There are many kinds of filters, and the common fast filters with quartz sand as filter material have a long history of use. On this basis, people have developed other types of fast filters from different process angles. On this basis, the French company Deliman developed the V-type filter in 1970s. V-type filter adopts thick quartz sand filter layer with uniform particles; The gas and water without expanding the filter layer are used for backwashing and cleaning the surface to be filtered at the same time; Air cushion is used to distribute air, and special long-handle filter head is used to distribute air and water. It has the characteristics of good effluent quality, high filtration rate, long operation cycle, good backwashing effect, energy saving and convenient automatic management. Therefore, it was widely used in continental Europe in 1970s. At the end of 1980s, it was introduced and used in Nanjing, Xi and Chongqing. Since 1990s, almost all newly-built large and medium-sized water purification plants in China have adopted V-type filter, especially the newly-built water purification plants in Guangdong Province. From September to 1994, our company designed, constructed and installed the V-type filter for the first time in the construction of Shakou Water Plant (500,000 m3/d). After that, we designed and installed the V-type filter. We have helped Gaoming, Zhongshan Xiaolan, Zhongshan Dongfeng, Shunde Longjiang, Sanshui, Guangning, Shantou and Huizhou to design and install V-type filters. In the process of design, construction, installation and automatic control of V-type filter in recent ten years, we have accumulated some practical experience and the following work experience:
First of all, learn and master the structure, working principle and technological characteristics of V-type filter.
Filter is an important link in the water purification process of water plant, and the regeneration of filter capacity is the key to the stable and efficient operation of filter. If better backwashing technology is adopted, the filter can always work in the best working condition, which not only saves water and energy, but also improves the water quality, increases the pollution interception ability of the filter layer, prolongs the working cycle and increases the water output. Advanced air-water backwashing and surface sweeping technology are adopted to regenerate the filtration capacity of V-type filter. Therefore, the filtration cycle of this filter is about 75% longer than that of pure water washing filter, and the sewage interception can be increased by 1 18%, while the backwashing water consumption can be reduced by more than 40% compared with pure water washing filter. When washing the filter with air, the air is forced into the filter layer by the blower, thus improving the filtering performance of the filter from the following aspects:
(1) The addition of compressed air increases the shear force on the surface of the filter material, so that dirt that is not easy to peel off during washing can be peeled off under the high shear force of the sharp rise of bubbles, thus improving the backwashing effect.
(2) After the bubbles move in the filter layer to produce mixing, the filter material particles can continue to swirl and spread, which promotes the circulating mixing of particles in the filter layer, thus obtaining a mixed filter layer with uniform grading, and its porosity is higher than that of the graded filter layer of graded filter material, which improves the filtration performance and thus improves the pollution interception ability of the filter layer.
(3) With the addition of compressed air, bubbles explode in the granular filter material, which intensifies the collision and friction between particles of filter material, and gives full play to the shearing effect on the surface of filter material particles during water washing, thus enhancing the efficiency of water washing and cleaning.
(4) The movement of bubbles in the filter layer reduces the contact resistance between filter particles during water washing, which greatly reduces the strength of water washing, thus saving the energy consumption of water washing.
To sum up, when air and water are backwashed, the ability to peel off dirt and intercept dirt is greatly enhanced due to the fierce action of bubbles. In the actual backwashing of the filter, we observed that when the backwashing time was about 5 minutes, the dirt in the filter layer peeled off more than 95%, so the backwashing effect of the V-type filter was positive. In addition, during backwashing, raw water enters the filter through the small holes at the bottom of two V-shaped tanks opposite to the backwashing drainage tank, which washes the surface of the filter layer, pushes the dirt and impurities washed up by the filter layer to the drainage tank, and washes some places where the horizontal speed is equal to zero, and the floating sediment in these places is precipitated again. In addition, the surface cleaning of the filter accelerates the washing speed of backwashing water, and the raw water is used to feed the water consumption and electric energy after backwashing and filtering, which also saves the washing water. Providing washing water is a special advantage of raw water surface cleaning. In fact, when a filter is backwashed, it also plays a role in preventing other filters from running under the maximum output load.
Second, reasonable selection of design parameters
After understanding and mastering the working principle of the above V-type filter, the designed V-type filter can give full play to its advantages. The structural dimensions required by the process must be strictly guaranteed. Therefore, it is very important to choose reasonable design parameters to design the filtration process. In recent ten years, several V-type filters designed by us have been put into operation, and the actual operation effect is generally good. This proves that the design parameters we have chosen are ideal, which are briefly introduced as follows:
1, adoption of main design parameters
Filter material: seasonable sea sand, preferably quartz sand from the seaside battlefield with strong seawater scouring intensity. The particle size is 0.95 ~ 65438 0.35 mm; Uneven coefficient k80 =1.0 ~1.3; The thickness of the filter layer is1.2 ~1.5m. ..
Filtration speed: 7 ~ 15m/h, sand surface depth1.2 ~1.3m. ..
Backwashing strength: compressed air15 ~161/m2.s; Water recoil is 4 ~ 51/m2.s; The water surface cleanliness is1.5 ~1.8/m2.s. ..
Filter head: QS type long handle filter head is adopted, and the length of filter head is 28.5cm；; There are 36 notches on the filter cover; The upper part of the filter handle has a φ2mm air hole, and the lower part has a slit with a length of 65mm and a width of 1mm; The material is ABS engineering plastic. The filter heads are evenly distributed on the filter plate, with 48 ~ 56 per square meter.
Both the filter plate and the filter beam are reinforced concrete prefabricated parts. The filter plate shall be rectangular or square, but the side length shall not exceed 1.2m, the width of the filter beam shall be 10cm, and the height and length shall be determined according to the actual situation.
2. Determination of filter structure size and elevation.
According to the fluid flow characteristics, in order to ensure the uniform distribution of gas and water in the filter plane during backwashing, the length-width ratio of the filter plane size is better to be slightly larger. Generally: length: width = 4: 1 ~ 3.5: 1 (the width does not include the central gas-water distribution tank, which is generally 0.7 ~ 0.9m). In general, the length of the pool should not be less than11m. The central air-water distribution box of the filter divides the width of the filter into two halves, and the width of each half shall not exceed 4 meters.
In order to ensure that the pressure at any point below the filter plate is equal during backwashing, the air pressed under the filter plate can form an air cushion layer as soon as possible, and a highly suitable space should be left between the filter plate and the bottom of the pool. We usually design the height of the clear water pool under the filter plate to be 0.85 ~ 0.95 meters. This height is enough to make the air fully mixed through the aperture of the filter head and evenly distributed in the whole filtering area, thus ensuring the normal filtering work of the filter and the regeneration effect of the filter.
The water to be filtered passes through the water inlet main canal, two pneumatic rubber valves and a manual gate valve, and then enters the V-shaped groove from two side holes through the overflow weir and flows into the filter screen. We design the middle square hole (represented by W 1) as the water inlet controlled by manual gate valve. This gate valve is normally open (only closed during filter maintenance), and surface cleaning water enters from this square hole through the overflow weir when the filter is backwashed. We designed two square holes with the same size (W2 1 and W22 respectively, W 1=W2), and used pillow-shaped inflatable rubber valve to control water filtration into the square holes. When backwashing the filter, these two holes are blocked by the pillow-shaped inflatable rubber valve. We design the areas of these three water inlets as follows: w1:w 21= w1:w22 =1:3; The flow rate of the water inlet is controlled at 0.40 ~ 0.5m/s; Using these two principles to modify each other, the size of the air inlet is finally determined.
Surface cleaning is realized by spraying from the small hole at the bottom of the V-shaped groove. According to the properties of the jet, in order to make the surface cleaning effect best, the jet is preferably semi-submerged. Therefore, it is very important to determine the center elevation of the small hole at the bottom of the V-shaped groove. According to our experience, the elevation of the center of the small hole is 0.8 ~ 1.2 cm lower than the backwashing water level. We once visited a water plant designed by French company Deliman. The elevation of the center of the small hole they designed was lower than the backwashing water level1.3cm.. When the filter is backwashed, the surface cleaning effect is not as good as that of the filter we designed.
Other aspects of the filter design are basically the same as those introduced in the relevant materials, so I won't repeat them here.
Third, the construction and installation practice
The quality of filter construction and installation is directly related to whether the filter can meet the requirements of process design and run normally after it is completed and put into production. V-type filter has strict requirements for construction and installation: the horizontal error of filter plate should not be greater than 2 mm; The horizontal error between filters shall not be greater than 5 mm; The distance error between beam center and anchor bar is 2mm；; The manufacturing error of plate size is 2 mm; It is required that the levelness error of the weir top of the central drainage channel is not greater than 2mm；; All internal dimensions of the filter must be strictly controlled. Therefore, in order to ensure the construction and installation quality requirements of the filter, in addition to the strict management and control of the civil construction of the whole pool, the most important thing is to strictly control the production and installation of the filter plate and beam. Only when the filter plate and the filter beam are leveled, the filter head will be leveled in essence. We often make prefabricated parts of filter plates and filter beams. In the prefabrication yard, we carefully make filter plates and filter beams with steel molds, steel bars and concrete to ensure the levelness and thickness of a single filter plate and filter beam, and maintain them to ensure good quality. The installation of filter beam is the key to make the filter surface of the whole pool high. We will accurately and smoothly embed the embedded iron piece for installing the filter beam in the bottom of the pool, then weld a short pipe of DN 100 steel on the embedded iron piece, weld a DN80 steel pipe on the embedded iron piece below the prefabricated filter beam, put the DN80 steel pipe into the DN 100 steel pipe, level it with a level, and then weld the pipe into a whole. Then the DN200 pipe is used as the mold, and cement mortar is poured into the mold, so that a protective film is formed on the outer side of DN 100 and DN80 pipes to prevent the steel pipes from rusting and strengthen their supporting strength. On the basis of installing the filter beam, strictly control the levelness of the filter plate with a level meter. The horizontal error of the filter surface of the whole tank is not more than 5 mm We use 905 joint sealant developed and produced by East China Survey and Design Institute of the Ministry of Electric Power (905 mortar prepared according to the ratio of cement: sand: 905 adhesive =11:0.5) to seal the gaps between the filter plates and between the filter plates and the tank wall. The water-tight and air-tight sealing performance is ensured, thus ensuring the success of gas-water backwashing.
Four, automatic control of production operation
The automatic control of V-type filter filtration and regeneration is the guarantee of normal production and operation of the filter. We use a real-time multi-task distributed control system composed of programmable controller and industrial computer (PLC+IPC) to control the filtration and backwashing of the filter.
1, filter control
We installed a water level sensor and a head loss sensor in the corresponding part of the filter. The filtration of the filter is to measure the water level and head loss of the filter through them, and send the water level value and the opening of the filtered water valve to the special module installed in each PLC cabinet. The adjusting module can adjust the opening of the valve to make the filter reach the balance of water inlet and outlet, so as to realize automatic filtration with constant water level and constant filtration speed.
2, backwashing control
The backwashing of a group of filters is controlled by an ordinary PLC. When the filtration reaches the set value of filtration cycle or filter pressure difference (water head), the filter puts forward a backwashing request, and PLC forms a backwashing queue according to the priority order of the filter. Once responding to the filter's request, PLC performs the whole backwashing process. In a group of filter plates, two filters are not allowed to backwash at the same time. When one filter backwashs, the signals of other filters requesting backwashing are stored in the public PLC, and then the filters are backwashed in turn according to the storage order.
When the filter is backwashed, the control process of public PLC is as follows: ① Close the water inlet valve of the water to be filtered; when the water level of the filter drops to the top of the sand washing drainage tank, close the filtered water control valve and open the backwashing drainage valve; (2) Start the blower, open the filter backwashing air valve after 5 seconds, and pre-wash the filter for 1 min; (3) Open the backwashing water valve, start the backwashing water pump, and backwash gas and water for 7 minutes at the same time; (4) Close the backwashing air valve, after 5 seconds, stop the blower, open the air diaphragm valve for exhaust, rinse with clean water for 5 minutes, and then stop the backwashing water pump. After 5 seconds, close the water backwashing valve, then close the backwashing drain valve, open the water inlet valve of the water to be filtered, and the filter tank resumes filtering. The whole backwashing process lasts about 25 minutes.
In addition, PLC can also control the opening of the filter. It determines the opening of the filter according to the water inflow of the filter, and determines the opening and stopping of a filter according to the principle of stopping first.
Verb (abbreviation of verb) Selection of main equipment for V-type filter
The selection of special instruments and pneumatic valves is the key to realize the automatic control of V-type filter. Backwash water valve and air valve for normal operation of V-type filter; We use pneumatic butterfly valves as clean water valves and sewage valves. The water supply company can decide whether to use domestic valves or imported valves according to its actual situation, but it must choose good quality valves. At present, the quality of some domestic manufacturers has reached the world advanced level, not worse than imported valves. It's not expensive. It's worth a try. We think that the pillow pneumatic rubber valve has better filtering effect on the water inlet valve, simple manufacture and reliable action. Other supporting equipment: blower, air compressor, water pump, etc. Domestic products can meet the requirements, there is no need to import.
To sum up, we think that the advantages of V-type filter are the use of homogeneous filter material and advanced gas-water backwashing and surface purging technology. In addition to the application of this technology in newly-built water purification plants, it can also be extended to the transformation of old plants, relying on scientific and technological progress, adopting new science and technology, carrying out technical transformation, giving full play to its maximum potential, and greatly increasing water production in a short time, that is, realizing the water supply industry to "improve water quality, improve water supply safety and reliability, and reduce drug consumption, energy consumption and leakage consumption." A better way. Its main characteristics are: using quartz sand homogeneous filter material with coarse particle size and thick filter layer to intercept and receive pollution and prolong the working cycle of the filter; Gas-water backwashing and surface cleaning make the filter layer not expand or slightly expand; Its water distribution system is a long-handled filter head water distribution system; Through the operation, the automatic control mode of "ordinary flushing PLC+ each filter PLC" is realized. The main design parameters are as follows: the plane size is12m× 7m; ; The design filtration speed is 8.04m/h; The density of filter heads is 54/m2; The thickness of filter material layer is1.2 m.
When the V-type filter runs in automatic mode, PLC controls the constant liquid level of the filter by controlling the opening of the filtered water outlet door, and starts backwashing when one of the following conditions is met: the running time of the filter reaches the set value; The loss of filter head reaches the set value; Flushing command from PLC—XBT keyboard in the console or monitoring computer in the central control room.
The backwashing mode of V-type filter has its own characteristics, and the backwashing is divided into three processes: ① air pre-scrubbing [a blower, air supply 1 min, Q air = 22.5 m3/(m2 h)]; (2) Gas-water mixed washing (two blowers, one washing water pump, washing for 6 min, Q gas = 55m3/(m2 h), Q water = 7.5m3/(m2 h)); (3) Water rinsing (two rinsing pumps, rinsing for 6 min, Q water =15m3/(m2 h)); The constant horizontal cleaning intensity Q water is about 5.2m3/(m2 h).
During the operation and management, it was found that the elevation of the horizontal surface of the V-shaped pool at the inlet of the filter was too low, and the surface sweeping intensity was slightly low, which led to poor horizontal sweeping effect and floating and retention of foam scum. The filter head is blocked, which makes cleaning very inconvenient; The failure of filter adjustment often occurs. The main countermeasures are to clean the water level gauge and filter head loss meter regularly, maintain the sensitivity and properly control the stability of filter inlet, which increases the workload of filter maintenance and management.
In addition, the direct filtration performance of V-type filter in low turbidity period (raw water turbidity < < 20 NTU) and the influence of polyaluminum dosage on direct filtration were investigated in production. The results show that reasonable control of PAC dosage will produce the following effects: ① the increase of filtration head loss will be slowed down, and the curve of head loss with time is approximately flat, which can effectively prevent premature plugging of filtration layer; (2) Increase the penetration depth of floc in the filter layer, and give full play to the advantages of homogeneous filter material and deep filter bed of V-type filter.
This article comes from: Environmental Science and Technology Network (www.cnjlc.com) Detailed source reference: jlc.com/h2o/3/20070706626.html