Environmental Engg. 3(2+1)

Lesson-13 Water treatment - Filtration

Theory of filtration

The process of filtration forms the most important stage in the purification of water.  It usually consists in allowing water to pass through a thick layer of sand.  It has been noticed from experience that during the process of filtration, the following effects occur on water:

1. The suspended and colloidal impurities which are present in water in a finely divided state are removed to a great extent

2. The chemical characteristics of water are altered

3. The number of bacteria present in water is also considerably reduced.

The theory of filtration to explain why such effects take place is based on the following four actions:

1. Mechanical straining

2. Sedimentation

3. Biological metabolism

4. Electrolytic changes

Mechanical straining

The suspended particles which are unable to pass through the voids of sand grains are arrested and removed by the action of mechanical straining.

Sedimentation

The voids between sand grains of filter act more or less like small sedimentation tanks.  The particles of impurities, arrested in these voids, adhere to particles of sand grains, mainly for the following two reasons:

1. Due to the presence of a gelatinous film or coating developed on sand grains by previously caught bacteria and colloidal matter and

2. Due to the physical attraction between the two particles of matter.

Biological metabolism

The growth and life process of the living cells is known as biological metabolism and the action of filter is explained on the basis of biological metabolism.  When bacteria are caught in the voids of sand grains, a zoological film is formed around the sand grains.  The film contains large colonies of living bacteria.  The bacteria feed on the organic impurities contained in water.  They convert such impurities into harmless compounds by the complex biochemical reactions

Electrolytic changes

The action of filter is also explained by the ionic theory.  It states that when two substances with opposite electric charges are brought into contact with each other, the electric charges are neutralized and in doing so, new chemical substances are formed.  It is observed that some of the sand grains of filter are charged with electricity of some polarity.  Hence, when particles of suspended and dissolved matter containing electricity of opposite polarity come into contact with sand grains, they neutralize each other and it ultimately results in the alteration of chemical characteristics of water.  After some interval of time, the electrical power of sand grains gets exhausted.  At that time, it becomes necessary to clean the filter and restore it with its property.

Filter sand

The sand to be used for filter should be free from clay, loam, vegetable matter, organic impurities, etc.  It should also be uniform in nature and size.  The filter sand is classified on the basis of its effective size and uniformity coefficient.

The effective size of sand indicates the size of sieve in mm through which ten per cent of the sample by weight will pass.

The uniformity coefficient of sand is the ratio of sieve size in mm through which 60 per cent of the sample of sand by weight will pass to the effective size of sand.  For instance, suppose the effective size of sand is 0.50 mm. If 60 percent of sand from the same sample passes through 0.60 mm sieve, the uniformity coefficient will be 0.60 / 0.50 = 1.20

Instead of sand, sometimes anthrafilt is used. It is made from anthracite which is a stone-coal that burns nearly without flame or smoke.  It almost entirely consists of carbon.  This material is found to possess many advantages such as low cost in handling, high rate of filtration, durability, better efficiency, etc.  But as sand is readily available, the usual practice is to recommend bed of sand for filters.

Classification of filters

The filters are classified into the following categories:

1. Slow sand filters

2. Rapid sand filters

3. Pressure filters

SLOW SAND FILTERS

Purpose

In case of slow sand filtration, the water is allowed to pass slowly through a layer of sand placed above the base material and thus the purification process aims at simultaneously improving the biological, chemical and physical characteristics of water.  The slow sand filtration is very well suited for rural areas in developing countries because of its simple operation and maintenance procedures.  It thus provides safe drinking water at low recurrent cost.

Rate of filtration

The rate of filtration for a normal slow sand filter varies from 100 to 200 litres per hour per m² of filter area.

Efficiency of slow sand filters

1. 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 per cent of bacterial load from raw water and this percentage may be as high as 99.50 to 99.90, when pre-treatment has been given to the raw water.  However, for complete removal of bacteria, disinfection is essential.

2. 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 per cent of colour of raw water.

3. 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 and bring down is turbidity below 50 ppm

 RAPID SAND FILTERS (GRAVITY TYPE)

Purpose

The great disadvantage of a slow sand filter is that it requires considerable space for its installation.  This requirement makes it uneconomical for places where land values are high.  The area required for slow sand filter, only for a moderate town of 15000 population, works out to be 1000 m² and with future expansion, other additional equipment etc., the area required for water supply project would be about 2000 m² or so.

The difficulty of requiring more space for slow sand filters led the engineers and scientists to find out means to increase the rate of filtration.  It was observed that rate of filtration can be increased in two ways:

1. By increasing the size of sand so that friction to water passing through filter media is minimized and

2. By allowing water to pass under pressure through the filter media

The former is achieved in rapid sand filters (gravity type) and it is the most popular method of filtration for public water supply projects.  The latter principle is adopted in the working of pressure filters.

Efficiency of rapid sand filter

1. Bacterial load:  The rapid sand filters are less effective in the removal of bacterial load.  It is expected that they remove about 80 to 90 per cent of bacterial impurity present in water.

2. Colour:  The rapid sand filters are highly efficient in colour removal and the intensity of colour can be brought down below 10 on cobalt scale.

3. Turbidity:  The rapid sand filters can remove turbidity to the extent of 35 to 40 ppm.  As water entering rapid sand filter is invariably given the treatment in coagulation sedimentation tank, it posses less turbidity.  This turbidity is easily brought down to permissible limits by rapid sand filters.

 PRESSURE FILTERS

The term pressure filter indicates that a filter is enclosed in space and water passes under pressure greater than atmospheric pressure.  This pressure can be developed by pumping and it may vary from 0.3 to 0.7 N/mm².

Rate of filtration

The rate of filtration of pressure filters is high as compared to that of rapid sand filters.  It is about 6000 to 15000 litres per hour per m² of filter area as compared to that of 3000 to 6000 litres per hour per m² of rapid sand filters.

Efficiency

The pressure filters are found to be less efficient than rapid sand filters in terms of bacterial load, colour and turbidity.

 Suitability

The pressure filters are not suitable for public water supply projects.  But they can be installed for small water supply water projects such as colonies of a few houses, industrial plants, private estates, swimming pools, railway stations, etc.

Double filtration

Sometimes the water is filtered twice to achieve better results.  This known as double filtration and it may be carried out in different ways as follows:

1. The water is allowed to pass through two or more slow sand filters arranged one after the other.

2. The water is allowed to pass through two or more rapid sand filters arranged one after the other.

3. The water is allowed to pass through a rapid sand filter before it is sent  to a slow sand filter.

In practice, the last alternative is most commonly adopted to increase the rate of filtration   The rapid sand filter in such a case is known as a roughing filter.  The coarse materials are used in the construction of a roughing filter and consequently, its rate of filtration is as high as 7000 litres per hour per m² of filter area.  The roughing filters generally do not require water treated with the coagulant.

The double filtration, especially of last combination as stated above, is adopted at places where land available for the installation of slow sand filters is restricted.  The installation of roughing filters practically doubles the capacity of slow sand filters.