Wastewater Influent & Effluent Quality

Wastewater operators must understand both the water entering the treatment plant and the water leaving it. Influent quality determines the load placed on treatment processes, while effluent quality shows how effectively the plant is removing pollutants and maintaining stable operation.

A single laboratory result rarely explains plant performance by itself. Operators should interpret concentration together with flow, mass loading, treatment conditions, historical trends, and related parameters.

Influent Versus Effluent

Influent is wastewater entering a treatment process or treatment plant.

Effluent is treated wastewater leaving a process or treatment plant.

Important comparisons include:

• influent versus primary effluent;
• influent versus secondary effluent;
• secondary effluent versus final effluent;
• current results versus historical trends.

Why Influent Quality Matters

Influent determines the hydraulic and pollutant load placed on the plant.

Important influent characteristics can include:

• flow;
• BOD;
• COD;
• TSS;
• ammonia;
• nitrogen;
• phosphorus;
• pH;
• alkalinity;
• temperature.

Why Effluent Quality Matters

Effluent quality reflects the combined performance of:

• biological treatment;
• clarification;
• filtration where provided;
• nutrient removal;
• disinfection;
• other downstream processes.

Concentration and Loading Are Different

A concentration such as mg/L describes how much pollutant is present in a unit volume.

A mass loading such as lb/day describes the total pollutant mass entering or leaving the plant each day.

Mass Loading Formula

A common wastewater calculation is:

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

Loading Example

Influent flow is 2.5 MGD and influent BOD is 220 mg/L.

Loading = 2.5 × 220 × 8.34

Loading = 4,587 lb/day

The plant receives approximately 4,587 lb/day of BOD.

Flow Changes Can Change Loading

Even if concentration remains constant, pollutant loading increases when flow increases.

For example, doubling flow at the same concentration approximately doubles mass loading.

Dilution Can Lower Concentration Without Lowering Mass

Wet-weather infiltration and inflow can dilute wastewater.

Influent BOD concentration may decrease while total hydraulic load rises sharply.

Operators should therefore review both:

• mg/L concentration;
• lb/day loading.

Biochemical Oxygen Demand

BOD estimates the amount of oxygen microorganisms use while degrading biodegradable organic material under defined test conditions.

Influent BOD is an important indicator of organic loading.

High Influent BOD

A higher influent BOD load can increase:

• oxygen demand;
• aeration requirement;
• biomass production;
• sludge production.

Effluent BOD

Effluent BOD provides information about how much biodegradable or oxygen-demanding material remains after treatment.

Unexpectedly high effluent BOD can be associated with:

• insufficient biological treatment;
• solids carryover;
• toxicity;
• poor aeration;
• hydraulic overload.

BOD Removal Efficiency

A common relationship is:

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

BOD Removal Example

Influent BOD is 200 mg/L and effluent BOD is 20 mg/L.

Removal = (200 - 20) ÷ 200 × 100

Removal = 90%

Do Not Use Removal Percentage Alone

A high removal percentage can still produce an unacceptable effluent concentration if influent loading is extremely high.

Operators should evaluate:

• influent concentration;
• effluent concentration;
• mass loading;
• permit requirements.

Chemical Oxygen Demand

COD measures oxygen equivalent associated with chemically oxidizable material.

COD results are generally available faster than BOD results.

COD and BOD

COD often exceeds BOD because COD measures a broader group of oxidizable substances.

The relationship between BOD and COD can provide useful process information, but the ratio is wastewater-specific.

COD Trend Changes

A sudden COD increase can indicate:

• industrial discharge;
• high-strength waste;
• process upset;
• sampling change.

Total Suspended Solids

TSS measures suspended particulate material.

Influent TSS contributes to:

• primary sludge;
• biological solids loading;
• downstream solids handling.

Effluent TSS

Effluent TSS is strongly influenced by clarification and solids separation.

High effluent TSS can result from:

• clarifier solids carryover;
• poor settling;
• hydraulic overload;
• sludge blanket problems;
• filamentous growth;
• mechanical problems.

TSS Removal Efficiency

The same removal formula can be used:

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

Solids Can Increase BOD

Effluent suspended solids can contain biodegradable organic material.

A clarifier solids problem may therefore increase both:

• effluent TSS;
• effluent BOD.

Interpret BOD and TSS Together

If both effluent BOD and TSS rise at the same time, investigate solids separation before assuming biological oxidation alone has failed.

pH

Wastewater pH affects:

• biological activity;
• chemical reactions;
• nitrification;
• chemical treatment.

Sudden Influent pH Change

A rapid pH change can indicate:

• industrial discharge;
• chemical spill;
• cleaning discharge;
• sampling or instrument error.

Alkalinity

Alkalinity helps buffer wastewater against pH change.

It is especially important when nitrification is occurring.

Nitrification Consumes Alkalinity

As ammonia is biologically oxidized, alkalinity is consumed.

A commonly used operator relationship is approximately:

7.14 mg/L alkalinity as CaCO3 consumed per mg/L ammonia-nitrogen oxidized

Low Alkalinity Can Limit Nitrification

If alkalinity becomes too low, pH may fall and nitrifying organisms may lose activity.

Ammonia

Influent ammonia represents an important nitrogen load.

Effluent ammonia provides information about nitrification performance.

High Effluent Ammonia

Possible causes include:

• low dissolved oxygen;
• low temperature;
• insufficient solids retention;
• low alkalinity;
• toxic inhibition;
• high ammonia loading.

Nitrite and Nitrate

During nitrification:

1. ammonia is oxidized to nitrite;
2. nitrite is oxidized to nitrate.

Patterns in ammonia, nitrite, and nitrate can help operators understand nitrification performance.

High Nitrite

An increase in nitrite can indicate incomplete nitrification or an imbalance between nitrifying populations.

Total Nitrogen

Total nitrogen includes several nitrogen forms.

Plants designed for nitrogen removal must evaluate multiple parameters rather than ammonia alone.

Phosphorus

Wastewater phosphorus may include:

• orthophosphate;
• organic phosphorus;
• particulate phosphorus.

Effluent Phosphorus

Effluent phosphorus can be affected by:

• biological phosphorus removal;
• chemical addition;
• solids separation;
• influent loading.

Solids Carry Phosphorus

Because some phosphorus is associated with solids, poor clarification can increase effluent phosphorus.

Temperature

Wastewater temperature affects:

• biological reaction rates;
• nitrification;
• oxygen transfer;
• settling behavior.

Cold-Weather Effects

Lower temperatures generally reduce biological reaction rates.

Nitrification can be particularly sensitive to cold conditions.

Dissolved Oxygen

Dissolved oxygen is an important process indicator in aerobic biological treatment.

Low DO can contribute to:

• poor organic removal;
• poor nitrification;
• undesirable biological conditions.

High DO Is Not Always Better

Excessive aeration can waste energy and may not improve treatment once process oxygen requirements are met.

Influent Flow

Flow is one of the most important wastewater operating parameters.

Flow affects:

• hydraulic detention time;
• clarifier loading;
• aeration loading;
• disinfection contact time;
• mass loading calculations.

Average Flow Versus Peak Flow

A plant may handle average daily flow successfully but experience problems during short peak-flow periods.

Peak flow can cause:

• shorter detention time;
• clarifier overload;
• solids washout;
• lower disinfection contact time.

Wet-Weather Flow

Wet weather can increase influent flow through:

• infiltration;
• inflow;
• storm-related collection-system conditions.

Wet-Weather Influent Characteristics

During wet weather, operators may observe:

• higher flow;
• lower BOD concentration;
• lower TSS concentration;
• greater total hydraulic load;
• rapid loading changes.

Industrial or High-Strength Waste

A sudden high-strength discharge can cause:

• higher BOD or COD;
• oxygen demand;
• pH change;
• toxicity;
• biological upset.

Toxic Influent

Toxic material can reduce biological activity even when ordinary influent loading appears normal.

Possible signs include:

• rapid oxygen-use change;
• loss of nitrification;
• poor settling;
• unexpected effluent deterioration.

Influent Sampling

Influent samples should represent the wastewater actually entering the plant.

Sampling can be affected by:

• location;
• time of day;
• flow variation;
• industrial discharge patterns.

Composite Samples

Composite sampling can provide a more representative picture of wastewater that changes significantly over time.

Grab Samples

Grab samples represent conditions at a specific time.

They can be valuable for parameters that can change rapidly or require immediate measurement.

Effluent Sampling

Final effluent sampling should represent the actual discharge being evaluated.

Operators should understand:

• sample point;
• sampling method;
• flow conditions;
• applicable facility requirements.

Compare Concentration and Load

Consider two days:

Day 1:

• flow = 1 MGD;
• BOD = 200 mg/L.

Load = 1 × 200 × 8.34 = 1,668 lb/day

Day 2:

• flow = 2 MGD;
• BOD = 120 mg/L.

Load = 2 × 120 × 8.34 = 2,002 lb/day

Although concentration decreased, total BOD loading increased.

Hydraulic Loading and Organic Loading Are Different

Hydraulic loading describes water volume or flow.

Organic loading describes pollutant mass such as BOD or COD entering the process.

High Flow with Low Concentration

This condition can still create problems through:

• short detention time;
• clarifier overload;
• solids washout.

Low Flow with High Concentration

This may create high-strength biological loading even though hydraulic loading is moderate.

Effluent Quality Is Often a Delayed Response

Influent changes may not affect final effluent immediately because wastewater must travel through:

• primary treatment;
• biological reactors;
• clarifiers;
• downstream processes.

Consider Process Detention Time

When investigating an effluent problem, review influent conditions from the period that would reasonably affect the current effluent.

Trend Influent and Effluent Together

Useful trends include:

• flow;
• BOD;
• COD;
• TSS;
• ammonia;
• pH;
• alkalinity;
• temperature;
• effluent nutrient results.

Example: Influent Load Increases but Effluent Remains Stable

This indicates that treatment is successfully absorbing the increased load within its available capacity.

Example: Influent Stable but Effluent TSS Rises

This points more strongly toward a solids-separation or clarification problem.

Example: Effluent BOD and TSS Rise Together

Review:

• secondary clarifier performance;
• sludge blanket;
• settleability;
• hydraulic loading;
• solids inventory.

Example: Effluent Ammonia Rises

Review:

• temperature;
• DO;
• alkalinity;
• solids retention;
• ammonia loading;
• toxicity.

Example: Effluent Phosphorus Rises

Review:

• chemical feed where used;
• biological phosphorus-removal conditions;
• solids carryover;
• influent phosphorus loading.

Example: Effluent Quality Worsens During Rain

Review:

• peak flow;
• clarifier hydraulic loading;
• solids washout;
• collection-system infiltration and inflow;
• disinfection contact time.

Removal Efficiency Can Be Misleading at Low Influent Concentration

If influent concentration is unusually low, even good effluent quality may produce a lower calculated percent removal.

Always interpret percentage removal with actual concentrations and loading.

Mass Removal

Mass removed can be estimated as:

Mass Removed = Influent Load - Effluent Load

Mass Removal Example

Influent BOD load is 4,000 lb/day and effluent BOD load is 400 lb/day.

Mass Removed = 4,000 - 400

Mass Removed = 3,600 lb/day

Permit and Operational Limits

Effluent requirements depend on the facility's permit and applicable requirements.

Operators should not assume that one numerical limit applies to every wastewater treatment plant.

Operational Targets Can Be More Conservative

A facility may use internal operating targets that provide warning before effluent approaches a permit limit.

Trend Toward a Limit

A deteriorating trend can be important even before a numerical requirement is exceeded.

Operators should respond to:

• rising effluent BOD;
• rising TSS;
• rising ammonia;
• declining treatment stability.

Laboratory QA/QC Matters

Unexpected influent or effluent results should be checked against:

• sample collection;
• preservation;
• holding time;
• instrument calibration;
• laboratory QA/QC.

Do Not Adjust the Process from One Questionable Result

Before making major operating changes, compare the result with:

• related laboratory parameters;
• online process measurements;
• historical trends;
• current plant observations.

Common Influent and Effluent Interpretation Mistakes

• Looking at concentration without flow.
• Looking at flow without pollutant loading.
• Assuming lower influent concentration always means lower plant load.
• Using removal efficiency without reviewing actual effluent concentration.
• Ignoring the effect of solids carryover on effluent BOD.
• Ignoring process lag between influent and effluent.
• Ignoring wet-weather hydraulic loading.
• Interpreting one laboratory result without related process data.
• Assuming one permit limit applies to every treatment facility.
• Ignoring laboratory QA/QC when results change unexpectedly.

A Practical Influent Review

1. Review flow.
2. Review BOD or COD.
3. Review TSS.
4. Calculate major mass loadings.
5. Review ammonia and nutrients where relevant.
6. Review pH and alkalinity.
7. Review temperature.
8. Compare with historical and seasonal patterns.

A Practical Effluent Review

1. Review BOD and TSS.
2. Review ammonia and nutrient results where applicable.
3. Review pH.
4. Review disinfection-related parameters.
5. Compare with influent loading.
6. Compare with process-control data.
7. Compare current results with permit requirements and internal operating targets.
8. Review trends rather than one result alone.

A Practical Effluent Deterioration Review

1. Verify the laboratory result.
2. Review influent conditions from the appropriate earlier time period.
3. Review current flow and recent peak flow.
4. Review biological-process conditions.
5. Review clarifier performance.
6. Review solids inventory and settleability.
7. Review nutrient-removal conditions where applicable.
8. Identify whether the primary cause is hydraulic, biological, chemical, or solids-separation related.

What to Remember for the Exam

• Influent quality describes wastewater entering treatment, while effluent quality describes wastewater leaving treatment.
• Concentration and mass loading are different concepts.
• A common loading formula is lb/day = MGD × mg/L × 8.34.
• Higher flow increases mass loading when concentration remains constant.
• Dilution can lower concentration even while total pollutant loading increases.
• BOD represents biodegradable oxygen demand, while COD measures a broader range of chemically oxidizable material.
• Effluent BOD can rise because of poor biological treatment or solids carryover.
• TSS is strongly affected by clarification and solids separation.
• Removal Efficiency, % = (Influent - Effluent) ÷ Influent × 100.
• Removal percentage should always be interpreted with actual effluent concentration and loading.
• pH and alkalinity affect biological treatment and nitrification.
• Nitrification consumes approximately 7.14 mg/L alkalinity as CaCO3 per mg/L ammonia-nitrogen oxidized.
• High effluent ammonia can indicate low DO, low temperature, insufficient solids retention, low alkalinity, high loading, or toxicity.
• Phosphorus can leave the plant with suspended solids, so poor clarification can increase effluent phosphorus.
• Peak flow can reduce detention time and overload clarifiers even when average flow is acceptable.
• Wet weather can lower influent concentration while increasing hydraulic loading.
• Influent changes may affect effluent only after process detention time has passed.
• Mass removed equals influent load minus effluent load.
• Wastewater effluent limits are facility-specific and should be interpreted using the applicable permit.
• Good influent and effluent interpretation combines concentration, flow, loading, process conditions, laboratory quality, permit requirements, and trends.