Membrane and Desalination Technologies - TCE Moodle Website

338 N.K. Shammas and L.K. Wang
for springtime turnover of a reservoir, resulting in subsequent degradation in feed water
quality.
Ideally, the pilot study should be conducted during the time of year yielding the most
difficult water quality to treat, so that design parameters resulting from the study, such as flux
and chemical cleaning frequency, would be conservative for year round operation. Some
water quality effects can be accurately modeled, such as that of temperature on productivity.
Multi-season piloting is advantageous if scheduling and cost constraints will allow. In fact,
some states require multi-season piloting for membrane treatment facilities.
The duration of the pilot study is also an important scheduling consideration. In general,
piloting through at least three cleaning cycles is recommended practice. The target cleaning
frequency for hollow-fiber MF/UF systems is typically at least 30 days of continuous
operation. Through the first cleaning cycle, the membrane flux and backwash frequency are
usually targeted to provide for 30 days of operation before cleaning is required. The second
cycle provides an opportunity for optimization and operational improvement. The third cycle
establishes repeatability if the operating conditions remain the same. Note that because new
membranes typically perform better than membranes that have been previously fouled and
subsequently cleaned, it may be beneficial to add an extra cycle; to study the effects of
repeated fouling and cleaning on membrane performance. Because this strategy represents
three operational cycles of at least 30 days, pilot test duration of 90 days (or 3 months) is
recommended, if possible. For spiral-wound NF/RO membrane systems, longer cleaning
intervals are desirable, which results in fewer operational cycles than for a MF/UF pilot of
similar duration.
For more thorough MF/UF pilot studies, approximately 4–7 months (3,000–5,000 h) of
cumulative operational time is usually recommended. A longer pilot study may be appropriate
for newer, less proven membrane filtration systems, or for applications in which the water
quality is extremely variable. NF/RO pilot studies generally range from about 2–7 months
(1,500–5,000 h) of cumulative operational time, with longer studies used for waters of
variable quality. The state may also have minimum requirements for the duration of pilot
studies.
2.2. Testing Objectives
Membrane flux and system productivity are typically the most important design parameters
to optimize, as these dictate the number of membranes (and hence a large portion of
the capital cost) required for the full-scale plant. Because these two parameters are
inversely related to a certain extent, the pilot testing process may help to establish the
optimum balance. For example, typically operating at higher fluxes increases the rate of
fouling, in turn requires more frequent backwashing and chemical cleaning. However, the
system productivity is limited by the backwashing (where applicable) and chemical cleaning
frequency. Backwashing and chemical cleaning not only use filtrate as process water
(thus affecting the productivity), but also represents time during which filtrate cannot be
produced (thus affecting overall system productivity). The effect of more frequent backwashes
and chemical cleanings on the system productivity may be an important