CHAPTER ONE Introduction & Waste Characterization

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Why Treating Wastewater?

Domestic and industrial processes use andpollute water => wastewaterMinimise effects of discharge on environmentRemove pollutants for recycling and/or reuse of

water

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Objectives of Wastewater Treatment

Ensure good water quality in natural environmentRemove pollutants most efficiently and

economicallyAvoid or minimise other environmental impacts

like: + solid disposal + gas emission + odour creation + noise generation

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Wastewater CharacterisationWhat is (in) Wastewater?1. Identify wastewater sources and flows2. Specify likely key pollutants3. Select suitable sampling strategies4. Measure pollutant concentrations5. Calculate pollutant loads6. Identify main components to be removed

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Sources and Flow Rates

Essential step to identify problem areaHow to define sources & flows?

1. Use “systems/mass balance” approach2. Utilize wastewater audits3. Anticipate future requirements4. Reduce > Reuse > Recycle5. Simple is better than complex

Source reduction can drastically improve wastewater situation

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CHARACTERISTICS OF INDUSTRIAL AND MUNICIPLE WASTE WATER

Physical Characteristics

Chemical Characteristics

Biological Characteristics Toxicity

Organic Matter Inorganic Matter

Measurement of Organic Compound

Types of PollutantsPhysical: solids, temperature, colour, turbidity, salinity, odourChemical:organiccarbohydrates, fats, proteins, toxins…inorganicalkalinity, N, P, S, pH, metals, salts…gaseous H2S, CH4, O2 …Biological: plants (algae, grass, etc.), microorganisms (bacteria, viruses)

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Measurement Techniques

Physical, chemical or biological methodsSummary of basic methods in APHA (US)(American

Public Health Assosiation): “Standard Methods for the Examination of Water and Wastewater”

Many instrument methods in use Flow Injection Analysis(FIA).

Good laboratory practice essential eg. dilution, weighing, filtration, standards

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Solids

Solids separated by filtration into non/soluble and by high temperature oxidation into non/volatile

Solids often form large percentage of total organic material

Solids degradation often slow due to mass transfer limitations

Sources: food processing, abattoirs rural industries, domestic

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Solids Fractions

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Practical Exercises: SolidsIn solids analysis, the following measurements were obtained: –Sample size: 50 mL –After filtration/evaporation: 12mg filter cake, 2.5mg solids in filtrate –After high temperature oxidation: 2.0 mg filter cakeWhat is TSS, VSS and TS in the sample?SolutionTSS : 12 mg / 50 ml = 0.24 mg/ml VSS : (12 – 2.0 mg) / 50 ml = 0.2 mg/lTS : (12+2.5) mg /50 ml = 0.29 mg/l

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Odour

Often very small amounts cause nuisance (eg. H2S approx. 10 ppb)

Physical/chemical measurement difficultOlfactometer determines dilution necessary until

no odour detected

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Temperature

Industrial WW often elevated temperatureAffects treatment performance of many treatment

systemsGas eg. O2 solubility is lower at higher

temperatureEffluent temperature usually specified in licence

limits

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Salinity

Affects ecosystems in receiving watersReduces O2 solubilityRestricts reuse applications (eg. irrigation)Critical for downstream water utilisationCauses density currents, stratification etc.

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Colour & Turbidity

Colour of ww & biological treatment: - light brown-gray => fresh, aerob - dark brown-black => old, anaerobSoluble dyes (stains) also cause colouring, very

difficult to removeTurbidity measures light-transmissionCaused by colloidal or suspended matterCan be correlated with suspended solids

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Organic Matter

Largest component group in most ww: 75 % of TSS, 40 % of TDS (domestic ww)

Composition highly industry dependentTypes: + carbohydrates + proteins + oil & grease + organic toxins (priority pollutants, eg. pesticides)

+ others eg. surfactants, dyes etc.Mostly biodegradable, some very slowly

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Carbohydrates

Composition: C, H, OSoluble: sugars, alcohols, Volatile fatty acids (VFA)

(ex. acetic acid, propionic acid or butyric acid) rapidly biodegradable

Insoluble: starches, cellulose, fibres (relatively) slowly biodegradable

Sources: sugar mills, breweries, dairy factories, canneries etc.

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ProteinsComposition: C, H, O, N (16%), S, P.Solubility varies with protein type and ww conditions

(eg: pH, salt conc.).Quite rapidly biodegradable to amino acids except

when insoluble.Anaerobic degradation creates H2S and other sulphur

components => odor. Sources: dairy factories, meat processing

(abattoirs),food processing.

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Oil & Grease

Composition: C, H, OHydrophobic substances: grease, fat, oilMostly insoluble, floating, easily adsorbed on

surfacesSlowly biodegradable, even when hydrolysed to

glycerol and fatty acidsSources: meat processing, food production,

chemical factories19

Toxics (Priority Pollutants)

Organic toxic chemicals, pesticides, herbicides, solvents, etc.

Inorganic substances eg. As, Se, heavy metals (Cd, Cr, Pb, Hg, Ag etc.)

Normally very low effluent limitsSources: chemical factories, metal manufacturing,

tanneries, agriculture, etc.

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Measurement of Organic Content

Mostly overall content measured:–Total organic carbon: TOC–Biochemical oxygen demand: BOD–Chemical oxygen demand: CODBOD & COD most commonly used for design and

effluent specificationsOil & grease: organic solvent extractionPriority pollutant analysis highly specific

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Biochemical Oxygen Demand (BOD)

Measures oxygen required for biological oxidation of organics

BOD: oxygen uptake by microorganism during aerobic growth in ww sample

Standard BOD: 5 day incubation @ 20°CSamples require a series of dilutions to achieve

suitable oxygen consumption

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Calculation(1.6)

Seeding: Provides micro-organisms to oxidize organic matter

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25

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Modeling of BOD ReactionThe rate of BOD oxidation (exertion) is modeled based on the assumption that the amount of organic material remaining at any time t is governed by a first-order function: d BODr / dt = - k1 BODr 2-58

Integrating between the limits of UBOD and BODt and t=0 and t=t yields:

BODr = UBOD (e-k1t) 2-59

Where BODr = amount of waste remaining at time t (days) expressed in oxygen equivalent , mg/l k1 = First order reaction constant (1/d)

UBOD = Total or ultimate carbonaceous BOD, mg/L t = TimeThus the BOD exerted up to time t is given by: BODt = UBOD – BODr = UBOD - UBOD (e-k1t) = UBOD (1- e-k1t ) 2-60

k1T = k1 20 T-20approx 27

Example 2-9: Calculation of BOD

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Determine the 1-day BOD and ultimate first-stage BOD for a wastewater at 20oC whose 5-day BOD at 20oC is 200 mg/L. The reaction constant k (base e)=0.23d-1 at 20oC . What would have been the 5-day BOD if the test had been conducted at 25oC?

Solution:1)Determine the ultimate carbonaceous BOD

2) Determine the 1-day BOD at 20°C

3) Determine the 5-day BOD at 25°C

LmgUBODUBODeUBOD

eUBODBODUBODBODx

tkr

/293)36.01()1(200

)1(23.05

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LmgeBOD

eUBODBODx

tkt

/1.60)795.01(293)1(293

)1(123.0

1

1

L/mg224)e1(293BOD

d29.0)047.1(23.0k5 x 29.0

5

12025125

Practical Concerns with BOD Test

Only partial degradation of organicsCannot be used for mass balancingVery high (>1000mg/L) and very low (<10mg/L)

values often unreliableIndustrial wastewater can contain inhibitors,

leading to low BOD resultsNitrification needs to be inhibited to avoid

measuring partial oxidation of NH4+

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Chemical Oxygen Demand (COD) 1/2

Also measures oxygen required, but for chemical oxidation of organics

COD: chemical oxidants used for oxidation of organics to CO2, H2O & NH3

Standard COD: K2Cr2O7 2- /H2SO4 @ 145°CDuring oxidation dichromate is used up and remaining

oxidant is measured Spectrophotometrically to determine oxidant used

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Chemical Oxygen Demand (COD) 2/2

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Chemical Oxygen Demand (COD)Oxygen equivalent of organic material that can be oxidized using

dichromate in a hot acid solution.

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The excess dichromate is titrated withStandard ferrous ammonium sulfate

Theoretical Oxygen Demand (ThOD)stoichiometric: 1 mole of O2 required to convert 1 mole of carbon to CO2

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BOD, COD, and TOC

Typical range of BOD/COD for untreated municipal WW is 0.3 - 0.8

Typical range for BOD/TOC is 1.2 - 2.0

If BOD/COD > 0.5WW is considered to be easily treatable by biological means

If BOD/COD < 0.3WW may have some toxic components and/or the addition of

acclimated micro-organisms may be required

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Wastewater Flow Terms

Equivalent person (EP): average wastewater amount produced per person

Typically 1 EP equivalent to 200-250 l/d per person for domestic households

Average Dry Weather Flow (ADWF): average flow over 7 days without rain

Peak Dry Weather Flow (PDWF): maximal flow during day (1.5-3 x ADWF)

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Typical Diurnal Flow Pattern

36Infiltration: is a ground water that enter sanitary sewer systems through cracks and/orLeaks in the sanitary sewer pipes.

WW Flowrates

Rated capacity of a WW treatment plant is based on the average annual daily flowrate plus a factor of safety

Factors that can influence the plant design:WW characteristicsConstituent concentrationsFlowrates

Mass loading [kg/d] = (concentration [g/m3])*(flowrate [m3/d])(103 g/kg)

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Design Flowrates and Mass Loadings

Process units and hydraulic conduits must be sized to accommodate the anticipated peak flowrates that will pass through the plant

Many WW treatment units are designed/sized based on the detention time or overflow rate (flowrate per unit surface area) required to achieve specific BOD and TSS levels in the effluent

Over-sizing the equipment can decrease the efficiency of the plant

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Sampling & Measurements

On-line measurements where possibleAppropriate sampling crucial to achieve relevant resultsSampling schedule based on expected (or measured) variance over timeAutomatic sampling often essential

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Composite Sampling

Reduces analysis costs and levels out concentration fluctuations

Composite samples should be taken proportional to flow

Individual samples can be collected and composited later

Ensure appropriate sample conservation/ storage from sampling time until analysis

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Flow Proportional Composite Sample

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Design Flowrates and Mass Loadings

A number of operating conditions must be considered in the design of a WW treatment facility:Peak hydraulic flowrateMinimum hydraulic flowrateMaximum mass loadingMinimum mass loadingSustained mass loading

Refer to Table 3-20 in Metcalf and Eddy

Initial and future

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Nitrogen Compounds & Units

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Phosphorus Compounds & Units

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Typical Municipal WW

TSS: 220 mg/LVSS: 165 mg/LBOD5: 220 mg/LCOD: 500 mg/LTKN: 40 mg-N/LAmmonia: 25 mg-N/LTP: 8 mg-P/LOrtho-phosphate: 5 mg-P/L

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