Page 115 - Membranes for Industrial Wastewater Recovery and Re-Use

94 Membranes for Industrial Wastewater Recovery and Re-use

Table 3.4 Water quality variables of importance for controlling corrosion
Parameter, optimum range Rationale

Alkalinity, consistency Variations in concentration can influence formation of
metal carbonate complexes and result in increased metal
release.
Ammonia, 0 mg I-’ Can complex with metals, increasing solubility and metal
release, and can also promote biological growth and
biocorrosion. Corrosive to copper alloys causing metal loss
and stress corrosion cracking at levels greater than 2 mg 1-l
as N. Reduces effectiveness of chlorine through chloramine
formation. Increases with cycles ofconcentration.
Calcium, consistency Can form calcite on pipe walls, possibly preventing corrosion.
Variations in water quality promotes dissolution of scales
and thus metal release.
Chloride, consistency Can increase corrosion rates under conditions of low dissolved
oxygen. Can form complexes with metals increasing solubility
and metal release. Influences corrosion ofmost metals
including mild steel, copper, stainless steel. Chlorides over
200 mg I-’ can cause stress corrosion cracking and/or pitting
corrosion.
Dissolved oxygen, consistency Can serve as an electron acceptor for corrosion reactions.
However it also reacts to form protective oxide layers that
prevent corrosion. Localized differences can promote corrosion.
Iron, consistency Can combine with phosphate and can also deactivate polymers
used to inhibit calcium phosphate scaling.
Magnesium, consistency Can interfere with deposition of calcium complexes on pipe
wall.
Organic carbon (TOC). consistency Can coat pipe surfaces and prevent metal release. It can also
form metal complexes and increase metal release and can
promote biological growth and biocorrosion.
Orthophosphate, 0.5-5 mg I-’ Can form a passivating layer on the internal surface of heat
as PO4 exchangers.
pH, 7.3-7.8 pH units Acidity increases metal solubility and promotes COz liberation
from alkalinity. pH may be locally higher at pipe surface due
to OH- generation.
Silica, below 150 ppm Can act to passivate surfaces, depending on the concentration,
thus decreasing the potential for metal release. Total silica
refers to colloidal and ionic silica in water. Polymeric silica is
non-reactive but it can revert to silica monomers after heating
in boiler water.
Sulphate, consistency Can interfere with formation of some scales, thus increasing
the potential for metal release.
Temperature, consistency Corrosion rates increase with increasing temperature.
Influences solubility, rates of microbiological activity, water
density and associated mixing efficiency.

3.7.9 Volumes and quality of aqueous process waste streams
Aqueous process waste streams include blowdown water from cooling
operations and boilers. The quantity of water generated is related to operational
variables including the source of the make-up water and the cycles of