Reverse osmosis system

In constructing a system for reverse osmosis many problems have to be solved:

1. The system must be designed to give a high liquid flux reducing the concentration potential.

2. The packaging density must be high to reduce pressure vessel cost.

3. Membrane replacement costs must be minimized.

4. The usually fragile membranes must be supported as they have to sustain a pressure of 20-100 atm.

Table 10.2

Comparison of the various techniques

Table 10.2

Comparison of the various techniques

Packing

Useful

Water flux

density

pH

Ease of

NaCI

at 40 atm.

Modul concept

(m2/m3)

range

cleaning

rejection

(m3/m2/day)

Plate and frame

450

2-8

fair

very good

0.5

Large tubes

150

2-8

very good

very good

0.5

Spiral

750

2-8

good

very good

0.5

Hollow fine fibers

7.5-15' 103

0-12+)

fair

good/fair

+) Polyamide

Four different system designs have been developed to meet the solution to problem 4. These are the plate and frame technique, large tube technique, spiral wound technique and the hollow fine fiber technique.

The various techniques are compared in Table 10.2. The most widely used membrane is the cellulose acetate membrane made by the Loeb-Sourirajan technique. This membrane is asymmetrical and consists of a thin dense skin of approximately 0.2 fj on an approximately 100 p thick porous support.

Polyamide membranes have also been developed. They are considerably more resistant to high pH-values, but give a smaller flux. During the last two decades there has been an intense research activity in the development of membranes, resulting in several new types. Cellulose acetate-butyrate resin, cellulose acetate-methacrylate, polyacryl-acid and cellulose nitrate-acetate, are among the recently developed membrane materials, which are more resistant to pH and temperature, but do not reduce the initial fluxes. Several natural materials could also be of use as membranes and extensive laboratory investigations may hold promise for the application of such natural membranes soon (Kraus et al., 1967).

Table 10.3 gives the characteristics of some widely used types of membranes.

Table 10.3.

Characteristics of membrane material Material pH-stabllity Chlorine Biological Temp. % ion se-

resistance resistance range°C paration

Table 10.3.

Characteristics of membrane material Material pH-stabllity Chlorine Biological Temp. % ion se-

resistance resistance range°C paration

Polyamide

4-11

not good

good

<35

>90

Cellulose acetate

2-8

good

not good

<30

90

Cell, tri-acetate

4-8

fair

fair

<30

90

Polyacrylic acid

2-11

fair

good

<40

>90

Cell, acetate-

butyrate

2-10

good

fair

<35

90

Combined mem

branes

2-12

fair

good

<50

>90

As mentioned in Section 10.1 it cannot be excluded that new and better membranes will be developed in the coming decade, which will make the use of reverse osmosis economically attractive for removal of inorganic nitrogen ions, i.e., ammonium and nitrate. This will have particular interest, where production of drinking water quality from municipal waste water will be needed due to problems of water shortage.

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