Recycling Treated Municipal Wastewater for Industrial Water Use

Section 2
Implementation Considerations
generate corrosive by-products [California State Water Resources Control Board,
1980].
Sulfide-producing bacteria and sulfate-reducing bacteria are the most common
corrosion-causing organisms in cooling systems using reclaimed water. These
anaerobic sulfide producers occur beneath deposits and cause pitting corrosion that is
most severe on mild and stainless steels. Serious corrosion is caused by thiobaccillus
bacteria, an acid-producer that converts sulfides to sulfuric acid. Similarly, nitrifying
bacteria can convert ammonia to nitric acid, thus causing pH depression, which
increases corrosion on most metals.
Removal of BOD and nutrients during treatment reduces the potential of the
reclaimed water to sustain microorganisms. Chlorine is the most common biocide
used to control biological growth because of its low cost, availability, and ease of
operation. Chlorination is also used as a disinfectant to reduce potential pathogens in
the reclaimed water. Frequent chlorination and shock treatment are generally
adequate.
Non-oxidizing microbiocides are generally required in addition to chlorine because of
the high nutrient content typically found in wastewater. Since most scale inhibitors
and dispersants are anionic, either anionic or nonionic biocides are usually used.
Low-foaming, nonionic surfactants enhance microbiological control by allowing the
microbiocides to penetrate the biological slimes. Chemical coagulation and filtration
during the phosphorus removal treatment phase significantly reduce the
contaminants that can lead to fouling. Chemical dispersants are also used as
required.
In many cases, power plants utilize disinfected secondary effluent for cooling water,
although additional treatment often is needed for recirculating cooling systems.
Additional treatment may include lime or alum treatment, filtration, ferric chloride
precipitation, ion exchange, and reverse osmosis. In some cases, only additional
chemical treatment is necessary, which may include many of the chemicals mentioned
above and others, such as phosphonates or calcium phosphate for destabilization,
polyacrylates for suspended solids dispersion, and anti-foaming agents for dispersion
of foam caused by phosphates and some organic compounds.
Boiler Feed Water. The use of reclaimed water for boiler feed water often requires
extensive additional treatment and is not a common use of reclaimed water. Quality
requirements for boiler-feed makeup water are dependent upon the pressure at which
the boiler is operated. Generally, the higher the pressure, the higher the quality of
water required.
Reclaimed water must be treated to remove hardness. Calcium and magnesium salts
are the principal contributors to scale formation and deposits in boilers. Excessive
alkalinity contributes to foaming and results in deposits in heater, reheater, and
turbine units. Bicarbonate alkalinity, under the influence of boiler heat, may lead to
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TM1-Sec2_0707.doc