This is far superior to that of a conventional septic tank,
which has a COD removal of 30–50%. However, in this
research, the CODtot removal performance was modest
due to a lower ambient temperature. The low temperature
was expected to negatively affect the suspended COD
removal.[25] Luostarinen et al. [9] found that the UASBseptic
tanks worked more as a settling tank, accumulating
suspendedCODwithout further conversion, when the operating
temperature was lower than 14◦C. Similar results were
reported by Al-Jamal and Al-Jamal and Mahmoud [26] who
used a UASB-septic tank to treat strong sewage withCODtot
of 905 mg L−1 at 17.3◦C. In their study, an average 51% and
24%ofCODtot andCODdis removal efficiency was achieved
with a 2-day HRT.
R1 was separated into two areas (sedimentation area
and baffled flow area). In such an arrangement, most TS is
expected to be collected in the first area, and the second area
in R1 was similar to a two-phase UASB.Asufficient upflow
velocity is needed in UASB systems to provide abundant
contact between anaerobic sludge and wastewater.[2] However,
some studies have demonstrated that the removal
efficiency of ABR was determined by HRT, not by liquid
upflow velocity like UASB.[13] The imposed upflow
velocity in the three compartments in R1 in this research
was very low at 0.006, 0.009 and 0.02mh−1, respectively.
Under a given HRT, the upflow velocity increases in direct
relation to the reactor height. The limited upstream velocity
results in large but shallow tanks, which makes ABR-septic
tanks beneficial in that they can be efficiently buried under
ground, consequently making good use of the underground
area, at the same time achieving better thermal insulation.
Another advantage of R1 was that the two main functions
of a septic tank e.g. sedimentation and anaerobic digestion,
are separated, which not only increases the efficiency
of solids removal, but also improves the performance of
sludge digestion.