Page 363 - Water and wastewater engineering

9-2 WATER AND WASTEWATER ENGINEERING

9-1 INTRODUCTION

Delineation of Membrane Processes
Reverse osmosis (RO), nanofiltration (NF), and electrodialysis are membrane processes that use
the differences in permeability of water constituents as a separation technique. The membrane is
a synthetic material that is semipermeable; that is, it is highly permeable to some constituents and
less permeable to others. To remove a constituent from the water, the water is pumped against
the surface of a membrane resulting in a separation of product and waste streams as shown in
Figure 9-1.
Four types of pressure driven membranes are generally recognized: microfiltration (MF),
ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO). The hierarchy of the pro-
cesses is identified by the types of materials rejected, operating pressures, and nominal pore
sizes on an order-of-magnitude basis. These are shown schematically in Figure 9-2 . Unlike NF/
RO that are pressure driven, electrodialysis (ED) and electrodialysis reversal (EDR) processes
are electrical voltage-driven.
The focus of the discussion for this chapter is RO and NF because they remove ions and have
been used widely in softening water. MF and UF separate suspended particles (colloidal matter,
microorganisms, and viruses) from the water. They are discussed in Chapters 12 and 26. ED/EDR
are reviewed briefly.
In the past, there was a distinction made between RO and NF membranes based on their
original manufactured properties and permeation capabilities. The differences have blurred
with the introduction of new RO membranes. The new RO membranes, called “loose” RO,
“softening membranes,” and “low-pressure” RO, have discriminating characteristics similar to
the NF membranes. Although the distinctions are important from a theoretical point of view,
the remainder of the discussion will treat NF/RO systems together for design and operational
considerations.

Feed stream
Feed-concentrate channel
Waste stream containing
impermeable components
(concentrate or reject)

Permeate channel

Semipermeable
membrane

Product stream containing
permeable components
(permeate)
FIGURE 9-1
Schematic of separation process through reverse osmosis or nanofiltration membrane.