Filtration Fundamentals & Filter Operation

Filtration is one of the most important particle-removal barriers in drinking water treatment. After coagulation, flocculation, and clarification have removed much of the suspended material, filtration removes particles that remain in the water before final treatment and distribution.

Successful filter operation depends on more than simply passing water through media. Operators must understand filtration rate, head loss, turbidity, filter-run development, backwashing, filter ripening, media condition, and the effect of upstream treatment on filter performance.

Purpose of Filtration

Filtration removes suspended and particulate material that remains after earlier treatment processes.

Filtration can help remove:

• fine floc;
• suspended solids;
• turbidity-causing particles;
• some microorganisms associated with particles.

Filtration as a Treatment Barrier

Filtration should be viewed as part of a multiple-barrier treatment system.

Good filter performance depends heavily on upstream processes such as:

• coagulation;
• flocculation;
• clarification.

A filter should not be expected to compensate indefinitely for poor pretreatment.

Granular-Media Filtration

Many drinking water plants use granular-media filters.

Filter media may include materials such as:

• sand;
• anthracite;
• other approved granular media;
• multiple layers of different media.

Single-Media and Multimedia Filters

A single-media filter uses one principal filtering material.

A multimedia filter uses more than one type of media to provide different particle-removal and hydraulic characteristics through the filter depth.

How Granular Filters Remove Particles

Particles are removed through several interacting mechanisms rather than simple surface straining alone.

These mechanisms can include:

• physical interception;
• attachment to media;
• settling within media pores;
• other particle-media interactions.

Depth Filtration

In a properly operating granular filter, particles can be captured throughout part of the filter bed rather than only on the upper surface.

This is called depth filtration.

Filter Bed

The filter bed provides the media through which water passes.

Important filter-bed characteristics include:

• media type;
• media size;
• media depth;
• uniformity;
• condition of the bed.

Underdrain System

The underdrain system supports filter operation by:

• collecting filtered water;
• distributing backwash water where applicable;
• supporting the filter media or support system.

Filtration Rate

Filtration rate relates filter flow to filter surface area.

A common relationship is:

Filtration Rate = Flow ÷ Filter Area

Filtration-Rate Example

A filter receives 1,440,000 gallons per day and has a surface area of 1,000 ft².

Convert the flow to gallons per minute:

1,440,000 gal/day ÷ 1,440 min/day = 1,000 gpm

Then:

Filtration Rate = 1,000 gpm ÷ 1,000 ft²

Filtration Rate = 1.0 gpm/ft²

The acceptable operating rate depends on the filter design and applicable operating requirements.

Increasing Flow Increases Hydraulic Loading

If filter area remains constant while flow increases, filtration rate increases.

Higher hydraulic loading can affect:

• particle capture;
• head loss;
• filter-run length;
• turbidity performance.

Filter Influent Quality

Water entering the filter should already have received effective upstream treatment where the plant design includes coagulation and clarification.

High solids loading can shorten filter runs and increase the chance of poor filtered-water quality.

Filtered-Water Turbidity

Filtered-water turbidity is one of the most important indicators of filter performance.

Operators should review:

• individual filter turbidity;
• combined filtered-water turbidity where applicable;
• trends over the filter run;
• turbidity after backwash.

Why Individual Filter Data Matter

A combined reading can hide poor performance from one filter if other filters are producing lower-turbidity water.

Individual-filter monitoring helps identify a developing problem earlier.

Head Loss

Head loss represents resistance to flow through the filter.

As particles accumulate in the media, resistance generally increases.

Head-Loss Development

During a typical filter run:

• the clean filter begins with relatively low head loss;
• particles accumulate;
• head loss gradually increases;
• eventually the filter requires backwashing or reaches another operating limit.

Rapid Head-Loss Increase

Head loss increasing faster than normal can indicate:

• high solids loading;
• poor coagulation;
• poor clarification;
• media fouling;
• abnormally high filtration rate.

Filter Run

A filter run is the period between placing a clean or backwashed filter into service and taking it out of service for the next backwash or another operational reason.

Filter-Run Length

Filter-run length can be affected by:

• raw-water quality;
• coagulation;
• clarification;
• filtration rate;
• media condition;
• seasonal conditions.

Shortening Filter Runs

If filter runs become progressively shorter, operators should not assume that the filters themselves are always the cause.

Review upstream treatment first.

Filter Breakthrough

Breakthrough occurs when particles begin passing through the filter in increasing amounts.

A rising filtered-water turbidity trend can indicate breakthrough.

Possible Causes of Breakthrough

• excessive filter loading;
• poor pretreatment;
• media problems;
• hydraulic disturbances;
• excessively long filter runs;
• damaged underdrains or other internal problems.

Filter Ripening

After a filter is backwashed and returned to service, its performance may require a period of stabilization.

This period is often called filter ripening.

Filtered-water turbidity can temporarily be higher during this period than during stable filter operation.

Post-Backwash Turbidity

Operators should closely observe turbidity after returning a filter to service.

Unexpected or prolonged turbidity can indicate:

• poor backwash performance;
• media disturbance;
• inadequate filter-to-waste operation where used;
• upstream treatment problems.

Backwashing

Backwashing reverses or alters the normal hydraulic conditions through the filter to remove accumulated material from the media.

The objective is to clean the filter without damaging the media or support system.

Why Backwashing Is Necessary

As solids accumulate:

• head loss increases;
• available pore space decreases;
• filter performance can deteriorate.

Backwash Initiation

A filter may be backwashed based on plant-specific operating criteria such as:

• head loss;
• filtered-water turbidity;
• filter-run time;
• operational scheduling;
• other plant-specific limits.

Operators should not rely on elapsed time alone if process conditions indicate an earlier problem.

Backwash Flow

Backwash water must provide enough hydraulic action to remove trapped solids.

Insufficient backwash can leave deposits in the bed.

Excessive backwash can:

• lose media;
• disturb media layers;
• damage equipment.

Bed Expansion

In many granular filters, backwash flow expands the media bed so trapped particles can be released.

Appropriate bed expansion depends on:

• media type;
• media size;
• water temperature;
• backwash rate.

Temperature and Backwash

Water temperature affects viscosity and therefore affects media expansion during backwash.

A backwash rate that produces acceptable expansion at one temperature may behave differently when water becomes much colder or warmer.

Surface Wash and Air Scour

Some filters use additional cleaning systems such as:

• surface wash;
• air scour;
• combined air and water cleaning.

The exact sequence depends on filter design.

Mudballs

Mudballs are accumulations of material that can form within a filter bed when deposits are not removed effectively.

They can contribute to:

• poor filtration;
• uneven flow;
• media deterioration.

Media Cracking

Cracks or separations in the filter bed can create pathways that allow water to pass with reduced particle contact.

Channeling

Channeling occurs when water follows preferential pathways through the filter rather than distributing uniformly through the media.

This can reduce treatment effectiveness.

Possible Causes of Channeling

• poor media condition;
• mudballs;
• cracks;
• uneven backwashing;
• underdrain problems.

Media Loss

Filter media can be lost if hydraulic conditions during backwash or operation are excessive or if equipment is damaged.

Operators should monitor media depth and condition where appropriate.

Media Mixing

In multimedia filters, improper backwashing can disrupt the intended media arrangement.

Changes in media distribution can affect filtration performance.

Underdrain Problems

Damaged or blocked underdrains can cause:

• uneven filtration;
• uneven backwash distribution;
• media disturbance;
• poor filter cleaning.

Visual Signs During Backwash

Operators should look for:

• uneven boiling or expansion;
• dead areas;
• excessive media movement;
• unusual wash-water appearance.

Backwash-Water Clarity

The appearance of backwash water can help indicate whether solids are being removed.

However, visual clarity alone should not replace the plant's established backwash criteria.

Filter-to-Waste

Some treatment systems route initial filtered water to waste after backwash until filter performance stabilizes.

This can reduce the chance that post-backwash turbidity reaches finished water.

Returning a Filter to Service

Before returning a filter to normal service, operators should confirm conditions required by plant procedures, which can include:

• acceptable turbidity;
• stable flow;
• proper valve position;
• completed backwash sequence.

Filter Valve Operation

Improper valve sequencing can cause:

• hydraulic surges;
• media disturbance;
• incorrect flow paths;
• equipment damage.

Rate-of-Flow Control

Some filters use automatic or manual controls to maintain the desired filtration rate.

Operators should understand how control valves and instrumentation respond to changing head loss.

Sudden Flow Changes

Rapidly increasing or decreasing filter flow can disturb the bed and release accumulated particles.

Stable hydraulic operation generally supports stable filtration.

Filter Loading After Another Filter Is Removed

When one filter is taken out of service, remaining filters may receive additional flow.

Operators should consider whether this raises individual filtration rates significantly.

Calculation: Flow Redistribution

A plant produces 4.0 MGD through four identical filters.

At equal loading:

4.0 MGD ÷ 4 = 1.0 MGD per filter

If one filter is removed and total plant flow remains 4.0 MGD:

4.0 MGD ÷ 3 = 1.33 MGD per remaining filter

Each operating filter now receives approximately 33 percent more flow than before.

Why Flow Redistribution Matters

The increased loading can:

• increase filtration rate;
• increase head-loss development;
• shorten filter runs;
• increase breakthrough risk.

Upstream Coagulation and Filter Performance

Poor coagulation can produce particles that are difficult for filters to capture efficiently.

Possible results include:

• high turbidity;
• short filter runs;
• rapid head loss.

Clarifier Carryover

If the clarifier sends excessive floc to the filters, solids loading increases.

The correct response may be to fix clarification rather than simply backwashing more frequently.

Raw-Water Changes

Storms, seasonal changes, and algae events can change:

• particle loading;
• coagulant demand;
• clarifier performance;
• filter loading.

Use Trends Across the Treatment Train

Operators should compare:

• raw-water turbidity;
• settled-water turbidity;
• individual filter turbidity;
• head loss;
• filter run length;
• backwash frequency.

Example: Head Loss Increases Quickly but Turbidity Remains Low

Possible causes include:

• high solids loading;
• effective particle capture with excessive upstream solids;
• media fouling.

Review upstream clarification and the physical condition of the media.

Example: Turbidity Rises Without High Head Loss

Possible causes include:

• poor coagulation;
• hydraulic disturbance;
• channeling;
• media or underdrain problems.

Example: One Filter Has Higher Turbidity Than the Others

Because all filters receive similar source water, investigate that filter specifically for:

• media problems;
• valve problems;
• underdrain problems;
• uneven backwashing;
• instrument problems.

Example: All Filters Deteriorate at the Same Time

A common upstream cause is more likely.

Review:

• raw-water conditions;
• coagulation;
• flocculation;
• clarification.

Example: Filter Runs Suddenly Become Much Shorter

Review whether:

• raw-water turbidity increased;
• coagulation changed;
• clarifier carryover increased;
• filter flow increased;
• media became fouled.

Example: Post-Backwash Turbidity Remains High

Review:

• backwash effectiveness;
• media condition;
• filter-to-waste sequence where applicable;
• upstream water quality;
• instrument accuracy.

Example: Backwash Appears Uneven

Possible causes include:

• blocked underdrains;
• media accumulation;
• uneven air or water distribution;
• valve problems.

Example: Media Is Found in Backwash Waste

Possible causes include:

• excessive backwash rate;
• incorrect sequence;
• damaged equipment;
• abnormal media condition.

Instrument Verification

If filter turbidity changes unexpectedly, verify:

• sample flow;
• instrument cleanliness;
• calibration;
• comparison with another measurement where appropriate.

Do Not Ignore a Real Process Problem Because an Instrument Might Be Wrong

Verification should occur promptly. Operators should evaluate both the instrument and the treatment process.

Routine Filter Inspection

Operators should routinely review:

• filter turbidity;
• head loss;
• flow rate;
• valve operation;
• backwash performance;
• media condition;
• abnormal noise or vibration.

Filter Records

Useful records include:

• start and end time of each filter run;
• filter flow;
• head loss;
• turbidity;
• backwash time;
• backwash duration;
• abnormal observations.

Filter Performance Trends

Historical data can reveal:

• gradually shortening runs;
• increasing post-backwash turbidity;
• increasing head-loss development;
• differences among filters.

Preventive Maintenance

Filter reliability depends on equipment such as:

• valves;
• flow meters;
• turbidimeters;
• underdrains;
• backwash pumps;
• air-scour equipment.

Maintenance and Treatment Are Connected

A mechanical or instrumentation failure can directly affect treated-water quality.

Common Filtration Mistakes

• Assuming filters can permanently compensate for poor coagulation or clarification.
• Looking only at combined filtered-water turbidity.
• Ignoring rapid head-loss development.
• Backwashing only by elapsed time without considering actual process conditions.
• Using excessive backwash flow.
• Ignoring uneven backwash patterns.
• Returning a filter to service without reviewing post-backwash turbidity.
• Changing filter flow too rapidly.
• Ignoring media and underdrain condition.
• Treating every filter problem as a filter-only problem.

A Practical Filter Review

1. Review individual filter turbidity.
2. Review filtration rate.
3. Review head loss.
4. Review filter-run length.
5. Review upstream clarified-water quality.
6. Inspect valve operation.
7. Review recent backwash performance.
8. Inspect media condition where appropriate.
9. Compare the filter with other filters.
10. Review historical trends.

A Practical High-Turbidity Review

1. Verify the turbidity measurement.
2. Determine whether one filter or all filters are affected.
3. Review raw and settled-water turbidity.
4. Review coagulation and clarification.
5. Review filter flow and head loss.
6. Check for hydraulic disturbances.
7. Review media and underdrain condition.
8. Take controlled corrective action.

A Practical Short-Filter-Run Review

1. Compare current runs with historical runs.
2. Review raw-water solids loading.
3. Review coagulation performance.
4. Review clarification carryover.
5. Review filtration rate.
6. Review media condition.
7. Review backwash effectiveness.

A Practical Backwash Review

1. Confirm the correct filter is isolated.
2. Verify the backwash sequence.
3. Observe water and media movement.
4. Look for dead zones and uneven expansion.
5. Confirm adequate cleaning.
6. Avoid excessive media loss.
7. Return the filter to service according to plant procedure.
8. Monitor post-backwash turbidity closely.

What to Remember for the Exam

• Filtration removes particulate material remaining after upstream treatment.
• Granular filters often provide depth filtration rather than simple surface straining.
• Filtration rate equals flow divided by filter surface area.
• Higher flow through the same filter area increases filtration rate.
• Filtered-water turbidity is a key indicator of filter performance.
• Individual filter monitoring can identify problems hidden by combined measurements.
• Head loss generally increases as solids accumulate in a filter.
• Rapid head-loss development can indicate high solids loading or media problems.
• A filter run is the operating period between backwashes or other removal from service.
• Breakthrough is associated with increasing passage of particles through the filter.
• Filter ripening occurs after backwash as filter performance stabilizes.
• Backwashing removes accumulated solids from filter media.
• Insufficient backwashing leaves deposits, while excessive backwashing can disturb or remove media.
• Water temperature can affect media expansion during backwash.
• Mudballs, cracks, and channeling can reduce effective filtration.
• Underdrain problems can cause uneven filtration and uneven backwashing.
• Removing one filter from service can increase loading on the remaining filters.
• Poor coagulation and clarification commonly shorten filter runs and increase filter loading.
• Filter troubleshooting should compare turbidity, head loss, flow, upstream treatment, backwash performance, and media condition.
• Good filter operation depends on stable hydraulics, effective pretreatment, proper backwashing, reliable instrumentation, maintenance, and trend analysis.