Slow And Filters And Rapid Sand Environmental Sciences Essay

Water Purification is a process in which the unhealthy water that is contaminated, unclean, adulterated, and poisoned is converted into safe drinking water through various stage processes rendering it safe for drinking and everyday use. Water Purification is done almost every day in every part of this world. The increasing population and industrialization is generating more and more threat to the drinking water sources as a lot of harmful chemicals are being released either intentionally or unintentionally into the sources. In most of the cases we cannot see these hazardous chemicals, but they pose a great threat to our health and lives.

To treat water before being brought to use, various purification methods have been developed. They vary in their design, method of filtration, equipment used, efficiency, location and various other factors.

Natural water is available from various sources like ground water, lakes, rivers and pond. It is very hard to find a pure water source in nature. Making the need to treat water and purify it before consumptions makes important. One of the objectives of water treatment and purification is to treat water from the best possible source to improve its various physical qualities, improving its taste and odour and eliminating all of the harmful contaminants.

Various impurities can be classified as suspended impurities(remain in suspension), dissolved impurities(dissolved impurities and responsible for taste and odour) and colloidal impurities( electrically charged and never settle). Table 1 shows the various constituents of the impurities present in water and there effects. Ss+Tech+guide.

Water for domestic use should be :

Colorless and have no suspended solids;

Odour free and must taste good;

Free from disease causing organisms;

Free of halmful salts and objectionable minerals;

Free from dissolved gases which can he harm full;

Should be non-corrosive in nature; Ss+Tech+guide.

Table 1: Suspended and dissolved impurities in water ss+tech+guide

One of the most important parts of the treatment process is Filtration. Filtration is a process in which solids particles in suspension in liquid medium are passed through porous media through which only liquid can pass away. It is required in conjugation with some kinds of chemical treatments to disinfect water of biological disease causing pathogens. [2] Crittenden, J. C. (2005). Water treatment: Principles and design (2nd ed., p. 868). New Jersey: John Wiley & Sons, Inc.

The various types of filters used are

Gravity Filters

Vacuum Filters

Pressure Filters

Centrifugals and Centrifuges


Suspended matter present in water makes the appearance of water cloudy; this physical characteristic is known as turbidity. This suspended mater may consist of floc, microorganisms, algae, iron, silt and manganese precipitates and also some precipitates which remain after the various earlier stages.

These suspended materials are filtered by passing water through granular beds, mostly composed of layers of sand, gravel, coal. As the process goes on and on and when the head lost is more than the permissible value the filter bed is backwashed or top soil is scrapped off and brought back into operation. But there need to be a second tank which will be operation as the first one is being backwashed or top soil is scraped off.

The various other important processes in the water purification process are:








Water requirements of a city/town can be divided into five categories: domestic or

residential use

institutional use

public or civic use

industrial use

water system losses


A sand filter is very simple in its opertion and design. It is merely a bed of sand with support of layer of gravel, in a big box like structure. Provided with various fitting for inlet and outlet of water. A set of valves to control head during various situations. Below the gravel layer are underdrains required to remove the filtered water. Before dissipaion into the sand bed the energy of the water is to be reduced so that it doesnot erode the sand bed, also reffered to as “short circuiting”. There is headloss during the fileration process, it occours mainly due to accumulation of particles at top surface or due to microbial growth at he surface. The filters are backwashed when stage is reached below which any head loss cannot be tolerated. For backwash the outlet valves are closed and water is made to move in opposite direction. Manual of slow sand filters


Thogh both slows sand filters and rapid sand filters function under the action of gravity and mostlly have same constituent parts yet they differ in various factors which are explaind in Table 2 below.

Slow Sand Filters

Rapid Sand Filters


Rate of Filtration

1-3-10 m/day

100-125-300 m/day


Depth of Bed

0.30m of gravel

1.0m of sand

0.45m of gravel

0.75m of sand


Size of Sand

0.25 to 0.35 mm

Uniformity Coeff:

2 to 3

0.45mm and higher

Uniformity Coeff:

1.5 and Lower


Length of run

20 to 30 to 60 days

12 to 24 to 48 hours


Penetration of Suspended Matter




Preperatory treatment of Water

Generally aeration

Flocullation and Sedimentation


Method of Cleaning


Back washing


Cost of Construction


Depreciation of plants








Amount of Wash Water

0.2 to 0.6% of water filtered

1-4-6% of water filtered

Table 2: Comparison between Slow and Rapid Sand Filters. Class notes


3.1: Introduction

Slow Sand Filters are the original form of Filtration. The history date backs to 1804 when John Gibb of Scotland used the technique of filtration to treat water for his bleachery. He used to sell the surplus water to the public. The first large scale use of sand filters was in London to treat water of River Thames in 1820.

Slow Sand filters are a viable water treatment solution. Though there are some difficulties faced involved in implementation and working of the system. It possess high initial cost and its use has seen a declining phase due to few resasons but is again gaining importance after the outbreak of Cryptosporidium in Milwaukee(1993) as they are superior to Rapid Sand Filters in removing pathogenic microorganisms. The various advantages of slow sand filters over Rapid sand filters are that it is highly efficient in removal of bacteria and viruses responsible for transmitting water related diseases. Also there is no usage of chemicals involved neither do we need any highly skilled and continuous supervision. Undeniably slow filteration is best suited for rural areas as there are low running costs. Slow sand filters

What happens in Slow Sand Filters is that influent water seeps through the sand bed under the action of gravity. The sand used is less uniform as compared to that used in the Rapid Sand Filters. The most of the filtration is performed in the top few centimetres of the sand bed. The process involves two stages, one being the filtration stage and the other being the regeneration stage. Slow sand filters are not backwashed as Rapid Sand Filters, in place the filter is drained off and the top few centimetres of sand media are scraped off and the filter is placed back to work. 4 prelimi

3.2 Advantages

The various advantages which slow sand filters posses are that theyare really very simple in design and operation. They also require minimum chemical and power requirements thereby making it an appropriate technique for removal of suspended organic and inorganic matter. They are also very effective in removal of pathogenic organisms from water source. Because of this reason they are gaining back importance as compared to Rapid sand filters. It also helps in reduction of bacteria and various organic levels, thus helping to reduce the amount of disinfectans to be used and thereby reducing the disinfection byproducts in treated water. Other advantages which they posses are that ther are minimal sludge handling problems, no expert supervison is required and also local labor and materials can be brought to use.

3.3 Disadvantages

On the other hand it possess few limitations such as they require a large land area using large quantities of filter media. It also requires more of manual labour for cleaning. The sand filter can get easily clogged if turbudity levels are high in source water. Even if the nutrient content is low it may hinder in the turbudity removal action as some nutrient are needed for promotion of biological ecosystem growth. They do not completely remove all organic chemicals and dissolved inorganic sustances. Also very fine clay particles are not easily removed using slow sand filters. As the water temperature drops the biological activity drops and similar is the case in slow sand filters that if the influent water temperature is low it will decrease the biological activity within the filter making it less effective.

3.4 Factors influencing Efficiency:

The removal of microbes and organism is slow sand filters are performed by biological processes. And the various factors affecting the growth and functionality of bacterial growth also affect eh efficiency of slow sand filters. The various factors can as stated as below’

Time: the minimum and bed depth should not be less than 0.7m and flow rate around 0.1 and 0.3 m per hour, as higher the time for reaction on the filter bed higher would be the efficiency attained.

Oxygen: Oxygen is important for bacteria as it uses it for its activity. If the amount of oxygen in incoming water is low or there is high organic content it would cause the filter to be less effective.

Temperature: temperature is not an issue in hot countries but in countries with cold conditions they affect the efficiency as lower the temperature lower will be the biological activity in the filter bed.

Maturation: Sufficient time should be provided for the biological activities to take place. Also the filter should not be cleaned too frequently. Ss tech guide for slow sand filters


4.1 Design Principles

4.1.1Sizing the Filter Bed

The very first step in the design is to size the bed. The depth and base area are two parameters which help to drive the rest of the design. Bed area is determined by the Hydraulic Loading Rate (HLR) selected. Hlr should be so selected such that it considers both the cariation over daily cycle and the increase each year. Pilot plant studies performed can help in determining performance characteristics. Area

The bed area is calculated using the following equation.

In which HLR= hydraulic loading rate (m3/m2/hr or mgad)

Q = flow (mil L/d or mgd)

A = bed area (m2 or acres) Depth of Sand

The depth of sand bed is determined using the number of years the filter is to be designed before there is any need for resanding. Year of operation is determined as follows

Where Y = years of operation

Di = depth of initial sand bed

Df = Depth of final sand bed before rebuilding is required

R = depth of removal

F = frequency


The various functions for which hydraulic analysis are required are to distribute raw water on the sand bed, collection of water, drainage of headwater, control of flow through the filter, and measurement of the headloss. DISTRIBUTION

The influent water needs to be distributed throughout the surfaceare uniformly as if entire influent water is delivered at same point, short circuiting of flow would occour. To control this kinetic energy of flow must be either distributed or dissipated. It is a matter of judgment to choose exit velocity and pipe size as there is not much of flow criteria in existence. Figure below shows a system of pipes for how distribution of water may be done.

Fig : showing an approach for distributin of water over sand bed. Collection

A system of underdrains need to designed for the collection of filtered water. Drainage

For the need to scrap the sand bed, the headwater must be drained off to a level below the the sand bed surface. The figure below explains how the drainage occours in two stages. One being done at the top surface removing the top portion of headwater and the second being at the bottom removing the rest of remaining water.

Fig: Shows drainage points BACKFILLING

After the drainage has been done and the scrapinf of the sand bed has been done there is need to backfill. Fot this we may use the filtered water from other sand filters operating in parallel. The filtered water is fed back to the recenlty scraped filter system using a system of valves as shown in the following figure. The Part a shows the filters working in normal condition with all valves closed. Part b shows the backfilling of filter 1 from filter 2 nad the filter 3 continues with its operations. The backfilling is done till the depth of water reaches around 1 ft above the sand saurface. It should be boted that amount of water in storage tanks should be sufficient to satisfy the demand as filter 1 and 2 are not in operation.

fig. showing the various stages of backfilling. FLOW MEASUREMENT

There is a need to monitor the influent and effluent flow throughout the operation so as to maintain a constant supply and hydraulic head to satisfy the demand. For this various flow measuring devices are installed at various points like at inlet to the pland and at oultel. Also they are applied at the various inlets and outlets to individual filters. Also these meters help in keeping a record for the water demands of the community. Fot it various euipments are used such as …. Flow Control

There is a 24 need to control the flow of water. There is a need to keep a check on the influent amount of water so that a constant supply is maintained. Also there is need to control flow at the effluent end of the plant to adjust according to the daily nad hourly variations in water demand. Headloss Measurement

Headloss measurement is important for the continous operation of the filter and to maintain a constant head throughout the supply. When the loss of head if below the required value is reached the scraping of filter is done followed by backfilling. To measure the headloss Piezometers need to be installed. One of it needs to be connected above the sand bed and the other to the tailwater basin.

3.5.1 Design Capacity

Slow sand filtration is best suited and cost effective for population range between 30,000 to 40,000 as compared to rapid sand filtration. It shoul not be used for population higher this range because for it we need to build more number of filteraton units, and for that larger area would be required. Also if there is need for additional pre-treatment facilities like sedimentation, as always is the case, more area would be required.

3.5.2 Quality considerations of influent water

The limit for turbudity of influent water should not be higher than 20 Nephelometric Turbidity Units (NTU). But for ideal conditions it should be less than 10 NTU. If the turbudity for influent water is higher than 20 NTU, the water need to be pretreated with facilities such as grit chamber, settlink tank or a combination of various other pre-filtration systems as shown in figure below. Ss tech guide

Figure 2: Schematic sequence of water treatment facilities when using slow

sand filter ss tech guide Pretreatment Facilities

Fot the pretreatment of the water we may make use of Grit chambers, sedimentation tanks or roughing filters. Each of these is explained as follows. Grit Chambers: These are also called as coarse sedimentation tanks. The maximum speed of water flow is around 0.75m/s. In case of the retention time it need not be very long, just a few minutes would be sufficient. In this coarse particles are removed before water goes to the settling tanks. Sedimentation tank: Settling of fine suspended particles is done in the sedimentation tanks. The detention time is around 2 hours. These are further of various types like type I, II, III and IV.

3.5.6 Design criteria

For the design of slow sand filters various design criteria shown in Table 3 can be used. Also to make sure there is adequate water supply and slow sand filter function properly, the flow rate should be maintained between 0.1 to 0.3 cubic meter of water per hour per square metre area of filter media.

Table 3: General design criteria ss tech guide

3.5.6 Determination of size and number of slow sand filters: From the point of effluent quality, the weakest part is the edge of the filter bed. As the raw water may leak and pass the sand filter if attention is not taken while the design and operation of the filter. The way to minimise this is that the filters should not be made too small. The recommended size is around 5 m2, a workable size is around 100 m 2 , with a maximum of 200 m2.

The system needs to have a minimum of 2 filters, to make sure one is in operation while the other is being cleaned. But a recommended value of 4 is good to ensure an increased and sufficient amount of treated water supply. The following empirical formula can be used to calculate an approximate number of filters required.

n = ½(A) (1/3), or

n = ¼ (Q) (1/2)


n – Number of filter units

A – Total required area in m2

Q – Average daily water demand expressed in m3/hr

3.5.7 Filter bed and filtration sand size of slow sand filter: There are various alteranatives to sand in sand filter bed like crushed coral or burnt rice husk. But the use of sand remains the most efficient and effective. The sand to be used is expressed in its effective size (D10) and its uniformity Coefficient.

Where D10 is defined as the sieve opening that permits passage of 10% of sand by weight. And Uniformity coefficient is defined as the ratio between D60 and D10.

UC = D60 / D10.

The values of effective size and uniformity coefficient can be determined by performing sieve analysis as follows. Also the value of D10 for a continually operated filter is around 0.15 to 0.30mm.

A mixture of 4 or 5 smaples is taken randomly to form a representative sample for the sieve analysis. It is washed to remove any impurities.

Take 500 gm of the sample and using a mechanical sieving shaker, sieve it for 15 minutes.

Sand retained on each sieve is weighed and added to the previous one.

The grain shape for the sand filter should be perfectly round. It should be free from any clay, soil or organic matter. If the influent water is expected to be high in carbon dioxide, then there should be less than 2% of calcium and magnesium carbonates.

Three important considerations should be kept in mind before deciding on the thickness of the filter bed:

Immidetialtely below the top surface lies a zone in which purifying bacteria thrive. The thickness of this zone is around 0.3 and 0.4 m(in case of high filtration rate)

Under this zone chemical reactions take place. It may be described as mineral oxidation zone, in which organic materials released in zone 1 are chemically degrade. Thickness of this zone is around 0.4 and 0.5 m(when water has high organic content). In total the thickness of zone 1 and 2 should not be less than 0.7m in any case.

With Continuous operation for one to three months the filter starts to get clogged and the top 1-2 cm of sand bed is scrapped off including the filter skin. This layer is not replaced immediately, but done after some time. So provisions should be made for successive cleaning throughout the period.

3.5.8 Under-drains

Under drains helps in efficient operation of filter. It assists in both providing support to the filter medium and to drain of the treated water to escape from the underside of the filter. Since it is laid below the sand bed, the under drainage system cannot be cleaned or repaired without any major disturbance to the sand bed. So care should be taken while designing that it doesn’t get choked by granular material.

One of the simplest designs consists of the main and lateral drains. Lateral drains consists of perforated PVC pipes or glazed pipes laid with open joints covered with gravel with successively increasing grain size to prevent intrusion to filter medium. In large filters it is mostly constructed of concrete as compared to PVC pipes in small filters. Figure … shows various arrangements for construction of under drain. Special considerations need to be taken care of while designing under drain system for perforated pipes as explained in table…

Figure: Arrangements for main drain

Figure: various types of filter bottoms

Table: Criteria for dimensioning under drain using perforated pipes

Factors to be considered for the gravel layer supported by the under drains.

The gravel layer is built of number of layers, with fine at top to coarse at the bottom with gradually graded grains not differing by a factor of not more than 1.41.

The bottom layer of gravel should have a grain diameter of at least double the size of the openings into the drainage system.

Each successive layer should be graded such that the its smaller dia (D10) particle diameter are not less than four times than those of layer immediately below.

If the joints are 8mm or less wide, the following values for D10 and D90 would be sufficient with three layers.



Top Layer



Second Layer



Third Layer



In slow sand filters the gravel should conform to specifications similar to filtering medium, like it also should be hard, rounded, free from sand, clay, dirt and with specific gravity of at least 2.5. Also after immersion in concentrated hydrochloric acid for 24 hrs. The weight lost should not be more than 5% by weight. For the thickness of the layer, it should be at least 3 times the diameter of the largest stone. The gravel layer should be placed with care as any movement may disturb the filter sand above or may choke the under drain.

3.5.6 Factors influencing slow sand filtration efficiency

Since the process of removal of microbes and organisms in filter is completely biological, the efficiency depends on various factors such as time, temperature, oxygen and the beneficial microbes. They are explained as follows

Time: For biological processes to take place time plays a critical role. So sufficient amount of time should be provided for the reactions to take place. Time is determined by the depth of sand bed provided and the flow rate. The sand depth should not be less than 0.7m and flow in between 0.1 and 0.3 m per hour.

Oxygen: Oxygen I required for the activity of bacteria in the filter bed. So sufficient amount of oxygen level should be maintained in incoming water as low amount would affect the efficiency of the slow sand filter.

Temperature: Hot temperature is best for the bacterial activity. Its not a problem in hot countries, but in cold countries attention need to be given to the temperature conditions.

Maturation: Bacterial growth is not that fast during the initial stages of the filter and also after every time it is cleaned. So it is suggested not to clean the filter bed too often.

3.5.7 Effluent Quality

The performance of slow sand filter in terms of various factors like colour, turbudity, organic matter, etc. are given in table….

Table.. Performance of slow sand filter

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