Every time water goes down a toilet, shower, sink, washing machine, or dishwasher, it becomes part of a wastewater system. What happens next is one of the least visible—but most important—parts of modern water infrastructure.
Wastewater travels through sewer pipes or onsite septic systems, where it must be managed before the water can be discharged or reused.
Quick answer: Wastewater treatment removes solids, reduces organic material and nutrients, manages contaminants, and usually disinfects the treated water before it is discharged or reused. Municipal treatment commonly includes screening and settling, biological secondary treatment, and additional treatment when required by discharge permits or reuse goals.
Wastewater management protects waterways, supports public health, and increasingly provides another source of water through recycling and reuse.
In This Guide
- What Is Wastewater?
- Types of Wastewater
- Wastewater vs. Stormwater
- How Sewer Systems Work
- Combined Sewer Systems
- How Wastewater Is Treated
- Preliminary & Primary Treatment
- Secondary Treatment
- Advanced Treatment
- Disinfection
- Sludge & Biosolids
- What Happens to Treated Water?
- Water Reuse & Recycling
- Potable Reuse
- Environmental Protection
- Wastewater System Challenges
- What Households Can Do
- Septic Systems
- Wastewater & Conservation
- Frequently Asked Questions
What Is Wastewater?
Wastewater is water that has been changed by household, commercial, institutional, or industrial use and requires appropriate management before discharge or reuse.
At home, wastewater comes from:
- Toilets
- Kitchen sinks
- Bathroom sinks
- Showers
- Bathtubs
- Washing machines
- Dishwashers
This water can contain:
- Human waste
- Food particles
- Grease
- Soap and detergents
- Suspended solids
- Nutrients
- Household chemicals
- Other substances washed into drains
Wastewater does not simply disappear after it leaves the house. It continues into sewer or septic infrastructure and eventually becomes part of the larger water cycle again.
What Are the Main Types of Wastewater?
Domestic Wastewater
Domestic wastewater comes from homes and residential buildings.
It includes wastewater from:
- Bathrooms
- Kitchens
- Laundry
- Toilets
Homes connected to public sewers send this water toward municipal treatment facilities. Homes without sewer service often use onsite septic systems.
Municipal Wastewater
Municipal wastewater systems collect wastewater from many sources, including:
- Homes
- Schools
- Offices
- Restaurants
- Hospitals
- Stores
- Other community facilities
Industrial Wastewater
Industrial wastewater can vary dramatically according to the activity producing it.
Sources include:
- Food processing
- Manufacturing
- Mining
- Metal processing
- Chemical manufacturing
- Energy production
Industrial facilities may need to pretreat wastewater before sending it to a municipal sewer, or they may operate their own permitted treatment and discharge systems.
Is Stormwater the Same as Wastewater?
Not necessarily.
Stormwater is rain or melting snow that flows across surfaces rather than infiltrating into the ground.
It can pick up:
- Oil
- Sediment
- Trash
- Fertilizer
- Pesticides
- Road salt
- Animal waste
- Other pollutants
In many communities, stormwater travels through a drainage system separate from the sanitary sewer.
A storm drain does not necessarily lead to a wastewater treatment plant. In separate storm-sewer systems, runoff may discharge to nearby streams, rivers, lakes, or coastal waters after whatever stormwater controls the community uses.
How Do Sewer Systems Work?
A sanitary sewer system uses underground pipes to collect wastewater and move it toward a treatment facility.
The system can include:
- Household service laterals
- Neighborhood sewer mains
- Larger interceptor sewers
- Pump stations
- Wastewater treatment plants
Gravity moves much of the wastewater through the network, while pumps are used where terrain makes gravity flow impractical.
What Is a Combined Sewer System?
Some older cities use combined sewer systems.
These systems carry both:
- Sanitary wastewater
- Stormwater runoff
through the same pipes.
During ordinary dry weather, the combined flow goes to a wastewater treatment plant.
During heavy rainfall, however, the volume can exceed the capacity of the sewer or treatment system.
When this happens, a combined sewer overflow, or CSO, may release a mixture of stormwater and untreated or partially treated sewage through permitted overflow points.
This is one reason cities invest in green infrastructure. Rain gardens, permeable pavement, green roofs, trees, and other measures can capture or slow stormwater before it enters combined sewer pipes.
How Does a Wastewater Treatment Plant Work?
Treatment plants are designed to remove or reduce substances before treated water is discharged or reused.
A simplified municipal process often includes:
- Screening and grit removal
- Primary settling
- Biological secondary treatment
- Secondary settling
- Additional treatment when required
- Disinfection where required
- Solids treatment
Not every treatment plant follows exactly the same sequence. Technology depends on plant design, discharge permits, receiving-water conditions, nutrient requirements, and whether the water will be reused.
Preliminary and Primary Treatment
Screening
Wastewater first passes through screens that remove large materials such as:
- Rags
- Plastic
- Wipes
- Other debris
Grit Removal
Grit chambers allow dense materials such as:
- Sand
- Gravel
- Small stones
to settle out before they damage pumps or interfere with later treatment.
Primary Settling
Wastewater then may enter large settling tanks called primary clarifiers.
Heavier solids settle to the bottom while oils, grease, and floating material can be removed from the surface.
The remaining liquid continues to biological treatment.
Secondary Biological Treatment
Secondary treatment uses biological processes to remove much of the biodegradable organic material remaining in wastewater.
Microorganisms consume organic matter as part of their normal biological activity.
Activated Sludge
One common process mixes wastewater with microorganisms in aerated tanks.
Air or oxygen helps support biological treatment.
After treatment, the mixture enters a secondary clarifier where biological solids settle out.
Other Biological Processes
Municipal systems can also use:
- Trickling filters
- Biological contactors
- Membrane bioreactors
- Oxidation ditches
- Stabilization ponds or lagoons
EPA establishes minimum technology-based secondary treatment standards for publicly owned treatment works, using measures such as biochemical oxygen demand, suspended solids, and pH.
Advanced Treatment and Nutrient Removal
Some wastewater requires additional treatment beyond conventional secondary treatment.
Advanced treatment can address:
- Nitrogen
- Phosphorus
- Fine suspended particles
- Dissolved organic compounds
- Salts
- Specific contaminants
Technologies can include:
- Sand and media filtration
- Biological nutrient removal
- Activated carbon
- Microfiltration
- Ultrafiltration
- Reverse osmosis
- Advanced oxidation
“Tertiary treatment” is often used as a general term for additional treatment after secondary treatment, but it is not a single universal process. A plant designed for river discharge may use very different advanced treatment from a facility producing recycled water.
How Is Treated Wastewater Disinfected?
Many systems disinfect wastewater before discharge or reuse.
Common methods include:
- Chlorine
- Ultraviolet light
- Ozone in some applications
Disinfection is intended to reduce the number of microorganisms remaining after earlier treatment.
Where chlorine is used, additional treatment may be needed to reduce residual chlorine before discharge to sensitive waterways.
What Happens to Sewage Sludge and Biosolids?
Wastewater treatment not only produces treated water.
It also concentrates solids.
Solids removed during primary and secondary treatment may undergo processes such as:
- Thickening
- Digestion
- Dewatering
- Drying
- Additional stabilization
Anaerobic Digestion
Some treatment plants use microorganisms in oxygen-free digesters to break down organic solids.
This process can generate biogas containing methane.
Some facilities capture that gas and use it for:
- Heating
- Electricity generation
- Other energy needs
What Are Biosolids?
Biosolids are treated sewage solids that meet applicable requirements for specific beneficial uses.
Depending on treatment quality and local regulations, wastewater solids may be:
- Applied to land
- Composted
- Incinerated
- Landfilled
- Managed through other approved methods
Sludge and biosolids are not interchangeable terms. Wastewater sludge becomes biosolids only after treatment and when it meets the requirements governing its intended use.
What Happens to Treated Wastewater?
Treated wastewater may be discharged under permit into:
- Rivers
- Lakes
- Estuaries
- Coastal waters
- Other receiving waters
Increasingly, communities also treat water for reuse.
What Is Water Reuse or Water Recycling?
Water reuse, also called water recycling or reclamation, treats previously used water so it can be used again for an appropriate purpose.
Potential applications include:
- Landscape irrigation
- Agricultural irrigation where permitted
- Industrial cooling
- Power-plant cooling
- Toilet flushing in some systems
- Street cleaning
- Groundwater replenishment
- Environmental restoration
- Some drinking-water supply applications after advanced treatment
Water reuse is not one treatment standard. The required treatment depends on how the reclaimed water will be used. Water used for landscape irrigation has different requirements from water intended to supplement a drinking-water supply.
Can Wastewater Become Drinking Water?
Yes, under carefully engineered and regulated potable-reuse systems.
Advanced-treated recycled water can be incorporated into drinking-water supplies through different approaches.
Indirect Potable Reuse
Advanced-treated recycled water may first enter an environmental buffer such as:
- A groundwater basin
- A reservoir
before later entering a drinking-water treatment system.
Direct Potable Reuse
Direct potable reuse introduces advanced-treated recycled water into a drinking-water treatment or distribution framework without first relying on a long-term environmental buffer.
Regulation differs by state.
Potable reuse should not be described as simply “cleaning sewage and putting it back in the tap.” Modern systems can use multiple independent treatment barriers, continuous monitoring, membranes, advanced oxidation, and additional safeguards.
Why Water Reuse Is Becoming More Important
Water reuse can:
- Reduce freshwater withdrawals
- Diversify community water supplies
- Increase local water reliability
- Reduce some wastewater discharges
- Support agriculture and industry
- Improve resilience during dry periods
EPA released an updated Water Reuse Action Plan 2.0 in 2026 as part of its continuing national work on water reuse.
Reuse requirements and applications still vary considerably by state and locality.
Why Wastewater Treatment Matters for the Environment
Reducing Organic Pollution
Untreated organic material can consume dissolved oxygen as it decomposes.
Low dissolved oxygen can stress fish and other aquatic life.
Controlling Nutrients
Too much nitrogen and phosphorus can contribute to:
- Excessive algae growth
- Reduced water clarity
- Oxygen depletion
- Changes in aquatic ecosystems
Reducing Suspended Solids
Suspended material can:
- Increase turbidity
- Carry pollutants
- Cover aquatic habitat
- Reduce light penetration
Supporting Aquatic Life
Healthy rivers and lakes depend on appropriate levels of:
- Dissolved oxygen
- Nutrients
- Temperature
- Sediment
- Chemical pollutants
Wastewater treatment is one part of maintaining those conditions.
What Challenges Do Wastewater Systems Face?
Aging Infrastructure
Sewer pipes, pumps, and treatment equipment can remain in service for decades.
Aging systems can experience:
- Leaks
- Pipe failures
- Infiltration of groundwater
- Stormwater entering sanitary sewers
- Pump failures
Heavy Rain and Flooding
Extreme rainfall can place major pressure on wastewater infrastructure.
Problems can include:
- Combined sewer overflows
- Sanitary sewer overflows
- Flooded pump stations
- Excessive inflow and infiltration
Energy Use
Wastewater treatment uses energy for:
- Pumping
- Aeration
- Mixing
- Filtration
- Disinfection
- Solids handling
Aeration is often one of the major energy uses at biological treatment plants.
Skilled Workforce
Wastewater systems depend on trained:
- Operators
- Engineers
- Laboratory staff
- Electricians
- Mechanics
- Maintenance teams
Emerging Contaminants
Wastewater facilities also face growing questions about substances that conventional treatment plants were not originally designed specifically to remove.
Examples include:
- PFAS
- Pharmaceutical residues
- Personal-care product chemicals
- Other trace organic compounds
Removal varies by contaminant and treatment process.
What Can Households Do to Protect Wastewater Systems?
Only Flush Human Waste and Toilet Paper
EPA's guidance is straightforward: toilets are not trash cans.
Do not flush:
- Wipes
- Paper towels
- Dental floss
- Feminine hygiene products
- Diapers
- Condoms
- Cat litter
- Cotton swabs
Even products marketed as “flushable” can create sewer and septic problems. The safest household rule is to flush only human waste and toilet paper.
Keep Fats, Oils, and Grease Out of Drains
Cooking fats, oils, and grease can cool and accumulate inside pipes.
Instead:
- Let grease cool.
- Place it in an appropriate container.
- Dispose of it according to local guidance.
- Wipe greasy cookware before washing.
Do Not Pour Hazardous Chemicals Down the Drain
Avoid disposing of substances such as:
- Paint
- Solvents
- Pesticides
- Motor oil
- Antifreeze
- Gasoline
Use local household hazardous-waste collection programs when available.
Use Medicines Responsibly
Do not automatically flush unused medication.
Use pharmacy, law enforcement, or community drug take-back programs where available, and follow current FDA or local disposal guidance.
How Are Septic Systems Different?
A septic system treats household wastewater onsite rather than sending it through a public sewer.
A typical system includes:
- A septic tank
- A drain field or soil treatment area
Solids settle in the tank while wastewater flows toward the drain field, where soil provides additional treatment.
Septic Maintenance Matters
Homeowners should:
- Have systems inspected according to local recommendations.
- Pump tanks when appropriate.
- Avoid flushing wipes and non-degradable materials.
- Keep grease and harmful chemicals out of drains.
- Avoid driving or parking over drain fields.
- Direct roof and surface drainage away from the drain field.
EPA does not recommend routine septic additives as necessary for a properly functioning domestic septic system. Good maintenance and appropriate household use are more important.
How Does Water Conservation Help Wastewater Systems?
Water conservation and wastewater management are closely connected.
Most indoor water that enters a home eventually becomes wastewater.
Reducing unnecessary use can lower the amount that must be:
- Pumped
- Treated
- Disinfected
- Discharged or reused
Useful household steps include:
- Repairing leaks
- Using efficient showerheads
- Taking shorter showers
- Running full laundry loads
- Running full dishwasher loads
- Using efficient fixtures
See our Water Conservation Guide and Shower Water Use Guide.
Wastewater and the Water Cycle
Wastewater treatment is ultimately part of the larger water cycle.
Treated water may return to a river, lake, ocean, aquifer, or reuse system.
From there it can:
- Evaporate
- Move downstream
- Recharge groundwater
- Enter another water-supply system
- Be reused for agriculture or industry
There is no such thing as “used-up” water. Water can become contaminated or unsuitable for a particular purpose, but treatment and natural processes allow it to continue moving through Earth's water system.
Frequently Asked Questions About Wastewater
What is wastewater?
Wastewater is water altered by household, commercial, institutional, or industrial use that requires appropriate management before discharge or reuse.
Is sewage the same as wastewater?
Sewage usually refers to wastewater containing human waste and other materials from sanitary systems. Wastewater is the broader term and can also include industrial and other used waters.
Is stormwater wastewater?
Not automatically. Stormwater is runoff from rain or snowmelt. In separate sewer systems, it travels through different infrastructure, while combined sewer systems carry stormwater and sanitary wastewater together.
Where does wastewater go after it leaves my house?
If you have municipal sewer service, wastewater travels through sewer pipes toward a treatment plant. Homes without sewer service often use onsite septic systems.
What happens during primary wastewater treatment?
Screening and grit removal capture debris and dense particles, while primary settling removes many solids and floating materials.
What happens during secondary wastewater treatment?
Secondary treatment primarily uses biological processes to reduce biodegradable organic material, followed by separation of biological solids.
What is tertiary wastewater treatment?
Tertiary, or advanced, treatment broadly refers to additional processes beyond conventional secondary treatment. It may include nutrient removal, filtration, activated carbon, membranes, or other technologies depending on the treatment objective.
Is treated wastewater clean enough to drink?
Ordinary treated wastewater discharged from a municipal plant should not automatically be considered drinking water. Potable reuse requires substantially more treatment, monitoring, and regulatory safeguards.
Can wastewater be recycled into drinking water?
Yes. Advanced water-reuse systems can treat reclaimed wastewater for indirect or direct potable reuse under appropriate regulatory programs.
What is reclaimed water?
Reclaimed or recycled water is previously used water that has been treated so it can be used again for an approved purpose.
What is a combined sewer overflow?
A combined sewer overflow can occur when heavy rain overwhelms a sewer system that carries both stormwater and sanitary wastewater, causing excess mixed flow to discharge through designated overflow points.
What are biosolids?
Biosolids are treated sewage solids that meet applicable requirements for specific uses or disposal methods.
Does wastewater treatment remove everything?
No treatment process completely removes every possible substance. Performance depends on the contaminant and the technologies used at the specific plant.
Do wastewater plants remove PFAS?
Conventional municipal treatment plants were generally not designed specifically for PFAS, and removal varies. Specialized treatment may be required for significant PFAS reduction or destruction.
Can I flush wipes labeled flushable?
The safest practice for sewer and septic systems is to flush only human waste and toilet paper. Wipes can contribute to clogs and equipment problems.
Can cooking grease go down the drain with hot water?
No. Hot water may temporarily keep grease liquid, but it can cool and solidify farther down the plumbing or sewer system.
Should I flush unused medicine?
Not routinely. Use drug take-back options where available and follow current disposal guidance for the specific medication.
How often should a septic tank be pumped?
The proper interval depends on tank size, household size, wastewater volume, solids accumulation, and local guidance. Regular inspection is more useful than assuming one interval applies to every home.
Do septic tank additives eliminate the need for pumping?
No. EPA does not consider additives necessary for normal domestic septic-system operation, and they do not eliminate the need for proper inspection and maintenance.
Does saving water help wastewater treatment plants?
Yes. Reducing unnecessary indoor water use lowers the amount of wastewater that must be transported and treated.
Final Thoughts
Wastewater infrastructure is largely invisible, but it is one of the systems that makes modern communities possible.
Every day, wastewater systems must:
- Collect used water
- Move it through sewer networks
- Remove solids
- Reduce organic material
- Manage nutrients and other contaminants
- Treat residual solids
- Disinfect water where required
- Return or reuse the treated water responsibly
At the same time, wastewater is increasingly being viewed not only as something to dispose of, but as a potential source of:
- Reclaimed water
- Nutrients
- Biogas and energy
- Recoverable resources
The modern goal is no longer simply to move sewage away from people. It is to manage water as a resource—protecting waterways, recovering useful resources where practical, and treating water to the level required for its next use.