Industrial Pretreatment Fundamentals
Learn industrial pretreatment fundamentals, including industrial discharges, interference, pass through, prohibited discharges, local limits, monitoring, sampling, slug loads, operator warning signs, and wastewater treatment impacts.
Industrial pretreatment helps protect publicly owned wastewater treatment systems from industrial discharges that can interfere with treatment, pass through the plant without adequate removal, damage equipment, create safety hazards, or contaminate biosolids and receiving waters.
Wastewater treatment operators do not need to perform every pretreatment-program function, but they should understand how industrial discharges can affect plant operation and why pretreatment controls are important. Operators are often among the first people to notice unusual influent conditions, biological inhibition, pH changes, high-strength loads, oil, color, odors, or other signs of an abnormal discharge.
What Industrial Pretreatment Means
Industrial pretreatment is treatment or control applied to industrial wastewater before it enters a publicly owned treatment works or another receiving wastewater system.
The objective is to reduce or control pollutants that could:
- interfere with wastewater treatment;
- pass through treatment;
- damage collection or treatment equipment;
- create worker hazards;
- cause permit or effluent problems;
- affect sludge or biosolids quality.
Industrial Wastewater Is Different from Domestic Wastewater
Domestic wastewater is relatively predictable compared with many industrial discharges.
Industrial wastewater can contain unusual concentrations of:
- organic matter;
- metals;
- oil and grease;
- acids or bases;
- solvents;
- toxic compounds;
- high-temperature wastes;
- other process-specific pollutants.
Why Pretreatment Matters to Plant Operators
An industrial discharge can affect:
- influent pH;
- BOD and COD loading;
- dissolved oxygen demand;
- biological activity;
- settling;
- nitrification;
- sludge production;
- effluent quality.
Interference
Interference occurs when a discharge disrupts or inhibits wastewater treatment or otherwise causes problems with the operation or performance of the treatment system.
Examples can include:
- toxicity to biological treatment;
- extreme pH;
- excessive organic loading;
- oil or grease that interferes with equipment;
- solids that clog or damage the system.
Pass Through
Pass through refers to pollutants entering the treatment plant and leaving in the plant effluent without adequate treatment, potentially contributing to a discharge violation or other water-quality problem.
A pollutant can pass through because:
- the treatment process is not designed to remove it;
- the concentration is too high;
- the pollutant is resistant to treatment;
- the process has been upset.
Interference and Pass Through Are Different
Operators should understand the distinction:
- interference disrupts treatment or plant operations;
- pass through involves pollutants leaving the treatment system without adequate removal.
A discharge can potentially cause one or both.
Prohibited Discharges
Wastewater systems may prohibit certain discharges because they can create serious operational, safety, or treatment problems.
Examples of problematic characteristics can include:
- flammable or explosive conditions;
- corrosive conditions;
- excessive solids;
- extreme temperatures;
- toxic pollutants;
- oil and grease;
- other substances that interfere with treatment.
Extreme pH
Strongly acidic or alkaline wastewater can:
- damage collection-system materials;
- create unsafe conditions;
- inhibit biological treatment;
- change chemical-treatment performance.
pH as an Early Warning Indicator
A sudden influent pH change can indicate an abnormal industrial or commercial discharge.
Operators should compare pH with:
- flow;
- conductivity;
- odor;
- color;
- COD;
- biological-process response.
High-Strength Organic Waste
Industrial wastewater can contain BOD or COD concentrations much higher than ordinary domestic wastewater.
A sudden high-strength discharge can create an organic shock load.
Organic Shock Effects
Possible effects include:
- rapid dissolved oxygen depletion;
- increased aeration demand;
- higher sludge production;
- clarifier loading;
- higher effluent BOD.
Mass Loading
Operators should evaluate industrial discharges using mass loading as well as concentration.
A common relationship is:
Loading, lb/day = Flow, MGD × Concentration, mg/L × 8.34
Industrial Load Example
An industrial discharge contributes 0.15 MGD at 1,200 mg/L COD.
COD Load = 0.15 × 1,200 × 8.34
COD Load = 1,501.2 lb/day
A relatively small flow can therefore contribute a substantial pollutant mass if the concentration is high.
Small Flow Does Not Mean Small Impact
Operators should never evaluate an industrial discharge only by its flow rate.
A low-volume discharge can have high:
- organic strength;
- toxicity;
- metal concentration;
- oil and grease;
- acidity or alkalinity.
Toxic or Inhibitory Discharges
Toxic compounds can interfere with microorganisms used for biological treatment.
Possible signs include:
- sudden loss of BOD removal;
- loss of nitrification;
- unusual dissolved oxygen behavior;
- poor settling;
- changes in microscopic biological activity;
- effluent deterioration.
Unexpectedly High DO
If biological microorganisms become inhibited, oxygen uptake can decrease.
If aeration continues at the same rate, dissolved oxygen may rise.
High DO should therefore be interpreted together with:
- loading;
- biological activity;
- effluent quality;
- recent industrial-discharge history.
Metals
Industrial wastewater may contain metals that can:
- pass through treatment;
- accumulate in solids;
- inhibit biological processes at sufficient concentrations;
- affect sludge or biosolids management.
Oil and Grease
Oil and grease can create:
- collection-system deposits;
- pump problems;
- surface scum;
- oxygen-transfer problems;
- treatment interference.
Solvents and Volatile Compounds
Some industrial wastes can contain volatile compounds.
These can create concerns involving:
- worker exposure;
- flammability;
- odor;
- treatment interference.
Temperature
High-temperature industrial discharges can affect:
- collection-system safety;
- biological treatment;
- dissolved oxygen;
- equipment.
Solids and Debris
Industrial discharges containing excessive solids can:
- block sewers;
- increase grit or sediment;
- increase clarifier loading;
- increase sludge production.
Categorical Requirements
Some industrial users are subject to specific federal pretreatment requirements based on industrial category.
Operators do not necessarily administer these requirements, but they should understand that industrial discharge controls may differ according to the type of industry.
Local Limits
Wastewater systems can establish local discharge limits to protect:
- treatment processes;
- workers;
- collection systems;
- effluent quality;
- sludge or biosolids quality.
Why Local Limits Matter Operationally
Local limits are intended to reflect the receiving treatment system and its ability to manage particular pollutants.
They help prevent excessive pollutant loads from reaching the plant.
Industrial User Monitoring
Industrial discharges may be monitored for parameters such as:
- flow;
- pH;
- BOD;
- COD;
- TSS;
- oil and grease;
- metals;
- other industry-specific pollutants.
Sampling
Representative sampling is important when evaluating industrial wastewater.
Sampling considerations can include:
- sample location;
- timing;
- composite versus grab sampling;
- preservation;
- documentation.
Grab Samples
A grab sample represents wastewater conditions at a particular time.
It can be appropriate for parameters that can change rapidly or cannot be reliably preserved in a composite sample.
Composite Samples
Composite samples combine multiple portions collected over time or flow.
They can provide a better representation of average discharge conditions for some parameters.
Flow-Proportional Sampling
Flow-proportional sampling can help represent pollutant loading when industrial flow changes significantly during the sampling period.
Slug Discharge
A slug discharge is a sudden or concentrated discharge that can create a significant temporary load on the wastewater system.
Possible slug characteristics include:
- high organic concentration;
- extreme pH;
- toxic material;
- oil or grease;
- high solids;
- other concentrated pollutants.
Slug Load Versus Normal Variation
Wastewater flow and concentration naturally vary, but a slug load is unusually concentrated or sudden and can exceed the process's ability to absorb the change smoothly.
Operator Response to a Suspected Slug Load
Operators should follow facility procedures and may need to:
- verify abnormal measurements;
- collect appropriate samples;
- document time and conditions;
- notify responsible staff;
- protect critical treatment processes;
- monitor plant response.
Do Not Delay Documentation
Slug events can pass quickly.
Useful information can disappear if operators wait too long to record:
- time;
- flow;
- pH;
- odor;
- color;
- process changes.
Collection-System Warning Signs
Industrial problems can first appear in the collection system as:
- unusual odors;
- corrosion;
- grease buildup;
- unexpected color;
- high temperature;
- pump problems.
Plant Influent Warning Signs
Operators may observe:
- rapid pH changes;
- unusual color;
- chemical odor;
- oil sheen;
- foam;
- temperature changes;
- unexpected conductivity changes.
Biological Warning Signs
Biological-process indicators can include:
- falling oxygen uptake;
- rising DO without aeration changes;
- poor nitrification;
- poor floc formation;
- poor settling;
- higher effluent BOD.
Primary Treatment Effects
Some industrial discharges can increase:
- primary sludge production;
- scum;
- odor;
- solids carryover.
Activated Sludge Effects
Industrial wastes can affect:
- dissolved oxygen demand;
- microbial activity;
- sludge settling;
- nitrification;
- sludge production.
Attached-Growth Process Effects
Toxic discharges can also affect biofilm processes such as:
- trickling filters;
- rotating biological contactors;
- other attached-growth systems.
Clarifier Effects
High industrial solids or biological upset can increase:
- clarifier blanket depth;
- solids carryover;
- effluent TSS.
Disinfection Effects
Industrial contaminants can alter disinfectant demand or interfere with downstream water quality.
High suspended solids can also reduce disinfection effectiveness.
Solids and Biosolids Effects
Pollutants removed from wastewater can concentrate in sludge.
Industrial pretreatment therefore helps protect not only the liquid treatment process but also solids-management options.
Example: Influent pH Drops Suddenly
Review:
- instrument accuracy;
- upstream industrial activity;
- collection-system reports;
- conductivity;
- biological response.
Example: COD Rises Sharply but Flow Is Stable
This suggests stronger wastewater rather than a purely hydraulic event.
Calculate the mass load and investigate potential high-strength sources.
Example: DO Falls Rapidly
Possible causes include:
- high organic load;
- industrial high-strength waste;
- aeration failure;
- high return load.
Example: DO Rises While Effluent Quality Deteriorates
This can indicate biological inhibition.
Review:
- influent toxicity indicators;
- pH;
- industrial discharges;
- microbiological activity;
- nitrification performance.
Example: Oil Sheen Appears at the Plant
Review:
- influent appearance;
- collection-system sources;
- industrial discharge history;
- scum handling;
- process effects.
Example: Nitrification Suddenly Fails
Possible causes include:
- toxic inhibition;
- low dissolved oxygen;
- low temperature;
- low pH or alkalinity;
- insufficient sludge age.
If other operating conditions are stable, investigate potential inhibitory discharges.
Example: Clarifier Settling Suddenly Deteriorates
Review:
- industrial shock load;
- biological condition;
- hydraulic loading;
- sludge age;
- RAS and WAS operation.
Example: Sludge Metal Concentration Increases
Investigate potential industrial sources because some metals can become concentrated in wastewater solids.
Example: Strong Solvent-Like Odor Appears
Operators should treat unusual chemical odors seriously and follow plant safety and notification procedures.
Do not enter potentially hazardous areas without appropriate atmospheric evaluation and protection.
Safety Comes First
Unknown industrial discharges can create safety hazards.
Potential hazards include:
- flammable vapors;
- toxic gases;
- corrosive liquids;
- oxygen-deficient atmospheres;
- chemical exposure.
Do Not Investigate Unknown Chemicals by Smelling Them Directly
Operators should use proper instruments, PPE, and facility procedures when abnormal chemical conditions are suspected.
Communication
Effective pretreatment protection depends on communication among:
- treatment plant operators;
- collection-system staff;
- laboratory staff;
- pretreatment personnel;
- industrial users;
- management.
Operator Records
Useful records during suspected industrial events can include:
- date and time;
- influent flow;
- pH;
- temperature;
- conductivity;
- color and odor observations;
- DO;
- laboratory results;
- process changes;
- notifications.
Trend Analysis
Repeated industrial discharges may follow patterns related to:
- work shifts;
- production cycles;
- days of the week;
- cleaning operations;
- batch releases.
Trend data can help identify recurring sources.
Compare Flow and Concentration
Operators should distinguish between:
- high concentration at low flow;
- moderate concentration at high flow;
- simultaneous increases in both.
Mass loading provides a common way to compare these conditions.
Pretreatment Does Not Replace Plant Process Control
Even with a strong pretreatment program, wastewater plants still experience normal loading changes and occasional unusual events.
Operators must continue to monitor and control the treatment process.
Plant Process Control Supports Pretreatment
Good plant records can help identify industrial impacts by showing when:
- influent quality changed;
- biological treatment changed;
- effluent quality changed;
- recovery began.
Common Industrial Pretreatment Mistakes
- Assuming small industrial flow means small treatment impact.
- Looking only at concentration and ignoring mass loading.
- Confusing interference with pass through.
- Ignoring unusual pH, color, odor, or temperature changes.
- Assuming high DO always indicates good biological treatment.
- Waiting for final effluent deterioration before investigating abnormal influent.
- Failing to document short-duration slug events.
- Ignoring impacts on sludge and biosolids.
- Investigating unknown chemical discharges without proper safety precautions.
- Failing to communicate abnormal conditions to pretreatment and collection-system personnel.
A Practical Industrial-Discharge Review
- Verify abnormal influent measurements.
- Record the time and location.
- Review flow.
- Review pH, temperature, conductivity, color, and odor.
- Review BOD or COD where available.
- Calculate relevant mass loading.
- Observe biological-process response.
- Review clarifier and effluent conditions.
- Notify responsible pretreatment or management personnel.
- Continue monitoring until the plant stabilizes.
A Practical Suspected Toxic-Shock Review
- Protect operator safety.
- Verify instrument data.
- Review influent pH and other indicators.
- Check dissolved oxygen behavior.
- Review nitrification and biological performance.
- Collect appropriate samples according to procedure.
- Notify responsible staff.
- Protect unaffected treatment units where possible.
- Monitor recovery.
A Practical High-Strength-Load Review
- Verify flow and concentration.
- Calculate mass loading.
- Review dissolved oxygen demand.
- Check aeration capacity.
- Monitor sludge production.
- Monitor secondary clarification.
- Review solids-handling capacity.
- Monitor final effluent.
A Practical Pretreatment Trend Review
- Review historical influent data.
- Identify recurring abnormal periods.
- Compare events with industrial operating schedules where available.
- Compare flow, pH, COD, and conductivity.
- Review plant biological response.
- Document recurring patterns for pretreatment staff.
What to Remember for the Exam
- Industrial pretreatment controls industrial wastewater before it enters the public wastewater treatment system.
- Pretreatment helps prevent interference, pass through, equipment damage, safety hazards, and solids-quality problems.
- Interference disrupts treatment or plant operation.
- Pass through occurs when pollutants leave the treatment plant without adequate removal.
- Industrial wastewater can contain high organic loads, metals, oil and grease, extreme pH, toxic compounds, solvents, and other unusual pollutants.
- Small industrial flows can create large pollutant loads when concentrations are high.
- Mass loading in lb/day equals MGD × mg/L × 8.34.
- A sudden pH change can be an early warning of an abnormal discharge.
- Toxic inhibition can reduce biological oxygen uptake and may cause DO to rise even while treatment deteriorates.
- Slug discharges are sudden or concentrated discharges that can place unusual stress on the treatment plant.
- Representative industrial wastewater sampling can include grab, composite, or flow-proportional methods depending on the parameter and objective.
- Local limits help protect the collection system, treatment process, workers, effluent quality, and solids management.
- Industrial pollutants can affect both liquid treatment and sludge or biosolids.
- Operators should document short-duration abnormal events promptly.
- Unusual chemical odors, vapors, or liquids should be treated as potential safety hazards.
- Pretreatment personnel, collection-system staff, laboratory staff, and plant operators should communicate about abnormal conditions.
- Trend analysis can reveal recurring industrial-discharge patterns.
- Operators should compare concentration, flow, and mass loading together.
- Pretreatment protects the plant, but it does not replace normal process control and monitoring.
- Good industrial-discharge response combines safety, monitoring, calculations, sampling, communication, documentation, and treatment-process observation.