Civil Engineering Reference
In-Depth Information
In operation, the EDR process requires the addition of a timing-control unit, au-
tomatic valves to interchange the product and brine streams, and relays to reverse the
polarity of the DC power supply.
Fouling
The EDR process has greatly reduced membrane scaling and fouling. In many plants,
the need for chemicals to control scaling and fouling has been eliminated through use
of an EDR system.
Fouling of ED / EDR membranes is still a problem when dissolved organic and
inorganic compounds in the raw water are above recommended level. Organic fouling
is caused by the precipitation of large, negatively charged molecules on the anion-
permeable membranes in the feed channels. Inorganic fouling is caused by the precip-
itation (scaling) of inorganic compounds (such as CaSO
4
and CaCO
3
) in the
concentrate compartments. Also the negative effects of ED membrane fouling include:
Cations such as Fe and Mn can absorb on the cation-permeable membranes.
•
Attached ions can neutralize or even reverse the fixed charge of the membranes,
causing significant reduction in efficiency.
•
Fouling increases the membrane stack electrical resistance, increasing electrical
consumption and operating costs.
•
The following items can result in membrane fouling:
Metals such as Fe, Mn, and Cu
•
Dissolved gases such as O
2
,CO
2
, and H
2
S
•
Silica (colloidal and soluble)
•
Fine particulates and colloidal material of a wide range of sizes and composition
•
Divalent cations (Ca, B)
•
Many of these foulants present in raw water may be controlled by pretreatment steps
which usually stabilize the ED / EDR process. However, according to Katz, the devel-
opment of the EDR process has helped to solve the pretreatment problem more readily
in that it provides self-cleaning of the vital membrane surfaces as an integral part of
the desalting process.
29
Pretreatment
The overall requirements for pretreatment in ED / EDR are less stringent than for RO
due to the nature of the salt separation and the larger passages provided. In ED / EDR,
the ions (impurities) move through the membranes, while the desalted water moves
through the membranes' feed messages, flushing out solids. With the EDR process,
precipitated salts in the brine compartments can be more readily dissolved and flushed
out of the system, using polarity reversal without the need for chemical pretreatment.
However, high removals of suspended solids, iron, manganese, organic matter, and
hydrogen sulphide are still critical to pretreatment of the feedwater (USAID Desali-
nation Manual, 1980) if it contains the following ions: Fe
0.3 mg / L, Mn
0,1 mg
