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Unit 2-D

YEAR 3 SEMESTER 2

2015

SHIROMI KARUNARATNE

EMAIL-SHIROMI.K@SLIIT.LK

MOBILE- 0776368620

CE3610 Environmental

Engineering:Week 2

mailto:Email-shiromi.k@sliit.lkmailto:Email-shiromi.k@sliit.lkmailto:Email-shiromi.k@sliit.lk

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the sequel to last week …………..

Water sources

 – in Sri Lanka

 – Water use and fit-for-purpose treatment

•Wastewater

 – Wastewater collection system; centralised vs decentralised systems, recycling

•Guidelines for water and wastewater quality

•Resource management of air, water, soil (multimedia)

 – Air quality management

 – Solid waste management

•History of water supply and sanitation

 – Waterborne diseases, history and current issues

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Material to be covered

Water Quality: Physical, Chemical andMicrobiological

Water Quality and Health

Raw water sources, water treatment anddistribution

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Water Quality:

Physical water quality parameters

Chemical water quality parameters and

Microbiological water quality parameters

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Review of water chemistry Chemistry is the basis of many problems addressed in environmental

engineering, e.g.

•design of water and WW treatment processes

•remediation of environmental pollution

 –alkalinity, pH: acid/base reactions

 –water treatment chemicals to remove impurities

•precipitation

•oxidation •kinetics – how long does the reaction take to occur?

•Chapter 5, Davis and Cornwell 5th Ed

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The common term for pH is alkalinity

The pH is an indication for the acidity of a substance. It idetermined by the number of free hydrogen ions (H+) in

a substance. Acidity is one of the most important properties of water.

Water is a solvent for nearly all ions. The pH serves as anindicator that compares some of the most water-solubleions.

The outcome of a pH-measurement is determined by aconsideration between the number of H+ ions and thenumber of hydroxide (OH-) ions. When the number of H+ions equals the number of OH- ions, the water is neutral.It will than have a pH of about 7.

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Continued….

The pH of water can vary between 0 and 14. When the pH of asubstance is above 7, it is a basic substance. When the pH of asubstance is below 7, it is an acid substance. The further the pH liesabove or below 7, the more basic or acid a solution is.

The pH is a logarithmic factor; when a solution becomes ten times

more acidic, the pH will fall by one unit. When a solution becomes hundred times more acidic the pH will fall by two units.

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Mastering the some chemistry will bea great help in understanding, andsolving, environmental engineering

challenges

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Water Chemistry

•Physical properties of water:

The basic properties of water relevant to water treatment are densityand viscosity.

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Water Quality: from an environmental

engineering point of view

Impurities in water:

•Dissolved substances: A substance which is truly insolution (homogenous; solvated by the liquid)

•Suspended solids are large enough to settle out of

solution or be removed by filtration •Colloidal particles are in the size range between

dissolved substances and suspended particles

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Dissolved substances: definition

Dissolved substances are substances that cannot be remove

the liquid without a phase change, such as:

 –distillation

 –adsorption

 –precipitation

 –gas stripping  –liquid extraction

 –reverse osmosis (membrane filtration with pore size in ionic size range,

•

Find the definitions by your s

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Dissolved substances

Can be simple atoms (e.g. charged ions)

 –Sodium ion: Na+ (cation)

 –Chloride ion: Cl- (anion)

or molecules dissolved in water through hydrogen bonding

 –Dissolved gases, e.g. dissolved oxygen O2

 –Complex molecules, e.g. sugar

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Suspended solids and colloids Suspended solids

•Large enough to be removed by physical methods

 – Centrifugation

 – Sedimentation

 – filtration

Colloidal substances

•Removal by high ultracentrifuge or tight membrane processes.

•Exhibit Tyndall effect (light scattering by particles), , reflected lightcausing turbidity

Both suspended and colloidal particles have a surface charge, NOan ionic charge

Surface charge is the electrical potential diffe

between the inner and outer surface of the d

phase in a colloid.

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Particulates in water 

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Effect of water treatment

Water treatment effectively changes size distribution.

Note much lower overall number of particles in treated

water

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Units for water quality measurements Soluteschange the density of a solution

 – Weight percent is % in g/kg solution (w/w) – Weight volume is % in g/L of solution (w/v)

For dilute solutions, assume that the density does notchange, so concentrations are expressed in

weight/volume (milligrams per litre, mg/L) 1 % = 10,000 mg/L

Consider example 5.1 P222 Davis and Cornwell

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Molarity, normality, equivalentweight

Molarity is the number of moles per litre of a solution

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E.g. 5.2 Davis and Cornwell

Commercial sulfuric acid comes as a 93% weight % solution.

Find the concentration of the solution in mg/L, molarity andnormality (normality relevant to an acid/base reaction)

H2SO4 has a specific gravity of 1.839 g/cm3

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Chemical reactions relevant to water qualityand treatment

•Precipitation

•e.g. water hardness

•Acid/base

•e.g. pH correction

•Ion association

 – e.g. environmental processes - heavy metals

•Oxidation-reduction

 – e.g. water chlorination

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Precipitation reactions Precipitation - dissolved ions reacting to form a solid

compound

 – e.g. Ca2+ + CO32- ↔ CaCO3(s)

 – note that ions in solution are often referred to as acompound when in fact a compound does not exist,simply the ions (e.g. CaSO4 and NaCl exist in solution as4 ions)

Ca2+ + SO42- ↔ CaSO4(s)

Na+ + Cl- ↔ NaCl (s)

The solubility of a compound is given by its solubilityconstant (Ks)

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Acids and bases

When acids enter the water, the ions willseparate. For instance, hydrogen chloride willseparate into hydrogen and chlorine ions (HCLinto H+ + CL-).

Bases also undergo separation of their ions whenenter the water. When sodium hydroxide enters

the water it will separate into sodium andhydroxide ions (NaOH into Na+ + OH-).

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When an acid substance ends up in water, it will give upa hydrogen ion to the water. The water will thenbecome acid.

The number of hydrogen ions that the water will receivedetermines the pH. When a basic substance enters thewater it will take up hydrogen ions.

This will lower the pH of the water.When a substance is strongly acidic it will give up moreH+ ions to the water. Strong bases will give up more OH-

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Acid/base reactions

• Acid/base – acids dissociate to form H+ (hydrogen ion) (and the

conjugate base)

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Rusting

In an oxidation-reduction or redox reaction, one atom ocompound will steal electrons from another atom ocompound. A classic example of a redox reactionrusting. When rusting happens, oxygen steals electronfrom iron. Oxygen gets reduced while iron gets oxidized

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Buffers and alkalinity

•Buffering capacity of a natural water refers to its ability to take upacid or base without a pH change.

•Alkaline water (pH>7) is different to water having a high alkalinity!

•Carbonate buffer system important in many aspects of water andwastewater treatment and environmental chemistry

•Alkalinity is experimentally determined as the sum of titratable

bases to pH 4.5.

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Water Quality Parameters

Sri Lanka Standard Specification for potable

water SLS 614 : Part I : 1983 and SLS 614 : Part II :1983

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ADWG: Water Quality

Physical

Associated with appearance of water such as turbidity, colour,temperature, particulate matter, taste and odor

Chemical

Differences are not visible but apparent in more subtle ways

e.g. hardness prevents lathering of soaps and shampoos

Microbiological Major cause of water-borne diseases

so highest priority is to ensure microbiological quality

Radiological

ADWG: Water Quality

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Turbidity

•The presence of suspended material such as clay, silt, finely

divided organic material, plankton, and other particulate material water

 – measured in Nephelometric Turbidity Units (NTU)

 – Tyndall effect due to light scattering by particles

 – useful for control of risks of micro-organisms

 – 

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Colour 

•True colour (filtered); apparent colour includes particulates

•ADWG

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Taste and Odour (organoleptic properties)

•Can be caused by organic compounds, inorganic salts, or

dissolved gases

•Chlorine which is used to disinfect water often imparts a tasteand/or odour 

•Chlorine can also react with other substances in the water toindirectly cause taste or odour (e.g. bromide, phenol)

•Contaminants, spills (e.g. hydrocarbons)

•Cyanobacteria

•ADWG: Drinking water should be free from any objectionable tastand odour at point of use.

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Temperature

•Temperature affects taste –10-15oC is most palatable.

 –Fridge taste???

•Above ground water pipes

•Effect on microbiological growth

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Chemical Characteristics Major ions (major components of TDS)

•Chloride

 – Naturally occurring salt, seawater

 – No health guideline, taste issue at >250mg/L

 – High Cl- promotes corrosion

 – Values up to 350mg/L in water supplies, guideline is 250mg/L

•Sulfate

 – Taste effects at >250mg/L; Laxative effects >500 mg/L

•Carbonate, bicarbonate

 – Alkalinity

•Sodium

 – Can affect individuals suffering from heart, kidney problems at >20 mg/L. Aestheticlimit, 180 mg/L

•Calcium, Magnesium

 – Hardness

•Potassium

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Chemical Characteristics

Trace ions

•Fluoride

 – Excessive fluoride may produce fluorosis (mottling) of teeth,acceptable limit 0.8 to 1.3 mg/L; ADWG guideline is 1.5 mg/L

 – Drinking water in much of the developed world is fluoridated

to prevent tooth decay (controversial) •Bromide

 – Reacts with disinfectants to form disinfection by-products

 – Interferes with some oxidation reactions in water treatment

i i i

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Chemical Characteristics

Metals

•Iron: Causes staining of laundry (iron staining), affects the taste of beverages

such as tea and coffee; easily removed by conventional water treatment •Manganese: Brownish color to water and stains laundry when used for washin

off taste at 0.1 mg/L, Mn is concentrated in biofilms in distribution pipes, can beremoved by oxidation and conventional water treatment

•Lead: Seriously damages health, nervous system, especially in children, possibcarcinogen

 – Occurs in old water pipes (solder)

 – ADWG value = 0.01 mg/L Prolonged exposure to relatively small quantities maresult in serious illness or death.

•Zinc, copper: Detrimental to health, undesirable taste

•Arsenic: Lung and urinary bladder cancer; skin pigment changes and scalinghyperkeratosis, reduced blood flow to skin, nerve damage

•Other heavy metals: Cd, Cr, Ni, Hg, Ag

Ch i l Ch i i

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Chemical Characteristics

Toxic inorganic substances: Nitrates, cyanides

•Toxic organic substances: Pesticides, insecticides, solvents,

pharmaceuticals, endocrine disruptors, industrial chemicals, petroleum

hydrocarbons (usually present in trace quantities – why?)

•

Mi bi l i l h t i ti

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Microbiological characteristics

•Water for drinking and cooking must be made free

from disease-causing organisms (pathogens).

 – viruses, bacteria, protozoa (amoeba, cryptosporidium,giardia) and helminths (worms), cyanobacteria

•Origin is either infected human or animal discharge

•The specific disease-causing organisms present in wate

are not easily identified.

 – surrogates (indicator organisms) are used to detectcontamination in routine tests total coliforms,thermotolerant coliforms or E.coli test.

Protection of drinking ater s pplies from

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Protection of drinking water supplies fromcontamination

Multiple barrier approach

•Catchment protection

•Water treatment

 – coagulation (conventional treatment)

 – filtration (conventional media)

 – advanced treatment (e.g. ozone for protozoa)

•Disinfection

 – primary disinfection

 – post-treatment disinfection

•Distribution systems

 – Closed systems

 – Maintenance of disinfectant residual, redosing of reservoirs and tanks

•Multiple barrier approach is guided by circumstances of the watersupply and distribution system, cf Melbourne, Adelaide, Perth

Drinking water problems caused by organic

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Drinking water problems caused by organiccarbon

•Presence of natural organic matter

 – Disinfection by-products

 – Disinfectant decay

 – Bacterial regrowth

•The use of polyelectrolytes as water treatment chemicals(coagulant aids)

Water quality problems caused by organic

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Water quality problems caused by organiccarbon

•Accelerates chlorine, chloramine decay

 – makes the disinfectant less effective

•Promotes bacterial regrowth

•Reacts with disinfectants to form carcinogenicdisinfection by-products

•Binds with ferric ions in water to cause discolouredwater

•Fouls membranes

•Can indirectly lead to production of objectionableodours and tastes

Problems caused by dissolved organic

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Problems caused by dissolved organiccarbon (DOC)

•For effective disinfection and pathogen control, chemicaldisinfectants are used

•Chlorine is the most commonly used

•Chlorine kills certain bacteria but also stays as a residual tomaintain disinfecting power later.

•This characteristic is important to protect any possible accidental

or subsequent contamination that happens in the system •This is compromised when other agents that can degrade the

disinfectant are present in the system

•One of the components that decays chlorine is natural organicmatter (NOM). This is measured as dissolved organic carbon (DOC)

DOC (continued)

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DOC (continued) •Dissolved organic carbon (NOM)

 – accelerates the decay of chlorine

 – provides precursors that react with chlorine to form carcinogenicdisinfection by-products, e.g. trihalomethanes (THMs) and haloacetic acid

•DOC provides food for micro-organisms to grow in distribution pipes(biofilms)

 – these biofilms harbour micro-organisms and also cause decay of chlorine(especially in small diameter pipes)

 – sloughing of biofilms causes discoloured water

 – off odours

 – nitrification

•Therefore DOC (NOM) needs to be removed

 – water treatment processes

*Decay of chlorine will be demonstrated in the laboratory when you dthe experiment

Recent research has focused on DOC

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Recent research has focused on DOCremoval and characterisation

•Organic carbon can be removed by:

 – Enhanced coagulation

 – Granular activated carbon

•Biologically Activated Carbon (BAC)

•Granular activated carbon (non-biological mode)

•Powdered Activated Carbon (PAC)

 – Ion Exchange process such as MIEX®

 – Advanced oxidation processes

 – Complete removal of organic carbon – cost would be exorbitant

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Some properties related to NOM

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Some properties related to NOMcan be measured as:

•Bacterial regrowth:  – Biodegradable Dissolved Organic Carbon (BDOC)

 – Assailable Organic Carbon

 – Bacterial Regrowth Potential

•Disinfection by-products formation:

 – THM formation potential

 – NDMA formation potential

•Membrane Fouling

 – Fouling index

•Disinfection decay characteristics

 – Fast and slow reacting agents

Bacterial Regrowth

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Bacterial Regrowth

•Bacterial Regrowth: Growth of bacteria in the treatedwater

•Bacteria grows if

 – disinfectant concentration is too low or bacteria isresistant

 – Sufficient food available (organic carbon forheterotrophic bacteria) and other nutrients (mainlynitrogen or phosphorus).

 – Other environmental conditions are suitable (e.g.temperature)

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Water Treatment Processes

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Water Treatment Processes

Conventional water treatment

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Conventional water treatment

system

•Coagulation: adding and rapidly mixing of chemical coagulants intothe raw water.

•Flocculation: slow mixing of the chemicals with the water to assist inbuilding up particles of floc.

•Sedimentation: allow the floc to settle out (gravity)

•Filtration: remove almost all of the suspended matter that remains bypassing the water through filters.

•Disinfection: destruction or inactivation of waterborne pathogens.

Design of Coagulation/Flocculation

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Design of Coagulation/Flocculation

•Major reason – to remove microorganisms and turbidityenhanced coagulation to remove DOC

•In the fresh water bodies small particles (includingorganic compounds, colloids microorganisms etc) exist

 – usually the particle surfaces are negatively charged •To bring them together and make them settle (to

remove by gravity), the charges must be neutralised. This achieved by coagulants.

Colloid stability

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Colloid stability

•Colloids are too small to be trapped in the filteand too small to settle in a reasonable period oftime.

•Surface is negatively charged

 – Colloidal particles repel each other (repulsiveinterparticle forces)

•When the surface charge is larger, the stabilityis stronger

Colloid destabilization

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Colloid destabilization

•Addition of positively charged cations such as Na+1,Ca+2, Mg+2, Al+3, or Fe+3 to destabilise the surfacecharge of colloids and allow aggregation (flocculation)into particles

•Schulze and Hardy rule: one mole of trivalent ion can

reduce the charge of as much as 30 to 50 moles of adivalent ion and as much as 1,500 to 2,500 moles of amonovalent ion

Coagulation

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Coagulation

•A coagulant is a substance (chemical) that is added to the waterto accomplish coagulation

•There are three key properties of a coagulant

 – Trivalent cation

 – Non-toxic

 – Insoluble in the neutral pH range

•Two common coagulants used are aluminium (Al3+) and ferric ion(Fe3+)

•Coagulant aids- These assist coagulation by creating bettercoagulation conditions,

 – e.g optimum pH,

 – formation of particulate nuclei,

 – bridging between small flocs to create bigger flocs.

Coagulation

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Coagulation

•Four basic types of Coagulant aids:

•(i) pH adjusters: most commonly used

• -for lowering pH – sulfuric acid [H2SO4]

• - for raising pH – lime [Ca(OH)2] or soda ash (Na2CO3) orsodium hydroxide (NaOH; caustic soda)

•(ii) Activated silica: produces a stable suspension of particles thathave a negative surface charge. The activated silica can unite wit

the positively charged aluminium or with iron flocs, resulting in alarger, denser floc that settles faster

•(iii) Clays: Clays can act much like activated silica in that theyhave a slight negative charge and can add weight to the flocs.Cheaper than AS

Coagulation

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Coagulation

•(iv) Polymers:

•- Polymers can have a negative charge (anionic), positive charge(cationic), positive and negative charge (polyamphotype), or nocharge (nonionic)

•- Polymers are long-chain carbon compounds of high molecularweight that have many active sites. The active sites adhere to flocs, joining them together and producing a larger, tougher floc thatsettles better. This process is called interparticle bridging.

from Bolto and Gregory, 2007,

Water Research, 2301-2324

Mixing and Flocculation

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Mixing and Flocculation

•Rapid mixing

 – To disperse the chemicals throughout the entirewater body

 – Achieved by injection of chemicals into the mostturbulent zone

 – In a pure water Alum + water Al(OH)3 in 1-7 second

 – In natural water we need to mix with particles beforethis so that the intermediary product (Al7(OH)174+)that forms within 0.01-1 second combines withparticles to neutralize the particles.

Determination of optimum

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Determination of optimumoperating conditions

Flocculation

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occu a o

•Bring particles into contact, to collide, stick

together and grow to a size that will readilysettle

•Accomplished by slow, gentle mixing

•Enough mixing must be provided to keep thefloc in suspension

•Too much mixing will shear the floc particlesand disperses it

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1. Coagul

Rapid mix

Destabilis

colloids

Fast mixincharge on

hydrolysis

2. Adsorp

particles d

Waals for

3. FlocculSlow mixi

Polyelect

bridges be

particles t

flocs

Shear forc

Potential health implications of trace

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contaminants in polyelectrolytes

•Trace contaminants come from manufacturing processes

•Residual monomers, starting materials and reaction by-products  – polyacrylimide and epichlorohydrin/dimethylamine (epi/DMA)

compounds

•Suspected carcinogens/mutagens

•Some countries (Japan, Switzerland) do not allow the use ofpolymers due to the contaminants

•Some (West Germany, France, and Canada) have stringent limits

•In US in some states require notification to health authorities whenusing these polyelectrolytes

Practice questions

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q

1. What is DOC?

2. What are the problems associated with DOC?

Lecture 2: Practice Questions

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4. Why do water utilities want to control turbidity to 0.1 NTU?

5. Why are the following compounds controlled in the distributionsystem and what are their possible origins?

•Iron and Manganese •Aluminium

•Copper

•Hardness

6. What is the reason for the multi-barrier approach to pollutantremoval?

7. Which components of water have the greatest effect on alkalinity? 8. Why is discoloured water an issue in utilities? What are possible

causes?

9. Which of the following compounds would you expect to be removeby the conventional water treatment method ofcoagulation/flocculation. Why? Na+; Cl+; finely divided clay particles; colloidal suspension of dairy waste; Fe(OH)3 formed from oxidation ofgroundwater containing reduced iron; bromide?

Practice questions (Lecture-2)

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q ( )

10. Charge neutralisation in a coagulation tank

is usually achieved very quickly. Why? 11. What are the primary concerns in drinking

water?

12. What properties are needed for acoagulant?

13. From your experimental experience, howmuch time should we allow in flocculation?

14. How long did you wait in the lab forsedimentation?