Membrane and Desalination Technologies - TCE Moodle Website

380 N.K. Shammas and L.K. Wang
saline or freshwater aquifers and may have unknown long-term environmental affects. The
use of evaporation ponds is generally limited to areas with low precipitation and high
evaporation rates, as well as an abundance of inexpensive and available land.
Another option for dealing with NF/RO concentrate is the concept of “zero liquid discharge,”
which involves sufficiently concentrating the residual stream through the use of such
technologies as crystallizers and evaporators to allow remaining solids to be landfilled. While
zero liquid discharge is typically an expensive option, it does offer a number of advantages,
including avoiding the discharge permitting process and the ability to be utilized at any
location independent of factors such as the proximity to a suitable surface water body or
available land for evaporation ponds. In addition, zero liquid discharge maximizes facility
recovery and has minimal environmental impact. Although still fairly uncommon, this option
has become increasingly feasible relative to other methods as environmental and discharge
regulations have become more stringent.
Some MF/UF systems that are operated in a suspension mode and waste the unfiltered flow
generate a continuous concentrate stream. This stream differs from that associated with NF/
RO systems in two significant ways: MF/UF systems concentrate suspended rather than
dissolved solids; and the concentrate stream represents only a small fraction of total feed
flow. Note that NF/RO membranes represent a barrier to particulate matter, and thus these
systems will also concentrate suspended solids; however, because suspended solids rapidly
foul the semipermeable membranes (which cannot be backwashed), most particulate matter is
typically removed with prefilters. An MF/UF concentrate stream has characteristics similar to
those of backwash residuals, and therefore can be considered comparable for the purposes of
treatment and disposal. Alternatively, these two residuals streams may be blended together.
7. INITIAL START-UP
The primary objectives of the initial phase are to ensure that the installation is successfully
completed and that the equipment is in proper working order and ready to produce potable
water that achieves all target quality standards. A well-planned initial start-up phase can thus
help facilitate the proper execution of the membrane filtration system start-up and create a
smooth transition from testing to drinking water production. This section discusses important
general start-up considerations, pointing out any significant differences between NF/RO and
MF/UF systems. For the purposes of this discussion, MCF systems are considered to be
similar to MF/UF systems, except as otherwise noted.
7.1. Temporary System Interconnections
During the start-up process, the filtered water produced from the membrane units may not
be acceptable for distribution. Therefore, facility design should include provisions for the
recycle and/or temporary disposal of feed and filtrate water. Provisions for disposal usually
consist of a removable pipe spool or a tee with a “dump” valve placed in the common feed or
filtrate line. This water can usually be diverted to the sanitary sewer. If the feed and filtrate
piping are interconnected, the connection is removed and replaced with blind flanges after
startup has been completed. Some facilities simply divert and recycle the treated water to the