Key Water Quality Parameters for General and Industrial Water Treatment
1. Overview
Water quality is not determined by a single parameter, but by the interplay of acid-base equilibria, ionic strength, redox conditions, nutrient loading and dissolved salts. In particular, pH, alkalinity, hardness, conductivity and ORP jointly govern corrosion, scaling, disinfection efficiency, biological activity and the performance of installations such as RO, demineralisation, cooling towers and boilers.
Parameter | Unit | What is measured | Importance in water treatment |
pH | pH units | Activity of hydrogen ions | Corrosion, scaling, disinfection, solubility of metals |
Alkalinity | mg/L or ppm as CaCO₃ | Acid-binding capacity (HCO₃⁻, CO₃²⁻, OH⁻) | pH buffering, corrosion control, precipitation behaviour |
Total hardness (GH) | mg/L or ppm as CaCO₃ | Primarily Ca²⁻ and Mg²⁻ | Scaling, membrane fouling, heat transfer |
Conductivity | µS/cm or mS/cm | Ability of water to conduct electric current | Indicator of dissolved ions and process control |
ORP / Redox | mV | Oxidising or reducing capacity | Disinfection, redox chemistry, biological processes, corrosion tendency |
Ammonia | mg/L as NH₃-N or NH₄⁺/NH₃ | Reduced nitrogen | Toxicity, nitrification, process loading |
Nitrite | mg/L as NO₂⁻-N or NO₂⁻ | Intermediate product of nitrification | Incomplete oxidation, acute toxicity, process instability |
Nitrate | mg/L as NO₃⁻-N or NO₃⁻ | End product of nitrification | Nutrient loading, compliance, biological conversion |
Phosphate | mg/L as PO₄-P or PO₄³⁻ | Phosphorus compounds in solution | Biofouling, algal bloom, corrosion inhibition (orthophosphate) |
2. pH
Definition
pH is the measure of hydrogen ion activity, defined as pH = −log a(H⁺). It determines whether water is acidic, neutral or alkaline, and directly affects the solubility of metals, the carbonate equilibrium, ammonia toxicity and the efficiency of oxidants and disinfectants.
Chemical significance
Many water treatment reactions are pH-dependent. A lower pH generally increases corrosion risk and metal solubility, whereas a higher pH increases the likelihood of carbonate precipitation and scaling. Furthermore, at higher pH the ammonium/ammonia equilibrium shifts towards the more toxic free ammonia form.
Reference values
For drinking water a pH of 6.5–8.5 is commonly maintained as a practical and material-technically favourable range. In industrial systems the optimal pH is process-dependent and is selected based on material compatibility, scaling risk, disinfection requirements and membrane performance.
3. Alkalinity
Definition
Alkalinity is the acid-neutralising capacity of water, primarily determined by bicarbonate (HCO₃⁻), carbonate (CO₃²⁻) and hydroxide (OH⁻). Minor contributions may also come from ammonia and conjugate bases of weak acids. Unlike hardness, alkalinity is not a measure of calcium and magnesium content, but of the water’s ability to neutralise added acids and buffer pH fluctuations.
Chemical significance
The carbonate system follows the equilibrium:
CO₂ + H₂O ⇌ H₂CO₃ ⇌ HCO₃⁻ + H⁺ ⇌ CO₃²⁻ + 2H⁺
This makes alkalinity act as a buffer against acidification. In distribution networks and process installations, sufficient alkalinity helps maintain pH stability, while excessively high alkalinity in the presence of calcium increases the risk of CaCO₃ precipitation.
Technical importance
- Low alkalinity: greater risk of sudden pH shifts and corrosive water
- Moderate alkalinity: favourable for stable process control and predictable chemistry
- High alkalinity: increased scaling risk, especially during heating or CO₂ stripping
4. Total Hardness (GH)
Definition
Total hardness is primarily caused by calcium and magnesium ions (Ca²⁻ and Mg²⁻) and is expressed as mg/L CaCO₃. Hardness is a measure of multivalent cations, not of buffering capacity.
Chemical significance
Calcium and magnesium react with carbonate, phosphate and other anions to form sparingly soluble salts. A key reaction in scaling is:
Ca²⁻ + CO₃²⁻ → CaCO₃ (s)
As hardness, pH and temperature increase, the risk of deposits grows, negatively impacting heat exchangers, boilers, membranes and pipework.
Technical importance
- Drinking water: affects taste and tendency to form limescale
- Cooling and boiler systems: excessive hardness reduces heat transfer and increases maintenance costs
- Membrane systems: hardness contributes to scaling and flux loss
5. Conductivity
Definition
Conductivity is a measure of how well water conducts an electric current. It is an indirect measure of the content of dissolved ionic substances such as chloride, nitrate, sulphate, phosphate, sodium, calcium, magnesium, iron and aluminium.
Chemical significance
The higher the concentration of dissolved ions, the higher the conductivity generally is. Conductivity is not a specific contamination parameter, but a rapid integral indicator of ionic strength, salt loading and process changes.
Technical importance
The WHO considers conductivity a basic parameter that should be monitored regularly in drinking water control. In RO and demineralisation plants, conductivity is used to monitor membrane performance, salt breakthrough and product water quality.
Practical interpretation
- Low conductivity: few dissolved ions — typical of permeate or demineralised water
- Moderate conductivity: normal for many drinking and process waters
- Sharp increase: possible contamination, salt intrusion, chemical dosing or membrane/resin breakthrough
6. ORP / Redox Potential
Definition
ORP (Oxidation-Reduction Potential), also known as redox potential, is a measure of the tendency of an aqueous system to accept or donate electrons, expressed in millivolts (mV). A positive ORP indicates a more oxidising environment; a lower or negative ORP indicates reducing conditions.
Chemical significance
Many important water reactions are redox reactions, including disinfection, iron and manganese removal, sulphide reduction and the biological conversion of ammonia to nitrite and nitrate. ORP is influenced by dissolved oxidants and reductants, as well as by pH, temperature and water composition, making it primarily an integral process parameter rather than a direct concentration measurement of any single substance.
Technical importance
In water and wastewater treatment, ORP is used for continuous process monitoring in effluent control, neutralisation, cooling towers and other treatment steps. A higher ORP generally indicates a stronger oxidising environment, but the desired setpoint depends on the oxidant used, contact time, pH and the water matrix.
7. Ammonia (NH₃ / NH₄⁺)
Definition
Ammonia in water exists as free ammonia (NH₃) and as ammonium (NH₄⁺). The ratio between the two forms is strongly dependent on pH and temperature.
Chemical significance
The acid-base equilibrium is:
NH₃ + H₂O ⇌ NH₄⁺ + OH⁻
At higher pH the equilibrium shifts towards NH₃, the form that is far more toxic to aquatic organisms. The presence of ammonia may indicate fresh nitrogen loading, biological breakdown of organic matter or incomplete nitrification.
Technical importance
- Drinking water and source water: indicator of contamination or insufficient oxidation/nitrification
- Biological treatment: key parameter for nitrification loading and oxygen demand
- Aquatic systems: harmful even at low free NH₃ concentrations
8. Nitrite (NO₂⁻)
Definition
Nitrite is an intermediate product in the biological oxidation of ammonia to nitrate. In stable, oxygen-rich water, nitrite is generally further oxidised to nitrate.
Chemical significance
An elevated nitrite level often indicates incomplete nitrification, oxygen deficiency, toxic inhibition or a temporary biological imbalance. Because nitrite is a reactive intermediate, it functions as an alarm parameter for process instability.
Technical importance
- Drinking water: undesirable due to health risk (methaemoglobinaemia) — regulated by law
- Wastewater treatment: indicator that nitrification is not proceeding to completion
- Aquatic systems: acutely toxic through disruption of oxygen transport in blood
9. Nitrate (NO₃⁻)
Definition
Nitrate is the oxidised end product of nitrification and is generally stable and highly soluble in water. It is used as an indicator of nutrient loading, agricultural influence, biological conversion and residual contamination.
Chemical significance
Because nitrate is the end product of oxidation of reduced nitrogen, a rising nitrate level means that ammonia and nitrite have already been converted. Under anoxic conditions nitrate can be reduced via denitrification, but in many drinking and process waters it remains dissolved.
Technical importance
High nitrate levels are undesirable in drinking water and are relevant for compliance and source water management. In open systems and aquatic applications, excessive nutrient loading indirectly promotes biological growth and eutrophication.
10. Phosphate (PO₄³⁻)
Definition
Phosphate is an essential nutrient for microbial and algal growth and enters water from food, wastewater, organic breakdown, detergents or chemical dosing.
Chemical significance
Phosphate can form sparingly soluble precipitates with calcium:
3Ca²⁻ + 2PO₄³⁻ → Ca₃(PO₄)₂ (s)
In addition to precipitation behaviour, phosphate plays a major role in biological fouling. In some drinking water and distribution applications orthophosphate is deliberately applied as a corrosion inhibitor, but excessive phosphate loading can promote biofouling or algal problems.
Technical importance
- Surface water and aquatic systems: strong influence on algal blooms (eutrophication)
- Industrial circuits: increases risk of biofilm formation and fouling
- Distribution networks: orthophosphate can be applied in a controlled manner for corrosion management
11. Indicative Assessment Frameworks
The zones below are indicative and must always be interpreted in relation to application, temperature, material selection, water source and process design. Industrial installations are typically governed by project-specific or vendor-defined limit values.
Parameter | Low zone | Desired / normal zone | High / critical zone |
pH | <6.5: increased corrosion risk | 6.5–8.5 generally suitable for drinking water and many applications | >8.5: higher risk of scaling and altered chemistry |
Alkalinity | <50 mg/L: limited buffering | 50–150 mg/L as CaCO₃ — generally manageable | >150–250 mg/L: strongly buffered, greater precipitation risk |
Total hardness | <70 mg/L: soft water | 70–210 mg/L as CaCO₃: common range | >210 mg/L: higher scaling risk |
Conductivity | Very low: demi/permeate water | Application-dependent | Rising values indicate more dissolved salts or breakthrough |
ORP | Low ORP: reducing conditions | Setpoint is process-dependent | High ORP: stronger oxidising environment |
Ammonia | Not detectable or very low | Traces possible depending on source/process | Elevated: toxicity or incomplete conversion |
Nitrite | Ideally not detectable | Low traces possible during start-up | Elevated: alarm for incomplete nitrification |
Nitrate | Low preferred | Moderate, source- and process-dependent | High: compliance and nutrient issue |
Phosphate | Low in fouling-sensitive systems | Controlled at specific dosing | High: increased biofouling and algal risk |
12. Relevance by Application
Application | Critical parameters | Why |
Drinking water | pH, conductivity, nitrate, nitrite, ammonia, alkalinity | Health, taste, corrosion control, distribution stability |
Cooling water | pH, alkalinity, hardness, conductivity, ORP, phosphate | Corrosion, scaling, biofouling and oxidative control |
Boiler feed water | Conductivity, hardness, alkalinity, pH | Protection against deposits and corrosion, higher efficiency |
RO / demineralisation | Conductivity, pH, alkalinity, ammonia, hardness | Membrane performance, rejection, scaling and product water quality |
Biological treatment | Ammonia, nitrite, nitrate, pH, ORP | Nitrification/denitrification, process stability and oxygen balance |
Aquatic systems | Ammonia, nitrite, nitrate, pH, alkalinity, phosphate, hardness | Toxicity, buffering, nutrient loading and species-specific water chemistry |
13. Measurement and Interpretation Notes
- Conductivity must always be interpreted with temperature correction, as ion mobility and therefore the measured value are temperature-dependent.
- pH, ORP and conductivity measurements require correct calibration and sensor maintenance to avoid drift, fouling and erroneous trends.
- When interpreting ammonia results, pH and temperature must always be considered, because it is free NH₃ rather than total ammonia that determines toxicity.
- For corrosion and scaling analysis, a single isolated parameter is rarely sufficient. The combination of pH, alkalinity, hardness, conductivity and system temperature provides far greater predictive value.
14. References
The following sources were consulted in compiling this data sheet. Click the link to access the source directly.
ID | Source name | URL |
WHO | World Health Organization — Guidelines for Drinking-water Quality | |
MERI | Typical Water Quality Parameters Explained (NJ Meadowlands) | |
Chem PG | Water Quality Control Part II: Parameters of natural waters | |
WHO IRIS | Guidelines for Drinking-water Quality (IRIS repository) | |
FAO | Water Quality Monitoring, Standards and Treatment | |
Yokogawa | pH/ORP Measurement for Reverse Osmosis | |
WHO Nitrate | Nitrate and Nitrite — WHO Fact Sheet | |
RS Hydro | ORP / Redox Potential Measurement | |
Hach | Water Hardness — Water Quality Parameter Overview | |
Metrohm | Conductivity, pH, alkalinity, and hardness in tap water |
Technical Data Sheet — Water Parameters v1.0 | Compiled on the basis of scientific and industrial sources