Wastewater Lagoons & Treatment Ponds

Wastewater lagoons and treatment ponds use large basins and relatively long detention times to provide biological and physical treatment. Compared with many mechanical treatment plants, lagoon systems can have simpler equipment, but successful operation still depends on hydraulic loading, organic loading, dissolved oxygen, algae, sludge accumulation, weather, and careful observation.

Lagoon performance changes with temperature, sunlight, wind, flow, and wastewater strength. Operators should understand these natural influences and use process data, visual observations, and laboratory results together.

What a Wastewater Lagoon Is

A wastewater lagoon is a basin designed to treat wastewater through biological activity, settling, natural aeration, mechanical aeration, or combinations of these processes.

Different lagoon designs can include:

• facultative lagoons;
• aerated lagoons;
• maturation or polishing ponds;
• other treatment pond configurations.

Facultative Lagoons

A facultative lagoon commonly contains both aerobic and anaerobic zones.

The upper portion can receive oxygen from:

• algal photosynthesis;
• surface reaeration;
• wind mixing.

Deeper portions can contain lower-oxygen or anaerobic conditions.

Aerated Lagoons

Aerated lagoons use mechanical or diffused aeration to provide oxygen and mixing.

Aeration can help:

• support biological oxidation;
• reduce septic conditions;
• improve mixing;
• increase treatment capacity.

Polishing Ponds

Polishing or maturation ponds can provide additional treatment after earlier processes.

They may help improve:

• solids settling;
• organic removal;
• pathogen reduction;
• overall effluent quality.

Hydraulic Detention Time

Lagoon systems often rely on relatively long detention times.

A simplified relationship is:

Detention Time = Lagoon Volume ÷ Flow

Detention-Time Example

A lagoon has a usable volume of 12 million gallons and receives 0.4 MGD.

Detention Time = 12,000,000 ÷ 400,000

Detention Time = 30 days

This is the theoretical detention time.

Actual Detention Time

Actual hydraulic behavior can differ because of:

• short-circuiting;
• dead zones;
• sludge accumulation;
• uneven inlet or outlet conditions.

Short-Circuiting

Short-circuiting occurs when part of the flow moves from inlet to outlet faster than intended.

This can reduce treatment effectiveness by decreasing actual contact time.

Dead Zones

Areas with little circulation can develop:

• solids accumulation;
• low dissolved oxygen;
• odor;
• reduced useful lagoon volume.

Organic Loading

Lagoon performance depends on the amount of biodegradable material applied to the system.

A common mass-loading relationship is:

Loading, lb/day = Flow, MGD × Concentration, mg/L × 8.34

BOD Loading Example

A lagoon receives 0.75 MGD with influent BOD of 180 mg/L.

BOD Loading = 0.75 × 180 × 8.34

BOD Loading = 1,126 lb/day

Why Mass Loading Matters

A concentration alone does not show the total treatment load.

If flow doubles while BOD concentration stays the same, daily BOD mass loading also doubles.

Dissolved Oxygen

Dissolved oxygen is an important lagoon operating parameter.

Low dissolved oxygen can contribute to:

• odors;
• poor treatment;
• septic conditions;
• reduced oxidation of organic matter.

Daily Oxygen Variation

In lagoons containing significant algae, dissolved oxygen can vary during the day.

Photosynthesis can increase oxygen during daylight.

At night, respiration continues while photosynthesis stops, so dissolved oxygen can decrease.

Morning Versus Afternoon DO

Dissolved oxygen measured in the early morning can be lower than the afternoon value.

Operators should consider sampling time when comparing lagoon DO results.

Algae

Algae are common in many lagoon systems.

They can provide oxygen through photosynthesis, but excessive algae can also affect effluent quality.

Algae and Suspended Solids

Algal cells can contribute to effluent suspended solids.

A lagoon can therefore have good biological treatment but still produce elevated TSS because of algae.

Algae and pH

Photosynthetic activity can influence carbon dioxide and pH.

During strong daytime photosynthesis, pH can increase.

Seasonal Effects

Lagoon treatment is strongly influenced by season.

Cold weather generally reduces biological reaction rates.

Warm weather generally increases biological activity.

Winter Operation

Cold-weather conditions can produce:

• slower biological treatment;
• lower oxygen demand from biological activity;
• ice cover in some climates;
• reduced mixing;
• changes in algae populations.

Spring Turnover

Seasonal changes can mix material that accumulated during colder periods.

Operators may observe:

• temporary odor;
• higher solids;
• changes in color;
• changes in dissolved oxygen.

Wind

Wind can provide surface mixing and reaeration.

Wind direction can also influence:

• surface scum;
• floating algae;
• localized solids accumulation.

Sunlight

Sunlight supports algal photosynthesis.

Changes in cloud cover and season can therefore affect oxygen production and lagoon biology.

Sludge Accumulation

Settleable solids accumulate on the lagoon bottom over time.

Excessive sludge can:

• reduce effective lagoon volume;
• reduce detention time;
• increase odor;
• create localized anaerobic conditions.

Sludge Depth

Periodic sludge-depth measurements can help determine:

• where solids are accumulating;
• how much usable volume remains;
• whether sludge removal planning is needed.

Sludge Does Not Disappear

Biological decomposition can reduce some solids, but inert and slowly degradable material can remain and accumulate.

Inlet Condition

The inlet should distribute wastewater without causing severe short-circuiting or erosion.

Operators should inspect for:

• debris;
• erosion;
• localized solids buildup;
• blocked piping.

Outlet Condition

Outlet structures should be maintained to provide the intended water level and effluent withdrawal.

Problems can include:

• blockage;
• algae accumulation;
• debris;
• damaged structures.

Lagoon Water Level

Water level affects lagoon volume and detention time.

Unexpected changes can indicate:

• flow changes;
• leaks;
• outlet problems;
• excessive evaporation;
• rainfall effects.

Aeration Equipment

Aerated lagoons may use:

• surface aerators;
• diffused-air systems;
• blowers;
• mixers.

Aerator Failure

If aeration equipment fails, operators should monitor:

• dissolved oxygen;
• odor;
• mixing;
• effluent quality.

Too Little Aeration

Possible effects include:

• low dissolved oxygen;
• odors;
• reduced treatment;
• solids deposition in unintended areas.

Too Much Aeration

Excessive aeration can:

• waste energy;
• resuspend settled solids;
• increase equipment wear.

Mixing

Mixing helps distribute:

• oxygen;
• organic load;
• microorganisms;
• temperature.

However, some lagoon configurations also depend on settling, so mixing should match the process design.

Odors

Lagoon odors can indicate:

• low dissolved oxygen;
• septic influent;
• overloading;
• sludge accumulation;
• seasonal turnover;
• stagnant zones.

Foam and Floating Material

Operators should inspect unusual:

• foam;
• scum;
• floating solids;
• algae mats.

These observations can provide clues about process changes.

Color

Lagoon color can provide operational clues but should not be used alone to diagnose treatment.

Color can be influenced by:

• algae;
• suspended solids;
• biological conditions;
• industrial wastes.

pH

pH can vary because of:

• influent conditions;
• algal photosynthesis;
• biological reactions;
• industrial discharges.

Hydraulic Overloading

High flow can:

• reduce detention time;
• increase short-circuiting;
• carry more solids to the outlet;
• reduce treatment performance.

Wet-Weather Flow

Infiltration and inflow can increase lagoon flow during or after rainfall.

Operators should compare:

• rainfall;
• influent flow;
• water level;
• effluent quality.

Organic Overloading

Excessive organic loading can increase oxygen demand beyond the system's ability to supply oxygen.

Possible signs include:

• low DO;
• odor;
• dark water;
• deteriorating effluent quality.

Shock Loads

A sudden high-strength discharge can disturb lagoon biology.

Possible sources include:

• industrial discharges;
• septage;
• slug loads;
• other concentrated wastes.

Toxic Loads

Toxic or inhibitory wastes can reduce biological activity.

Operators may observe:

• poor BOD removal;
• unusual color;
• odor;
• changes in algae;
• lower biological activity.

Effluent BOD

Effluent BOD is an important indicator of treatment performance.

A rising trend can indicate:

• higher influent load;
• lower biological activity;
• hydraulic overloading;
• short-circuiting;
• seasonal effects.

Effluent TSS

Effluent TSS can increase because of:

• algae;
• solids washout;
• hydraulic surges;
• sludge disturbance.

Ammonia

Some lagoon systems may provide nitrification under favorable conditions.

Nitrification can be affected by:

• temperature;
• dissolved oxygen;
• pH;
• detention time;
• toxic substances.

Removal Efficiency

A simplified relationship is:

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

BOD Removal Example

Influent BOD is 180 mg/L and effluent BOD is 36 mg/L.

Removal Efficiency = (180 - 36) ÷ 180 × 100

Removal Efficiency = 80%

Do Not Use Percentage Alone

Operators should also evaluate actual effluent concentration and permit requirements.

Example: DO Is Low Early in the Morning

Possible causes include normal nighttime respiration combined with:

• high organic load;
• low aeration;
• heavy algae respiration;
• warm water.

Example: DO Is High in the Afternoon but Low Before Sunrise

This pattern is consistent with strong daytime photosynthesis and nighttime respiration.

Example: Effluent TSS Is High During an Algal Bloom

Algal cells may be contributing significantly to suspended solids.

Review:

• effluent algae;
• season;
• water temperature;
• downstream polishing processes.

Example: Strong Odor Develops

Review:

• dissolved oxygen;
• organic loading;
• sludge accumulation;
• aerator operation;
• septic influent;
• stagnant areas.

Example: Effluent Quality Declines After Heavy Rain

Possible causes include:

• hydraulic overloading;
• shorter detention time;
• short-circuiting;
• solids washout.

Example: Lagoon Water Level Falls Unexpectedly

Investigate:

• outlet condition;
• leakage;
• evaporation;
• influent flow measurement.

Example: Aerated Lagoon Becomes Septic

Review:

• aerator operation;
• blower output;
• organic loading;
• dissolved oxygen;
• mixing.

Example: Sludge Depth Increases Over Time

This is expected in many lagoon systems.

Operators should track accumulation and plan for removal before useful treatment volume is seriously reduced.

Example: One Area of the Lagoon Has Persistent Solids Accumulation

Possible causes include:

• poor circulation;
• dead zones;
• inlet hydraulics;
• wind patterns;
• failed mixing equipment.

Example: Effluent Ammonia Increases in Winter

Lower temperature can reduce nitrification rates.

Review:

• temperature;
• dissolved oxygen;
• pH;
• loading;
• detention time.

Lagoon Inspection

Routine observation should include:

• water level;
• color;
• odor;
• foam;
• algae;
• inlet condition;
• outlet condition;
• embankments;
• aeration equipment.

Embankments

Lagoon embankments should be inspected for:

• erosion;
• animal burrows;
• vegetation problems;
• seepage;
• structural damage.

Vegetation

Vegetation should be managed so operators can inspect embankments and structures.

Excessive vegetation can:

• hide erosion;
• encourage burrowing animals;
• interfere with access.

Safety

Lagoon hazards can include:

• open water;
• steep or slippery banks;
• electrical equipment;
• aerators;
• biological exposure;
• remote work locations.

Electrical Safety

Aeration equipment often operates near water.

Operators should follow electrical safety and lockout/tagout procedures during inspection and maintenance.

Process Records

Useful records can include:

• influent flow;
• influent BOD;
• effluent BOD;
• effluent TSS;
• dissolved oxygen;
• pH;
• temperature;
• water level;
• aerator operation;
• sludge depth;
• weather.

Trend Seasonal Performance

Comparing data by season can help distinguish normal seasonal behavior from abnormal process failure.

Common Lagoon Operation Mistakes

• Assuming lagoon systems require little operator attention.
• Ignoring dissolved oxygen variation over the daily cycle.
• Interpreting algae only as a problem instead of recognizing their role in lagoon oxygen production.
• Ignoring sludge accumulation and loss of useful volume.
• Ignoring hydraulic short-circuiting.
• Evaluating concentration without considering mass loading.
• Ignoring seasonal temperature effects.
• Running excessive aeration without considering energy and solids resuspension.
• Ignoring embankment maintenance.
• Waiting for permit violations before responding to changes in odor, DO, or effluent quality.

A Practical Lagoon Review

1. Review influent flow.
2. Review organic loading.
3. Measure dissolved oxygen.
4. Review pH and temperature.
5. Observe color, algae, and odor.
6. Inspect aerators where present.
7. Inspect inlet and outlet structures.
8. Review effluent BOD and TSS.
9. Review sludge accumulation.
10. Compare with seasonal historical data.

A Practical Low-DO Review

1. Verify the DO measurement.
2. Consider sampling time.
3. Review organic loading.
4. Check aeration equipment.
5. Review temperature.
6. Look for stagnant zones.
7. Check for septic influent or shock loads.

A Practical Odor Review

1. Identify where the odor is strongest.
2. Review dissolved oxygen.
3. Check aeration and mixing.
4. Review sludge accumulation.
5. Check influent conditions.
6. Review seasonal turnover or weather changes.

A Practical High-Effluent-TSS Review

1. Verify the laboratory result.
2. Review algae conditions.
3. Review flow and recent rainfall.
4. Check for solids washout.
5. Review aeration and mixing.
6. Inspect outlet conditions.

What to Remember for the Exam

• Wastewater lagoons use long detention time, biological activity, settling, and natural or mechanical aeration for treatment.
• Facultative lagoons can contain aerobic upper zones and lower-oxygen or anaerobic lower zones.
• Aerated lagoons use mechanical or diffused aeration to supply oxygen and mixing.
• Polishing ponds provide additional treatment after earlier processes.
• Theoretical detention time equals lagoon volume divided by flow.
• Short-circuiting reduces actual treatment time.
• Organic loading can be calculated as MGD × mg/L × 8.34.
• Dissolved oxygen can vary significantly between day and night in lagoons with algae.
• Algae can supply oxygen through photosynthesis but can also increase effluent TSS.
• Cold weather reduces biological reaction rates.
• Sludge accumulation reduces effective lagoon volume and detention time.
• Hydraulic overloading can reduce treatment efficiency and increase solids washout.
• Organic overloading can create low DO and odor problems.
• Wet weather can increase lagoon flow through infiltration and inflow.
• Aeration equipment should provide adequate oxygen without unnecessary energy use or excessive solids resuspension.
• Lagoon embankments, inlet structures, and outlet structures require regular inspection.
• Effluent BOD, TSS, DO, pH, temperature, and flow are important operating indicators.
• Removal efficiency equals influent concentration minus effluent concentration, divided by influent concentration, times 100.
• Seasonal trends are important when evaluating lagoon performance.
• Good lagoon operation depends on hydraulic control, oxygen balance, sludge monitoring, seasonal awareness, equipment maintenance, and careful observation.