Municipal Wastewater Reuse for Cooling

7/30/2019 Municipal Wastewater Reuse for Cooling

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Inorganic phosphate, present in many treated wastewaters, can enhance corrosion

protection. Much of this phosphate, however, is often complexed with iron and organicsand may not be present in a form available to provide corrosion protection.

Galvanic Corrosion

Assuming that dissimilar metals have not been coupled as a result of system design,

galvanic corrosion can be very subtle in a cooling system. Consider the effects of awastewater containing 0.2 mg/l of copper or nickel entering a cooling system which

operates at ten cycles of concentration. Levels of copper or nickel in the cooling systemwould reach 2.0 mg/l. Much of this metal would plate out on steel with copper or nickel

becoming a cathodic site to the steel anode. Localized attack due to the galvanic action ofdissimilar metals could rapidly penetrate a steel heat exchanger tube with a tube wall

thickness of 1/16" or less.

Under-deposit Corrosion

Deposits in cooling systems result from the settling of suspended solids, accumulation of

corrosion products, precipitation of salts such as calcium carbonate, sulfate, or phosphate,and magnesium silicate, and from bacterial growth on heat exchange surfaces. Such

deposits cause a localized area of metal to be shielded from the bulk cooling water flowcreating two different chemical environments or a differential concentration cell. The

effect of localized corrosion or pitting attack can be severe on thin walled heat exchangertubes.

Microbiologically Induced CorrosionThe most common corrosion causing organisms in cooling systems are sulfide producingbacteria (Desulfoviborio, Clostridium, and Thiobacillus). These are anaerobic bacteria

which utilize a sulfur source for metabolism and produce hydrogen sulfide. Beinganaerobic, sulfide producers occur beneath deposits causing pitting corrosion, most

severe on mild and stainless steels.

In cooling waters containing high ammonia content, nitrifying bacteria can proliferate.These bacteria including Nitrobacter and Nitrosomonas can convert ammonia and

ammonia compounds to nitric acid. Such occurrences have caused pH depressions,increasing corrosion on most metals. The need for good microbiological control programs

when using a wastewater makeup source cannot be too strongly emphasized.

Salt Precipitation

In all open recirculating cooling systems the solubility of many salts can quickly be

exceeded and must be controlled by blowdown and with chemical scale inhibitors. Themost common cooling water precipitates are calcium carbonate, calcium sulfate, calcium

orthophosphate, silica, and magnesium silicate. Scale formation is strongly influenced bypH and temperature. With the exception of silica these scales generally show inverse


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solubility to increasing pH and temperature. One result is that scale first appears in

hottest spots of a cooling efficiency. Chemical scale inhibitors include polyphosphates,organic phosphonates, and carboxylated polymers. These materials function by holdingthe precipitating species in solution or by modifying crystal growth so that the precipitate

is in a non-adherent form. All cooling systems must still blowdown so that dissolvedsolids do not concentrate beyond the capabilities of chemical scale inhibitors. Most

cooling systems also feed acid for pH control in the range of 6.5-8.5. When a wastewatermakeup is being considered, salt solubilities must be predicted.

Because of high phosphate content in most secondary wastewaters, calcium

orthophosphate is commonly the most troublesome salt. Phosphate is also troublesomedue to its tendency to precipitate as iron phosphate or as an iron/calcium/phosphate

complex. In the presence of aluminum, aluminum phosphate precipitation will occur.

Suspended Solids

Suspended material tends to drop out in low flow areas of a cooling system or onto

biologically active sites. The amount of suspended solids which chemical treatment cankeep dispersed depends upon characteristics of the cooling system. When there are low

cooling water velocities and certainly when "shell side" cooling is present, suspendedsolids levels below approximately 50 mg/l the cooling water must be maintained. In

systems with consistently high flow velocities chemical treatments can effectivelydisperse suspended solids of several hundred mg/l.

Microbiological FoulingThe high nutrient value of wastewater plus warm and well aerated conditions are ideallysuited for bacteria and algae growth. The area most sensitive to microbiological growth is

heat exchange surfaces where even a slight growth will dramatically reduce equipmentefficiency and provide sites for suspended material to adhere.

On cooling tower distribution decks exposure to sunlight promotes algae growth; and if

not controlled, distribution nozzles can become clogged. In many cooling systems using amakeup water with high nutrient content, cooling tower decks are covered.

Cooling tower fill, the tower's heat transfer media, can also be susceptible to

microbiological fouling. Splash fill due to turbulent flow which it promotes is much lesssusceptible to microbiological fouling. Due to quiescent flow conditions in film filled

towers, algal, bacterial, and fungal material can more easily attach to the fill surface.With a large surface area and tight spacing in film fill, flow can be interrupted, and a

uniform layer of microbiological growth will add considerable weight to the fill sectionof the cooling tower.


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makeup, concern has also been expressed regarding the effects of these biocidal agents

carried by drift to work areas and surrounding neighborhoods. Solutions to theseconcerns must involve effective disinfection and bacteria removal before wastewaterenters the cooling system, effective microbiological control with the cooling system,

effective foaming control, improved mechanical drift eliminator sections of the tower,and placement of the cooling tower where cooling tower drift will not contact workers or

surrounding neighborhoods.

Discharge Limitations

Due to the concentrating mechanism of the cooling system, dissolved and suspended

solids present in makeup water can conceivably concentrate to levels unacceptable fordischarge. Heavy metals, for example, present in low concentrations within the makeup

may increase in concentration by 5-10 times causing cooling tower blowdown to beunacceptable for discharge even when diluted by other plant waste streams.

Consistent Makeup Water Quality

A cooling system is dynamic due to constant evaporation, makeup, and blowdown. Thecooling tower operator attempts to find a steady state where water and treatment chemical

use rates are optimized. A set of narrow chemical control ranges are usually developed asguidelines. Control parameters will include pH, calcium, T.D.S., silica and desired

residuals for corrosion and scale inhibitors. Upsets in makeup water chemistry canquickly change chemistry within the cooling system particularly if the system is being

monitored only one time per shift or less. Even after upsets have been discovered there

will be some lag time before the problem is corrected. The result is frequently increasedrates of corrosion, fouling, or scaling. Thus, the cooling tower operator will be veryinterested in the reliability of wastewater treatment steps and holding capacity or flow

equalization which precedes the cooling system.

Treatment

It is important to consider the composition of wastewater makeup relative to cooling

system characteristics and its operation.

Most secondary municipal wastewaters will contain high nutrient content and high andfluctuating levels of phosphate. Biofouling and calcium phosphate scaling are the most

common problems associated with a wastewater makeup source. High temperaturecooling systems are more prone to scaling. Low flow conditions, shell side and jacket

cooling are more prone to deposition of suspended material. On the other hand, heatexchangers with tube side cooling and high flow velocities are self cleaning; suspended

solids deposition is not a major problem. Carbon steel is much more susceptible tocorrosion and fouling than copper and copper alloys. Although stainless steel is very

resistant to general corrosion, sulfide producing bacteria or concentration of chloridesbeneath deposits can cause severe pitting attack. Ammonia will also accelerate corrosion


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of copper and copper alloys. Wastewater makeup can be successful if proper physical and

chemical treatment of makeup and recirculating cooling water is selected andimplemented.

Table 1: Criteria In Examining A Wastewater Makeup Source

Makeup Water Cooling System

Nutrient Value Metallurgy

Phosphate Flow Velocities

Suspended Solids Maximum Temperatures

TDS Cooling Tower Construction

Metals Content Varying OperationAmmonia/sulfide pH Process In-leakage

Scaling Potential Side Stream Filtration

Bacteriological Content Operator Attention/Sophistication

Consistency of Composition

Within the recirculating cooling water system, treatment must include acceptable levelsof the following:

1. Steel corrosion inhibition2. Copper corrosion inhibition

3. pH control4. Calcium phosphate scale inhibitors or dispersants

5. Bio-mass control with oxidizing and non-oxidizing microbiocides6. Suspended solids dispersants

7. Calcium carbonate scale inhibitors, surfactants to enhance microbiological control,side stream filtration, and antifoams are also often required

Table 2: Steel Corrosion Inhibition

Control Range for Film

Inhibitor Maintenance

Chromate 5-20 ppmInorganic Phosphate 10-30 ppm

Zinc 3-5 ppm

Polysilicate 10-20 ppm

Molybdate 5-20 ppm


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Several of these inhibitors when used together have a synergistic effect. For example, 1-2

ppm of zinc fed with chromate, phosphate, or molybdate allow lower levels of chromate,phosphate, or molybdate.

When chromate is not permitted, steel corrosion inhibition should take advantage of theinorganic phosphate present in makeup water. Remember, however, that much of this

phosphate will be complexed with iron and organics; it may not all be available forcorrosion inhibition. Even when there is a phosphate removal step on the makeup water,

some phosphate (usually 1-2 ppm) will enter the cooling system and concentrate to 5-15ppm or higher. Because of this calcium phosphate content, successful treatment must

include reliable pH control and calcium phosphate dispersants. The pH should becontrolled between 6.8-7.5. Dispersants may permit operation at high pH; however, there

will be much greater risk of calcium phosphate deposition particularly if phosphate levelsfluctuate in makeup water.

Steel corrosion inhibition with a wastewater makeup should be accomplished with the

addition of 2-4 ppm zinc, 5-15 ppm. molybdenum, or polysilicate at 10-30 ppm, or azinc/ molybdenum combination. These materials will act synergistically with the

phosphate which is already present. Zinc is also strongly influenced by pH; zinc salts canbegin to precipitate when pH is greater than 7.5. Molybdenum is insensitive to pH but

loses efficiency in high TDS (greater than 5000 ppm) waters.

There are some "all organic" steel corrosion control programs currently available. These

programs generally require relatively high pH operation (above 7.5 up to 9.0). Withwastewater makeup, containing some phosphate, the pH ranges required for manyorganic programs would likely cause calcium phosphate scale.

To summarize, successful steel corrosion control programs with a wastewater makeup

will require strict pH control below approximately pH 7.5 to prevent calcium phosphateprecipitation. Phosphate present in makeup water should be supplemented with zinc,

molybdenum, or polysilicate to achieve synergistic corrosion control.

Copper Corrosion Control

Even with high ammonia content in makeup waters, corrosion protection of copper and

copper alloys can be achieved with proper addition of copper inhibitors such astolyltriazol or benzotriazole. These materials are normally maintained at 1.5-3.0 ppm.

High ammonia may require levels of 3-5 ppm.

Calcium Phosphate Inhibition

Strict pH control is the best method of calcium phosphate inhibition. There are also many

new polymeric materials which increase the solubility of calcium phosphate or distort thecrystal growth of calcium phosphate so that a non-adherent precipitate is formed.


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Sulfonated polymers such as sulfonated styrene maleic anhydride (SSMA) are proving to

be the most effective calcium phosphate inhibitors. Other acrylate-based copolymers andpolymaleics are also effective. To date, however, only incremental improvements havebeen made in this technology. When using a wastewater makeup, a suitable polymeric

inhibitor should be applied to maintain approximately 15-40 ppm. These materials shouldnot be the sole source of calcium phosphate inhibition. Consistent pH control is the first

requisite.

Because of the constant risk of calcium phosphate precipitation, a wastewater makeupsource cannot tolerate sloppy pH control. It is desirable to increase pH (to reduce the

aggressiveness of the cooling water); however, too high a pH will still cause calciumphosphate precipitation. Non-chromate programs should operate in the range of 7.0-7.5.

If phosphate residuals are consistently below 15-20 ppm and good calcium phosphatedispersants are applied, pH control may be raised to pH 7.0-8.0.

Microbiological Control

Because of their cost effectiveness, oxidizing biocides, chlorine or bromine, should be thebasis for microbiological control in the recirculation cooling water. In wastewaters with

some ammonia content, chloramines and bromamines will be formed; bromamines arethe most effective microbiocidal agents. These materials are typically fed to maintain 0.1-

0.3 ppm free available halogen on a continuous basis or slug fed on time per day (or moreoften) to achieve a free available halogen residual of 1.0 ppm for 2-4 hours. With

wastewater makeup, total bacterial counts should be maintained below 10' colonies per

milliliter in the recirculating cooling water. Non-oxidizing microbiocides will generallybe required in addition to chlorine or bromine because of the high nutrient content ofwastewaters. Remember that most polymeric scale inhibitors and dispersants added to the

cooling water are anionic; thus, suitable non-oxidizing biocides should also be anionic ornon-ionic and be effective in the pH range anticipated within the cooling water.

Suspended Solids Dispersion

Low molecular weight (approximately 1000) acrylate based polymers provide the bestavailable suspended solids dispersion. Levels will vary from 5-40 ppm depending upon

the needs of the cooling system and makeup water characteristics. Low foaming, non-ionic surfactants enhance microbiological control programs by allowing penetration of

biological slimes. This type of treatment may be necessary with a wastewater makeup.

Foaming in the cooling systems with a wastewater makeup is common and can beattributed to wetting agents present in makeup or added to enhance microbiological

control. Organic components of makeup waters or organic microbiocides will also causefoaming. Silicone or hydrocarbon based antifoams may be added periodically.


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Summary

Treated municipal wastewater can and is successfully used to replace fresh water as acooling tower makeup in utility and industrial cooling water systems. Success firstdepends upon a thorough knowledge of potential problems possibly related to the

contaminants in the wastewater and how they can and should be controlled. It alsorequires matching the entire cooling water system characteristics and operation with the

water treatment chemicals to enable continuous efficient operation.

Using wastewater in place of fresh water not only enables water conservation but canlead to overall cost reduction for cooling tower operation. Chemical treatment costs for

corrosion, scaling, and microbiological control will, however, likely increase. Greaterattention and testing means higher labor costs with wastewater versus fresh water. Plus,

the potential for scaling, fouling, and corrosion is always higher with wastewatermakeup. Still the use of municipal wastewater as cooling tower makeup is a viable

approach in fresh water short regions and should be considered. However, only withsufficient knowledge and experience in matching it with the cooling system will it stay

protected.

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Municipal Wastewater Reuse for Cooling