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WASTEWATER

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  1. WASTEWATER

  2. SEWAGE COLLECTION SYSTEMS • combined sewers • separated sewers

  3. COMBINED SEWERS - wastewater and stormwater in the same system - in older towns - volumes of stormwater >> domestic sewage - large diameter + special shape is needed - treatment plants can’t handle the huge volumes - bypass is needed (combined sewer overflow) - = mixed wastewater directly to the receiving water - pollution of receiving water body - overflow can be stored and disinfected

  4. COMBINED SEWERS

  5. SEPARATED SEWERS - stormwater is carried in separate storm sewers to a point of storage or disposal (can be treated or discharged to receiving waters directly) - stormwater is always driven by gravity - pipes or open ditches - wastewater goes to municipal treatment plant, and after treatment is is discharged to surface waters - closed pipes under the surface - wastewater flows by gravity, under pressure, or because of vacuum

  6. SEPARATED SEWERS

  7. POSSIBLE RECIPIENTS • surface water • subsurface water • soil

  8. WASTEWATER TREATMENT Decreasing the pollutant concentration of wastewaters before entering into the recipient

  9. Hungary UTILITY GAP

  10. POLLUTANTS IN WASTEWATER - Microorganisms (pathogens) - Nutrients - Metals - Other inorganic substances - Radioactive substances - Biologically degradable organic substances - Other organic substances - Odor and taste matters - Heat

  11. 30 mg/l • 50 mg/l • 0.1 mg/l • 0.5 mg/l • mg/l • 20-700 mg/l NITROGEN ORGANICALLY BOUND INORGANIC AMMONIUM (NH4-N) NITRATE (NO3-N) NITRITE (NO2-N) TOTAL KJELDAHL NITROGEN (TKN) NON-OXIDISED FORMS (NH4-N + orgN) TOTAL NITROGEN (TN) No exact values!!!

  12. NITROGEN NITRIFICATION NH4+ + 1,5 O2 NO2- + H2O + 2H+ + energy Nitrosomonas NO2-+ 0,5 O2 NO3- + energy Nitrobacter necessary: ammonium-N aerobic condition (DO) bacteria – pH, T, toxic matters, operation

  13. NITROGEN DENITRIFICATION 2 NO3-+ org C + 2H+  CO2 + H2O + N2+ energy NO3-  NO2-  NO  N2O  N2 necessary: organic carbon (easily degradable) anoxic condition (DO=0, NO3-, NO2-) bacteria – pH, T, toxic matters, operation

  14. PHOSPHORUS MAIN SOURCE DETERGENTS (50%) HUMAN EXCRETA (50%) ORGANICALLY BOUND SOLID INORGANIC POLYPHOSPHATES ORTHOPHOSPHATES (PO43-) DISSOLVED TP

  15. OTHER POLLUTION PARAMETERS TOTAL SOLIDS (TS) • SUM OF PARTICULAR AND DISSOLVED MATTERS (residue remaining after a wastewater sample has been evaporated and dried at 105 ºC) TOTAL SUSPENDED SOLIDS (TSS) • Portion of the TS retained on a filter (0.45 ; 1.58 m), measured after drying at 105 ºC)

  16. OTHER POLLUTION PARAMETERS - EXTRACTABLE MATTERS (fats and oils) - DETERGENTS (surface active matters) - METALS (dissolved or particulate, heavy metals) Fe, Hg, Ni, Cd, Zn, Cu, Cr, Pb, Mn, As -pH

  17. pH OF WATER Dissociation of water = self ionisation 2H2O = H3O+ + OH- [H3O +][OH-] = Kw = 10-14 (constant at constant temperature) pH = parameter describing acidity or basicity pH = - log [H+] range: 0 - 14 natural waters: pH = 6.5-9 importance: natural processes take place in a certain interval of pH how can we influence pH addition of acid/base addition of salts NH4+ + H2O = NH3 + H3O+

  18. BIOLOGICAL PARAMETERS - viruses - bacteria (Total Coliform, Faecal Coliform, Streptococcus, Salmonella) - most often indicator parameters

  19. POPULATION EQUIVALENT converting industrial pollutant load - expressing it as a municipal load The population equivalent is a unit of measurement of organic biodegradable pollution representing the average load of that pollution produced by one person in one day; in the EU Directive it is fixed at 60 grams of B0D5 per day. e.g. industry producing ww of 1000 pe means 60000 g/d BOD5 load

  20. CONVENTIONAL TREATMENT OF SEWAGE PHYSICAL (MECHANICAL) SEPARATION OF SETTLEABLE and FLOATING SOLIDS BIOLOGICAL SEPARATION OF SUSPENDED OR DISSOLVED SOLIDS WITH THE USE OF MICROORGANISMS CHEMICAL PRECIPITATION, COAGULATION, FLOCCULATION

  21. MECHANICAL TREATMENT SEPARATION OF COARSE MATERIALS SCREENING - floating debrish, leaves, nylon sacks, etc. GRIT (SAND) TRAP - sand (inorganic, inert substances) SETTLING OF OTHER SUBSTANCES SEDIMENTATION TANK - organics (COD, BOD, TP, TN) (OR FLOTATION)

  22. in a frame SCREENING BAR SCREENS aim: defending the subsequent instruments retain floating debrish (wood, rags, etc.) long, narrow metal bars 25 mm (1 in.) apart vertical or tilted cleaned automatically (75-80°) or by hands (60°, 1-2 times a day with rake)

  23. BAR SCREENS COARSE SCREENS 50-100 mm free gap  20-30 mm FINE SCREENS 10-50 mm free gap  1.5-15 mm BOD removal  5%

  24. SEDIMENTATION DISCRETE PARTICLES (do not tend to flocculate) HEAVIER PARTICLES FALL TO THE BOTTOM v < vcr  SETTLING CRITICAL VELOCITY – KEEPS THE PARTICLE SUSPENDED

  25. GRIT TRAP / SAND TRAP SAND SPOILS THE PUMPS  HAVE TO BE REMOVED HEAVIER PARTICLES FALL TO THE BOTTOM v < 0.3 m/s (1 ft/s)  SETTLING

  26. SEDIMENTATION BASIN PRETREATMENT (no dissolved substances removed) AIM: removal of suspended solids smaller than sand REMOVES 25-35% OF THE ORGANIC MATTER SPECIFIC LOADING : 1-3 m3/h/m2

  27. SECONDARY (BIOLOGICAL) TREATMENT WITH THE USE OF MICROORGANISMS ORGANIC MATTER microorganisms oxidise the organic matter while producing new cell material, CO2 and H2O NITROGEN ammonification, nitrification, denitrification PHOSPHORUS Biological P-removal

  28. CONVERSIONS IN BIOLOGICAL WWTREATMENT biological growth hydrolysis decay Slowly degradable material hydrolysis Easily degradable material biological growth Biomass decay Inert material

  29. TYPE OF REACTORS • activated sludge reactors • microorganisms are suspended in water (mobilised) • different conditions at different places • biofilm reactors • microorganisms are hold on a fixed surface • different conditions are at the same place but at other time

  30. BIOLOGICAL PARAMETERS Activated sludge processes Aeration basin systems Oxidation ditch Aerated sewage lagoons Biofilm processes Trickling filters Rotating biological contactors (biological rotors) Thin biofilm systems

  31. SECONDARY TREATMENT Biological reactor Secondary settling Disinfection

  32. SECONDARY TREATMENT Main aim is to remove BOD (organic matter) to avoid oxygen depletion in the recipient Microbial action Aerobic/anaerobic microorganisms that decompose organic material Aerobic degradation is much faster and easier to control Activated sludge treatment – bacteria suspended in the wastewater(most common type of biological WWT) Sludge contains bacteria Activated because they are hungry (spend some time without food (easily biodegradable organics) in the settling tank)

  33. ACTIVATED SLUDGE TREATMENT Organic material + O2 + nutrients + microorganisms  new cells + CO2 + H2O Same process occurs in nature Protection of water quality Controlled process Intensified process Bacteria are reused – recycling from secondary clarifier (recycling sludge or returned activated sludge) Microbial growth (continuous food supply) – bacteria have to be removed  waste activated sludge (excess sludge)

  34. AERATION TANK Oxygen has to be provided – aeration tank (reactor) Wastewater = liquid containing food (organic pollution) Biomass (bacteria – concentrated by recycling) Combination of the liquid and microorganisms undergooing aeration = mixed liquor

  35. aeration tank sedimentation basin Q3, S3, X3 Q1, S1, X1 inlet effluent V2,X2, S2 O2 Q4, S4, X4 Q5, S5, X5 recycling sludge excess sludge ACTIVATED SLUDGE PLANT Q: wastewater volume (m3/d) S: BOD5 concentration = soluble substrate (mg/L  S1=120-400 mg/L) X: concentration of biomass (sludge) (mg/L, g/L  X2=3-6 g/L) V: volume (m3)

  36. Q, S0, X0 (Q-Qw), S, Xe V,X, S Qw, S, XR Qr, S, XR • Treatment efficiency (in terms of soluble BOD): • E = (S0-S)/S0 • Recycle rate: • ratio of return sludge volume to raw wastewater volume • R = Qr/Q

  37. TERTIARY TREATMENT (PHYSICO-CHEMICAL TREATMENT) pH control Nitrogen removal (ammonia stripping) Stripping (volatile organic compounds) Filtration Adsorption Chemical phosphorus removal

  38. Stripping: Mass transfer of a gas from the liquid phase to the gas phase Adsorption is the physical and/or chemical process in which a compound is accumulated at an interface between phases (solid-liquid interface) Adsorbate: the substance being removed from the liquid phase to the interface Adsorbent:the solid phase on which the accumulation occurs

  39. CHEMICAL PHOSPHORUS REMOVAL • Phosphorus removal (chemical precipitation) • Al3+ + PO43-AlPO4 • = converting of dissolved P compounds to a low solubility metal • phosphate (through use of a metal salt) • Precipitants: • Aluminium salts • Iron salts • Lime

  40. CHEMICAL PRECIPITATION OF PHOSPHORUS Precipitation chemicals precipitate the dissolved inorganic phosphates as insoluble compounds (to be more exact: compounds with small solubility) At the same time metal-hydroxides are formed  jelly-like flocs which bind the precipitated metal phosphates and any other suspended substances in the water (coagulation-flocculation) This also removes organically combined P, as the amount of suspended matter is greatly reduced by chemical precipitation

  41. Coagulation: - destabilization of the colloidal particles Flocculation: - increase the size of flocs

  42. CHEMICAL TREATMENT • as the only treatment process • primary (direct) precipitation • or in combination with biological treatment processes • pre-precipitation • simultaneous precipitation • post-precipitation significant part of the organic pollutants is connected to suspended solids  increasing of their removal efficiency in the primary settling tank results low organic pollutant load in the activated sludge processes

  43. PRE-PRECIPITATION metal salt min min activated sludge basin screen grit chamber flocculator sedimentation sedimentation BOD removal  90% TP removal > 90%

  44. PRE-PRECIPITATION Direct precipitation followed by a biological treatment stage Introduced to biological treatment plants to reduce the loading to the biological stage Reduction in energy consumption and in hydraulic retention time

  45. SIMULTANEOUS PRECIPITATION metal salt min screen grit chamber sedimentation sedimentation activated sludge basin BOD removal: 90% TP removal: 75-90%

  46. SIMULTANEOUS PRECIPITATION Phosphorus is chemically precipitated at the same time as biological treatment in an activated sludge process The biological stage also serves as a flocculation tank, with both the biological and chemical sludge being separated in a subsequent stage Results 1 mg/L TP

  47. POST-PRECIPITATION metal salt 20 min 10 min coagulation tank and flocculator activated sludge basin screen sedimentation grit chamber sedimentation sedimentation BOD removal  90% TP removal > 95%

  48. POST-PRECIPITATION Phosphorus is separated from biologically treated water in a separate post-treatment stage TP below 0.5 mg/L

  49. WASTEWATER TREATMENT - We aim to change the phase of the dissolved (or liquid) pollutants - Usually solid-liquid phase separation - Result is watery sludge containing pollutants – concentrated (only 1% of the total volume) - Sludge has to be treated (40-60% of the total operation cost!!!) - Composition and volume of sludge vary as do those of the wastewater influent

  50. SEWAGE SLUDGE Inorganic sludge • Screen residue • Sand Organic sludge • Primary sludge (removed in the mechanical treatment) • Secondary (excess biological) sludge (from the biological treatment stage) • Return sludge • Teriary sludge (chemical sludge from post-precipitation)