Recycling Treated Municipal Wastewater for Industrial Water Use

Industrial Water Quality Concerns
Section 3: Recycled Wastewater System Components and Costs
Recycling Treated Municipal Wastewater for Industrial Water Use
Industrial uses of recycled wastewater include cooling, process water, stack scrubbing, boiler feed,
washing, transport of material, and as an ingredient in a product. Cooling is the predominant recycled
wastewater application, accounting for more than 90 percent of the total volume of recycled wastewater in
the U.S. used for industrial purposes. As shown in the demand analysis of Section 2, cooling water is also
the predominant industrial water use in Minnesota. Cooling and boiler feed water are water uses
applicable to multiple industry categories. The constituents of concern for these water uses and some
process water uses are listed in Table 3.2 and are discussed in more detail below.
Cooling Water
The constituents of concern for cooling water uses include: pathogenic microorganisms, inorganic matter
that leads to scale formation, dissolved solids that can cause corrosion, and organic matter and nutrients
that promote biological growth and the formation of slimes. These problems are caused by constituents in
ground or surface waters and potable water, as well as recycled wastewater, but the concentrations of
some constituents in recycled wastewater may be higher. These constituents need to be controlled in the
supply to the cooling systems and may also have to be removed from the blowdown prior to discharge,
depending on the cycles of concentration and discharge option (surface water, land application, or sewer
system) and the corresponding limits.
Pathogenic microorganisms in water supplied to cooling towers must be eliminated prior to use so there is
no hazard to workers and to the public in the vicinity of cooling towers from aerosols and windblown
spray. Biocides are added to all cooling waters onsite to prevent slimes and otherwise inhibit
microbiological activity, which has the secondary effect of eliminating or greatly diminishing the
potential health hazard associated with aerosols or windblown spray. Biocide addition is required for
recycled wastewater and traditional water supplies. Aerosols produced in the workplace or from cooling
towers also may present hazards from the inhalation of VOCs. This same hazard exists with traditional
water supplies that could have VOCs present. There has been no indication that VOCs have created health
problems at any existing recycled wastewater site. Closed-loop cooling systems using recycled
wastewater present minimal health concerns unless there is inadvertent or intentional misuse of the water.
All cooling water systems should be operated and maintained to reduce the Legionella threat, regardless of
the origin of the source water. There have been no reported cases to show that recycled wastewater is more
likely to contain Legionella pneumophila bacteria than waters of non-sewage origin.
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
through chemical addition or other treatment processes is common for facilities using potable, supplies or
their own permitted ground or surface supply with naturally hard water. The higher concentrations of
these inorganic constituents in recycled wastewater may require more extensive treatment than with an
existing supply.
High levels of dissolved solids, ammonia, and heavy metals in cooling water can cause serious corrosion
problems. Corrosion potential is higher in recycled wastewater where total dissolved solids (TDS)
concentrations are between 100-400 mg/L more than in traditional water supplies [Puckorius and Hess,
1991; Tchobanoglous et al., 2003]. Of particular concern in Minnesota are high chloride levels. Many of
Minnesota’s recycled wastewater supplies may have high chloride levels as a result of softening system
salt brine disposal from homes and commercial and industrial businesses. This was evident in the
sampling of Twin Cities metro area WWTPs that identified higher levels of chlorides in communities
served by potable ground water supplies without centralized softening treatment. Chloride concentration
in WWTP effluent has also been linked to the influence of infiltration and inflow (I&I) and chlorides
imparted from road salt used for winter deicing.
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