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Membrane Processes for Reclamation of Municipal Wastewater 441
Table 10.5
Consumption of chemical and power and cost per cubic meter product for
the pilot trials (13)
Item Consumption Cost (S$)
Chemicals
Flocculants (as Al) 2.77 g 0.0174
NaOCl 0.41 g 0.0027
H 2SO 4 0.56 g 0.0019
Subtotal 0.022
Power 0.194 kW-h 0.0248
Total 0.047
Note: S$ Singapore dollar; 1 US dollar = 1.75 Singapore dollar.
3.3.2. Transmembrane Pressure as a Function of Time
The effect of alum dosing concentration on stabilization of the plant operation was
intensively investigated. Figure 10.4 illustrates the TMP of the UF membranes as a function
of time throughout the study under different concentrations of alum dosage. It should be
pointed out that a TMP band appeared because TMP increased with time within one cycle of
the operation mode (20–30 min). The high level and low level corresponded to the TMP at the
time immediately before and after a backwash operation, respectively. Figure 10.4a indicated
that TMP was 160–210 mbar when alum concentration was 4.8 mg/L and the water recovery
was 87.5% at the beginning, then TMP increased to 200–270 mbar as alum concentration
decreased to 1.3 mg/L. TMP tended to slightly increase with time during these two tests.
However, TMP increased sharply and was unstable when alum concentration further
decreased to 0.6 mg/L. Figure 10.4b illustrated that TMP was stable in the range of
200–240 mbar as alum concentration was enhanced to 2.5 mg/L during 29 April–6 May.
High TMP on 7 May appeared because the air lock in the feed line caused the feed pump
shutdown; further, the plant shutdown when water level in the feed tank became gradually
low due to the clogging of the strainer. After that, the water recovery was increased to 90.1%
on 10 May while the alum concentration remained the same. TMP had been stable in the
range of 220–260 mbar since then. The results showed that alum concentration was critical
for maintaining stable TMP and achieving a smooth operation of the plant, which could be
explained as follows from two aspects.
On the one hand, within a backwash interval, a colloidal fouling of the membrane was
formed. In the dead-end UF process for this particular application, alum was dosed into the
feed line just prior to feed pump, then well mixed with the feed inside the pump. It took a few
seconds for the flocculating feed to reach the membranes. Theoretically, flocs formed should
be much larger than the membrane pores before they reached the membrane surface with an
appropriate dosing concentration of alum. During the filtration operation, the flocs gradually
cumulated on the membrane surface and formed a temporary cake layer of suspended solids.
After the backwash, the back washable suspended solids could be easily removed and the