What is Wastewater Treatment?

What happens when you flush the toilet? Or when you pour spoiled milk down the sink? Or when the local brewery needs to dispose of its process water? The water used within households, businesses, and industries must be collected and treated before returning to the environment through a process called wastewater treatment. In its simplest definition, wastewater treatment uses physical, chemical, and biological processes to remove pollution or convert it to less harmful forms before discharging into a natural body of water (lake, river, ocean, etc.).

Wastewater treatment removes several pollutants that can damage our natural ecosystems and impact human health, including solids, dissolved organics, nutrients, and pathogenic organisms. Solid pollutants come in many forms, such as human-made objects (rags, plastics, etc.), inorganic solids (silt, clay, sand), or organic solids (decaying animal or plant matter, feces). Dissolved organics are dissolved organic compounds from decaying animal or plant matter that generally do not settle out like solids. Nutrients, such as nitrogen or phosphorus, make their way into wastewater from many sources, including urine, feces, and human-made products. Finally, pathogenic organisms that can cause disease often thrive in untreated wastewater. In high concentrations, each of these pollutants can be destructive to humans and our environment.

water cycle diagram

The Water Cycle: The human use of water occurs in a cycle. Water is utilized for consumption from one of many sources (lakes, rivers, groundwater, etc.). Before it is distributed to our residences, business, and industries, it must be treated to meet specific standards to ensure the water is safe for consumption. After the water is utilized, the water must again be treated in a process called wastewater treatment. After removing any dangerous contaminants, the water can be discharged back into our environment.

Before being discharged to a stream, estuary, or other receiving water, wastewater effluent must meet a set of standards set forth by the National Pollution Discharge Elimination System (NPDES). The NPDES was created by the EPA under the Clean Water Act of 1972, which issues permits to facilities stating the level of pollutants the facility can discharge based on the requirements of the receiving water. The Clean Water Act also included standards for water quality of surface waters, protection from industrial and municipal waste, and funding for the development of more advanced wastewater treatment facilities. Although the history of wastewater goes back much further, the Clean Water Act established significant changes to limit the impact of human activity on receiving waters and improve our natural ecosystems.

What are the stages of wastewater treatment?

wastewater treatment stages
Stages of Wastewater Treatment

wastewater collections system

Collections System

All waste generated from society, such as garbage and recycled goods, is collected and managed with various processes to protect the environment. Wastewater is no different. A wastewater collection system is a series of pipes and lift stations that bring used water from houses, businesses, or industries to the wastewater treatment facility. Civil engineers design collection systems in a way that allows gravity to do the work, using the laws of physics to direct water to the wastewater treatment facility. However, in many instances, lift stations are installed in strategic areas to pump water to a higher elevation to support this gravity-led process. Depending on the design, municipalities can either be sanitary sewer systems that transport strictly wastewater or combined sewer systems that collect both stormwater and wastewater. Combined sewer systems often bring additional difficulties, such as high flows, to wastewater treatment.

wastewater treatment headworks


The collection system brings wastewater to the beginning of the treatment facility, also known as the headworks. The headworks are responsible for removing large debris and heavy solids from the water, such as wood, rags, plastics, sand, gravel, grease, and personal hygiene products. This stage prepares the wastewater for the downstream stages by removing solids that may damage pumps and other equipment. Specialized pumps bring raw wastewater from influent wells up to a series of bar screens and grit removal chambers. These two common pieces of equipment will help remove a portion of the incoming solids (total suspended solids, TSS) and organic material (biochemical oxygen demand, BOD). Sensors for TSS, TDS (total dissolved solids), turbidity, and pH can be used to monitor incoming wastewater for EPA violations and loads that may be dangerous to downstream biological processes.

wastewater primary treatment

Primary Treatment

After the larger debris is removed, the organic and inorganic solids can be removed in a process called primary treatment, reducing influent BOD by 25-40% and influent TSS by 50-70%. Circular or rectangular clarifiers are most often used to separate solids from the water via gravity separation. The speed of flow in these tanks allows solids to settle at the bottom of the tank to form sludge, which is then pumped to another section of the treatment facility for biosolids treatment. The separated water then flows over the weirs at the surface of the clarifiers and onto the next treatment step. The biosolids collected from both primary and secondary treatment are collected, reduced in volume, and then either disposed of or created into fertilizers. Before the Clean Water Act, many wastewater treatments plants only utilized primary treatment. However, secondary treatment has become a requirement with the increase in industry and city population.

wastewater secondary treatment

Secondary Treatment

With endless tank configurations and the utilization of live microorganisms, secondary treatment is perhaps the most complex stage of wastewater treatment. In this stage, dissolved pollutants like organic matter or nutrients are removed using the biological processes of certain microorganisms (bacteria, protozoa, etc.). Different environments can be created within large tanks with high solids (i.e., activated sludge basins) to accumulate the desired type of microorganism. Aeration is added to some tanks to provide oxygen and grow bacteria which remove organics and ammonia, while aeration can be purposefully limited to assist with nitrogen or phosphorus removal. The number and order of these tanks are configured to meet goals specific to the facility. After the activated sludge basins, another set of clarifiers is used to separate the activated solids from the water via secondary clarification.

This location utilizes the most online sensors to monitor the activated sludge system. Sensors for DO (dissolved oxygen), TSS, ammonium, nitrate, ORP (oxidation-reduction potential), pH, and orthophosphate monitor the conditions within each tank. Whether the environment is aerobic (oxygen-rich), anoxic (low oxygen, nitrate available), or anaerobic (no oxygen, no nitrate), these parameters are maintained at optimum levels. In addition, sensors are used to automatically control pumps, aeration, and chemical dosing. DO and ammonium sensors can control aeration to provide precise treatment. Similarly, online measurements of TSS, nitrate, and orthophosphate are used to control pumps and chemical dosing to assist with solids control and nutrient removal. Carbon parameters, such as BOD, COD, and TOC are commonly monitored as they are quantifications of the organic material in wastewater. BOD (biochemical oxygen demand) is the current NPDES standard for this measurement, however, COD (chemical oxygen demand) and TOC (total organic carbon) can be substituted for BOD, if a long-term correlation with BOD (COD:BOD or TOC:BOD) has been established.

wastewater tertiary treatment

Tertiary Treatment

At this point, solids and dissolved pollutants are mostly removed from the water. However, a final treatment step called tertiary treatment can include several processes to disinfect the water or further remove pollutants to achieve very stringent treatment goals. Depending on the body of water, wastewater facilities may have strict discharge limits to further protect the body of water. For example, discharge limits near freshwater lakes have very low total phosphorus (TP) limits to limit eutrophication in freshwater systems, while coastal wastewater facilities will have low total nitrogen (TN) limits in a saltwater system.

Disinfection is an important step to kill or deactivate pathogenic organisms before they return to the environment. Chlorine and UV disinfection are the most common methods, both of which damage the cell walls and eliminate the ability of pathogens to reproduce. Chlorine disinfection has been used for decades to disinfect wastewater. Many forms of chlorine can be dosed at the beginning of contact tanks, providing enough time for the chlorine to kill bacteria and dissipate or dechlorinate before the final effluent. Chlorine analyzers monitor chlorine levels after the contact tanks to ensure enough chlorine is dosed, but also to ensure low amounts of chorine are discharged. UV disinfection systems are becoming more prevalent as they do not require storing and dosing hazardous chlorine chemicals. UV banks emit light in a series of channels to deactivate bacteria before discharge. UVT% sensors can control the UV banks to dose the exact amount of light required to achieve the required disinfection.

Tertiary treatment can also include another filtration step utilizing processes like sand filtration, membrane filters, or disk filters, which can remove solids and, thus, other pollutants down to extremely low levels.

Check out YSI’s educational handbooks:

phosphorus in wastewater handbook

Phosphorus in Wastewater

dissolved oxygen in water handbook

Dissolved Oxygen Handbook

titration handbook

Titration Handbook

pH handbook

pH Handbook

The City of Lafayette Water Resource Recovery Facility (WRRF) utilizes instrumentation as part of the their infrastructure to improve water quality, protect the community, and enhance their sustainability. As part of their laboratory, field sampling, and online process monitoring and control instrumentation, they choose YSI.

See how they use these solutions as a way to optimize their process, meet compliance standards, decrease energy use and costs, and improve their efficiency.

What types of sensors and analyzers are used in wastewater treatment?

From online sensors for aeration basins to BOD sensors in the lab, YSI offers instrumentation designed specifically for the rigorous wastewater environment. Sensors and analyzers provide the data to make informed process decisions, ensure the facility is operating correctly, and optimize treatment. Several types of sensors are available to measure key wastewater parameters, and they can be broadly categorized into one of the following types of sensors below.

lab instrumentation wastewater testing

Lab Instrumentation

Wastewater laboratories utilize many instruments to analyze water quality. Grab samples throughout the wastewater system give snapshots of the conditions of the water and the data can be used to calibrate or verify online sensor readings. In addition, grab samples from the final effluent can be used for compliance reporting for NPDES permitting, ensuring wastewater facilities are meeting their treatment goals. Laboratory sensors are essential in wastewater because lab conditions are controlled, and measurements can be taken isolated from the elements. As a result, many lab techniques are exceptionally accurate, repeatable, and considered the truest representation of the measured parameter.

wastewater sample benchtop meter

YSI’s MultiLab line is a digital benchtop system that can connect and measure up to three sensors simultaneously. Biochemical Oxygen Demand (BOD) is one of the most critical parameters for wastewater treatment as it is the current standard for measuring the amount of organic material and is the permitted parameter by the NPDES. With up to three YSI OBOD or BOD sensors, lab technicians and operators can save significant time processing samples. In addition, the MultiLab can also measure other critical parameters such as ammonium, nitrate, pH, ORP, and more!

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wastewater sample titrators

YSI Titrators can add precision and accuracy to wastewater labs for key parameters that require titration, like alkalinity, TKN (Total Kjeldahl Nitrogen), and volatile fatty acids. The TitroLine automated titrators, TITRONIC manual titrators, autosamplers, and TitriSoft software make these measurements easy and repeatable.

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total organic carbon analyzers

Due to its rapid analysis time and versatility in various applications, TOC analysis is becoming more frequently used in wastewater. TOC analysis is the fastest way to non-specifically determine water quality and can be utilized in both secondary and tertiary treatment phases of wastewater treatment.

OI Analytical offers several TOC analyzers that meet the analysis needs of any operation. Our laboratory analyzers, the 1030 series and the Model 1080, provide technicians with a way to reliably measure TOC with a low cost of ownership.

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wastewater lab chemistry analyzer

Other common parameters measured in wastewater effluent include total, available & free cyanide, total Kjeldahl nitrogen, and total phosphorus. Our automated chemistry analyzer, the FS3700, can handle the simultaneous analysis of two analytes at once. The system is modular, easy to connect to LIMS via the Flowview software, and offers autosampler and autodilutor options for additional automation.

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water quality meters sensor systems


Handheld sensor systems are a staple for wastewater facilities as they provide immediate in situ measurement of critical wastewater parameters anywhere throughout the wastewater process. Many operators use handheld systems for routine checks at specific locations, spot sampling to troubleshoot process issues, or to verify online sensors. YSI’s handheld meters have provided the wastewater industry with the best quality and reliability in portable measurement for decades.

wastewater portable sampling meters

The ProDIGITAL line from YSI is the ultimate in handheld sampling with several variations available, each providing digital sensor measurements and a rugged handheld meter that can withstand the harsh wastewater environment. The ProSolo provides digital optical dissolved oxygen measurement, ideal for monitoring aeration within the activated sludge basins. The ProSwap and ProDSS offer user-replaceable sensors on single-port or 4-port bulkheads, respectively, allowing for the measurement of multiple parameters on a single system.

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water quality sampling systems for wastewater

The YSI Professional Series includes the ProQuatro, Pro20, and Pro20i, which can provide similar measurements and rugged design at a lower cost. These systems feature analog measurement systems and electrochemical dissolved oxygen sensors.

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wastewater online process monitoring control

Online Systems

Wastewater treatment facilities are utilizing more online sensor systems than ever before. The benefits of 24/7, continuous data have been demonstrated in many facilities, and operators are always looking for new ways online sensors can improve their processes. Learn more about the benefits of online instrumentation in this section.

wastewater online monitoring sensor system

YSI’s IQ SensorNet is a monitoring and control system for analytical sensors and analyzers. Up to 20 parameters can be displayed and output to SCADA with a single IQ network. With a fully customizable layout, sensors and modules can be distributed across a wastewater treatment facility in many flexible configurations. IQ SensorNet includes robust instrumentation designed specifically for wastewater. Sensors and analyzers are available for dissolved oxygen, pH, ORP, TSS, ammonium, nitrate, orthophosphate, sludge level, chlorine, TOC, and much more.

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wastewater flow level measurement, electromagnetic, hydrostatic, ultrasonic meters

Flow and Level Measurement

Sensors measuring flow and level are essential to wastewater treatment operations. They provide data for flow rate or water level throughout the wastewater facility or within the collections system. There are many different technologies for measuring flow or level, so choose the technology that best suits your specific application.

water flow sensors electromagnetic meters

MJK’s MagFlux electromagnetic flow meters provide great precision and accuracy in closed, pressurized pipe systems. The MagFlux is accurate down to very low flow rates and easily installed into pipe systems with a 3x pipe diameter upstream length and 2x pipe diameter downstream length requirement.

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wastewater level sensors

MJK offers several options for level measurement. The Expert hydrostatic level sensors are designed to provide stable and accurate measurements in tanks, lift stations, and open channels. Ultrasonic, non-contact sensors can measure water level from above the liquid. Both level measurements are often used to calculate open-channel flow with a weir or flume. MJK’s float switches also provide a reliable alarm or pump signal to control the level within a tank.

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online process monitoring system for water

How can online sensors improve your wastewater treatment process?

Online sensor systems, like YSI IQ SensorNet, are the newest advancement in sensors for wastewater. With more stringent effluent permitting, pressure to reduce operational costs, and a shrinking work force, continuously monitoring sensors are becoming essential for operating wastewater treatment facilities of all sizes. To show how online sensors can improve your facility, here are the biggest benefits of utilizing online instrumentation.

continuous water wastewater data

Access to Continuous Data: With 24/7 continuous monitoring, operators can get baseline data and trends for key parameters throughout the entire day. Trending data allows operators to analyze high loads or flows and adjust the process accordingly.

water monitoring process efficiency

Monitoring Process Efficiency: With strategically placed online sensors, continuous data can measure how efficiently the process is removing pollutants. For example, monitoring influent and effluent orthophosphate can indicate how much phosphorus is removed with their process.

reduce wastewater energy and chemical costs

Reduced Energy and Chemical Costs: Process control with online sensors allows for the input of the exact energy and chemicals needed to remove pollutants. Many wastewater facilities will over-aerate activated sludge basins or overdose chemicals to achieve goals, which can lead to significantly higher operational costs.

effluent water monitoring

Monitoring for Effluent Permitting: Online sensors can be utilized to monitor parameters for effluent permitting. Continuous data can provide instant alerts when a parameter is out of compliance and provide peace of mind during normal operation.

wastewater treatment capacity

Increased Treatment Capacity: With process variables being more tightly monitored, we can be more precise with our process and treat higher loads, which may have been previously unachievable.

reduce wastewater maintenance costs

Reduced Maintenance Costs: With process variables being more tightly monitored, we can be more precise with our process and treat higher loads, which may have been previously unachievable.

Finally, wastewater optimization tools utilizing machine learning or digital twins are becoming more common, creating a push toward a more data-driven future. Online sensors are at the forefront of this transition, meaning reliable and accurate instrumentation is increasingly important. Wastewater operators must have the analytical instrumentation to provide the best data to optimize the performance of their facilities and keep their facility running smoothly. Whatever the application, YSI has the instrumentation for your wastewater needs.

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Additional Resources

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