Water Treatment Engineering - Chapter 3: Conventional Water Treatment Design

Screening

Aeration

Coagulation

Flocculation

Sedimentation

Filtration

Disinfection

Introduction to Disinfection

• Disinfection is a process used to make water hygienic and suitable for consumption.
• For high-quality groundwater, disinfection may be the only required treatment.
• It is the final step before water enters the distribution system.
• Disinfection kills or inactivates pathogens, which are microscopic organisms ranging from 1-5 microns (µm).
• Pathogens can cause diseases like cholera, dysentery, giardiasis, and typhoid.
• Handling pathogenic organisms is a primary concern in water treatment.
• Disinfection is defined as the selective destruction/inactivation or removal of pathogenic organisms.
• Sterilization is the complete destruction of all living organisms, but it is not necessary (or cost-effective) for water treatment.
• Disinfection can create objectionable tastes and disinfection by-products (DBPs).
• Untreated water may contain:
  • Viruses (e.g., infectious hepatitis, poliomyelitis)
  • Bacteria (e.g., cholera, typhoid fever, dysentery, Legionnaires’ disease)
  • Intestinal parasites (e.g., giant roundworm, amoebic dysentery, giardiasis, cryptosporidiosis)

Types of Disinfection

Primary Disinfection

• Primary disinfection involves the removal or inactivation of pathogens through various treatment methods.
• Pre-chlorination, coagulation, settling, filtration, and post-chlorination all contribute to primary disinfection.

Secondary Disinfection

• Secondary disinfection involves leaving a residual disinfectant to protect against possible contamination in the distribution system.
• Only disinfectants that create a persistent residual can be used (e.g., chlorine, chlorine dioxide, chloramines).
• Secondary disinfection protects against microbiological recontamination, reduces bacterial regrowth, controls biofilm formation, and indicates distribution system integrity.
• Loss of residual disinfectant may indicate a compromised system.
• Chlorine, chlorine dioxide, and monochloramine provide a persistent residual.
• UV and ozone do not provide a residual and cannot be used for secondary disinfection.
• Large distribution systems may need chlorine booster stations to maintain minimum residual levels.
• Rapid decay of disinfectant residual may be due to incrustation, sediment accumulation, and biofilm activity.
• Free chlorine residual is more powerful but decays faster than combined chlorine residual.

Disinfection Methods

• Pathogens can be removed or inactivated using physical and chemical processes.

Removal Processes

• Conventional filtration (chemical coagulation, rapid mixing, flocculation, sedimentation, rapid rate sand filtration).
• Membrane filtration (passing water through membranes with fine pores).

Inactivation Processes

• Inactivation creates a harsh environment for organisms, typically using strong oxidizing chemicals.
• These chemicals destroy or impair pathogens by diffusing through the cell wall.
• Methods include:
  • Ultraviolet light (expensive but doesn't produce harmful byproducts and is effective against cysts).
  • Heat (boiling water for five minutes).
  • Ozone (powerful oxidizing agent).

Chlorination

• Chlorination offers advantages such as reliability and low cost.
• Chlorine residual serves as a tracer and protects against contamination (secondary disinfection).
• Oxidant chemicals like chlorine can also:
  • Control biological growth in tanks and pipes.
  • Control tastes and odors.
  • Remove color and aid flocculation.
  • Precipitate iron and manganese

Chlorine Compounds

• Chlorination is effective for both primary and secondary disinfection.
• Methods include:
  • Gas chlorination (adding chlorine gas).
  • Hypochlorination (using chlorine compound solutions, like sodium hypochlorite).
  • Chlorine dioxide (becoming more common due to efficiency and lower DBP production).

Gas Chlorination

• Uses chlorine gas as the disinfectant, especially in larger installations.
• Not used in small systems due to safety considerations and initial cost.
• Chlorine gas:
  • Has a pungent odor and greenish-yellow color.
  • Is highly toxic and detectable at concentrations as low as 0.3 ppm.

Hypochlorination

• Common in small water systems.
• Uses liquid chlorine compounds (hypochlorite) instead of chlorine gas.
• Advantages include:
  • Reduced danger (compared to chlorine gas).
  • No need for elaborate equipment or a separate room.
  • Hypochlorite raises pH, while gaseous chlorine suppresses it.
• Types of hypochlorite:
  • Calcium hypochlorite ($Ca(OCl)_2$): a dry, white, or yellow-white granular material.
  • Sodium hypochlorite ($NaOCl$): commonly called bleach, a water-based solution of sodium hydroxide and chlorine.

Chlorine Dioxide Disinfection

• Chlorine dioxide is a greenish-yellow gas with a chlorine-like odor.
• Advantages over chlorine include:
  • Lower formation of trihalomethanes (THMs).
  • Higher effectiveness in killing bacteria, viruses, and Cryptosporidium.
• Disadvantages:
  • Higher cost limits widespread use.
  • Used for taste and odor problems and oxidizing iron and manganese.

Chemistry of Chlorination

• Chlorine reacts with compounds in the water and with the water itself.
• The results of these reactions (chlorine residual) kill microorganisms.

Reactions of Chlorine Gas with Water

• The reaction depends on the type of chlorine and the pH of the water.
• Chlorine gas is toxic, heavy, corrosive, and an irritant.
• At high concentrations, it can be fatal.
• The chemical reaction when chlorine gas enters the water is:
  $Cl<em>2 + H</em>2O >>> HOCl + HCl$
• Chlorine reacts with water to form hypochlorous acid (HOCl) and hydrochloric acid (HCl).
• Hypochlorous acid may further break down depending on pH:
  $HOCl >>> H^+ + OCl^-$
• The concentration of hypochlorous acid and hypochlorite ions depends on pH.
• Higher pH increases hypochlorite ions and reduces hypochlorous acid.
• Hypochlorous acid is the most effective form of free chlorine residual.
• Disinfection is more efficient at low pH (more hypochlorous acid).

Hypochlorites

• Hypochlorites (bleach) are used instead of chlorine gas in some plants.
• They are less pure and less dangerous than chlorine gas.
• However, they decompose in strength over time in storage.
• Temperature, light, and physical energy can break down hypochlorites.
• Types of hypochlorites: sodium hypochlorite, calcium hypochlorite, and commercial bleach.
• They react with water to form hypochlorous acid.
• Reactions:
  • Calcium hypochlorite:
    $Ca(OCl)<em>2 + 2H</em>2O >>> 2HOCl + Ca(OH)_2$
  • Sodium hypochlorite:
    $Na(OCl) + H_2O >>> HOCl + Na(OH)$

Chloramines

• Some plants use chloramines instead of hypochlorous acid.
• Chloramines are produced by adding chlorine gas or hypochlorite to water to form hypochlorous acid, then adding ammonia.
• Three types of chloramines can form: monochloramine, dichloramine, and trichloramine.
• Reactions:
  • Monochloramine:
    $NH<em>3 + HOCl >>> NH</em>2Cl + H_2O$
  • Dichloramine:
    $NH<em>2Cl + HOCl >>> NHCl</em>2 + H_2O$
  • Trichloramine:
    $NHCl<em>2 + HOCl >>> NCl</em>3 + H_2O$
• The extent of these reactions depends on pH.
• Monochloramines and dichloramines can be used as combined chlorine residual.
• Chloramines are weaker but more stable than chlorine.
• Effective at killing bacteria and some protozoa, but less effective against viruses.

Breakpoint Chlorination Curve

• The amount of free chlorine residual is less than the chlorine added due to reactions with impurities.
• The breakpoint chlorination curve shows chlorine residual versus dosage after a specified contact time.
• The curve can be divided into four stages:
  • Stage I: No chlorine residual; all chlorine is used up by reducing agents (iron, manganese, nitrites, sulfides, dissolved organics). No disinfection occurs.
  • Stage II: Chlorine reacts with ammonia and organic matter, forming monochloramine and chloro-organic compounds. These have some disinfectant properties and are called combined chlorine residual.
    $NH<em>3 + HOCl = NH</em>2Cl + H_2O$
  • Stage III: Adding more chlorine decreases the residual as it oxidizes chloro-organic compounds and ammonia. $2NH<em>3 + 2Cl = N</em>2 + 6HCl$
    • Monochloramine changes to dichloramine and trichloramine.
    • Chloramine residual declines to a minimum (breakpoint).
    • Chlorine demand is the difference between chlorine added and chlorine residual. Demand is maximum at the breakpoint.
  • Stage IV: Beyond the breakpoint, any chlorine addition results in free chlorine residual.
    • The curve is a straight line with a one-to-one slope.
    • The chlorine demand of the water has been satisfied.
    • Breakpoint chlorine dosage is unique for each water sample.
    • Depending on the constituents in the water, some stages may be absent.

Chlorine Demand

• Chlorine reacts with compounds in the water, forming trihalomethanes and reacting with reducing agents (hydrogen sulfide, ferrous ions, manganous ions, nitrite ions).
• Example reactions with hydrogen sulfide:
  1. $H<em>2S + Cl</em>2 + O<em>2 >>>> S</em>2 + H_2O + 2Cl^-$
     • Hydrogen sulfide reacts with chlorine and oxygen to form elemental sulfur, water, and chloride ions.
     • Elemental sulfur precipitates and causes odor problems.
  2. $H<em>2S + 4Cl</em>2 + 4H<em>2O >>>> H</em>2SO_4 + 8HCl$
     • Hydrogen sulfide reacts with chlorine and water to form sulfuric acid and hydrochloric acid.
• The total amount of chlorine used up is the chlorine demand.
• Sufficient chlorine must be added to meet the demand and leave enough to kill microorganisms.

Dosage of Chlorine

• The amount of chlorine needed varies by source.
• Waters should be clear and free of organic matter and suspended solids.
• Chlorine oxidizes organic matter and kills bacteria.
• Residual chlorine protects against future contamination.
• Insufficient chlorine will be used up in oxidation, leaving none for disinfection.
• Disinfection is carried out by the residual chlorine.
• A sufficient quantity should leave an adequate chlorine residual without causing taste complaints.
• Applied dosage rates are usually 0.5 to 2.0 mg/L.
• Contact times are between 10 and 30 minutes, with 30 minutes recommended.
• After contact, the water should have a chlorine residual of 0.2 to 0.5 mg/L.
• Higher dosages may be needed for new water mains, wells, or reservoirs.
• New water mains need disinfection before commissioning, and older mains after repairs.
• Pipelines should be flushed with clean water and a foam swab to remove slime.
• Disinfection involves filling pipelines with water containing 20 mg/L (or 50 mg/L) chlorine for at least 24 hours.
• Wells, reservoirs, and storage tanks should be disinfected after construction, repairs, cleaning, or maintenance.
• Walls should be brushed with a strong chlorine solution (50-100 mg/L).
• Reservoirs and storage tanks may be washed with high-pressure water jets.
• After cleaning, fill with water containing at least 20 mg/L chlorine and let stand for at least 24 hours before draining.
• Refill with fresh water to remove chlorine taste.

Chlorine Practices

• Methods of feeding chlorine into water include:
  • Plain Chlorination
  • Super Chlorination
  • Breakpoint Chlorination
  • De-chlorination

Plain Chlorination

• Adding chlorine to surface water when no other treatment is required.
• Used for pure lakes and springs.
• A rate of 0.8 mg/L/hour at 15 N/cm$^2$ pressure maintains a residual chlorine of 0.2 mg/L.

Breakpoint Chlorination

• Free chlorine residual is produced beyond the breakpoint.
• Dosage depends on chlorine demand and desired free chlorine residual.
• Free chlorine residual is 25 times more effective and needs less contact time.
• Combined chlorine requires approximately 100 times longer contact time for equivalent bactericidal action.

Super Chlorination

• Similar to breakpoint chlorination but with high levels of free residual.
• Used in emergencies (epidemics, breakdowns, water main repairs, heavily polluted water).
• Heavy doses (10–15 mg/L) destroy resistant organisms and cysts.
• Excess chlorine must be removed by dechlorination.

Dechlorination

• Reducing high chlorine residuals after superchlorination to acceptable levels.
• Used after disinfecting tanks and pipes.
• Reducing compounds like sulfur dioxide, sodium thiosulphate, and sodium bisulphate are used to neutralize chlorine.
• Prolonged storage and adsorption on charcoal or activated carbon are also effective.

Points of Chlorination

• Chlorine is used at various stages, from raw water pumping to the distribution system.
• Terms include pre-, post-, and re-chlorination.

Pre-Chlorination

• Applying chlorine to raw water before other treatment processes.
• Also called source water chlorination.
• Goals include:
  • Starting disinfection early.
  • Controlling biological growth on filters, pipes, and basins.
  • Promoting improved coagulation.
  • Preventing mud balls and slime in filters.
  • Oxidizing iron and manganese for removal.
  • Reducing taste, odor, and color problems and minimizing post-chlorination dosage.

Post-Chlorination

• Applying chlorine to treated water for residual and secondary disinfection.
• Provides protection against contamination in the distribution or customer plumbing system.
• Water regulations specify guidelines for chlorine residual before water leaves the plant.
• A free chlorine residual of 0.20 mg/L is recommended.

Re-Chlorination

• Applying chlorine in stages to boost residual in large distribution systems.
• Used when it is not possible to maintain the minimum residual of 0.2 mg/L.

Factors Affecting Chlorine Dosage

• High turbidity inhibits disinfection by providing hiding places for bacteria.
• Factors include:
  • Chlorine Concentration: Higher concentration is more effective and faster.
  • Contact Time: Longer contact time is more effective. Decreased concentration requires increased contact time.
  • Type of Chlorine Residual: Free chlorine is more effective than combined chlorine.
  • Temperature: Higher temperature is generally more effective.
  • pH: Disinfection is more effective at low pH since HOCl is more effective than OCl ions.
  • Substances in Water: Turbidity provides shelter to organisms.

Example Problem 1

• A water pump delivers 760 lit/min.
  • Desired chlorine dosage: 2.0 mg/L
  • Pump operates 18 hours per day.
• Calculations:
  • A) Chlorine dosage per day:
    $Q * Cl dosage = 760 \frac{lit}{min} * 2 \frac{mg}{lit} = 1520 \frac{mg}{min} = 1520 \frac{mg}{min} * \frac{1440}{day} = 2.19 \frac{kg}{day}$
  • B) Chlorine amount per week (24 hours/day):
    $2.19 \frac{kg}{day} * 7 day = 15.32 \frac{kg}{week}$
  • Chlorine demand per week (18 hours/day):
    $\frac{18}{24}*15.32 \frac{kg}{week} = 11.5 \frac{kg}{week}$

Example Problem 2

• Chlorinator feeds filtered water at 12 kg/d.
• Chlorine residual: 0.55 mg/L
• Average flow: 5.0 ML/d
• Calculations:
  • Demand = ?
  • Increased chlorine feed: 13 kg/d
  • Expected increase in free chlorine residual = ?
• Solution
  $Dosage, C = \frac{M}{Q} = \frac{12 \frac{kg}{d}}{5.0 \frac{ML}{d}*\frac{kg}{ML}} = 2.40 \frac{mg}{L}$
• Demand = dosage - residual = (2.4-0.55) = 1.85mg/L
• $Cincrease = \frac{(13-12) \frac{kg}{d}}{5.0 \frac{ML}{d}*\frac{kg}{ML}} = 0.20\frac{mg}{L}$
• Since demand is not changed, the increase in residual will also be 0.2 mg/L.