Page 254 - Membranes for Industrial Wastewater Recovery and Re-Use

System design aids 22 3

4.3.5 Problem in submerged membrane filtration design
A dead-end hollow fibre submerged microfiltration membrane module having a
membrane area A, of 46 m' is being used to provide a flow of 600 m3 day-l. The
process operates at a flux J of 32 1 m-* h-l and an aeration rate Qa of 50 m3 h-'
per module, and the water viscosity is around 1.1 5 x Pa s.
The resistance of the cleaned membrane is 7 x 10" m-l. As a result of fouling
by the suspended matter, the operational (reversible) fouling resistance R,,,
increases by 1.55 x lo1' m-l per minute. The residual (irreversible) fouling
resistance Rirr increases at an average of 5.4 x lo9 m-l per minute over the
entire cleaning cycle (i.e. the period between chemical cleans), and the system is
chemically cleaned once a pressure of 1 bar is reached.
Rackwashing (with permeate) at three times the operating flux for 20 seconds
every 12 minutes completely removes the reversible fouling. Cleaning in place by
flushing with hypochlorite for 10 minutes and then soaking for a further 40
minutes removes the irreversible fouling.
Calculate (a) the cleaning cycle time, and so (b) the total minimum membrane
area requirement and (c) the capital cost, assuming CAPEX, €k = €80 +
membrane cost/l50 given that the membranes cost €40 perm'.
The aeration energy demand (kWh per m3 permeate) is related to the aerator
flow by:

E, = 0.02Qa//A,,, for Q, > 25 m3 h-'

under the operating conditions outlined, Q,, J and A, taking SI units. If electrical
costs are 8p per kWh, what is the total electrical energy demand? How
significantly is this affected by the backflush energy demand?

Solutrofl
The solution proceeds through determination of the pressure changes and
operational cycle times from Equation (2.10), and using Equation (2.23) to
determine pumping energy demand and then adding the aeration energy to this.
The figures must be adjusted to account for downtime and loss of permeate used
for backflushing.
The pressure profile is depicted in Fig. 4.23. The filtration backflush cycle
pressure change APb can be calculated from the rate of increase in the reversible
fouling resistance, 5, = 1.55 x loll m-l min-l, andfromEquation (2.10):

pb = pJ(Rm + Rr + Rx)

where R,, R,, and Ki, refer to the hydraulic resistances of the membrane,
reversible fouling component and irreversible fouling component respectively.
R, = &t, if the resistance increases linearly with time. Given that Ri, << Rr over
the course of a backflush cycle, the change in pressure over the cycle is:
APb PJErtr