Water & Wastewater Chemistry Fundamentals

Water and wastewater treatment depends on chemistry. Operators use chemical principles when adjusting pH, feeding treatment chemicals, controlling corrosion, disinfecting water, monitoring biological processes, removing contaminants, and interpreting laboratory results.

Operators do not need to become chemists, but they should understand the basic terms and relationships that explain why treatment processes behave the way they do.

Matter and Elements

Matter is anything that has mass and occupies space.

Water, air, treatment chemicals, minerals, sludge, and dissolved substances are all forms of matter.

An element is a substance made of only one type of atom.

Examples important in water and wastewater treatment include:

• hydrogen;
• oxygen;
• carbon;
• nitrogen;
• phosphorus;
• chlorine;
• calcium;
• iron.

Atoms

An atom is the basic unit of an element.

Atoms contain:

• protons;
• neutrons;
• electrons.

Protons have positive charge, electrons have negative charge, and neutrons have no electrical charge.

Molecules

A molecule contains two or more atoms chemically bonded together.

Water is written as H2O, meaning each molecule contains two hydrogen atoms and one oxygen atom.

Compounds

A compound contains two or more different elements chemically combined in fixed proportions.

Examples include:

• water, H2O;
• carbon dioxide, CO2;
• sodium chloride, NaCl;
• calcium carbonate, CaCO3.

Ions

An ion is an atom or group of atoms with an electrical charge.

An ion forms when electrons are gained or lost.

Cations

A cation has a positive charge.

Examples include:

• calcium, Ca2+;
• magnesium, Mg2+;
• sodium, Na+;
• ammonium, NH4+.

Anions

An anion has a negative charge.

Examples include:

• chloride, Cl-;
• nitrate, NO3-;
• bicarbonate, HCO3-;
• sulfate, SO4 2-.

Why Ions Matter

Dissolved ions influence:

• conductivity;
• hardness;
• alkalinity;
• corrosion;
• chemical reactions;
• water taste;
• treatment performance.

Solutions

A solution is a uniform mixture in which one substance is dissolved in another.

The substance being dissolved is the solute.

The material doing the dissolving is the solvent.

In water treatment, water is commonly the solvent.

Dissolved Versus Suspended Material

Dissolved material exists at the molecular or ionic level and generally does not settle by gravity.

Suspended material consists of particles dispersed in water.

This distinction affects which treatment processes are effective.

Concentration

Concentration describes how much of a substance is present in a given amount of solution.

Common water and wastewater units include:

• mg/L;
• µg/L;
• percent;
• molar concentration in specialized chemistry work.

Milligrams per Liter

mg/L is one of the most common concentration units used by operators.

For dilute water solutions:

1 mg/L is approximately equal to 1 part per million, or ppm.

This approximation works well because one liter of water has a mass close to one kilogram under common conditions.

Micrograms per Liter

µg/L is used for much lower concentrations.

There are:

1,000 µg in 1 mg

Therefore:

1 mg/L = 1,000 µg/L

Percent Concentration

Percent means parts per hundred.

For example:

5% = 5 parts out of 100

Percent solutions may be expressed by weight, volume, or another defined basis, so operators should know exactly how a chemical product concentration is stated.

Dilution

Dilution reduces concentration by adding more solvent without adding more solute.

A common dilution relationship is:

C1 × V1 = C2 × V2

where:

• C1 = initial concentration;
• V1 = initial volume;
• C2 = final concentration;
• V2 = final volume.

Dilution Example

Suppose 2 gallons of a 10% solution are diluted to a final volume of 10 gallons.

C1 × V1 = C2 × V2

10% × 2 gal = C2 × 10 gal

C2 = 2%

The chemical mass has not disappeared. It is distributed through a larger total volume.

Temperature and Chemistry

Temperature can affect:

• reaction rates;
• gas solubility;
• biological activity;
• chemical equilibrium;
• instrument response.

Many reactions proceed faster at higher temperature, although the exact relationship depends on the process.

Acids

An acid increases hydrogen-ion activity in water.

Acidic solutions generally have pH below 7 under typical aqueous conditions.

Examples relevant to treatment include:

• carbonic acid;
• sulfuric acid;
• hydrochloric acid.

Bases

A base can accept hydrogen ions or increase hydroxide-ion concentration.

Basic solutions generally have pH above 7 under typical aqueous conditions.

Examples include:

• sodium hydroxide;
• lime compounds;
• carbonate and bicarbonate species.

pH

pH describes the hydrogen-ion condition of a solution.

pH affects:

• corrosion;
• coagulation;
• disinfection;
• biological treatment;
• chemical solubility;
• precipitation.

The pH Scale Is Logarithmic

The pH scale is logarithmic rather than linear.

A change of one pH unit represents approximately a tenfold change in hydrogen-ion activity.

Therefore, pH 6 is not just slightly more acidic than pH 7.

Neutralization

When an acid reacts with a base, the process is called neutralization.

Neutralization can change:

• pH;
• alkalinity;
• chemical form of dissolved compounds.

Alkalinity

Alkalinity is the capacity of water to neutralize acids.

Important alkalinity species include:

• bicarbonate;
• carbonate;
• hydroxide.

Alkalinity helps resist rapid pH change.

Alkalinity Is Not the Same as pH

pH describes the current acid-base condition.

Alkalinity describes the water's ability to resist added acid.

Two waters can have similar pH values but very different alkalinity.

Buffering

A buffer reduces rapid changes in pH when acid or base is added.

Bicarbonate alkalinity is an important buffering system in many water and wastewater processes.

Why Alkalinity Matters in Wastewater Treatment

Biological nitrification consumes alkalinity.

If alkalinity becomes too low:

• pH can fall;
• nitrification can slow;
• process stability can decrease.

Hardness

Hardness is caused mainly by dissolved calcium and magnesium ions.

Hardness can affect:

• scale formation;
• soap use;
• industrial water applications;
• treatment chemistry.

Hardness Versus Alkalinity

Hardness and alkalinity are different water-quality characteristics.

Hardness mainly describes concentrations of multivalent metal ions such as calcium and magnesium.

Alkalinity describes acid-neutralizing capacity.

Calcium Carbonate Equivalent

Hardness and alkalinity are commonly reported as mg/L as CaCO3.

This does not necessarily mean that all of the measured material is physically calcium carbonate.

It provides a common chemical basis for comparing different species.

Chemical Reactions

A chemical reaction changes substances into different chemical forms.

Operators encounter reactions during:

• disinfection;
• coagulation;
• precipitation;
• corrosion;
• oxidation;
• biological treatment.

Reactants and Products

Reactants are substances present before a chemical reaction.

Products are substances formed by the reaction.

Reaction Rate

The speed of a reaction may depend on:

• temperature;
• concentration;
• pH;
• mixing;
• contact time;
• catalysts.

Mixing and Chemical Reactions

Good mixing brings reactants into contact.

Poor mixing can cause:

• uneven chemical distribution;
• local overdosing;
• incomplete reaction;
• poor treatment performance.

Precipitation

Precipitation occurs when dissolved substances react or change conditions and form an insoluble solid.

Precipitation is important in:

• softening;
• phosphorus removal;
• iron and manganese treatment;
• scale formation.

Solubility

Solubility describes how much of a substance can remain dissolved under specified conditions.

Solubility may change with:

• temperature;
• pH;
• chemical form;
• other dissolved substances.

Saturation

A solution is saturated when it contains approximately the maximum amount of a substance that can remain dissolved under the existing conditions.

If conditions change, material may precipitate or additional material may dissolve.

Oxidation

Oxidation involves loss of electrons by a substance.

Examples relevant to operators include:

• oxidation of iron;
• oxidation of manganese;
• oxidation of reduced sulfur compounds;
• chemical disinfection reactions.

Reduction

Reduction involves gain of electrons.

Oxidation and reduction always occur together in a redox reaction.

Oxidizing Agents

An oxidizing agent accepts electrons from another substance.

Examples used in treatment include:

• chlorine;
• ozone;
• oxygen;
• permanganate in some applications.

Oxidation-Reduction Potential

Oxidation-Reduction Potential, or ORP, is a measurement related to the tendency of a solution to support oxidation or reduction reactions.

ORP can be useful as a process indicator, but it should be interpreted with other process information rather than used alone.

Dissolved Gases

Important dissolved gases include:

• oxygen;
• carbon dioxide;
• hydrogen sulfide;
• nitrogen.

Dissolved Oxygen

Dissolved oxygen is essential to aerobic biological treatment.

DO affects:

• microbial activity;
• nitrification;
• odor formation;
• redox conditions.

Gas Solubility and Temperature

As water temperature increases, the solubility of many gases generally decreases.

This is one reason warm water can hold less dissolved oxygen than cold water.

Carbon Dioxide

Dissolved carbon dioxide participates in the carbonate chemistry of water.

It can influence:

• pH;
• alkalinity relationships;
• corrosion tendency.

Hydrogen Sulfide

Hydrogen sulfide can form under low-oxygen or anaerobic conditions.

It is associated with:

• odor;
• corrosion;
• worker-safety hazards.

Conductivity

Conductivity measures the ability of water to conduct electrical current.

Conductivity generally increases as dissolved ionic content increases.

It can be useful for identifying changes in dissolved solids or process streams.

Total Dissolved Solids

Total Dissolved Solids, or TDS, represents dissolved material remaining in water after suspended material is removed and an appropriate analytical procedure is performed.

TDS includes dissolved:

• salts;
• minerals;
• metals;
• other ionic and molecular substances.

Conductivity and TDS Are Related but Not Identical

Conductivity responds mainly to ions that carry electrical charge.

TDS represents total dissolved material determined or estimated by an analytical method.

Conductivity can be used as a useful indicator, but the relationship varies with water chemistry.

Suspended Solids

Suspended solids are particles that are not dissolved.

They may include:

• silt;
• biological solids;
• precipitated chemicals;
• organic particles.

Colloids

Colloids are very small particles that remain suspended because they do not settle readily by gravity.

Coagulation and flocculation are used to destabilize and combine many colloidal particles into larger floc.

Chemical Equilibrium

Some chemical reactions can proceed in both directions.

At equilibrium, forward and reverse reactions continue but the overall concentrations remain relatively stable.

Changing:

• pH;
• temperature;
• concentration;

can shift the equilibrium and change the form of chemicals present.

Chemical Species

A chemical element or compound can exist in different forms, called species.

For example, inorganic carbon can exist as:

• dissolved carbon dioxide;
• carbonic acid;
• bicarbonate;
• carbonate.

The relative amount of each form depends strongly on pH.

Nitrogen Species

Nitrogen can exist as:

• organic nitrogen;
• ammonia or ammonium;
• nitrite;
• nitrate;
• nitrogen gas.

Treatment processes convert nitrogen between these forms.

Phosphorus Species

Phosphorus may occur in:

• organic forms;
• orthophosphate;
• condensed phosphate forms.

Chemical and biological treatment can change phosphorus form and remove it from wastewater.

Mass Is Conserved

In ordinary treatment reactions, matter is not created or destroyed.

It is converted from one form to another.

This principle is important in:

• mass loading;
• chemical dosing;
• material balances;
• solids production.

Mass Balance Concept

A simple mass balance can be expressed as:

Input = Output + Accumulation

If material is also consumed or generated by reaction, those terms must be included in a more complete balance.

Example of Concentration and Mass

If a flow contains 10 mg/L of a substance, increasing the flow while concentration remains constant increases the total mass entering the process.

This is why operators must distinguish:

• concentration;
• mass loading.

Concentration Is Not the Same as Loading

Concentration tells how much material exists per unit volume.

Loading describes the total mass delivered over time.

A lower concentration at much higher flow can produce a larger total load.

Chemical Dose Versus Residual

Dose is the amount of chemical applied.

Residual is the amount remaining after reactions and demand have occurred.

This distinction is especially important in disinfection.

Chemical Demand

Demand is the amount of chemical consumed by reactions before a measurable residual remains.

For chlorine:

Chlorine Demand = Chlorine Dose - Chlorine Residual

Example of Chemical Demand

If chlorine dose is 3.0 mg/L and measured residual is 1.2 mg/L:

Demand = 3.0 - 1.2 = 1.8 mg/L

The consumed chlorine reacted with substances in the water.

Chemistry and Corrosion

Corrosion tendency is influenced by factors such as:

• pH;
• alkalinity;
• dissolved oxygen;
• chloride;
• temperature;
• water stability;
• pipe material.

Chemistry and Scale

Scale forms when dissolved substances precipitate onto surfaces.

Scaling can affect:

• pipes;
• valves;
• heat exchangers;
• chemical systems;
• instruments.

Chemistry and Biological Treatment

Microorganisms depend on chemical conditions such as:

• pH;
• alkalinity;
• dissolved oxygen;
• nutrient availability;
• temperature.

Changes in chemistry can therefore produce significant biological process changes.

Chemistry and Disinfection

Disinfection performance depends on:

• disinfectant dose;
• residual;
• contact time;
• pH;
• temperature;
• water quality.

Chemical Storage Can Change Product Quality

Treatment chemicals may change during storage because of:

• age;
• temperature;
• sunlight;
• contamination;
• chemical decomposition.

Operators should follow manufacturer and facility storage requirements.

Do Not Mix Chemicals Without Knowing Compatibility

Incompatible chemicals can cause:

• heat;
• toxic gas;
• fire;
• precipitation;
• violent reaction.

Chemical compatibility must be established before materials are combined or stored together.

Sampling and Chemistry

A laboratory result is only meaningful if the sample represents the process.

Results can be affected by:

• sample location;
• sample timing;
• container;
• preservation;
• holding time;
• analysis method.

Instrument Readings Need Context

Online instruments may measure:

• pH;
• conductivity;
• DO;
• chlorine residual;
• ORP.

Unexpected readings should be checked against:

• process conditions;
• historical trends;
• independent measurements;
• calibration status.

Common Chemistry Mistakes

• Confusing concentration with total mass loading.
• Confusing pH with alkalinity.
• Confusing hardness with alkalinity.
• Assuming all dissolved substances are suspended solids.
• Assuming a chemical dose equals the final residual.
• Ignoring temperature effects on reactions and dissolved gases.
• Ignoring mixing when evaluating chemical-feed performance.
• Changing chemical feed based on one questionable instrument reading.
• Mixing chemicals without checking compatibility.
• Ignoring the units attached to laboratory and chemical-feed values.

A Practical Chemistry Review Sequence

1. Identify the chemical or water-quality parameter.
2. Confirm the units.
3. Determine whether the value represents concentration, mass, dose, or residual.
4. Review pH and temperature where relevant.
5. Consider alkalinity and buffering where acid-base chemistry is involved.
6. Consider oxidation-reduction conditions where relevant.
7. Review flow if total loading matters.
8. Check process mixing and contact conditions.
9. Verify unusual instrument or laboratory results.
10. Evaluate the process response before making additional changes.

What to Remember for the Exam

• Atoms are the basic units of elements, and molecules contain chemically bonded atoms.
• Cations are positively charged ions, while anions are negatively charged ions.
• A solution contains a solute dissolved in a solvent.
• Dissolved material is different from suspended material.
• mg/L is a common concentration unit, and for dilute water solutions 1 mg/L is approximately 1 ppm.
• 1 mg/L equals 1,000 µg/L.
• Dilution reduces concentration by increasing total solution volume while chemical mass remains present.
• pH describes acid-base condition and uses a logarithmic scale.
• Alkalinity is the capacity of water to neutralize acid and helps buffer pH.
• pH and alkalinity are not the same measurement.
• Hardness is caused mainly by calcium and magnesium and is different from alkalinity.
• Hardness and alkalinity are commonly reported as mg/L as CaCO3.
• Precipitation converts dissolved material into an insoluble solid.
• Oxidation is loss of electrons, while reduction is gain of electrons.
• Oxidation and reduction occur together in redox reactions.
• Gas solubility generally decreases as water temperature increases.
• Conductivity generally increases as dissolved ionic concentration increases.
• Concentration and mass loading are different concepts.
• Dose is the amount of chemical applied, while residual is what remains after demand and reactions.
• Chemistry affects corrosion, scale formation, biological treatment, disinfection, and chemical-feed performance.