The process of water filtration is carried out in mechanical plants known as filters.
The filters are of various types, but basically, two main categories known as:
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1. Rapid Sand Filters; and
2. Slow Sand Filters.
Types # 1. Rapid Sand Filters:
There are a few designs of rapid sand filters, but most of them are improvisation to suit the local needs. All of them operate on the basic principle of passing the raw water through beds of sand and other media so that these beds retain suspended matter.
a. Rapid Gravity Sand Filters:
The most common filters are of the Rapid Gravity Sand-type which are operated with coagulants. They give good result when the incoming water has less than 10 JTU of turbidity, if the filters are operated for a short time to produce an acceptable filtrate when the settled water has a turbidity of up to 50 JTU.
In this, the flow of water is through sand. The column of water above the filter bed (water- head) is about 2 metre thick. The flow of water through the sand is streamline flow, meaning that the water flows along a line followed by a liquid, along least resistance, and the loss of head is proportional to the velocity. The reduction of the water-head due to filtration is also a function of the viscosity of water and the porosity of the sand. Therefore, the temperature and the cleanliness of the filter sand too control the performance of these filters.
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The filter sand is of grain size of 0.4 to 1.0 millimetre. The rapid gravity sand filter can only operate effectively as intended when coagulants have been earlier used for the water in the settlement stage. Interception of the suspended matter by sand is a complex process, but most of the suspended matter is removed by adhesion to the surface of the sand grains. As the filter becomes dirty due to, and in the course of, the filtration, the pores between the sand and gravel particles diminish in size. Under such a condition, the velocity in the waterways increases and the suspended matter is carried deeper into the filter.
The filtering action proceeds at depths and is only to a limited extend influenced by the formation of a film on the surface of the sand. If the filter works normally, the sticky floe particles do not pass through the intricate pattern of channels between the sand grains, and thus perfectly clear water emerges after filtration.
In the early part of the use of the filter, when the sand is clean and the floe particles are stopped at the higher levels of the sand bed, the pore space in the top layer of the sand gets clogged due to filling of the floe and dirt. This results in the increase of the hydrostatic pressure (pressure due to the water column above the sand bed). Under this condition, the floe particles penetrate to deeper levels of the sand bed.
Consequently, the loss of water head due to filtration of water increases. If the head loss in the sand at any point exceeds the static head of water on the filter, a vacuum gets induced. Expulsion of the air from the pore spaces between the sand grains, and the resultant binding of the filter bed are caused. Washing of the sand bed and the filter removes the vacuum and thereafter the filter would start working normally. The use of coarser sand is preferable in the filter as the turbidity of the filtrate occurs at lower loss of head than if fine sand is used.
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The whole system of a rapid gravity filter is enclosed in an open-topped concrete chamber. The filter unit consists of a bed of sand supported on a bed of gravel. The sand grains need to be sharp, fresh, siliceous and free from weathering, and of uniform size of 0.5 to 1.5 millimetre.
A set up of under-drains are embedded in these filters. The sand layer has a thickness varying between half and one metre. The supporting gravel bed may have a thickness of half metre, making the total thickness of the filter medium to 1 to 1.5 metre. The size of the gravel should be graded between 5 to 50 millimetres.
The bottom set up of the filters can be designed to suit the users’ need. The components of the filter such as collectors, nozzles and the porous plate at the bottom can be of any locally available designs and can vary based on the filter design. Likewise, the numbers of these components as well as the valves to control the inlet, outlet, raw water, filtrate and the drain to waste can also be variable to suit the size of the filters and the requirement.
The most important factor and requirement in the filters are the number of orifices that are uniformly arranged to collect the filtrate water, and deliver the air and the input water to the filter bed. The rapid gravity filter units can be constructed over an area of about one hundred square-metre. More than this area may result in complications such as raw water’s higher velocity, scour of sand bed by the passing raw water and so on, which may affect the performance of the filters.
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Higher capacity filter system can be achieved by increasing the number of the filter units, and not by increasing the area of the filter unit. The velocity of the raw water over the sand bed may be manipulated to about 10 to 15 kilometres per hour. Washing and replenishment of the sand, and scouring of the sand bed to release the interstitial air as also other maintenance of the filter can be carried out by machines and appropriate designs can be given to such machines. The idea is to carry out these exercises in minimum time so that the filtration process goes on unhampered.
b. Multi-Layer Filter or Mixed Media Filter:
This filter type is a refinement of the Rapid Gravity Sand-type filter. In this, instead of a bed of sand supported on gravel with particles of roughly similar density but greater in size as in the rapid gravity filter, layers consisting of media of varying densities are employed. A very coarse upper layer of lightweight material can provide increased void spaces to accommodate the larger impurities removed first from the incoming water. Usually anthracite coal or vesicular pumice, both of which are lighter than the sand, is placed as a layer over a sand layer in the filter design.
The void spaces between the sand grains of the sand bed in the rapid gravity filter being small, there is limited or no capacity to retain the impurities. Therefore, the rapid gravity filter gets dry quickly and very frequently. In the multi-layer filter, this problem is overcome due to the improved design of the filter, which consists the lighter but coarser layer on top over a bed of sand arranged over the gravel bed.
This arrangement removes the floe at the top itself, before it reaches the sand layer that lies quite down. With the turbidity of input water less than 5 JTU, this filter gives good performance and the filter runs longer than the rapid gravity sand filter. The filtrate water that comes out of this filter is of superior quality.
The Pressure Filters are very similar to the Rapid Gravity type filters, except that the pressure filters are enclosed in steel pressure cylinders (Figure 9.17).
The filter shell cylinders are of usually 2.4 metre diameter, and they can be installed either vertically or horizontally. The horizontal units have a length of 15 metres. Therefore, these units have 15 x 2.4 = 36 square-metre sand area. They are used where hydraulic conditions in the system make their adoption desirable and they can be installed at any point in a pressure pipeline without interfering with the hydraulic gradient.
The rapid gravity filters always are preferable to the pressure filters because not only one can see and monitor them closely due to the direct visibility, but also it is possible to regulate the flow, mix the coagulants and perform pre-treatment of the input water in the rapid gravity filters.
The pressure filters are common under such conditions where the input water from the reservoir is clear and it is transported in long and steeply inclined pipelines. In the pressure filters, water can be filtered without preliminary treatment in settling tanks and without disrupting the hydraulic gradient.
In keeping with the adage that ‘the necessity is the mother of inventions’, in the middle of the 20th Century, Iraq had indigenously designed simple form of treatment plants with the aid of inexpensive locally available materials, to cut down the delays due to import of the filter-plant equipments.
These filters comprised three basic elements of functions, namely a settling basin, a rapid gravity filter and a high-level tank. The settling basins are of common horizontal-flow type, and the rapid gravity filter is the same. The high-level tanks are built on concrete platform structures or from bolted steel plates in such a way that they can take the desired load.
These filters have advantages. The flow control is automatically provided because the increased loss of water head due to the filter becoming dirty is balanced by a fall of water level in the pure water tank; this averts the need for the flow controlling devices. Second, constant output is ensured by the high lift pump, since whatever is the state of the filter, the speed of operation has to match the constant speed of the pump. Third, there is no artificial head loss in this type of filters and therefore there is significant gain in the efficiency.
Finally, the low cost of Iraq-type filters and the use of locally available materials makes it an attractive initial proposition. The disadvantage of the Iraq-type filters is that they are small in size and therefore where bulk filtered water is needed, one has to go for more parallel similar units; this offsets the advantage of the basic low-cost of individual filter units.
Types # 2. Slow Sand Filters:
In these filters, a bed of about one metre thick sand is spread over graded gravel bed in which under-drains of open-jointed tiles are buried. The sand is spread to the full depth of the bed near the outer walls. The important difference of the slow sand filters from the rapid sand filters is that the sand in the slow sand filters is finer than the latter.
Further, in the slow sand filters, no pre-treatment or chemicals are necessary. The raw water needs to be strained to remove coarse suspended materials and can be directly fed into the filter for filtration. Locally available sand can be used. The water column above the sand bed can be of about 1.5 metre.
From top to bottom, the filter beds consist of sand, fine-size gravel, medium-size gravel and coarse gravel of about lm, 5 cm, 5 cm and 15 cm thickness respectively. The size of the sand should be of 0.2 to 0.4 millimetre, of the fine-size gravel can be 5 to 10 millimetre, of medium- size gravel can be 10 to 25 millimetre and coarse gravel is generally of about 25 to 80 millimetre size.
The main advantage of slow sand filters is that the output water is of high quality without bacteria. Since no chemical treatment is required before filtration, there is no chance of chemical traces affecting the quality of the filtrate. However, they occupy large areas and the construction cost is higher. Highly turbid water gets stagnated in the slow sand filter, which is another disadvantage.
The main difference between the slow sand filters and the rapid sand filters is that the filtering action in the former takes place mainly at or near the surface of the sand. A thin veneer of impurities gets built up at the top of the sand bed and this requires frequent cleaning. The filtration is slow and the loss of water head is small depending upon the cleanliness of the sand bed’s top.
The higher water-head required over the sand bed is achieved by making adjustments using the outlets. When the filtration rate goes down, the top layer (about 10 cm) of the sand bed is scraped, taken out and washed to remove the veneer impurities. The process of filtration is continued after restoring the sand bed thickness to the original level. Similarly, the gravel beds would also require periodic servicing.