Page 372 - Water and wastewater engineering

REVERSE OSMOSIS AND NANOFILTRATION 9-11

TABLE 9-1
Typical NF/RO membrane process selections based on application
Raw water Objective Process
Groundwater Softening NF or “low-pressure” RO
Brackish water Desalinization RO, or “low-pressure” RO, NF
Seawater Desalinization RO
Surface water NOM* removal NF
Groundwater Specific contaminant RO
or surface water removal (i.e., arsenic, nitrate,
radionuclides)

*NOM natural organic matter.

The concentrate itself is extremely high in total dissolved solids (TDS). Disposal methods
include discharge to the municipal sewer system, ocean discharge, and deep well injection. In
warm, dry climates evaporation ponds may be appropriate. As with ion exchange, disposal of the
concentrate is a major issue in the selection of this technology and should be addressed early in
the design process.

Process Design
Membrane Process Selection. As shown in Table 9-1 the initial choice of a conventional RO
process or an NF (or low-pressure RO) is dependent on the raw water source and the product
water quality objectives. Some preliminary design and economic analyses are required to refine
the selection.
For water softening applications, the NF or low-pressure RO have some inherent advan-
tages over conventional RO. As noted in Figure 9-2 , these systems remove polyvalent ions (that
is, those that cause hardness) but not monovalent ions. This results in a potential reduction in
the TDS in the concentrate stream with a consequent amelioration of its disposal problems. In
addition the operating pressures for NF and low-pressure RO are less than RO with a consequent
reduction in energy operating costs.
If the feed water is not to be treated with chlorine, then a PA membrane is the typical
membrane selected. If pretreatment with chlorine is required, then a CA membrane is more
appropriate.
Side-by-side pilot testing of RO and/or NF units provides the best means of selecting an
appropriate unit. Pilot testing will also provide information on chemical costs and concentrate
disposal.

Operating Pressures. For NF membranes the feed water pressure ranges from 350 to
1,000 kPa. Low-pressure and brackish water RO units have feed water pressures in the range
of 1,000 to 4,000 kPa. RO units treating seawater operate at pressures in the range of 5,500 to
8,500 kPa (Bergman, 2005).