Recycling Treated Municipal Wastewater for Industrial Water Use

Section 2
Implementation Considerations
Craddock Consulting Engineers 2-15
In Association with CDM & James Crook
TM1-Sec2_0707.doc
minimal health concerns unless there is inadvertent or intentional misuse of the
water.
There is no indication that reclaimed water is more likely to contain Legionella
pneumophila bacteria than waters of non-sewage origin. All cooling water systems
should be operated and maintained to reduce the Legionella threat, regardless of the
origin of the source water.
In general, the major problems related to power plants employing municipal effluents
as makeup water are scale formation, corrosion, foaming, and biological fouling due
to high residual organic substrate and nutrient concentrations in the wastewater.
These problems are caused by contaminants in potable water as well as reclaimed
water, but the concentrations of some contaminants in reclaimed water may be
higher.
Cooling water should not lead to the formation of scale, i.e. hard deposits in the
cooling system. Such deposits reduce the efficiency of the heat exchange. The
principal causes of scaling are calcium (as carbonate, sulfate, and phosphate) and
magnesium (as carbonate and phosphate) deposits. Scale control for reclaimed water
is achieved through chemical means and sedimentation. Acidification or addition of
scale inhibitors can control scaling. Acids (sulfuric, hydrochloric, and citric acids and
acid gases such as carbon dioxide and sulfur dioxide) and other chemicals (chelants
such as EDTA and polymeric inorganic phosphates) are often added for pH and
alkalinity control to increase the water solubility of scale-forming constituents, such as
calcium and magnesium. Lime softening removes carbonate hardness and soda ash
removes noncarbonate hardness. Other methods used to control scaling are alum
treatment and sodium ion exchange.
High levels of dissolved solids, ammonia, and heavy metals in reclaimed water can
cause serious increased corrosion rates [Puckorius and Hess, 1991]. The
concentrations of TDS in municipally treated reclaimed water can increase electrical
conductivity and promote corrosion. Ammonia can induce corrosion in copper-based
alloys. Dissolved gases and certain metals with high oxidation states also promote
corrosion. For example, heavy metals, particularly copper, can plate out on mild
steel, causing severe pitting. Corrosion also may occur when acidic conditions
develop in the cooling water. Corrosion inhibitors such as chromates,
polyphosphates, zinc, and polysilicates can be used to reduce the corrosion potential
of the cooling water. These substances may have to be removed from the blowdown
prior to discharge. An alternative to chemical addition is ion exchange or reverse
osmosis.
Reclaimed water used in cooling systems should not supply nutrients or organic
matter that promote the growth of slime-forming organisms. The moist environment
in the cooling tower is conducive to biological growth. Microorganisms can
significantly reduce the heat transfer efficiency, reduce water flow, and in some cases