Page 369 - Water and wastewater engineering

9-8 WATER AND WASTEWATER ENGINEERING

Hollow
Feed
tube fibers

Concentrate Permeate
Permeate
Pretreated
source
water

Flow
screen
FIGURE 9-5
Typical hollow fiber NF/RO membrane module.
(Adapted from U.S. AID, 1980.)

Excessive concentrations of iron and manganese, if oxidized, will form precipitates that will
foul the membrane. Scaling will also occur as a result of increasing recovery because the concen-
tration of limiting salts will increase to their solubility limit, and they will precipitate. The most
common scales of limiting salts are calcium carbonate and calcium sulfate. Others of concern
are calcium fluoride, calcium orthophosphate, strontium sulfate, barium sulfate, and amorphous
silica (MWH, 2005).
An additional negative impact is the result of the accumulation of solutes that form a bound-
ary layer of high concentration at the membrane surface. The concentration at the surface of the
membrane becomes higher than the concentration in the bulk feed water. This effect is called
polarization. It has the following negative impacts (MWH, 2005):
• Water flux is lower because the osmotic pressure gradient is higher.

• Rejection is lower.
• Solubility limits of solutes are exceeded leading to precipitation and scaling.

These issues may be ameliorated by pretreatment of the raw water and operational procedures.

9-4 RO AND NF PRACTICE

Process Description
The smallest physical unit of production capacity is the membrane element. The membrane
elements are enclosed in pressure vessels as shown in Figure 9-6 . A group of pressure vessels
operating in parallel is called a stage. The arrangement of one or more stages is called an array.
In a multistage process the stages are arranged in series. The number of pressure vessels
decreases in each succeeding stage to maintain the water velocity in the feed channel as permeate is