Waster water treatment

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Physical unit operation-Screening, flow equalization, mixing, flocculation,sedimentation. Chemical unit processes-Chemical precipitation.Biological unit processes: Aerobic attached growth and aerobicsuspended growth treatment processes, anaerobic suspended growthtreatment process. Low Cost Sanitation System: septic tanks, soak pit,stabilization ponds

Transcript of Waster water treatment

  • 1. Module- IVUnit Operations for Waste WaterTreatment Bibhabasu MohantyAsst. Prof. Dept. of civil EngineeringSALITER, Ahmedabad

2. Course ContentPhysical unit operation-Screening, flow equalization, mixing, flocculation, sedimentation. Chemical unit processes-Chemical precipitation. Biological unit processes: Aerobic attached growth and aerobic suspended growth treatment processes, anaerobic suspended growth treatment process. Low Cost Sanitation System: septic tanks, soak 3. INTRODUCTIONWe will start with an overview of treatment processes1) Why do we treat water and wastewater? The main objectives of the conventionalwastewater treatment processes are thereduction in biochemical oxygen demand,suspended solids and pathogenic organisms. 4. Also necessary to remove nutrients such as Nand P, toxic components, non-biologicallydegradable compounds and dissolved solids. Removal of these materials are necessary for thesimple reason that discharge to the environmentwill result in damage of some sort. 5. 2) What are the materials in water and wastewater thatwe must remove?There are a wide range of these pollutants(contaminants) ranging from municipal sewageto highly specific industrial wastes. The usualapproach in discussing treatment schemes is tocategorize pollutants into general classes so thata general class of treatment methods can beapplied. 6. Note that many pollutants fall into severalcategories. For example, some biodegradable organic matter(one category) is in the form of suspended solids(another category). So removal of SS sometimes results in the removal of organic matter. 7. 3) To what level do we need to remove contaminants? The degree to which drinking water must betreated depends on the raw water quality andthe desired quality of the finished water. Similarly the degree of treatment of awastewater depends on the quality of the rawwaste and the required effluent quality. 8. BOD5 = 30 mg/L monthly average Suspended Solids = 30 mg/L monthly average pH (if there is industrial input) = 6 9continuousFor drinking water treatment the requirementsare, of course, much more stringent with manymore categories and lower contaminant limits. 9. Turbidity (a measure of suspended solids): lessthan 0.5 NTU in at least 95% of samples takeneach month. Lead: 0.005 mg/L Copper: 1.3 mg/L Total Coliform: no coliform detection in morethan 5% of samples collected each month. 10. 4) How are these contaminants removed fromwater and wastewater? Contaminant removal is accomplished by a series ofunit processes or unit operations. Unit operation is a physical ,chemical or biologicaltreatment process. The system of integrated unit processes or unitprocesses used to treat a water or wastewater iscalled a treatment train. 11. General overview of plant components Raw Wastewater Influent PRELIMINARY Preliminary Residuals PRIMARYA (i.e., grit, rags, etc.) ClarifierSECONDARY Biological (e.g., attached-grwothB Primary Sludge TreatmentUsually to LandfillSuspended-Growth,System Constructed Wetland, etc.) WastewaterClarifierClarifier Treatment ResidualsDISINFECTION C Secondary SludgeBiosolids Processingand DisposalClean Wastewater EffluentDischarge to Receiving Waters 12. Figure Location of physical unit operations in a wastewater-treatment plant flow diagram 13. PhysicalChemicalBiologicalProcesses Processes ProcessesScreening Precipitation AerobicSedimentation ChlorinationAnaerobicFiltrationDisinfectionDifferent unit operation process 14. SCREENINGFigure Definition sketch for types of screens used in wastewatertreatment 15. 1. damage subsequent process equipmentCoarse screen2. reduce overall treatment process is to remove3. contaminate waterwaysmaterials1. protect process equipmentFine screen 2. eliminate materials that mayis used toinhibit the beneficial reuse ofbiosolids 16. A. Bar RacksTypical design information for mechanically cleaned bar racksU.S. customary units SI unitsCleaning method Cleaning methodParameterUnitManual Mechanical Unit ManualMechanicalBar size Widthin0.2 0.6 0.2 0.6mm 5 -155 -15 Depthin1.0 1.5 1.0 1.5mm 25- 38 25 38Clear spacing between In1.0 2.0 0.6 3.0mm25 50 15 75barsSlope from vertical30 450 3030 450 30Approach velocity Maximumft/s1.0 2.0 2.0 3.25 m/s 0.3 0.6 0.6 1.0 Minimumft/s1.0 1.6m/s 0.3 0.5Allowable headlossin 66 - 24 mm 150150 - 600 17. Mechanically cleaned bar screens Chain - driven screens Continuous beltscreens 18. B. Fine screensDescription of fine screenType of Screening surfaceScreeningSizeSize rangeScreen device classificationinmmamedium Application Stainless-steel inclinedMedium 0.01 0.10.25 2.5wedgewire Primary treatment(fixed)screenDrum Stainless-steelCoarse0.1 0.2 2.5 5wedgewirePreliminary treatment (rotary)Stainless-steel wedgewireMedium 0.01 0.10.25 2.5 screen Primary treatmentRemoval of residual Stainless-steel and Fine6 35secondarypolyester suspended solidsscreen clothsHorizontalCombined sewerreciprocati Medium 0.06 0.171.6 4Stainless-steeloverflows/ngbarsstormwaterCombined sewerTangential Fine0.0475 1200Stainless-steeloverflowsmesh 19. Fine screens for Combined Sewer Overflows(CSO)horizontaldrum typetype 20. flow equalizationDescription/ Application (1) dry weather flows to reduce peak flows and loads (2) wet weather flows in collection system :inflow/infiltration (3) Combined stormwater and sanitary system flowsBenefits and Disadvantages Benefits shock loading, pH, inhibiting substance improved consistency in solid loading reduce filtration surface area/ process reliabilityDisadvantages large land areas have to be covered for odor control capital cost is increased 21. Location(a) Locate in-line (b) off-line 22. Volume Requirements for Equalization Basin Schematic mass diagrams for the volume of Equalization Basin 23. Mixing Important unit operation in wastewatertreatment including1. Mixing of one substance completely withanother2. blending of miscible liquids3. flocculation of waste particles4. continuous mixing of liquid suspensions5. heat transfer 24. Mixing operations classified as continuous rapid(< 30 s)continuous (i.e. ongoing)Continuous rapid mixing Most often used when one substance is to bemixed with another. Mainly used for blending of chemicals withwaste water (addition of alum & addition ofdisinfectants) Blending of miscible liquids Addition of chemicals to sludge. 25. Continuous mixing Used where the contents of a reactor or holdingtank or basin must be kept in suspension such asin equalization basins, flocculation basins,aerated lagoons, suspended growth biologicaltreatment process, and aerobic digester. 26. Types of mixers used for rapid mixingStatic mixersHigh-speed induction mixersPressurized water jetsTurbine and propeller mixers 27. Flocculation 28. Sedimentation 29. Objectives of SedimentationTo separate solids from liquid using the force of gravity. In sedimentation, only suspended solids (SS) are removed.UseSedimentation is used in water and wastewater treatment plants 30. Types of SettlingType I settling (free settling)Type II settling (settling of flocculated particles)Type III settling (zone or hindered settling)Type IV settling (compression settling) 31. Particle Settling Theory where : gravitational force, MLT-2 : density of particle, ML-3 : density of water, ML-3 : acceleration due to gravity, LT-2 : volume of particle, M3 : frictional drag force, MLT-2 where : drag coefficient : cross-sectional area, L2 : particle settling velocity, LT-1 32. Type I (Discrete sedimentation)Occurs in dilute suspensions, particleswhich have very little interaction with eachother as they settle.Particles settle according to Stokes lawDesign parameter is surface overflow rate(Q/As) 33. Discrete Particle Settling The rate at which clarified water is produced is equal towhere ;: flowrate, /sec: surface of the sedimentation basin: particle settling velocityRearranging upper Eq : Q/A = overflow rate critical velocity " 34. Type II (flocculent sedimentation) Particles flocculate as they settle Floc particle velocity increase with time Design parameters: 1. Surface overflow rate 2. Depth of tank or, 3. Hydraulic retention time 35. 2. Flocculent Particle Settling Figure Definition for the analysis of flocculent settling 36. The settling velocity in flocculentsettling where: height of the settling column: time reguired for a given degree of removal to be achieved, T The fraction of particles removed is given by where : total height of settling column : distance between curves of equal percent removal : TSS removal 37. Type III settling (zone or hindered settling)Is the settling of an intermediate concentration of particlesThe particles are close to each otherInterparticle forces hinder settling of neighbouring particlesParticles remain in fixed position relative to each otherMass of particles settle as a zone 38. 3. Hindered (zone) settling and compressionwhere: area required for sludge thickening: flowrate into tank: initial height of interface in column: time to reach desired underflowFigure Graphical analysis of hindered concentration interface settling curve 39. Type IV settling (compression settling)Settling of particles that are of highconcentrationParticles touch each otherSettling occurs by compression of thecompacting massIt occurs in the lower depths of finalclarifiers of activated sludge 40. Chemical unit processes-precipitation Widely used, technology for the removal of metals andother inorganics, suspended solids, fats, oils, greases,and some other organic substances from wastewater. Precipitation is a method of causing contaminants thatare either dissolved or suspended in solution to settleout of solution as a solid precipitate, which can then befiltered, centrifuged, or otherwise separated from theliquid portion. 41. It can be used on a small or large scale. A beaker full of waste, a 50,000