Essay on Classification of Filters | Water Treatment | Water Engineering

In this essay we will discuss about the classification of filters:- A. Gravity Filters B. Pressure Filters.

Essay # A. Gravity Filters:

1. Slow Sand Filters:

Construction of Slow Sand Filters:

These are watertight shallow tanks about 2.5 m to 4 m deep and having surface area 100 sq.m to 2000 sq.m. in plan. These tanks contain 60-90 cm thick bed of sand (filtering media) supported on 30-60 cm thick gravel bed. Generally three to four layers of gravel having thickness of 15-20 cm are used.

The coarsest gravel is placed in the bottom and the smallest size gravel is used in the topmost layer. The size of the bottom layer gravel is 40-60 mm the size of the intermediate layer 20-40 mm and 6 mm to 20 mm (if two intermediate layers are used). The size of the topmost gravel layer is 3-6 mm. The gravel is supported on a bed of concrete sloping towards a central longitudinal drain connected by a system of open jointed under-drains.

Fig. 13.1 shows the plan and sections at various places:

Inlet:

The inlet of the slow sand filter consists of a chamber fitted with sluice valve or one equilibrium float valve. Mostly the inlet pipe is carried vertically in the body of the filter with the mouth of the inlet pipe flush with the water level.

Under Drainage System:

The gravel is laid on the top of the under-drainage system constructed in the bottom of the filter to collect the filtered water. The under-drainage system mainly consists of a central drain which is connected by lateral pipes as shown in Figs. 13.1 (a), (b) and (c).

The laterals are constructed with the open jointed pipes, usually placed 3.5 m apart on the bottom of the floor sloping towards the main central drain. These laterals collect the filtered water through the gravel bed and discharge it into the main drain, which carries it to the clear water reservoir. If the size of the filter is large more than one main drains are constructed in the side or intermediate positions.

Depth of Water:

There is no hard and fast rule for the fixation of water depth. It varies in different countries, but the depth of water equal to the thickness of the filtering media has been found to give best results in India.

Outlet Water after passing through the filter bed is collected in the outlet chamber, where a regulating arrangement is provided to obtain a regular discharge from the filter. The regulating arrangement may consist of a telescopic pipe or adjustable weir operated by means of a float, so that the depth of mouth of outlet pipe from the water level inside the outlet chamber should always remain constant.

Miscellaneous Appurtenances:

In addition to the arrangements described above, some miscellaneous appurtenances are also used in the filters. In some filters vertical air pipe is provided inside the sand layer. Compressed air is passed through this pipe and it helps in the proper functioning of the filter sand, by removing the mud clogs by stirring them.

Some devices are provided to control the water depth above the sand bed for efficient functioning of the filter, at constant level. Flow gauge is also provided to measure the flow of the water. Other equipment’s to measure rate of filtration, loss of head. Loss of head or resistant offered in the beginning is less and increases with the time. When the rate of filtration reduces to the specified limit of 0.7 to 1.2 m, the cleaning of the filter is done.

Operation of Filter:

The water from the sedimentation tanks enters the slow sand filter through a submersible inlet as shown in Fig. 13.2. This water is uniformly distributed over the sand bed without causing any disturbances. The water passes through the filtering media at an average rate of 100 to 200 lit/m²/hr.

This rate of filtration is continued until the difference between the water level on the filter and in the outlet chamber is slightly less than the depth of water above the sand. The difference between the water above the sand bed and in the outlet chamber is called loss of head.

During filtering as the filtering media gets clogged due to impurities which stay in the pores, the resistance to the passage of water and loss of head also continuously increases. When loss of head reaches its permissible limit the working of filter is stopped, and about 2-3 cm sand from the top of bed is scrapped and replaced with clean sand before putting back into service to the filter. The scrapped sand is washed with water, dried and stored for return to the filter at the time of the next washing.

Result of Slow and Filtration:

The main object of slow sand filtration is to remove bacteria and suspended matter from the water. Slow sand filters are very efficient in this respect and remove 98 to 99% of bacteria of the raw water and completely all the suspended matter, which cannot be removed by the sedimentation process.

Slow sand filtration also removes odours, tastes and colour from the water. But slow sand filters cannot remove pathogenic bacteria completely therefore disinfection of filtered water is necessary to assure complete safety against water borne diseases. The rate of filtration of slow sand filter is also very slow and it requires large area for its construction.

Slow sand filters are best suited for the filtration of water for small plants and for small towns. These filters remove the turbidity, bacteria, low colour and tastes very efficiently. As these filters require large area for their construction, it increases their initial cost too high.

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 may remove about 98 to 99% of bacterial load from raw water. However, for complete removal of bacteria, disinfection is essential.

(ii) 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.

(iii) Turbidity:

The slow sand filters can remove turbidity to the extent of about 50 ppm. For water having greater turbidity than 60 ppm, it is necessary to give preliminary treatment.

2. Rapid Sand Filter:

Construction of Rapid Sand Filter:

Fig. 13.3 clearly shows the constructional details of a rapid sand filter. It essentially consists of an open watertight basin of masonry or concrete. On slopy floor of concrete, under drainage system, consisting of one central longitudinal conduit with lateral pipes connecting from all sides is laid. All pipes of under-drainage system have holes on their bottom side, from where water enters in these pipes.

This under drainage system is embedded in 60 cm to 70 cm thick graded gravel. The size of gravel varies from 2.5 cm at the bottom to 0.5 cm at the top. The size of gravel depends on the rate of filtration; more the rate of filtration, larger is the gravel size. The main function of gravel layer is to support the sand layer over it and prevent the sand particles from entering into under-drain pipes.

Second function of it is to uniformly distribute the wash water on the sand bed. On the top of gravel an equally thick layer of sand is spread. Some wash water gutters are placed a little above the sand bed to collect the wash water, and carry it away to the drain through a set of pipes.

Fig. 13.3 shows the Section through a Rapid gravity filter. It essentially consists of an open watertight rectangular tank, constructed with masonry or concrete. The depth of the tank varies from 2.5 to 3.5 m. The surface of each filter is kept between 10 m² to 80 m².

Morrell and Wallace have given the following formula to get approximately the number of rapid gravity filter units:

where N is the number of units and Q the quantity of water in million litres/day. At every filter units there must be atleast two filter units.

The filter media consists of coarse sand layers of effective size varying from 0.35 mm to 0.55 mm, having uniformity coefficient D₆₀/D₁₀ ranging from 1.2 to 1.8. The finer variety should be near top and coarser variety near the bottom. The base material consists of gravel in thickness of60-90 cm. Usually five to six layers of 10-15 cot each are used.

The size of the coarser gravel is about 40 mm and that of smallest is 3 mm. The gravel must be packed in uniform grading for proper and efficient function of the filter. The quantity of wash water mainly depends on the filter media.

Miscellaneous Appurtenances:

Rapid gravity filters are also provided with various types of appurtenances which are summarised as follows:

(a) Flow Rate Controller:

These are provided in the filters to obtain uniform rate of flow irrespective of the loss of head in the filter. The venturi-rate controller shown in Fig. 13.4 is usually employed for this purpose. It essentially works on the principle of venturi. One typical type known as simplex-rate of flow controller is shown in the figure.

It mainly consists of balanced disc-valve connected to a flexible diaphragm and to a lever and weight above. The water enters the valve through the venturi-tube. The pressure is transmitted from the throat of the tube to the under-side of the diaphragm which further transmits it to the lever arm. The rate of flow is directly recorded by the level after swinging the counter weight.

(b) Compressed Air:

Before allowing the back wash water to pass, the sand grains are thoroughly agitated by passing the compressed air in it. Air compressor unit of the required size is installed. It should be able to supply the compressed air at the rate of600 to 800 litres/min/m² at least for five minutes.

The pressure should be sufficient to overcome the frictional resistances of air pipes and the column of water present above the distribution system. Generally the compressed air is supplied through the laterals or through separate pipe system.

(c) Wash Water Troughs:

The suspended impurities removed by the filter are accumulated at the surface and upto some depth inside the filter media sand. When the quantity of these impurities increases the rate of filtration is reduced. The removal of the impurities (suspended solids etc.) is done by back washing of the filter. The dirty wash back water is collected in the V-shaped wash water troughs or gutters provided near the water surface as shown in Fig. 13.3.

These troughs are usually placed 1.5 to 2.0 m apart. For efficient working these troughs should be of large size and laid in slope at about half the thickness of sand bed from the top of the sand filter.

(d) Miscellaneous Accessories:

Various types of accessories such as head loss indicator, meters for measuring the flow, constant level equipment’s etc. are also provided in the rapid gravity filters.

Operation of Filter:

The water from coagulation sedimentation tank enters the filter unit through inlet pipe and is uniformly distributed on the whole sand bed. Water after passing through the sand bed is collected through the under drainage system in the filtered water well.

The outlet chamber in this filter is also equipped with filter rate controller. In the beginning the loss of head is very small. But as the bed gets clogged, the loss of head increases, and the rate controller adjusts it to a limit. When the limit is reached the rate of Alteration becomes very low and the filter bed requires its washing.

Washing of Filter:

It is done by the back flow of water through the sand bed. Refer to Fig. 13.5 it is done as follows:

First the valve A is closed and the water is drained out from the filter leaving a few centimetre depth of water on the top of sand bed. Keeping all valves closed the compressed air is passed through the separate pipe system for 2-3 minutes, which agitates the sand bed and stir it well causing the loosening of the dirt, clay etc. inside the sand bed.

Now valve C and B are opened gradually, the wash water from the wash-water tank, rises through the laterals, the strainers, gravel and sand bed. Due to back flow of water the sand expands and all the impurities are carried away with the wash water to the drains through the channels which are kept for this purpose.

This washing process is continued till the sand bed appears clearly. The washing of filter is done generally after 24 hours and it takes about 10 minutes and during back washing the sand bed expands by about 50%.

Surface Wash:

Now-a-days in recent development, the washing of filter bed is done only at the surface by applying clear water at the rate of 270 litres/m²/min under a pressure of 0.7 to 1.1 kg/cm² through nozzles. These nozzles are kept vertical on the surface of sand bed. If both the surface wash of and back washing is done simultaneously, very good results are obtained and the total quantity of water required, is also less than that required for back washing alone.

The Under-Drainage System:

In the bottom of the filter under drainage system is laid to collect the filtered water and to distribute wash water uniformly on the filter bed of sand.

There are various types of under-drainage systems, but the most common are as follows:

(a) The manifold and pipe laterals:

This system is most widely used. It essentially consists of one central C.I. main pipe of about 50 cm diameter, to which 7.5 to 10 cm diameter laterals are connected on both sides at 15 to 20 cm centre to centre spacing. All laterals are of perforated pipes, having 6 mm to 12 mm diameter holes drilled to their bottom at an angle of 30° to 60° with the vertical as shown in Fig. 13.6.

The perforated laterals are mostly used with Wagner block which is placed between them.

(b) Strainer Pipe System:

In this system, the laterals are attached to the manifold as in the case of perforated pipe system, but holes are not drilled into the laterals. Instead of drilling holes, strainers are placed on the lateral drains, as shown in fig 13.7(a) & (b). A strainer is a small brass pipe closed at its top by a perforated cap.

Such strainers one placed at about 15 cm apart on lateral drains instead of drilling holes into them. Various forms of strainers have been devised by different manufacturers of filter units. In certain cases, the strainers are even fixed directly on the central drain without any laterals.

When pipe and strainer system is provided, compressed air is used during back washing of filter. This results in the saving of the wash water.

When no strainers are used (i.e., in the case of perforated pipe system). The agitation of sand particles is done by water-jet, and the back wash is therefore, required at a high velocity. The rate of water application being of the order of 700 to 800 lit/min/sq. m of filter area, which is known as high velocity wash.

Where as in the case of pipe and strainess system, wash water is required at low rate of about 250 to 300 lit/min/sq. m of filter area, because compressed air assists in agitating the sand particles. This is known as slow velocity wash.

The following points may also be considered and kept in mind, while designing the sizes of the pipes to be used in the above system:

(i) The total cross-sectional area of perforations should be about 0.2 percent of the total filter area.

(ii) The cross-sectional area of each lateral should be about 2-4 times the total cross- sectional area of perforations in it.

(iii) The cross-sectional area of the manifold should be about twice the cross-sectional areas of the lateral-drains.

(v) Maximum permissible velocity in the manifold to provide the required amount of wash water, is about 1.8 to 2.4 m/sec.

(c) Vitrified Tile Blocks:

This is also known as Leopold block system. It essentially consists of vitrified tile blocks having orifices and holes in them for the passage of water. All these tiles are lid in the bottom, over which gravel layer is land as shown in Fig. 13.8.

(d) The Wheeler Filter Bottom:

This is very cheap system. In this system a false bottom is prepared above the real bottom by R.C.C. This false bottom consists of solid, inverted, truncated pyramids with water connections at the apex of each pyramid, and the pockets filled with cement or glazeo earthenware spheres as shown in Fig. 13.9.

(e) The Porous Plate Bottom:

In this case also a false bottom is prepared with porous plates. The sand layer is directly laid over it and there is no necessity of gravel layer. In this type of under-drainage system, the rate of filtration and wash water distribution is very uniform. In addition to the above, this system has further advantage that there is no fear of corrosion of metals (Fig. 13.10).

Results of Rapid Gravity Filtration:

Rapid sand filters remove suspended matter, colour, odour and bacteria from water. As coarser sand is used in rapid sand filter the water must pass through sedimentation with coagulation process.

The maximum turbidity of raw water should not be more than 35 to 40 p.p.m. This filter cannot remove bacteria completely; therefore, filtered water must be disinfected. The rate of filtration is usually 3000 to 6000 litres/m²/hour. The wash water pressure should not exceed 10 metres.

Essay # B. Pressure Filters:

It is a rapid sand filter placed within a closed, watertight steel cylinder. The water passes through the sand bed under a pressure greater than atmospheric. All the operation of this filter is similar to rapid gravity filters, except that the coagulated water is directly applied to the filter without mixing and flocculation.

These filters are used on small supplies where water is received under pressure, which can be used to force the water through the filter. Mostly pressure filters are used for industrial plants, because these are much suited for such places. But these are not economical on large scale.

The given Figs. 13.11 and 13.12 show pressure filters. The cylindrical shell containing filter sand and gravel bed may be kept vertical or horizontal. The size of vertical filters varies from 0.3 m to 2.75 m in diameter and height may be 2-2.5 metres. The horizontal units are generally 2 to 3 metres in diameter and length upto 9 metres. The rate of filration is 6,000 to 15,000 litres/hour/m² area of filter bed.

The cleaning of filter bed is done by back washing similar to rapid gravity filter. Generally air pressure is maintained on the surface of water to increase the rate of filtration.

Advantages of Pressure Filters:

(i) It is a compact unit and now-a-days automatic pressure filter units have been designed, requiring no manual operation.

(ii) These units are ideal for small estates and small water works.

(iii) These filters are very flexible, because the rate of filtration can be changed by change of compressed air pressure over the water.

(iv) These filters require very small area for their installation.

(v) Small number of fittings is required in these filters.

(vii) As the filtered water comes out under pressure no further pumping is required to lift the water.

(viii) No sedimentation and coagulant tanks are required with these filters.

Disadvantages of Pressure Filters:

(i) Overall capacity of the filter is small, though the rate of filtration per sq.m is high.

(ii) Due to the heavy cost on water treatment, they cannot be used for treating large quantity of water at the water works.

(iii) As the process of filtration and back washing is done in the closed tank, proper quality control and inspection is not possible.

(iv) The inspection, cleaning, change of filter media, gravel and repair of under drainage system is difficult.

(v) Due to circular shape in plan, the design and fixing of back water channels is difficult.

(vi) Their efficiency in removal of bacteria and turbidity is poor.

(vii) They require additional pumps to pump the water in them.