Page 114 -

MIXING, COAGULATION, AND FLOCCULATION 6,5

• Increased solids. The higher coagulant dosages directly result in increased sludge vol-
umes. Chapter 17 discusses the relationship between coagulant dosages and sludge pro-
duction.
• Poorer dewatering characteristics. The increased metal (A13+ or Fe z+ or 3+) concen-
trations typically result in poorer dewatering characteristics. As a result, a change to
enhanced coagulation may result in lower ultimate, dewatered solids concentrations.
• Increased concrete~metal corrosion. The lower pH of the coagulated water for TOC
removal will be significantly more aggressive on concrete and metals as compared to
the more neutral pH of water that has been coagulated for turbidity removal.

If pH is lowered to improve coagulation, it is typically necessary to raise the pH in
the final effluent from the plant to provide a less corrosive finished water. The pH may
be adjusted at one or more points in the treatment, including rapid mixing, prefiltration,
and postfiltration. If the pH is lowered to improve coagulation and organics removal, it
is often recommended to readjust the pH after the filtration process as compared to pre-
filtration. This is due to the fact that some organic matter may be adsorbed onto the floc
that may carry over from the clarification process, and any prefiltration pH adjustment
may then result in the "release" of this organic matter, which could pass through the fil-
ters and contribute to subsequent DBP formation.
For plants where only a small increase in pH is required, liquid caustic soda is most
commonly used because of its ease of handling. When a large increase in pH is required,
lime is normally the most economical choice. Lime, however, may add turbidity to a fin-
ished water; therefore, if lime is used for postfiltration pH adjustment, it is generally best
to use a lime saturator to minimize the potential of turbidity addition. Also, in some wa-
ters, the utilization of soda ash for precoagulation alkalinity adjustment often helps the
overall coagulation process.

COAGULATION AND MEMBRANE TECHNOLOGY

Low-pressure membrane technology (micro- and ultrafiltration) is becoming significantly
more prevalent in the drinking water industry (see Chapter 13). Low-pressure membranes
are purely size-exclusionary devices. As a result, anything smaller than membrane pore
sizes (approximately 0.01 to 0.1/xm) will pass through the membrane, Therefore, mem-
brane feedwaters with dissolved materials, such as organics and metals, require some form
of additional treatment.
Often, in these cases, the most economical pretreatment process is simple coagulation.
Potential coagulants for membrane pretreatment include those also used for conventional
water treatment. Additionally, organic adsorption media such as PAC and MIEX, or ox-
idants such as potassium permanganate, chlorine, or chlorine dioxide can be applied up-
stream of a low-pressure membrane (assuming appropriate membrane compatibility) for
enhanced dissolved material removal.
Similar to a direct filtration mode of operation for conventional technology, the goal
of coagulation for membrane pretreatment is to produce a pinpoint floc that is capable of
adsorbing dissolved matter, but minimizes solids loading onto the membrane filtration
process. As noted briefly above, it is important to quantify membrane compatibility and
performance with the coagulant of choice. Each commercially available membrane uti-
lizes different membrane materials. As a result, the compatibility and performance of a
coagulant for membrane filtration pretreatment will likely vary between membrane sys-
tem and raw water supplies. As such, there are no specific guidelines for membrane sys-
tem precoagulation except the general guidelines that are associated with conventional
treatment.