Converting wastewater into an effluent for return to the water cycle This article is about the treatment of any type of wastewater from a range of sources. For treatment of wastewater from households or municipalities (domestic wastewater), see Sewage treatment.

Wastewater treatment is a process which removes and eliminates contaminants from wastewater. It thus converts it into an effluent that can be returned to the water cycle. Once back in the water cycle, the effluent creates an acceptable impact on the environment. It is also possible to reuse it. This process is called water reclamation.[1] The treatment process takes place in a wastewater treatment plant. There are several kinds of wastewater which are treated at the appropriate type of wastewater treatment plant. For domestic wastewater the treatment plant is called a Sewage Treatment. Municipal wastewater or sewage are other names for domestic wastewater. For industrial wastewater, treatment takes place in a separate Industrial wastewater treatment, or in a sewage treatment plant. In the latter case it usually follows pre-treatment. Further types of wastewater treatment plants include Agricultural wastewater treatment and leachate treatment plants.

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One common process in wastewater treatment is phase separation, such as sedimentation. Biological and chemical processes such as oxidation are another example. Polishing is also an example. The main by-product from wastewater treatment plants is a type of sludge that is usually treated in the same or another wastewater treatment plant.[2]: Ch.14  Biogas can be another by-product if the process uses anaerobic treatment. Treated wastewater can be reused as reclaimed water.[3] The main purpose of wastewater treatment is for the treated wastewater to be able to be disposed or reused safely. However, before it is treated, the options for disposal or reuse must be considered so the correct treatment process is used on the wastewater.

The term "wastewater treatment" is often used to mean "sewage treatment".[4]

Types of treatment plants

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Wastewater treatment plants may be distinguished by the type of wastewater to be treated. There are numerous processes that can be used to treat wastewater depending on the type and extent of contamination. The treatment steps include physical, chemical and biological treatment processes.[5]

Types of wastewater treatment plants include:

• Sewage treatment plants
• Industrial wastewater treatment plants
• Agricultural wastewater treatment plants
• Leachate treatment plants

Sewage treatment plants

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Industrial wastewater treatment plants

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Agricultural wastewater treatment plants

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Leachate treatment plants

[edit] Main article: Leachate § Treatment

Leachate treatment plants are used to treat leachate from landfills. Treatment options include: biological treatment, mechanical treatment by ultrafiltration, treatment with active carbon filters, electrochemical treatment including electrocoagulation by various proprietary technologies and reverse osmosis membrane filtration using disc tube module technology.[15]

Unit processes

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The unit processes involved in wastewater treatment include physical processes such as settlement or flotation and biological processes such oxidation or anaerobic treatment. Some wastewaters require specialized treatment methods. At the simplest level, treatment of most wastewaters is carried out through separation of solids from liquids, usually by sedimentation. By progressively converting dissolved material into solids, usually a biological floc or biofilm, which is then settled out or separated, an effluent stream of increasing purity is produced.[2][page needed][16]

Phase separation

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Phase separation transfers impurities into a non-aqueous phase. Phase separation may occur at intermediate points in a treatment sequence to remove solids generated during oxidation or polishing. Grease and oil may be recovered for fuel or saponification. Solids often require dewatering of sludge in a wastewater treatment plant. Disposal options for dried solids vary with the type and concentration of impurities removed from water.[17]

Sedimentation

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Solids such as stones, grit, and sand may be removed from wastewater by gravity when density differences are sufficient to overcome dispersion by turbulence. This is typically achieved using a grit channel designed to produce an optimum flow rate that allows grit to settle and other less-dense solids to be carried forward to the next treatment stage. Gravity separation of solids is the primary treatment of sewage, where the unit process is called "primary settling tanks" or "primary sedimentation tanks".[18] It is also widely used for the treatment of other types of wastewater. Solids that are denser than water will accumulate at the bottom of quiescent settling basins. More complex clarifiers also have skimmers to simultaneously remove floating grease such as soap scum and solids such as feathers, wood chips, or condoms. Containers like the API oil-water separator are specifically designed to separate non-polar liquids.[19]: 111–138 

Biological and chemical processes

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Oxidation

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Oxidation reduces the biochemical oxygen demand of wastewater, and may reduce the toxicity of some impurities. Secondary treatment converts organic compounds into carbon dioxide, water, and biosolids through oxidation and reduction reactions.[20] Chemical oxidation is widely used for disinfection.[21]

Biochemical oxidation (secondary treatment)

[edit] This section is an excerpt from Secondary treatment. Secondary treatment (mostly biological wastewater treatment) is the removal of biodegradable organic matter (in solution or suspension) from sewage or similar kinds of wastewater.[22]: 11  The aim is to achieve a certain degree of effluent quality in a sewage treatment plant suitable for the intended disposal or reuse option. A "primary treatment" step often precedes secondary treatment, whereby physical phase separation is used to remove settleable solids. During secondary treatment, biological processes are used to remove dissolved and suspended organic matter measured as biochemical oxygen demand (BOD). These processes are performed by microorganisms in a managed aerobic or anaerobic process depending on the treatment technology. Bacteria and protozoa consume biodegradable soluble organic contaminants (e.g. sugars, fats, and organic short-chain carbon molecules from human waste, food waste, soaps and detergent) while reproducing to form cells of biological solids. Secondary treatment is widely used in sewage treatment and is also applicable to many agricultural and industrial wastewaters.

Chemical oxidation

[edit] Main article: Advanced oxidation process

Advanced oxidation processes are used to remove some persistent organic pollutants and concentrations remaining after biochemical oxidation.[19]: 363–408  Disinfection by chemical oxidation kills bacteria and microbial pathogens by adding hydroxyl radicals such as ozone, chlorine or hypochlorite to wastewater.[2]:   These hydroxyl radical then break down complex compounds in the organic pollutants into simple compounds such as water, carbon dioxide, and salts.[23]

Anaerobic treatment

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Anaerobic wastewater treatment processes (for example UASB, EGSB) are also widely applied in the treatment of industrial wastewaters and biological sludge.

Polishing

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Polishing refers to treatments made in further advanced treatment steps after the above methods (also called "fourth stage" treatment). These treatments may also be used independently for some industrial wastewater. Chemical reduction or pH adjustment minimizes chemical reactivity of wastewater following chemical oxidation.[19]: 439  Carbon filtering removes remaining contaminants and impurities by chemical absorption onto activated carbon.[2]:   Filtration through sand (calcium carbonate) or fabric filters is the most common method used in municipal wastewater treatment.

See also

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• Water portal

• List of largest wastewater treatment plants
• List of wastewater treatment technologies
• Water treatment

References

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Wastewater

What is a septic tank and how does it work?

A septic tank is an economical system that is used for the partial treatment of wastewater generated from domestic properties and commercial buildings that aren’t connected to the main sewage system.

If you have questions about what septic tanks are, how they work and what they are used for, the answers you need are here.

Let's start at square one:

• Your septic tank is made up of a series of components that are used to treat wastewater on-site, most likely in a rural area that is off the main sewage network (off-mains).
   
• Your septic tank works by collecting wastewater from your home, retaining solid materials, removing wastewater pollutants, and returning the treated effluent to your property’s soil or the surrounding environment via infiltration.
   
• Across the UK, people use many different terms to describe septic tanks. The most common are sewage tanks, septic systems, residential septic tanks and sewage treatment plants. Note that a sewage treatment plant is a different system than a septic tank.

Now that you have the basics under your belt, read on to discover how standalone septic tanks and more advanced treatment systems work.

How does a septic tank work?

A septic tank works by collecting wastewater from your toilets and drains. It retains solids and scum within the tank. While some of the more resilient pollutants are biologically treated, the effluent liquid from your septic tank is driven to a drainage field.

In your drainage field, bacteria further break down the residual pollutants and the now fully-treated effluent returns to your property’s soil and groundwater.

That is the high-level answer. For a fuller understanding of how a standalone septic tank works, we can break the wastewater treatment process into 10 steps:

1. Domestic sewage from your toilet (called blackwater) and wastewater from your shower, bathtub, sink, washing machine, and dishwasher (called greywater) runs into the main drainage pipe that leads to your septic tank.
    
2. Heavier solid matter sinks to the bottom of your septic tank and forms sludge.
    
3. Lighter matter such as fats, oils, and greases (FOGs) float to the top of your septic tank to form scum.
    
4. Anaerobic organisms (microorganisms that do not need oxygen) in your septic tank feed on organic wastewater pollutants, converting them to inert matter.
    
5. Cleaner liquids pass through an effluent filter (or other retention mechanisms), further purifying the effluent.
    
6. The filtered liquid flows through a pipe toward the drainage field (or a secondary filter)
    
7. The perforated pipes from your drainage field allow the partially treated wastewater to seep into a layer of gravel.
    
8. The microorganisms present, as well as other subsurface mechanisms, further break down contaminants as wastewater percolates through the layer of gravel and into your native soil.
    
9. In your native soil, any remaining impurities are removed. These impurities may include harmful bacteria, parasites, viruses, and nutrients.
    
10. Treated wastewater reaches groundwater, which eventually drains into nearby streams, rivers, and lakes.
     

Regulations changed in

As part of the England's General Binding Rules, septic tanks are no longer allowed to discharge directly into streams, rivers, or ditches. To continue discharging into surface water, your property must upgrade to a sewage treatment plant (like our Ecoflo biofilter), divert its effluent to a drainage field, or connect to the utility sewage network.

Contact us to discuss your requirements of Wastewater Treatment Tanks. Our experienced sales team can help you identify the options that best suit your needs.

How a septic tank works with a secondary filter

A passive secondary filter (such as the Ecoflo biofilter) works by receiving wastewater from your septic tank and lowering concentrations of organic matter, suspended solids, pathogens, and, in some cases, nutrients like phosphorus or nitrogen.

Using natural forces and mechanisms, treatment of the wastewater occurs in the filter itself. Then, the system discharges the purified effluent downstream, where it eventually returns to your soil, a ditch, or a watercourse.

Septic tanks with a secondary treatment filter are most often used on properties with:

• limited or no space for a conventional drainage field
• poor drainage field soils with water oversaturation or similar
• very high permeability soils that limit effective treatment via infiltration
• impermeable soil or bedrock that prevents treated wastewater from infiltrating into the ground
• shallow soil that is unsuitable for wastewater percolation
• sensitive environments that require additional wastewater treatment processes to further improve the final effluent
• strict local environmental regulations

What are the parts of a septic system?

To add depth to your understanding of how a septic system works, you may want to take a closer look at the functions of its individual parts.

A septic tank has four main parts: a pipe that carries wastewater from your home, the actual septic tank, a pipe that connects your septic tank to your drainage field (or secondary system), and the drainage field (or secondary system).

Septic tank inlet pipe

The plumbing in your home is connected to the main sewer line that slopes toward your septic tank. In most cases, the sewer line is constructed from PVC (plastic). When you flush your toilet, take a shower, wash clothes, or do the dishes, the wastewater generated enters the sewer line and flows into your septic tank.

Septic tank

A septic tank is an underground and watertight container made of concrete, polyethylene or fibreglass. It provides primary wastewater treatment with the help of naturally occurring forces and bacteria that break down solids and organic material. The tank then pipes partially purified effluent downstream for further treatment or final dispersal.

A septic tank may seem complicated, but the way it works is actually quite simple. It holds the wastewater long enough for it to separate into three layers:

• Top layer
  Fats, oils, and greases float to the surface and form a layer of scum.
   
• Bottom layer
  Solid particles settle on the bottom of the tank and form sludge.
   
• Middle layer
  Liquid wastewater flows between the scum and sludge layers. It exits the tank through a pipe that leads to your advanced treatment system or septic drain field.

Across all three stages, helpful bacterial colonies are formed to break down most of these solids and turn them into inert matter.

Septic tank effluent filter

An effluent filter, such as the one in a Rewatec septic tank, is a cylindrical device installed before the outlet. It traps suspended solids (fine particles) that would otherwise clog your drainage field or secondary treatment filter.

Effluent filters need to be regularly cleaned. The exact timing depends on the design of your septic tank and how much you use it. As a minimum, we recommend that you ask a professional to clean your filter every time they service or desludge your tank.

Please note that not all septic tanks have an integrated effluent filter however they do all have a separation mechanism such as a baffle wall.

Septic tank vent

A septic tank must be continuously vented; the premises’ vent stack(s) is the conventional route that allows gases to safely escape your septic tank. If a vent stack is not present or it is not in working order, a ventilation mechanism must be introduced.

Bacteria in your tank generate gases as they break down wastewater contaminants. The most common is hydrogen sulfide, (which smells like rotten eggs). These gases must be released to prevent a build-up of pressure that could stop or reverse the flow of wastewater.

Septic tank access ports

An access port or access lid is a covered opening on the top of your septic tank. This opening allows waste removal professionals to empty your septic tank. It also provides easy access to important components, including the effluent filter where applicable.

Septic tank risers

A septic tank riser is a shaft made from concrete, polyethylene, or fibreglass. When your septic tanks’ access ports are below ground level, you can install a riser to bridge the gap and allow entry into your system. Otherwise, you would have to excavate the tanks' lids before every desludging.

Septic tank outlet pipe

When raw wastewater enters your septic tank, an equal volume of liquid exits (liquid displacement) through an outlet pipe that leads to a secondary treatment system or your drainage field.

If your property has uneven terrain, such as slopes or embankments, then gravity alone may not be enough to move wastewater through the outlet pipe. Your septic tank will need to include a pump station (sometimes called a sewage pumping station) that pressurises the flow of wastewater from your septic tank.

Drainage field

A drainage field is an underground network of perforated pipes that receives wastewater from your septic tank.

It works by accepting, treating, and dispersing wastewater. After wastewater seeps through the perforated pipes in your drainage field, it percolates through a bed of gravel and then into your native soil. Bacteria in these layers break down contaminants and the clean wastewater is discharged into the ground.

Secondary wastewater treatment systems

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