was 1.26 g L−1 and the mass ratio of volatile solid (VS) to
TS (VS/TS) was 0.4. The initial pH was 7.55.
The three reactors were all started up during summer,
from July to September, for a period of 3 months. The influent
was domestic sewage delivered via peristaltic pumps in
such a manner to control the hydraulic retention time (HRT)
to a fixed period of 36 h during the start-up phase. When the
value ofCODtot/CODdis (totalCOD/dissolved COD) in the
effluent became stable, the start-up phase was over. HRTs
of 36, 24 and 12 h were used in the following ‘steady-state’
experiment, and the reactor operation was divided into three
phases according to the experimental design. The reactors
were operated under a temperature of 16◦Cduring this stage,
which is the average temperature in septic tanks in winter in
Harbin, China, according to previous investigations of this
research group.[16] The operating conditions of the three
phases (P1, P2 and P3) are listed in Table 1. There were 10
days for adaptation to occur to the new flow rate each time
the HRT was changed.
Different volumes of primary sludge from a municipal
wastewater treatment plant (Harbin, China) were added into
reactors from the top of reactors twice a day to keep a stable
TS value in the three phases.