Membrane and Desalination Technologies - TCE Moodle Website

Membrane Systems Planning and Design 355
Because MF, UF, and MCF systems do not directly affect the pH or remove alkalinity, these
processes do not generally require chemical conditioning to stabilize the filtrate.
While the use of membrane filtration does not specifically necessitate disinfection posttreatment
as a result of process considerations, the need for post-disinfection is generally
required by regulation for primary and/or secondary disinfection. However, in some states,
the use of membrane filtration may reduce primary disinfection (i.e., CT) requirements, thus
helping to control DBP formation. Because membrane filtration is often the last major process
in the treatment scheme, it is common to apply a disinfectant to the filtrate prior to entry into a
clearwell and/or the distribution system (24). This application is particularly important if
disinfectants were either neutralized or not added at all prior to the membrane filtration
process to avoid damaging oxidant-intolerant membranes.
For NF and RO membrane processes, if the disinfectant is applied prior to filtrate pH
adjustment, post-disinfection may have the additional benefit of oxidizing sulfide to sulfate,
thus reducing the potential for both sulfide precipitation and taste and odor concerns.
Corrosion inhibitors may also be added prior to distribution, particularly for NF and RO
systems that produce more corrosive water.
4. BASIC PRINCIPLES OF SYSTEM DESIGN AND OPERATION
The material presented in this section is intended to provide an overview of the basic
principles underlying membrane filtration system design and operation. Although all of the
types of membrane filtration (i.e., MF, UF, NF, RO, and MCF) utilize pressure (or vacuum) as
a driving force, there are fundamental differences in the models used to describe systems
using porous membranes (MF, UF, and MCF) and semi-permeable membranes (NF and RO).
The basic principles of these respective models, along with some general concepts that are
applicable to all membrane filtration systems, are discussed in this section.
4.1. General Concepts
There are a number of general concepts that are applicable to all types of pressure-driven
membrane filtration systems and which serve as the underlying basic principles for system
design and operation. These concepts include
1. Flux
2. Recovery
3. Flow balance
Membrane filtration system throughput or productivity is typically characterized by the
system flux, which is defined as the filtrate flow per unit of membrane filtration area, as shown
in Eq. (1):
J ¼ Qp
; (1Þ
Am
where J is the flux; gal/ft 2 /d, Qp is the filtrate flow, gpd; and Am is the membrane surface area, ft 2 .