Wastewater Process Control, Loading & Shock Loads

Wastewater process control is the systematic adjustment of treatment operations to maintain stable performance as flow, pollutant loading, weather, equipment condition, and wastewater characteristics change. Operators should base process decisions on measurements, trends, observations, and an understanding of how one treatment unit affects another.

Stable treatment does not mean plant conditions remain constant. Wastewater plants continuously experience changing hydraulic and organic loads. Good process control recognizes these changes early and responds before they become effluent problems.

Process Control Begins with Loading

Operators should distinguish between concentration and mass loading.

A wastewater concentration tells how much of a constituent is present in a given volume. Mass loading describes the total quantity entering a process over time.

Mass Loading

A common operator relationship is:

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

BOD Loading Example

A plant receives 2.4 MGD with influent BOD of 210 mg/L.

BOD Load = 2.4 × 210 × 8.34

BOD Load = 4,203.4 lb/day

This value represents the approximate daily BOD mass entering the plant.

Why Concentration Alone Can Be Misleading

Suppose BOD remains at 210 mg/L but flow increases from 2.4 MGD to 3.6 MGD.

BOD Load = 3.6 × 210 × 8.34

BOD Load = 6,305.0 lb/day

The concentration did not change, but the daily organic load increased by about 50 percent.

Hydraulic Loading

Hydraulic loading describes the amount of wastewater flow applied to a treatment unit.

High hydraulic loading can affect:

• detention time;
• clarifier settling;
• filter loading;
• contact time;
• solids washout.

Organic Loading

Organic loading describes the amount of biodegradable material applied to a biological process.

High organic loading can increase:

• oxygen demand;
• sludge production;
• aeration requirements;
• secondary clarifier loading.

Hydraulic and Organic Loading Can Change Separately

During heavy rain, flow can increase while influent BOD concentration decreases because of dilution.

Even then, operators should calculate the actual mass load rather than assuming loading decreased.

Influent Monitoring

Important influent data can include:

• flow;
• BOD;
• COD;
• TSS;
• pH;
• temperature;
• ammonia;
• conductivity or other plant-specific indicators.

BOD

Biochemical oxygen demand is commonly used to estimate biodegradable organic strength.

Higher BOD generally means greater biological oxygen demand and potentially greater treatment loading.

COD

Chemical oxygen demand can often provide a faster indication of wastewater strength than BOD.

Operators may use COD trends to identify changing loads or unusual influent conditions.

TSS

Total suspended solids indicate the suspended particulate load entering or leaving treatment processes.

High influent TSS can increase:

• primary sludge production;
• biological solids loading;
• clarifier loading;
• solids-handling demand.

pH

Extreme pH can interfere with biological treatment and can indicate an abnormal discharge.

Operators should investigate sudden pH changes, especially when they occur with changes in:

• odor;
• color;
• conductivity;
• biological performance.

Temperature

Temperature affects biological reaction rates.

Cold wastewater can reduce:

• biological activity;
• nitrification rates;
• overall process response speed.

Dissolved Oxygen

Dissolved oxygen is a major process-control parameter in aerobic biological treatment.

Low DO can indicate:

• high organic loading;
• insufficient aeration;
• blower problems;
• diffuser fouling;
• rapid oxygen demand.

High DO Is Not Automatically Better

Excessive aeration can waste energy.

Good process control maintains adequate oxygen for the biological process without unnecessary aeration.

Process Trends

One measurement shows current conditions. A trend shows direction.

Useful trends include:

• flow versus BOD loading;
• BOD loading versus DO;
• flow versus clarifier performance;
• temperature versus ammonia removal;
• sludge wasting versus solids inventory;
• rainfall versus influent flow.

Rate of Change Matters

A parameter that changes gradually may allow operators time to respond.

A sudden change can indicate:

• equipment failure;
• shock load;
• industrial discharge;
• hydraulic surge;
• instrument failure.

Normal Operating Range

Operators should understand the normal range for important plant parameters.

This helps identify abnormal conditions before final effluent quality deteriorates.

Shock Load

A shock load is a sudden change in wastewater quantity or quality that places unusual stress on the treatment process.

Shock loads can be:

• hydraulic;
• organic;
• toxic or inhibitory;
• chemical;
• thermal.

Hydraulic Shock Load

A hydraulic shock occurs when flow increases rapidly.

Possible effects include:

• shorter detention time;
• clarifier overload;
• solids washout;
• reduced disinfection contact time;
• higher downstream flow rates.

Common Hydraulic Shock Sources

• heavy rainfall;
• infiltration and inflow;
• pump-station releases;
• equalization-basin discharge;
• industrial or commercial flow changes.

Organic Shock Load

An organic shock occurs when biodegradable pollutant loading increases sharply.

Possible effects include:

• rapid oxygen demand;
• lower dissolved oxygen;
• higher biological activity;
• more sludge production;
• higher effluent BOD if treatment capacity is exceeded.

Toxic Shock Load

A toxic or inhibitory shock can reduce or stop normal biological activity.

Possible warning signs include:

• sudden loss of oxygen uptake;
• unexpectedly high DO despite high loading;
• poor settling;
• loss of nitrification;
• unusual odor or color;
• rapid deterioration of effluent quality.

High DO Can Sometimes Signal a Biological Problem

If aeration remains unchanged but microorganisms suddenly stop consuming oxygen, DO may rise.

Operators should not automatically interpret rising DO as improved treatment.

pH Shock

A sudden acidic or alkaline discharge can inhibit biological treatment.

Review:

• influent pH;
• alkalinity;
• industrial discharge history;
• biological response.

Temperature Shock

Rapid temperature changes can affect biological activity.

Large temperature changes can result from:

• industrial discharges;
• seasonal events;
• unusual process flows.

Shock Loads May Be Short but Effects Can Last

A toxic discharge may pass through the influent quickly while biological recovery takes much longer.

Operators should continue monitoring after the immediate influent condition has ended.

Process Response Should Be Controlled

Operators should avoid making several large process changes simultaneously unless an emergency procedure requires it.

Multiple changes make it difficult to determine:

• which adjustment helped;
• which adjustment made conditions worse;
• how the process is responding.

Verify the Data First

Before making major changes, verify abnormal measurements where practical.

Check:

• instrument calibration;
• sample location;
• sample condition;
• related process indicators.

Mechanical Problems Can Look Like Process Problems

Examples include:

• failed blower causing low DO;
• failed RAS pump causing rising clarifier blanket;
• blocked chemical line causing low chemical dose;
• failed mixer causing poor contact.

Process Problems Can Look Like Mechanical Problems

Heavy solids loading can increase pump load or clog equipment even when the equipment itself is mechanically sound.

Clarifier Response to High Flow

High hydraulic loading can increase:

• surface overflow rate;
• solids carryover;
• sludge blanket instability.

Surface Overflow Rate

A simplified relationship is:

Surface Overflow Rate = Flow ÷ Clarifier Surface Area

Surface Overflow Example

A clarifier with 4,000 ft² of surface area receives 2.0 MGD.

Surface Overflow Rate = 2,000,000 ÷ 4,000

Surface Overflow Rate = 500 gpd/ft²

If flow rises to 3.0 MGD:

Surface Overflow Rate = 3,000,000 ÷ 4,000

Surface Overflow Rate = 750 gpd/ft²

The hydraulic loading increased by 50 percent.

Detention Time

A simplified relationship is:

Detention Time = Volume ÷ Flow

When flow increases and basin volume remains constant, theoretical detention time decreases.

Example: Detention Time During High Flow

A basin contains 500,000 gallons.

At 1.0 MGD:

Detention Time = 500,000 ÷ 1,000,000 = 0.5 day = 12 hours

At 2.0 MGD:

Detention Time = 500,000 ÷ 2,000,000 = 0.25 day = 6 hours

Flow-Paced Chemical Feed

When chemical dose should remain constant, chemical mass feed generally must change with flow.

A common relationship is:

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

Chemical Feed Example

A plant applies 8 mg/L of a treatment chemical at 1.5 MGD.

Feed = 1.5 × 8 × 8.34

Feed = 100.1 lb/day

If flow rises and chemical mass feed remains unchanged, the applied mg/L dose decreases.

Sludge Production

Higher organic and suspended-solids loads can increase solids production.

Operators should consider downstream capacity for:

• sludge pumping;
• thickening;
• digestion;
• dewatering;
• storage.

Solids Inventory

Biological processes often require controlled solids inventory.

Operators should use plant-specific indicators such as:

• MLSS;
• sludge age;
• WAS rate;
• clarifier blanket;
• settling characteristics.

Do Not React to MLSS Alone

MLSS should be considered together with:

• influent loading;
• sludge settling;
• effluent quality;
• sludge wasting;
• process objectives.

Secondary Clarifier Loading

A biological process can perform well in the aeration basin while final effluent deteriorates because the secondary clarifier cannot separate the solids effectively.

Process control must therefore include both biology and solids separation.

Wet-Weather Operation

During wet weather, operators should closely monitor:

• influent flow;
• clarifier loading;
• sludge blankets;
• effluent TSS;
• disinfection conditions;
• solids washout.

Industrial Discharges

Industrial or commercial discharges can cause changes in:

• BOD;
• COD;
• pH;
• temperature;
• toxicity;
• oil and grease;
• nutrients.

Unusual COD-to-BOD Relationship

A sudden change in COD without a similar BOD change can provide a clue that influent wastewater composition changed.

Operators should use plant history and additional testing before drawing conclusions.

Septage and High-Strength Waste

Accepted hauled wastes can create concentrated loads.

Operators should understand:

• where the waste enters;
• when it is received;
• how much is added;
• how it affects plant loading.

Equalization

Flow or load equalization can reduce rapid variations entering downstream processes.

Equalization can help moderate:

• hydraulic peaks;
• organic peaks;
• chemical-feed changes.

Return Flows

Solids-processing return flows can add concentrated pollutants back to the liquid treatment process.

Return flows may contain:

• ammonia;
• solids;
• BOD or COD;
• other concentrated constituents.

Internal Loads Matter

Plant influent is not always the only important load.

Internal recycle streams can significantly affect treatment, especially during certain solids-handling operations.

Example: DO Falls but Influent Flow Is Normal

Review:

• influent BOD or COD;
• blower output;
• diffusers;
• temperature;
• return flows;
• recent industrial discharges.

Example: DO Rises Suddenly

Possible causes include:

• reduced organic loading;
• increased aeration;
• loss of biological oxygen uptake;
• instrument error.

Example: Influent pH Drops Suddenly

Review:

• industrial discharges;
• collection-system events;
• alkalinity;
• biological response;
• instrument accuracy.

Example: Effluent TSS Rises During a Rain Event

Possible causes include:

• clarifier hydraulic overload;
• solids washout;
• blanket disturbance;
• higher RAS requirements.

Example: Effluent Ammonia Increases

Review:

• temperature;
• dissolved oxygen;
• sludge age;
• pH and alkalinity;
• toxic loads;
• influent ammonia loading.

Example: BOD Load Increases but Flow Does Not

The influent wastewater has become stronger.

Review potential sources such as:

• industrial discharge;
• septage;
• return flows;
• sampling changes.

Example: Flow Increases but BOD Concentration Falls

This often occurs during wet weather.

Calculate total BOD mass loading before deciding whether biological loading actually increased or decreased.

Example: Sludge Production Suddenly Increases

Review:

• influent TSS;
• organic loading;
• chemical addition;
• biological growth;
• primary treatment performance.

Example: One Instrument Shows an Extreme Change

If related process indicators remain normal, verify the instrument before making a major process adjustment.

Use Multiple Indicators

Good process control rarely depends on one number.

For example, an operator evaluating biological treatment may consider:

• flow;
• BOD or COD loading;
• DO;
• pH;
• temperature;
• solids inventory;
• settling;
• effluent quality.

Leading and Lagging Indicators

Some measurements provide early warning before final effluent deteriorates.

Examples can include:

• influent flow;
• influent COD;
• dissolved oxygen;
• clarifier blanket depth;
• equipment alarms.

Final effluent results may reflect problems that began hours earlier.

Process Changes Need Time

Biological processes may not respond immediately to an adjustment.

Operators should understand the process response time before making repeated changes.

Document Adjustments

Record:

• what changed;
• why it changed;
• when the adjustment occurred;
• what result followed.

Shift Handoffs

Important process changes should be communicated clearly between operators.

A useful handoff includes:

• current process condition;
• recent adjustments;
• abnormal influent events;
• equipment problems;
• parameters requiring close monitoring.

Process-Control Records

Useful records can include:

• influent and effluent flow;
• BOD;
• COD;
• TSS;
• pH;
• temperature;
• DO;
• ammonia;
• sludge wasting;
• clarifier blanket;
• chemical feed;
• weather;
• operator adjustments.

Common Process-Control Mistakes

• Looking only at concentration and ignoring mass loading.
• Confusing hydraulic loading with organic loading.
• Changing several major process settings at the same time.
• Reacting to one abnormal result without verification.
• Assuming high DO always means treatment is good.
• Ignoring internal return loads.
• Waiting for final effluent deterioration before responding.
• Trying to correct mechanical failures only with process adjustments.
• Ignoring weather and wet-weather hydraulic loading.
• Failing to document process changes and results.

A Practical Daily Process-Control Review

1. Review current and previous flow.
2. Review influent strength.
3. Calculate important mass loads.
4. Review dissolved oxygen and biological conditions.
5. Review clarifier performance.
6. Review sludge wasting and solids inventory.
7. Review chemical feed.
8. Review equipment status.
9. Review final effluent quality.
10. Compare current conditions with trends.

A Practical Shock-Load Review

1. Verify the abnormal measurement.
2. Determine when the change began.
3. Identify whether the event is hydraulic, organic, toxic, chemical, or thermal.
4. Review influent flow and strength.
5. Check pH, DO, temperature, and related indicators.
6. Check mechanical equipment.
7. Protect critical downstream processes.
8. Make controlled process adjustments.
9. Monitor recovery.
10. Document the event.

A Practical High-Flow Review

1. Verify influent flow.
2. Review rainfall and collection-system conditions.
3. Calculate hydraulic loading.
4. Review detention time.
5. Monitor clarifier blankets and effluent TSS.
6. Review disinfection capacity.
7. Watch for solids washout.

A Practical High-Organic-Load Review

1. Verify influent BOD or COD.
2. Calculate mass loading.
3. Review dissolved oxygen.
4. Check aeration equipment.
5. Review biological solids inventory.
6. Monitor clarifier performance.
7. Review solids-handling capacity.
8. Monitor final effluent.

A Practical Toxic-Shock Review

1. Identify the first abnormal indicator.
2. Verify pH, DO, conductivity, or other relevant measurements.
3. Review industrial or hauled-waste activity.
4. Observe biological response.
5. Protect unaffected treatment units where possible.
6. Follow facility emergency or pretreatment procedures.
7. Continue monitoring after the influent event passes.
8. Document recovery.

What to Remember for the Exam

• Wastewater process control uses measurements, calculations, observations, and trends to maintain stable treatment.
• Mass loading in lb/day equals MGD × mg/L × 8.34.
• Concentration alone does not describe total pollutant loading.
• Hydraulic loading describes water flow applied to a process.
• Organic loading describes pollutant mass applied to a biological process.
• Higher flow reduces detention time when basin volume remains constant.
• Surface overflow rate equals flow divided by clarifier surface area.
• High organic loading can increase oxygen demand and sludge production.
• Hydraulic shock loads can cause reduced detention time and solids washout.
• Organic shock loads can rapidly increase oxygen demand.
• Toxic shock loads can inhibit biological activity and may cause DO to rise if oxygen uptake falls.
• Sudden pH changes can indicate abnormal industrial or chemical discharges.
• Wet-weather flow can increase hydraulic loading even when wastewater concentration is diluted.
• Internal return flows can create significant pollutant loads.
• Operators should verify abnormal measurements before making major changes when practical.
• Mechanical failures can appear as process-control problems.
• Good process control uses multiple related indicators rather than one number.
• Leading indicators can warn of problems before final effluent deteriorates.
• Biological processes need time to respond to operational changes.
• Good operators make controlled adjustments, monitor the response, and document the result.