Reverse Osmosis and Other Treatment Technologies - Global Drinking Water Management and Conservation - page 56

Table 4.2 Desalination
technologies — classi ed

Thermal methods Membrane processes

Distillation Electrodialysis

Freeze desalination Reverse osmosis

Ion exchange

Each technology has its own set of advantages and disadvantages depending on
the permitted plant size, concentration of organic and inorganic material in treated
water, desired quality of water produced, and the availability of energy and
chemicals to treat the water and their costs.

4.3.1 Reverse Osmosis

Reverse osmosis is a three-stage process represented in Fig. 4.5 . In stage one, water
is pretreated and screened to remove particles that would clog the RO membranes.
In stage two, water is forced under high pressure through several semi-permeable
membranes. The membranes restrict salts and other contaminants while allowing
water molecules to pass through to the

final stage is referred to as
the posttreatment stage. During the posttreatment stage freshwater may have its pH
level adjusted or be combined with chlorine or various chemicals for disinfection
and transportation. The end result is high-quality water

final stage. The

fit for human consumption
with approximately 10 - 500 parts per million of dissolved solids.
Recent advancements in membrane technology have allowed the cost of puri-
fying water to drop substantially while at the same time increasing the quality of the
water. In the past, RO plant operators had dif

culty in keeping the membrane
surface clean, particularly when treating seawater, surface water, or wastewater.
Often the highly contaminated water would clog the membranes and reduce the

flow capacity of the plant. As a result, higher operating costs due to cleaning
chemicals, down time, and increased labor cost to clean the membranes made
desalination a costly method of water puri

cation.
To prevent the RO membranes from clogging, an Ultra- ltration pretreatment

membrane has been developed. The ultra- ltration membrane consists of

Fig. 4.5 Flow diagram of a reverse osmosis system (California Coastal Commission 1993 )