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Membrane Systems Planning and Design 369
5.2.3. Organic Carbon
Another water quality constituent that influences membrane flux is the organic carbon
content, which is typically expressed in terms of either total (TOC) or dissolved organic
carbon (DOC). Organic carbon in the feed water can contribute to membrane fouling, either
by adsorption of the dissolved fraction onto the membrane material or obstruction by the
particulate fraction. Thus, lower fluxes may be necessary if membrane filtration is applied to
treat water with significant organic carbon content. The tendency for a membrane to be
affected by TOC is partially influenced by the nature of the organic matter in the water. TOC
can be characterized as either hydrophilic or hydrophobic in composition, and studies suggest
that the hydrophobic fraction contributes more significantly to membrane fouling. The
character of the organic carbon content can be roughly quantified by measuring the specific
ultraviolet absorbance (SUVA) of the water, as calculated using the following equation:
SUVA ¼ UV254
; (29Þ
DOC
where SUVA is the specific UV absorbance, L/mg m, UV254 is the UV absorbance at 254 nm,
1/m, and DOC is the dissolved organic carbon, mg/L.
Because TOC is more commonly measured than DOC in drinking water treatment, SUVA
is sometimes estimated using values for TOC in place of those for DOC.
Higher SUVA values tend to indicate a greater fraction of hydrophobic organic material,
thus suggesting a greater potential for membrane fouling. Generally SUVA values exceeding
4 L/mg m are considered somewhat more difficult to treat. However, organic carbon (as well
as turbidity) can often be removed effectively via coagulation and pre-settling, particularly if
more hydrophobic in character, thus minimizing the potential for membrane fouling and
facilitating operation at higher fluxes. Coagulation can also be conducted in-line (i.e., without
pre-settling) with MF/UF systems. Pretreatment using the injection of PAC may also reduce
DOC in the membrane feed; however, because spiral-wound membrane modules cannot be
backwashed, PAC should not be used in conjunction with NF/RO systems unless provisions
are made to remove the particles upstream.
5.2.4. Dissolved Solids
The total dissolved solids (TDS) and the particular species of dissolved solids present in
the membrane feed are both critical considerations for NF/RO systems. Species such as silica,
calcium, barium, and strontium, which can precipitate as sparingly soluble salts, can cause
scaling and a consequent rapid decline in flux under certain conditions. Scaling is typically
controlled using pretreatment chemicals such as an acid to lower the pH and/or a proprietary
scale inhibitor. However, the total quantity of dissolved solids of any species also influences
system operation, as the NDP required to achieve a target flux is related to the osmotic
pressure of the system, which is directly proportional to the TDS. Thus, as the TDS increases,
so does the required feed pressure.
TDS is generally not a significant consideration for MF, UF, and MCF systems, since these
processes do not remove dissolved solids. In some cases, however, the use of upstream