Page 198 - Facility Piping Systems Handbook for Industrial, Commercial, and Healthcare Facilities

WATER TREATMENT AND PURIFICATION

4.34 CHAPTER FOUR

Membrane Selection
System performance is determined by considering the following factors that influence the
capacity of the individual membranes selected:
1. Operating pH
2. Chlorine tolerance
3. Temperature of the feedwater
4. Feedwater quality, usually measured as SDI
5. Types of impurities and previous feedwater treatment
6. Membrane flux
7. Number of operating hours
8. Resistance to biodegradation and ability to be sanitized
9. Rejection characteristics. Typical RO systems remove the following contaminants to the
following levels:
Inorganic ions 93–99%
Dissolved organics (>300 MW) >99%
Particulates >99%
Microorganisms >99%
The selection of a system configuration must consider the following:

1. Maximum recovery
2. Fouling properties and resistance
3. Production rate per unit volume
There are only a small number of polymers that have the necessary characteristics to
function as semipermeable membranes.
1. Thin film composite of various polymer materials
2. Polyamide
3. Cellulose acetate
4. Cellulose triacetate
5. Polysulfone
Typical characteristics and a comparison of these membranes are given in Table 4.8.

TABLE 4.8 Comparison of Reverse Osmosis Polymers

Thin film Cellulose Cellulose
composite Polyamide acetate triacetate Polysulfone
pH Stability 2–12 4–11 2–8 4–7.5 3–11
Chlorine tolerance Fair–poor Poor Good Fair–good Good
Biological resistance Good Good Poor Fair–good Good
Temperature limit for 122 (45) 95 (35) 95 (35) 86 (30) 95 (35)
stability, °F (°C)
Typical rejection ionic >90 >90 90 90 90
species, %
Flux High Low Low–medium Low–medium High

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