Wastewater treatment facilities, also known as water resource

recovery facilities (WRRF), typically take used water from the

collection system and perform treatment processes to cleanse

the water before discharging the treated water to the environment.

There are a number of potential physical, chemical, and biological

processes at each step, governed by the quality of the treated water

required to meet receiving water use designations or reuse requirements.


Wastewater sources include:


  • Sanitary wastewater, called sewage.  Sewage includes gray water from sinks, showers, and washers, and black water from toilets which has been in contact with fecal material.  Sanitary wastewater is very treatable but some microconstituents from pharmaceutical and personal care products (PPCP) may pass through conventional treatment processes and impact the environment.


  • Industrial wastewater includes used water from manufacturing processes.  Industrial wastewater may contain components that are difficult to remove or large quantities of conventional pollutants that can overload treatment processes.


  • Infiltration and Inflow (I&I), two types:

    • Infiltration is ground water which enters the sanitary sewer system through cracks and joints;

    • Inflow is rain and melted precipitation which enters the sewer system through legal and illegal cross connections with the storm sewer system.





In the United States, all wastewater discharged to surface water requires a minimum of secondary treatment for removal of solids and oxygen demand.  More stringent treatment, e.g. for removal of nutrients nitrogen and phosphorus, may be required depending on the receiving stream. 


WRRF consist of a variety of physical, biological, and chemical processes, including: 

  • Preliminary treatment typically consists of two steps:

    • Screening - Straining out debris and floatables using mechanical screening devices;

    • Grit removal - Gravity separation of inorganic particulates known as grit. 


  • Primary Treatment

    • ​Gravity separation or straining to remove organic particulates;

    • Pumping of sludge to sludge treatment;

    • Many smaller WRRF do not have this step. Screened and de-gritted wastewater flows directly to secondary treatment.


  • Secondary Treatment - Biologically-mediated transformation of soluble, colloidal, and particulate oxygen-demanding pollution including organics and ammonia-nitrogen by suspended growth or attached growth processes

    • Suspended growth (Activated Sludge) – Flocculated bacteria are suspended in wastewater as part of the activated sludge process (ASP). The ASP consists of aeration tanks and settling tanks.  Air or pure oxygen is injected into aeration tanks provide oxygen for growth of bacteria, Sludge (bacteria) is intentionally wasted from aeration tanks or settling tanks to maintain optimal bacteria population (waste activated sludge or WAS), and sludge is recirculated from the settling tanks to the aeration tanks to maintain high concentrations of bacteria in contact with the wastewater (return activated sludge or RAS). There are many configurations of the ASP, including configurations which remove nutrients.

    • Attached growth (Trickling filter) – Bacteria grow on solid media and consume pollutants from wastewater which is trickled over the media.


  • Tertiary Treatment 

    • Removal of particulates by filtration through sand media or by straining through cloth media;

    • Biologically-mediated removal of nitrate from nitrified secondary treatment effluent by a version of the trickling filter process which facilitates growth of bacteria by injecting synthetic organic carbon upstream.


  • Disinfection – Elimination of pathogenic organisms from treated wastewater

    • Destruction of pathogenic bacteria by a strong oxidant, usually Chlorine or a compound containing chlorine.

    • Inactivation of bacteria using ultraviolet light to disrupt reproduction.

    • Ozonation using ozone gas to destroy harmful organisms.


  • Final Effluent Discharge – Aeration of treated effluent to increase dissolved oxygen.

    • Mechanical injection of air, OR;

    • Cascading water over a stepped effluent structure.


  • Sludge Storage – Storage of untreated sludge prior to processing or treated sludge after processing.

    • Storage of WAS and/or primary sludge prior to further treatment;

    • Storage of digester and/or dewatered sludge.


  • Sludge Thickening – Separation of solids from water to reduce the mass and volume for further processing.  The product is in a liquid form.

    • Separation of solids from water by gravity sedimentation, and / or;

    • Mechanically using a porous belt or centrifuge.


  • Sludge digestion – Biologically-mediated destruction of sludge solids to reduce the mass of sludge. 

    • Aerobic digestion – an extension of the ASP which converts sludge solids into carbon dioxide (CO2) and water;

    • Anaerobic digestion – A process which destroys organics in the absence of oxygen generating CO2, methane (CH4), and other organic by-products.  The CH4 can be recovered as a source of fuel for use in boilers and generators.


  • Sludge dewatering – Mechanical separation of solids from water to reduce the mass and volume for disposal or re-use using a porous belt or centrifuge. The product in cake form does not flow.


  • Liquor treatment and control – Biologically-mediated transformation of ammonia in reject water side-streams from sludge processing to mitigate the impact of recycle streams on main-stream treatment. Processes typically utilise short-cuts in the nitrogen cycle.


  • Pumping stations – Effluent pumping is sometimes required to discharge treated wastewater to receiving streams when the gravity flow of effluent is insufficient at high stream flow. 

  • Smart Water Interfaces include:

    • Real-time data or data logging from rainfall gauges or stream level data;

    • Programmable Logic Controller (PLC) or micro-processor-based control systems at the treatment plant for monitoring and controlling all stages of treatment;

    • Instruments used for process monitoring, e.g. flow, level, pH, solids, ammonia, nitrate, chlorine residual).



Sanitation systems including wastewater collection and treatment are generally regarded as one of the greatest engineering achievements of the 20th century.  Wastewater treatment restores the biological and aesthetic quality of receiving waters making streams safe for designated uses including recreation and public water supply. 





A control system consists of field instruments, final controlling equipment, controllers such as programmable logic controllers (PLC), and a communication system. Field instruments (measuring devices), such as DO sensors, gather the data. A supervisory control and data acquisition (SCADA) or distributed control system (DCS) acquires, analyses, and displays the data.


A final controlling equipment typically consists of a signal conditioner, an actuator, a final controlling element, and a digital signal translator. Two-way communication is also required because the final controlling element must receive commands from the controller and provide operational information to the controller.