Thesis - faculty.ait.ac.th - Asian Institute of Technology

The reactors were continuously aerated with the help of stone diffusers placed at the
bottom of the reactors. The polyethylene hollow fibre membrane was kept in the upper end
of the reactors. Peristaltic pumps were used to withdraw permeate from these membrane
modules. The reactors were also equipped with the pH meter to monitor pH continuously.
The pH of the yeast and bacterial reactor was maintained within the range of 3.5 to 3.8 and
6.8 to 7, respectively with the help of an external dosing pump. The DO in the reactors was
maintained around 2-4 mg/L.
Both the bioreactors were operated with periodic air backwashing. The filtration
cycle of the reactor consists of 25 minutes of filtration, 3 minutes of air backwashing at a
pressure of 300 kPa, and 1 minute of air release. The operation of filtration, backwash and
release air was alternatively controlled with an intermittent controller and solenoid valves.
The trans-membrane pressure (TMP) was measured using a mercury manometer.
Sampling from the reactors was done from the sampling port. The sludge from the
reactor could be withdrawn from a sampling port present in the bottom of the reactor. The
treated leachate was collected in a container kept at the side of the reactor. The treated
effluent corresponds to permeate from the membrane bioreactor.
3.6.3 Parametric Studies
The experiments were done by varying the volumetric loading. The different
organic loading rates (OLR) used in this experiment are summarized in Table 3.6. Each
volumetric loading was maintained at least for 25 days. Furthermore, the excess sludge
was periodically withdrawn to maintain a mean biomass concentration of 10,000 to 12,000
mg/L of MLSS. The pH in the YMBR system and BMBR system was maintained around
3.5 to 3.8 and 6.8 to 7.0, respectively. The varied mean hydraulic retention time in which
the experiment was conducted are 12, 16, 20 and 24 hours. The hydraulic loading varied
from 6.75 to 17.88 kg COD/m 3 .d .
Table 3.6 Experimental Operating Conditions of YMBR and BMBR Systems
Stage Time
(days)
1
1-25
2
26-60
3
61-149
4
150-181
3.6.4 Molecular Weight Distribution
Mean HRT
(h)
24
20
16
12
62
OLR
(kg COD/m 3 .d)
6.75-8.33
7.60-11.14
9.54-14.40
13.92-17.88
To investigate the composition of organic content in the leachate on the basis of their
molecular weight, ultrafiltration membrane (UF) was used. Ultrafiltration was performed
in a 300 ml cell, using flat circular membrane of 76 cm diameter with molecular weight
cut-off (MWCO) of 5,000, 10,000, and 50,000 Daltons (Da). Nitrogen gas was used to
apply pressure in the UF cell at about 2 bars (Gourdon, et al., 1989; Huang, at el., 2000).
The three types of UF with molecular weight cutoff ranges are presented in Table 3.7. The
fractionated organics in the treated leachate were also measured to find out the organic
removal efficiency of the membrane bioreactor in terms of their molecular weight. The
organics were fractionated into four groups based on their molecular weight (MW): (1)