Recycling Treated Municipal Wastewater for Industrial Water Use
Recycling Treated Municipal Wastewater for Industrial Water Use Recycling Treated Municipal Wastewater for Industrial Water Use
Section 2 Implementation Considerations requirements can lead to inconsistencies in permit requirements across the state and may result in requirements that represent “moving targets,” leaving proponents unsure (and perhaps unable) to determine the requirements that may be placed on them. A statewide set of comprehensive water reuse regulations would provide definitive information to assist industries and municipalities in the planning and implementation of projects. On the other hand, a case-by-case system can allow for greater flexibility in developing reuse projects. 2.3 Technical Considerations The main technical issues that must be addressed with water reuse involve water quality and conveyance of water from the WWTP to the industry. The most costeffective source would be one where the wastewater effluent water quality meets all the industrial water quality criteria and conveyance requirements are minor. In most cases, some additional treatment is required to meet the industrial process requirements or health-related criteria. This section will provide some general industry water quality requirements and issues with reclaimed water use. Treatment requirements and technologies are summarized for several industrial uses. Conveyance considerations are very site specific and are not discussed in this Technical Memorandum. However, an assessment of conveyance requirements will be conducted as part of Task 2 of this project. Water Quality Industrial Water Quality Concerns Due to the myriad of industrial processes that use water and site-specific conditions, regulatory agencies generally prescribe water reuse requirements on an individual case basis, except for some common widespread uses such as cooling water. Reclaimed water from conventional wastewater treatment processes is of adequate quality for many industrial applications that can tolerate water of less than potable quality, and it has the important advantage of being a reliable supply. Industries are often located near populated areas that generate large volumes of wastewater. Industrial uses of reclaimed water include cooling, process water, stack scrubbing, boiler feed, washing, transport of material, and as an ingredient in a product. Cooling is the predominant reuse application, accounting for more than 90 percent of the total volume of reclaimed water used for industrial purposes. Cooling Water. Pathogenic microorganisms in reclaimed water used in cooling towers present potential hazards to workers and to the public in the vicinity of cooling towers from aerosols and windblown spray. In practice, however, biocides are usually 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. Aerosols produced in the workplace or from cooling towers also may present hazards from the inhalation of VOCs, and although little definitive research has been done in this area, there has been no indication that VOCs have created health problems at any existing water reuse site. Closed-loop cooling systems using reclaimed water present 2-14 Craddock Consulting Engineers In Association with CDM & James Crook TM1-Sec2_0707.doc
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
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Section 2<br />
Implementation Considerations<br />
Craddock Consulting Engineers 2-15<br />
In Association with CDM & James Crook<br />
TM1-Sec2_0707.doc<br />
minimal health concerns unless there is inadvertent or intentional misuse of the<br />
water.<br />
There is no indication that reclaimed water is more likely to contain Legionella<br />
pneumophila bacteria than waters of non-sewage origin. All cooling water systems<br />
should be operated and maintained to reduce the Legionella threat, regardless of the<br />
origin of the source water.<br />
In general, the major problems related to power plants employing municipal effluents<br />
as makeup water are scale <strong>for</strong>mation, corrosion, foaming, and biological fouling due<br />
to high residual organic substrate and nutrient concentrations in the wastewater.<br />
These problems are caused by contaminants in potable water as well as reclaimed<br />
water, but the concentrations of some contaminants in reclaimed water may be<br />
higher.<br />
Cooling water should not lead to the <strong>for</strong>mation of scale, i.e. hard deposits in the<br />
cooling system. Such deposits reduce the efficiency of the heat exchange. The<br />
principal causes of scaling are calcium (as carbonate, sulfate, and phosphate) and<br />
magnesium (as carbonate and phosphate) deposits. Scale control <strong>for</strong> reclaimed water<br />
is achieved through chemical means and sedimentation. Acidification or addition of<br />
scale inhibitors can control scaling. Acids (sulfuric, hydrochloric, and citric acids and<br />
acid gases such as carbon dioxide and sulfur dioxide) and other chemicals (chelants<br />
such as EDTA and polymeric inorganic phosphates) are often added <strong>for</strong> pH and<br />
alkalinity control to increase the water solubility of scale-<strong>for</strong>ming constituents, such as<br />
calcium and magnesium. Lime softening removes carbonate hardness and soda ash<br />
removes noncarbonate hardness. Other methods used to control scaling are alum<br />
treatment and sodium ion exchange.<br />
High levels of dissolved solids, ammonia, and heavy metals in reclaimed water can<br />
cause serious increased corrosion rates [Puckorius and Hess, 1991]. The<br />
concentrations of TDS in municipally treated reclaimed water can increase electrical<br />
conductivity and promote corrosion. Ammonia can induce corrosion in copper-based<br />
alloys. Dissolved gases and certain metals with high oxidation states also promote<br />
corrosion. For example, heavy metals, particularly copper, can plate out on mild<br />
steel, causing severe pitting. Corrosion also may occur when acidic conditions<br />
develop in the cooling water. Corrosion inhibitors such as chromates,<br />
polyphosphates, zinc, and polysilicates can be used to reduce the corrosion potential<br />
of the cooling water. These substances may have to be removed from the blowdown<br />
prior to discharge. An alternative to chemical addition is ion exchange or reverse<br />
osmosis.<br />
Reclaimed water used in cooling systems should not supply nutrients or organic<br />
matter that promote the growth of slime-<strong>for</strong>ming organisms. The moist environment<br />
in the cooling tower is conducive to biological growth. Microorganisms can<br />
significantly reduce the heat transfer efficiency, reduce water flow, and in some cases