Comparison and Selection of Wastewater Treatment Methods

Since the 20th century, various technologies have been produced in the field of water treatment, especially since the 1970s, sewage and wastewater treatment technologies have been rapidly developed
.

At present, China’s water pollution control urgently needs a large number of applicable technologies.

Sewage wastewater treatment technology can be divided into physical method, chemical method, biological method or different combinations of these three methods according to the principle of treatment technology
.

It can also be divided into primary treatment (removal of non-water-soluble substances), secondary treatment (removal of dissolved organic matter) and advanced treatment (removal of nutrients and trace toxic organic matter) according to the treatment objects and goals

The most commonly used physical methods for sewage and wastewater treatment are precipitation and filtration processes.
Because of their low operating costs and mature technology, they are widely used in the primary treatment and pretreatment of sewage and wastewater
.

When the organic or inorganic insoluble pollutants in sewage and wastewater can be removed by precipitation, the precipitation separation technology should be the first choice

The most commonly used chemical methods are chemical precipitation and chemical oxidation
.

The chemical precipitation method may include coagulation sedimentation for removing fine particles and colloids, using the hydrolysis bridging effect of the flocculant to form flocs with micro particles and colloids in the water, and purifying pollutants through precipitation

Biological method (or biochemical method) is based on the principle of biochemistry and uses the metabolism of microorganisms to remove organic pollutants in wastewater
.

This type of method is currently a large number of process methods used in wastewater treatment, including a variety of different technologies, and is mainly used in secondary treatment and advanced treatment of wastewater

(1) The main parameters and conditions The biological treatment technology of sewage and wastewater has a wide variety of applications and is widely used.
It is used to degrade the organic matter and nitrogen and phosphorus in the wastewater that cause eutrophication of the water body
.

These technologies can be divided into different types according to different treatment objects, application conditions and the characteristics of the process itself, and are suitable for the treatment of different wastewater

The essence of biological treatment of sewage and wastewater is to use the metabolism of microorganisms to decompose pollutants in the water.
Therefore, the core of the biological treatment process is the relationship between the growth of microorganisms, the decomposition of organic matter and environmental conditions
.

The main parameters are specific substrate degradation rate (sludge load), specific growth rate of microorganisms (sludge yield), biochemical ratio (chemical oxygen demand and biochemical oxygen demand should have a certain proportional relationship: domestic sewage is usually 0.

(2) Urban domestic sewage treatment process Urban domestic sewage treatment is a large-scale and wide-ranging water pollution control problem
.

The main pollutants removed are suspended matter, dissolved organic matter and nitrogen and phosphorus

There are many processes for urban domestic sewage treatment, but the most suitable process for urban sewage treatment plants with a daily treatment capacity of more than 200,000 tons is the activated sludge method and its improved process
.
Mainly the fully mixed activated sludge process and oxidation ditch process with blast aeration
.
The fully mixed activated sludge process is a city sewage treatment process with wide applicability, while the oxidation ditch process is a derivative process of the activated sludge process
.
The advantage of the oxidation ditch is that the process system is simple, and only one or several aeration rotating brushes or rotating discs and trench-shaped biological oxidation tanks are used
.
Continuously operating oxidation ditch requires secondary sedimentation tanks, and alternately operating oxidation ditch does not require secondary sedimentation tanks
.
Generally, the advantages of good effluent water quality and low sludge production in the oxidation ditch are conditional.
When the designed sludge load is low, the oxidation ditch is close to the delayed aeration method, and a part of the oxygen is used to stabilize the organisms in the ditch.
Sludge, under these conditions, does have the advantages of good effluent quality and low sludge output.
However, the energy consumption of processing 1 ton of sewage will naturally be higher at this time, and due to the increase in equipment volume, infrastructure costs will also increase
.
The disadvantage of the oxidation ditch process is that the water temperature loss caused by the rotating brush aeration is relatively large.
When used in the northern cold regions, it is necessary to consider adding anti-freezing measures, which will increase the cost
.

For urban sewage treatment plants with a daily treatment capacity of 50,000 to 200,000 tons, in addition to the above two types of processes, the sequential batch activated sludge process (SBR process) can also be used
.
The SBR method is to operate a group of activated sludge tanks in the cycle sequence of "water intake → aeration reaction → sedimentation → drainage → sludge discharge" to achieve the purpose of continuous water intake and continuous treatment
.
The reason why this kind of process is rarely used in a treatment plant with a capacity of more than 200,000 tons is because the larger the processing capacity, the more pools and the more complicated the operation control program
.
At the same time, due to the increase of the pool wall, the infrastructure investment has increased
.
The advantage of the SBR method is that each single tank can be used as both an aeration tank and a sedimentation tank.
It can be operated in aerobic or anoxic state in different operation stages, and it can remove carbon or nitrogen and phosphorus
.
Due to the variability of the multi-pool combination and its operating conditions, the SBR process can be applied to different water quality and different treatment requirements
.
For the SBR process with denitrification, since denitrification in the same tank will consume part of the organic carbon, it can save the energy consumption of carbon removal and reduce the operating cost
.
For sewage plants with a daily processing capacity of more than 50,000 tons, the classic SBR process is mainly used, while the current application experience of DAT-IAT process, Unitank process, CAST, CASS process and ICEAS process derived from SBR process is still relatively small
.

Urban sewage treatment plants with a daily treatment capacity of 10,000 to 50,000 tons can adopt many technologies.
In recent years, various new processes have been continuously produced, but the overall application is still dominated by the activated sludge method
.
According to different requirements, it can be divided into the following categories
.

①For the purpose of removing organic carbon: activated sludge method, oxidation ditch, SBR process, biological filter, aerated biological filter, contact oxidation method
.

② for the purpose of denitrification to remove carbon: A / O method, oxidation ditch, operated alternately oxidation ditch, SBR process, CASS (CAST) process, like the process of UNITANK
.

③ carbon to remove nitrogen and phosphorus removal for the purpose of: A ² / O method, oxidation ditch alternate operation, SBR process and the like
.
It is worth noting the A/O method in the above process [refers to Anoxic (anoxia)/Oxic (aerobic); Anaerobic (anaerobic)/Oxic (aerobic), anoxic, anaerobic stage, use and denitrification and dephosphorization) ; Aerobic section, used to remove organic matter in water], oxidation ditch and SBR method
.
The A/O method is an anoxic-aerobic biological treatment method.
This process uses the process of microorganisms under anoxic conditions to oxidize organic carbon with combined oxygen to reduce nitrate to nitrogen, thereby achieving the purpose of denitrification
.
At the same time of denitrification, the combined oxygen is used, so the oxygen consumed by the oxidation of organic carbon in the subsequent aerobic process is reduced; under anoxic conditions, some particulate organic matter or macromolecular organic matter will be hydrolyzed, which is conducive to the follow-up.
The degradation of organic matter in the oxygen biological treatment process
.
Therefore, the A/O method can not only remove nitrogen, but also can save energy, improve the treatment effect and inhibit the growth of filamentous bacteria under the same conditions
.
Oxidation ditch and SBR process can also achieve the effect of A/O method under certain operating conditions
.
The operation and management of the oxidation ditch process is simpler, while the SBR process has a plug flow reaction mode during the aerobic oxidation process for each single cell and the quasi-static precipitation state during precipitation, making the treatment facility more compact and occupying a smaller area
.

For the treatment of domestic sewage with a daily processing capacity of less than 10,000 tons, in order to operate stably and facilitate management, in addition to the above-mentioned processes, a fixed-growing biological treatment process can also be used
.
Due to the small volume of the treatment facility, the biological filler will not increase a lot of infrastructure costs
.
The use of this type of process can avoid the loss of microorganisms, and is especially suitable for small sewage treatment stations that lack specialized management and operating experience
.
The most commonly used in the fixed growth biological treatment process are biological contact oxidation method, aerated biological filter and biological fluidized bed
.
The contact oxidation method has a large number of application examples and operating experience
.
Biological fluidized beds and biological aerated filters are relatively new and lack experience in large-scale applications
.
The former has high treatment efficiency and is suitable for areas where land is scarce; the latter has better effluent quality and can be used where wastewater reuse is considered
.
The scale of processing less than 10,000 tons per day provides development space for the equipmentization of sewage treatment devices
.
On this scale, various equipmentized sewage treatment devices can be designed and produced .

The A ² /O method (referring to anaerobic/anoxic/aerobic) process is generally used for the dephosphorization of domestic sewage
.
The phosphorus removal capacity of the biological phosphorus removal process is limited.
Generally, the total phosphorus concentration in the effluent can reach 1mg/L.
To reduce the total phosphorus concentration in the effluent to 0.
5mg/L, a combination of chemical phosphorus removal and biological phosphorus removal is required.
Treatment method
.

Anaerobic biological treatment technology is widely used in the digestion of sludge produced by urban domestic sewage treatment
.
In recent years, starting from energy saving, it has also been applied to the treatment of urban domestic sewage in some countries, but it is still in the exploratory stage
.
Anaerobic biological treatment in wastewater treatment is mainly used in industrial wastewater treatment
.
Since anaerobic microorganisms convert organic carbon into methane when it utilizes organic carbon, the yield of microorganisms used per unit of carbon source is low, so anaerobic biological treatment can first be used to treat high-concentration and easily biodegradable organic wastewater, such as beer production wastewater, slaughter wastewater and aquaculture wastewater
.
Anaerobic biological treatment also has some special applications in the treatment of industrial wastewater.
For example, anaerobic biological treatment can be used as pretreatment, and the hydrolysis and decomposition of some complex organic matter under anaerobic conditions can be used to improve the biodegradability of wastewater: For the waste water prone to foaming, anaerobic biological treatment can be used to hydrolyze and modify the foam-producing substances, and then anaerobic treatment can be carried out
.

The development of science and technology provides opportunities for the development of wastewater treatment technology
.
For large-scale urban sewage treatment, as the process technology has basically stabilized, the main development will be in the automatic monitoring, operation control and management of sewage treatment plants
.
In recent years, European and American countries have greatly improved the treatment efficiency and stability of urban sewage plants through integrated equipment and automatic control of sewage plants
.
The development of small and medium-sized domestic sewage treatment technology will focus on the integration and equipmentization of treatment systems, and the development of combined and assembled sewage treatment devices, so as to move towards a serialized and standardized production model
.
The development of industrial wastewater treatment technology will focus on the treatment of high-concentration, difficult-to-degrade, and high-salinity wastewater.
Among them, the development of new flocculants, the combined application of anaerobic biological treatment and aerobic biological treatment, and membrane separation technology will become the development Hot spot
.
For industrial wastewater bioenhancement technology containing refractory toxic organics, that is, targeted degradation by adding preferred strains or genetically engineered bacteria has a good development prospect
.