However, this difference in the
batch experiments (ABmax and maximum removal in the
batch recirculation experiments) is 10% of the maximum
CODt removal. Accordingly, 10–14% of the removed CODt in
the UASB reactor is slowly biodegradable COD, which was not
converted in the grey water treatment in the UASB reactor.
The results of digestibility tests confirmed the previous
conclusion, as 7–12% of the CODt of the sludge in the UASB
reactor was digested after batch digestion for a period of
76–84 days.