Drinking Water Laboratory Analyses

Drinking water operators rely on laboratory and field analyses to evaluate source water, treatment performance, disinfection, corrosion control, and finished-water quality. A single result rarely tells the whole story. Operators should understand what each parameter represents, how it is measured, and how several results can be interpreted together.

This article focuses on common drinking water analyses and their operational meaning rather than detailed regulatory limits, which should always be checked against current applicable requirements.

Why Drinking Water Analyses Matter

Laboratory and field testing can help operators answer questions such as:

• Is treatment removing particles effectively?
• Is disinfection performing as expected?
• Is finished-water chemistry stable?
• Are source-water conditions changing?
• Are minerals or metals causing operational problems?
• Do results suggest contamination or treatment failure?

Field Tests Versus Laboratory Tests

Some parameters are best measured immediately in the field because they can change rapidly.

Examples include:

• temperature;
• pH;
• chlorine residual;
• some dissolved-gas measurements.

Other analyses are commonly performed in a laboratory using specialized instruments, reagents, incubation, or filtration.

Turbidity

Turbidity is an optical measurement related to light scattering by suspended and colloidal particles.

It is commonly reported in:

NTU

What Can Cause Turbidity?

Turbidity can be caused by:

• silt;
• clay;
• organic particles;
• precipitated minerals;
• biological material;
• treatment floc.

Why Turbidity Matters

Turbidity is important because particles can:

• indicate poor clarification or filtration;
• interfere with disinfection;
• carry microorganisms;
• cause visible water-quality problems.

Turbidity Measurement

A turbidimeter measures light scattered by particles in the sample.

Measurement can be affected by:

• dirty sample cells;
• scratches;
• air bubbles;
• settling;
• instrument calibration.

Air Bubbles and Turbidity

Air bubbles can scatter light and create a false high turbidity result.

Operators should handle samples carefully and follow instrument procedures.

Filter Effluent Turbidity

Individual filter turbidity can be useful for identifying:

• filter breakthrough;
• poor filter ripening;
• media problems;
• hydraulic disturbances.

Turbidity Trend

A rising turbidity trend may be more important than one isolated value.

Review:

• raw-water turbidity;
• settled-water turbidity;
• individual filter turbidity;
• combined filter effluent;
• treatment changes.

pH

pH describes the acid-base condition of water.

pH affects:

• coagulation;
• disinfection chemistry;
• corrosion;
• mineral precipitation;
• chemical-feed performance.

pH Measurement

Reliable pH measurement requires:

• clean electrode;
• proper calibration;
• appropriate buffer solutions;
• adequate stabilization time.

Unexpected pH Results

If pH changes suddenly, review:

• chemical-feed rates;
• source-water changes;
• alkalinity;
• instrument calibration;
• sample location.

Alkalinity

Alkalinity is the acid-neutralizing capacity of water.

It is commonly reported as:

mg/L as CaCO3

Why Alkalinity Matters

Alkalinity affects:

• pH stability;
• coagulation;
• corrosion control;
• chemical-feed response.

Low Alkalinity

Low-alkalinity water can experience larger pH changes when acids or acid-forming treatment chemicals are added.

Hardness

Hardness is caused mainly by calcium and magnesium.

It is commonly reported as:

mg/L as CaCO3

Why Hardness Matters

Hardness can influence:

• scale formation;
• softening treatment;
• corrosion-control chemistry;
• customer water characteristics.

Hardness Is Not the Same as Alkalinity

Hardness describes mainly calcium and magnesium concentration.

Alkalinity describes acid-neutralizing capacity.

The values may be similar in some waters but they represent different chemical properties.

Conductivity

Conductivity measures the ability of water to conduct electrical current.

It generally increases as dissolved ionic concentration increases.

Conductivity as a Trend Indicator

A sudden conductivity change can indicate:

• source-water change;
• chemical addition;
• blend change;
• contamination;
• instrument problem.

Total Dissolved Solids

Total Dissolved Solids, or TDS, represent dissolved substances in water according to the analytical method used.

TDS may include:

• salts;
• minerals;
• metals;
• other dissolved substances.

Conductivity and TDS

Conductivity and TDS are related but not identical.

Conductivity mainly responds to ions, while TDS represents total dissolved material measured or estimated by the method.

Temperature

Temperature affects:

• reaction rates;
• gas solubility;
• disinfection performance;
• instrument response;
• biological activity.

Chlorine Residual

Chlorine residual is chlorine remaining after chemical demand has been satisfied.

Residual can be measured as:

• free chlorine;
• combined chlorine;
• total chlorine.

Free Chlorine

Free chlorine generally refers to chlorine present as hypochlorous acid and hypochlorite ion.

The relative amount of these species depends strongly on pH.

Combined Chlorine

Combined chlorine includes chloramine compounds formed when chlorine reacts with ammonia or other nitrogen compounds.

Total Chlorine

A general relationship is:

Total Chlorine = Free Chlorine + Combined Chlorine

Chlorine Demand

A simple relationship is:

Chlorine Demand = Chlorine Dose - Chlorine Residual

Chlorine Demand Example

If chlorine dose is 2.8 mg/L and residual is 1.1 mg/L:

Demand = 2.8 - 1.1 = 1.7 mg/L

Chlorine Samples Should Be Tested Promptly

Chlorine continues reacting after sample collection.

Delaying analysis can produce a lower residual than actually existed at the sampling point.

Colorimetric Chlorine Testing

Many chlorine analyses use a colorimetric reaction.

Potential errors include:

• dirty sample cells;
• incorrect reagent;
• wrong timing;
• sample color or turbidity;
• instrument calibration problems.

Microbiological Testing

Microbiological analyses evaluate indicators of possible microbial contamination.

Common indicator organisms include:

• total coliform bacteria;
• Escherichia coli, or E. coli.

Why Indicator Organisms Are Used

Testing every possible pathogen directly would be impractical.

Indicator organisms are used because their presence can indicate problems with:

• source contamination;
• treatment;
• distribution-system integrity;
• sample collection.

Total Coliform

Total coliform bacteria are a broad group used as indicators of sanitary condition and distribution-system integrity.

E. coli

E. coli is a more specific indicator associated with fecal contamination.

An unexpected microbiological result requires careful review of both system conditions and sampling technique.

Microbiological Sampling Technique

Important practices include:

• use the correct sterile bottle;
• avoid touching bottle interior or cap interior;
• use required dechlorinating agent when applicable;
• collect from the correct location;
• follow required flushing and disinfection procedures.

False Microbiological Positives

Contamination during sampling can create misleading positive results.

Possible sources include:

• dirty faucet;
• touching sterile surfaces;
• poor hand hygiene;
• contaminated sampling equipment.

Nitrate

Nitrate is an oxidized form of nitrogen.

Sources can include:

• agricultural activity;
• septic systems;
• wastewater impacts;
• natural sources.

Nitrate Reporting Basis

Nitrate can be reported as:

• nitrate as N;
• nitrate as NO3.

These are not numerically equivalent.

Operators must confirm the reporting basis before comparing results.

Nitrite

Nitrite is an intermediate nitrogen species and may be analyzed separately from nitrate.

As with nitrate, results may be reported on a nitrogen basis.

Iron

Iron can occur naturally in source water or enter water through corrosion.

Iron can contribute to:

• red or brown staining;
• discolored water;
• deposits;
• customer complaints.

Iron Oxidation

Dissolved reduced iron can be oxidized into less-soluble forms.

After oxidation, particulate iron must be removed by appropriate treatment.

Manganese

Manganese can cause:

• dark staining;
• black or brown deposits;
• distribution-system accumulation.

Manganese Oxidation

Like iron, dissolved manganese can be oxidized into less-soluble forms.

Effective treatment requires both:

• chemical conversion;
• particle removal.

Color

Water color can result from:

• natural organic matter;
• iron;
• manganese;
• industrial or source-water contaminants.

Apparent and True Color

Apparent color can include effects from suspended particles.

True color is determined after turbidity-related interference is removed according to the analytical method.

Odor

Odor can be associated with:

• natural organic compounds;
• algae;
• hydrogen sulfide;
• chlorine;
• treatment reactions.

Odor observations should be interpreted cautiously because human perception varies.

Fluoride

Where fluoride is present naturally or added as part of treatment, analytical monitoring helps confirm concentration and feed-system performance.

Unexpected fluoride results should prompt review of:

• chemical-feed rate;
• flow pacing;
• instrument or laboratory method;
• chemical strength.

Aluminum

Aluminum may be monitored where aluminum-based coagulants are used or where source-water chemistry makes it operationally relevant.

Elevated finished-water aluminum can be associated with:

• coagulation conditions;
• pH;
• chemical dose;
• solids carryover.

Coagulant Residuals

Residual metal concentrations can provide information about chemical treatment performance.

They should be interpreted together with:

• pH;
• coagulant dose;
• turbidity;
• settling;
• filtration.

Organic Matter

Natural organic matter can affect:

• color;
• coagulant demand;
• disinfectant demand;
• formation of disinfection byproducts.

UV Absorbance and Organic Indicators

Some treatment plants use measurements related to ultraviolet absorbance or organic carbon to monitor natural organic matter.

These results can help evaluate:

• source-water changes;
• coagulation effectiveness;
• disinfectant demand.

Total Organic Carbon

Total Organic Carbon, or TOC, measures carbon present in organic compounds.

TOC can be useful for evaluating:

• raw-water organic loading;
• treatment removal;
• potential disinfectant demand.

Corrosion-Control Monitoring

Useful chemical measurements may include:

• pH;
• alkalinity;
• calcium;
• hardness;
• conductivity;
• orthophosphate where used;
• metal concentrations.

Orthophosphate

Where orthophosphate is used for corrosion control, monitoring can help operators evaluate:

• chemical-feed performance;
• distribution-system residual;
• process consistency.

Lead and Copper Sampling

Lead and copper analyses require specialized sampling procedures because results can be strongly affected by:

• sampling location;
• stagnation conditions;
• premise plumbing;
• sample collection technique.

Operators should follow the specific applicable sampling protocol rather than general process-sampling practices.

Source-Water Monitoring

Source-water analyses can help track changes in:

• turbidity;
• temperature;
• pH;
• alkalinity;
• hardness;
• iron;
• manganese;
• organic matter;
• nutrients;
• conductivity.

Seasonal Source-Water Changes

Seasonal changes can affect:

• temperature;
• turbidity;
• organic matter;
• algae;
• chemical demand.

Trend data help operators anticipate treatment changes.

Finished-Water Monitoring

Finished-water testing evaluates whether the treatment process is producing stable water.

Useful parameters may include:

• turbidity;
• pH;
• disinfectant residual;
• alkalinity;
• conductivity;
• microbiological indicators.

Distribution-System Monitoring

Water quality can change after leaving the treatment plant.

Distribution monitoring may detect:

• disinfectant decay;
• microbiological problems;
• corrosion;
• iron or manganese release;
• stagnation;
• intrusion or pressure-related problems.

Compare Related Results

A result becomes more useful when compared with related measurements.

Examples include:

• turbidity versus filter operation;
• chlorine residual versus dose and contact time;
• pH versus alkalinity;
• iron versus color complaints;
• conductivity versus source-water change.

Example: High Filter Turbidity

If filter turbidity increases, review:

• raw-water turbidity;
• coagulant dose;
• settled-water quality;
• filter run time;
• head loss;
• backwash history;
• instrument condition.

Example: Low Chlorine Residual

Possible causes include:

• low chlorine dose;
• high chlorine demand;
• long water age;
• high organic matter;
• sample delay;
• instrument or reagent problem.

Example: Sudden Conductivity Increase

Possible causes include:

• source-water change;
• chemical-feed change;
• blending change;
• contamination;
• instrument error.

Example: Iron Increase

Possible causes include:

• source-water iron increase;
• poor oxidation or filtration;
• distribution-system corrosion;
• release of deposits.

Unexpected Results Should Be Verified

Before making a major process adjustment, review:

• sample location;
• collection time;
• instrument calibration;
• laboratory QC;
• related process data;
• historical trend.

Units Matter

Common mistakes include confusing:

• mg/L and µg/L;
• nitrate as N and nitrate as NO3;
• hardness as CaCO3 and actual calcium concentration;
• free chlorine and total chlorine.

Detection Limits

A laboratory result reported as non-detect does not necessarily mean the concentration is exactly zero.

It means the analyte was not detected at or above the applicable reporting threshold.

Trend Data

Operators should trend important drinking-water data to identify:

• seasonal changes;
• gradual treatment deterioration;
• distribution-system changes;
• instrument drift;
• chemical-feed problems.

Common Drinking Water Laboratory Mistakes

• Interpreting one result without reviewing related process data.
• Confusing pH with alkalinity.
• Confusing hardness with alkalinity.
• Confusing free chlorine with total chlorine.
• Delaying chlorine testing after sample collection.
• Allowing bubbles or dirty cells to affect turbidity testing.
• Using poor microbiological sampling technique.
• Confusing nitrate as N with nitrate as NO3.
• Ignoring sample location when evaluating distribution-system results.
• Changing treatment based on one unverified unusual result.
• Ignoring units and reporting basis.

A Practical Drinking Water Result Review

1. Confirm the parameter and units.
2. Confirm sample location and time.
3. Review instrument calibration or laboratory QC.
4. Compare with historical data.
5. Compare with related process measurements.
6. Review current treatment conditions.
7. Repeat or verify unusual results when appropriate.
8. Document the operational interpretation and response.

A Practical Filter-Water Review

1. Review raw-water turbidity.
2. Review coagulation conditions.
3. Review settled-water turbidity.
4. Review individual filter turbidity.
5. Review filter head loss.
6. Review filter run time and backwash history.
7. Verify the turbidimeter if the result does not match process behavior.

A Practical Disinfection Review

1. Review chlorine dose.
2. Review free and total residual as applicable.
3. Review pH.
4. Review temperature.
5. Review contact conditions.
6. Review source-water or organic-matter changes.
7. Verify prompt sample analysis and instrument condition.

What to Remember for the Exam

• Drinking water laboratory analyses support treatment control, disinfection, corrosion control, and finished-water monitoring.
• Turbidity measures light scattering by suspended and colloidal particles and is commonly reported in NTU.
• Dirty sample cells, scratches, bubbles, and poor calibration can affect turbidity results.
• pH affects coagulation, disinfection, corrosion, and mineral chemistry.
• Alkalinity describes acid-neutralizing capacity and is commonly reported as mg/L as CaCO3.
• Hardness is caused mainly by calcium and magnesium and is different from alkalinity.
• Conductivity generally increases as dissolved ionic concentration increases.
• Free, combined, and total chlorine are different measurements.
• Total chlorine equals free chlorine plus combined chlorine.
• Chlorine residual should be measured promptly because chlorine continues reacting after sampling.
• Total coliform and E. coli are important microbiological indicators.
• Microbiological sampling technique is critical because contamination during collection can create misleading results.
• Nitrate results may be reported as nitrogen or as nitrate, so the reporting basis must be confirmed.
• Iron can cause red or brown staining and may result from source water or corrosion.
• Manganese can create dark deposits and staining.
• Oxidized iron and manganese must be physically removed after chemical conversion.
• TOC and other organic-matter indicators can help evaluate source-water changes and treatment demand.
• Distribution-system water quality can differ from finished water leaving the plant.
• Unexpected results should be verified using sampling information, calibration, QC, trends, and related process data.
• Operators should always confirm units and reporting basis before interpreting laboratory results.