Taste, Odor & Advanced Drinking Water Treatment

Taste and odor problems are among the most noticeable drinking-water complaints because customers can detect changes even when the water appears clear. Some taste and odor conditions originate in source water, while others develop during treatment, storage, or distribution.

Advanced drinking-water treatment is used when conventional processes such as coagulation, clarification, filtration, and disinfection do not adequately address a particular contaminant or treatment objective. Operators should understand the basic purpose of advanced technologies even when they do not operate every type of process.

Taste and Odor Are Operational Clues

A taste or odor complaint can indicate a change in:

• source-water quality;
• algae or biological activity;
• dissolved gases;
• treatment chemistry;
• disinfection conditions;
• distribution-system conditions.

The operator's first task is to identify where the condition originates rather than immediately increasing chemical feed.

Source-Water Taste and Odor

Surface waters can develop taste and odor problems because of:

• algae;
• cyanobacteria;
• decaying organic matter;
• seasonal turnover;
• runoff;
• changes in source-water temperature.

Groundwater Taste and Odor

Groundwater problems can include:

• hydrogen sulfide;
• dissolved minerals;
• iron;
• manganese;
• other source-specific constituents.

Hydrogen Sulfide

Hydrogen sulfide is commonly associated with a rotten-egg odor.

Possible treatment approaches can include:

• aeration;
• oxidation;
• filtration;
• other source-specific processes.

Earthy or Musty Odors

Earthy or musty conditions can be associated with biological activity in source water.

Operators should review:

• source-water conditions;
• seasonal history;
• algae observations;
• raw-water analytical data;
• treatment performance.

Treatment Can Create Taste and Odor

Taste and odor can also result from treatment conditions.

Possible causes include:

• excessive chemical feed;
• disinfectant reactions;
• poor chemical mixing;
• stagnant treatment units;
• inadequate residuals removal.

Distribution-System Taste and Odor

If water leaves the treatment plant in good condition but complaints develop farther into the system, investigate:

• water age;
• storage tanks;
• disinfectant residual;
• sediment;
• biofilm;
• corrosion;
• local plumbing conditions.

Determine Whether the Problem Is Widespread

A useful first question is whether the complaint affects:

• one customer;
• one neighborhood;
• one pressure zone;
• the entire system.

The geographic pattern can help locate the source.

Compare Raw, Finished, and Distribution Water

Sampling at different points can help determine where the condition develops.

Compare:

• raw water;
• water after major treatment processes;
• finished water;
• distribution-system locations.

Do Not Diagnose by Odor Description Alone

Customer descriptions are useful clues, but different compounds can produce similar sensory descriptions.

Operational decisions should also use:

• analytical testing;
• process data;
• source-water observations;
• distribution-system information.

Conventional Treatment Has Limits

Conventional treatment can be highly effective for particles and many treatment objectives, but some dissolved contaminants are not removed efficiently by normal coagulation, sedimentation, and filtration.

Advanced treatment may be needed when a contaminant requires a different removal mechanism.

What Advanced Treatment Means

Advanced drinking-water treatment refers broadly to processes used beyond or in addition to conventional treatment to address specific contaminants or treatment objectives.

Examples include:

• activated carbon adsorption;
• ion exchange;
• membrane treatment;
• advanced oxidation;
• specialized biological treatment;
• other contaminant-specific processes.

Treatment Selection Must Match the Contaminant

No advanced treatment process removes every contaminant equally well.

Operators should understand that treatment selection depends on:

• contaminant type;
• concentration;
• water chemistry;
• flow;
• treatment objectives;
• residuals management.

Activated Carbon

Activated carbon is used to adsorb selected dissolved compounds onto a highly porous carbon surface.

Applications can include control of:

• taste and odor compounds;
• some organic contaminants;
• selected treatment byproduct precursors or other compounds.

Adsorption

Adsorption occurs when substances accumulate on the surface of another material.

It is different from absorption, in which material enters the bulk of another substance.

Powdered Activated Carbon

Powdered activated carbon, often abbreviated PAC, can be added as a treatment chemical.

It may be applied during periods of:

• taste and odor events;
• seasonal source-water problems;
• specific contaminant concerns.

PAC Dose

PAC performance can depend on:

• dose;
• contact time;
• compound being removed;
• water chemistry;
• application point.

PAC Must Eventually Be Removed

Powdered carbon becomes a suspended solid after addition.

It must be removed through the plant's solids-separation processes.

Granular Activated Carbon

Granular activated carbon, often abbreviated GAC, can be used as a fixed treatment bed.

Water passes through the carbon and selected compounds adsorb to the media.

GAC Capacity

GAC does not have unlimited capacity.

As adsorption sites become occupied, contaminant removal can decline.

Breakthrough

Breakthrough occurs when a contaminant begins appearing in treated water as the treatment media loses effective capacity.

Operators should monitor influent and effluent water quality to detect breakthrough.

GAC Operational Considerations

Operators may need to monitor:

• flow;
• pressure or head loss;
• influent concentration;
• effluent concentration;
• media age;
• backwashing where applicable.

Ion Exchange

Ion exchange uses a resin containing exchangeable ions.

Selected dissolved ions in the water are exchanged with ions held by the resin.

Applications can include:

• softening;
• removal of selected inorganic contaminants;
• other specialized treatment.

Ion Exchange Capacity

Resin capacity is finite.

When exchange sites become exhausted, the target ion begins to appear in treated water.

Regeneration

Many ion exchange systems restore resin capacity through regeneration.

Operators should understand:

• regenerant concentration;
• regeneration sequence;
• rinse requirements;
• waste stream management.

Membrane Treatment

Membrane processes use a selective barrier to separate constituents from water.

Different membrane technologies remove different ranges of material.

Pressure-Driven Membranes

Pressure-driven membrane systems can include processes designed for removal of:

• particles;
• microorganisms;
• dissolved salts;
• specific dissolved contaminants.

Membrane Feed Pressure

Pressure is an important operating variable in many membrane systems.

Changes in pressure can indicate:

• fouling;
• flow changes;
• membrane resistance;
• equipment problems.

Membrane Fouling

Fouling occurs when material accumulates on or within a membrane and reduces performance.

Possible foulants include:

• particles;
• scale;
• organic matter;
• biological growth.

Pretreatment Protects Membranes

Membrane systems often require pretreatment to reduce fouling or scaling.

Poor pretreatment can cause:

• higher pressure;
• lower production;
• more frequent cleaning;
• shorter membrane life.

Membrane Recovery

Recovery describes the fraction of feed water converted to treated product water.

A simplified relationship is:

Recovery, % = Product Flow ÷ Feed Flow × 100

Recovery Example

A membrane system receives 1,000 gpm of feed water and produces 750 gpm of product water.

Recovery = 750 ÷ 1,000 × 100

Recovery = 75%

The remaining flow becomes concentrate or another residual stream, depending on the process.

Concentrate

Some membrane processes produce a concentrate stream containing constituents removed from the product water.

Operators must consider how this residual stream is managed.

Advanced Oxidation

Advanced oxidation processes create highly reactive chemical species that can break down certain difficult organic contaminants.

These systems can use combinations of:

• oxidants;
• ultraviolet energy;
• other process conditions.

Advanced Oxidation Is Not Ordinary Disinfection

Although some advanced oxidation systems may also affect microorganisms, the treatment objective can include destruction of difficult dissolved organic compounds.

Advanced Oxidation Depends on Water Chemistry

Performance can be affected by substances in the water that consume reactive oxidants.

Operators should monitor:

• chemical dose;
• water quality;
• contact conditions;
• treatment performance.

Ozone

Ozone is a strong oxidant used in some drinking-water treatment plants.

Applications can include:

• oxidation;
• taste and odor control;
• treatment of selected organic compounds;
• disinfection.

Ozone Must Be Generated On Site

Ozone is unstable and is generally produced at the treatment facility rather than transported as a stored bulk disinfectant.

Ozone Safety

Ozone gas presents respiratory hazards.

Systems require appropriate:

• monitoring;
• ventilation;
• destruction of off-gas where applicable;
• safety procedures.

Ultraviolet Treatment

Ultraviolet light can be used for disinfection and can also be part of some advanced oxidation systems.

UV performance depends on factors such as:

• UV intensity;
• water clarity;
• lamp condition;
• flow;
• reactor condition.

UV Transmittance

Water containing substances that absorb UV light can reduce the amount of useful UV energy reaching the treatment target.

UV Lamp Fouling

Deposits on lamp sleeves can reduce treatment performance.

Operators should monitor:

• lamp status;
• UV intensity;
• cleaning systems;
• fouling.

Biological Drinking-Water Treatment

Some drinking-water processes use controlled biological activity to transform or remove selected contaminants.

Successful biological treatment requires management of conditions such as:

• dissolved oxygen;
• temperature;
• nutrients;
• contact conditions;
• biological activity.

Advanced Treatment Produces Residuals

Advanced treatment does not make contaminants disappear without consequences.

Residuals may include:

• spent activated carbon;
• regeneration waste;
• membrane concentrate;
• backwash water;
• chemical residuals.

Residuals Management Is Part of Treatment

A treatment process should be evaluated not only by what it removes from drinking water, but also by what residual stream it creates.

Taste and Odor Troubleshooting

A systematic investigation should determine:

• when the problem began;
• where it is occurring;
• whether raw water is affected;
• whether finished water is affected;
• whether only the distribution system is affected.

Example: Raw and Finished Water Both Have Musty Odor

This suggests the condition is entering with source water and is not being adequately removed by treatment.

Review:

• source-water conditions;
• algae or biological activity;
• activated carbon treatment where available;
• oxidation strategy;
• treatment performance.

Example: Raw Water Has Odor but Finished Water Does Not

The treatment process is successfully controlling the condition at the time of sampling.

Continue monitoring because source-water conditions can change.

Example: Finished Water Is Normal but Distribution Complaints Occur

Investigate:

• water age;
• storage tanks;
• disinfectant residual;
• sediment;
• local pipe conditions;
• customer plumbing.

Example: GAC Effluent Concentration Slowly Increases

This can indicate media exhaustion and contaminant breakthrough.

Review:

• influent concentration;
• effluent concentration;
• media age;
• flow;
• replacement or reactivation schedule.

Example: Membrane Pressure Increases

Possible causes include:

• fouling;
• scaling;
• blocked pretreatment;
• flow changes;
• membrane deterioration.

Example: Membrane Product Flow Declines

Review:

• feed pressure;
• temperature;
• fouling;
• recovery;
• pretreatment;
• membrane condition.

Example: Ion Exchange Breakthrough Occurs Early

Possible causes include:

• higher influent concentration;
• poor regeneration;
• resin fouling;
• excess flow;
• channeling.

Example: PAC Dose Increases but Odor Does Not Improve

Review:

• application point;
• contact time;
• actual compound causing the odor;
• raw-water concentration;
• PAC feed calibration.

Example: Ozone Treatment Performance Declines

Review:

• ozone production;
• gas transfer;
• water flow;
• source-water demand;
• instrument condition.

Example: UV Intensity Alarm Occurs

Possible causes include:

• lamp aging;
• lamp failure;
• sleeve fouling;
• sensor fouling;
• water-quality change.

Advanced Treatment Process Control

Operators should identify the key variables controlling each process.

Examples include:

• GAC: flow, influent concentration, effluent concentration, media condition;
• ion exchange: loading, breakthrough, regeneration;
• membranes: pressure, flow, recovery, fouling;
• advanced oxidation: chemical dose, energy, contact conditions;
• UV: intensity, flow, lamp condition.

Do Not Operate Advanced Treatment by One Number

A single pressure, concentration, or flow reading rarely describes the complete condition of a treatment process.

Operators should use multiple related measurements and trends.

Trend Performance

Useful trends can include:

• influent versus effluent contaminant concentration;
• head loss;
• pressure;
• media age;
• chemical consumption;
• product flow;
• residuals production.

Treatment Removal Efficiency

A simplified removal-efficiency relationship is:

Removal Efficiency, % = (Influent - Effluent) ÷ Influent × 100

Removal-Efficiency Example

A process receives water containing 20 µg/L of a contaminant and produces treated water containing 5 µg/L.

Removal Efficiency = (20 - 5) ÷ 20 × 100

Removal Efficiency = 75%

This calculation describes percentage removal, but operators must also compare the actual treated concentration with the applicable treatment objective or requirement.

Removal Percentage Can Be Misleading Alone

A high percentage removal does not automatically mean treated water meets the required goal.

For example, 90 percent removal of an extremely high influent concentration can still leave an unacceptable effluent concentration.

Advanced Treatment and Energy Use

Some advanced processes can require significant energy for:

• high-pressure pumping;
• UV systems;
• ozone generation;
• air systems.

Energy Trends Can Reveal Problems

Increasing energy use at the same production rate can indicate:

• fouling;
• higher pressure requirements;
• equipment deterioration.

Advanced Treatment Maintenance

Equipment can include:

• media vessels;
• high-pressure pumps;
• membranes;
• UV lamps;
• ozone generators;
• chemical-feed systems;
• instrumentation.

Instrumentation Is Critical

Advanced treatment often depends heavily on instrumentation.

Operators should verify:

• calibration;
• sample flow;
• sensor condition;
• alarm settings;
• data trends.

Common Taste, Odor, and Advanced Treatment Mistakes

• Assuming every taste or odor complaint originates at the treatment plant.
• Diagnosing odor only from customer descriptions.
• Increasing chemical dose before locating the source of the problem.
• Assuming conventional filtration removes every dissolved contaminant.
• Confusing adsorption with absorption.
• Ignoring activated-carbon breakthrough.
• Ignoring pretreatment for membrane systems.
• Operating ion exchange without monitoring regeneration and breakthrough.
• Evaluating treatment only by percent removal instead of final concentration.
• Ignoring residual streams created by advanced treatment.

A Practical Taste and Odor Review

1. Document the complaint location and time.
2. Determine whether one area or the entire system is affected.
3. Check raw-water conditions.
4. Check finished-water conditions.
5. Review source-water biological activity and dissolved gases.
6. Review treatment chemical feed.
7. Review storage and distribution conditions.
8. Use analytical testing where appropriate.
9. Apply treatment based on the identified cause.

A Practical Advanced-Treatment Review

1. Identify the contaminant or treatment objective.
2. Review influent concentration.
3. Review the treatment mechanism.
4. Verify flow and loading.
5. Review operating variables such as pressure, dose, or contact time.
6. Measure treated-water concentration.
7. Review residuals production.
8. Compare performance with historical trends.

A Practical Media-Treatment Review

1. Measure influent concentration.
2. Measure effluent concentration.
3. Review media age and loading.
4. Review flow.
5. Check head loss.
6. Look for breakthrough.
7. Regenerate, replace, or reactivate media according to the process design.

A Practical Membrane Review

1. Review feed-water quality.
2. Review pretreatment.
3. Review feed pressure.
4. Review product flow.
5. Calculate or review recovery.
6. Check for fouling or scaling.
7. Review concentrate flow.
8. Compare current operation with clean-system performance.

What to Remember for the Exam

• Taste and odor problems can originate in source water, treatment processes, storage, distribution, or customer plumbing.
• Operators should determine where a taste or odor problem begins before selecting treatment.
• Hydrogen sulfide commonly produces a rotten-egg odor.
• Earthy or musty conditions can be associated with biological activity in source water.
• Conventional treatment does not remove every dissolved contaminant effectively.
• Advanced treatment should be selected according to the contaminant and treatment objective.
• Activated carbon removes selected dissolved compounds through adsorption.
• PAC is added as a powder, while GAC is commonly used as a treatment bed.
• Activated-carbon breakthrough occurs when treatment capacity becomes exhausted.
• Ion exchange uses resin to exchange selected dissolved ions and often requires regeneration.
• Membranes use selective barriers and can require substantial pretreatment and pressure.
• Membrane recovery equals product flow divided by feed flow times 100.
• Membrane fouling can increase pressure and reduce product flow.
• Advanced oxidation uses highly reactive chemistry to treat selected difficult compounds.
• Ozone can be used for oxidation, taste and odor control, selected contaminant treatment, and disinfection.
• UV treatment depends on intensity, water quality, flow, and equipment condition.
• Advanced treatment can create residual streams such as spent carbon, regeneration waste, or membrane concentrate.
• Removal efficiency equals influent concentration minus effluent concentration, divided by influent concentration, times 100.
• Percentage removal should always be considered together with the actual treated-water concentration.
• Good advanced-treatment operation depends on contaminant-specific process control, monitoring, maintenance, residuals management, and trend analysis.