Municipal wastewater is wastewater collected through a municipal sewer system. It mainly comes from households, commercial buildings and institutions, and may also include permitted industrial discharges and stormwater or infiltration, depending on the sewer system. Municipal wastewater is treated to meet applicable effluent quality and regulatory requirements before the treated water is discharged to an approved receiving water body, reused, or otherwise managed.
Municipal wastewater treatment plant processes
Activated sludge
Activated sludge is a suspended-growth biological treatment process widely used in municipal wastewater treatment. Wastewater is mixed with activated sludge in an aeration tank, where microorganisms consume biodegradable organic matter under controlled aeration and mixing conditions. The mixed liquor then flows to a secondary clarifier, where biological solids are separated from the treated water. Part of the settled sludge is returned to the aeration tank as return activated sludge (RAS), while excess sludge is removed as waste activated sludge (WAS).
Common aeration systems include:
Mechanical aeration: Surface or submerged mechanical aerators provide oxygen transfer and mixing.
Diffused aeration: Blowers supply air through piping to diffusers installed in the aeration tank. Aeration output can be adjusted according to process demand, often using dissolved oxygen (DO) measurements and blower control.
Biofiltration
Biofilm treatment is an attached-growth biological process in which microorganisms grow on the surface of fixed or moving media. As wastewater contacts the biofilm, microorganisms degrade biodegradable organic pollutants and, depending on process conditions, may also support nitrification and other biological nutrient removal processes.
Common attached-growth technologies include trickling filters, rotating biological contactors (RBCs), fixed-bed biofilm reactors and moving-bed biofilm reactors (MBBRs). Excess biomass may periodically detach from the media and is typically separated in a downstream solids-separation process.
Oxidation ditch
An oxidation ditch is a modified activated sludge process that commonly operates under extended-aeration conditions. Wastewater and activated sludge circulate continuously through a loop-shaped or racetrack-shaped channel, where mechanical aerators or diffused aeration systems provide oxygen and mixing.
After biological treatment, the mixed liquor typically flows to a secondary clarifier, where the treated water is separated from the biological solids. Part of the settled sludge is returned to the oxidation ditch as return activated sludge (RAS), while excess sludge is removed as waste activated sludge (WAS).
Oxidation ditches are commonly used where stable biological treatment and relatively long solids retention times are required. However, they may require a larger footprint than some other activated sludge configurations.
Municipal water treatment process
Municipal wastewater treatment typically consists of several liquid-treatment stages:
- Preliminary treatment – screening and grit removal to protect downstream equipment.
- Primary treatment – sedimentation or clarification to remove settleable solids and floating materials.
- Secondary treatment – biological treatment and solids separation to remove biodegradable organic matter and suspended solids.
- Tertiary or advanced treatment, where required – additional treatment for nutrients, fine suspended solids, dissolved contaminants or specific reuse requirements.
- Disinfection, where required – reduction of pathogenic microorganisms before discharge or reuse.
Sludge generated during primary and secondary treatment is handled in a separate solids-treatment process.
Primary treatment
Primary treatment removes large solids, such as grit and silt. This is usually the first stage to avoid clogging and damage to valves, pumps, channels and other components of the treatment facility. Wastewater first enters a settling tank, also known as a grit removal unit (e.g., a grit separator), which is then screened to remove large amounts of organic material. It starts with a coarse screen, followed by finer fine screens, screen compactors and drum filters. Screening is sometimes combined with maceration, in which the raw sewage is chopped and the solids are crushed into tiny particles. The screened effluent is then sent to settling tanks to settle most of the suspended material.
Secondary treatment
Wastewater wastes include toxic, organic and inorganic wastes. Organic wastes are usually decomposed by biological processes. This requires the cultivation of cultures and other microorganisms that continually multiply under the right conditions and feed on organic wastes and help break down certain chemicals. For example, when ammonia is oxidized, it is converted to nitrogen compounds such as nitrates. This is often referred to as nitrification. Inorganic waste can be treated biologically to a small degree, but in most cases it requires chemical treatment.
The nutrients provided by the untreated effluent promote the growth of microbial populations. But in addition to nutrients, there must be the right temperature, pH and dissolved oxygen to make the environment optimal for bacterial growth.
Tertiary treatment
Tertiary or advanced treatment may be added after secondary treatment when stricter discharge limits or water reuse requirements must be met. Depending on the treatment objective, these processes may include filtration, chemical phosphorus removal, biological nutrient removal, membrane filtration, activated carbon adsorption or other advanced treatment technologies.
The required treatment configuration depends on the influent characteristics, effluent limits and intended use of the treated water.
Disinfection
Disinfection is used to reduce pathogenic microorganisms before treated wastewater is discharged or reused where required. Common disinfection methods include chlorination using chlorine or sodium hypochlorite, ultraviolet (UV) disinfection and ozone. Chlorine dioxide may also be used in certain applications as a separate oxidizing disinfectant.
On-site sodium hypochlorite generators produce sodium hypochlorite solution from brine by electrolysis, providing an alternative to transporting and storing commercial hypochlorite solutions.
Sludge treatment
Primary sludge and waste activated sludge generated during wastewater treatment are transferred to a separate solids-treatment system. Depending on the plant design, sludge treatment may include thickening, stabilization or digestion, conditioning, dewatering and further drying or beneficial use.
In plants using anaerobic digestion, thickened sludge is treated in digesters where microorganisms break down biodegradable organic matter and produce biogas containing methane. The stabilized sludge can then be dewatered using equipment such as centrifuges, belt filter presses, filter presses or screw presses to reduce its volume and facilitate subsequent handling.
Summary
Municipal wastewater treatment plants allow most city residents to use tap water in their homes without having to worry about their sewage. Municipal wastewater is treated by a variety of methods, including physical, chemical and biological processes, and then safely discharged into rivers and oceans.