Introduction to the Most Comprehensive Sewage Treatment Processes

This article provides you with a detailed introduction to various sewage treatment processes.

Basic methods of sewage treatment

Classified by the nature of the processing method

Physical methods: sedimentation method, filtration, oil separation, air flotation, centrifugal separation, magnetic separation

Chemical methods: coagulation precipitation method, Zhonghe method, oxidation-reduction method, chemical precipitation method

Physicochemical methods: adsorption, ion exchange, extraction, stripping, stripping

Biological methods: activated sludge method, biofilm method, anaerobic process, biological nitrogen and phosphorus removal process

According to the water quality status and the direction of treated water

Primary treatment: Mechanical treatment (pre treatment stage)

Coarse and fine grids, sand settling tanks, initial settling tanks, air flotation tanks, and regulating tanks

Secondary treatment: the main process is biochemical treatment (main body)

Activated sludge process, CASS process, A2/O process, A/O process, SBR, oxidation ditch, hydrolysis acidification tank.

Third level treatment: control eutrophication and reuse

Advanced catalytic oxidation, biological aerated filter, fiber filter, activated sand filtration, reverse osmosis, membrane treatment

There are generally disinfection tanks for reclaimed water reuse: ultraviolet ozone disinfection tank, chlorine dioxide disinfection tank

Basic process flow of sewage treatment

Primary treatment of sewage

Primary treatment: Mechanical treatment (pre treatment stage)

Regulating tank, coarse and fine grids, sedimentation tank, initial sedimentation tank, air flotation tank, hydrolysis acidification tank

Regulating pool

The function of the regulating pool

In order to ensure the normal operation of subsequent treatment structures or equipment, it is necessary to adjust the amount and quality of sewage.

2. Mixing acidic and alkaline wastewater in a regulating tank can achieve neutralization.

3. The short-term discharge of high-temperature sewage can also be adjusted to balance the water temperature.

a grille

It is a metal frame made of a set of parallel metal bars, slanted on the channel through which wastewater flows or at the inlet of the pump station's collection tank, used to intercept large suspended or floating solid pollutants, in order to avoid blocking the sludge discharge pipes of the water pump and sedimentation tank. The interception effect depends on the width of the gap and the nature of the water.

Classified by specifications:

Coarse grid (50-100mm), medium grid (10-40mm), and fine grid (3-10mm)

Grit chamber

1. Function: Separating dense inorganic particles from sewage, protecting water pumps and pipelines from wear and tear, reducing the volume of sludge treatment structures, increasing the organic content of sludge, and increasing the value of sludge as fertilizer.

2. Type of sedimentation tank: Aerated sedimentation tank, horizontal flow sedimentation tank

Aerated sedimentation tank:

Aerated sand settling tank is a rectangular water tank where air is introduced on one side, causing sewage to move in a swirling motion. The flow rate gradually decreases from the periphery to the center, and sand particles are separated from water in the sand collection tank at the bottom of the tank. The organic matter in the sewage and sludge washed down from the sand particles are still in a suspended state, and as the water flows into the treatment structure behind.

Horizontal flow sedimentation tank:

The horizontal flow sedimentation tank is actually a channel wider and deeper than the human flow channel and the outlet channel. When sewage flows through it, due to the increase of the water flow section, the water flow speed decreases, and the inorganic particles carried in the wastewater sink under the action of gravity, thus achieving the purpose of separating inorganic particles in the water.

Advantages and disadvantages of horizontal flow sedimentation tank and aeration sedimentation tank:

The biggest drawback of ordinary sand settling tanks is the inclusion of some organic substances in the retained sand. The presence of these organic substances makes the sand prone to decay and odor, especially during high summer temperatures, which has adverse effects on the treatment of sand settling and the surrounding environment. Another disadvantage of ordinary sedimentation tanks is their poor retention of sand particles wrapped in organic matter.

The advantage of an aerated sand settling tank is that its sand removal efficiency is stable and less affected by changes in inlet flow rate. The hydraulic rotation effect has a good separation effect between sand particles and organic matter. In the sediment discharged from the aeration sedimentation tank, organic matter only accounts for about 5%, and it will not decay or stink after long-term use. During the process of aeration and sand settling, it can also play a role in floating oil and blowing off volatile organic compounds, as well as pre aeration and oxygenation to oxidize some organic compounds.

Sedimentation tank

1. Working principle of sedimentation tank: Water purification is achieved by utilizing the principle that suspended impurity particles in the water flow can separate from the water flow when the settling speed is greater than the downward flow speed of the water flow, or the settling time is less than the time when the water flow flows out of the sedimentation tank.

2. Sedimentation tank structure: inlet and outlet areas: to ensure uniform flow of water through the sedimentation tank, avoid short flow and reduce the adverse effects of turbulence on sedimentation, while reducing dead water areas and improving the volume utilization rate of the sedimentation tank.

Sedimentation area: The area where precipitated particles are separated from wastewater.

Sludge area: It is the area where sludge is stored, concentrated, and discharged.

Buffer zone: It is a water layer area that separates the sedimentation zone and the sludge zone, ensuring that particles that have already settled do not float again due to water flow agitation.

3. The difference between sedimentation tanks and sedimentation tanks: Sedimentation tanks are generally facilities for separating mud and water in front of biochemical structures in sewage treatment plants. The separated precipitates are mostly sand with larger particles, with a larger specific gravity, high inorganic composition, and low water content. Sewage inevitably mixes with mud and sand during migration, flow, and collection. If the sand in sewage is not pre settled and separated for removal, it will affect the operation of subsequent treatment equipment. The main issue is the wear and tear of the pump, blockage of the pipeline network, interference and even destruction of the biochemical treatment process.

Sedimentation tanks are generally structures for separating sludge and water before or after biochemical treatment, mostly for separating fine-grained sludge. Before biochemistry, it is called the primary sedimentation tank, and the precipitated sludge has a higher inorganic content. The moisture content of the sludge is lower than that of the secondary sedimentation tank sludge. The sedimentation tank located after biochemical treatment is generally called a secondary sedimentation tank, which is mostly organic sludge with a high moisture content.

4. Type of sedimentation tank: horizontal flow sedimentation tank, vertical flow sedimentation tank, amplitude flow sedimentation tank, oblique flow sedimentation tank

a. Horizontal sedimentation tank: With simple structure and good sedimentation effect, it occupies a large area and has many problems with sludge discharge. Currently, it is used in large, medium, and small sewage treatment plants.

b. Vertical flow sedimentation tank: It has a small footprint, convenient sludge discharge, and is easy to manage. However, the depth of the tank is too large, construction is difficult, and the cost is high. Therefore, it is generally only suitable for use in small and medium-sized sewage treatment plants.

c. Amplitude flow sedimentation tank: most suitable for use in large water treatment plants, with standardized sludge discharge machinery and good operating results, but requiring high construction quality and management level.

d. Diagonal flow sedimentation tank: mainly suitable for initial sedimentation tanks, widely used in water supply treatment, with high sedimentation efficiency, short residence time, small land occupation, and the disadvantage of easy growth of algae, difficult sludge discharge, easy blockage, and inconvenient maintenance.

Air flotation tank

Air flotation method: By introducing air into sewage, small bubbles are generated as carriers, causing pollutants such as emulsified oil and small suspended solids in the sewage to adhere to the bubbles. The purpose of separating impurities and purifying sewage is achieved by collecting foam or scum on the water surface through the floating action of bubbles.

Hydrolysis acidification tank

The main purpose is to transform the insoluble organic matter in the original wastewater into soluble organic matter, especially in industrial wastewater. The main purpose is to transform the difficult to biodegradable organic matter into easily biodegradable organic matter, improve the biodegradability of the wastewater, and facilitate subsequent aerobic treatment.

Secondary treatment of sewage

Activated sludge process

The degradation of organic matter by activated sludge mainly occurs during the aeration stage, which can be divided into two stages: adsorption stage and stabilization stage. In the adsorption stage, the organic matter in the sewage is mainly transferred to the activated sludge, which is due to the large specific surface area of the activated sludge and the presence of sticky substances such as polysaccharides on the surface. In the stable stage, the organic matter on the transfer channel activated sludge is mainly utilized by microorganisms. When organic matter in sewage is in a suspended and colloidal state, the adsorption stage is very short, usually around 15-45 minutes, while the stable stage is longer.

SBR method

SBR method, also known as sequencing batch activated sludge method, is a modification of continuous activated sludge method. Its reaction mechanism and pollutant removal mechanism are basically the same as traditional activated sludge method, but the operation is different.

Disadvantages of classic SBR reactors:

1. For the application of a single SBR reactor, a larger regulating pool is required;

2. Frequent automatic switching of inlet and outlet valves for multiple SBR reactors;

3. Unable to meet the treatment requirements for continuous inflow and outflow of large sewage treatment projects;

4. The idle rate of equipment is relatively high;

5. The head loss of sewage lifting is significant.

CASS process

The CASS process is called the circulating activated sludge process. On the basis of the sequencing batch activated sludge process (SBR), the reaction tank is designed into two parts along the length of the tank. The front part is a biological selection zone, also known as a pre reaction zone, and the rear part is the main reaction zone. The rear part of the main reaction zone is equipped with a lifting automatic skimming device. The entire process of aeration, sedimentation, drainage, and other processes are cyclically operated in the same tank, eliminating the secondary sedimentation tank and sludge reflux system of conventional activated sludge method; Simultaneously, it can continuously feed water and intermittently discharge water.

technological process:

1. Aeration stage: oxygen is charged into the reaction tank from the aeration device. At this time, the organic pollutants are oxidized and decomposed by microorganisms, and the NH3-N in the sewage is converted into NO3-N through the nitrification of microorganisms.

2. Sedimentation stage: At this point, aeration is stopped and microorganisms utilize the remaining DO in the water for oxidative decomposition. The reaction tank gradually transforms from aerobic state to anoxic state, and denitrification starts. The activated sludge gradually settles to the bottom of the tank, and the upper water becomes clear.

3. Decanting stage: After sedimentation is completed, the decanter placed at the end of the reaction tank begins to work, gradually discharging the supernatant from top to bottom. At this time, the reaction tank gradually transits to anaerobic state to continue denitrification.

4. Idle stage: The idle stage refers to the stage when the decanter rises to its original position.

Advantages of CASS process:

1. Simple process flow, small footprint, low investment, and low operating costs

2. Strong driving force for biochemical reactions

3. Good sedimentation effect

4. Flexible operation and strong impact resistance

5. Less prone to sludge bulking

6. Wide application range, suitable for phased construction

7. Low sludge production and stable sludge properties

Disadvantages of CASS process:

1. The complex relationship between microbial populations needs to be studied

2. Difficulty in improving biological denitrification efficiency

3. Difficulty in improving phosphorus removal efficiency

4. The control method is relatively single

AO process

A/O process: The sewage undergoes two biological treatment processes, anaerobic and aerobic, to achieve the simultaneous removal of BOD, nitrogen, and phosphorus.

Advantages of A/O process:

1. High efficiency

2. Simple process, low investment, and low operating costs

3. Anoxic denitrification process has high degradation efficiency for pollutants

4. High volumetric load

5. Strong load impact resistance of anaerobic/aerobic processes

Disadvantages of A/O process:

Due to the lack of an independent sludge reflux system, it is not possible to cultivate sludge with unique functions, and the degradation rate of difficult to degrade substances is relatively low

2. To improve denitrification efficiency, it is necessary to increase the internal circulation ratio, which increases operating costs

AAO process

A2/O sewage treatment system: The sewage undergoes three biological treatment processes: anaerobic, anaerobic, and aerobic (referred to as A2/O), achieving the simultaneous removal of BOD, nitrogen, and phosphorus.

Advantages of A2/O process:

1. High pollutant removal efficiency, stable operation, and good impact load resistance.

2. Good sludge settling performance.

3. The organic combination of anaerobic, anaerobic, and aerobic environmental conditions and different types of microbial communities can simultaneously remove organic matter, nitrogen, and phosphorus.

4. The denitrification effect is influenced by the reflux ratio of the mixed liquid, while the phosphorus removal effect is influenced by the DO and nitrate oxygen entrained in the reflux sludge, so the denitrification and phosphorus removal efficiency cannot be very high.

5. In the process of simultaneous deoxygenation and phosphorus removal to remove organic matter, this process is the simplest and the total hydraulic retention time is also less than other similar processes.

Under the alternating operation of anaerobic anoxic aerobic, filamentous bacteria do not proliferate in large numbers, and SVI is generally less than 100, so sludge bulking will not occur

7. The phosphorus content in sludge is high, usually above 2.5%

Disadvantages of A2/O process:

1. The volume of the reaction tank is larger than that of the A/O denitrification process

2. The internal return flow of sludge is large, and the energy consumption is high

3. High cost for small and medium-sized sewage plants

4. Poor economic benefits of biogas recycling and utilization

5. Chemical phosphorus removal is required for sludge exudate

Oxidation ditch process

Oxidation ditch: It is an improved activated sludge method in which the aeration tank is in the shape of a closed ditch, in which the mixture of sewage and activated sludge circulates and flows. Therefore, it is called an "oxidation ditch" or "annular aeration tank".

The HRT and SRT of the oxidation ditch are relatively long, and organic matter can be completely removed. The discharged sludge is highly stable, and the overall power density is low, saving energy. However, problems such as sludge bulking, uneven flow rate, sludge deposition, foam and sludge floating are easy to occur.

AB process

The sewage enters section A directly through the drainage system, through the grid and sedimentation tank. This section is an adsorption section with high load, short mud age, and a short hydraulic retention time of about 30 minutes, which is conducive to the growth and reproduction of microorganisms with fast proliferation speed. After the wastewater is treated in section A, BOD is removed by 40% to 70%, and the biodegradability is improved, which is beneficial for the operation of section B; The A stage sludge has a high yield and strong adsorption capacity. Heavy metals, difficult to degrade substances, as well as plant-based nutrients such as nitrogen and phosphorus, may be removed through the adsorption of sludge.

After the sewage flows out of section A, it enters section B. Section B is a biological oxidation section and belongs to the traditional activated sludge method. It generally operates at a lower load, with a retention time of about 2-6 hours and a longer sludge age of 15-20 days. Nitrification and partial denitrification occur in section B, the activated sludge sedimentation efficiency is good, and the effluent SS and BOD are generally less than 10mg/L.

The AB method mainly has the following characteristics: there is no initial sedimentation tank, and the A section composed of an adsorption tank and an intermediate sedimentation tank is a primary treatment system; Section B consists of an aeration tank and a secondary sedimentation tank; A. The two sections of section B each have independent sludge reflux systems, which are completely separate and each has a unique microbial community, which is conducive to functional stability.

Tertiary treatment of sewage

After secondary biological treatment, the effluent generally contains around 30mg/L of BOD, 60mg/L of COD, NH315-25mg/L, P3-8mg/L, SS30mg/L, as well as bacteria and heavy metals. It must be treated, otherwise it can easily lead to eutrophication of the water body and have an impact on fish, crops, freshwater water quality, and treatment costs.

The tertiary treatment methods include sand filtration, coagulation, microfiltration, reverse osmosis, electrodialysis, ion exchange, disinfection, activated carbon adsorption, nitrogen and phosphorus removal, etc.

Removal of suspended solids

1. Particle size: The secondary effluent SS is composed of a 1um~1mm biological flocculent and an unflocculated colloidal substance. Generally, it is removed through coagulation, sand filtration, microfiltration, and reverse osmosis.

2. Coagulation and sedimentation: By adding coagulants and rapidly stirring for coagulation, and slowly stirring for flocculation, small particles and colloidal substances are destabilized and condensed, forming larger particle flocs for precipitation and removal.

Dissolved organic matter removal

1. Activated carbon adsorption: Activated carbon has a huge surface area and small pores, which can adsorb organic matter, heavy metal ions, etc.

2. O3 oxidation treatment: Secondary treatment water shall be treated for reuse, so as to remove organics, chroma, sterilization and disinfection in sewage.

Removal of soluble inorganic salts

Hazard: It is corrosive and prone to scaling. SO42- reduces to produce H2S, causing soil compaction and salinization. Therefore, desalination is required before water reuse and agricultural use.

Desalination technology: reverse osmosis, electrodialysis, ion exchange.

Disinfection treatment of sewage

Reason: Regardless of the process, the bacteria in the effluent will exceed the standard, causing harm.

Application occasions: agricultural irrigation with sewage, upstream of discharge water source, tourist attraction, and epidemic season.

Disinfection methods: liquid chlorine, ozone, sodium hypochlorite, and ultraviolet light