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 aerobic
suspended growth treatment processes, anaerobic suspended growth
treatment process. Low Cost Sanitation System: septic tanks, soak pit,
stabilization ponds

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Waster water treatment

  1. 1. Module- IVUnit Operations for Waste Water Treatment Bibhabasu Mohanty Asst. Prof. Dept. of civil Engineering SALITER, Ahmedabad
  2. 2. Course Content Physical 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. 3. INTRODUCTIONWe will start with an overview of treatment processes1) Why do we treat water and wastewater? The main objectives of the conventional wastewater treatment processes are the reduction in biochemical oxygen demand, suspended solids and pathogenic organisms.
  4. 4.  Also necessary to remove nutrients such as N and P, toxic components, non-biologically degradable compounds and dissolved solids. Removal of these materials are necessary for the simple reason that discharge to the environment will result in “damage” of some sort.
  5. 5. 2) What are the materials in water and wastewater that we must remove? There are a wide range of these pollutants (contaminants) ranging from municipal sewage to highly specific industrial wastes. The usual approach in discussing treatment schemes is to categorize pollutants into general classes so that a general class of treatment methods can be applied.
  6. 6.  Note that many pollutants fall into several categories. 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. 7. 3) To what level do we need to remove contaminants? The degree to which drinking water must be treated depends on the raw water quality and the desired quality of the finished water. Similarly the degree of treatment of a wastewater depends on the quality of the raw waste and the required effluent quality.
  8. 8.  BOD5 = 30 mg/L monthly average Suspended Solids = 30 mg/L monthly average pH (if there is industrial input) = 6 – 9 continuousFor drinking water treatment the requirements are, of course, much more stringent with many more categories and lower contaminant limits.
  9. 9.  Turbidity (a measure of suspended solids): less than 0.5 NTU in at least 95% of samples taken each month. Lead: 0.005 mg/L Copper: 1.3 mg/L Total Coliform: no coliform detection in more than 5% of samples collected each month.
  10. 10. 4) How are these contaminants removed from water and wastewater? Contaminant removal is accomplished by a series of unit processes or unit operations. Unit operation is a physical ,chemical or biological treatment process. The system of integrated unit processes or unit processes used to treat a water or wastewater is called a treatment train.
  11. 11. General overview of plant components Raw Wastewater Influent PRELIMINARY Preliminary Residuals PRIMARY A (i.e., grit, rags, etc.) Clarifier SECONDARY Biological (e.g., attached-grwoth B Primary Sludge TreatmentUsually to Landfill Suspended-Growth, System Constructed Wetland, etc.) Wastewater Clarifier Clarifier Treatment Residuals DISINFECTION C Secondary Sludge Biosolids Processing and Disposal Clean Wastewater Effluent Discharge to Receiving Waters
  12. 12. Figure Location of physical unit operations in a wastewater-treatment plant flow diagram
  13. 13. Physical Chemical BiologicalProcesses Processes ProcessesScreening Precipitation AerobicSedimentation Chlorination AnaerobicFiltration Disinfection Different unit operation process
  14. 14. SCREENINGFigure Definition sketch for types of screens used in wastewatertreatment
  15. 15. 1. damage subsequent process equipmentCoarse screen 2. reduce overall treatment process is to remove 3. contaminate waterways materials 1. protect process equipmentFine screen 2. eliminate materials that mayis used to inhibit the beneficial reuse of biosolids
  16. 16. A. Bar RacksTypical design information for mechanically cleaned bar racks U.S. customary units SI units Cleaning method Cleaning methodParameter Unit Manual Mechanical Unit ManualMechanicalBar size Width in 0.2 – 0.6 0.2 – 0.6 mm 5 -15 5 -15 Depth in 1.0 – 1.5 1.0 – 1.5 mm 25- 38 25 – 38Clear spacing between In 1.0 – 2.0 0.6 – 3.0 mm 25 – 50 15 – 75barsSlope from vertical ° 30 – 45 0 – 30 30 – 45 0 – 30Approach velocity Maximum ft/s 1.0 – 2.0 2.0 – 3.25 m/s 0.3 – 0.6 0.6 – 1.0 Minimum ft/s 1.0 – 1.6 m/s 0.3 – 0.5Allowable headloss in 6 6 - 24 mm 150 150 - 600
  17. 17. Mechanically cleaned bar screens Chain - driven screens Continuous belt screens
  18. 18. B. Fine screensDescription of fine screen Type of Screening surfaceScreening Size Size range Screen device classification in mma medium Application Stainless-steel inclined Medium 0.01 – 0.1 0.25 – 2.5 wedgewire Primary treatment (fixed) screen Drum Stainless-steel Coarse 0.1 – 0.2 2.5 – 5 wedgewire Preliminary treatment (rotary) Stainless-steel wedgewire Medium 0.01 – 0.1 0.25 – 2.5 screen Primary treatment Removal of residual Stainless-steel and Fine 6 – 35㎛ secondary polyester suspended solids screen clothsHorizontal Combined sewerreciprocati Medium 0.06 – 0.17 1.6 – 4 Stainless-steel overflows/ ng bars stormwater Combined sewerTangential Fine 0.0475 1200㎛ Stainless-steel overflows mesh
  19. 19. Fine screens for Combined Sewer Overflows(CSO) horizontal drum type type
  20. 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 reliability Disadvantages · large land areas · have to be covered for odor control · capital cost is increased
  21. 21. Location(a) Locate in-line (b) off-line
  22. 22. Volume Requirements for Equalization Basin Schematic mass diagrams for the volume of Equalization Basin
  23. 23. Mixing Important unit operation in wastewater treatment including 1. Mixing of one substance completely with another 2. blending of miscible liquids 3. flocculation of waste particles 4. continuous mixing of liquid suspensions 5. heat transfer
  24. 24.  Mixing operations classified as continuous rapid (< 30 s) continuous (i.e. ongoing)Continuous rapid mixing Most often used when one substance is to be mixed with another. Mainly used for blending of chemicals with waste water (addition of alum & addition of disinfectants) Blending of miscible liquids Addition of chemicals to sludge.
  25. 25. Continuous mixing Used where the contents of a reactor or holding tank or basin must be kept in suspension such as in equalization basins, flocculation basins, aerated lagoons, suspended growth biological treatment process, and aerobic digester.
  26. 26. Types of mixers used for rapid mixingStatic mixersHigh-speed induction mixersPressurized water jetsTurbine and propeller mixers
  27. 27. Flocculation
  28. 28. Sedimentation
  29. 29. Objectives of SedimentationTo separate solids from liquid using the force of gravity. In sedimentation, only suspended solids (SS) are removed.UseSedimentation is used in water and wastewater treatment plants
  30. 30. Types of SettlingType I settling (free settling)Type II settling (settling of flocculated particles)Type III settling (zone or hindered settling)Type IV settling (compression settling)
  31. 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. 32. Type I (Discrete sedimentation)Occurs in dilute suspensions, particleswhich have very little interaction with eachother as they settle.Particles settle according to Stoke’s lawDesign parameter is surface overflow rate(Q/As)
  33. 33. Discrete Particle Settling The rate at which clarified water is produced is equal to where ; : flowrate, ㎥/sec : surface of the sedimentation basin : particle settling velocity Rearranging upper Eq : Q/A = overflow rate “ critical velocity "
  34. 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. 35. 2. Flocculent Particle Settling Figure Definition for the analysis of flocculent settling
  36. 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. 37. Type III settling (zone or hindered settling)Is the settling of an intermediate concentration of particlesThe particles are close to each otherInterparticle forces hinder settling of neighbouring particlesParticles remain in fixed position relative to each otherMass of particles settle as a zone
  38. 38. 3. Hindered (zone) settling and compression where : 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. 39. Type IV settling (compression settling)Settling of particles that are of highconcentrationParticles touch each otherSettling occurs by compression of thecompacting massIt occurs in the lower depths of final clarifiers of activated sludge
  40. 40. Chemical unit processes-precipitation Widely used, technology for the removal of metals and other inorganics, suspended solids, fats, oils, greases, and some other organic substances from wastewater. Precipitation is a method of causing contaminants that are either dissolved or suspended in solution to settle out of solution as a solid precipitate, which can then be filtered, centrifuged, or otherwise separated from the liquid portion.
  41. 41.  It can be used on a small or large scale. A beaker full of waste, a 50,000 tank, a 1,000,000 gallon lagoon or a lake can be batch treated with chemicals. Chemical precipitation can be used in a continuous treatment system on flows ranging from a trickle to 1 gallon/minute, 1,000 gallons/minute and more. Precipitation is assisted through the use of a coagulant, an agent which causes smaller particles suspended in solution to gather into larger aggregates. Frequently, polymers are used as coagulants.
  42. 42.  When colloidal matter such as emulsified oil or metal bearing particles are treated with metal salts and lime or NaOH, the metal salts act as primary coagulants. The positively charged metal ions combine with the negative colloid particles and neutralize their charge. The particles then repel each other less strongly and tend to coagulate or collect into larger particles.
  43. 43. Chemicals for precipitationLime – Calcium Oxide, CaOFerrous Sulfate – Fe(SO4)3Alum or Filter Alum – Al2(SO4)3.14H2OFerric Chloride – FeCl3Polymer
  44. 44. Advantages Chemical precipitation is a well-established technology with ready availability of equipment and many chemicals. Some treatment chemicals, especially lime, are very inexpensive. Completely enclosed systems are often conveniently self-operating and low maintenance.
  45. 45. Disadvantages Competing reactions, varying levels of alkalinity and other factors typically make calculation of proper chemical dosages impossible. Chemical precipitation may require working with corrosive chemicals, increasing operator safety concerns. The addition of treatment chemicals, especially lime, may increase the volume of waste sludge up to 50 percent. Large amounts of chemicals may need to be transported to the treatment location. Polymers can be expensive.
  46. 46. APPLICATION OF DIFF. CHEMICALSLime – Calcium Oxide, CaOProduces calcium carbonate in wastewater which acts as a coagulant for hardness and particulate matter. Often used in conjunction with other coagulants, since: (1) by itself, large quantities of lime are required for effectiveness, and (2) lime typically generates more sludge than other coagulants.
  47. 47. Ferrous Sulphate – Fe(SO4)3Typically used with lime to soften water. The chemical combination forms calcium sulfate and ferric hydroxide. Wastewater must contain dissolved oxygen for reaction to proceed successfully.Ferric Chloride – FeCl3Reacts with alkalinity or phosphates to form insoluble iron salts.
  48. 48. Alum or Filter Alum – Al2(SO4)3.14H2O Used for water softening and phosphate removal. Reacts with available alkalinity (carbonate, bicarbonate and hydroxide) or phosphate to form insoluble aluminium salts.Polymer High molecular weight compounds (usually synthetic) which can be anionic, cationic, or non-ionic. When added to wastewater, can be used for charge neutralization for emulsion-breaking, or as bridge- making coagulants, or both. Can also be used as filter aids and sludge conditioners.
  49. 49. Biological unit processes In the case of domestic wastewater treatment, the objective of biological treatment is: – To stabilize the organic content – To remove nutrients such as nitrogen and phosphorus Types: Attached Growth Aerobic Processes Suspended Growth Anoxic Processes Combined Systems Anaerobic Processes Combined Aerobic-Anoxic- Anaerobic Processes Aerobic Pond Processes Maturation Facultative Anaerobic
  50. 50. Attached Growth Process What can this process do? 1. Remove Nutrient 2. Remove dissolved organic solids 3. Remove suspended organic solids 4. Remove suspended solids
  51. 51. Cross-section of an attached growthbiomass film Oxygen (the natural or forced draft)Wastewater Organic/ nutrient Biomass : viscous, jelly-like substance containing bacteria filter media
  52. 52. Major Aerobic Biological ProcessesType of Common Name UseGrowthSuspended Activated Sludge (AS) Carbonaceous BOD removal (nitrification)Growth Aerated Lagoons Carbonaceous BOD removal (nitrification)Attached Trickling Filters Carbonaceous BOD removal. nitrificationGrowth Roughing Filters (trickling Carbonaceous BOD removal filters with high hydraulic loading rates) Rotating Biological Carbonaceous BOD removal (nitrification) Contactors Packed-bed reactors Carbonaceous BOD removal (nitrification)Combined Activated Biofilter Process Carbonaceous BOD removal (nitrification)Suspended & Trickling filter-solidsAttached contact processGrowth Biofilter-AS process Series trickling filter-AS process
  53. 53. Activated Sludge Process The aeration tank contains a suspension of the wastewater and microorganisms, the mixed liquor. The liquor is mixed by aeration devices (supplying also oxygen) A portion of the biological sludge separated from the secondary effluent by sedimentation is recycled to the aeration tank Types of AS Systems: Conventional, Complete-Mix, Sequencing Batch Reactor, Extended Aeration, Deep Tank, Deep Shaft
  54. 54. Advantages/Disadvantages Advantages Disadvantages Flexible, can adapt to  High operating costs minor pH, organic and (skilled labor, electricity, temperature changes etc.) Small area required  Generates solids requiring Degree of nitrification is sludge disposal controllable  Some process alternatives Relatively minor odor are sensitive to shock problems loads and metallic or other poisons  Requires continuous air supply
  55. 55. Trickling Filters• The trickling filter or biofilter consists of a bed of permeable medium of either rock or plastic• Microorganisms become attached to the media and form a biological layer or fixed film. Organic matter in the wastewater diffuses into the film, where it is metabolized. Periodically, portions of the film slough off the media
  56. 56. Flow Diagram for Trickling Filters Recirculation= A portion of the TF effluent recycled through the filter Recirculation ratio (R) = returned flow (Qr)/ influent flow (Q) Qr Recycle Final clarifierQInfluent Primary Wast clarifier sludg Trickling filter
  57. 57. Advantages/Disadvantages Advantages Disadvantages Good quality (80-90%  High capital costs BOD5 removal) for 2-  Clogging of distributors or stage efficiency could beds reach 95%  Snail, mosquito and Moderate operating costs insect problems (lower than activated sludge) Withstands shock loads better than other biological processes
  58. 58. Rotating Biological Contactors It consists of a series of circular disks of polystyrene or polyvinyl chloride that are submerged in wastewater and rotated slowly through it The disk rotation alternately contacts the biomass with the organic material and then with atmosphere for adsorption of oxygen Excess solids are removed by shearing forces created by the rotation mechanism
  59. 59. Advantages/Disadvantages Advantages Disadvantages Short contact periods  Need for covering units Handles a wide range of installed in cold climate to flows protect against freezing Easily separates biomass from waste stream  Shaft bearings and Low operating costs mechanical drive units Short retention time require frequent maintenance Low sludge production Excellent process control
  60. 60. Major Anaerobic Biological ProcessesType of Common Name UseGrowthSuspended Anaerobic Contact Process Carbonaceous BODGrowth removal Upflow Anaerobic Sludge- Carbonaceous BOD Blanket (UASB) removalAttached Anaerobic Filter Process Carbonaceous BODGrowth removal, waste stabilization (denitrification) Expanded Bed Carbonaceous BOD removal, waste stabilization
  61. 61. Anaerobic Contact ProcessUntreated wastewater is mixed with recycled sludge solids and then digested in a sealed reactorThe mixture is separated in a clarifierThe supernatant is discharged as effluent, and settled sludge is recycled
  62. 62. Advantages/Disadvantages Advantages Disadvantages Methane recovery  Heat required Small area required  Effluent in reduced Volatile solids chemical form destruction requires further treatment  Requires skilled operation  Sludge to be disposed off is minimal
  63. 63. Upflow Anaerobic Sludge Blanket Wastewater flows upward through a sludge blanket composed of biological granules that decompose organic matter Some of the generated gas attaches to granules that rise and strike degassing baffles releasing the gas Free gas is collected by special domes The effluent passes into a settling chamber
  64. 64. ACTIVATED SLUDGEAdvantages/Disadvantages Advantages Disadvantages Low energy demand  Long start-up period Low land requirement  Requires sufficient Low sludge production amount of granular seed Less expensive than other sludge for faster start-up anaerobic processes  Significant wash out of High organic removal sludge during initial efficiency phase of process  Lower gas yield than other anaerobic processes
  67. 67. Low Cost Sanitation System Septic Systems
  68. 68. Septic tankAll household wastewater systems will have a septic tankMicrobial action digests solid wastesLiquids flow through tank to disposal areaTank size -1000 gallon liquid capacity (4-BR house or less) -Add 250 gallons per additional bedroom
  69. 69. System Components SourceWell Tank Drainfield Treatment in Soil Groundwater
  70. 70. ManholeInlet Outlet Scum Liquid Solids Solids Sludge Sludge
  71. 71. Role of the septic tankAnaerobic fermentation of solidsReduce the load of pathogens in the effluentHold the effluent for 2-3 days for improved safetyRetain solid material to prevent blockage of further disposal system
  72. 72. The soak-away Absorption field treatment of septic tank effluent Absorption trench system Distribution box septic tank
  73. 73. The field requires periodic maintenance.Diversion of the flow at distribution box and repacking of the rock fill, removal of plant roots etc.
  74. 74. Connection to a sewage system: what are the alternatives? Conventional sewage connection…expensive Small bore sewage system: less expensive Use road-side drains, and hope for the best…. Unfortunately this is the common outcome
  75. 75. Where do we go from here:The effluent must be disposed of in a sanitary mannerThe system should be inexpensive and easy to manageTropical areas do have long hours of sunlight, why not exploit this.We can by using oxidation ponds…
  76. 76. Soak pit Soak Pit, also known as a soak away or leach pit, is a covered, porous-walled chamber that allows water to slowly soak into the ground. Pre-settled effluent from a Collection and Storage/Treatment technology is discharged to the underground chamber from where it infiltrates into the surrounding soil.
  77. 77.  The Soak Pit can be left empty and lined with a porous material (to provide support and prevent collapse), or left unlined and filled with coarse rocks and gravel. The rocks and gravel will prevent the walls from collapsing, but will still provide adequate space for the wastewater. In both cases, a layer of sand and fine gravel should be spread across the bottom to help disperse the flow. The soak pit should be between 1.5 and 4m deep, but never less than 1.5m above the ground water table.
  78. 78. As wastewater (pre-treated grey water or black water) percolates through the soil from the Soak Pit, small particles are filtered out by the soil matrix and organics are digested by micro-organisms.Thus, Soak Pits are best suited to soils with good absorptive properties; clay, hard packed or rocky soils are not appropriate.
  79. 79. Advantages/Disadvantages Advantages Disadvantages Can be built and repaired  Pre-treatment is required to with locally available prevent clogging, although materials. eventual clogging is Small land area required. inevitable. Low capital cost; low  May negatively affect soil and operating cost. groundwater properties. Can be built and maintained with locally available materials. Simple technique for all users.
  80. 80. Health Aspects Soak Pit is not used for raw sewage, and as long as the previous Collection and Storage/Treatment technology is functioning well, health concerns are minimal. The technology is located underground and thus, humans and animals should have no contact with the effluent. It is important however, that the Soak Pit is located a safe distance from a drinking water source (ideally 30m). Since the Soak Pit is odourless and not visible, it should be accepted by even the most sensitive communities.
  81. 81. Maintenance A well-sized Soak Pit should last between 3 and 5 years without maintenance. To extend the life of a Soak Pit, care should be taken to ensure that the effluent has been clarified and/or filtered well to prevent excessive build up of solids. The Soak Pit should be kept away from high-traffic areas so that the soil above and around it is not compacted.
  82. 82.  When the performance of the Soak Pit deteriorates, the material inside the soak pit can be excavated and refilled. To allow for future access, a removable (preferably concrete) lid should be used to seal the pit until it needs to be maintained. Particles and biomass will eventually clog the pit and it will need to be cleaned or moved.
  83. 83. Stabilization ponds One of the most ancient wastewater treatment methods known to humans are waste stabilization ponds, also known as oxidation ponds or lagoons. Theyre often found in small rural areas where land is available and cheap. Such ponds tend to be only a meter to a meter and a half deep, but vary in size and depth, and may be three or more meters deep.
  84. 84.  Consist of shallow man-made basins comprising a single or several series of anaerobic, facultative or maturation ponds. The primary treatment takes place in the anaerobic pond, which is mainly designed for removing suspended solids, and some of the soluble element of organic matter (BOD). Secondary stage in the facultative pond most of the remaining BOD is removed through the coordinated activity of algae and heterotrophic bacteria. The main function of the tertiary treatment in the maturation pond is the removal of pathogens and nutrients (especially nitrogen).
  85. 85. New Light cells Algae CO2 O2 nutrients BacteriaOrganic Newmatter cells
  86. 86. Three processes of stabilization:anaerobic Anaerobic Facultative Maturation Suspended Settleable & solids Sludge layer Soluble solids
  87. 87. The second process is facultative Anaerobic Facultative Maturation Suspended & Settleable Soluble solids solids Sludge layer
  88. 88. In the maturation pond, pathogens are reduced:the water an be released to a river Anaerobic Facultative Maturation Organic matter Pathogenic organisms
  89. 89. Application and suitability Stabilization ponds are particularly well suited for tropical and subtropical countries because the intensity of the sunlight and temperature are key factors for the efficiency of the removal processes. It is also recommended by the WHO for the treatment of wastewater for reuse in agriculture and aquaculture, especially because of its effectiveness in removing nematodes (worms) and helminth eggs.
  90. 90. Septic systems can be apermanent solution to wastewater management if homeowners understand andmaintain their systems.