precipitation method is one of the most basic methods in sewage treatment. it uses the density difference between suspended particles and water in water to produce a sinking effect under the action of gravity to achieve a solid-liquid separation process.
sedimentation tank classification
depending on the nature of the water and the degree of treatment required, the precipitation treatment process can be a single step in the overall water treatment process, and it can also be the only treatment method. in a typical water treatment plant, the precipitation method can be used in the following areas:
grit chamber (sewage pretreatment)
the grit chamber is often used as a pretreatment to remove inorganic particles that tend to settle in water. the grit chamber is a pretreatment unit that uses physical methods to separate and separate sand from water. its function is to separate out particulate matter with a relative density of more than 1.5 and a particle size of 0.2 mm or more from the water, mainly including inorganic sand and gravel. and a small amount of dense organic particles such as nuclear skin, seed and so on.
the grit chambers are generally installed before lifting equipment and processing equipment to protect the pumps and pipes from wear, prevent the clogging of subsequent water structures and the reduction of the volume of the sludge treatment structures, and at the same time reduce the inorganic components in the activated sludge and improve activated sludge activity.
counterflow grit chamber
the advection grit chamber is actually a channel that is wider and deeper than the inflow channel and the outflow channel. when the water passes through, the flow rate decreases due to the increase of the cross-section of the water, and the inorganic particles entrained in the water sink under the action of gravity. in order to achieve the purpose of separating inorganic particles in water.
aerated grit chamber
the aerated grit chamber introduces air on one side of the rectangular basin. it is a swirling movement of water. the flow rate gradually decreases from the periphery to the center. the sand is separated from the water in the sand trap at the bottom of the tank, organic matter in the water and from the sand. the washed-out sludge is still in a suspended state, with the flow of water entering the subsequent treatment structure. the advantage of the aerated grit chamber is that the sand removal efficiency is stable and less affected by changes in the flow rate of incoming water. the residence time of the aerated grit chamber is generally 1-3 minutes. if both have the effect of pre-aeration, the depth of the pool can be extended. the residence time is 15-30 minutes.
primary sedimentation tank (primary wastewater treatment)
the primary sedimentation tank can remove suspended solids from the water more economically and efficiently, and at the same time remove part of the suspended organic matter, so as to reduce the organic load of the subsequent biological treatment structure. sometimes the primary sedimentation tank is also used separately, and the water is discharged after being treated in a primary treatment.
the primary sedimentation tanks are generally installed after the sedimentation tanks of the sewage treatment plant and before the aeration tanks. the primary role of the primary sedimentation tanks is to remove solid suspended particles in the sewage to reduce the organic load of biological treatment and improve the activated sludge. microbial activity. wastewater passes through the grid to trap large suspended and suspended organic matter, and after passing through the grit chamber to remove suspended particles with a density greater than 1.5g/cm3, there are still many suspended particles with a slightly lower density or smaller particle size. mainly organic.
when the primary sedimentation tank is used to treat municipal sewage, the sedimentation time is generally 1.5-2 hours, the removal rate of influent bod5 can reach 20%-30%, and the removal rate of suspended solids ss can reach more than 50%.
1, operation management considerations
(1) determine the scraping method and the length of the scraping cycle according to the form of the primary sedimentation tank and the form of the scraper. avoid depositing sludge that causes excessively long residence time for the sludge, or too much scraping or muddy sludge to disturb the subsiding sludge.
(2) the primary sedimentation tank generally adopts intermittent sludge discharge. it is necessary to pay attention to summarizing experience and manually mastering the number of sludge discharges and sludge discharge time based on experience. when continuous mud drainage is used, attention should be paid to the flow of sludge discharged and the color of the discharged sludge.
(3) the inspection is to observe whether the water output from each pool is even, and whether the outflow from the water stream is even or not, whether the sluice is blocked by scum, and timely adjustment or repair.
(4) during the inspection, observe whether the scum in the scum bucket can be discharged smoothly, and whether the scum scraper and the scum bucket baffle fit properly, and timely adjust or repair.
(5) during the inspection, pay attention to the sound of scraping mud, scraping slag, and mud discharge equipment, and check whether the parts are loose, and adjust or repair them in time.
(6) sludge pipes should be flushed at least once a month to prevent silt, grease, etc. from causing fouling in the pipelines, especially at the valves. the number of flushes should also be increased in the winter. the primary sedimentation tank is evacuated each year for thorough cleaning and inspection.
(7) according to the provisions of the primary sedimentation tank routine testing items for timely analysis and testing, especially the ss and other important items to be compared in a timely manner to determine whether the ss removal rate is normal, if it is necessary to take the necessary rectification measures.
(8) routine testing items for primary sedimentation tanks: water temperature, ph value, codcr, bod5, ts, ss, solids content and volatile solids content of the inflow and outflow water.
2. causes of the effluent of the primary sedimentation pond containing fine suspended particles
in order to give full play to the role of primary sedimentation tanks, the excess sludge from many wastewater treatment plants is discharged from primary sedimentation tanks. therefore, the primary suspended sediments have small suspended particles in the effluent: hydraulic loading or long-term overloading; because the short water stream 2 has reduced the residence time, the flocs have been in the water stream before the sedimentation; the activated sludge in the aeration tank is excessively aerated, and the sludge itself is oxidized to disintegrate; precipitation particles.
the solutions corresponding to the above reasons include: increasing the adjustment pool, evenly distributing the influent hydraulic load; adjusting the inhomogeneity of the inflow and outflow facilities, alleviating the impact of the impact load, and overcoming the short-flow condition; adjusting the operating parameters of the aeration tank, in order to improve the sludge flocculation performance, such as timely replenishment of nutrients in the absence of nutrients, the sludge ageing caused by sludge aging should be shortened sludge age, aeration should be adjusted for excessive aeration, and flocculant should be added to improve certain difficult-to-precipitate suspended particles. the sedimentation performance; the digester, concentrated pool supernatant uniformly into the primary sedimentation tank, eliminating its negative impact; so that the secondary sedimentation tank sludge evenly into the primary sedimentation tank, eliminating the impact of excess sludge return.
secondary sedimentation tank (solid-liquid separation after biological treatment)
sedimentation tank is a treatment structure that uses gravity sedimentation to remove suspended particles with higher density than water from water. it is one of the most widely used treatment units in water treatment and can be used for primary treatment of water, post-treatment of biological treatment, and depth. deal with. the secondary sedimentation tank is set after the aeration tank, before the deep treatment or discharge.
according to the direction of flow, sedimentation tanks can be classified into three types: advection flow, radial flow and vertical flow. the sedimentation tank consists of five parts, the influent zone, the sedimentation zone, the buffer zone, the sludge zone and the effluent zone. the role of the influent zone and the outflow zone is to make the water flow evenly through the sedimentation tank, avoiding short flow and reducing the adverse effects of turbulence on the sedimentation, reducing the deadwater zone and increasing the volumetric utilization rate of the sedimentation tank; the precipitation zone is also called clarification. the zone, which is the working area of the sedimentation tank, is the area where the sedimentable particles can separate from the water; the sedimentation area is the area where the sludge is stored, concentrated, and discharged; and the buffer zone is the water layer area that separates the sedimentation area and the sludge area. the precipitated particles do not float again due to the agitation of the water stream.
advection type secondary sedimentation tank
the surface of the advection sedimentation tank is generally rectangular in shape. after the water enters the sedimentation area through energy dissipation and rectification in the influent area, the water flows slowly and horizontally. the suspended solids in the water gradually sink to the bottom of the pool. the sedimentation area overflows the overflow and passes through the outfall. out of the pool. the basic requirements are as follows:
(1) the length of the advection sedimentation tank is mostly 30-50m, and the tank width is usually 5-10m. the effective depth of the sedimentation area generally does not exceed 3m, and is mostly 2.5-3m. in order to ensure the even distribution of water in the pool, the aspect ratio is generally not less than 4 and the length to depth ratio is 8-12.
(2) when using mechanical scraping mud, a sludge bucket is provided at the influent end of the sedimentation tank. the vertical sludge bucket slope at the bottom of the tank can not be less than 0.01, generally 0.01-0.02. the speed of the scraper cannot exceed 1.2m. /min, generally 0.6-0.8m/min.
(3) the horizontal flow rate is the speed at which only the water flows in the tank. when the flat sedimentation tank is used as the primary settling tank, the maximum horizontal flow rate is 7mm/s and the surface load is 1-3m3/(m2*h); when it is used as the secondary settling tank the maximum horizontal flow rate is 5mm/s.
(4) the inlet should have rectification measures. common inflow methods include overflow weir-perforated rectification wall (board) type, bottom hole inflow-baffle combination, submerged hole inflow-baffle combination and submerged hole inflow-perforated rectification. wall (board) modular four. when using a perforated rectification wall, the total area of the openings in the commutating wall is 6%-20% of the cross-section, the flow rate at the orifice is 0.15-0.2m/s, and the orifice should be used as a divergent shape.
(5) baffles shall be provided at the entry and exit points, which are 0.1-0.15m above the water surface. the depth of submerged entry baffle should not be less than 0.25m, generally 0.5-1m; the depth of submerged exit baffles is generally 0.3-0.4m. the baffle at the inlet is 0.5-1.0m away from the inlet and the outlet baffle is 0.25-0.5m away from the outlet faucet.
(6) when the advection-type sedimentation tank has a small volume, a perforated pipe can be used to drain mud. the perforated pipes are mostly arranged in the collecting bucket and can also be arranged on the bottom of the horizontal pool. when the sedimentation tank uses multiple buckets of mud, the mud bucket has a square or approximately square rectangle, and the number of rows generally cannot exceed two rows. large-scale sedimentation tanks with general advection type are generally equipped with a scraper, scraping bottom sludge from the outlet end to the sludge bucket on the inflow end, and scraping the scum to the slag collection tank at the outlet end.
(7) the height of the buffer layer is 0.5m for the non-mechanical sludge in the advection-type sedimentation tank. when using mechanical sludge, the upper edge of the buffer layer shall be 0.3m higher than the mud scraper.
vertical type secondary sedimentation tank
the vertical sedimentation tank has a round or square shape. the sewage enters the pool from the inlet of the central tube, and is distributed on the horizontal section through the reflection plate's blockage and slowly flows upwards. the suspended particles with the sedimentation velocity higher than the rising velocity of the water sink into the sludge hopper, and the supernatant is overflowed from the outlet orifice around the pool top to the outside of the pool. the basic requirements of the vertical sedimentation tank are as follows:
(1) in order to ensure the bottom-up vertical flow of the water flow in the tank and prevent the flow of water from being in a radial flow state, the ratio of the diameter of the round pool or the length of the square side to the effective depth of the sedimentation area is generally not more than 3, and the diameter of the pool is generally 4 - 7m, maximum not more than 10m. when the diameter of the round pool or the side of the square pool is less than or equal to 7m, the effluent of the precipitated effluent will flow along the periphery; when the tangential ≥7m, the radiant collection branch may be increased.
(2) the upflow velocity of the water flow in the vertical flow sedimentation tank is 0.5-1mm/s and the precipitation time is 1-1.5h. the flow velocity in the center tube should generally be greater than 100 mm/s, with a bell or reflector plate at the lower outlet. the bottom of the reflection plate is at least 0.3m away from the mud surface, the diameter and height of the bell mouth are 1.35 times the diameter of the center tube, the diameter of the reflection plate is 1.3 times the diameter of the bell mouth, and the inclination angle between the surface of the reflection plate and the horizontal plane is 17°.
(3) the height of the gap between the bottom of the bellmouth and the surface of the reflector is 0.25-0.5m. the velocity of the water in the crevice when used as the primary settling pond should not exceed 30mm/s. the velocity of the water in the gap when used as the second settling pond should not be greater than 20mm/s.
(4) the inclination of the conical storage hopper is 45°-60°, the diameter of the sludge discharge pipe can not be less than 200mm, the distance between the mud discharge nozzle and the bottom of the tank is less than 0.2m, and the upper end of the open mud discharge pipe can not exceed 0.4. m. the scum baffle is submerged to a depth of 0.3-0.4m, 0.1-0.25m above the water surface and 0.25-0.5m away from the sump.
radial secondary sedimentation tank
the flow state of the water flow in the radial flow sedimentation tank is a radial flow type, so the sewage enters the sedimentation tank from the center or the periphery. the inlet pipe of the center influent radial flow sedimentation tank is suspended below the bridge or buried in the concrete of the tank floor. the sewage first enters the center pipe of the tank body, and then enters the sedimentation tank, and is rectified after passing through the rectifier plate around the central pipe. evenly radiating and flowing around, the supernatant overflows through the outlet sluice located around the sedimentation tank, the sludge settles to the bottom of the pool, and there is a sludge scraper or hanging suction machine to hang the mud bucket in the center of the sedimentation tank. evacuate again with gravity or a pump.
peripheral influent radial flow sedimentation tank the influent canal is arranged around the sedimentation tank, and the supernatant flows out through the outlet enthalpy arranged around or in the middle of the sedimentation tank. the sludge is discharged in the same way as the center influent radial sedimentation tank. .
the basic requirements are as follows:
(1) the arrangement of the inlet and outlet water includes three forms: water outlet around the center inlet, outlet water from the surrounding inlet center, and outlet water around the surrounding inlet.
(2) the scraper's rotation speed is generally 1-3r/h, and the peripheral scraper's linear speed cannot exceed 3m/min, usually 1.5m/min.
(3) the radial-flow sedimentation tank of the surrounding influent is highly efficient, and the surface load is increased by about one time compared with the radial-flow sedimentation tank of the center water inlet and the surrounding outlet water.
settlement tank operation precautions:
(1) check that the corrosion protection of metal components such as a scraper or suction scraper is in good condition and that it is operating in a sewage condition.
(2) observe whether there is water leakage after settling in the sedimentation tank. observe the sedimentation observation. check whether there is uneven sedimentation in the sedimentation tank. observe the outflow condition of the outflow triangle to find the settlement of the sedimentation tank.
(3) check the load operation of the scraper or suction scraper. the main observation is whether the vibration, noise, and driving motor operating conditions are normal, and whether the line speed, angular velocity, etc. are within the set range.
(4) test and determine if the scraping, suctioning, and scraping functions of the scraper or suction scraper are normal. observe whether the scum on the surface of the sedimentation tank can be discharged in time, and observe the suction and scraping effect when the mud discharge amount changes within a certain range.
(5) determine the inlet and outlet ss, respectively, and verify that the sedimentation tank's function under the designed inlet load meets the design requirements. for example, whether the return sludge concentration in the secondary sedimentation tank and the sludge concentration in the primary sedimentation tank are within a reasonable range.
(6) check whether the automatic control system related to the sedimentation tank can be normally linked. for example, the automatic shut-down function of the initial settling tank and the automatic discharge of excess sludge or scum function from the second settling tank according to the mud position can be obtained.
article source: new environmental protection classroom
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