Page 255 - Membranes for Industrial Wastewater Recovery and Re-Use

224 Membranes for Industrial Wastewater Recovery and Re-use

Backjlushing (every 12 minutes at 75
LMH) for purtialflla recovery

t

Figure 4.23 Pressureprofile forsubmergedfiltration
So, converting flux to SI units and employing the appropriate viscosity value
for 15°C:

APb = 1.15 X X 6.94 X X 1.55 X lo1' X 12/10' bar
= 0.148 bar

The cleaning cycle time is also obtained from Equation (2.10). In this case the
maximum prcssurc is givcn as 1 bar (lo5 Pa) and pJR, has already been
calculated as 0.148 bar. Thus:
pJ(Rm + Rir) = lo5 - 1480 = 9.85 x lo4

&,ti, = 9.85 x lO'/pJ - R,

so,
9.85 x 104
t. - - 7 x 10")
" - 5.4 x 109 ( 1.15 x 10-3 x 6.94 x 10-6
l
= 2160 minutes (36 hours).
Note that if the value for the irreversible fouling rate Ri, had applied only to the
filtration cycle itself, then ti, would be extended by 100 x 0.33/12.33 = 2.7% -
the ratio of the backflush period tb (0.33 min) to the total backflush cycle time t,
+ tb (12 + 0.33 min).
The required feed flow rate is dictated by the product flow rate and total
downtime. The downtime tD over the course of a complete cycle is given by the
sum of the cleaning period t, and backflush periods:

tD = t, + tb x ti,/(& + tb) 50 + 0.33 x 2160/12.33
= 108 min (1.8 h) over the 36 hour cleaning cycle

At a product flow Qp of 600 m3 per day, this downtime is associated with a
volume of