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CHEE 370 Waste Treatment Processes

CHEE 370 Waste Treatment Processes. Disinfection. CSTR no Recycle The Activated Sludge Process Trickling Filters. What we have covered so far:. Type II Settling. Type III Settling. Bar Racks. Type I Settling. To reduce the sludge volume sent to the AD.

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CHEE 370 Waste Treatment Processes

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  1. CHEE 370Waste Treatment Processes Disinfection

  2. CSTR no Recycle • The Activated Sludge Process • Trickling Filters What we have covered so far: Type II Settling Type III Settling Bar Racks Type I Settling To reduce the sludge volume sent to the AD Sludge stabilization by methanogenic bacteria • Fertilizer • Incineration • Landfills

  3. Disinfection • Selective destruction of disease-causing organisms • In WW treatment, we are concerned with: • Some bacteria • Viruses • Amoebic cysts • Overall objective is to prevent waterborne diseasessuch astyphoid, cholera, and dysentery • Disinfection is not the same thing as sterilization • Textbook Chapter 12; Table 12-28

  4. Mechanisms of Disinfection • Damage the cell wall • Alteration of cell permeability (i.e. phenol) • Alteration of colloidal nature of protoplasm (i.e. heat) • Inhibition of enzyme activity (i.e. chlorine or any oxidant)

  5. Methods of Disinfection • Chemical agents: • Chlorine and its compounds, bromine, iodine, ozone, phenol, alcohols, soaps and detergents, hydrogen peroxide, acids, bases. • Most common disinfectants are oxidizing chemicals • Chlorine is the most commonly used disinfectant in North America

  6. Methods of Disinfection • Physical Agents: • Heat (boiling), UV • The use of UV systems has been increasing dramatically in the last few years • Mechanical Means: • Filtration, ultrafiltration, nanofiltration

  7. Selecting the Appropriate Type of Disinfection Considerations: • Germicidal and viricidal effect • Needs to be sufficient to achieve the treatment objectives • Stable Residual • Important in drinking water treatment (or cases of water reuse) to prevent re-contamination during distribution • Safety • Some chemicals are highly toxic, such as chlorine • Formation of disinfection by-products

  8. Measuring the Effectiveness of Disinfection • Total Coliform • Species ferment lactose and produce CO2 gas when incubated at (35 ± 0.5) °C for (24 ± 2) h • Species produce a colony within (24 ± 2) h to (48 ± 3) h when incubated in a medium that facilitates growth • Fecal Coliform • Species produce gas or colonies when incubated at the higher temperature of (44.5 ± 0.2) °C for (24 ± 2) h • Total coliform is used as the indicator for drinking water and wastewater effluent

  9. Disinfection Using Chlorine • Chlorine can be added as a: • Gas (Cl2(g)) - lots of plants use this method. Chlorine gas is highly toxic. • Sodium hypochlorite (NaOCl) - Commonly known as bleach • Calcium hypochlorite (Ca(OCl)2) - solid • Fairly complex chemistry in water and wastewater

  10. Chlorine in Water • There are three basic reactions occurring when chlorine is added to water: • Dissolution: Cl2(g) Cl2 (l) • Very fast reaction • Hydrolysis: Cl2(l) + H2O  HOCl + H++ Cl- • Very fast reaction • Chlorine reacts with water to generate hypochlorous acid, hydrogen ions (acid), and chloride • Dissociation: HOCl  H+ + OCl- • Hypochlorous acid is a weak acid that can dissociate into hydrogen ions and hypochlorite.

  11. Speciation of Chlorine as a Function of pH

  12. Influence of Chlorine Species • Hypochlorous acid is a much stronger oxidant than hypochlorite (40 - 80 x the killing efficiency) • For a given total chlorine concentration: CT=Cl2(l) + HOCl + OCl- = Free Residual Chlorine • Lower disinfection efficiencies are obtained at high pH • Either longer exposure times or higher dosages are required if hypochlorite is the predominant species

  13. Reactions of Hypochlorous Acid with Ammonia • Hypochlorous acid is a very strong oxidant that can react with ammonia present in the effluent • Can be very significant if the plant is not nitrifying • The reactions result in the formation of different species, with differing levels of disinfection (oxidation) capabilities • Need to account for these side reactions to provide adequate disinfection

  14. Reactions of Hypochlorous Acid with Ammonia • NH3 + HOCl  NH2Cl (monochloramine) + H2O • NH2Cl + HOCl  NHCl2 (dichloramine) + H2O • NHCl2 + HOCl  NCl3 (nitrogen trichloride) + H2O • Free Residual Chlorine= Cl2(l) + HOCl + OCl- • Combined Residual = Summation of all Chloramines • Total Residual Chlorine = Free Residual Chlorine + Combined Residual Chlorine • All concentrations are expressed as Cl2

  15. Breakpoint Chlorination Dose

  16. Breakpoint Chlorination • If a target residual chlorine concentration is to be maintained, it is necessary to provide a dose such that the demand is satisfied and the desired residual is obtained • Breakpoint chlorination leads to the effective removal of nitrogen from the effluent wastewater

  17. Kinetics of Bacterial Death • Several factors influence the rate of bacterial kill: • The type of disinfectant (we are considering chlorine) • Temperature (moderate effect) • pH (strong effect) • Presence of organic matter (can have a strong effect) • Concentration and types of organisms (strong effect) • The concentration at which chlorine is present • The contact time between the chlorine and the organisms to be killed

  18. Effect of Contact Time • The most common expression to reflect the effect of contact time is Chick’s Law:

  19. Effect of Concentration • The rate of kill will be maximum if both the contact time and the disinfectant concentration are high • It is possible to have adequate kill rates if C is low, but t is high (and vice-versa) • It is common to assess the product of C*t when dealing with disinfection systems

  20. Estimating the Kill Efficiency • A correlation often used to estimate the amount of residual chlorine required to achieve a certain kill efficiency is: • Collins Model • N = number of organisms • [C] = total chlorine residual, mg/L; [t] = time, minutes

  21. Practice Problem • Consider a chlorination tank with a detention time of 30 minutes. We want to destroy 99.9999% of the coliforms in the effluent. What residual chlorine concentration is required?

  22. MOE Guidelines • MOE Guidelines for Chlorine Contact Chambers • 30 minutes for average annual flow • 15 minutes for peak hourly flow • Design the chamber based on the larger volume

  23. MOE Guidelines • MOE regulates the chlorine concentration in the plant effluent • Total Residual Chlorine (TRC) ≤ 0.02 mg/L • If the required residual chlorine concentration for adequate disinfection is higher than what the MOE allows, it is necessary to perform dechlorination • Dechlorination involves the addition of sulphur dioxide, which leads to the conversion of hypochlorous acid to chloride

  24. Chlorine as a Disinfectant • Advantages: • Reliable • Cheap • Simple • Provides a stable residual • Limitations: • Extremely toxic and corrosive • Can influence water taste and odor • Forms trihalomethanes by reacting with organic matter in the WW - these compounds are known carcinogens

  25. UV Disinfection • Physical method of inactivating pathogens • Mechanism of UV Disinfection: • Radiation with a wavelength of ~ 260 nm penetrates the cell wall and cell membrane of microorganisms • The radiation is absorbed by cell material such as DNA and RNA, promoting changes that prevent cell replication • DNA - interferes with cell replication • RNA - interferes with protein and enzyme production

  26. UV Disinfection - Mechanism • Schematic of the effect of UV on DNA

  27. UV Disinfection - Dosage • For chlorine disinfection, the “Ct product” is an indication of disinfecting potential • For UV disinfection, a similar quantity is UV dosage:

  28. UV Design Factors • There is an “intensity field” in an UV reactor, as each lamp radiates in all directions • The water, non-biological materials, and other lamps will absorb the emitted radiation • Effect is termed “dissipation” • Bacteria are present in flocs in the effluent • The higher the TSS concentration in effluent, the higher the dosage required to reach all the bacteria • It is necessary to perform pilot tests for UV systems to determine the effectiveness

  29. Full-Scale UV System

  30. UV Disinfection • Advantages of UV disinfection over chlorine disinfection: • No toxic by-products of disinfection are known to be formed • Short detention times • UV disinfection requires a six-to-10-second contact time, compared to a 15-to-30-minute contact time for chlorine • UV disinfection presents no dangers in terms of handling chemicals

  31. Ozone • More commonly used for the disinfection of water supplies, rather than for municipal WW treatment • As ozone is unstable, a gap electrode is used to generate O3 from O2 on site • 3 O2 -> 2 O3 • Using air: 0.5 - 3% ozone by weight • Using pure oxygen: 1 - 6% ozone by weight

  32. Ozone as a Disinfectant • Advantages: • Does not react with ammonia • No bad taste or colouration • No chemical residue • Aerates water to near O2 saturation • Limitations: • Must reach a threshold concentration to facilitate disinfection • Problems can result if there are high hydraulic loadings • Organics can interfere with the process • Raises the electricity costs

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