Civil Engineering Reference
In-Depth Information
Raw Water
Concentrate
Permeate
Air
Scrubbing
Discharge
Permeate
UF
Balance
Tank
Raw
Water
Tank
Permeate
Tank
Compressed
Air
NaOCl
Tank*
Dosing
Pump
Transfer
Pump
UF Feed
Pump
Prefilter
Reverse
Filtration
Discharge
Reverse
Filtration
Pump
* Optional
Fig. 15-11.
Backwash system using compressed air for scrubbing (Reprinted from
Water Treat-
ment Membrane Processes,
by permission. Copyright
1998, American Water Works Associa-
tion, AWWA Research Foundation, Lyonnaise des Eaux, and Water Research Commission of
South Africa.)
screen capable of removing particles above 100 to 200
is all that is required for MF
pretreatment. More extensive solids removal pretreatment should be considered if the
raw-water suspended solids concentration is above 200 mg / L.
Adjustment of pH is normally not required, because virtually every membrane now
manufactured has a wide range of tolerance. If scaling is a concern, steps to balance
the alkalinity of the water should be taken. If iron and manganese are present, oxidation
should be performed to force precipitation to occur prior to entering the membrane.
Care must be taken to determine the compatibility of the membrane material to
oxidants and cleaning agents. The Memcor polypropylene hollow-fiber membranes
have a low tolerance for combined chlorine and cannot tolerate any free chlorine.
Membranes constructed of polysulphone, PVDF, and PAN tend to be more tolerant to
chlorine.
Temperature Effects
Feedwater temperature affects the flow through MF and UF
membranes. The filtrate flow rate at any given pressure changes with temperature and
is related to the change in water viscosity with temperature. The following correction
factors can be used as a guide to approximate flow rate changes. Membrane manufac-
turers should be consulted to determine expected performance at a specific location.
Temperature
C
Correction Factor
1
5
10
15
20
25
30
0.58
0.66
0.77
0.88
1.00
1.13
1.26
