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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
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.