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1. WASTEWATER TREATMENT TECHNOLOGIES1.1. WASTEWATER TREATMENT PROCESSESThe typical contaminants in the domestic effluent are fecal discharges (containing E. coli andother microbes and nutrients in the form of ammonia, urea, etc.), dissolved and suspended solids, anddissolved gases (H 2 S, NH 3, etc.). Similarly, typical contaminants in the industrial effluent aresuspended and dissolved solids (inorganic, including heavy metals and organic), dissolved complexorganic compounds, color bearing compounds (dyes and pigments) and dissolved gases. The mainobjective of the effluent treatment is to either decompose the contaminant into environmentallyharmless products or to stabilize the contaminant into its most stable thermodynamic form. Variousphysical, chemical and biological methods are used to remove contaminants from wastewater, andtherefore the wastewater processes can be classified as physical, chemical and biological unitoperations.1.1.1. Physical processesPhysical forces are applied to remove contaminants in physical unit operations. The physicalprocess includes unit operations for screening, flow equalization, sedimentation, flotation andgranular-medium filtration.(a) ScreeningScreening is the first physical operation carried out on the incoming effluent. It removes coarseparticles and other floating rubbish materials such as plastic pieces, loose sheets and bags etc. This isachieved by allowing the wastewater to flow through the rakes or screens depending on the extent oflarge solids present in the incoming effluent. Particles larger than 10 mm are generally removed outthis stage. Since, after this stage, the effluent is pumped using centrifugal pumps, the removal of largesolids protect the subsequent pumping equipment. The material retained from the manual ormechanical cleaning of bar racks and screens is referred to as ‘screenings’, and is either disposed of byburial or incineration, or returned into the waste flow after grinding.(b) Flow equalizationFlow equalization is a technique used to improve the effectiveness of wastewater treatmentprocesses by leveling out operation parameters such as flow and pollutant levels over a period of time.Variations are damped until a near-constant flow rate is achieved, minimizing the downstream effectsof these parameters.(c) SedimentationSedimentation, a fundamental and widely used unit operation in wastewater treatment, involvesthe gravitational settling of heavy particles suspended in a mixture. Sedimentation takes place in asettling tank, also referred to as a clarifier. This process is used for the removal of grit, particulatematter in the primary settling basin, as well as biological flocks in the activated sludge settling basin(or secondary clarifier).(d) FlotationFlotation is a unit operation used to remove solid particles from a liquid phase by introducing afine gas, usually air bubbles. The gas bubbles either adhere to the liquid or are trapped in the particlestructure of the suspended solids, raising the buoyant force of the combined particle and gas bubbles.Particles that have a higher density than the liquid can thus be made to rise. In wastewater treatment,flotation is used mainly to remove suspended matter and to concentrate biological sludge. The chiefadvantage of flotation over sedimentation is that very small or light particles can be removed morecompletely and in a shorter time. Once the particles have been floated to the surface, they can be3
skimmed out. Flotation, as currently practiced in WWTP, uses air exclusively as the floating agent.Furthermore, various chemical additives can be introduced to enhance the removal process.(e) Granular medium filtrationThe filtration of effluents is used for the supplemental removal of suspended solids. Thewastewater to be filtered is passed through a filter bed consisting of granular material such as sand,and/or anthracite.1.1.2. Chemical processesChemical processes used in wastewater treatment are designed to bring about some form ofchange by means of chemical reactions. They are always used in conjunction with physical unitoperations and biological processes. The chemical process includes unit operations for chemicalprecipitation, adsorption, disinfection and dechlorination.(a) Chemical precipitationChemical coagulation of raw wastewater before sedimentation promotes the flocculation of finesolids into more readily settleable flocks, thereby enhancing the efficiency of suspended solid removalas compared to plain sedimentation without coagulation. The degree of clarification obtained dependson the kind and quantity of coagulant used and the care with which the process is controlled.Coagulant selection for enhanced sedimentation is based on performance, reliability and cost.(b) DisinfectionDisinfection refers to the selective destruction of disease-causing microorganisms. This processis of importance in wastewater treatment owing to the nature of wastewater, which harbors a numberof human enteric organisms that are associated with various waterborne diseases. Commonly usedmeans of disinfection include ultraviolet (UV) light, gamma ray or electron beam and variouschemical agents. The most common chemical disinfectants are the oxidizing chemicals, and of these,chlorine is the most widely used.1.1.3. Biological processesBiological unit processes are used to convert the fine dissolved organic matter in wastewaterinto flocculent settleable solids. In these processes, micro-organisms, particularly bacteria, convert thecolloidal and dissolved organic matter into various gases and into cell tissue which is then removed insedimentation tanks. Biological processes are usually used in conjunction with physical and chemicalprocesses. Biological processes used for wastewater treatment are activated sludge process, aeratedlagoon, trickling filters, rotating biological contactors, pond stabilization, nutrient removal process andanaerobic digestion.(a) Aerobic activated-sludge processThe activated-sludge process is an aerobic continuous-flow system containing a mass ofactivated microorganisms that are capable of stabilizing organic matter. Aeration basin is used foraerobic process. The aerobic environment is maintained in the aeration basin by means of diffused ormechanical aeration, which also serves to keep the contents of the reactor mixed. After a specificretention time, the mixed liquor passes into the secondary clarifier, where the sludge is allowed tosettle and a clarified effluent is produced for discharge.(b) Anaerobic digestionAnaerobic digestion involves the biological conversion of organic matter in the absence ofmolecular oxygen to a variety of end-products including methane and carbon dioxide. The process4
Page 1 and 2: Radiotracer Applications inWastewat
Page 3 and 4: The following States are Members of
Page 5 and 6: TRAINING COURSE SERIES No. 49RADIOT
Page 7 and 8: EDITORIAL NOTEThe use of particular
Page 9 and 10: PRACTICAL RECOMMENDATIONS .........
Page 11: The success of radiotracer applicat
Page 16 and 17: 1.3.3. Primary clarifiersA clarifie
Page 18 and 19: esidence times of phases, mixing ch
Page 20 and 21: FIG.8. Secondary clarifier: periphe
Page 22 and 23: processes that occur in natural env
Page 24 and 25: The RTD and MRT are parameters that
Page 26 and 27: (b) Perfect mixers in series modelA
Page 28 and 29: and practically it has better sense
Page 30 and 31: The experimental RTD curve gives ma
Page 32 and 33: FIG.18. Experimental and CFD calcul
Page 34 and 35: Optical tracers can be divided in t
Page 36 and 37: Table 1b. Liquid tracers.- Field of
Page 38 and 39: TABLE 2. SOME WATER RADIOTRACERSIso
Page 40 and 41: The detection efficiency (ϕ) is de
Page 42 and 43: Since on-line radiotracer technique
Page 44 and 45: source. The dose received is direct
Page 46 and 47: 4. CASE STUDIESWastewater from indu
Page 48 and 49: FIG.26. Scheme of equalizer-clarifi
Page 50 and 51: FIG.29. Exp. RTD of S2 and S3 jet m
Page 52 and 53: FIG.33. Experimental RTD curves mea
Page 54 and 55: 1. Radiotracer testsRadiotracer Br-
Page 56 and 57: 4.00E-033.50E-03ExperimentalModel3.
Page 58 and 59: 3. Conclusions• The aeration tank
Page 60 and 61: FIG.50. Experimental response curve
Page 62 and 63:
4.8. DIAGNOSIS OF A CYLINDRICAL TWO
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FIG.56. RTD simulation for experime
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The radiotracer was chosen to trace
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(a) D1 - D3 (b) D1 - D4(c) D1 - D6
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(d) ConclusionsThe diagnosis of dig
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• average gamma energy of 400 keV
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To estimate the MRT of the mud insi
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FIG.71. Experimental RTD curve and
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FIG.75. Biological aeration tank.FI
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4.12.2. Radiotracer experiment(a) L
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FIG.80. The experimental RTD curve
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FIG.84. Experimental RTD curve of l
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FIG.88. Characteristic curves of se
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tracer background level detection.
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As seen from Fig. 94, the tanks-in-
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FIG.99. Experimental RTD curves wit
Page 94:
The experimental RTD curves obtaine
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BIBLIOGRAPHYBORROTO J.I., DOMINGUEZ
Page 102:
CONTRIBUTORS TO DRAFTING AND REVIEW
Page 106:
I S S N 1 0 1 8 - 5 5 1 8
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