Slow And Filters And Rapid Sand Environmental Sciences Essay

slake And separates And rapid gritrock Environmental Sciences EssayWater Purification is a procedure in which the ulcerous piddle that is contaminated, unclean, adulterated, and poisoned is converted into safe drinking pee through mixed stage processes rendering it safe for drinking and e actuallyday enforce. Water Purification is through almost every day in every part of this world. The increasing population and industrialization is generating more and more threat to the drinking body of pissing bloodlines as a lot of harmful chemicals atomic number 18 being released either intentionally or unintentionally into the sources. In most of the cases we arse non see these hazardous chemicals, but they idiosyncrasy a great threat to our health and lives.To treat weewee before being brought to recitation, mixed purification methods have been developed. They vary in their plan, method of filtration, equipment used, efficiency, billet and variant different factors. http// www.waterpurificationmethods.com/Natural water is available from various sources kindred 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 dress hat come-at-able 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 hang impurities(remain in suspension), dissolved impurities(dissolved impurities and creditworthy for taste and odour) and colloidal impurities( electrically charged and never settle). skirt 1 shows the various constituents of the impurities present in water and on that lay effects. Ss+Tech+guide.Water for domestic use should be Colorless and have no suspend solidsOdour free and must taste good foreswear from disease ca exploitation organismsFree of halmful salts and objectionable mineralsFree from dissolved gases which can he harm fullShould be non-corrosive in nature Ss+Tech+guide.Table 1 Suspended and dissolved impurities in water ss+tech+guideOne 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 necessitate 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 nates Wiley Sons, Inc.The various types of imbues used areGravity FiltersVacuum FiltersPres sure FiltersCentrifugals and CentrifugesPROCESS OVERVIEWSuspended matter present in water makes the appearance of water waterlogged this physical characteristic is known as turbidity. This suspended mater may consist of floc, microorganis ms, algae, iron, silt and manganese precipitates and to a fault 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 guts, gravel, coal. As the process goes on and on and when the pass lost is more than the permissible value the filter bed is backwashed or carrousel terra firma is scrapped off and brought back into public presentation. But on that point need to be a second tank which will be operation as the freshman one is being backwashed or top soil is scraped off.The various other important processes in the water purification process areScreeningAerationCoagulationFloculationSedimentationDisinfectionStabilisationWater requirements of a city/town can be divided into five categories domestic orresidential useinstitutional usepublic or civic useindustrial usewater system lossesDESCRIPTIONA sand filter is very simple in its opertion and design. It is merely a bed of sand with back 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 interrogative during various situations. Be diminished the gravel layer are underdrains needful to remove the filtered water. Before dissipaion into the sand bed the cleverness of the water is to be reduced so that it doesnot dilapidate 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 harvest-home at he surface. The filters are backwashed when stage is reached on a lower floor which both head loss cannot be tolerated. For backwash the outlet valves are closed and water is make to move in opposite direction. Manual of boring sand filtersChapter 2 COMPARISONThogh both slows sand filters and rapid sand filters function under the follow up of gravity and mostlly have same constituent parts yet they differ in various factors which are explaind in Table 2 below.Slow mother wit FiltersRapid guts Filters1.Rate of Filtration1-3-10 m/day100-125-300 m/day2.Depth of Bed0.30m of gravel1.0m of sand0.45m of gravel0.75m of sand3.Size of gritrock0.25 to 0.35 mmUniformity Coeff2 to 30.45mm and higherUniformity Coeff1.5 and Lower4.Length of run20 to 30 to 60 days12 to 24 to 48 hours5.Penetration of Suspended MatterSuperficialDeep6.Preperatory treatment of WaterGenerally aerationFlocullation and Sedimentation7.Method of CleaningScrapingBack process8.Cost of ConstructionOperationDepreciation of plantsHigherLowerLowerLowerHigherHigher9.Amount of Wash Water0.2 to 0.6% of water filtered1-4-6% of water filteredTable 2 Comparison between Slow and Rapid vertebral column Filters. Class notesChapter 3 SLOW SAND FILTERS3.1 IntroductionSlow 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. interchangeablely there is no usage of chemicals involved neither do we need whatsoever highly skilled and straight supervision. Undeniably slow filteration is best suited for rural areas as there are low running costs . Slow sand filtersWhat 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 cen quantifytres 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 prelimi3.2 AdvantagesThe 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 facilitates in lessening of bacteria and various organic levels, thus jocking to reduce the pith 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 sacrificeling problems, no expert supervison is required and also local labor and materials can be brought to use.http//www.nesc.wvu.edu/pdf/dw/publications/ontap/2009_tb/slow_sand_filtration_dwfsom40.pdf3.3 DisadvantagesOn 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 of arms 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. The y 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 drop-off the biological activity within the filter making it less effective. http//www.nesc.wvu.edu/pdf/dw/publications/ontap/2009_tb/slow_sand_filtration_dwfsom40.pdf3.4 Factors influencing EfficiencyThe 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 belowTime the minimum and bed depth should not be less than 0.7m and lead rate rough 0.1 and 0.3 m per hour, as higher the time for response on the filter bed higher would be the efficiency attained.Oxyge n Oxygen is important for bacteria as it uses it for its activity. If the amount of oxygen in elect(postnominal) 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 savory countries but in countries with cold judicial admissions they affect the efficiency as lower the temperature lower will be the biological activity in the filter bed. purulence Sufficient time should be provided for the biological activities to recognise place. Also the filter should not be cleaned too frequently. Ss tech guide for slow sand filters http//ebookbrowse.com/ss-tech-guide-slow-sand-filters-pdf-d108621291CHAPTER 4- excogitate CONSIDERATIONS4.1 Design Principles4.1.1Sizing the Filter BedThe very first step in the design is to coat 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 selec ted such that it considers both the cariation over cursory cycle and the increase each year. Pilot plant studies performed can help in determining performance characteristics.4.1.1.1 AreaThe 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)4.1.1.2 Depth of SandThe 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 followsWhere Y = years of operationDi = depth of initial sand bedDf = Depth of final sand bed before rebuilding is requiredR = depth of removalF = frequency4.1.2 HYDRAULICSThe various functions for which hydraulic analysis are required are to distribute raw water on the sand bed, collection of water, waste pipe of headwater, control of flow through the filter, and measurement of the headloss.4.1.2.1 DISTRIBUTIONThe influent water needs to be distributed t hroughout 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. epitome below shows a system of pipes for how distribution of water may be make.Fig showing an prelude for distributin of water over sand bed.4.1.2.2 CollectionA system of underdrains need to designed for the collection of filtered water.4.1.2.3 DrainageFor the need to scrap the sand bed, the headwater must be drained off to a level below the the sand bed surface. The attend below explains how the waste pipe occours in two stages. One being done at the top surface removing the top portion of headwater and the second being at the git removing the rest of remaining water.Fig Shows drainage points4.1.2.4 BACKFILLINGAfter the drainage has been done and the s crapinf 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 federal official 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 nicotinamide adenine dinucleotide the filter 3 continues with its operations. The backfilling is done till the depth of water reaches around 1 ft supra 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.4.1.2.4 FLOW MEASUREMENTThere is a need to monitor the influent and sewer water flow throughout the operation so as to maintain a constant supply and hydraulic head to satisfy the demand. For this various flow bar 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 .4.1.2.5 Flow ControlThere 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.4.1.2.6 Headloss MeasurementHeadloss 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.4.1.2.73.5.1 Design CapacitySlow 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 down payment, as always is the case, more area would be required.3.5.2 Quality considerations of influent waterThe 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 guideFigure 2 Schematic sequence of water treatment facilities when using slowsand filter ss tech guide3.5.2.1 Pretreatment FacilitiesFot the pretreatment of the water we may make use of Grit chambers, sedimentation tanks or roughing filters. Each of these is explained as follows.3.5.2.1.1 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.3.5.2.1.2 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 criteriaFor 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 even up metre area of filter media.Table 3 General design criteria ss tech guide3.5.6 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 interpreted 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 = (A) (1/3), orn = (Q) (1/2)Where,n Number of filter unitsA Total required area in m2Q Average daily water demand evince in m3/h r3.5.7 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 phonation 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 f or 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 bedImmidetialtely 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. T his 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-drainsUnder 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 doesnt get clotted 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. Spe cial considerations need to be taken care of while designing under drain system for perforated pipes as explained in tableFigure Arrangements for main drainFigure various types of filter bottomsTable Criteria for dimensioning under drain using perforated pipesFactors 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.D10(mm)D90(mm) overtake Layer11.4Second Layer45.6Third Layer1623In slow sand filters the gravel shou ld 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 demigod 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 efficiencySince 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 followsTime 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 provide d 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 QualityThe 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