Envi Chapter 4

University of San Carlos - Department of Chemical Engineering

University of San Carlos - Department of Chemical Engineering

2000 B.C. in India: water was to be heated, boiled, or filtered to remove impurities 1450 B.C. in Egypt: drawings depicts people siphoning liquid from a canister “… whosoever wishes to investigate medicine properly should – consider the water that the inhabitants use – for water contributes much to health”

Hippocrates (460 to 354 B.C.)

University of San Carlos - Department of Chemical Engineering

Raw Water

Aeration

CaCO3

Lime Soda Ash Chlorine

Chlorine

Gases to atmosphere

Softening

Filtration

Disinfection

Storage

Typical plant treating hard groundwater.

Mg(OH)2

Raw Water University of San Carlos - Department of Chemical Engineering

Chlorine Ammonia Alum Polymers Chlorine

Pre-sedimentation

Mixing, flocculation, settling

Filtration

Adsorption

Chlorine

Typical plant treating turbid surface water with organics

Disinfection

Storage

Gases to atmosphere

University of San Carlos - Department of Chemical Engineering

Characteristics Major sources of Public Drinking Water Surface Water - include streams and rivers, natural lakes, and constructed lakes - exposed to plant and animal life and to human influences from land - contains a wide variety of microorganisms and natural organics

Groundwater - free of significant levels of organics - low levels of microbial contamination - contain significant levels of dissolved inorganics (e.g. carbonates, iron and manganese)

University of San Carlos - Department of Chemical Engineering

Treatment Processes Objective: to produce a safe, aesthetically pleasing water

Gas Transfer (Aeration) used to remove dissolved gases in water or to add oxygen to water to convert undesirable substances to a more manageable form

CO2 – results in a corrosive water – may interfere with other treatment process

H2S – imparts an unpleasant taste and odor to water

University of San Carlos - Department of Chemical Engineering

Iron and Manganese – in the absence of oxidizing agents both are stable in water

Oxidation Reaction

4 Fe 2+ + O2 + 10 H 2O → 4 Fe(OH ) 3 + 8 H + 2 Mn 2+ + O2 + 2 H 2O → 2 MnO2 + 4 H +

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Liquid – Gas Contact Systems -designed to drive the water – gas mixture toward equilibrium -provide supersaturation or oxygen for oxidation purposes Accomplished by: Dispersing the water into the air Dispersing the air into the water Liquid film

Liquid film Bulk Liquid Cs < Ct Bulk Liquid

Gas film

Bulk Liquid Cs > Ct Bulk Liquid

Gas film

(a) (b) Figure 1. Water Dispersed in air: (a) desorption and (b) absorption.

University of San Carlos - Department of Chemical Engineering

Gas film

Gas film Bulk Liquid Cs < Ct Bulk Gas

Bulk Liquid Cs > Ct Bulk Gas

Liquid film

Liquid film

(a) (b) Figure 2. Air Dispersed in water: (a) desorption and (b) absorption.

University of San Carlos - Department of Chemical Engineering

Devices for Liquid – Gas Contact Fountains Cascade Towers Tray Towers Diffused Aerators

University of San Carlos - Department of Chemical Engineering

Solids Separation Clarification Sedimentation Discrete Particles – whose size, shape and specific gravity do not change with time Flocculating Particles – whose surface properties are such they aggregate with other particles Dilute Suspensions – the concentration of particles is not sufficient to cause significant displacement of water as they settle Concentrated Suspensions – there is velocity field interference

University of San Carlos - Department of Chemical Engineering

Coagulation Table 1. Settling velocities of various size particles* Particle diameter (mm)

Size typical of

Settling Velocity

10

Pebble

0.73 m/s

1

Coarse Sand

0.23 m/s

0.1

Fine Sand

1.0 x 10-2 m/s (0.6m/min)

0.01

Silt

1.0 x 10-4 m/s (8.6m/day)

0.0001

Large Colloid

1.0 x 10-8 m/s (0.3m/yr)

0.000001

Small Colloid

1.0 x 10-13 m/s (3 m/million yr)

*

Spheres with specific gravity of 2.6 in water at 20oC

Stable – colloidal suspensions that do not agglomerate naturally Large surface – to – volume ratio – most important factor contributing to the stability of colloidal suspension Coagulants – induces agglomeration

University of San Carlos - Department of Chemical Engineering

Major Forces Acting on Colloids Electrostatic potential Van der Waals Force

Figure 3. Reduction of collloidal electrostatic repulsion by addition of trivalent aluminum ions.

University of San Carlos - Department of Chemical Engineering

Flocculation gentle mixing to speed the agglomeration process

University of San Carlos - Department of Chemical Engineering

Softening Chemical Precipitation - calcium hardness to calcium carbonate - magnesium hardness to magnesium hydroxide

Lime-soda Process

Caustic soda Process

University of San Carlos - Department of Chemical Engineering

Lime-soda Process

Ca 2 + + 2( HCO3 ) − + CaO + H 2O → 2CaCO3 + 2 H 2O Mg 2 + + 2( HCO3 ) − + CaO + H 2O → 2CaCO3 + Mg 2 + + CO32 − Mg 2 + + CO32 − + CaO + H 2O → CaCO3 + Mg (OH ) 2 SO42 −  SO42 −          Mg 2 + + 2Cl −  + CaO + H 2O → Ca 2 + + 2Cl −  + Mg (OH ) 2   − − 2 NO3  2 NO3 

 SO42 −   SO42 −      2+  −  +  −  Ca +  2Cl  + Na2CO3 → CaCO3 + 2 Na +  2Cl    − −  2 NO3   2 NO3 

University of San Carlos - Department of Chemical Engineering

Caustic soda Process

CO2 + 2 NaOH → 2 Na + + CO32 − + H 2O Ca 2 + + 2( HCO3 ) − + 2 NaOH → CaCO3 + 2 Na + + CO32 − + 2 H 2O Mg 2 + + 2( HCO3 ) − + 4 NaOH → Mg (OH ) 2 + 4 Na + + 2CO32 − + 2 H 2O Mg 2 + + SO42 − + 2 NaOH → Mg (OH ) 2 + 2 Na + + SO42 −

University of San Carlos - Department of Chemical Engineering

Stabilization Addition of Acid

2CaCO3 + H 2 SO4 → 2Ca 2 + + 2( HCO3 ) − + SO42 − Mg (OH ) 2 + H 2 SO4 → Mg 2 + + SO42 − + 2 H 2O Recarbonation

CaCO3 + CO2 + H 2O → Ca 2 + + 2( HCO3 ) − Mg (OH ) 2 + 2CO2 → Mg 2 + + 2( HCO3 ) −

University of San Carlos - Department of Chemical Engineering

Disinfection Disinfection – operations aimed at killing or rendering harmless, pathogenic microorganisms Sterilization – the complete destruction of all living matter

Chlorination

Cl2 + H 2O → H + + HOCl Ca (OCl ) 2 → Ca 2 + + 2OCl − NaOCl → Na + OCl −

University of San Carlos - Department of Chemical Engineering

Ozone high voltage

O2    → O + O O + O2 ⇔ O3 Chlorine Dioxide Effective in oxidizing phenolic compounds Generated on-site in aqueous form by the chlorination of sodium chlorite at low pH

Irradiation with ultraviolet light

University of San Carlos - Department of Chemical Engineering

Dissolved-Solids Removal Inorganic Materials - demineralization and desalinization Ion – exchange Microporous Membranes reverse osmosis electrodialysis

Organic Materials Adsorption Chemical Oxidation

University of San Carlos - Department of Chemical Engineering

Activity for the Day……. 4. What are the characteristics of a good disinfectant? 5. Why is aeration used in water-treatment plants? Is it more commonly used in groundwater or surface water? Why?