Slow And Filters And Rapid Sand Environmental Sciences Essay

Water Purification is a procedure in which the unhealthy H2O that is contaminated, dirty, adulterated, and poisoned is converted into safe imbibing H2O through assorted phase processes rendering it safe for imbibing and mundane usage. Water Purification is done about every twenty-four hours in every portion of this universe. The increasing population and industrialisation is bring forthing more and more menace to the imbibing H2O beginnings as a batch of harmful chemicals are being released either deliberately or accidentally into the beginnings. In most of the instances we can non see these risky chemicals, but they pose a great menace to our wellness and lives.

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To handle H2O before being brought to utilize, assorted purification methods have been developed. They vary in their design, method of filtration, equipment used, efficiency, location and assorted other factors. hypertext transfer protocol: //

Natural H2O is available from assorted beginnings like land H2O, lakes, rivers and pool. It is really difficult to happen a pure H2O beginning in nature. Making the demand to handle H2O and sublimate it before ingestions makes of import. One of the aims of H2O intervention and purification is to handle H2O from the best possible beginning to better its assorted physical qualities, bettering its gustatory sensation and smell and extinguishing all of the harmful contaminations.

Assorted drosss can be classified as suspended drosss ( remain in suspension ) , dissolved drosss ( dissolved drosss and responsible for gustatory sensation and smell ) and colloidal drosss ( electrically charged and ne’er settle ) . Table 1 shows the assorted components of the drosss present in H2O and there effects. Ss+Tech+guide.

Water for domestic usage should be:

Colorless and have no suspended solids ;

Odour free and must savor good ;

Free from disease doing beings ;

Free of halmful salts and obnoxious minerals ;

Free from dissolved gases which can he harm full ;

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

Table 1: Suspended and dissolved drosss in H2O ss+tech+guide

One of the most of import parts of the intervention procedure is Filtration. Filtration is a procedure in which solids atoms in suspension in liquid medium are passed through porous media through which merely liquid can go through off. It is required in junction with some sorts of chemical interventions to disinfect H2O of biological disease doing pathogens. [ 2 ] Crittenden, J. C. ( 2005 ) .A Water intervention: Principles and design ( 2nd ed. , p. 868 ) . New Jersey: John Wiley & A ; Sons, Inc.

The assorted types of filters used are

Gravity Filters

Vacuum Filters

Pressure Filters

Centrifugals and Centrifuges


Suspended affair nowadays in H2O makes the visual aspect of H2O cloudy ; this physical feature is known as turbidness. This suspended mater may dwell of floc, micro-organisms, algae, Fe, silt and manganese precipitates and besides some precipitates which remain after the assorted earlier phases.

These suspended stuffs are filtered by go throughing H2O through farinaceous beds, largely composed of beds of sand, crushed rock, coal. As the procedure goes on and on and when the caput lost is more than the allowable value the filter bed is backwashed or top dirt is scrapped away and brought back into operation. But at that place necessitate to be a 2nd armored combat vehicle which will be operation as the first 1 is being backwashed or top dirt is scraped off.

The assorted other of import procedures in the H2O purification procedure are:








Water demands of a city/town can be divided into five classs: domestic or

residential usage

institutional usage

public or civic usage

industrial usage

H2O system losingss


A sand filter is really simple in its opertion and design. It is simply a bed of sand with support of bed of crushed rock, in a large box like construction. Provided with assorted suiting for recess and mercantile establishment of H2O. A set of valves to command caput during assorted state of affairss. Below the crushed rock bed are underdrains required to take the filtered H2O. Before dissipaion into the sand bed the energy of the H2O is to be reduced so that it doesnot gnaw the sand bed, besides reffered to as “ short circuiting ” . There is headloss during the fileration procedure, it occours chiefly due to accretion of atoms at top surface or due to microbic growing at he surface. The filters are backwashed when phase is reached below which any caput loss can non be tolerated. For slipstream the mercantile establishment valves are closed and H2O is made to travel in opposite way. Manual of slow sand filters

Chapter 2: Comparison

Thogh both slows sand filters and rapid sand filters function under the action of gravitation and mostlly have same component parts yet they differ in assorted 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 crushed rock

1.0m of sand

0.45m of crushed rock

0.75m of sand


Size of Sand

0.25 to 0.35 millimeters

Uniformity Coeff:

2 to 3

0.45mm and higher

Uniformity Coeff:

1.5 and Lower


Length of tally

20 to 30 to 60 yearss

12 to 24 to 48 hours


Penetration of Suspended Matter




Preperatory intervention of Water

By and large aeration

Flocullation and Deposit


Method of Cleaning


Back rinsing


Cost of Construction


Depreciation of workss








Sum of Wash Water

0.2 to 0.6 % of H2O filtered

1-4-6 % of H2O filtered

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

Chapter 3: Slow Sand Filters

3.1: Introduction

Slow Sand Filters are the original signifier of Filtration. The history day of the month dorsums to 1804 when John Gibb of Scotland used the technique of filtration to handle H2O for his bleachery. He used to sell the excess H2O to the populace. The first big graduated table usage of sand filters was in London to handle H2O of River Thames in 1820.

Slow Sand filters are a feasible H2O intervention solution. Though there are some troubles faced involved in execution and working of the system. It possess high initial cost and its usage has seen a worsening stage due to few resasons but is once more gaining importance after the eruption of Cryptosporidium in Milwaukee ( 1993 ) as they are superior to Rapid Sand Filters in taking infective micro-organisms. The assorted advantages of slow sand filters over Rapid sand filters are that it is extremely efficient in remotion of bacteriums and viruses responsible for conveying H2O related diseases. Besides there is no use of chemicals involved neither do we necessitate any extremely skilled and uninterrupted supervising. Undeniably slow filteration is best suited for rural countries as there are low running costs. Slow sand filters

What happens in Slow Sand Filters is that inflowing H2O seeps through the sand bed under the action of gravitation. The sand used is less unvarying as compared to that used in the Rapid Sand Filters. The most of the filtration is performed in the top few centimeters of the sand bed. The procedure involves two phases, one being the filtration phase and the other being the regeneration phase. Slow sand filters are non backwashed as Rapid Sand Filters, in topographic point the filter is drained away and the top few centimeters of sand media are scraped off and the filter is placed back to work. 4 prelimi

3.2 Advantages

The assorted advantages which slow sand filters posses are that theyare truly really simple in design and operation. They besides require minimal chemical and power demands thereby doing it an appropriate technique for remotion of suspended organic and inorganic affair. They are besides really effectual in remotion of infective beings from H2O beginning. Because of this ground they are deriving back importance as compared to Rapid sand filters. It besides helps in decrease of bacteriums and assorted organic degrees, therefore assisting to cut down the sum of disinfectans to be used and thereby cut downing the disinfection byproducts in treated H2O. Other advantages which they posses are that ther are minimum sludge managing jobs, no expert supervison is required and besides local labour and stuffs can be brought to utilize.

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3.3 Disadvantages

On the other manus it possess few restrictions such as they require a big land country utilizing big measures of filter media. It besides requires more of manual labor for cleansing. The sand filter can acquire easy clogged if turbudity degrees are high in beginning H2O. Even if the food content is low it may impede in the turbudity removal action as some food are needed for publicity of biological ecosystem growing. They do non wholly take all organic chemicals and dissolved inorganic sustances. Besides really all right clay atoms are non easy removed utilizing slow sand filters. As the H2O temperature drops the biological activity beads and similar is the instance in slow sand filters that if the inflowing H2O temperature is low it will diminish the biological activity within the filter doing it less effectual. hypertext transfer protocol: //

3.4 Factors act uponing Efficiency:

The remotion of bugs and being is slow sand filters are performed by biological procedures. And the assorted factors impacting the growing and functionality of bacterial growing besides affect eh efficiency of slow sand filters. The assorted factors can every bit stated as below ‘

Time: the lower limit and bed deepnesss should non be less than 0.7m and flow rate around 0.1 and 0.3 m per hr, as higher the clip for reaction on the filter bed higher would be the efficiency attained.

Oxygen: Oxygen is of import for bacteriums as it uses it for its activity. If the sum of O in incoming H2O is low or there is high organic content it would do the filter to be less effectual.

Temperature: temperature is non an issue in hot states but in states with cold conditions they affect the efficiency as lower the temperature lower will be the biological activity in the filter bed.

Maturation: Sufficient clip should be provided for the biological activities to take topographic point. Besides the filter should non be cleaned excessively often. Ss tech usher for slow sand filters http: //


4.1 Design Principles

4.1.1Sizing the Filter Bed

The really first measure in the design is to size the bed. The deepness and base country are two parametric quantities which help to drive the remainder of the design. Bed country is determined by the Hydraulic Loading Rate ( HLR ) selected. Hlr should be so selected such that it considers both the cariation over day-to-day rhythm and the addition each twelvemonth. Pilot works surveies performed can assist in finding public presentation features. Area

The bed country is calculated utilizing the undermentioned equation.

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

Q = flow ( mil L/d or mgd )

A = bed country ( M2 or estates ) Depth of Sand

The deepness of sand bed is determined utilizing the figure of old ages the filter is to be designed before there is any demand for resanding. Year of operation is determined as follows

Where Y = old ages of operation

Di = deepness of initial sand bed

Df = Depth of concluding sand bed before reconstructing is required

R = deepness of remotion

F = frequence


The assorted maps for which hydraulic analysis are required are to administer natural H2O on the sand bed, aggregation of H2O, drainage of headwater, control of flow through the filter, and measuring of the headloss. Distribution

The inflowing H2O demands to be distributed throughout the surfaceare uniformly as if full influent H2O is delivered at same point, short circuiting of flow would occour. To command this kinetic energy of flow must be either distributed or dissipated. It is a affair of judgement to take issue speed and pipe size as there is non much of flow standards in being. Figure below shows a system of pipes for how distribution of H2O may be done.

Figure: demoing an attack for distributin of H2O over sand bed. Collection

A system of underdrains need to designed for the aggregation of filtered H2O. Drain

For the demand to trash the sand bed, the headwater must be drained off to a degree below the the sand bed surface. The figure below explains how the drainage occours in two phases. One being done at the top surface taking the top part of headwater and the 2nd being at the underside taking the remainder of staying H2O.

Figure: Shows drainage points BACKFILLING

After the drainage has been done and the scrapinf of the sand bed has been done there is demand to backfill. Fot this we may utilize the filtered H2O from other sand filters runing in analogue. The filtered H2O is fed back to the recenlty scraped filter system utilizing a system of valves as shown in the undermentioned figure. The Part a shows the filters working in normal status 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 deepness of H2O ranges around 1 foots above the sand saurface. It should be boted that sum of H2O in storage armored combat vehicles should be sufficient to fulfill the demand as filter 1 and 2 are non in operation.

fig. demoing the assorted phases of backfilling. FLOW MEASUREMENT

There is a demand to supervise the influent and wastewater flow throughout the operation so as to keep a changeless supply and hydraulic caput to fulfill the demand. For this assorted flow measurement devices are installed at assorted points like at recess to the pland and at oultel. Besides they are applied at the assorted recesss and mercantile establishments to single filters. Besides these metres help in maintaining a record for the H2O demands of the community. Fot it assorted euipments are used such as aˆ¦ . Flow Control

There is a 24 demand to command the flow of H2O. There is a demand to maintain a cheque on the inflowing sum of H2O so that a changeless supply is maintained. Besides there is need to command flow at the outflowing terminal of the works to set harmonizing to the day-to-day nicotinamide adenine dinucleotides hourly fluctuations in H2O demand. Headloss Measurement

Headloss measuring is of import for the continous operation of the filter and to keep a changeless caput throughout the supply. When the loss of caput if below the needed value is reached the scraping of filter is done followed by backfilling. To mensurate 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 effectual for population scope between 30,000 to 40,000 as compared to rapid sand filtration. It shoul non be used for population higher this scope because for it we need to construct more figure of filteraton units, and for that larger country would be required. Besides if there is demand for extra pre-treatment installations like deposit, as ever is the instance, more country would be required.

3.5.2 Quality considerations of inflowing H2O

The bound for turbudity of inflowing H2O should non be higher than 20 Nephelometric Turbidity UnitsA ( NTU ) . But for ideal conditions it should be less than 10 NTU. If the turbudity for inflowing H2O is higher than 20 NTU, the H2O demand to be pretreated with installations such as grit chamber, settlink armored combat vehicle or a combination of assorted other pre-filtration systems as shown in figure below. Ss tech usher

Figure 2: Conventional sequence of H2O intervention installations when utilizing decelerate

sand filter US Secret Service tech usher Pretreatment Facilities

Fot the pretreatment of the H2O we may do usage of Grit Chamberss, deposit armored combat vehicles or roughing filters. Each of these is explained as follows. Grit Chambers: These are besides called as coarse deposit armored combat vehicles. The maximal velocity of H2O flow is about 0.75m/s. In instance of the keeping clip it need non be really long, merely a few proceedingss would be sufficient. In this harsh atoms are removed before H2O goes to the settling armored combat vehicles. Sedimentation armored combat vehicle: Settling of mulct suspended atoms is done in the deposit armored combat vehicles. The detainment clip is about 2 hours. These are farther of assorted types like type I, II, III and IV.

3.5.6 Design standards

For the design of slow sand filters assorted design standards shown in Table 3 can be used. Besides to do certain there is equal H2O supply and slow sand filter map decently, the flow rate should be maintained between 0.1 to 0.3 three-dimensional metre of H2O per hr per square metre country of filter media.

Table 3: General design standards ss tech usher

3.5.6 Determination of size and figure of slow sand filters: From the point of outflowing quality, the weakest portion is the border of the filter bed. As the natural H2O may leak and go through the sand filter if attending is non taken while the design and operation of the filter. The manner to understate this is that the filters should non be made excessively little. The recommended size is around 5 M2, a feasible size is around 100 m 2, with a upper limit of 200 M2.

The system needs to hold a lower limit of 2 filters, to do certain one is in operation while the other is being cleaned. But a recommended value of 4 is good to guarantee an increased and sufficient sum of treated H2O supply. The undermentioned empirical expression can be used to cipher an approximative figure of filters required.

n = A? ( A ) ( 1/3 ) , or

n = A? ( Q ) ( 1/2 )


n – Number of filter units

A – Sum needed country in M2

Q – Average day-to-day H2O demand expressed in m3/hr

3.5.7 Filter bed and filtration sand size of slow sand filter: There are assorted alteranatives to sand in sand filter bed like crushed coral or burned rice chaff. But the usage of sand remains the most efficient and effectual. The sand to be used is expressed in its effectual size ( D10 ) and its uniformity Coefficient.

Where D10 is defined as the screen gap that permits transition of 10 % of sand by weight. And Uniformity coefficient is defined as the ratio between D60 and D10.

UC = D60 / D10.

The values of effectual size and uniformity coefficient can be determined by executing sieve analysis as follows. Besides the value of D10 for a continually operated filter is about 0.15 to 0.30mm.

A mixture of 4 or 5 smaples is taken indiscriminately to organize a representative sample for the sieve analysis. It is washed to take any drosss.

Take 500 gram of the sample and utilizing a mechanical sieving shaker, sieve it for 15 proceedingss.

Sand retained on each screen is weighed and added to the old one.

The grain form for the sand filter should be absolutely round. It should be free from any clay, dirt or organic affair. If the inflowing H2O is expected to be high in C dioxide, so there should be less than 2 % of Ca and Mg carbonates.

Three of import considerations should be kept in head before make up one’s minding on the thickness of the filter bed:

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

Under this zone chemical reactions take topographic point. It may be described as mineral oxidization zone, in which organic stuffs released in zone 1 are chemically degrade. Thickness of this zone is around 0.4 and 0.5 m ( when H2O has high organic content ) . In entire the thickness of zone 1 and 2 should non be less than 0.7m in any instance.

With Continuous operation for one to three months the filter starts to acquire clogged and the top 1-2 centimeter of sand bed is scrapped off including the filter tegument. This bed is non replaced instantly, but done after some clip. So commissariats should be made for consecutive cleansing throughout the period.

3.5.8 Under-drains

Under drains helps in efficient operation of filter. It assists in both supplying support to the filter medium and to run out of the treated H2O to get away from the bottom of the filter. Since it is laid below the sand bed, the under drainage system can non be cleaned or repaired without any major perturbation to the sand bed. So attention should be taken while planing that it does n’t acquire choked by farinaceous stuff.

One of the simplest designs consists of the chief and sidelong drains. Lateral drains consists of pierced PVC pipes or glazed pipes laid with unfastened articulations covered with crushed rock with in turn increasing grain size to forestall invasion to filtrate medium. In big filters it is largely constructed of concrete as compared to PVC pipes in little filters. Figure aˆ¦ shows assorted agreements for building of under drain. Particular considerations need to be taken attention of while planing under drain system for pierced pipes as explained in tableaˆ¦

Figure: Agreements for chief drain

Figure: assorted types of filter undersides

Table: Criteria for dimensioning under drain utilizing pierced pipes

Factors to be considered for the crushed rock bed supported by the under drains.

The crushed rock bed is built of figure of beds, with all right at top to coarse at the underside with bit by bit graded grains non differing by a factor of non more than 1.41.

The bottom bed of crushed rock should hold a grain diameter of at least dual the size of the gaps into the drainage system.

Each consecutive bed should be graded such that the its smaller Defense Intelligence Agency ( D10 ) atom diameter are non less than four times than those of bed instantly below.

If the articulations are 8mm or less broad, the undermentioned values for D10 and D90 would be sufficient with three beds.

D10 ( millimeter )

D90 ( millimeter )

Top Layer



Second Layer



Third Layer



In slow sand filters the crushed rock should conform to specifications similar to filtrating medium, like it besides should be difficult, rounded, free from sand, clay, soil and with specific gravitation of at least 2.5. Besides after submergence in concentrated hydrochloric acid for 24 hour. The weight lost should non be more than 5 % by weight. For the thickness of the bed, it should be at least 3 times the diameter of the largest rock. The crushed rock bed should be placed with attention as any motion may upset the filter sand above or may choke the under drain.

3.5.6 Factors act uponing slow sand filtration efficiency

Since the procedure of remotion of bugs and beings in filter is wholly biological, the efficiency depends on assorted factors such as clip, temperature, O and the good bugs. They are explained as follows

Time: For biological procedures to take topographic point clip plays a critical function. So sufficient sum of clip should be provided for the reactions to take topographic point. Time is determined by the deepness of sand bed provided and the flow rate. The sand deepness should non be less than 0.7m and flux in between 0.1 and 0.3 m per hr.

Oxygen: Oxygen I required for the activity of bacteriums in the filter bed. So sufficient sum of O degree should be maintained in incoming H2O as low sum would impact the efficiency of the slow sand filter.

Temperature: Hot temperature is best for the bacterial activity. Its non a job in hot states, but in cold states attending demand to be given to the temperature conditions.

Maturation: Bacterial growing is non that fast during the initial phases of the filter and besides after every clip it is cleaned. So it is suggested non to clean the filter bed excessively frequently.

3.5.7 Effluent Quality

The public presentation of slow sand filter in footings of assorted factors like coloring material, turbudity, organic affair, etc. are given in tableaˆ¦ .

Table.. Performance of slow sand filter


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