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Chapter 13 Rapid filtration
13.1 Elements of a rapid sand filter A rapid sand filter (also called a rapid gravity sand filter) can be located before a constructed treatment wetland. The mechanisms of filtration in a rapid sand filter are essentially the same as those in a slow sand filter, except that the biological processes are minimised. This results from the much shorter filter run times between cleanings, preventing the establishment of mature biological growths. When rapid sand filters were initially introduced, the filtration rates (loading rates) were between 3 and 4 m3 /m2 /h. At these relatively high rates of filtration, the following observations were made: • Coagulation was required in most cases to prevent the impurities from being drawn deep into the filter bed; • Surface cleaning was no longer adequate, because impurities were drawn deep into the sand bed; and • Clogging occurred much more rapidly with filters requiring cleaning at intervals between 2 and 3 d instead of intervals between 30 and 100 d for slow sand filters. For rapid sand filtration to be acceptable, it was necessary to develop a method of cleaning the full depth of the filter, rapidly and economically. The method adopted was to remove the impurities from the sand bed by a reverse flow of water, either preceded or accompanied by some form of agitation to loosen the impurities from the sand grains. Moreover, a rapid sand filter comprises the following construction elements: • A sand bed in which filtration occurs; • A support for the sand bed; • An underdrain system to remove filtered water, and to admit backwash water (and air for agitation, if used); • An inlet for water;
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Wetland systems to control urban runoff
• An outlet for used wash water; and • Means for controlling the flow through the filter. In normal operation, the inlet valve and filtered water valve are open, and all other valves are closed. Water enters through the inlet valve, passes down through the sand and subsequently the underdrain system, and out through the filtered water valve. To start a backwash cycle, using air scour followed by water scour, the inlet valve is closed, but filtering is allowed to continue for a time to avoid excessive loss of settled water. After a reasonable drawdown period during which some of the settled water in the filter chamber is filtered instead of wasted, the filtered water valve is closed and the waste valve opened, thus dumping into the drain that portion of the settled water, which was above the level of the backwash collecting trough. The air valve is then opened to admit air under pressure into the underdrain system where it is distributed evenly underneath the filter. The air bubbling upwards through the sand bed causes agitation, which loosens the impurities from the sand grains. At the end of the air scour, the air valve is closed and the backwash valve opened to admit water to the underdrain system at a rate sufficient to wash the sand. The water passes upwards through the sand and carries with it impurities into the backwash collection troughs, and hence into the drain. In order to finish the backwash cycle, the backwash and waste valves are closed, and the inlet valve is opened. When the water has reached a satisfactory level in the filter basin, the filtered water valve is again opened and filtering is resumed.
13.2 Sand bed The sand bed in a conventional rapid sand filter consists of clean silica sand (depth between 0.60 and 0.75 m). The effective size of the sand used is between 0.9 and 1.0 mm. This size is required, because it is necessary to use a coagulant aid, and water is cold in winter. This arrangement produces a tough floc. Anthracite is often used as a filter media in the USA. The crushed anthracite has a density of about 1400 kg/m3 (compared with silica sand, which has a density of 2650 kg/m3 ), and it does therefore not require high backwash rates to achieve fluidisation of the filter bed. When a sand bed is washed, there is a tendency for stratification to occur, with the larger particles migrating to the bottom of the bed, and the smaller particles migrating to the top. In normal operation, the water is therefore filtered first through the fine sand and later through the coarse sand. The length of a filter run depends on the amount of storage voids available in the sand bed for retention of impurities. If the storage space in the fine sand layer becomes clogged before the
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storage space in the coarse sand layer (located deeper in the filter), the length of run is likely to be shorter than ideal. Dual media filters aim to overcome this problem. If anthracite with a diameter of 2 mm is placed over sand with a diameter of 0.6 mm, for example, the anthracite, being lighter, will form a surface layer after backwashing, so that the water will be filtered through the coarse anthracite and subsequently through the finer sand, and ideal conditions are nearly approached. Mixed media filters use three different materials. The bottom layer of the filter consists of fine garnet (density = 4200 kg/m3 ), the middle layer of silica sand (2650 kg/m3 ) and the upper layer of coarse anthracite (1400–1700 kg/m3 ). The three materials are not so closely graded that they form separate layers, so mixing takes place, giving a gradual gradation of void sizes from large voids near the surface to fine voids near the bottom. The sand support bed requires a layer of coarse garnet sand to prevent the fine garnet sand from penetrating the gravel bed and distributors. The sand support commonly consists of layers of gravel placed over the filter compartment floor. The purpose of this support is threefold: • To provide uniform drainage conditions for removing filtered water from the sand bed; • To prevent sand from entering the underdrainage system; and • To assist in the even distribution of backwash water. In earlier filters, the bottom layer of gravel consisted of material with a diameter between 50 and 70 mm, but the trend nowadays is towards smaller gravel in the bottom layers; a maximum of between 15 and 25 mm is often used. The total thickness of gravel is between 300 and 450 mm for many installations. The uppermost layer of the gravel must prevent the intrusion of the sand; it can consist of coarse sand with an effective diameter between 1 and 2 mm. With mixed media filters, the uppermost layers of the support consist of (high-density) coarse sand. Much attention should be given to the design of the gravel layers to ensure even backwash flow, effective cleaning of the sand and freedom from gravel displacement.
13.3 Underdrain system The underdrain system is hydraulically designed to carry the backwash water. The backwash water flow rates through sand beds in different systems range from about 6 to 16 l/s/m2 , whereas filtering rates are only between 1.3 and 4.1 l/s/m2 . The backwash water and the air for scouring the sand are distributed as evenly as practically possible over the full extent of the sand bed.
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Wetland systems to control urban runoff
Where the distribution nozzles are used as controls, the flow distribution will be uneven unless the combined head loss through the nozzles, gravel bed and fluidised sand to a depth equal to the original depth of the sand bed is a monotonic increasing function of the upflow rate. The total head loss in the distribution channel should be ≤10% of the nozzle head loss, so that the backwash flow rates through the nozzles should not vary by ≥5% as a result of friction losses in the underdrains. Too much coarse gravel in a gravel bed will permit free horizontal movement of backwash water in the space between the nozzles and the sand bed, and therefore partially negate the distribution value of the nozzles. Where air scour is used in conjunction with a drainage system, which uses nozzles, the nozzles are specially designed to distribute the air evenly. These nozzles are fitted with hollow stems, which project down into the laterals or into the plenum. A small air control orifice is drilled in the stem of each nozzle just below the filter floor. The pressure difference, which can be sustained across this orifice, is dictated by the length of the stem below the hole. The greater the allowable pressure difference, the less is the effect of extraneous disturbing influences on the air flow rate. A hood over the top of the nozzle can help to prevent water from entering the top of the stem and interfering with the air distribution. During the air scour cycle, air is introduced into the underdrain system; it escapes through the nozzles into the filter. If water and air are to be used simultaneously for filter cleaning, care must be taken to ensure that the necessary water can be distributed to the various parts of the filter in such a way that the height of the waves in the water surface is small in comparison to the head used for forcing the air through the control orifice (otherwise, uneven air scour will result). This condition can best be attained in a plenum distribution system, particular care being taken to dissipate the energy of the incoming water, which could cause excessive waves and turbulence. For the low head loss arrangement, backwash water can freely enter the lower gravel layers. The head loss, which controls the flow distribution, occurs in the upper gravel layers. If air scour is required, air is supplied through a separate layer of perforated air pipes set in one of the gravel layers. The washwater is forced up through the filter bed and emerges laden with impurities removed from the sand. The troughs to collect the dirty water are placed as low as practically possible, in view of the need to avoid excessive disturbance of the hydraulic flow pattern in the expanded sand bed. The troughs discharge the water into the gullet, which is the main collection channel. If it is expected that some of the displaced impurities may be too heavy or may settle too rapidly to be carried up into washwater collection troughs. The gullet wall may be used as a weir. The depth of water over the sand during the backwash is kept to a minimum, so that the horizontal velocity thus induced may
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carry the impurities over into the gullet. This is known as a cross-wash system. In order to intensify the horizontal velocity, water is sometimes introduced at the surface of the sand on the side opposite the gullet. Backwash procedures are equally as important as the design of the underdrain, sand support and sand bed in planning a system, in which the sand is thoroughly and evenly washed, without the occurrence of mud balls and gravel movement. A filter backwash is started in response to one or more of the following conditions: excessive head loss, effluent turbidity or run time. In most installations, the head loss criterion is the most frequent reason for backwash initiation; the usual head loss limit is set at about 2 m. If the floc in the water is too weak, it sometimes happens that it is not retained in the filter bed until the desired maximum head is attained, and a turbidity breakthrough occurs. In conditions where the pretreatment by clarification is very good, or the followthrough rates are low, the floc trapped in the sand bed may become so well gelled with time that thorough washing would be difficult, if the operator was to wait until the usual maximum head loss was reached. In such a case, the time limitation would control the initiation of backwashing. Common practice has been to use an air scour method in which air is bubbled through the sand bed at a rate between 5 and 15 l/s/m2 for a period of about 300 s (to let friction dislodge the impurities from the sand grains), followed by a low to medium-rate water backwash at 7–10 l/s/m2 until the water becomes clear after about 600 s. A recent trend is to use a period of combined water and air scour during which the air loosens the impurities, while a low-rate water backwash of about 6 l/s/m2 carries them to the surface and prevents their penetration, which sometimes occurs with air scour only. This period is then followed by a medium-rate backwash at a rate between 9 and 12 l/s/m2 to clear out the loosened impurities. Satisfactory combined water and air scour can be achieved, only if particular attention has been paid to the design of the water and air distribution systems. In hot climates, where the raw water may be polluted with nutrients, the growth of algae in the sedimentation and filter basins may be a problem. This can be overcome by chlorination of the water before sedimentation. The viscosity of the warmer water found in hot climates is lower than that in temperate climates, and therefore the backwash rate needed for efficient filter cleaning is greater than that with cooler water. The flow control for rapid sand filters is achieved in three different ways. Most systems include some means of automatic flow control; control valves operated by signals from level-sensing or flow-sensing elements. Care should be taken to avoid control conditions, which lead to controller instability, such as ‘hunting’ caused by continual over-correction. Flow control systems in water treatment plants are usually operated hydraulically or pneumatically. The damp conditions often occurring in filter control
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Wetland systems to control urban runoff
galleries are not conducive to the reliable long-term operation of electrical equipment. In a treatment plant equipped with downstream flow control, a flow measuring device (e.g., venturi, orifice or weir) is used to provide a signal, which operates the flow control valve. The control system adjustment may be set by hand, so that the rate of flow is kept close to a pre-determined rate. The plant inflow control valve is then operated by a device, which senses the level in the channel feeding the filters. The efficiency of turbidity removal in a filter is greatly reduced by sudden changes in flow. The total head loss across the filter and its control valve is equal to the difference between the water level in the filter chamber and the hydraulic grade level downstream of the control valve. The flow is controlled by motion of the filter control valve, which automatically adjusts to compensate for changes in the resistance to flow in the filter bed and, because the filter bed clogs slowly, only a relatively slow movement is required. This allows the valve action to be well damped, so that ‘hunting’ will not occur, and sudden flow variations may be avoided. With this control system, there is a sudden change of the flow through the pretreatment portion of the plant when a filter is taken out of service for backwashing, and an even greater change as the filter chamber is refilled after backwashing. These sudden changes of flow can cause a marked deterioration in the settlement efficiency of any clarifier. The system of upstream flow control with flow splitting avoids shock loading on the pre-treatment units. The plant inflow can either be set to a given flow rate or automatically controlled by the demand for water (perhaps from the level in the filter water storage). The flow to each filter is controlled by a flow-splitting device, through which the flow rate is a function of the level in the distribution channel. In this way, the flow from the pre-treatment section is split equally among all operating filters. A filter that is relatively clean can filter water efficiently at a higher flow rate than one in which clogging with impurities is well advanced. A filter, which is starting to pass turbid water, can continue to produce acceptable water for a time, if the flow rate is reduced. For some installations, it would appear that the best use could be made of a filter, if a high flow rate could be applied while it is relatively clean, with the rates of flow being gradually reduced as the filter becomes clogged. This is what happens if all filters are subjected to a common head loss. The total head loss through each filter, together with its underdrain and flow-limiting orifice, is equal to the difference in head between the influent header and the common effluent weir. Filters are sometimes backwashed in a fixed sequence. The time for backwashing the next filter in the sequence is judged from the rising level of the water in all the filter basins. The improvement in performance, coupled with the absence of
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automatic control equipment, makes this filtration control ideal for use in countries with limited technology.
13.4 Hydraulics of filtration The hydraulics of filtration are an important area for design and operation considerations. Accurate prediction of head loss and bed expansion during backwashing is important. Otherwise, filter design may prove to be inaccurate, and the sand in the bed may also be lost. After the first backwash, rapid (gravity) sand filters will stratify (or be stratified by design in the case of multi-media filters). Each layer in the filter will consist of different sized particles. After a filter has been in operation for a period of time, the head loss builds up as a result of filter clogging. When the head loss reaches an unacceptable level, the filter is stopped and backwashing is commenced. Unfortunately, backwashing of vertical-flow constructed wetlands is virtually impossible. However, the degradation of rhizomes results in hollow passages where liquid can rapidly pass through. This counteracts clogging, and makes backwashing unnecessary. When a bed of uniform particles is subjected to backwashing, the bed just begins to open when the backwash velocity reaches a critical value. At this stage, the effective weight of the (submerged) particles is exactly balanced by the upward drag on the particles resulting from the upflow velocity. As the velocity increases, the bed opens up further (porosity increases). This increase in velocity does not improve the cleaning action, but is thought to be important in allowing sufficient open space for trapped suspended matter to be washed away. For a stratified bed, expansion takes place successively for each media type. The surface strata are expanded at a lower rate of backwash than deeper ones. The bed is fully expanded when the upflow (backwashing) velocity of a filter equals the critical velocity for the largest particles.
13.5 Summary of rapid sand filtration Rapid (gravity) sand filters operate at rates some ten times those of slow sand filters. It follows that impurities are drawn deep into the bed. Hence, cleaning is automated and hydraulic (i.e. not labour intensive). The rapid sand filter takes up a relatively small land area (in comparison to a slow sand filter or wetland), the water requires pre-treatment and the operator skill level required is high.
Chapter 13 Rapid filtration
13.1 Elements of a rapid sand filter A rapid sand filter (also called a rapid gravity sand filter) can be located before a constructed treatment wetland. The mechanisms of filtration in a rapid sand filter are essentially the same as those in a slow sand filter, except that the biological processes are minimised. This results from the much shorter filter run times between cleanings, preventing the establishment of mature biological growths. When rapid sand filters were initially introduced, the filtration rates (loading rates) were between 3 and 4 m3 /m2 /h. At these relatively high rates of filtration, the following observations were made: • Coagulation was required in most cases to prevent the impurities from being drawn deep into the filter bed; • Surface cleaning was no longer adequate, because impurities were drawn deep into the sand bed; and • Clogging occurred much more rapidly with filters requiring cleaning at intervals between 2 and 3 d instead of intervals between 30 and 100 d for slow sand filters. For rapid sand filtration to be acceptable, it was necessary to develop a method of cleaning the full depth of the filter, rapidly and economically. The method adopted was to remove the impurities from the sand bed by a reverse flow of water, either preceded or accompanied by some form of agitation to loosen the impurities from the sand grains. Moreover, a rapid sand filter comprises the following construction elements: • A sand bed in which filtration occurs; • A support for the sand bed; • An underdrain system to remove filtered water, and to admit backwash water (and air for agitation, if used); • An inlet for water;
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• An outlet for used wash water; and • Means for controlling the flow through the filter. In normal operation, the inlet valve and filtered water valve are open, and all other valves are closed. Water enters through the inlet valve, passes down through the sand and subsequently the underdrain system, and out through the filtered water valve. To start a backwash cycle, using air scour followed by water scour, the inlet valve is closed, but filtering is allowed to continue for a time to avoid excessive loss of settled water. After a reasonable drawdown period during which some of the settled water in the filter chamber is filtered instead of wasted, the filtered water valve is closed and the waste valve opened, thus dumping into the drain that portion of the settled water, which was above the level of the backwash collecting trough. The air valve is then opened to admit air under pressure into the underdrain system where it is distributed evenly underneath the filter. The air bubbling upwards through the sand bed causes agitation, which loosens the impurities from the sand grains. At the end of the air scour, the air valve is closed and the backwash valve opened to admit water to the underdrain system at a rate sufficient to wash the sand. The water passes upwards through the sand and carries with it impurities into the backwash collection troughs, and hence into the drain. In order to finish the backwash cycle, the backwash and waste valves are closed, and the inlet valve is opened. When the water has reached a satisfactory level in the filter basin, the filtered water valve is again opened and filtering is resumed.
13.2 Sand bed The sand bed in a conventional rapid sand filter consists of clean silica sand (depth between 0.60 and 0.75 m). The effective size of the sand used is between 0.9 and 1.0 mm. This size is required, because it is necessary to use a coagulant aid, and water is cold in winter. This arrangement produces a tough floc. Anthracite is often used as a filter media in the USA. The crushed anthracite has a density of about 1400 kg/m3 (compared with silica sand, which has a density of 2650 kg/m3 ), and it does therefore not require high backwash rates to achieve fluidisation of the filter bed. When a sand bed is washed, there is a tendency for stratification to occur, with the larger particles migrating to the bottom of the bed, and the smaller particles migrating to the top. In normal operation, the water is therefore filtered first through the fine sand and later through the coarse sand. The length of a filter run depends on the amount of storage voids available in the sand bed for retention of impurities. If the storage space in the fine sand layer becomes clogged before the
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storage space in the coarse sand layer (located deeper in the filter), the length of run is likely to be shorter than ideal. Dual media filters aim to overcome this problem. If anthracite with a diameter of 2 mm is placed over sand with a diameter of 0.6 mm, for example, the anthracite, being lighter, will form a surface layer after backwashing, so that the water will be filtered through the coarse anthracite and subsequently through the finer sand, and ideal conditions are nearly approached. Mixed media filters use three different materials. The bottom layer of the filter consists of fine garnet (density = 4200 kg/m3 ), the middle layer of silica sand (2650 kg/m3 ) and the upper layer of coarse anthracite (1400–1700 kg/m3 ). The three materials are not so closely graded that they form separate layers, so mixing takes place, giving a gradual gradation of void sizes from large voids near the surface to fine voids near the bottom. The sand support bed requires a layer of coarse garnet sand to prevent the fine garnet sand from penetrating the gravel bed and distributors. The sand support commonly consists of layers of gravel placed over the filter compartment floor. The purpose of this support is threefold: • To provide uniform drainage conditions for removing filtered water from the sand bed; • To prevent sand from entering the underdrainage system; and • To assist in the even distribution of backwash water. In earlier filters, the bottom layer of gravel consisted of material with a diameter between 50 and 70 mm, but the trend nowadays is towards smaller gravel in the bottom layers; a maximum of between 15 and 25 mm is often used. The total thickness of gravel is between 300 and 450 mm for many installations. The uppermost layer of the gravel must prevent the intrusion of the sand; it can consist of coarse sand with an effective diameter between 1 and 2 mm. With mixed media filters, the uppermost layers of the support consist of (high-density) coarse sand. Much attention should be given to the design of the gravel layers to ensure even backwash flow, effective cleaning of the sand and freedom from gravel displacement.
13.3 Underdrain system The underdrain system is hydraulically designed to carry the backwash water. The backwash water flow rates through sand beds in different systems range from about 6 to 16 l/s/m2 , whereas filtering rates are only between 1.3 and 4.1 l/s/m2 . The backwash water and the air for scouring the sand are distributed as evenly as practically possible over the full extent of the sand bed.
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Wetland systems to control urban runoff
Where the distribution nozzles are used as controls, the flow distribution will be uneven unless the combined head loss through the nozzles, gravel bed and fluidised sand to a depth equal to the original depth of the sand bed is a monotonic increasing function of the upflow rate. The total head loss in the distribution channel should be ≤10% of the nozzle head loss, so that the backwash flow rates through the nozzles should not vary by ≥5% as a result of friction losses in the underdrains. Too much coarse gravel in a gravel bed will permit free horizontal movement of backwash water in the space between the nozzles and the sand bed, and therefore partially negate the distribution value of the nozzles. Where air scour is used in conjunction with a drainage system, which uses nozzles, the nozzles are specially designed to distribute the air evenly. These nozzles are fitted with hollow stems, which project down into the laterals or into the plenum. A small air control orifice is drilled in the stem of each nozzle just below the filter floor. The pressure difference, which can be sustained across this orifice, is dictated by the length of the stem below the hole. The greater the allowable pressure difference, the less is the effect of extraneous disturbing influences on the air flow rate. A hood over the top of the nozzle can help to prevent water from entering the top of the stem and interfering with the air distribution. During the air scour cycle, air is introduced into the underdrain system; it escapes through the nozzles into the filter. If water and air are to be used simultaneously for filter cleaning, care must be taken to ensure that the necessary water can be distributed to the various parts of the filter in such a way that the height of the waves in the water surface is small in comparison to the head used for forcing the air through the control orifice (otherwise, uneven air scour will result). This condition can best be attained in a plenum distribution system, particular care being taken to dissipate the energy of the incoming water, which could cause excessive waves and turbulence. For the low head loss arrangement, backwash water can freely enter the lower gravel layers. The head loss, which controls the flow distribution, occurs in the upper gravel layers. If air scour is required, air is supplied through a separate layer of perforated air pipes set in one of the gravel layers. The washwater is forced up through the filter bed and emerges laden with impurities removed from the sand. The troughs to collect the dirty water are placed as low as practically possible, in view of the need to avoid excessive disturbance of the hydraulic flow pattern in the expanded sand bed. The troughs discharge the water into the gullet, which is the main collection channel. If it is expected that some of the displaced impurities may be too heavy or may settle too rapidly to be carried up into washwater collection troughs. The gullet wall may be used as a weir. The depth of water over the sand during the backwash is kept to a minimum, so that the horizontal velocity thus induced may
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carry the impurities over into the gullet. This is known as a cross-wash system. In order to intensify the horizontal velocity, water is sometimes introduced at the surface of the sand on the side opposite the gullet. Backwash procedures are equally as important as the design of the underdrain, sand support and sand bed in planning a system, in which the sand is thoroughly and evenly washed, without the occurrence of mud balls and gravel movement. A filter backwash is started in response to one or more of the following conditions: excessive head loss, effluent turbidity or run time. In most installations, the head loss criterion is the most frequent reason for backwash initiation; the usual head loss limit is set at about 2 m. If the floc in the water is too weak, it sometimes happens that it is not retained in the filter bed until the desired maximum head is attained, and a turbidity breakthrough occurs. In conditions where the pretreatment by clarification is very good, or the followthrough rates are low, the floc trapped in the sand bed may become so well gelled with time that thorough washing would be difficult, if the operator was to wait until the usual maximum head loss was reached. In such a case, the time limitation would control the initiation of backwashing. Common practice has been to use an air scour method in which air is bubbled through the sand bed at a rate between 5 and 15 l/s/m2 for a period of about 300 s (to let friction dislodge the impurities from the sand grains), followed by a low to medium-rate water backwash at 7–10 l/s/m2 until the water becomes clear after about 600 s. A recent trend is to use a period of combined water and air scour during which the air loosens the impurities, while a low-rate water backwash of about 6 l/s/m2 carries them to the surface and prevents their penetration, which sometimes occurs with air scour only. This period is then followed by a medium-rate backwash at a rate between 9 and 12 l/s/m2 to clear out the loosened impurities. Satisfactory combined water and air scour can be achieved, only if particular attention has been paid to the design of the water and air distribution systems. In hot climates, where the raw water may be polluted with nutrients, the growth of algae in the sedimentation and filter basins may be a problem. This can be overcome by chlorination of the water before sedimentation. The viscosity of the warmer water found in hot climates is lower than that in temperate climates, and therefore the backwash rate needed for efficient filter cleaning is greater than that with cooler water. The flow control for rapid sand filters is achieved in three different ways. Most systems include some means of automatic flow control; control valves operated by signals from level-sensing or flow-sensing elements. Care should be taken to avoid control conditions, which lead to controller instability, such as ‘hunting’ caused by continual over-correction. Flow control systems in water treatment plants are usually operated hydraulically or pneumatically. The damp conditions often occurring in filter control
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Wetland systems to control urban runoff
galleries are not conducive to the reliable long-term operation of electrical equipment. In a treatment plant equipped with downstream flow control, a flow measuring device (e.g., venturi, orifice or weir) is used to provide a signal, which operates the flow control valve. The control system adjustment may be set by hand, so that the rate of flow is kept close to a pre-determined rate. The plant inflow control valve is then operated by a device, which senses the level in the channel feeding the filters. The efficiency of turbidity removal in a filter is greatly reduced by sudden changes in flow. The total head loss across the filter and its control valve is equal to the difference between the water level in the filter chamber and the hydraulic grade level downstream of the control valve. The flow is controlled by motion of the filter control valve, which automatically adjusts to compensate for changes in the resistance to flow in the filter bed and, because the filter bed clogs slowly, only a relatively slow movement is required. This allows the valve action to be well damped, so that ‘hunting’ will not occur, and sudden flow variations may be avoided. With this control system, there is a sudden change of the flow through the pretreatment portion of the plant when a filter is taken out of service for backwashing, and an even greater change as the filter chamber is refilled after backwashing. These sudden changes of flow can cause a marked deterioration in the settlement efficiency of any clarifier. The system of upstream flow control with flow splitting avoids shock loading on the pre-treatment units. The plant inflow can either be set to a given flow rate or automatically controlled by the demand for water (perhaps from the level in the filter water storage). The flow to each filter is controlled by a flow-splitting device, through which the flow rate is a function of the level in the distribution channel. In this way, the flow from the pre-treatment section is split equally among all operating filters. A filter that is relatively clean can filter water efficiently at a higher flow rate than one in which clogging with impurities is well advanced. A filter, which is starting to pass turbid water, can continue to produce acceptable water for a time, if the flow rate is reduced. For some installations, it would appear that the best use could be made of a filter, if a high flow rate could be applied while it is relatively clean, with the rates of flow being gradually reduced as the filter becomes clogged. This is what happens if all filters are subjected to a common head loss. The total head loss through each filter, together with its underdrain and flow-limiting orifice, is equal to the difference in head between the influent header and the common effluent weir. Filters are sometimes backwashed in a fixed sequence. The time for backwashing the next filter in the sequence is judged from the rising level of the water in all the filter basins. The improvement in performance, coupled with the absence of
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automatic control equipment, makes this filtration control ideal for use in countries with limited technology.
13.4 Hydraulics of filtration The hydraulics of filtration are an important area for design and operation considerations. Accurate prediction of head loss and bed expansion during backwashing is important. Otherwise, filter design may prove to be inaccurate, and the sand in the bed may also be lost. After the first backwash, rapid (gravity) sand filters will stratify (or be stratified by design in the case of multi-media filters). Each layer in the filter will consist of different sized particles. After a filter has been in operation for a period of time, the head loss builds up as a result of filter clogging. When the head loss reaches an unacceptable level, the filter is stopped and backwashing is commenced. Unfortunately, backwashing of vertical-flow constructed wetlands is virtually impossible. However, the degradation of rhizomes results in hollow passages where liquid can rapidly pass through. This counteracts clogging, and makes backwashing unnecessary. When a bed of uniform particles is subjected to backwashing, the bed just begins to open when the backwash velocity reaches a critical value. At this stage, the effective weight of the (submerged) particles is exactly balanced by the upward drag on the particles resulting from the upflow velocity. As the velocity increases, the bed opens up further (porosity increases). This increase in velocity does not improve the cleaning action, but is thought to be important in allowing sufficient open space for trapped suspended matter to be washed away. For a stratified bed, expansion takes place successively for each media type. The surface strata are expanded at a lower rate of backwash than deeper ones. The bed is fully expanded when the upflow (backwashing) velocity of a filter equals the critical velocity for the largest particles.
13.5 Summary of rapid sand filtration Rapid (gravity) sand filters operate at rates some ten times those of slow sand filters. It follows that impurities are drawn deep into the bed. Hence, cleaning is automated and hydraulic (i.e. not labour intensive). The rapid sand filter takes up a relatively small land area (in comparison to a slow sand filter or wetland), the water requires pre-treatment and the operator skill level required is high.