Unit 2-DW-PF.pdf

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Unit 2-

Year 3 Semester 2

2015

Shiromi Karunaratne

[email protected]

Mobile- 0776368620

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CE3610 Environmental

Engineering:Week 2

mailto:[email protected]:[email protected]:[email protected]


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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, re

• •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 and Microbiological

• Water Quality and Health

• Raw water sources, water treatment and distribution

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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 isdetermined by the number of free hydrogen ions (H+) in asubstance.

• Acidity is one of the most important properties of water. Water is asolvent for nearly all ions. The pH serves as an indicator thatcompares some of the most water-soluble ions.

• The outcome of a pH-measurement is determined by a consideratiobetween the number of H+ ions and the number 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 osubstance is above 7, it is a basic substance. When the pH osubstance is below 7, it is an acid substance. The further theabove or below 7, the more basic or acid a solution is.

• The pH is a logarithmic factor; when a solution becomes tenmore acidic, the pH will fall by one unit. When a solution behundred times more acidic the pH will fall by two units.

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• Mastering the some chemistry will be a great understanding, and solving, environment

engineering challenges

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Water Chemistry•Physical properties of water:

The basic properties of water relevant to water treatment areand viscosity.

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Water Quality: from an environmental enginpoint of view

• Impurities in water:• •Dissolved substances: A substance which is truly in so

(homogenous; solvated by the liquid)

• •Suspended solids are large enough to settle out of solbe removed by filtration

• •Colloidal particles are in the size range between dissosubstances and suspended particles

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Dissolved substances: definition• Dissolved substances are substances that cannot be removed

the liquid without a phase change, such as:

• –distillation

• –adsorption

• –precipitation

• –gas stripping

• –liquid extraction

• –reverse osmosis (membrane filtration with pore sizeionic size range, ~10Å)

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•

Find the definitions by your self


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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 bond

• –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 membraneprocesses.

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

• Both suspended and colloidal particles have a surface

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Surface charge is the electrical potential diffe

between the inner and outer surface of the dphase in a colloid.


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Particulates in water

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Effect of water treatment

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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 Solutes change 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 not change, soconcentrations are expressed in weight/volume (milligrams per litremg/L)

• 1 % = 10,000 mg/L

• Consider example 5.1 P222 Davis and Cornwell

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Molarity, normality, equivalent weight

•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 quality and treat

• •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 a compoundwhen in fact a compound does not exist, simply the ions (e.g. CaSO4and NaCl exist in solution as 4 ions)

• Ca2+ + SO42- ↔ CaSO4(s)

• Na+ + Cl- ↔ NaCl (s)• The solubility of a compound is given by its solubility constant (Ks)

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Acids and bases• When acids enter the water, the ions will separate. For

instance, hydrogen chloride will separate into hydrogen

and chlorine ions (HCL into H+ + CL-).

• Bases also undergo separation of their ions when enter thewater. When sodium hydroxide enters the water it willseparate into sodium and hydroxide ions (NaOH into Na+ +OH-).

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• When an acid substance ends up in water, it will give up a hydrogen io

water. The water will then become acid.

• The number of hydrogen ions that the water will receive determines tWhen a basic substance enters the water it will take up hydrogen ions

• This will lower the pH of the water.When a substance is strongly acidic it will give up more H+ ions to the

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 or compoun

electrons from another atom or compound. A classic example of a reis rusting. When rusting happens, oxygen steals electrons from iron.reduced while iron gets oxidized.

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Buffers and alkalinity

••Buffering capacity of a natural water refers to its ability totake up acid or base without a pH change.

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

• •Carbonate buffer system important in many aspects ofwater and wastewater 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 Quality8/17/2015


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Turbidity

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

divided organic material, plankton, and other particulatematerial in 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, ordissolved 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 objectionabletaste and 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. Aesthetic limit,180 mg/L

• •Calcium, Magnesium

• –Hardness

• •Potassium

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Chemical Characteristics

• •pH

• –6.5-8.5, new cement pipes can increase pH• –buffering capacity of water

• –alkalinity/acidity

• •Conductivity• –an indirect measure of total dissolved salts (TDS)

• –inverse of ???

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• Trace ions

• •Fluoride

• –Excessive fluoride may produce fluorosis (mottling) of teeth, acceptablelimit 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 preventtooth decay (controversial)

• •Bromide

• –Reacts with disinfectants to form disinfection by-products

• –Interferes with some oxidation reactions in water treatment

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Chemical CharacteristicsMetals

• •Iron: Causes staining of laundry (iron staining), affects the taste of beveragessuch as tea and coffee; easily removed by conventional water treatment

• •Manganese: Brownish color to water and stains laundry when used for washingoff 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, possiblecarcinogen

• –Occurs in old water pipes (solder)

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

• •Zinc, copper: Detrimental to health, undesirable taste

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

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

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• Toxic inorganic substances: Nitrates, cyanides

•

• •Toxic organic substances: Pesticides, insecticides, spharmaceuticals, endocrine disruptors, industrial chempetroleum hydrocarbons (usually present in trace quanwhy?)

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•


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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 water are not

easily identified.• –surrogates (indicator organisms) are used to detect contamination

in routine tests total coliforms, thermotolerant coliforms or E.coli test .

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Protection of drinking water supplies from contaminat


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Protection of drinking water supplies from contaminat

• 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 tan

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

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Drinking water problems caused by organic carbon


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Drinking water problems caused by organic carbon

••Presence of natural organic matter

• –Disinfection by-products

• –Disinfectant decay

• –Bacterial regrowth

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

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Water quality problems caused by organic carbon

• •Accelerates chlorine, chloramine decay

• –makes the disinfectant less effective

• •Promotes bacterial regrowth

• •Reacts with disinfectants to form carcinogenic disinfection by-products

• •Binds with ferric ions in water to cause discoloured water

• •Fouls membranes

• •Can indirectly lead to production of objectionable odours andtastes

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Problems caused by dissolved organic carbon (D


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Problems caused by dissolved organic carbon (D

• •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 osubsequent contamination that happens in the system

• •This is compromised when other agents that can degrade thedisinfectant are present in the system

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

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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 carcinogenic disinfectio

by-products, e.g. trihalomethanes (THMs) and haloacetic acids

• •DOC provides food for micro-organisms to grow in distributionpipes (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 youdo the experiment

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Recent research has focused on DOC removal and


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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 exorbitan

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Some properties related to NOM can be me


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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

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l h


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Bacterial Regrowth

• •Bacterial Regrowth: Growth of bacteria in the treated water

• •Bacteria grows if

• –disinfectant concentration is too low or bacteria is resistant

• –Sufficient food available (organic carbon for heterotrophic bacteria)and other nutrients (mainly nitrogen or phosphorus).

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

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W T P


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Water Treatment Processes

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C ll id d i


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Colloid dynamics

• •Colloids are too small to be trapped in the filter and too smsettle in a reasonable period of time.

• •Surface is negatively charged

• –Colloidal particles repel each other (repulsive interparticle f

••When the surface charge is larger, the stability is stronger

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Colloid stability


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• •Addition of positively charged cations such as Na+1, Ca+2, Mg+2, Al+to destabilise the surface charge of colloids and allow aggregation (flointo particles

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Colloid destabilization

Coagulation or Flocculation Value of the


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gElectrolyte

• The coagulation of a colloidal solution by an electrolyte doe

place until the added electrolyte has certain minimum concin the solution.

• The minimum amount of an electrolyte (millimoles) that muadded to one litre of a colloidal solution so as to bring aboucomplete coagulation or flocculation is called the Coagulatio

Flocculation Value of the Electrolyte.• Thus smaller is the flocculation value of electrolyte, greater

coagulation or precipitating power.

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• Different electrolytes have different coagulation values.

• The coagulation behaviour of various electrolytes was studieby Hardy and Schultz.

• – Schultz Law ,“Greater the valency of oppositely charged ions of the elecbeing added, the faster is coagulation” .

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Thus in case of positively charged sol thecoagulating power of anions is in the order of

and in case of negatively charged sols, thecoagulating power of cations is in the orderof Al3+ > Ba2+ > Na+

http://1.bp.blogspot.com/-YpsP_tgnDKs/UhJwYb_Yb1I/AAAAAAAAAUE/Y6W9ICfYgP0/s1600/order.PNGhttp://1.bp.blogspot.com/-YpsP_tgnDKs/UhJwYb_Yb1I/AAAAAAAAAUE/Y6W9ICfYgP0/s1600/order.PNG


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Coagulants

• •A coagulant is a substance (chemical) that is added to thewater to 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 ferriion (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.

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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 particlesthat have a negative surface charge. The activated silica can

unite with the positively charged aluminium or with iron flocs,resulting in a larger, 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

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Coagulation

• •(iv) Polymers:

••- Polymers can have a negative charge (anionic), positivecharge (cationic), positive and negative charge(polyamphotype), or no charge (nonionic)

• •- Polymers are long-chain carbon compounds of highmolecular weight that have many active sites. The active sitesadhere to flocs, joining them together and producing a larger,tougher floc that settles better. This process is calledinterparticle bridging.

• from Bolto and Gregory, 2007,

• Water Research, 2301-2324

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Mixing and Flocculation


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Mixing and Flocculation

• •Rapid mixing

• –To disperse the chemicals throughout the entire water body

• –Achieved by injection of chemicals into the most turbulent zon

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

• –In natural water we need to mix with particles before this sothat the intermediary product (Al7(OH)174+) that forms within0.01-1 second combines with particles to neutralize the particle

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Determination of optimum operating


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conditions

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Flocculation

• •Bring particles into contact, to collide, stick together and

grow to a size that will readily settle• •Accomplished by slow, gentle mixing

• •Enough mixing must be provided to keep the floc insuspension

• •Too much mixing will shear the floc particles and

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

flocsShear forc

Potential health implications of trace contaminants inl l l


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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

of polymers due to the contaminants• •Some (West Germany, France, and Canada) have stringen

limits

• •In US in some states require notification to healthauthorities when using these polyelectrolytes

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Practice questions


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Practice questions

• 1. What is DOC?

• 2. What are the problems associated with DOC?

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Lecture : Practice Questions

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

• 5. Why are the following compounds controlled in the distribution systemand what are their possible origins?

• •Iron and Manganese• •Aluminium

• •Copper

• •Hardness

• 6. What is the reason for the multi-barrier approach to pollutant removal

• 7. Which components of water have the greatest effect on alkalinity?

• 8. Why is discoloured water an issue in utilities? What are possible cause• 9. Which of the following compounds would you expect to be removed by

the conventional water treatment method of coagulation/flocculation.Why? Na+; Cl+; finely divided clay particles; a colloidal suspension of dairywaste; Fe(OH)3 formed from oxidation of groundwater containing reduceiron; bromide?

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Practice questions (Lecture-2)


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Practice questions (Lecture 2)

• 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 a coagulant?

• 13. From your experimental experience, how much timeshould we allow in flocculation?

• 14. How long did you wait in the lab for sedimentation?

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Unit 2-DW-PF.pdf

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