NAMS 2002 Workshop - ICOM 2008

which can adsorb on the surface of the membrane and reduce water flow. In a
typical RO plant, the normalized flow may drop by 15 to 25% in the first 60 days
of operation. In subsequent months, though, the decline may only be a few
percent. Cleaning can often recover a good portion of this loss. Detailed
operating data will be shown to examine this.
Although much progress has been made in controlling fouling, further research is
needed to understand and minimize this fouling. This paper will present some
detailed studies of membranes analyzed during the initial 30 days of operation
and characterize the organic material which leads to membrane fouling. It will
also evaluate common foulants found in commercial systems that can degrade
membrane performance.
High rejection of contaminants is the key to the use of RO processes. The
membrane which is selected must produce water that meets the water quality
targets. Depending on the use of the water, these targets can vary significantly.
For applications such as those in Singapore where the water is primarily used in
the wafer fabrication industry, it is critical to have low concentrations of organics,
as well as hardness and other salts. These very strict limits cannot be met by all
membranes. Typically, high rejection, low pressure composite polyamide
membranes, such as the Hydranautics ESPA2 membrane, have found much
use, since they give adequate rejection and operate at the lowest possible
pressure.
A recent survey of commercial plants shows that hardness ions are very highly
rejected, with rejections ranging from 99.88 to 99.99%. Similar rejection would be
seen for most ionized metals such as iron or manganese. Likewise the divalent,
negatively charged sulfate molecule has similar rejection rates. Monovalent ions
such, as sodium and chloride, have much lower rejection rates, in the range of 99
to 99.3%, and nitrate, which has a smaller hydrated radius, is the lowest rejected
anion, at about 94-97% rejection. These rejection rates are still much higher than
the values seen for brackish water treatment. Rejection of TOC is mostly in the
range of 99.6 to 99.7%. This is important for places such as Singapore, where
the permeate must contain less than 100 ppb of TOC. It is apparent, that these
membranes can easily achieve such values for a feed stream containing 10-15
ppm of TOC. From recent plant data, a detailed analysis of the rejection of
various compounds will be presented, and how these are meeting the recent
stringent demands of the end user.