Water Chemistry and Industrial Water Treatment

Introduction

Water is both a raw material and a process utility in mechatronic manufacturing, power generation, food processing, pharmaceuticals, cooling systems, and environmental control. Although natural water may appear clear, it contains dissolved ions, gases, suspended particles, microorganisms, and organic compounds. These constituents determine whether water is suitable for drinking, heat transfer, boiler operation, electronic assembly, or discharge to the environment.

Industrial water treatment is therefore an application of chemistry, fluid mechanics, materials engineering, instrumentation, and process control. A treatment system must remove harmful constituents while controlling energy consumption, chemical use, waste production, reliability, and cost. The correct process depends on the water source, required product-water quality, operating temperature and pressure, and environmental regulations.

This chapter develops the chemistry used to assess water quality and select treatment processes. It also examines hardness, alkalinity, salinity, dissolved gases, boiler and cooling-water problems, softening, demineralization, membranes, disinfection, and wastewater treatment.

Learning Objectives

By the end of this chapter, the reader should be able to:

1. Explain the chemical composition and important quality parameters of natural and industrial water.
2. Calculate hardness, alkalinity, salinity-related quantities, and basic treatment requirements.
3. Distinguish carbonate scaling, corrosion, fouling, and biological problems in boilers and cooling systems.
4. Select appropriate softening, demineralization, membrane, disinfection, and wastewater-treatment processes.
5. Interpret treatment data and evaluate process limitations, waste streams, safety, and regulatory obligations.

1. Water as a Chemical System

Water is a polar molecule. Its oxygen atom attracts electrons more strongly than hydrogen, producing partial charges and enabling hydrogen bonding. This polarity gives water a high dielectric constant and allows it to dissolve many ionic and polar substances. Natural water consequently becomes a dilute electrolyte solution rather than chemically pure H₂O.

The principal dissolved ions commonly found in fresh water are calcium Ca²⁺, magnesium Mg²⁺, sodium Na^+, potassium K^+, bicarbonate HCO₃^-, carbonate CO₃^2-, chloride Cl^-, sulfate SO₄^2-, and nitrate NO₃^-. Their concentrations depend on geology, rainfall, seawater intrusion, agriculture, industrial activity, and municipal discharges.

Water quality is usually described by measured parameters rather than by a single number. Important parameters include:

• pH and temperature;
• turbidity and suspended solids;
• electrical conductivity and total dissolved solids;
• hardness and alkalinity;
• dissolved oxygen, carbon dioxide, and hydrogen sulfide;
• nutrients such as nitrogen and phosphorus;
• metals, oils, solvents, pesticides, and other organic pollutants;
• microbial indicators and pathogens.

1.1 pH and acid–base chemistry

pH is defined as the negative logarithm of hydrogen-ion activity. In dilute solutions, activity is often approximated by concentration:

Here, a_H^+ is hydrogen-ion activity and [H^+] is hydrogen-ion concentration in mol L⁻¹. The approximation is most reliable for dilute solutions with relatively low ionic strength. At 25 °C, neutral water has pH approximately 7, but neutrality changes with temperature because the ionization constant of water changes.

The ionization equilibrium is:

At 25 °C:

Acidic water can increase corrosion, whereas strongly alkaline water can promote caustic attack, scaling, and skin hazards. pH must therefore be interpreted with alkalinity, temperature, conductivity, and the materials in contact with the water.

1.2 Electrical conductivity and total dissolved solids

Electrical conductivity indicates the ability of water to carry current through dissolved ions. It is temperature dependent and is commonly reported at a reference temperature, often 25 °C. Total dissolved solids (TDS) is a mass concentration, normally in mg L⁻¹. Conductivity can provide a rapid operational estimate of TDS, but the conversion factor depends on ion composition and must be calibrated for the particular water.

Neither conductivity nor TDS identifies individual contaminants. Water with low conductivity may still contain toxic neutral organics or microorganisms, while high conductivity generally indicates a substantial ionic load.

2. Hardness and Alkalinity

2.1 Hardness

Hardness is primarily caused by dissolved calcium and magnesium. It is conventionally reported as milligrams per litre as calcium carbonate, mg\,L^-1\ as\ CaCO₃. The conversion is based on equivalent concentration:

where C_i is the concentration of hardness-producing ion i in mg L⁻¹, E_i is its equivalent weight in g equivalent⁻¹, and 50.0 is the equivalent weight of CaCO₃ in mg per milliequivalent. For calcium, E_Ca²⁺ = 40.08/2 = 20.04, and for magnesium, E_Mg²⁺ = 24.31/2 = 12.16. The formula assumes that reported concentrations refer to the ions themselves, not to their salts.

Hardness is often classified approximately as follows:

These categories are descriptive rather than universal regulatory limits.

Temporary hardness is associated mainly with calcium and magnesium bicarbonates. Heating decomposes bicarbonate and may produce carbonate scale:

Permanent hardness is associated mainly with sulfates, chlorides, and nitrates of calcium and magnesium. It is not removed simply by boiling. Hardness causes soap consumption, deposits in heat exchangers, and reduced boiler efficiency.

2.2 Alkalinity

Alkalinity is the acid-neutralizing capacity of water. It is mainly due to hydroxide, carbonate, and bicarbonate, although borate, phosphate, silicate, and organic bases may contribute. Alkalinity is reported as mg\,L^-1\ as\ CaCO₃. It is not the same as pH: pH describes current hydrogen-ion activity, whereas alkalinity describes buffering capacity.

For a simplified carbonate system, total alkalinity is:

The concentrations in this equation are equivalent concentrations, commonly expressed in mol L⁻¹ as charge equivalents. The equation assumes that carbonate species dominate and that other weak-acid systems are negligible.

In a laboratory titration, alkalinity as calcium carbonate can be calculated from the acid volume:

where V_a is acid volume in mL, N_a is acid normality in equivalents L⁻¹, V_s is sample volume in mL, and 50,000 is the conversion factor to mg\,L^-1\ as\ CaCO₃. The equation assumes a standardized acid and a correctly identified titration endpoint.

Worked Example 1: hardness

A water sample contains 80 mg L⁻¹ calcium and 24 mg L⁻¹ magnesium. Calculate total hardness as CaCO₃.

Calcium contribution:

Magnesium contribution:

Therefore:

The water is very hard by the classification above. The result does not by itself indicate whether hardness is temporary or permanent; anion analysis is also required.

3. Salinity, Dissolved Gases, and Contaminants

Salinity represents the concentration of dissolved salts. In seawater, sodium chloride is dominant, but magnesium, sulfate, calcium, potassium, and bicarbonate are also important. High salinity increases conductivity and osmotic pressure, damages crops, reduces suitability for boilers, and accelerates corrosion under some conditions.

Dissolved oxygen is essential for aquatic ecosystems but often undesirable in boilers and closed water circuits because it promotes electrochemical corrosion. Carbon dioxide forms carbonic acid:

Carbon dioxide can lower pH and increase corrosion, particularly in condensate-return systems. Hydrogen sulfide is toxic, has a characteristic odor, and can cause corrosion. Removal may involve aeration, stripping, oxidation, adsorption, or chemical treatment, selected according to concentration and process conditions.

Contaminants can be grouped by treatment behavior:

Chemical oxygen demand (COD) is an operational measure of oxygen equivalent required to chemically oxidize substances in water. Biochemical oxygen demand (BOD) measures oxygen consumed by biological oxidation under specified test conditions. They are not direct measurements of the total mass of every organic compound and should not be interpreted as identical quantities.

4. Boiler-Water and Cooling-Water Problems

4.1 Boiler systems

Boilers convert water into steam. As steam leaves, nonvolatile dissolved solids remain in the boiler, causing concentration cycles and increasing the risk of deposits. Major boiler problems include:

• Scale: hard, adherent deposits such as calcium carbonate, calcium sulfate, and silica compounds. Scale increases thermal resistance and can cause tube overheating.
• Sludge: soft, loose deposits formed when precipitates do not adhere strongly. Sludge can obstruct circulation and must be removed by blowdown.
• Corrosion: attack caused by dissolved oxygen, low pH, carbon dioxide, chloride concentration, or unsuitable chemical control.
• Carryover: entrainment of boiler water droplets in steam, causing contamination of turbines, valves, and process equipment.
• Caustic embrittlement: localized cracking associated with concentrated alkaline conditions and susceptible stressed steel.

The heat-transfer consequence of scale can be expressed using the overall heat-transfer relationship:

where Q is heat-transfer rate in W, U is overall heat-transfer coefficient in W m⁻² K⁻¹, A is heat-transfer area in m², and \Delta T_lm is the logarithmic mean temperature difference in K. Scale adds thermal resistance, reducing U; therefore the same heat duty may require a higher metal temperature, increasing failure risk. The equation applies to steady heat exchange with an appropriately defined overall coefficient.

Control measures include pretreatment, softening or demineralization, deaeration, oxygen scavenging where permitted, phosphate or polymer programs, condensate monitoring, and controlled blowdown. Boiler treatment must follow the pressure, metallurgy, steam use, and applicable boiler-water standards.

4.2 Cooling-water systems

Cooling towers reject heat by sensible heat transfer and evaporation. Evaporation removes water but leaves most dissolved solids behind. The concentration factor, or cycles of concentration, is commonly approximated by:

A more useful steady-state formulation, when evaporation is the only significant dissolved-solids loss, is:

where M is makeup flow, B is blowdown flow, D is drift loss, and C is the concentration factor. If drift is negligible, C ≈ M/B. These relationships assume steady state and conservative dissolved solids; chemical reactions, precipitation, and side-stream treatment can invalidate the simple balance.

Cooling-water problems include corrosion, scale, microbiological slime, and suspended-solids deposition. Control requires a balanced program of filtration, biocide application, corrosion inhibitor, scale inhibitor, pH control, blowdown, and monitoring. Excessive biocide can create environmental hazards; insufficient treatment can cause rapid loss of heat-transfer performance.

5. Industrial Treatment Processes

Treatment is normally arranged as a train. A typical sequence is screening, equalization, coagulation–flocculation, clarification, filtration, softening or membrane treatment, disinfection, and final conditioning. Not every plant requires every step.

5.1 Coagulation, flocculation, and clarification

Coagulation destabilizes colloids by neutralizing charge or forming precipitates. Common coagulants include alum, ferric salts, and polymers. Flocculation provides gentle mixing so destabilized particles collide and form settleable flocs. Clarifiers then separate the floc by gravity.

Important operating variables include pH, dose, mixing intensity, temperature, particle concentration, and settling rate. Jar testing is commonly used to select dose and pH. Excess coagulant may increase sludge production and residual metal concentration.

5.2 Filtration

Media filters remove suspended solids and floc. Multimedia filters use layers of materials with different densities and particle sizes. Cartridge and bag filters provide fine particle removal but require replacement. Granular activated carbon adsorbs many organic compounds and can remove taste and odor; however, it does not reliably remove all dissolved ions and can become a site for microbial growth if poorly operated.

5.3 Lime–soda softening

Lime–soda treatment precipitates calcium carbonate and magnesium hydroxide. Simplified reactions include:

Lime, Ca(OH)₂, supplies hydroxide and carbonate-producing capacity; soda ash, Na₂CO₃, supplies carbonate for noncarbonate calcium hardness. The process can handle large flows but produces sludge and usually requires clarification and filtration.

5.4 Ion exchange softening

A sodium-cycle softener exchanges calcium and magnesium for sodium on a sulfonated polymer resin:

Here, R represents a fixed resin functional group. Regeneration uses concentrated sodium chloride:

Ion exchange is compact and effective for hardness removal, but it does not remove all anions or reduce total dissolved solids. Regeneration produces a salty waste stream. Sodium addition may be unsuitable for certain industrial processes and some drinking-water applications.

5.5 Demineralization

Demineralization removes cations and anions using separate or mixed ion-exchange beds. A hydrogen-cycle cation exchanger replaces cations with hydrogen ions, and an anion exchanger replaces anions with hydroxide ions. The released hydrogen and hydroxide form water. Mixed-bed systems can produce very low conductivity but require careful regeneration and separation of resins.

Demineralization is appropriate for high-purity boiler feed, laboratories, electronics manufacturing, and some process applications. Pretreatment is essential because suspended solids, iron, oil, and organics can foul or poison resin. Demineralization does not necessarily remove microorganisms or uncharged organic molecules.

5.6 Membrane processes

Membrane treatment separates substances using pressure, concentration, or electrical potential.

Reverse osmosis requires pressure greater than the osmotic-pressure difference. The water flux may be represented by:

where J_w is water flux, A_m is membrane water-permeability coefficient, \Delta P is applied pressure difference, and \Delta\pi is osmotic-pressure difference. The equation is an idealized expression; actual flux also depends on concentration polarization, temperature, fouling, and membrane compaction.

Membranes produce a permeate stream and a concentrated reject stream. They can reduce chemical use but require energy, pretreatment, cleaning, and responsible concentrate disposal.

5.7 Disinfection

Disinfection reduces viable pathogens; it is not equivalent to sterilization and does not necessarily remove chemical contaminants. Chlorination, chloramination, ozone, and ultraviolet irradiation are common methods.

For many disinfectants, exposure is described using CT:

where C_r is residual disinfectant concentration in mg L⁻¹ and t is effective contact time in minutes, giving CT in mg min L⁻¹. Required CT depends on organism, temperature, pH, disinfectant, and water quality. The simple product is useful for design screening but does not replace validated regulatory calculations because concentration decays and hydraulic short-circuiting may occur.

Chlorine is economical and leaves a measurable residual, but it can form disinfection by-products with natural organic matter. UV produces no persistent residual and requires low turbidity and reliable lamp performance. Ozone is a strong oxidant but has higher equipment complexity and limited residual protection.

6. Wastewater Treatment

Municipal and industrial wastewater treatment commonly follows four levels:

1. Preliminary treatment: screens, grit removal, and flow equalization.
2. Primary treatment: sedimentation and removal of settleable solids.
3. Secondary treatment: biological oxidation of biodegradable organic matter.
4. Tertiary or advanced treatment: nutrient removal, filtration, adsorption, membranes, advanced oxidation, or disinfection.

Activated sludge uses microorganisms suspended in an aeration tank. Organic matter is converted into biomass, carbon dioxide, and water. Nitrification oxidizes ammonium to nitrate, while denitrification reduces nitrate to nitrogen gas under anoxic conditions. Phosphorus may be removed biologically or by chemical precipitation.

Anaerobic digestion converts concentrated biodegradable sludge into biogas containing methane and carbon dioxide. It can reduce sludge volume and recover energy, but gas safety, odor, pathogen control, and process stability must be managed.

Industrial wastewater may require source segregation, oil separation, pH neutralization, metals precipitation, equalization, biological treatment, or specialized oxidation. A treatment plant should not be designed from COD or BOD alone; toxicity, biodegradability, salinity, temperature, hydraulic variability, and sludge characteristics also matter.

7. Worked Application: Selecting Treatment for a Factory

Consider an illustrative electronics factory using 500 m³ d⁻¹ of borehole water. Analysis gives hardness of 320 mg L⁻¹ as CaCO₃, conductivity of 1,100 µS cm⁻¹, turbidity of 8 NTU, dissolved oxygen of 7 mg L⁻¹, and detectable iron. The factory requires low-hardness water for cooling makeup and low-conductivity water for a boiler.

A defensible treatment train is:

1. Screening and cartridge protection to remove coarse material.
2. Oxidation and filtration to convert dissolved iron into filterable solids.
3. Multimedia filtration to reduce turbidity.
4. Ion-exchange softening for cooling makeup, with hardness monitoring after the softener.
5. Reverse osmosis for boiler makeup, preceded by antiscalant control or suitable pretreatment.
6. Mixed-bed polishing if the boiler specification requires very low conductivity.
7. Deaeration or membrane degassing, together with suitable oxygen control, for boiler feedwater.
8. Controlled cooling-tower blowdown and side-stream filtration.

The softener is not selected as the sole boiler treatment because it replaces calcium and magnesium with sodium and leaves substantial dissolved salts. Reverse osmosis reduces the broader ionic load, while polishing provides the final conductivity reduction. Iron removal is placed before ion-exchange and membrane units because iron fouling can reduce capacity and flux.

Monitoring should include raw-water turbidity, iron, hardness before and after softening, conductivity before and after reverse osmosis, permeate flow, differential pressure, boiler conductivity, dissolved oxygen, and cooling-water cycles of concentration. The plant should also track regenerant use, membrane-cleaning frequency, reject volume, and sludge disposal.

The outcome is not simply “pure water.” It is a controlled system in which each unit operation protects the next unit and produces water matched to a specific use. This distinction is central to economically and technically sound design.

8. Design, Safety, Environmental, and Professional Considerations

Treatment systems should be designed around a verified water analysis, design flow, peak flow, seasonal variation, required quality, and failure consequences. Online sensors are valuable, but they require calibration, validation, cleaning, and alarm management. A conductivity reading should not be accepted automatically if temperature compensation or sensor fouling is uncertain.

Chemical handling requires appropriate storage compatibility, secondary containment, labeling, ventilation, personal protective equipment, and emergency response. Acids and alkalis can generate heat during mixing. Chlorine compounds can release toxic gases if mixed with acids. Hydrogen sulfide and ozone require dedicated safeguards.

Environmental responsibility includes reducing chemical consumption, minimizing reject and brine discharge, beneficially using sludge where permitted, and preventing concentrated contaminants from being transferred from water to an unmanaged waste stream. Regulatory compliance commonly addresses discharge quality, residual disinfectants, hazardous sludge, worker exposure, drinking-water quality, and boiler safety. Local legal requirements must be consulted because numerical limits differ by jurisdiction and application.

Artificial intelligence can support anomaly detection, softener regeneration prediction, membrane-fouling diagnosis, and energy optimization. It should not replace chemical verification or safety interlocks. Training data may be biased toward normal operation, and a model can fail during unusual contamination events. Human review, traceable alarms, cybersecurity, and fail-safe control remain essential.

9. Challenges and Limitations

No single treatment process removes every contaminant. Ion exchange is selective and generates regenerant waste. Membranes are sensitive to fouling and require concentrate management. Disinfection performance decreases when turbidity shields microorganisms. Biological systems are vulnerable to toxic shocks and temperature changes. Chemical precipitation may create large sludge volumes. Activated carbon eventually saturates and must be regenerated or replaced.

Water chemistry is also dynamic. A source may change after rainfall, drought, industrial discharge, or seawater intrusion. Consequently, periodic laboratory analysis is necessary even when online instruments appear stable. Treatment design should include redundancy for critical equipment, bypass prevention, sampling points, preventive maintenance, and documented operating limits.

Chapter Summary

Water is a chemically active electrolyte system containing dissolved ions, gases, suspended solids, organic compounds, and microorganisms. pH describes hydrogen-ion activity, while alkalinity measures acid-neutralizing capacity. Hardness is mainly caused by calcium and magnesium and is conventionally reported as mg\,L^-1\ as\ CaCO₃. Salinity and conductivity indicate ionic content but do not identify individual contaminants.

Hardness causes scale and soap consumption; alkalinity buffers pH and influences carbonate precipitation. Dissolved oxygen and carbon dioxide can promote corrosion, while silica, sulfate, and concentrated salts create boiler and cooling-water problems. Boiler control requires pretreatment, blowdown, corrosion control, and monitoring. Cooling systems require control of cycles of concentration, scale, corrosion, suspended solids, and biological growth.

Treatment selection depends on contaminant type and required water quality. Coagulation and filtration remove particles; lime–soda and ion exchange reduce hardness; demineralization and reverse osmosis reduce dissolved ions; ultraviolet, chlorine, ozone, and other methods control microorganisms. Wastewater treatment combines physical, chemical, and biological processes, with sludge and concentrate management included in the design.

A successful industrial system is therefore a treatment train rather than an isolated device. Professional practice requires verified analysis, mass-balance thinking, instrumentation, safety management, regulatory compliance, waste minimization, and continuous performance evaluation.

Key Terms

Hardness: Concentration of mainly calcium and magnesium ions, commonly expressed as mg\,L^-1\ as\ CaCO₃.

Alkalinity: Acid-neutralizing capacity of water.

pH: Negative logarithm of hydrogen-ion activity.

Salinity: Concentration of dissolved salts in water.

Conductivity: Ability of water to conduct electrical current through dissolved ions.

Total dissolved solids: Mass concentration of dissolved substances.

Scale: Hard, adherent deposit formed on heat-transfer or process surfaces.

Sludge: Soft or loose precipitated material accumulated in treatment or boiler systems.

Blowdown: Controlled removal of concentrated boiler or cooling water.

Cycles of concentration: Ratio of circulating dissolved-solids concentration to makeup-water concentration.

Ion exchange: Reversible exchange of ions between water and charged resin sites.

Demineralization: Removal of dissolved cations and anions to produce low-conductivity water.

Reverse osmosis: Pressure-driven membrane process that rejects many dissolved salts.

Permeate: Water passing through a membrane.

Concentrate: Membrane reject stream containing retained contaminants.

Coagulation: Destabilization of colloids to enable particle aggregation.

Disinfection: Reduction of viable pathogens to an acceptable level.

BOD: Biochemical oxygen demand under specified biological test conditions.

COD: Chemical oxygen demand representing chemically oxidizable material under a defined test.

Fouling: Accumulation of unwanted material on a treatment surface or membrane.

Review Questions

Short Answer Questions

1. Why is hardness conventionally reported as calcium carbonate equivalent?
2. Distinguish pH from alkalinity.
3. State two causes of boiler corrosion.
4. What is the function of blowdown in a cooling tower?
5. Why is pretreatment important before reverse osmosis?

Long Answer Questions

1. Explain the chemical causes, effects, and control methods of boiler scale.
2. Compare lime–soda softening, ion-exchange softening, and reverse osmosis.
3. Describe the principal stages of a municipal or industrial wastewater-treatment plant.
4. Explain how dissolved oxygen and carbon dioxide influence corrosion.
5. Discuss the advantages and limitations of chlorine, ultraviolet irradiation, and ozone for disinfection.

Analytical / Application Questions

1. A water contains 100 mg L⁻¹ calcium and 30 mg L⁻¹ magnesium. Calculate its hardness as CaCO₃ and classify it using the table in this chapter.
2. An industrial plant has high hardness, moderate conductivity, iron, and turbidity. Propose a treatment train for cooling makeup and explain the position of each unit.
3. A cooling tower shows rising conductivity, visible deposits, and microbiological slime. Identify likely causes and propose a monitoring and corrective-action program.

Multiple Choice Questions with Answers

Question 1

Which parameter measures the acid-neutralizing capacity of water?

A. Hardness

B. Alkalinity

C. Conductivity

D. Turbidity

Correct Answer: B. Alkalinity

Question 2

A water contains 80 mg L⁻¹ calcium and 24 mg L⁻¹ magnesium. What is its approximate hardness as CaCO₃?

A. 120 mg L⁻¹

B. 199 mg L⁻¹

C. 298 mg L⁻¹

D. 520 mg L⁻¹

Correct Answer: C. 298 mg L⁻¹

Question 3

Which process primarily exchanges calcium and magnesium for sodium ions?

A. Ion-exchange softening

B. Ultraviolet disinfection

C. Activated-carbon adsorption

D. Aeration

Correct Answer: A. Ion-exchange softening

Question 4

Why does scale on a boiler tube create a serious operating risk?

A. It increases water solubility

B. It reduces thermal resistance

C. It increases heat-transfer resistance and may overheat the tube

D. It removes dissolved oxygen

Correct Answer: C. It increases heat-transfer resistance and may overheat the tube

Question 5

Which membrane process generally provides the greatest removal of dissolved salts among the listed pressure-driven processes?

A. Microfiltration

B. Ultrafiltration

C. Nanofiltration

D. Reverse osmosis

Correct Answer: D. Reverse osmosis

Question 6

In the expression CT=C_rt, what does C_r represent?

A. Residual disinfectant concentration

B. Carbon concentration

C. Cooling-water flow rate

D. Coagulant dose

Correct Answer: A. Residual disinfectant concentration

Question 7

Which statement about conductivity is correct?

A. It identifies every contaminant individually

B. It is unrelated to dissolved ions

C. It indicates ionic content but depends on ion composition and temperature

D. It measures only microorganisms

Correct Answer: C. It indicates ionic content but depends on ion composition and temperature

Question 8

What is the main purpose of cooling-tower blowdown?

A. Increase dissolved-solids concentration

B. Control accumulation of dissolved solids

C. Add biological nutrients

D. Eliminate all evaporation

Correct Answer: B. Control accumulation of dissolved solids

Question 9

Which treatment stage commonly uses microorganisms to oxidize biodegradable organic matter?

A. Secondary biological treatment

B. Screening

C. Cartridge filtration

D. Chlorine storage

Correct Answer: A. Secondary biological treatment

Question 10

Why is iron commonly removed before ion-exchange or membrane treatment?

A. Iron always lowers water temperature

B. Iron can foul resin and membrane surfaces

C. Iron prevents all microbial growth

D. Iron converts directly into sodium chloride

Correct Answer: B. Iron can foul resin and membrane surfaces