All societies produce waste, both solid and liquid. The liquid part of these wastes is the same water that has been contaminated as a result of various uses. In terms of production sources, sewage can be considered as a combination of wastes produced by water from residential, administrative and commercial and industrial facilities and discharged to underground water and surface water.

Untreated sewage contains pathogenic microorganisms and organic substances, which decompose the organic matter of untreated sewage to produce foul-smelling gases. Also, due to its nutrients, wastewater can stimulate the growth of aquatic plants. Therefore, purification and decontamination of wastewater is one of the necessary measures that must be taken before discharging into the environment.The ultimate goal of wastewater management is to protect the environment in a way that is in harmony with the principles of public health and economic, social and political issues.

Wastewater treatment and goals Purification Wastewater is water that is prepared for specific consumption and for any reason, quality It has lost itself for that consumption and is contaminated. One cubic meter of sewage can pollute 14 to 20 cubic meters of water. some of These pollutants prevent the use of water for purposes such as drinking, bathing, recreation, Agriculture, industrial uses, etc. The meaning of purification Wastewater accelerates natural forces under controlled conditions for the following primary purposes Is : Removal of suspended and floating materials Breakdown of biodegradable organic matter (substances Perishable organics) to less harmful minerals Destruction of pathogens and parasites Removal of toxic substances The most important goals of purification Wastewater also includes: Providing health conditions for people’s lives Keeping the environment clean Recovery and reuse of wastewater Due to population growth and lack of water resources, wastewater treatment and reuse It can be an effective step in reducing water consumption and preserving water resources.  
Types of sewage and its origin Components of wastewater And the types of sewage in each community is subject to the type of collection system that may include items be as follows: Domestic sewage Sewage from the unit residential, commercial, industrial and similar facilities are discharged, which is this type of sewage, sewage It is sanitary which is produced as a result of human use of water for sanitary purposes. Because this sewage in It is produced by human activities, sometimes it is also called human or domestic sewage will be Toilets, Bathing, washing dishes, washing surfaces and laundry are the main activities that They produce sanitary-human sewage. The volume of sanitary sewage is directly related to the per capita It has water consumption that is different according to the level of well-being and culture of the society. According to the standards Almost 80-90% of the water used becomes sewage. Industrial wastewater It is a sewer in which There is more industrial waste. This type of sewage is the most variable type of sewage because every Factories and industries, according to the nature of their produced wastewater, have special conditions of wastewater is. Industrial wastewater in the effect Water consumption in industrial activities or from industrial sources and during different stages of production and dangerous They are the most common type of sewage. Industrial wastewater depending on The type of industry and the amount of pollution have different methods for wastewater treatment.
Agricultural wastewater More to drain the lands It refers to agriculture and usually contains different pesticides, poisons and chemical fertilizers. floods Surface currents that It includes rainfall and snow melt, which is separate or shared with sewage A city is collected, purified and disposed of. permeation flow External water is through Seams, cracks, fractures or porous walls enter the sewer network.
Qualitative and quantitative characteristics Wastewater The amount of produced wastewater varies depending on the climatic conditions. Coefficient 50 to 80% can be chosen to convert used water into wastewater in Iranian cities . Domestic sewage from sewage devices Home hygiene such as toilets, washrooms, bathrooms, washing machines and dishwashers. Wastewater from kitchens or sewage from washing different parts of the house is formed. The characteristics of domestic sewage at the level of a country are almost the same and only their concentration depends on The amount of water consumption per capita is different in cities. 99.9% of wastewater consists of water and only Solids make up 1% of wastewater Wastewaters are divided into three categories according to the severity of pollution: weak, medium and Strongly divided
Physical characteristics of wastewater The most important physical characteristics of wastewater including total solids of wastewater, including total dissolved solids, suspended solids, and soluble solids It is sedimentation. Other physical characteristics of sewage Temperature, density, turbidity, smell, color and acidity of wastewater can be mentioned. Chemical characteristics Wastewater Chemical characteristics of wastewater including organic matter, measurement of organic content of wastewater, Inorganic substances and gases. Measuring the organic content of wastewater because of its importance in determining the amount of pollution Sewage and the design and operation of the sewage treatment plant are important. Measurement of organic content Wastewater including BOD and COD is wastewater that is used to control the quality of the effluent. Biological characteristics of wastewater Biological characteristics of wastewater, including important characteristics of wastewater Is. Knowing the organisms in wastewater and their role in wastewater treatment is important It is practical. In fact, wastewater microorganisms play the main role of purification and The treatment plant workers are the ones who perform the main operations of wastewater treatment.
Wastewater treatment steps Usually, wastewater treatment is done in three stages, which are: Preliminary clearance or primary treatment Primary treatment includes physical treatment such as garbage removal, grain removal and sedimentation It is primary. The primary purpose of municipal wastewater treatment is to remove suspended materials (solids). suspended and part of suspended BOD). This separation is usually done by physical operations such as garbage collection and sedimentation. Suspended solids are separated from the wastewater in the seeding pond and settling pond. A primary treatment system should remove approximately half of the suspended solids from the wastewater Slow and BOD related to these solids is almost 30% of the total organic load The entrance is sewage. Secondary treatment or Biological or secondary treatment of wastewater Secondary wastewater treatment includes biological treatment such as aeration and use of Bacteria for wastewater treatment. The main objective of secondary wastewater treatment is to separate biodegradable organic matter and Suspended solids. This separation is through the biological conversion of organic matter into flocs It is separated and removed from the sewage in the sedimentation stage. Management of secondary treatment facilities compared to the preliminary stage in terms of energy and cost needs more
The third stage purification or Tertiary or advanced treatment The quality of the effluent from the sewage treatment of the secondary treatment may always be consistent Not with effluent standards. These conditions are usually when discharging into small streams or when the ecosystem Zarif is the recipient of wastewater. Advanced wastewater treatment including clarification and additional treatments such as finishing Nitrification operation, wastewater passing through sand filters, use of active carbon, salt Desalination by ion exchange method is the removal of phosphorus and nitrogen from wastewater, etc. In fact, the purpose of advanced treatment is to remove pollutants that are present in primary treatment The secondary ones are not separated, but the most used is the removal of nitrogen and phosphorus as well as organic matter It is resistant to sewage.

Purification processes Wastewater

Pollutants in wastewater with physical, chemical and biological methods
It is separated. Each of these methods is usually referred to as physical unit operation.
Chemical unit processes and biological processes are categorized.

Physical unit operation

Purification methods in which the use of physical forces is dominant are single operations
It is called physical.

Units such as garbage collection, granulation, mixing, coagulation, sedimentation, floating
Making, refining and transferring gas are physical units.

Garbage collection unit

Garbage collector is the first physical unit in sewage treatment plants and in fact
A device with openings generally of the same size as those available for storing coarse suspended solids
It is used in sewage.

Seeding unit

Granulation is the first unit in the treatment plant where sedimentation is done.

Granulation pond for separating grains such as sand, gravel and other solids
Heavy that the rate of settling or their specific gravity is higher than the decaying organic solids in it
It is sewage, it is used.

Flow equalization unit

Current equalization or balancing unit to overcome gain problems
Vectoring caused by fluctuations in the flow rate, improving the performance of subsequent units of the case treatment plant
is used

Preliminary clearance

In order to improve the treatability of wastewater and separate fat and foam, operations such as pre-aeration and flocculation can be used in preliminary treatment.

Chemical unit processes

Treatment methods in which the separation or transformation of pollutants are done by adding chemicals or as a result of chemical reactions are called chemical unit processes.

Chemical precipitation, surface absorption and disinfection are the most common treatment processes in wastewater treatment plants.

Biological treatment of wastewater

The purification methods in which the separation of pollutants is done through biological activity are called biological unit processes.

Biological treatment is the most effective method to reduce organic matter in water and wastewater. Common wastewater processes are divided into three main groups based on the type of active microorganisms:

Aerobic processes
Anaerobic processes
Combined biological processes (aerobic, anaerobic, anoxic) which are advanced wastewater processes in removing nitrogen and phosphorus.
Depending on whether the biological treatment is carried out in suspended growth systems, adherent growth or a combination of them, each of the processes can be divided into other types.

Aerobic biological treatment of wastewater

In aerobic biological treatment, wastewater is removed by aerobic microorganisms in the presence of oxygen and wastewater treatment.

Biological purification by aerobic method has different types depending on the type of growth environment and purification method. Below we briefly explain some of the biological methods of wastewater.

Activated sludge process

Activated sludge is a suspended growth microbial culture system.

The reason for naming this method is due to the presence of live and active microorganisms settled in the sludge, which return to the system to increase the living mass and improve the purification process.

Extensively Aerated Activated Sludge (EASS)

The extended aeration process is a type of activated sludge in which the aeration time is longer and the organic loading is lower.

In this method, due to the long aeration, the amount of produced sludge is less and this process is used for small communities.

Fixed Growth Activated Sludge (IFAS)

In this tank, by using PVC packings, a suitable environment is created for the sticky growth of bacteria, and the aerobic conditions are also done by the deep aeration of the diffusers located at the bottom of the tank.

Compared to the old and conventional systems, this system requires very little space, and in these reactors, no complex mechanical and electromechanical devices are used.

Activated Sludge with Moving Bed Growth (MMBR)

Wastewater treatment using MMBR method is a kind of activated sludge, in this method, mobile media packing is used to increase the contact surface and growth level of microorganisms, which is suspended inside the tank.

Discontinuous reactor Sequential (SBR) Complete mixing is separate where all steps of the activated sludge process take place. The mixed liquid remains inside the reactor during all the cycles, so there is no need to The sedimentation tanks are not separate. callers circulating biological (RBC) RBC are a type of fixed bed bioreactors that consist of a series of plates The circles close to each other are made of polyester or polyvinyl chloride. These pages They spin slowly inside the sewer. Intermittent exposure of organic matter in wastewater The oxygen in the air causes the sewage to be purified. Membrane bioreactors (MBR) Wastewater treatment by MBR method is actually the same as activated sludge, with the difference that membrane systems are used to separate and settle suspended substances from wastewater, and the membrane system for separation can be done in an aeration unit or in a separate unit.