A rapid sand filter (gravity type) consists of the following parts:
1. 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 50 m2. Thus depending on the total surface area of filter required a number of small filter units are provided. These filter units are arranged in series. Further for flexibility of operation a minimum of four filter units should be provided for larger water treatment plants and a minimum of two filter units should be provided for smaller water treatment plants. The length to width ratio of the tank is normally kept between 1.25 to 1.35.
2. Filter Media:
The filter media consists of sand layer (or sand bed) 60 to 75 cm thick. The effective size of the sand varies from 0.45 to 0.70 mm. The uniformity coefficient, Cu, of sand varies from 1.3 to 1.7, the common value being 1.5. Due to higher value of effective size and lower value of uniformity coefficient the void space in the filter media is increased which results in a higher rate of filtration for this filter.
Preparation of Filter Sand:
ADVERTISEMENTS:
The sand to be used in the filter has to be of specified effective size and uniformity coefficient. However, the available sand or stock sand may not meet the required specifications of the size. As such from a sieve analysis of the stock sand, the coarse and fine portion of stock sand that must be removed in order to meet the size specifications can be computed.
From the desired or known values of the effective size D10, and the uniformity coefficient Cu, the desired value of D60, of the filter sand may be computed as-
To meet the specified composition the filter sand can contain 1/10 of the usable sand below D10 size. Hence the percentage below which the stock sand is too fine for use is –
Sand washer is essentially an upward flow settling tank in which an upward flow velocity is maintained to a value slightly less than the hydraulic subsidence value of the smallest particle size to be retained i.e., Dtoo fine. Thus particles of less than Dtoo fine size are floated out with the flowing water. The washed sand, settling to the bottom, is ejected hydraulically or withdrawn by gravity through shear gate.
Estimation of Thickness of Sand Bed:
The thickness of the sand bed should be such that the flocs do not break through the sand bed. The thickness of the sand bed can be checked against breakthrough of the floe through the sand bed by calculating the minimum thickness required by Hudson formula-
Q = rate of filtration in m3/hr/m2;
ADVERTISEMENTS:
d = sand size in mm
h = terminal loss of head in m
I = thickness of sand bed in m
ADVERTISEMENTS:
Bi = break through index whose value ranges between 4 × 10-4 to 6 × 10-3 depending on response to coagulation and degree of pre-treatment in filter influent.
3. Base Material:
The sand layer is supported on base material which consists of 45 to 60 cm thick gravel bed. The gravel bed is graded and it is laid in layers. The top most layer should be of small size gravel and the bottom most layer should be of big size gravel.
A typical section of base material is as indicated below:
However, the thickness of the layers of gravel of different sizes may be computed as indicated below.
ADVERTISEMENTS:
Estimation of Gravel Size Gradation:
To begin with, a size gradation of 2 mm at top and 50 mm at bottom is assumed. The required depth I, in cm of a component gravel layer of size d, in mm can be computed from the following empirical formula-
Thus the thicknesses of the layer of gravel of size 2 mm to be provided at the top = 9.2 cm
Next assuming the gravel size 5 mm. the depth h for gravel of size 5 mm is obtained as
l2 = 2.54 × 12(log510) = 21.3 cm
Thus the thickness of the next layer of gravel of size 5 mm = (21.3 – 9.2) = 12.1 cm.
By adopting the same procedure the thicknesses of the other layers of gravel of sizes varying upto 50 mm may be computed.
4. Under Drainage System:
In the case of rapid sand filters (gravity type) the under drainage system serves two purposes:
(a) It collects the filtered water uniformly over the area of gravel bed.
(b) It provides uniform distribution of backwash water without disturbing or upsetting the gravel bed and filter media.
There are various forms of under drainage systems for these filters out of which the following two systems are commonly adopted which are described below:
(a) Perforated Pipe System:
In this system, there is a central drain or manifold to which a number of lateral drains are attached on either side. The central drains as well as lateral drains are usually made of cast iron, but these may also be made of other materials such as plastic, asbestos cement, concrete, etc. The lateral drains are placed at a spacing of 15 to 30 cm.
The lateral drains are provided with perforations on the bottom side which make an angle of 30° with the vertical. The diameter of the perforations varies from 5 to 12 mm, and these are staggered. The spacing of perforations along the laterals may vary from 80 mm for perforations of 5 mm diameter to 200 mm for perforations of 12 mm diameter.
The lateral drains are supported on concrete blocks of thickness about 4 to 5 cm placed on the floor of the filter.
The perforated pipe system is economical and simple in operation. It, however, requires more quantity of water for back washing of filter, which is about 700 litres per minute per m2 of filter area. The water for back washing is obtained from a wash-water overhead tank. This is known as high velocity wash. The velocity of jet issuing from the perforations during back washing is, however, dissipated against the filter floor and in the surrounding gravel.
The following general rules may be observed in the design of an under drainage system consisting of central manifold and laterals:
(1) The ratio of length to diameter of the lateral should not exceed 60.
(2) The spacing of the laterals shall be from 15 to 30 cm.
(3) The cross-sectional area of the manifold should be preferably 1.5 to 2 times the sum of the cross-sectional areas of the laterals to minimise frictional losses and to give the best distribution.
(4) The diameter of perforations in the laterals should be between 5 and 12 mm. The perforations should be staggered, at a slight angle (usually 30°) with the vertical axis of the pipe.
(5) The spacing of perforations along the laterals may vary from 80 mm for perforations of 5 mm diameter to 200 mm for perforations of 12 mm diameter.
(6) The ratio of total area of perforations to the entire filter area may be about 0.003.
(7) The ratio of the total area of perforations in the under drainage system to the total cross-sectional area of the laterals should not exceed 0.5 for perforations of 12 mm diameter, and should decrease to 0.25 for perforations of 5 mm diameter.
(b) Pipe and Strainer System:
In this system also, there is a central drain or manifold with lateral drains attached to it on either side. In this system holes are drilled at the top of the laterals and each hole is provided with a strainer. A strainer is a small pipe of brass which is closed at top and contains holes on its surface. The strainers are either screwed or fixed on the top of the lateral drains. There are various forms of strainers devised by different manufacturers of filter units.
Some manufacturers provide umbrella shaped strainers. Some umbrella shaped strainers have a special air orifice, and are employed where an auxiliary air wash is used. In some cases strainers are fixed even on the central drain. The lateral drains as well as the strainers are generally placed at a spacing of 15 to 30 cm. All the strainers are usually placed at the same elevation.
When pipe and strainer system is adopted, compressed air is used for the purpose of back washing of the filter. This results in saving of wash-water, Thus it requires about 250 litres of water per minute per m2 of filter area for the purpose of back washing of filter. This is known as low velocity wash.
5. Appurtenances:
The important appurtenances provided with a rapid sand filter (gravity type) are as follows:
(a) Wash Water Troughs:
Wash water troughs are provided in the upper portion of the filter tank to collect the back wash water as it emerges from the sand and to conduct it to the wash water drain. These troughs may be of R.C.C, asbestos cement, plastic, cast iron and steel, out of which R.C.C. troughs are commonly used. The troughs span across the width or length of the tank.
The spacing of the troughs is kept between 1.2 to 2 m, so that the horizontal distance travelled by the dirty water over the surface of the sand bed is kept between 0.6 to 1.0 m before entering the trough.
The upper edge of the trough should be placed sufficiently near to the surface of the sand so that a large quantity of dirty water is not left in the filter after the completion of washing. At the same time, the top of the trough should be placed sufficiently high above the surface of the sand so that sand will not be washed into the trough.
During back washing the sand is expanded to about 130 to 150 percent of its undisturbed volume, and hence the top edge of the trough should be slightly above the highest elevation of the sand as expanded in washing. Further the bottom of the trough should be kept at least 5 cm above the top surface of the expanded sand.
The troughs may be rectangular, square, V-shaped or semi-circular in section. The troughs having rectangular section at the top and V-shaped or semi-circular bottom are also used. The trough should be large enough to carry all the water delivered to it with a minimum freeboard of 5 cm. Any submergence of the trough will reduce the efficiency of the wash.
The troughs are designed as free falling weirs or spillways. For free falling rectangular trough with level bottom, the following expression is used for fixing the size of the trough.
Q = 1.376 bh3/2 …(9.36)
In which,
Q = total water received by the trough in m3/s;
b = width of the trough in m; and
h = depth of water in the trough in m.
(b) Air Compressors:
During back washing of a filter the agitation of the sand grains is carried out either by water jet, or by compressed air, or by mechanical rakes. When compressed air is used, an air compressor of required capacity should be installed.
Generally it should have the capacity of supplying compressed air at the rate of 0.60 to 0.80 m3 per minute per m2 of filter area for 5 minutes. The pressure of the compressed air should be sufficient to overcome the frictional resistance offered by the air pipes and the depth of water lying above the air distribution system.
The compressed air may be supplied either through the laterals of the under drainage system or through a separate pipe system. If the compressed air is supplied through the laterals of the under drainage system then the pipe and strainer system of under drainage should be adopted. When a separate pipe system is to be provided for the supply of compressed air then the pipes and manifold of this system are placed immediately above the pipes of the under drainage system.
(c) Rate Control Device:
It is essential to maintain a constant rate of filtration irrespective of loss of head through the filter. This is so because a sudden increase in the rate of filtration may cause water to break through the filter material without proper treatment, and a sudden reduction in the rate of filtration may release a bubble of gas entrained in the sand, causing it to make a hole through the filter bed. In order to automatically control the rate of filtration the most commonly used device is Simplex rate controller.
It consists of a balanced valve connected to a flexible diaphragm (or disc) below, and to a lever with a movable weight above. The position of the valve controls the rate of flow through the device, and this position is regulated by means of the movable weight on the lever. The water enters this device through a venturi tube. A small pipe connection enables to transmit pressure at the throat of the venturi tube to the lower side of the diaphragm.
By setting the movable weight on the lever, the position of the valve is so adjusted that the desired rate of flow is achieved. Corresponding to this rate of flow there will be certain pressure difference between the upper and the lower sides of the diaphragm which will balance the pull of the lever and the weight, and thus keep the valve in position to provide the desired rate of flow. When the rate of flow increases the pressure at the throat of the venturi tube will decrease, but in the control chamber where the valve is located the pressure will increase.
Thus the upper side of the diaphragm will be subjected to a higher pressure and its lower side will be subjected to a lower pressure than before. The increased pressure difference between the upper and the lower sides of the diaphragm will force the valve to move down, thus reducing the rate of flow.
Once the same rate of flow as before is restored the pressure difference between the upper and the lower sides of the diaphragm will be same as before and it will balance the pull of the lever and the weight and thus keep the valve in new position to provide the same rate of flow as before. When the filter gets clogged with its use, and the rate of flow decreases, the action in the rate controller is reversed and the same rate of flow as before is restored.
(d) Miscellaneous Accessories:
The various accessories such as head loss indicators, meters for measuring the flow rates, etc., are also provided. The loss of head may be measured by inserting two piezometric tubes one above the sand bed in the filter tank, and the other in the effluent pipe between the filter and the rate controller. Alternatively a differential manometer may be connected between these two points to measure the loss of head. Meters are installed for measuring discharges at inlet and outlet of the filter, and also at back wash.