Wastewater Treatment Process Overview
Learn how municipal wastewater moves through preliminary, primary, biological, clarification, disinfection, and solids-handling processes, and how operators monitor flow, loading, treatment performance, and effluent quality.
Wastewater treatment removes or reduces pollutants before treated water is discharged, reused, or sent to another approved destination. A treatment plant does not rely on one process. Instead, it uses a sequence of physical, biological, and chemical processes that work together.
Operators need to understand how the entire treatment train is connected. A problem in preliminary treatment can affect pumps and downstream basins. Poor biological treatment can overload secondary clarifiers. Poor solids handling can return unwanted material to the liquid process. Good operation therefore depends on understanding both individual treatment units and the relationships among them.
What Enters a Wastewater Treatment Plant
Municipal wastewater can contain:
- organic matter;
- suspended solids;
- nutrients;
- microorganisms;
- oil and grease;
- grit and debris;
- household and commercial wastes;
- industrial contributions where allowed;
- infiltration and inflow.
Influent
Influent is wastewater entering the treatment plant or a treatment process.
Important influent characteristics can include:
- flow;
- biochemical oxygen demand;
- chemical oxygen demand;
- total suspended solids;
- pH;
- temperature;
- ammonia;
- other nutrients.
Effluent
Effluent is treated water leaving a treatment process or the treatment plant.
Operators compare influent and effluent conditions to determine how well treatment is working.
The Treatment Train
A municipal wastewater treatment plant can include:
- collection and influent pumping;
- preliminary treatment;
- primary treatment;
- biological treatment;
- secondary clarification;
- advanced or tertiary treatment where required;
- disinfection;
- effluent discharge or reuse;
- solids treatment and handling.
Not Every Plant Uses Every Process
Plant design varies.
Some facilities may not have primary clarification. Others may use:
- activated sludge;
- trickling filters;
- rotating biological contactors;
- lagoons;
- membrane systems;
- other biological processes.
Operators should understand their own process configuration and the purpose of every major unit.
Preliminary Treatment
Preliminary treatment removes material that can damage or interfere with downstream equipment.
Typical processes include:
- screening;
- comminution;
- grit removal;
- flow measurement.
Screening
Screens remove large objects such as:
- rags;
- plastics;
- wood;
- other debris.
Removing these materials protects pumps, valves, piping, and downstream equipment.
Grit Removal
Grit includes relatively heavy inorganic material such as:
- sand;
- gravel;
- coffee grounds;
- other dense particles.
Grit can wear pumps and accumulate in tanks or channels.
Preliminary Treatment Is Equipment Protection
The main objective is not to remove most dissolved pollution. It is to remove materials that interfere with plant operation.
Primary Treatment
Primary treatment removes settleable and floatable solids before biological treatment.
Primary clarifiers provide relatively calm conditions so:
- heavy solids settle;
- floatable material can rise;
- sludge can be collected;
- scum can be removed.
Primary Sludge
Solids removed in primary clarification are commonly called primary sludge.
This material becomes part of the plant's solids-handling system.
Primary Effluent
Water leaving primary treatment is called primary effluent.
It still contains:
- dissolved organic matter;
- fine suspended solids;
- nutrients;
- microorganisms.
Additional treatment is normally required.
Biological Treatment
Biological treatment uses microorganisms to transform biodegradable pollutants.
Microorganisms use organic matter as a source of energy and growth.
Secondary Treatment
The term secondary treatment generally refers to biological treatment followed by solids separation.
Common biological processes include:
- activated sludge;
- trickling filters;
- rotating biological contactors;
- other attached-growth or suspended-growth systems.
Activated Sludge
Activated sludge is a suspended-growth process in which microorganisms are maintained in an aeration basin.
Operators control conditions such as:
- dissolved oxygen;
- solids inventory;
- sludge age;
- return activated sludge;
- waste activated sludge;
- loading.
Attached-Growth Treatment
In attached-growth processes, microorganisms grow on media surfaces.
Wastewater contacts the biological growth, and pollutants are transformed as the water passes through or over the media.
Oxygen
Many biological wastewater processes require oxygen.
Low dissolved oxygen can cause:
- poor biological treatment;
- odors;
- settling problems;
- reduced nitrification where nitrification is required.
Too Much Aeration
Excessive aeration can waste energy and may create undesirable process conditions.
The objective is not maximum oxygen. It is enough oxygen for effective treatment under current loading conditions.
Organic Loading
Biological processes must handle the amount of biodegradable material entering the plant.
Loading can change because of:
- flow changes;
- industrial discharges;
- seasonal conditions;
- storm events;
- changes in population or plant use.
Biochemical Oxygen Demand
Biochemical oxygen demand, commonly abbreviated BOD, is an important wastewater parameter representing the oxygen microorganisms use while biologically degrading organic matter under specified test conditions.
High influent BOD generally represents a greater organic treatment load.
Total Suspended Solids
Total suspended solids, commonly abbreviated TSS, represent suspended material measured by a standard analytical method.
TSS is important for:
- influent characterization;
- clarifier performance;
- effluent quality;
- solids-loading calculations.
Secondary Clarification
After biological treatment, biological solids must usually be separated from treated water.
Secondary clarifiers provide settling conditions for this separation.
Clarifier Performance
Operators may monitor:
- sludge blanket depth;
- solids carryover;
- surface conditions;
- return sludge rate;
- effluent turbidity or suspended solids;
- hydraulic loading.
Return Activated Sludge
In activated sludge systems, settled biological solids are partly returned to the aeration basin.
This flow is called return activated sludge, or RAS.
Waste Activated Sludge
Excess biological solids must be removed from the process.
This flow is called waste activated sludge, or WAS.
Why Sludge Wasting Matters
Sludge wasting helps control:
- solids inventory;
- sludge age;
- biological population;
- clarifier loading.
Nutrient Removal
Some wastewater plants are designed to remove nutrients such as:
- nitrogen;
- phosphorus.
Nutrient removal can involve:
- biological reactions;
- chemical treatment;
- multiple treatment zones;
- specialized process control.
Nitrification
Nitrification is a biological process that converts ammonia to more oxidized forms of nitrogen.
Nitrifying organisms can be sensitive to:
- low dissolved oxygen;
- low temperature;
- toxic substances;
- insufficient sludge age;
- unfavorable pH.
Denitrification
Denitrification biologically converts nitrate to nitrogen gas under suitable conditions.
This process requires different environmental conditions from aerobic nitrification.
Phosphorus Removal
Phosphorus can be removed through:
- biological phosphorus removal;
- chemical precipitation;
- other treatment processes.
Tertiary and Advanced Treatment
Some plants require treatment beyond conventional secondary treatment.
Additional processes can include:
- filtration;
- nutrient removal;
- membrane treatment;
- advanced oxidation;
- other effluent-polishing processes.
Disinfection
Disinfection reduces pathogenic microorganisms in treated effluent before discharge or reuse where required.
Common technologies include:
- chlorine-based disinfection;
- ultraviolet disinfection;
- other approved methods.
Chlorination
When chlorine is used, operators should understand:
- dose;
- demand;
- residual;
- contact time;
- feed-system operation.
Chlorine Demand
A simplified relationship is:
Chlorine Demand = Chlorine Dose - Chlorine Residual
Ultraviolet Disinfection
UV systems depend on:
- UV intensity;
- lamp condition;
- water clarity;
- flow;
- reactor condition.
Solids Handling
Wastewater treatment transfers pollutants from the liquid stream into solids.
These solids must then be treated and managed.
Solids processes can include:
- thickening;
- stabilization;
- digestion;
- dewatering;
- storage;
- transport;
- final use or disposal.
Solids Handling Is Part of Treatment
A plant is not operating successfully if the liquid process performs well while solids accumulate without adequate treatment or removal.
Return Flows
Water separated during solids processing can be returned to the liquid treatment process.
These return flows may contain:
- ammonia;
- suspended solids;
- organic matter;
- other concentrated pollutants.
Operators should recognize that solids handling can affect liquid-process loading.
Flow Measurement
Flow is one of the most important operating measurements in a wastewater plant.
Flow affects:
- hydraulic loading;
- detention time;
- chemical feed;
- solids loading;
- process capacity.
Detention Time
A simplified relationship is:
Detention Time = Volume ÷ Flow
Detention-Time Example
A treatment basin has a volume of 600,000 gallons and receives 2.0 MGD.
Detention Time = 600,000 gal ÷ 2,000,000 gal/day
Detention Time = 0.30 day
Convert to hours:
0.30 × 24 = 7.2 hours
The theoretical detention time is approximately 7.2 hours.
Hydraulic Loading
Higher flow increases hydraulic loading on treatment units.
This can affect:
- screen channels;
- grit systems;
- clarifiers;
- biological basins;
- filters;
- disinfection systems.
Wet-Weather Flow
Rain events can increase wastewater flow because of infiltration and inflow.
Wet-weather conditions can cause:
- higher hydraulic loading;
- shorter detention time;
- clarifier stress;
- solids washout;
- changes in influent strength.
Organic Loading Calculation
A common operator relationship is:
Loading, lb/day = Flow, MGD × Concentration, mg/L × 8.34
Loading Example
A plant receives 2.5 MGD with influent BOD of 180 mg/L.
BOD Loading = 2.5 × 180 × 8.34
BOD Loading = 3,753 lb/day
This value represents the approximate daily BOD mass entering the plant.
Why Mass Loading Matters
A concentration alone does not show the total treatment burden.
For example, the same BOD concentration at twice the flow represents twice the daily BOD mass loading.
Removal Efficiency
A simplified treatment-removal calculation is:
Removal Efficiency, % = (Influent - Effluent) ÷ Influent × 100
Removal-Efficiency Example
Influent BOD is 200 mg/L and effluent BOD is 20 mg/L.
Removal Efficiency = (200 - 20) ÷ 200 × 100
Removal Efficiency = 90%
Percent Removal Is Not Enough by Itself
Operators should also evaluate the actual effluent concentration.
A high percent removal does not automatically mean the final effluent meets the required treatment objective.
Process Monitoring
Operators should monitor both individual processes and overall plant performance.
Useful parameters can include:
- influent flow;
- BOD or COD;
- TSS;
- pH;
- dissolved oxygen;
- ammonia;
- clarifier blanket depth;
- effluent quality;
- sludge production.
Trend Data
Trends often provide more information than one isolated measurement.
Useful trends include:
- flow versus effluent quality;
- organic loading versus dissolved oxygen;
- solids inventory versus clarifier performance;
- sludge wasting versus biological performance;
- rainfall versus influent flow.
Process Control Is Connected
A change in one process can affect several others.
For example, poor screening can increase debris loading on pumps. Poor biological treatment can increase clarifier solids. Poor clarification can increase effluent solids and interfere with disinfection.
Example: Influent Flow Suddenly Increases
Review:
- weather conditions;
- collection-system inflow;
- hydraulic loading;
- clarifier performance;
- disinfection capacity;
- solids washout.
Example: Effluent TSS Increases
Possible causes include:
- poor biological settling;
- clarifier overload;
- high sludge blanket;
- hydraulic surge;
- mechanical clarifier problems.
Example: Effluent BOD Increases
Review:
- influent organic loading;
- biological process conditions;
- dissolved oxygen;
- temperature;
- toxic or inhibitory loads;
- solids washout.
Example: Dissolved Oxygen Falls
Possible causes include:
- higher organic loading;
- blower problems;
- diffuser fouling;
- higher temperature;
- increased process demand.
Example: Sludge Blanket Rises
Review:
- clarifier loading;
- sludge settling;
- RAS pumping;
- WAS rate;
- hydraulic flow.
Example: Ammonia Breakthrough Occurs
Review:
- dissolved oxygen;
- temperature;
- sludge age;
- pH;
- toxic conditions;
- influent ammonia load.
Example: Disinfection Performance Declines
Review:
- effluent suspended solids;
- disinfectant dose;
- contact conditions;
- UV intensity where applicable;
- flow;
- instrument condition.
Instrumentation
Wastewater plants may use instruments for:
- flow;
- dissolved oxygen;
- pH;
- level;
- turbidity;
- chlorine residual;
- UV intensity;
- other process parameters.
Verify Unexpected Readings
An unusual instrument reading may represent a real process change or an instrument problem.
Operators should check:
- sensor condition;
- calibration;
- sample flow;
- related process measurements.
Laboratory Testing
Laboratory results help operators evaluate treatment performance.
Common wastewater analyses can include:
- BOD;
- COD;
- TSS;
- ammonia;
- nutrients;
- pH;
- other permit or process parameters.
Operating Records
Useful records include:
- flow;
- process observations;
- laboratory results;
- chemical feed;
- sludge wasting;
- equipment status;
- operator adjustments;
- weather conditions.
Maintenance Supports Treatment
Treatment depends on reliable equipment such as:
- pumps;
- screens;
- blowers;
- mixers;
- clarifier mechanisms;
- chemical-feed systems;
- disinfection equipment.
A mechanical failure can quickly become a process-control problem.
Common Wastewater Treatment Mistakes
- Treating each unit process as if it operates independently.
- Looking only at concentration and ignoring mass loading.
- Ignoring wet-weather hydraulic loading.
- Trying to correct a mechanical problem with process adjustments.
- Ignoring solids handling when evaluating liquid-process performance.
- Failing to distinguish hydraulic problems from biological problems.
- Using one laboratory result without reviewing trends.
- Ignoring return flows from solids processing.
- Failing to verify abnormal instrument readings.
- Waiting for final effluent problems before responding to upstream warning signs.
A Practical Plant Review
- Review influent flow.
- Review influent strength.
- Inspect preliminary treatment.
- Review primary treatment where present.
- Review biological process conditions.
- Review secondary clarifier performance.
- Review nutrient removal where applicable.
- Review disinfection.
- Review solids handling.
- Review final effluent quality.
A Practical Process-Upset Review
- Verify the abnormal result.
- Determine when the problem began.
- Review flow and loading.
- Review upstream processes.
- Check mechanical equipment.
- Check instrumentation.
- Review laboratory trends.
- Make controlled adjustments.
- Monitor the response.
- Document the result.
A Practical High-Flow Review
- Verify influent flow.
- Review weather and collection-system conditions.
- Review hydraulic loading on clarifiers.
- Watch for solids washout.
- Review disinfection contact conditions.
- Monitor final effluent quality.
A Practical Effluent-Quality Review
- Identify which parameter changed.
- Review influent conditions.
- Review biological treatment.
- Review clarification.
- Review tertiary treatment if present.
- Review disinfection.
- Check process and laboratory instruments.
- Compare with historical trends.
What to Remember for the Exam
- Wastewater treatment uses a sequence of physical, biological, and chemical processes.
- Preliminary treatment removes debris and grit that can damage or interfere with equipment.
- Primary treatment removes settleable and floatable solids.
- Secondary treatment generally combines biological treatment with solids separation.
- Activated sludge is a suspended-growth biological process.
- Attached-growth processes support microorganisms on treatment media.
- Secondary clarifiers separate biological solids from treated water.
- RAS returns settled activated sludge to the biological process.
- WAS removes excess biological solids from the process.
- Nitrification converts ammonia to more oxidized nitrogen forms under aerobic conditions.
- Denitrification converts nitrate to nitrogen gas under suitable conditions.
- Tertiary treatment provides additional polishing beyond secondary treatment.
- Disinfection can use chlorine, UV, or other approved processes.
- Wastewater treatment creates solids that require thickening, stabilization, dewatering, storage, and final management.
- Flow affects hydraulic loading, detention time, chemical feed, and process capacity.
- Theoretical detention time equals basin volume divided by flow.
- Mass loading in lb/day can be calculated as MGD × mg/L × 8.34.
- Removal efficiency equals influent concentration minus effluent concentration, divided by influent concentration, times 100.
- Wet-weather flow can reduce detention time and increase clarifier and process loading.
- Good wastewater treatment operation requires understanding the entire treatment train, not only individual unit processes.