epc_fa2011_lecture_7 - Settling and Sedimentation --...

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Unformatted text preview: Settling and Sedimentation -- Sedimentation is the separation from water, by gravitational settling, of suspended particles that are heavier then water. -- Particles that will settle within a reasonable period of time can be removed in a sedimentation basin (also called a clarifier). -- Sedimentation basins are usually rectangular or circular. 1 Types of Settling -- On the basis of the concentration and tendency of particles to interact, four types of the settling can occur in a sedimentation basin: 1) Discrete particle settling (Type 1 settling) 2) Flocculant settling (Type 2 settling) 3) Hindered settling (Zone settling) 4) Compression 2 Types of Settling Discrete Particle Settling (Type 1) -- 1 -- Settling velocity of a particle will accelerate until the drag force, FD, of the particle equals to the vertical force, FZ where FZ = FB - Fg. -- Then the settling velocity becomes stabilized at a constant velocity, vs, called a terminal settling velocity. Fg = (ρ s )gVp FD = C D A p (ρ )g FB = (ρ )gVp ν s2 2 Fz = FD = (ρ s - ρ )gVp = C D A p (ρ )g 4 4 d3 π ⋅r3 = π ⋅ 3 8 = 2d =3 2 3 Ap d π ⋅r2 = π ⋅ 4 Vp Fz = FB - Fg = (ρ s - ρ )gVp ⎡ 4g(ρ s - ρ ) ⋅ d ⎤ ∴ νs = ⎢ ⎥ ⎣ 3C D ⋅ ρ ⎦ ν s2 2 0.5 3 Types of Settling Discrete Particle Settling (Type 1) -- 2 -- Drag coefficient, CD =f(Laminar or Turbulent flow regime) = f(NR) v d v dρ NR = s = s ν μ (Stokes’ Law) -- Much of the Discrete Particle Settling in water and wastewater treatment process will follow the Stokes' Law! 4 Types of Settling Discrete Particle Settling (Type 1) -- 3 Design of a Sedimentation Basin (Rectangular shape) -- Procedure is to define a terminal settling velocity, vs and to design the basin so that all particles that have a terminal velocity equal to or greater than vs will be removed (or settle down in the sedimentation basin). Horizontal Velocity = vL = v0 (of water) Terminal settling velocity = vs vs = h0 t where t = detention time and t= Volume V L ⋅ W ⋅ h0 == Flow rate Q Q 5 Types of Settling Discrete Particle Settling (Type 1) -- 4 vs = h0 h0 h0 ⋅ Q Q Q = = = = L ⋅ W ⋅ h0 t L ⋅ W ⋅ h0 L ⋅ W A Q -- Thus the terminal settling velocity is equal to Surface Overflow Rate (SOR), or surface loading rate vs = Q = Surface Overflow Rate (SOR) A SOR = ft3/sec-ft2 (= ft/sec) or = m3/sec-m2 (= m/sec) or = gal/ft2-day Alum or iron floc 14.5 to 22.3 m3/day-m2 Lime softening floc 22.3 to 82.1 m3/day-m 6 Types of Settling Discrete Particle Settling (Type 1) -- 5 -- Discrete Particle Settling can be estimated by relationships based on vp ≥ vs All particles will be removed/settled vp < vs The particle won't be removed and carried out or settled in proportion to the ratio of their settling velocity to vs -- Thus, design parameters for the sedimentation zone are: 1. Particle settling velocity, vs 2. Flow velocity, v0 7 Types of Settling Discrete Particle Settling (Type 1) -- 5 Design of a Sedimentation Basin (Circular shape) -- The same design principles prevails vL = Qout dh 0 v s = dr vL h0 ∫ dh 0 = 0 Qin vL h0 Q 2π rh 0 vs r1 r0 Qout h0 = vs = vs, SOR, v0 ∴ dh 0 2π rh 0 v s = dr Q 2π h 0 v s r0 ∫ r dr Q r 1 2π h 0 v s Q r ⎡ r 2 ⎤ 0 π h 0 vs h Av (r0 2 - r1 2 ) = 0 s ⎢⎥= 2⎥ Q Q ⎢ ⎦r ⎣ 1 Q = Surface Overflow Rate (SOR) A 8 System & Reproducibility! Types of Settling Flocculant Settling (Type 2) -- 1 -- Refers to a rather dilute suspension of particles that flocculate during the sedimentation operation. -- By flocculating, the particles increase in mass and settle at a faster rate. Therefore, the actual settling rate cannot be calculated theoretically, but is determined from experimental observation of the settling rate. -- There is no adequate mathematical relationship that can be used to describe Type II settling. The Stokes' equation can not be used because the flocculant particles are continually changing in size, shape, and specific gravity, especially when water is entrapped in the floc. -- The overall settling efficiency depends not only on the individual particle’s settling rate, but also on the concentration of solids and the detention time. 9 Types of Settling Flocculant Settling (Type 2) -- 2 -- Sedimentation tests can be conducted using a cylindrical tube (=Settling Tube) with sampling taps equally spaced (normally 2 ft intervals) along the column depth. -- The tube should be at least 5 to 8 inches in diameter to minimize side-wall effects, and the height should be at least equal to the depth of the settling tank under design. -- After the test starts, samples are withdrawn from each port simultaneously at frequent intervals and analyzed for suspended solids concentrations. -- The percent removal is computed for each sample analyzed based on the initial suspended solids concentration and the suspended solids concentration of the sample. 10 Types of Settling Flocculant Settling (Type 2) -- 3 -- Percent removal is then plotted on grid paper as a number against time and depth of collection for the sample. -- Interpolations are made between the plotted points, and curves of equal percent removal (RA, RB, and so on) are drawn in a similar way as drawing contour lines (= isoconcentration line). This plot is called a Settling Diagram for Type II settling. 11 Types of Settling Flocculant Settling (Type 2) -- 4 -- The surface overflow rates, SOR, are then determined for the various settling times (t1, t2, and so on = detention time, θ) where the R curves intercept the x-axis. -- For example, for the isoconcentration curve R2 in the figure below, SOR becomes SOR = v s = H × conversion factors t2 where H = height of the tube t2 = intercept of the R2curve at x-axis 12 Types of Settling Flocculant Settling (Type 2) -- 5 -- Then the fractions of solids removed, %RTa at various times, t1, t2, and so on are then determined. (t1, t2, etc. are detention time (θ) for corresponding % Removal) -- In other word, it is saying that it will take 'x' amount of time (θ) to settle out 'y' % of suspended particles (%RTa) via Type 2 settling in the flocculator. -- General form for calculating the fractions of solids removed, %RTa is %R Ta = R a + Point Estimate H H1 H (R b − R a ) + 2 (R c − R b ) + 3 (R d − R c ) + L H H H Proportional Balancing w/ a higher resolution 13 Point Estimate Types of Settling Proportional Balancing w/ a higher resolution Flocculant Settling (Type 2) -- 6 -- For example, if we're using the same figure, for time t2 (for 50% removal curve), the percent fraction removed, %RT50, can be calculated by RT = R2 + H3 H H (R 3 - R 2 ) + 2 (R 4 - R 3 ) + 1 (R 5 - R 4 ) H H H where H3 = height that the particles of (R3 - R2) size would settle during t2 14 Types of Settling Flocculant Settling (Type 2) -- 7 -- In the design of an actual settling tank for field operation, a scale-up factor must be applied to the data obtained from the laboratory tests [= Bench-scale data]. a) Design Detention Time, t0 = Bench-scale detention time x Scale-Up Factor (=1.75) b) Design settling velocity, v0 or design SOR = Bench-scale settling velocity (vs) or Bench-scale SOR x Scale-Up Factor (=0.65) 15 Flocculant Settling (Type 2) Example A circular settling basin is to be designed to treat an industrial wastewater having 320 mg/L suspended solids and a flow, Q of 2 MGD. A batch settling test was performed using an 8-inch diameter cylindrical column that was 10 ft long and had withdrawal ports every 2 ft. The data of the percent removals are shown below. 1) Calculate design detention time, t0 and design SOR (= v0) if 65% of the suspended solids are to be removed. 2) Calculate the diameter of the settling basin satisfying 65% removal. 3) Calculate the design diameter of the settling basin if equipment is available in 5-ft increments of the settling basin diameter. 4) Calculate the depth of the tank 16 Flocculant Settling (Type 2) Example – Step 1 1) Plot/Mark the percent removal values on a grid paper as number against time and depth of the sample collected 17 Flocculant Settling (Type 2) Example – Step 2 2) Interpolate the percent removal values to locate/find points that are equal to a % removal. For example, to locate/find the 20% removal, interpolate plotted/marked removal depths at 2, 4, 6 and 8 ft depths to find 20% equivalents (=R1 in the figure). Repeat interpolation for 20, 30, 40, 50, 60, and 70% removal, and draw curves to the plot. 18 Flocculant Settling (Type 2) Example – Step 3 3) Next, 20% removal curve (= R1) intersects the xaxis at 13 min., thus the bench-scale detention time t1 is 13 min. for 20% removal. This bench-scale detention time, ti is also denoted as θ. Then, calculate the benchscale settling velocity, vs or SOR for t1 of 13 min. H × conversion factors t1 10 ft 1440 min = × 13 min day SOR = v s = = 1107.7 ft/day 19 Flocculant Settling (Type 2) Example – Step 4 Point Estimate Proportional Balancing w/ a higher resolution 4) Draw a perpendicular line to x-axis at t1 of 13 min., upward, and then locate the point midway between 20% (= R1) and 30% (= R2) removal curves to find the corresponding depth, which is 4.1 ft. Repeat for 30% 40% 50% 60% and and and and 40% 50% 60% 70% 2.6 1.6 1.1 0.7 ft ft ft ft 20 Flocculant Settling (Type 2) Example – Step 5 Point Estimate Proportional Balancing w/ a higher resolution 5) Next, calculate a total fraction removed, RT at t1 of 13 min., i.e., %RTa. (in this case, %RT20 for 20% removal) %R Ta = R a + H H1 H (R b − R a ) + 2 (R c − R b ) + 3 (R d − R c ) + L H H H 4.1 2.6 ( 30 - 20) + ( 40 - 30) 10 10 1.6 1.1 ( 50 - 40) + (60 - 50) + 10 10 0.7 + (70 - 60) = 30.1% 10 %R T20 = 20 + 21 Flocculant Settling (Type 2) Example – Step 6 6) Repeat the same procedure to calculate %RTa for remaining 30%, 40%, 50% and 60% removal curves at t2 through t5. Tabulate calculated bench-scale results. H × conversion factors t1 10 ft 1440 min = × 13 min day SOR = v s = = 1107.7 ft/day 22 Flocculant Settling (Type 2) Example – Step 7 7) All set, plot calculated bench-scale %RTa vs. bench-scale Detention times, θ to find the detention time required for the target -- 65% removal. Thus the design detention time, t0 becomes 1.34 x 1.75 = 2.35 hrs, where 1.75 is a scale-up factor for the design detention time. 23 Flocculant Settling (Type 2) Example – Step 8 8) Plot calculated bench-scale %RTa vs. bench-scale SOR or benchscale vsto find the SOR required for the target -- 65% removal. Thus the design SOR is 190 x 0.65 = 123.5 ft/day, where 0.65 is a scale-up factor for the design SOR or design settling velocity, v0. 24 Flocculant Settling (Type 2) Example – Step 9 9) Subsequently, the require surface area can be then calculated by Surface Area = 2 × 10 6 gal day × ft 3 = 2165 ft 2 123.5 ft 7.48 gal day × V L⋅W⋅H 1 t L⋅W A Q Q = Q× = Q× = Q× = Q× = Q× = Q× = A H H H H Q Q t Thus the diameter of the circular settling basin, D becomes ⎡4 ⎤ D = ⎢ × 2165 ⎥ ⎣π ⎦ 0. 5 = 52.5 ft Since the equipment is available in 5-ft increments, design diameter, D will be 55 ft. 25 Flocculant Settling (Type 2) Example – Step 10 10) With calculated design detention time and design diameter, the required depth of the circular settling basin becomes H= 2,000,000 gal ft 3 4 1 × 2.35 hr × ×× = 11.02 ft 24 hr 7.48 gal π (55 ft) 2 = Q× V1 × =H QA 11) Most importantly, summarize your design parameters! Design detention time, t0 (65% removal) = 2.35 hrs Design SOR, v0 (65% removal) = 123.5 ft/day Diameter of the circular settling basin, D = 55 ft ( 52.5 ft) Depth of the circular settling basin, H = 11.02 ft 26 Filtration -- Filtration is to remove discrete suspended solids presented in the source water, remove precipitated hardness, iron and manganese Filter any residuals after coagulation and sedimentation -- Critical to protect public health (SDWA/Safe Drinking Water Act & SWTR/Surface Water Treatment Rule) -- Giadia and Chryptosporidium cysts, about 1 µm -- 99.99% removal requirement by filtration (SWTR). The infamous Cryptosporidiosis outbreak in Milwaukee was due to filtration breakdown. 27 Filtration -- Goals of filtration is to obtain the following level of the water quality 28 Filtration Mechanisms -- Removal of suspended matter in a granular-medium filter is based on by one or combination of; By 1) 2) 3) 4) 5) Straining Sedimentation and inertial impaction Interception Adhesion (= electrostatic attraction) Flocculation 29 Filtration Mechanisms Granular-Medium Filter -- 1 1) by Straining 2) by Sedimentation and Inertial impaction 3) by Interception 30 Filtration Mechanisms Granular-Medium Filter -- 2 4) by Adhesion (= electrostatic attraction) 5) by Flocculation 31 Type of Filters -- 1 -- Diatomaceous earth -- Microstrainer -- Pressure filters -- Slow sand filters (SSF) -- Rapid sand filter (RSF) -- Multimedia filter 32 Type of Filters -- 2 -- Diatomaceous earth: 80-90% porosity, light -- Microstrainer: Uses woven steel, clogs easily -- Pressure filters: Effective, yet expensive -- Slow sand filters (SSF) - Gravitational, low flows - Medium is finer and less carefully graded than RSF - "Cake filtration" (i.e., Schmutzedecke) : solids removed at top of sand bed, and cake builds-up improves straining. - Typically cleaned by hand - rake cake off surface. - Can remove of suspended solids, turbidity, microorganisms without the need for chemical addition. - Cannot remove high turbidity, high levels of microorganisms, nor chemical contamination. - Require large surface areas. - Common in Europe. 33 Type of Filters -- 3 -- Rapid sand filter (RSF) - Gravitational, higher flows - Traditional silica sand, sand, anthracite, garnet sand and materials that are locally available such as crushed glass, slag metallic ores and others. (typical RSFs are a dual media filter consists of sand and anthracite) - Solids removed within granular bed. - Cleaned by backwash - Can remove of suspended solids, high turbidity, high levels microorganisms. - Require small surface areas. - Common in US and Canada. 34 Type of Filters -- 4 -- Multimedia filter - Gravitational, higher flows - Mixture of sand, anthracite, garnet sand. - Larger media particles (and larger pores) at top of filter and smaller media particles at bottom. - Solids removed within granular bed. - Less likely to clog at the surface of media - Cleaned by backwash - Can remove of suspended solids, high turbidity, high levels microorganisms. - Low head loss and longer filter run time (duration of the filter operation between cleanings) through improved hydraulic characteristics - Require small surface areas. - Common in US and Canada. 35 Filter Media -- 1 -- Specific Gravity (s.g.) of various media (with sand/s.g. 2.5) - Garnet (sand) is a high hardness, high density granular filter media which is normally used as the lower filtration layer of a multimedia filter bed. Light tan to reddish purple color. s.g.4.5 - Anthracite coal grains. Among different ranks of coal, anthracite coal is the hardest. (in comparison to lignite coal) s.g. 1.5 36 Filter Media -- 2 - Granular Activated carbon (GAC), sorbs organic material (= color, taste and odor control) s.g. 1.5 37 Filter Media -- 3 -- Brita filter / Granular Activated carbon (GAC) for color, taste and odor control 38 Media Size Characteristics -- 1 -- Filter media would have following desired qualities i) Coarse enough to retain large quantities of floc. ii) Sufficiently fine to prevent passage of suspended solids. iii) Deep enough to allow relatively long filter runs. iv) Graded to permit backwash cleaning (RSF). -- Small pore openings increase filtration efficiency, however head loss through the medium increases fast and results in reduced flow rates. -- Larger media increase pore size, reduced head loss, and increased flow rate, but sacrifice the filtration efficiency. -- To obviate such, filter media vary in diameter within a selected size range. Multimedia Filter 39 Media Size Characteristics -- 2 -- The effect of varying size ranges becomes important because of stratification during backwash operations. When the bed is expanded, small grains are lifted farther than larger grains and settle more slowly when the wash cycle is ended -- which is not an ideal condition. -- Mixed-media filters are not true depth filters (i.e., filtration efficiency depends on the depth of filter medium) but provide two or three filter surfaces with progressively small openings and thus permit effective use of a larger potion of the volume. 40 Backwash Process -- 1 -- During filtration, the filter bed will become more and more clogged. As the filter clogs, the water level will rise above the sand as it becomes harder to force water through the bed. -- Eventually, the water level will rise to the point that the filter bed must be cleaned. This point is called the terminal head loss. -- When this occurs, the operator turns off valves to stop the supply of water from the sedimentation tank and prevents any more water from entering the clear well. 41 Backwash Process -- 2 -- Operator then opens other valves to allow a large flow of washwater (= clean water stored in an elevated tank or pumped from a clear well) to enter below the filter bed. This rush of water forces the sand bed to expand and sets individual sand particles in motion. (= Fluidization) 42 Backwash Process -- 3 -- By rubbing against each other, the light colloidal particles that were trapped in the pore spaces are released and escape with the washwater. -- The washwater is a waste stream that must be treated again through the normal filtration process. -- After a few minutes, the washwater is shut off and filtration resumed. 43 Backwash Process -- 4 1 2 3 4 5 6 44 Backwash and Multimedia Filter -- 1 A) A) Smaller particles [that to be filtered] penetrate deeper into filter layers before being captured/filtered. Thus, less head loss builds up throughout filter media, and as a result, efficient use of the filter is made. (this is a desirable filtering condition) B) B) If all filter media have the same specific gravity, after a backwash [and fluidization], filter media will settle out exact opposite of the desired, i.e., larger size media will settle and be stacked first and smaller size media will settle last at the top of the filter bed. (this is a definitely undesirable filtering condition) With the finest sand media layer is on the top of filter bed (=smallest pores at the top), most of particles will clog right in the top layer of the filter bed before reaching to next media layer, and it would result in a less filter depth for particle removal, i.e., ineffective filtering and premature head loss. 45 Backwash and Multimedia Filter -- 2 C) C) Multimedia filter uses different media with different specific gravities. Larger but lighter media settle out more slowly than smaller but heavier media -- as a result, larger but lighter media remain on top after each backwash! typical RSFs are a dual media filter consists of sand and anthracite. 46 Head Loss Calculation (Rose Equation) -- 1 -- Rose Equation is used to calculate Head loss through a clean stratified sand filter with uniform porosity 1.067 C D v a 2 1 hL = D4 ϕ g εd Single uniform filter media layer or Per sieve size diameter 1.067 v a 2 D hL = ϕ ε4 g Multiple/Stratified filter media layers or Cumulative sieve size diameter Shape factor, ϕ (phi/’fai/) ∑ CD ⋅ f d ϕ(sphere) = 1 ϕ(rounded sand) = 0.82 ϕ(average sand) = 0.75 ϕ(crushed coal and angular sand) = 0.73 47 Head Loss Calculation (Rose Equation) -- 2 1) Calculate Reynolds number, NR using shape factor, ϕ and approach velocity, va NR = ϕ d ⋅ va ⋅ ρ μ = ϕ d ⋅ va ν 2) Depending on the range of calculated Reynolds number, NR, calculate a corresponding drag coefficient, CD 48 Monitoring Filter Performance -- Head loss is measured by pressure gauges and will give an indication about when to backwash. -- Turbidity Sensors inform the operator when the turbidity is > 1 NTU. -- Flow meter monitoring for total influent and effluent flows in addition to head loss monitoring. 49 Commonly Encountered Problems in the Filtration Operation -- Air binding caused by air dissolved in the water escape into the media to form bubbles Controlled by adjustment of pretreatment or chemical addition -- Turbidity Breakthrough Chemicals and polymers added to right before filtration -- Sand incrustation by entering water supersaturated with calcium carbonate, then coat individual grains and cement them together Adjust pH with acid and keep calcium in solution 50 Quick Recap 1) Giadia and Chryptosporidium cysts -- 99.99% removal requirement by filtration (SWTR). 2) Turbidity (less than 5 NTU) as the main operational filtration guideline 3) RSF and Multimedia filters! 4) Backwash process – it’s the specific gravity make it happen 5) Rose equations for calculating Head Loss for both singleand multi-media layer(s) 51 Disinfection -- Disinfection refers to the selective destruction of the disease causing organisms. Not all the organisms are destroyed during typical water treatment processes. -- Disinfection is different from sterilization, which is the destruction of all the organisms. -- The purpose of disinfection is: i) To prevent direct transmission of disease to people through water ii) To break the chain of disease and infection by destroying responsible infective agents. 52 Requirements for Disinfectant -- The disinfectant must be safe (neither toxic nor unpalatable for aesthetic reasons). -- It must be able to destroy the kind and the number of pathogens in a reasonable time period. -- Should be available at reasonable cost range. -- Provides the persistent residual protection after the disinfection process has been completed in WTP. 53 Common Disinfection Methods -- 1 1) Heating 2) Ultraviolet Irradiation (UV) 3) Application of Chemicals ----- Halogens Ozone Hypochlorination Chlorine dioxide (ClO2) 54 Common Disinfection Methods -- 2 1) Heating -- Applicable to very small amount of water to disinfect. For large quantity of water, this is too expensive or not feasible. -- The boiling point will destroy the major diseaseproducing, non-spore forming bacteria. Commonly used in beverage and dairy industry. (i.e. pasteurization and sterilization in milk) 55 Common Disinfection Methods -- 3 2) Ultraviolet Irradiation (UV) -- Effective in killing all types of bacteria and viruses by destroying their nuclei. (i.e., can not regenerate) -- The advantages of UV disinfection include no chemical handling, short retention time, and little maintenance. -- The disadvantages include no residual protection, high cost and ineffectiveness on turbid waters that rays cannot penetrate. 56 Common Disinfection Methods -- 4 3) Application of Chemicals a) Halogens -- Chlorine gas is the most widely used disinfectant. Difficult to handle, and may form THMs. Cl 2 (g) + H 2 O ⇔ HOCl + H + + Cl − -- Chlorine was named after the Greek word ‘chloros’, which means yellow-greenish and refers to the color of chlorine gas. -- Iodine is effective and does not react with ammonia and organic nitrogen compounds to form amines (= derivatives of ammonia) but expensive and has a physiological effect on thyroid activity. 57 Common Disinfection Methods -- 5 3) Application of Chemicals a) Halogens / Chloramine (NH2Cl) -- Chloramine is a combination of chlorine and ammonia that is considered a better disinfectant than chlorine alone. -- More stable than chlorine and lasts longer in the distribution system, provides increased protection from bacterial and viral contamination. -- Compared to chlorine, chloramine produces lower levels of DBP such as THMs. -- People often report improved taste and odor in their water when chloramine disinfection method is used instead of chlorine. 58 Common Disinfection Methods -- 5 3) Application of Chemicals b) Ozone -- A strong oxidizing agent -- It helps in removing taste and odor, reduces the concentration of precursors, (='forerunner,' organic compound capable of being formed into a THM) and produces fewer halogenated organics than other common oxidizing disinfectants. -- Requires a very little contact time for effective disinfection (i.e., very quick). 59 Common Disinfection Methods -- 6 3) Application of Chemicals b) Ozone – Cont.d -- It is a powerful oxidant stronger than hypochlorous acid. More effective than chlorine in destroying cysts and viruses, i.e., a more effective biocide -- Ozone dissipates rapidly, therefore no residual will be formed which might guard against the subsequent contamination of the distribution system. Ozone will not persist in the water. -- It is more costly than chlorine. -- Due to its instability, ozone has to be generated on site. 60 Common Disinfection Methods -- 7 3) Application of Chemicals c) Hypochlorination -- Refers to the addition of Calcium hypochlorite (Ca(OCl)2) Solid form containing 65% chlorine, most stable, easy to handle, easy to dissolve, easy long-term storage Sodium hypochlorite storage unit, HRSD VIP WWTP, Norfolk Sodium hypochlorite (NaOCl) Liquid for containing 5~15% chlorine, very corrosive and more expensive than chlorine gas, employed under emergency conditions (i.e., acute waterborne disease outbreaks) 61 Common Disinfection Methods -- 8 3) Application of Chemicals d) Chlorine dioxide (ClO2) -- More effective in inactivation of viruses than chlorine. -- It is an unstable and explosive gas and therefore must be generated on site. -- Very potentially toxic end products can be formed (chlorite and chlorate), but these residuals are believed to degrade faster than chlorine residuals and may not pose as serious threat. -- In case that ClO2 dose does not maintain a residual long enough to be useful in distribution system, chloramine (NH2Cl) is used in distribution system. 62 Factors Influencing the Actions of Disinfectants 1) Contact Time 2) Concentration and Type of Chemical Agents 3) Intensity and Nature of Physical Agent 4) Temperature 5) Number of Organisms 63 Factors Influencing the Actions of Disinfectants 1) Contact Time -- One of the most important variables in disinfection process. -- For a given concentration of disinfectant, the longer the contact time, the greater the kill would be. (Chick's Law of disinfection (1908) - a pseudo first-order reaction) dN = −kN t dt -- If N0 is the number of organisms at t=0, above equation can be integrated to yield a general solution Nt = e −kt N0 or ln Nt = −kt N0 64 Factors Influencing the Actions of Disinfectants 2) Concentration and Type of Chemical Agents -- Depending on the type of chemical agent, disinfection effectiveness is proportional to the concentration. Ct Concept -- The effect of concentration has been formulated empirically Cntp = k or Ct = k and Microorganisms killed by disinfectants is assumed to follow this Ct concept. -- Ct is widely used in SWTR as a criteria for cyst and virus disinfection. -- if n < 1, contact time is more important than concentration if n = 1, effect of time ≈ effect of concentration if n > 1, concentration is more important than contact time 65 Combined Residual Chlorination -- Breakpoint is the condition when the chlorine demand (I.e., Ct concept!) has been satisfied by continuous addition of chlorine to a water or wastewater. -- Further additions after this breakpoint will result in a residual that is directly propotional to the amount added beyond the breakpoint, and referred to as combined chlorine residual. -- Combined chlorine residual will be reduced slowly and therefore, persists for a longer time in the distribution system. (and provide prolonged disinfection to water after water treatment processes completed) 66 Dechlorination (for WW) -- If chlorinated effluents is discharged to receiving waters and aquatic ecosystems, the combined chlorine residual is removed to reduce the toxic effects of chlorinated effluents. -- Common agents for dechlorination are sulfur dioxide, SO2 (=most commonly used), sodium sulfite, Na2SO3, sodium thiosulfates, Na2S2O3, hydrogen peroxide, H2O2, etc. 67 Taste and Odor Control -- Taste and odors in water most likely caused by variety of parameters including minerals, metals, and salts from the soil, end products of biological reactions. Alkaline materials impart a bitter taste to water, while metallic salts may give a salty or bitter taste. -- Direct measurement of materials that produce taste and odor can be made if the causative agent is known, and by liquid or gas chromatography. -- Quantitative tests that employ human sense can also be used to measure taste and odor in form of Threshold Odor Number (TON). TON is the dilution factor required to produce a solution in which the odor is just perceptible. -- A TON of 3 or less is ideal. Untreated river water usually has a TON between 6 and 24. Treated water normally has a TON between 3 and 6. At TONs of 5 and above, peoples will begin to notice the taste and odor of their water. 68 ...
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