Summary of sewage treatment technology

1. Sewage treatment level

Sewage treatment levels include primary treatment (including primary intensive treatment), secondary treatment (including secondary intensive treatment) and advanced treatment.

2. Composition of sewage treatment process

(1) Physical processing section. (2) Biochemical treatment section.

The principle of sewage treatment process selection

The main technical and economic indicators of process selection include: investment per unit of water treatment, investment reduction per unit of pollution, power consumption and cost per unit of water treatment, power consumption and cost reduction per unit of pollutants, land area, reliability of operation performance, management and maintenance difficulty, overall environmental benefits, etc.

The municipal sewage treatment process should be optimized according to the treatment scale, water quality characteristics, environmental functions of receiving water bodies and local actual conditions and requirements after comprehensive technical and economic comparison.

The influent water quality of sewage should be determined realistically, the process design parameters should be optimized, the current water quality characteristics of sewage and the composition of pollutants must be investigated or determined in detail, and a reasonable analysis and prediction should be made. When the water quality composition is complex or special, the dynamic test of sewage treatment process should be carried out, and pilot study should be carried out if necessary.

The new process applied for the first time in China must go through pilot and production tests to provide reliable design parameters before being applied. When the same sewage plant is constructed in stages, the same process should be adopted in each stage as far as possible, and the construction scale of each stage should be as same as possible.

Sewage treatment method

Modern sewage treatment methods are mainly divided into four categories: physical treatment, chemical treatment, physical chemical treatment and biological treatment.

1 Physical treatment method

The physical treatment method is to separate and recover the insoluble and suspended pollutants (including oil film and oil beads) in sewage through physical action, without changing their chemical properties during treatment. Commonly used are filtration, precipitation, flotation and so on.

(1) Filtration method: The use of filtration media to intercept suspended matter in sewage. Filter media have screen, gauze, particles, commonly used filtration equipment such as grille, screen, microfilter and so on.

1) Grille and screen. In the drainage project, the waste water flow through the sewer to the water treatment plant, should first pass through a group of metal vertically parallel frame (grid), perforated plate or filter (screen) in the channel, so that the floating matter or suspended matter can not pass and is blocked in the grid, fine screen or filter material.

Grille plate

This step is the pretreatment of wastewater, the purpose of which is to recover useful substances; Initial flushing of wastewater to facilitate subsequent treatment and reduce the load on settling tanks or other treatment equipment; Protect pumping machinery from failure due to particulate matter blockage. Protect water pumps and other treatment equipment. The effect of grille interception mainly depends on the sewage quality and the size of the grille gap. There are two kinds of slag cleaning methods: manual and mechanical. Grid slag should be cleaned and treated in time.

The screen is mainly used to intercept fine suspended debris with a particle size of a few millimeters to tens of millimeters, such as fibers, pulp, algae, etc., usually woven with metal wire, chemical fiber, or perforated steel plate, the aperture is generally less than 5mm, the minimum can be 0.2mm. The screen filter device has rotary drum type, rotary type, rotary disk type, fixed vibration inclined screen and so on. No matter what kind of structure, it should not only be able to trap dirt, but also easy to discharge and clean the screen surface.

2) Granular media filtration (also known as Tong, filter, surprise material filtration). When the wastewater passes through the granular filter material (such as quartz sand) bed, the fine suspended matter and limbs are trapped in the surface of the filter material and the internal void. Commonly used filter media are quartz sand, anthracite and garnet. In the filtration process, the filter material also acts on physical retention, sedimentation and adsorption of suspended matter. The effect of filtration depends on the size of the pore size of the filter material, the thickness of the filter material layer, the filtration speed and the nature of the sewage.

When the waste water flows through the granular filter layer from top to bottom, the suspended particles with larger diameter are first trapped in the gap of the surface filter material, so that the gap of the filter material of this layer is smaller and smaller, and gradually form a filter membrane mainly composed of the trapped group particles, and it plays the main filtration role. This action belongs to the resistance trapping or screening action.

When the wastewater passes through the filter material layer, the numerous filter material surfaces provide a huge effective area for suspended solids to settle, forming countless small "sedimentation ponds", where suspended solids can easily settle down. This effect belongs to gravitational sedimentation.

Because the filter material has a huge surface area, there is an obvious physical adsorption between it and the suspended matter. In addition, the sand often has a negative surface charge in water, which can adsorb positively charged iron, aluminum and other limbs, thus forming a positively charged film on the surface of the filter material, and then adsorb negatively charged colloids such as colloids and a variety of organic matter, and contact flocculation occurs on the sand.

(2) precipitation method. Sedimentation method is to use the principle that the relative density of suspended matter in sewage and water is different, and the suspended matter is separated from the water by gravity sedimentation. According to the concentration of suspended particles in the water and the flocculation characteristics (that is, the ability to bond to each other) can be divided into four types:

1) Separation settlement (or free settlement). In the precipitation process, the particles do not polymerize with each other and settle separately. The position of the star is only affected by its own gravity in the water and the resistance of the water flow, and its shape, size and mass do not change, and the descent speed does not change.

2) Coagulation precipitation (or called flocculation settlement). Coagulation sedimentation refers to the condensation of colloids and fine suspended matter in wastewater into separable flocculants under the action of coagulants, and then gravity sedimentation is used to separate and remove. The characteristic of coagulation precipitation is that in the precipitation process, the particles contact and collide and aggregate with each other to form a larger floc, so the size and mass of the particles will increase with the increase of the depth, and the settling speed will also increase with the depth.

The commonly used inorganic coagulants are aluminum sulfate, ferrous sulfate, ferric chloride and polyaluminum; The commonly used organic flocculants are polypropylene cool glue, etc., and coagulants such as water glass and lime can also be used.

3) regional settlement (also known as crowded settlement and stratified settlement). When the content of suspended matter in wastewater is high, the distance between particles is small, and the cohesion force can make it gather into a whole and sink together, while the position of particles between each other does not change, so there is a clear interface between the clarified water and the mixed water, and gradually move downward, such settlement is called regional settlement. Sedimentation in sedimentation tanks with high turbidity water and secondary sedimentation tanks (in the middle and later stages of sedimentation) are mostly of this kind.

4) compression precipitation. When the concentration of suspended solids in the suspension is high, the particles contact and squeeze each other, and under the gravitational action of the upper particles, the water in the lower particle gap is squeezed out, and the particle population is compressed. Compression precipitation occurs in the sludge hopper or sludge thickening tank at the bottom of the sedimentation tank and proceeds very slowly. According to the different properties of the suspended substances in the water, there are two kinds of equipment: settling tank and settling tank.

The sedimentation tank is used to remove the relatively dense particles such as sand and cinder in the water. The sedimentation tank is generally located in front of the sewage treatment device to prevent other mechanical equipment for treating sewage from being worn out.

The sedimentation tank uses gravity to separate the suspended impurities from the water. It can separate particles with diameters of 20 to 100 µm and above. According to the direction of water flow in the sedimentation tank, it can be divided into three kinds: horizontal flow, radial flow and vertical flow.

① Flat-flow sedimentation tank. The waste water flows from one end of the pool, flows horizontally in the pool, the suspended matter in the water gradually sinks to the bottom of the pool, and the clarified water overflows from the other end.

② radial sedimentation tank. The pool is mostly round, large in diameter, generally more than 20 ~ 30m, suitable for large water treatment plants. After entering the central cylinder through the inlet pipe, the raw water flows radially to the periphery of the sedimentation tank through the hole on the wall of the cylinder and the annular perforated baffle. As the cross section continues to increase, the flow rate gradually becomes smaller, the particles settle down, and the clarified water overflows from its surroundings into the collection tank.

③ Vertical sedimentation tank. The cross-section is mostly circular, but also square and polygonal. Water flows into the pool from the lower mouth of the central pipe, distributes around the entire horizontal section through the barrier of the reflection plate, and slowly flows upward. The particles whose settling speed exceeds the rising velocity are sunk into the sludge hopper, and the clarified water spills out of the pond through the surrounding buried orifice.

In sewage treatment and utilization methods, precipitation (or floating) method is often used as a pre-treatment before other treatment methods. If the biological treatment method is used to treat sewage, it is generally necessary to remove most of the suspended substances through the pre-sedimentation tank in advance to reduce the load during biochemical treatment, and the effluent after biological treatment must still be treated by the secondary sedimentation tank for sludge water separation to ensure the quality of the effluent.

(3) Flotation method. The air into the sewage, and in the form of a small bubble from the water to become a carrier, the relative density of the sewage is close to the water of the small particles of pollution (such as emulsified oil, etc.) attached to the bubble, and with the bubble rise to the surface, and then with a mechanical method of skimming, so that the pollution in the sewage can be separated from the sewage. Hydrophobic substances float easily, while hydrophilic substances do not float easily. Therefore, sometimes in order to improve the efficiency of air flotation, it is necessary to add flotation agents to the sewage to change the surface characteristics of the pollutants, so that some hydrophilic substances are transformed into hydrophobic substances, and then air flotation is removed, this method is called "flotation".

Air flotation requires high dispersion of bubbles and a large amount, which is conducive to improving the effect of air flotation. The stability of the foam layer should be appropriate, which is convenient for the scum to stabilize on the water surface and does not affect the transportation and dehydration of the scum. There are two ways to create a bubble:

1) Mechanical method. The air is generated into tiny bubbles through microporous tubes, microporous plates, and turntables with holes.

2) Pressure dissolved gas method. The air is dissolved in water at a certain pressure, and reaches a saturated state, and then suddenly decompresses, and the supersaturated air escapes from the water in the form of tiny bubbles. At present, the air flotation technology of wastewater treatment mostly adopts pressure dissolved gas method.

The main advantages of the gas flotation method are: the operation capacity of the equipment is better than that of the sedimentation tank, and the solid-liquid separation can be completed in only 15 to 20 minutes, so it occupies less land and has higher efficiency; The sludge produced by the air float method is dry, not easy to decompose, and the surface is scraped, and the operation is convenient. The whole work is to circulate air into the water, increase the submerged oxygen in the water, and have obvious effects on removing organic matter, algal surfactants and odors in the water, and the effluent quality provides favorable conditions for subsequent treatment and utilization.

The main disadvantages of gas flotation are: large power consumption; The workload of equipment maintenance and management increases, and the operation part is often clogged. The scum is exposed to the surface of the water and is susceptible to climatic factors such as wind and rain.

In addition to the above two air flotation methods, the more commonly used methods are electrolysis gas flotation.

(4) Centrifugation. When sewage containing suspended pollutants is rotating at high speed, the centrifugal force of suspended particles (such as emulsified oil) and sewage is different, so as to achieve the purpose of separation. Commonly used centrifugal equipment are cyclone separator and centrifugal separator.