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Wastewater Characterisation and Treatment

Recommended text books:

Wastewater Engineering – Metcalf and Eddy

Standard Methods for Examination of Water and Wastewater

Contact: Benoit Guieysse

B.J.Guieysse@massey.ac.nz

RC.2.18

Lecture block outline

The big picture: “Understanding the nature of wastewater is essential in the

design and operation of collection, treatment, and reuse facilities – and in the

engineering management of environmental quality”

We need to know what’s in it before we can decide what to do with it!

Characterisation

Sampling

Bio pollutants

Chemical pollutants

Physical pollutants

Treatment

Disposal

Tertiary

Secondary

Primary

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1.0. What makes water polluted?

Indicators of water quality

Physical Solids, temperature, conductivity, color, turbidity, odor.

Chemical pH, alkalinity, hardness, organic compounds (including

BOD, COD, TOC), inorganic compounds (sulfate,

phosphorus, nitrate, nitrite, ammonium, Cl-, H2S, heavy

metals etc), dissolved oxygen etc

Biological Bacteria, algae, protozoa, viruses, coliforms, toxicity

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Water pollutantsClass Examples Examples and potential environmental

impactSuspendedsolids

Sand, clay Can lead to the development of sludgedeposits and anaerobic conditions in theaquatic environment

Biodegradableorganics

Sugar, proteins, syntheticchemicals

The uncontrolled biodegradation canlead to the depletion of natural oxygenresources and to the development ofseptic conditions

Pathogens Bacteria, fungi, algae,viruses, protozoans

Diseases, toxins

Nutrients Nitrate, nitrite, phosphate Uncontrolled algae growth, groundwaterpollution

Prioritypollutants

Heavy metals and toxicorganics such as As, Cu,phenolics etc

Acute and chronic toxic effects

2.0. Sources of pollution

Heavy industry: eg. chemicals, metals, COD,

nutrients

People/homes: eg. COD, N

Meat works, Dairy: eg. fats, oils, COD nutrients (N & P)

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3.0. Physical Characterization

Solids concentration

Turbidity

Colour

Transmittance

Temperature

Conductivity

Density

Specific gravity

Specific weight

3.1. Solids

A solid is a chemical or particle that is solid in a dry form under “normal condition of temperature and pressure”.

In water, a solid is “anything” that would remain after the water is being evaporated.

In water, a solid can be found dissolved or in suspension. The dissolved solid fraction (TDS) can pass through a 45µm pore size filter whereas the suspended solid (TSS) is retained by the filter.

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Solids size

1000nm 100nm 1nm

molecularcolloidalfinecoarse

Sizes of solids; source: sawyer, 1994.

Organic waste bacteria proteinsviruses

Colloidal materials are very fine solids of 0.001 – 0.5 μm that

cannot be removed by simple sedimentation process

Solids fractionation

Total solids content (TS): all the matter that remains as residue upon evaporation at 103-105oC

Settleable solids: settle to the bottom of a cone shaped container in 1hr (mL/L)

Total suspended solids (TSS): are in suspension in the liquid phase – ie, they are removed from the liquid by filtration (mg TSS/L)

Volatile suspended solids (VSS): are driven off at temp of 550oC(mg VSS/L)

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Sample Settleablesolids

FSSeg.

sand

TSS

DFSeg.

NaCl

TFSTVS

DVSeg.

sugar

VSSeg.

bacteria

TDS

Filter(glass fibre)

ImhoffCone

Oven105oC TS

Oven550oC

Oven550oC

Oven550oC

filtrate

Solids map

FiltrationSuspended solids include bacteria as well as waste material

Caught in the filter = suspended solidmeasured in mg SS/L

The filter needs to be dry before and after filtration!

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VolatisationVSS is an indication of bacteria/biomass content in the wastewater – important for monitoring biological wastewater treatment

Organic material will oxidise as gas at temp of 550 +/- 50oC – inorganic fraction remains as ashmeasured in mg VSS/L

Furnace is used to generate very high temperature – be careful!

Settable solids

Imhoff cone

solids accumulate in the bottom, measured in mL/L

Important measure of the quantity of material that can be removed by primary sedimentation

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Summary sheet for solids – Most important fractions are TS, TSS, VSS, TDS

Test DescriptionTotal solids (TS) Residue after a wastewater sample has been

evaporated and dried (103 to 105°C, 24h).

Total volatile solids Solids volatilized after the incineration (500°C) of theTS fraction

Total fixed solids Residue remaining after incineration of the TS (TS =TVS + TFS)

Total suspended solids (TSS) Portion of the TS retained of a filter of specified poresize

Volatile suspended solids (VSS) Volatile fraction of the TSS that has been incinerated

Fixed suspended solids (FSS) FSS = TSS – VSS

Total dissolved solids (TDS) TDS = TS – TSS. This fraction also comprises colloids,which size typically range from 0.001 to 1 µm.

Total volatile dissolved solids (VDS) Volatile fraction of TDS

Fixed dissolved solids (FDS) FDS = TDS – VDS

Settleable solids Suspended solids that will settle out of suspensionwithin a specified amount of time.

3.3. Turbidity

Turbidity is a measure of light-transmitting properties – it is a test of the

quality of effluent with respect to colloidal and residual suspended matter

Issues: High degree of variability observed depending on the light source

& varying light adsorbing properties of the suspended material

Difficult to compare turbidity values reported in literature – but turbidity

meters can be used to monitor relative plant performance

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Turbidity

1 NTU = 1 Formazin Turbidity Unit (FTU) for a given apparatus.

3.4. Conductivity

Electrical Conductivity (EC) is the measure of the ability of a solution to conduct

an electrical current. EC is related to total dissolved solids (as it is related to the

concentration of ions in solution).

TDS (mg/L) ≈ EC (millisiemens / m) x 10 x (0.55 – 0.70)

EC is used as a measure of salinity so it is an important parameter for

determining if water is suitable for irrigation

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Temperature :

1. Affects the reaction rate

2. Affects the solubility of

gases

3.5. Temperature

Temp. of domestic WWT systems can vary with seasons and can be

directly related to process operations

Oxygen solubility (and with it Dissolved Oxygen – DO) decreases with increasing temperature

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4. Chemical Properties

pH

Alkalinity

Nitrogen

Phosphorus

Metals

Soluble gases

Organics

4.1. pHpH is related to the hydrogen ion

concentration, it is defined as:

pH = -log [H+] = log(1/[H+])

The pH of pure water is equal to 7.

Solutions with pH < 7 are acidic,

solutions with pH > 7 are basic.

pH is naturally influenced by CO2

concentration as CO2 dissolves and

forms carbonic acid

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4.1. Alkalinity

The alkalinity of water is a measure of its capacity to neutralize acids (buffer

capacity)

Bicarbonate (HCO3-) represents the major form of alkalinity in natural water

(pH around 7), because it is formed upon the reaction of CO2 with calcium (or

magnesium) carbonate: CO2 + CaCO3(s) + H2O ⇄ Ca2+ + 2HCO3-

The same reactions also explain the presence of Ca2+ and Mg2+ in water, which

are responsible for the hardness of the water.

[Carbonate = CO32- / bicarbonate = HCO3

- / carbonic acid = H2CO3]

[OH- and CO32- only contribute significantly to alkalinity for pH > 9-10]

4.2. Nitrogen

Nitrogen is a fertilising agent. Elevatedquantities of inorganic N can cause algalblooms. Nitrogen is not often limiting in freshwater but is often limiting in sea water.

Essential to the growth of algae and other biological organisms – excessive

concentrations can cause excessive growth.

Interest in reducing N loads in effluents (typical domestic N content 20-50 mg

nN/L)

N is usually found in organic compounds (proteins) and inorganic compounds

such as ammonia (NH4+ or NH3 depending on the pH), nitrite, and nitrate.

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Nitrogen summary

MOST REDUCED NH3 → NO2 →.NO3 MOST OXIDISED

Form NotesAmmonia NH3 In equilibrium with NH4

+ (NH4+ ↔ NH3 + H+)

In wastewater, with pH less than 8, most is present as NH4+

Typical range in domestic WW is 15-40 mg/L. Much higher inagricultural WW and leachate.

Measured by colorimetry, titrimetrically or by ion specific electrodes.

Nitrate NO3- Most oxidised form of nitrogen. High concs are harmful to human

health (blue baby syndrome).

Typical range in treated effluents is 15-20mg/L.

Measured by colorimetry or by ion specific electrodes.

Nitrite NO2- Relatively unstable – easily oxidised to nitrate. (rarely above 1mg/L).

Extremely toxic to fish and other aquatic life.

Measured by colorimetry.

Organic nitrogen(Kjeldahl N)

Bound to carbon material (eg proteins).

Measured by the Kjeldahl method (NH3 is removed first).

Total Kjeldahlnitrogen

TKN Organic N + NH3/NH4+

Measured by the Kjeldahl method

Total nitrogen Organic N + NH3 + NH4+ + NO2

- + NO3-

4.3. Phosphorus

Phosphorus is a fertilising agent. Elevatedquantities of inorganic P can cause algal bloomsbecause Phosphorus is often limiting in freshwater.

Essential to the growth of algae and other biological organisms – excessive

concentrations can cause excessive growth…

Interest in reducing P loads in effluents (typical domestic P content 5 - 15mg

P/L)

P is normally found as orthophosphate (PO43-, HPO4

2-, H2PO4-, H3PO4) or

polyphosphate (polyP), which undergo hydrolysis to orthophosphate

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5. Organic carbon

Organic Matter

Proteins (40-60%)About 16% N

From animals and foodMay not be soluble

Carbohydrates (25-50%)Both readily and slowly biodegradable

eg,. Sugar and starch, cellulose

Fats (8-12%)Can clog pipes

Interfere with treatment

Compounds made of carbon, hydrogen and oxygen, together with nitrogen in

some cases. About 75% of SS in domestic wastewater are organic.

5.1. COD

COD is a measure of the oxygen equivalent of the organic material in wastewater

that can be oxidised chemically using dichromate in an acid solution.

Where

Rapid & Measured experimentally (2hr), chemical oxidation + heat + acid. Test

designed to that organic-N is released as NH4+ .

The amount of Cr2O7- used as converted into O2 equivalent.

( ) +++− ++−++→+++ 3422

272 2

2388 dCrcNHOHcdanCOHcdOdCrNOHC cban

23632 cband −−+=

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5.1. thOD

The ThOD of a known compound of known concentration (or mixtures) can be

calculated from the balanced reaction of oxidation of that compound with

oxygen where

1. Organic carbon is converted into CO2

2. Amino nitrogen (-III) is converted to ammonia

3. Non-amino nitrogen is converted to nitrate

4. Sulfur is converted to sulfate

5. Organic Cl, Br, or F are released as Cl-, Br- or F-

The COD test aims to measure the sum of the theoretical oxygen demands of

all organics present in the sample.

Measuring BODWe measure the oxygen consumed during degradation

of organic compounds by bacteria (bacteria “respire” and

consume organic food just like us). The amount of oxygen

is correlated to the population size and “health”

(activity), which increases with the amount of food =

pollutant!

t0 – sat with O2

Seed? (if bacteria limited)

Nutrients(to prevent growth limitation)

Vol: 300mlTemp = 20oC Measure DO at t0 and t5days

BOD5

Dilutedsample

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BOD Significance

• Similar to biochemical mechanisms naturally occurring in aqueous streams

• Integrated index of all carbon sources (can be hundreds)

• Universally accepted

• Legal and administrative uses for setting limits and monitoring

• Comparatively simple

• Can use for design

• Quantitative measure

• BOD/COD ratio = 0.4 - 0.8, but can vary widely with the waste

5.3. Fats Oils Grease (FOG)

FOG interfere with biological life and they can create unsightly

films.

FOG are of particular concern in agricultural wastewaters (eg.

dairy and meat processing wastewaters).

The FOG content is determined by extraction of a wastewater

sample with trichloro-trifluroethane, in which FOG are soluble.

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Known or suspected carcinogenicity, mutagenicity,

teratogenicity or high acute toxicity

US Environmental Protection Agency : 129 pollutants (inorganic

and organic) - Many are VOCs (volatile organic compounds)

6. Priority pollutants

http://www.mfe.govt.nz/issues/hazardous/

http://www.ermanz.govt.nz/hs/index.html

7. Heavy metals

One of the oldest known groups of pollutants

Definition hazy, some based on density (> 6 g/cm3), others on

atomic weight and atomic number

However, some metals that don’t fit these definitions are often

classed as ‘heavy metals’ e.g. aluminium, some heavy metals

aren’t even technically metals, e.g. arsenic, antinomy.

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Heavy metals in waters

1. Form stable complexes or compounds with other materials

2. Are often toxic to plants and animals in very low

concentrations

3. Include: Arsenic, cadmium, chromium, copper, lead,

mercury, nickel, selenium and zinc

Selenium toxicity in Kesterson Reservoir, California

Reservoir built in 1960’s to

attract native wildlife

Problem was the water source –

agricultural runoff from

surrounding area, from

mountains formed by shale with

naturally high selenium levels

The runoff and water in the

reservoir became concentrated

in selenium

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8. Biological Constituents

Most are only visible with microscope

algae

fungibacteria

viruses

protozoa

prokaryotes(single cell) eukaryotes

(single and multi cell)(internal compartments)

sub-cellularorganisms

helminths

Fundamental importance for human health(also important for biological treatment)

Biological pollutants

Large number of potential pathogens!

Impossible to monitor them all: need for indicator organisms

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Indicator organisms

An ideal indicator should:

1. Be present when faecal contamination is present

2. Be at least equal (most often much high) in population with the target organism

3. Exhibit similar survival characteristics as the target organism

4. Not reproduce outside the host

5. Allow fast, cheap quantification

6. Be a member of the intestinal micoflora of warmbloodedanimals

9. Sampling

Proper sampling and analytical techniques are of

fundamental importance in the characterisation of

wastewater.

Four goals of sampling:

Representative

Reproducible

Defensible

Useful!

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Sampling

Grab samples:

One sample from a sample point – only represents that point at

that time

Composite samples:

A collection of numerous individual discrete samples taken at

regular intervals over a period of time, usually 24 hours –

represents the average performance/ composition/ flow during

the collection period.