Filters are classified on the basis of: (1) The filtration rate; and (2) The driving force to overcome the frictional resistance encountered by the water flowing through filler.
On the basis of filtration rate the filters are classified as: (i) Slow sand filters; and (ii) Rapid sand filters.
The Rapid sand filters arc further classified as: (a) Rapid gravity filters; and (b) Pressure filters.
The driving force to overcome the frictional resistance encountered by the water flowing through filter can be either the force of gravity, or the applied pressure force.
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As such on the basis of the driving force the fillers arc classified as: (i) Gravity filters; and (ii) Pressure filters.
The gravity filters are further classified as: (a) Slow sand filters; and (b) Rapid sand filters.
Thus combining the above two classifications it may be noted that in all there are following three types of filters:
I. Slow Sand Filters:
Slow sand filters are the earliest type of filters which were developed in England by James Simpson in 1829. Since then these filters were widely used, till the last decade of the 19th century, when the rapid sand filters (gravity type) were developed. Slow sand filters are so called because the rate of filtration through them is usually one-twentieth (or less) of the rate of filtration through rapid gravity filters or pressure filters.
ADVERTISEMENTS:
Because of low filtration rates, slow sand filters require large areas of land and correspondingly large quantity of sand. In the case of slow sand filters bed cleaning is done by surface scrapping which involves a lot of labour. Thus slow sand filters are suitable when land, labour and filter sand are readily available at low cost.
The raw water having low turbidity (< 20 NTU) can be directly fed to slow sand filters without any pretreatment such as sedimentation. However, when the raw water has high turbidity then plain sedimentation would be necessary to reduce turbidity within reasonable limit so that the filters are not unduly loaded.
Normally sedimentation with coagulation is not adopted for the raw water to be fed to slow sand filters. However, highly turbid raw water may be pretreated by sedimentation with coagulation before being fed to slow sand filters without adversely affecting the quality of the filtered water obtained by these filters.
A slow sand filter consists of the following parts:
ADVERTISEMENTS:
(i) Enclosure Tank:
It is an open watertight rectangular tank constructed of stone or brick masonry or concrete. The depth of the tank is about 2.5 to 3.5 m.
The surface area of the tank may vary from 10 to 2000 m2 or more, which is determined on the basis of filtration rate which varies from 100 to 200 litres per hour per square metre. The floor of the tank is provided with a cross slope of 1 in 100 to 1 in 200 towards the central drain.
(ii) Filter Media:
ADVERTISEMENTS:
The filter media consists of sand layer (or sand bed) 90 to 110 cm thick. The effective size of sand varies from 0.25 to 0.35 mm with a common value of 0.3 mm. The uniformity coefficient Cu of sand varies from 3 to 5. The finer the sand, better will be turbidity removal efficiency as well as bacteria removal efficiency, but lower will be the filtration rate.
Thus finer sand is suitable when pretreatment of raw water is either not done or it is poorly done, high turbidity and bacteria removal efficiency is desired, and saving of wash-water is not an important factor.
However, if pretreatment of raw water is good and water to be treated is not highly polluted, then coarse sand is suitable. Further the sand should not contain more than 2% of calcium and magnesium computed as carbonate.
(iii) Base Material:
ADVERTISEMENTS:
The sand layer is supported on base material which consists of 30 to 75 cm thick gravel bed. The gravel bed is graded and it is laid in layers, each of 15 cm thickness. The top most layer should be of small size gravel and the bottom most layer should be of bigger size gravel.
A typical section of base material is as indicated below:
(iv) Under Drainage System:
The filter media and the base material are supported over the under-drainage system which collects the filtered water and delivers it to the clean water reservoir. It consists of a central drain and lateral drains. The lateral drains are placed at a distance of 2 to 3 m and they are stopped at a distance of about 50 to 80 cm from the walls of the tank. The lateral drains may consist of earthenware pipes or perforated pipes of 7.5 to 10 cm diameter laid with open joints, or they may consist of patented drain devices.
The lateral drains can also be formed by placing two rows of bricks at right angles to each other. The first layer of bricks is laid with the direction of its rows perpendicular to the central drain. The bricks in this layer are placed about 2.5 to 4 cm apart. This space acts as lateral drains. The second layer is placed with its rows at right angles to the first layer and the bricks in this layer arc closely spaced.
(v) Appurtenances:
For efficient working of slow sand filters various appurtenances are installed.
These include:
(a) Vertical air pipe passing through the layer of sand;
(b) Devices for controlling the depth of water above the filter media;
(c) Devices for measuring loss of head through the filter media;
(d) Devices for maintaining constant rate of flow through the filter.
Vertical air pipes passing through the layer of sand may be provided to help in proper functioning of filter layers.
The arrangements are made to regulate the inflow to the filter tank so that a constant depth of water (1 to 1.2 m) is maintained above the sand layer.
The difference between the water levels in the filter tank and the outlet chamber is known as filter head. The filter head represents the loss of head caused by the resistance offered by the filter media to the water flowing through it. For a newly constructed or freshly cleaned filter unit, the resistance offered is less, and hence the head loss is small being 100 to 150 mm, but it goes on increasing as the filter layers get clogged. The outlet chamber is provided with a gauge to measure the loss of head or the filter head.
In order to maintain a constant rate of flow through the filter under all the filter heads, an adjustable telescopic pipe is provided at the top of the outlet pipe. At the top end of the telescopic pipe a circular weir plate is provided.
The telescopic pipe is supported by an annular ring which floats on the water surface at a fixed distance above the submerged open end of the pipe. With the increase in the filter head the telescopic pipe is adjusted manually so that the flow of water into the outlet pipe is constant since it is under a fixed head over a circular weir of fixed length.
Working of Slow Sand Filter:
The water from the sedimentation tank is allowed to enter the filter through the inlet chamber. The depth of water over the filter media should neither be too small nor too large; it is generally kept equal to the thickness of the layer of sand. The water passes through the filter media and during this process it gets purified. The purified water passes through the underdrains to the outlet chamber from where it is taken to clear water storage tanks.
In a slow sand filter water is subjected to various purifying actions as it percolates through the filter media. Impurities present in water are removed by a combination of straining, sedimentation, biochemical and biological process. Shortly after the starting of filtration, a thin slimy layer called the schmutzdecke is formed on the surface of the sand bed.
It consists of a great variety of biological organisms which feed on the organic matter and convert it into simple, harmless substances. Considerable portion of inert suspended particles is mechanically strained out in this layer. Further during its passage through the sand bed the water becomes virtually free from suspended and colloidal matter, pathogenic bacteria and complex salts in solution. This results in simultaneous improvement in the physical chemical and bacteriological quality of water.
Slow sand filters are operated upto a maximum filter head of 75 cm or equal to 65 to 85% of the thickness of the sand bed. The beginning the filter head (also termed as filtration head or filtering head) is only 10 to 15 cm. But as the suspended matter is arrested and the thickness of schmutzdecke increases, the resistance to the flow of water increases, which results in the increase of the filter head. When the filter head reaches the maximum permissible value the filter requires cleaning.
Cleaning of Slow Sand Filter:
In order to clean the filter the water in the filter tank is drained off. The top layer of sand is scrapped manually and removed through a depth of 15 to 30 mm. The scraped sand is either removed manually for washing, or it is placed in a portable hydraulic ejector which forces it through a hose to a washer outside the filter bed.
The washed sand is dried and stored for future use. After this the filter is again put into operation. For that the filter tank is refilled with filtered water from below until the sand bed is completely covered with water. This prevents the entrapping of air in the sand bed.
Thereafter, the inlet valve is opened slowly to prevent disturbance of sand, till the desired level of water is reached. The filter is operated at 1/5 of the normal rate of filtration for 12 to 15 hours to help in the formation of the biological film around the sand grains.
During this period the filtered water is diverted to the wash water drain and it is not taken into use. Thereafter the filtered water is diverted to the clear water storage tanks, but the rate of filtration is kept 1/3 of the normal rate for another 3 to 4 days. After this period the filter is operated at its normal rate.
The slow sand filter requires cleaning after one to three months depending upon the impurities present in the raw water. In each cleaning, some depth of filter sand is removed due to which the thickness of the sand bed goes on decreasing.
The process of cleaning by scrapping is continued till the thickness of the sand bed reaches a minimum value of 600 mm. When this stage is reached, the top of the sand bed is provided with a fresh layer of clean new sand or the old washed sand, upto its original level.
Rate of Filtration:
The normal rate of filtration for a slow sand filter varies from 100 to 200 litres per hour per m2 of filter area.
Efficiency of Slow Sand Filters:
(i) Bacterial Load:
The slow sand filters are highly efficient in the removal of bacterial load from water. It is expected that they remove about 98 to 99% of bacterial load from raw water and this percentage may be as high as 99.50 to 99.90 when pretreatment has been given to the raw water. However, for complete removal of bacteria, disinfection is essential.
(ii) Turbidity:
The slow sand filters can remove turbidity to the extent of about 50 p.p.m. As such for water having higher turbidity it is necessary to give preliminary treatment to bring down its turbidity below 50 p.p.m.
(iii) Colour:
The slow sand filters are less efficient in the removal of colour of raw water. It is estimated that they remove about 20 to 25% colour of raw water. As such slow sand filters are not suitable for raw water having higher colour content.
(iv) Colloidal Matter:
The slow sand filters are not highly efficient in the removal of colloidal matter.
Guidelines for Design of Slow Sand Filters:
The ‘Manual on Water Supply and Treatment’, published by Ministry of Urban Development, Govt. of India, New Delhi gives the guidelines for the design of slow sand filters as indicated in Table 9.3.
Standards of Performance for Slow Sand Filters:
The following standards of performance for slow sand filters have been recommended:
(a) The filtrate should be clear with a turbidity of 1 NTU or less.
(b) The filtrate should be free from colour (3 or less on the cobalt scale).
(c) When the raw water turbidity does not exceed 30 NTU the filter runs should normally be not less than 6 to 8 weeks, with the filter head not exceeding 0.6 m.
(d) The initial loss of head should not normally exceed 50 mm. A higher head loss will indicate that the entire sand bed needs overhauling.
Disadvantages of slow sand filters and Development of rapid sand filters and pressure filters:
The main disadvantage of a slow sand filter is its low rate of filtration, on account of which large filter area is required and consequently large quantity of filter material is required.
This difficulty of requiring more space for slow sand filters led to find out means to increase the rate of filtration.
It was observed that the rate of filtration can be increased in two ways:
(i) By increasing the size of sand so that the resistance to the water passing through it is minimised; and
(ii) By allowing water to pass under pressure through the filter media.
On the basis of the first method rapid sand filters (gravity type) have been developed, and on the basis of the second method pressure filters have been developed.
II. Rapid Sand Filters (Gravity Type), or Rapid Gravity Filters:
Rapid sand filters (gravity type) are the most commonly used filters in large water supply schemes. In the case of these filters the raw water is usually fed to the filters only after it has been treated through sedimentation with coagulation.
Working and Back Washing of Rapid Sand Filters (Gravity Type):
The working and back washing of a rapid sand filter (gravity type) is controlled by six valves as indicated below:
Valve 1 — Inlet valve
Valve 2 — Filtered water storage tank
Valve 3 — Waste-water valve to drain water from inlet chamber
Valve 4 — Wash-water storage tank valve
Valve 5 — Waste-water valve to drain water from main drain
Valve 6 — Compressed air valve
For normal working of the filter only valves 1 and 2 are opened and all the other valves are kept closed. Valve 1 is opened to permit water from coagulation- sedimentation tank to enter the filter. Valve 2 is opened to carry the filtered water to the filtered water storage tank. The filter operates under gravity flow. The standing depth of water over the sand bed varies between 1 and 2 m.
Back Washing:
With more and more material being trapped in the sand bed, the pores are clogged and the loss of head through the filter bed becomes excessive. The filter is then back washed to remove the trapped material. The back washing of a filter is usually done when the loss of head through it has reached the maximum allowable value which is between 2.5 and 3 m.
The back washing of a rapid sand filter (gravity type) is carried out by passing air and water upwards through the filter bed. The following is the sequence of operations during back washing of the filter.
(i) Close valve 1, allow the filter to operate till the water level reaches the edge of the wash water troughs. Some operators permit the water level to fall to about 15 cm from the top of the sand.
(ii) Close valve 2.
(iii) Open valve 6, to allow compressed air to pass through the filter bed for about 2 to 3 minutes in the upward direction. This will break-up the surface scum and loosen the dirt.
(iv) Close valve 6.
(v) Open valve 4 to allow wash water to pass through the filter bed in the upward direction. The valve should be opened gradually to prevent dislodging of the gravel. Open valve 3 to carry dirty wash water through inlet chamber to wash water drain. Continue washing till the wash water emerging from the filter bed and flowing through the wash water troughs appears to be fairly clear.
(vi) Close valve 4.
(vii) Close valve 3, after the water in the filter has drained down to the edge of the wash water troughs. Allow a short period to permit material in the water to settle on the surface of the sand and form a very thin sticky layer.
(viii) Open valve 1 slightly, and open valve 5 to allow the filtered water to flow to the wash water drain for a few minutes.
(ix) Close valve 5 and open valve 2. Open valve 1 fully. The filter is now back in service.
Water used for back washing should be filtered water. The total wash water used should normally not exceed 2% of the treated water. The wash water should be applied at a pressure of about 5 m head of water as measured in the under drains. The rate of application of wash water may be 600 litres per minute per m2 of the filter surface area, equivalent to a rise in the filter tank of 60 cm per minute, for a period of 10 minutes. The capacity of the wash water storage tank should be sufficient to give a normal wash to two filter tanks for a period of 5 to 6 minutes.
The rapid sand filters (gravity type) get clogged very frequently and have to be back washed every 1 to 3 days. Normally 10 to 15 minutes are required in back washing, but in recommissioning a total time of about 30 minutes may be consumed.
Surface Wash:
The upper layer of the filter bed becomes the dirtiest because most of the impurities are trapped in this layer. This layer may not be properly washed by the conventional method of back washing. The inadequate washing of the top layer of the filter bed will lead to the formation of mud balls, cracks and clogged spots in the filters. These troubles may, however, be overcome by adequate washing of the upper layer of the filter bed by surface wash.
The surface wash is usually accomplished by applying wash water or mechanical agitation, or both, at or near the surface of the sand bed in the filter. The wash water is applied from above in the form of high pressure water jets directed on the surface of the sand bed. The two systems usually adopted for applying wash water on the surface of the sand bed are (a) fixed type surface wash system; and (b) rotary type surface wash system.
The fixed type surface wash system consists of pipes 25 mm or more in diameter arranged vertically at intervals of 0.6 to 0.9 m. The lower ends of the pipes are provided with nozzles and are kept about 0.1 m above the sand surface. An alternate fixed type surface wash system consists of pipes horizontally arranged at an interval of about 0.6 m at a height of 0.05 to 0.1 m above the sand surface. The horizontal pipes are perforated at intervals of about 0.3 m and provided with non-clogging orifices to prevent entry of filter media.
The rotary type surface wash system consists of rotating units suspended at a height of 50 to 75 mm at adequate intervals over the bed to provide complete coverage. Jet nozzles are located on side and bottom of arms and jet action of water causes the arms to rotate at a rate of 7 to 10 r.p.m.
The rate of application of surface wash water ranges from 200 to 400 litres per minute per m2 of filter area, under a pressure of 10 to 20 m head of water.
Loss of Head and Negative Head:
When water passes through filter media it is offered frictional resistance. In order to overcome the frictional resistance there occurs, some loss of head. In the beginning when the filter has been cleaned the loss of head is small being about 15 to 30 cm. As the water is filtered through it, impurities are arrested by the filter media, due to which the loss of head goes on increasing.
The loss of head can be measured by inserting two piezometric tubes, one connected to the filter above the sand and the other connected to the effluent pipe between the filter and the rate controller. The difference of water level in the two piezometric tubes indicates the loss of head.
As the loss of head increases a stage may come when the frictional resistance offered by the filter media may exceed the static head of water above the sand bed. This may happen when the thickness of the suspended matter deposited on the surface of the sand bed may be considerably increased. At this stage the lower portion of the filter media and the under drainage system are under partial vacuum or negative head. Water is then sucked through the filter media rather than being filtered through it.
The fall of water level in the piezometer tube below the centre line of the under drainage system indicates negative head. Due to the development of the negative head dissolved air and other gases present in the water may be released. These gases may either accumulate in the voids of filter media causing air binding, or they may break through the filter, leaving an opening through which water may pass with imperfect treatment.
A result of air binding is that water does not pass through the portion of the filter where the bubbles of air and gas have accumulated. This may result in loss of capacity of the filter or an overloading of other portions of the filter.
In rapid sand filters (gravity type) the maximum permissible loss of head is 3 to 4.5 m. However, the maximum loss of head usually allowed is 2.5 to 3 m. Thus when the loss of head in the filter reaches the maximum allowable value of 2.5 to 3 m, the back washing of the filter should be carried out. Further the maximum permissible negative head is 0.8 to 1.2 m.
Rate of Filtration:
The rate of filtration for a rapid sand filter (gravity type) varies from 3000 to 6000 litres per hour per m2 of filter area (or 50 to 100 litres per minute per m2 of filter area). The high rate of filtration results in considerable saving of space for the installation of filter as well as filter materials.
Efficiency of Rapid Sand Filters (Gravity Type):
(i) Bacterial Load:
The rapid sand filters (gravity type) are less efficient in the removal of bacterial load. It is expected that they remove about 80 to 90% of bacterial load present in water.
(ii) Turbidity:
The rapid sand filters (gravity type) can remove turbidity to the extent of 35 to 40 p.p.m. As water entering these filters is invariably given the treatment in coagulation sedimentation tank, it possesses turbidity less than 35 to 40 p.p.m.
(iii) Colour:
The rapid sand filters (gravity type) are highly efficient in colour removal. The intensity of colour can be brought down below 3 on cobalt scale.
Standards of Performance for Rapid Sand Filters (Gravity Type):
The following standards of performance for rapid sand filters (gravity type) have been recommended:
(a) The filtrate should be clear with a turbidity of 1 NTU or less.
(b) The filtrate should be free from colour (3 or less on the cobalt scale).
(c) The filter runs should normally be not less than 24 hours with a loss of head not exceeding 2 m.
(d) For an efficient filter, the wash water consumption should not exceed 2% of the quantity filtered in between washing.
III. Pressure Filters:
Pressure filters are the types of rapid sand filters which are in closed steel cylindrical tanks and through which water to be treated passes under pressure. This pressure is developed by pumping and it may vary from 0.3 to 0.7 N/mm2. These filters may be of horizontal type or of vertical type. The diameter of pressure filters varies from 1.5 to 3 m and their length or height varies from 3.5 to 8 m. At the top inspection windows (or manholes) are provided for the purpose of inspection.
The specifications for the sand and the gravel to be used for pressure filters are the same as those provided for rapid sand filters (gravity type). Further the thicknesses of sand bed and gravel layers as well as the under drainage system are also the same as in the case of rapid sand filters (gravity type).
The operation of pressure filters is similar to that of rapid sand filters (gravity type) except that in the case of pressure filters the coagulated raw water is fed directly to the filters without mixing, flocculation and sedimentation. The commonly used coagulant is alum, which is kept in a pressure container connected to the influent line to the filter.
The back washing of the pressure filters is also carried out in the same manner as in the case of rapid sand filters (gravity type). Automatic pressure filters are also available in which back washing is done automatically after a fixed interval of time or when the head loss has reached a given value. It may, however, be noted that the head loss through pressure filters is approximately the same as through rapid sand filters (gravity type). The term pressure filter does not imply that water is pumped through the filter under a high pressure loss.
Rate of Filtration:
The rate of filtration of pressure filters is high as compared to that of rapid sand filters (gravity type). It is about 6000 to 15000 litres per hour per m2 of filter area (or 100 to 250 litres per minute per m2 of filter area).
Efficiency of Pressure Filters:
In general pressure filters are found to be less efficient than rapid sand filters (gravity type) in terms of removal of bacterial load, colour and turbidity.
Suitability of Pressure Filters:
Pressure filters are not suitable for public water supply schemes, because of high cost, inefficiency of filtration and relatively poor quality of results obtained. However, these filters can be installed for small water supply schemes such as colonies of a few houses, industrial plants, private estates, swimming pools, railway stations, etc.
Advantages of Pressure Filters:
The various advantages of pressure filters are as follows:
(i) These are compact units. The modern automatic pressure filters are designed in such a way that no manual operation or supervision is required.
(ii) These filters are flexible in operation because the rate of filtration can be altered by changing the pressure of water fed to the filters.
(iii) No further pumping is required as the filtered water comes out under pressure.
(iv) They require very small space for their installation.
(v) Number of fittings required for these filters is less.
(vi) When these filters are employed, the coagulation sedimentation tanks are not required.
(vii) They are ideal for small water supply schemes.
Disadvantages of Pressure Filters:
The various disadvantages of pressure filters are as follows:
(i) It is difficult to keep close watch on the performance of these filters because the process of filtration and back washing are done in the closed tank.
(ii) It is difficult to inspect, clean and replace the sand, gravel and under drains of these filters.
(iii) In case of direct supply from these filters, it is not possible to provide adequate contact time for chlorine for disinfection of water.
(iv) Because of the curved shape of these filters it is difficult to provide wash water troughs effectively designed so that the material washed from the sand is discharged to waste and not flushed back to other portions of the sand bed.
(v) The overall capacity of these filters is small.
(vi) It is difficult to repair these filters.
(vii) These filters possess poor efficiency in the removal of bacteria, colour and turbidity.
(viii) They are costly and hence, they cannot be recommended for treating large quantity of water.
(ix) Additional pumps are required for feeding water to these filters.
Double Filtration—Roughing Filter:
Double filtration consists of the filtration of water through two or more filters.
It may be carried out in different ways as follows:
(i) Water is allowed to pass through two or more slow sand filters arranged in series.
(ii) Water is allowed to pass through two rapid sand filters (gravity type) arranged in series.
(iii) Water is allowed to pass through a rapid sand filter (gravity type) and then through a slow sand filter arranged in series.
In double filtration the preliminary filter is known as roughing filter which in most of the cases is rapid sand filter (gravity type). In the construction of roughing filter course materials are used and hence its rate of filtration is as high as 7000 litres per hour per m2 of filter area or more. The roughing filters generally do not require the use of coagulants.
In practice the last combination indicated above is most commonly adopted to increase the rate of filtration of an existing slow sand filter. It has been found that the installation of roughing filters practically double the capacity of slow sand filters and hence it is quite useful where land available for the installation of slow sand filters is restricted.