Iron, Manganese, Aeration & Oxidation

Learn how drinking water operators control iron and manganese using aeration, oxidation, contact time, filtration, pH control, chemical feed, monitoring, and troubleshooting.

Iron and manganese are naturally occurring elements that are common in many groundwater supplies and can also affect some surface waters. They may be present in dissolved forms that are not visible when water first enters the treatment plant. After oxidation, however, they can form colored particles that create staining, deposits, filter loading, and customer complaints.

Effective treatment requires more than simply adding an oxidant. Operators must understand the form of the metal, water chemistry, pH, oxidant demand, contact time, filtration, and the condition of downstream equipment. Aeration and chemical oxidation are often used to convert dissolved iron or manganese into forms that can be removed by filtration.

Iron in Drinking Water

Dissolved iron may enter a treatment plant in water that appears clear.

When the iron oxidizes, it can form reddish, orange, or brown particles.

Operational problems can include:

colored water;
staining;
sediment;
filter loading;
distribution-system deposits;
customer complaints.

Manganese in Drinking Water

Manganese can create:

black or dark-brown particles;
dark staining;
deposits;
filter problems;
distribution-system accumulation.

Manganese treatment can require careful control because oxidation behavior depends strongly on water chemistry and the treatment process.

Dissolved Versus Particulate Forms

A major operator concept is the difference between dissolved and particulate material.

Dissolved iron or manganese can pass through a particle filter if it has not first been converted into a removable form.

After oxidation, the resulting particles can be captured by appropriate filtration.

Basic Treatment Sequence

A common treatment approach is:

measure the raw-water condition;
apply aeration or an oxidant;
allow sufficient reaction time;
filter the oxidized particles;
verify finished-water quality.

Oxidation

Oxidation changes the chemical form of iron or manganese.

Possible oxidants used in drinking-water treatment include:

oxygen;
chlorine;
permanganate;
other approved treatment chemicals.

The appropriate oxidant depends on the water chemistry and treatment system.

Aeration

Aeration increases contact between water and air.

It can serve several purposes:

introduce oxygen;
support oxidation of some dissolved constituents;
remove certain dissolved gases;
change carbon dioxide concentration;
affect pH.

Types of Aeration

Aeration equipment can use different methods to increase air-water contact.

Examples can include:

cascade aeration;
tray aeration;
diffused air;
spray systems;
packed towers.

The appropriate equipment depends on the treatment objective.

Oxygen and Iron Oxidation

When reduced dissolved iron is exposed to oxygen under suitable conditions, it can be converted into an oxidized particulate form that can then be removed.

The reaction rate can depend on:

pH;
temperature;
oxygen availability;
contact time;
water chemistry.

Manganese Oxidation

Manganese oxidation can be slower or require different conditions than iron oxidation.

Operators should not assume that a treatment condition that removes iron effectively will automatically provide equally good manganese removal.

Importance of pH

pH can strongly influence oxidation reaction rates.

If oxidation is slow, increasing chemical dose without checking pH may waste chemical without correcting the real problem.

Contact Time

Oxidation reactions need time.

If water reaches the filters before the desired reaction has occurred, dissolved metal may pass through treatment.

A simplified hydraulic relationship is:

Detention Time = Volume ÷ Flow

Detention-Time Example

An oxidation contact tank has a usable volume of 60,000 gallons and receives 500 gpm.

Detention Time = 60,000 gal ÷ 500 gal/min

Detention Time = 120 minutes

This is a theoretical detention time. Actual hydraulic behavior can differ because of short-circuiting or incomplete mixing.

Oxidant Demand

Iron and manganese are not necessarily the only substances consuming oxidant.

Demand can also be created by:

hydrogen sulfide;
organic matter;
other reduced compounds;
ammonia in some waters.

Chlorine Demand

When chlorine is used, a simplified relationship is:

Chlorine Demand = Chlorine Dose - Chlorine Residual

A sudden increase in demand can indicate a source-water or treatment change.

Oxidation Before Filtration

If the treatment objective is particle removal, oxidation should occur early enough for removable particles to form before filtration.

Poor sequencing can allow dissolved material to pass the filter and oxidize later.

Problems from Incomplete Oxidation

Incomplete oxidation can contribute to:

finished water that initially appears clear;
color developing later;
distribution-system deposits;
customer staining complaints.

Filtration

Once iron or manganese has been converted into particulate material, filtration can remove it.

Operators should monitor:

filter influent quality;
filter effluent quality;
head loss;
filter run length;
backwash frequency;
media condition.

Higher Metal Loading Means Higher Solids Loading

If raw-water iron or manganese increases, successful oxidation can produce more solids.

This may cause:

faster head-loss development;
shorter filter runs;
more frequent backwashing;
greater residuals production.

Backwashing

Filters used for iron and manganese removal must be cleaned effectively.

Inadequate backwashing can result in:

accumulated deposits;
media fouling;
channeling;
reduced filter capacity;
poor finished-water quality.

Filter Media

Different treatment systems use different media.

Some media primarily capture oxidized particles, while other media can also support catalytic oxidation processes.

Operators should understand the specific media used at their plant and its operating requirements.

Catalytic Media

Some treatment media promote oxidation reactions at the media surface.

Performance can depend on:

pH;
oxidant availability;
media condition;
flow rate;
backwashing.

Aeration and Dissolved Gases

Aeration may also remove gases such as carbon dioxide or hydrogen sulfide.

This can influence treatment beyond iron and manganese removal.

Hydrogen Sulfide

Hydrogen sulfide can cause a rotten-egg odor and can increase oxidant demand.

Possible treatment approaches include:

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

Carbon Dioxide

Removing carbon dioxide through aeration can increase pH.

This can change:

oxidation behavior;
corrosion potential;
finished-water stability.

Aeration Can Change Several Parameters at Once

Operators should not evaluate aeration only by one treatment objective.

A change in aeration can affect:

dissolved oxygen;
dissolved gases;
pH;
oxidation;
downstream chemical demand.

Chemical Feed Calculation

A standard operator relationship is:

Chemical Feed, lb/day = Flow, MGD × Dose, mg/L × 8.34

Oxidant-Feed Example

A plant treats 2.2 MGD and applies an oxidant dose of 1.8 mg/L.

Chemical Feed = 2.2 × 1.8 × 8.34

Chemical Feed = 33.03 lb/day

This calculation determines chemical mass based on flow and dose. Converting that mass to solution volume may require chemical concentration and other product information.

Flow-Paced Feed

If plant flow changes, chemical feed should generally change if the target dose is to remain constant.

However, flow pacing alone cannot respond to a change in raw-water iron, manganese, or oxidant demand unless the target dose is also adjusted.

Measure Raw and Finished Water

Operators should compare raw-water and finished-water results.

Useful trends include:

raw iron versus finished iron;
raw manganese versus finished manganese;
oxidant dose versus residual;
pH versus treatment performance;
filter run length versus metal loading.

Source Changes

Systems using several wells should recognize that each well can have different:

iron concentration;
manganese concentration;
pH;
alkalinity;
hydrogen sulfide;
oxidant demand.

Well Changes Can Look Like Treatment Failures

If a different well is placed in service and finished-water quality changes, first determine whether raw-water chemistry also changed.

Blending Sources

When multiple source streams are blended, the combined concentration can be estimated by mass balance.

For two streams:

Ccombined = (Q1 × C1 + Q2 × C2) ÷ (Q1 + Q2)

Iron Blending Example

Well A supplies 0.75 MGD containing 0.4 mg/L iron.

Well B supplies 0.25 MGD containing 1.2 mg/L iron.

Ccombined = (0.75 × 0.4 + 0.25 × 1.2) ÷ 1.0

Ccombined = (0.30 + 0.30) ÷ 1.0

Ccombined = 0.60 mg/L

The blended raw-water iron concentration is approximately 0.60 mg/L before treatment, assuming complete mixing and no reaction before the sampling point.

Example: Raw Iron Increases

If raw iron increases while treatment settings remain unchanged, operators may observe:

greater oxidant demand;
greater filter solids loading;
shorter filter runs;
higher finished-water iron.

Example: Raw Iron Is Stable but Finished Iron Increases

This pattern points more strongly toward a treatment problem.

Review:

oxidant feed;
pH;
contact time;
filter condition;
backwash effectiveness.

Example: Raw Manganese Is Stable but Finished Manganese Rises

Review:

oxidation conditions;
pH;
chemical feed;
media condition;
filter run length.

Example: Filters Plug Rapidly After Oxidant Dose Is Increased

The increased oxidation may be creating more particulate solids.

Review:

whether the new dose is necessary;
raw-water concentration;
filter loading;
backwash frequency.

Example: Water Is Clear at the Plant but Brown at Customer Taps

Possible causes include:

incomplete iron removal;
distribution-system deposits;
corrosion products;
hydraulic disturbance in the distribution system.

Do not assume the treatment plant is the only possible source of color.

Example: Black Deposits Appear

Review:

manganese concentrations;
oxidation conditions;
filter performance;
distribution-system deposits.

Example: Rotten-Egg Odor Persists After Aeration

Possible causes include:

insufficient air-water contact;
high hydrogen sulfide concentration;
equipment fouling;
need for additional oxidation.

Example: Aeration Raises pH

Removal of carbon dioxide can increase pH.

This can improve some oxidation conditions but can also change finished-water stability.

Example: Oxidant Residual Falls

Possible causes include:

higher iron or manganese;
higher hydrogen sulfide;
other reducing compounds;
lower chemical feed;
feed-pump problems.

Verify the Chemical Feed System

If treatment deteriorates, check:

chemical supply;
solution concentration;
pump calibration;
feed tubing;
injection point;
actual flow.

Do Not Diagnose by Color Alone

Color and staining can provide useful clues, but treatment decisions should use measured water-quality data.

Similar visual symptoms can have different causes.

Laboratory Monitoring

Useful analyses can include:

raw and finished iron;
raw and finished manganese;
pH;
alkalinity;
oxidant residual;
other relevant source-water parameters.

Trend Filter Performance

Compare:

metal loading;
filter head loss;
run length;
backwash frequency;
finished-water quality.

This can show whether the treatment burden is changing over time.

Preventive Maintenance

Important equipment can include:

aerators;
blowers;
chemical-feed pumps;
contact tanks;
filters;
backwash systems;
instruments.

Aerator Maintenance

Aerator performance can deteriorate because of:

mineral deposits;
biological growth;
blocked openings;
blower problems;
corrosion.

Oxidized Deposits Can Accumulate in Equipment

Iron and manganese solids can accumulate in:

contact tanks;
pipes;
valves;
filter surfaces;
backwash systems.

Inspection and cleaning may be necessary.

Treatment Residuals

Iron and manganese removal produces solids that must be managed.

Residuals can appear in:

filter backwash water;
settled solids;
tank cleanout material.

Common Iron and Manganese Treatment Mistakes

Assuming clear raw water contains no dissolved iron or manganese.
Trying to filter dissolved metal before adequate oxidation occurs.
Increasing oxidant dose without checking pH and contact time.
Ignoring other substances that consume oxidant.
Ignoring additional filter loading created by successful oxidation.
Using flow pacing without adjusting target dose when source-water quality changes.
Assuming all wells have the same treatment demand.
Diagnosing staining only by color instead of laboratory data.
Ignoring aerator and filter maintenance.
Failing to compare raw and finished concentrations.

A Practical Iron and Manganese Review

Measure raw iron and manganese.
Review which source wells are operating.
Review plant flow.
Verify pH.
Verify aeration or oxidant feed.
Review reaction or contact time.
Review filter performance.
Measure finished iron and manganese.
Review distribution complaints and deposits if necessary.

A Practical Oxidation Review

Identify the constituent being oxidized.
Measure its raw-water concentration.
Review pH and other relevant chemistry.
Verify oxidant dose.
Verify actual chemical feed.
Review available contact time.
Check downstream filtration.
Measure the finished-water result.

A Practical Aeration Review

Identify the treatment objective.
Verify water flow.
Verify air flow or aerator operation.
Inspect for fouling or deposits.
Compare raw and treated dissolved-gas or metal conditions.
Review pH changes.
Review downstream filter performance.

What to Remember for the Exam

Iron and manganese may occur in dissolved forms that are not visible in raw water.
Oxidation converts dissolved iron or manganese into forms that can often be removed by filtration.
Aeration can provide oxygen, remove some dissolved gases, and influence water chemistry.
Iron oxidation and manganese oxidation do not necessarily occur equally well under the same conditions.
pH strongly affects many oxidation reactions.
Oxidation requires adequate reaction or contact time.
Theoretical detention time equals volume divided by flow.
Other reduced compounds can consume oxidant and increase treatment demand.
Chlorine demand equals chlorine dose minus chlorine residual.
Incomplete oxidation can allow dissolved material to pass through filters and oxidize later.
Successful oxidation increases particulate solids loading on filters.
Filter head loss and backwash frequency can increase as iron or manganese loading increases.
Some filter media support both particle removal and catalytic treatment.
Aeration can remove carbon dioxide and thereby change pH.
Chemical feed in lb/day can be calculated as MGD × mg/L × 8.34.
Different wells may have different iron, manganese, pH, and oxidant demand.
Blended concentrations can be estimated with flow-weighted mass balance.
Raw and finished metal concentrations should be trended together.
Color and staining are clues, but treatment decisions should use measured water-quality data.
Good iron and manganese control combines source monitoring, oxidation chemistry, aeration, contact time, filtration, backwashing, equipment maintenance, and finished-water verification.