Tertiary Treatment, Effluent Reuse & Advanced Treatment

Tertiary and advanced wastewater treatment provide additional treatment beyond conventional primary and secondary processes. These processes are used when a facility must achieve higher effluent quality, remove specific pollutants, support water reuse, or meet other treatment objectives.

Operators should understand that advanced treatment does not replace good primary and secondary treatment. Most tertiary processes perform best when upstream treatment is stable and produces consistent water quality.

What Tertiary Treatment Means

Tertiary treatment generally refers to treatment applied after secondary treatment to further improve effluent quality.

Tertiary treatment can target:

• suspended solids;
• nutrients;
• microorganisms;
• dissolved contaminants;
• other constituents that remain after secondary treatment.

Advanced Treatment

Advanced treatment is a broad term for processes designed to achieve treatment beyond conventional secondary treatment or to remove contaminants requiring specialized technology.

Examples can include:

• tertiary filtration;
• membrane treatment;
• advanced nutrient removal;
• activated carbon;
• advanced oxidation;
• specialized disinfection.

Why Additional Treatment Is Used

Additional treatment may be required to:

• meet stringent discharge requirements;
• reduce nutrient loading;
• prepare water for reuse;
• protect sensitive receiving waters;
• remove specific contaminants;
• improve disinfection performance.

Tertiary Filtration

Tertiary filtration removes suspended material remaining after secondary clarification.

Common treatment approaches can include:

• granular-media filters;
• cloth or disk filtration;
• membrane filtration;
• other solids-polishing systems.

Why Tertiary Filtration Helps

Reducing suspended solids can improve:

• final effluent clarity;
• disinfection performance;
• downstream reuse treatment;
• overall effluent quality.

Filter Loading

A simplified filtration-rate relationship is:

Filtration Rate = Flow ÷ Filter Area

Filtration-Rate Example

A tertiary filter receives 800 gpm and has 400 ft² of surface area.

Filtration Rate = 800 ÷ 400

Filtration Rate = 2.0 gpm/ft²

The acceptable operating rate depends on the specific filter design.

Head Loss

As solids accumulate in a filter, resistance to flow can increase.

Operators should monitor:

• head loss;
• effluent turbidity;
• filter run length;
• backwash frequency.

Backwashing

Many tertiary filters require periodic cleaning or backwashing.

Poor backwashing can lead to:

• shorter filter runs;
• solids accumulation;
• higher head loss;
• poor effluent quality.

Upstream Solids Affect Tertiary Filters

If secondary clarifiers carry excessive solids into tertiary filters, the filters may foul rapidly.

The correct response may require improving secondary clarification rather than simply increasing backwash frequency.

Nutrient Removal

Advanced wastewater treatment may include additional nitrogen or phosphorus removal.

Nitrogen removal can involve:

• nitrification;
• denitrification;
• other specialized processes.

Phosphorus removal can involve:

• biological phosphorus removal;
• chemical precipitation;
• filtration;
• combinations of processes.

Nitrification

Nitrification biologically converts ammonia to more oxidized nitrogen forms.

Important operating factors include:

• dissolved oxygen;
• temperature;
• pH;
• alkalinity;
• sludge age;
• toxic conditions.

Denitrification

Denitrification converts nitrate to nitrogen gas under suitable biological conditions.

The process generally requires:

• nitrate;
• an available carbon source;
• low dissolved oxygen conditions appropriate for the process;
• adequate contact time.

Phosphorus Precipitation

Chemicals can be added to convert dissolved phosphorus into solids that can be separated from the water.

Operators should monitor:

• chemical dose;
• mixing;
• pH;
• solids production;
• filtration or clarification performance.

Chemical Feed Calculation

A common relationship is:

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

Chemical Feed Example

A facility treats 4.0 MGD and applies a treatment chemical at 5 mg/L.

Chemical Feed = 4.0 × 5 × 8.34

Chemical Feed = 166.8 lb/day

Membrane Treatment

Membrane processes use selective barriers to separate contaminants from water.

Depending on membrane type and treatment objectives, membranes can remove:

• suspended particles;
• microorganisms;
• some dissolved substances;
• salts and other dissolved constituents in higher-pressure processes.

Membrane Fouling

Fouling occurs when material accumulates on or within membrane surfaces.

Possible foulants include:

• suspended solids;
• organic matter;
• scale;
• biological growth.

Membrane Monitoring

Operators can monitor:

• feed pressure;
• transmembrane pressure where applicable;
• product flow;
• recovery;
• water quality;
• cleaning frequency.

Membrane Recovery

A simplified relationship is:

Recovery, % = Product Flow ÷ Feed Flow × 100

Recovery Example

A membrane system receives 2.0 MGD and produces 1.5 MGD of product water.

Recovery = 1.5 ÷ 2.0 × 100

Recovery = 75%

Residual Streams

Advanced treatment can produce residual streams such as:

• filter backwash water;
• membrane concentrate;
• spent media;
• chemical sludge;
• cleaning waste.

Operators should understand where these residuals go and how they affect other plant processes.

Activated Carbon

Activated carbon can remove selected dissolved organic compounds through adsorption.

Wastewater applications can include polishing for:

• specific organic compounds;
• odor-producing compounds;
• other treatment objectives.

Adsorption Capacity

Activated carbon has finite adsorption capacity.

As capacity is used, contaminant breakthrough can occur.

Advanced Oxidation

Advanced oxidation processes generate highly reactive chemical species that can destroy certain difficult organic compounds.

Systems may use combinations of:

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

Disinfection

High-quality effluent or reclaimed water commonly requires effective disinfection.

Disinfection technologies can include:

• chlorine;
• ultraviolet light;
• ozone;
• other approved technologies.

Suspended Solids Can Affect Disinfection

Particles can interfere with disinfection by shielding microorganisms or increasing disinfectant demand.

Improved solids removal can therefore improve disinfection performance.

UV Treatment

UV performance depends on factors such as:

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

Chlorine Treatment

When chlorine is used, operators should understand:

• dose;
• demand;
• residual;
• contact time;
• dechlorination where required.

Water Reuse

Water reuse, also called water recycling or water reclamation, means treating wastewater or another used water source so the water can be used again for a beneficial purpose.

Reuse applications can vary widely and can include:

• landscape irrigation;
• industrial uses;
• environmental applications;
• groundwater recharge;
• other nonpotable uses;
• potable reuse systems with extensive additional treatment.

Treatment Depends on Intended Use

Not every reuse application requires the same water quality.

The required treatment depends on factors such as:

• source water;
• intended use;
• potential human exposure;
• environmental exposure;
• applicable requirements.

Nonpotable Reuse

Nonpotable reuse uses reclaimed water for purposes other than drinking.

Examples can include:

• irrigation;
• industrial cooling;
• process water;
• other approved uses.

Potable Reuse

Potable reuse involves reclaimed water becoming part of a drinking-water supply.

Because potential human exposure is much greater, potable reuse requires extensive treatment, monitoring, and multiple protective barriers.

Multiple Treatment Barriers

Advanced reuse systems can use several processes that provide overlapping protection.

Multiple barriers reduce reliance on any single treatment unit.

Treatment Redundancy

Redundancy means that more than one process may contribute to removing or controlling the same type of contaminant.

Operators should understand which barriers protect against:

• particles;
• microorganisms;
• dissolved contaminants;
• other treatment risks.

Reuse Is Not Simply Effluent Disposal

Reuse changes treated wastewater from a waste stream into a water resource intended for another beneficial use.

This can require additional:

• treatment;
• storage;
• monitoring;
• distribution infrastructure.

Reuse Storage

Storage can help balance differences between water production and reuse demand.

Operators should monitor storage for:

• water age;
• disinfectant conditions;
• algae;
• sediment;
• water-quality changes.

Reuse Distribution

Reclaimed-water distribution systems require appropriate operational controls to maintain water quality and prevent unintended connections or uses.

Specific requirements vary by jurisdiction and intended use.

Monitoring Advanced Treatment

Advanced systems often depend on multiple measurements.

Parameters can include:

• flow;
• turbidity;
• TSS;
• BOD or COD;
• nitrogen;
• phosphorus;
• pressure;
• UV intensity;
• disinfectant residual;
• process-specific contaminant measurements.

Online Instruments

Advanced processes often use continuous or frequent online measurements.

Operators should verify:

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

Alarm Response

An alarm should lead to a defined operational response.

Operators should know:

• what triggered the alarm;
• which treatment barrier is affected;
• whether flow should continue;
• what verification is required.

Process Integrity

Advanced treatment requires reliable equipment and controls.

A process can fail because of:

• mechanical failure;
• instrument failure;
• loss of chemical feed;
• membrane damage;
• poor upstream water quality.

Upstream Treatment Matters

Poor secondary treatment can increase:

• filter loading;
• membrane fouling;
• disinfectant demand;
• UV interference;
• advanced-treatment operating cost.

Example: Tertiary Filter Head Loss Rises Rapidly

Review:

• secondary clarifier effluent TSS;
• filtration rate;
• backwash performance;
• filter-media condition.

Example: Tertiary Effluent Turbidity Increases

Possible causes include:

• filter breakthrough;
• hydraulic overload;
• poor backwashing;
• upstream solids carryover;
• instrument problems.

Example: Membrane Pressure Increases

Possible causes include:

• fouling;
• scaling;
• poor pretreatment;
• blocked equipment;
• changes in feed-water quality.

Example: Membrane Product Flow Declines

Review:

• feed pressure;
• membrane condition;
• temperature;
• fouling;
• cleaning history.

Example: Reuse Demand Falls

If reclaimed-water production continues while demand decreases, storage volume and water age can increase.

Operators should monitor storage and distribution conditions.

Example: UV Intensity Drops

Possible causes include:

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

Example: Chlorine Demand Increases

Review:

• effluent organic matter;
• suspended solids;
• ammonia;
• process changes;
• chemical feed.

Example: Phosphorus Removal Declines

Review:

• chemical dose;
• mixing;
• pH;
• solids separation;
• influent phosphorus load.

Example: Nitrogen Removal Declines

Review:

• temperature;
• dissolved oxygen;
• carbon availability;
• pH and alkalinity;
• loading;
• process-zone conditions.

Residuals Management

Operators should account for residuals created by tertiary and advanced treatment.

These can affect:

• solids-handling capacity;
• plant recycle loads;
• disposal requirements;
• chemical use.

Energy Use

Advanced treatment can require substantial energy for:

• high-pressure pumping;
• membrane systems;
• UV equipment;
• ozone generation;
• aeration;
• advanced filtration.

Energy Trends

Increasing energy use at similar treatment flow can indicate:

• fouling;
• higher pressure requirements;
• equipment deterioration;
• poor process efficiency.

Process Records

Useful records can include:

• flow;
• influent and effluent quality;
• filter pressure or head loss;
• membrane pressure;
• chemical feed;
• UV intensity;
• disinfectant residual;
• reuse production;
• reuse demand;
• equipment alarms.

Common Tertiary and Advanced Treatment Mistakes

• Expecting advanced treatment to compensate indefinitely for poor secondary treatment.
• Ignoring upstream solids carryover.
• Operating filters only by elapsed time instead of actual performance.
• Ignoring membrane fouling trends.
• Using percentage removal without reviewing final concentration.
• Failing to account for residual streams.
• Assuming all water reuse applications require the same treatment.
• Ignoring intended end use when evaluating reclaimed-water quality.
• Ignoring alarms or instrumentation drift.
• Evaluating advanced processes independently instead of as multiple treatment barriers.

A Practical Tertiary-Treatment Review

1. Review secondary effluent quality.
2. Review tertiary influent flow.
3. Review filter or membrane loading.
4. Check head loss or pressure.
5. Review treated-water turbidity and solids.
6. Review nutrient performance where applicable.
7. Review disinfection.
8. Review residuals production.
9. Compare performance with historical trends.

A Practical Reuse-System Review

1. Confirm the intended reuse application.
2. Review source-water quality.
3. Review each treatment barrier.
4. Review disinfection.
5. Review storage conditions.
6. Review reclaimed-water distribution conditions.
7. Review process alarms.
8. Review final water-quality measurements.

A Practical Membrane Review

1. Review feed-water quality.
2. Review pretreatment performance.
3. Review feed pressure.
4. Review product flow.
5. Calculate or review recovery.
6. Check fouling indicators.
7. Review cleaning history.
8. Review concentrate flow.

What to Remember for the Exam

• Tertiary treatment provides additional treatment after secondary treatment.
• Advanced treatment targets pollutants or water-quality goals not adequately addressed by conventional treatment alone.
• Tertiary filtration removes suspended solids remaining after secondary clarification.
• Poor secondary clarification can rapidly overload tertiary filters.
• Filtration rate equals flow divided by filter area.
• Advanced nutrient removal can include nitrification, denitrification, biological phosphorus removal, and chemical phosphorus removal.
• Chemical feed in lb/day can be calculated as MGD × mg/L × 8.34.
• Membrane treatment uses selective barriers to remove particles, microorganisms, or dissolved constituents depending on membrane type.
• Membrane fouling can increase pressure and reduce product flow.
• Membrane recovery equals product flow divided by feed flow times 100.
• Advanced treatment produces residual streams that must be managed.
• Activated carbon removes selected dissolved compounds by adsorption.
• Advanced oxidation can destroy selected difficult organic contaminants.
• Good solids removal can improve disinfection performance.
• Water reuse means treating used water so it can be used again for a beneficial purpose.
• Required reuse treatment depends on source water, intended use, exposure, and applicable requirements.
• Nonpotable and potable reuse have different treatment objectives and risks.
• Multiple treatment barriers provide redundancy and reduce dependence on one process.
• Advanced systems depend heavily on reliable monitoring, alarms, maintenance, and instrumentation.
• Good tertiary and advanced treatment begins with stable upstream treatment and ends with verification of final water quality.