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cmutsvangwa:, Wastewater Engineering, Dept. of Civil and Water
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Chapter 2 WASTEWATER CHARACTERISATION
Wastewater is composed of domestic, industrial wastewater and
agricultural wastes Domestic wastewater Domestic wastewater is
composed of:
human body wastes (faeces and urine) sullage (laundry and
kitchen wastes)
A greater percentage of the wastewater is water (99.9%) and the
remainder 0.1% are solids. The composition of the wastewater and
human wastes are given in Tables 2.1 and Fig. 2.1 respectively.
Fig. 1: Composition of wastewater Source: Mara D., (1986),
sewage treatment in hot climates Table 2.1: Composition of human
faeces and urine n/n Component Unit Faeces Urine 1 Quantity (wet)
per person per day g 135-270 1000-1300 2 Quantity (dry solids) per
person per day 35-70 50-70 3 Moisture % 66-80 93-96 4 Organic
matter % 88-97 65-85 5 Nitrogen % 5-7 15-19 6 Phosphorous ( as
P205) % 3-5.4 2.5-5 7 Potassium (as K2O) % 1-2.5 3-4.5 8 Carbon %
44-55 11-17 8 Calcium (as CaO) % 4.5 4.5-6 Source: Mara D., (1986),
sewage treatment in hot climates
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Wastewater characterisation The wastewater components
include:
organic material inorganic material microbial content
Organic material These are the carbonaceous compounds in
wastewater which are oxidised both chemically and biologically to
yield energy and CO2+H2O. The energy is necessary for
microorganisms for growth, e.g.
moleculessmallenergyHCOOOHC ++++ 06 2226126 The above oxidation
reactions are carried out both microbial and by use of chemical
oxidation agents, and indicate the amount of organic material
present in a wastewater. If biological oxidation is employed, the
test is called BOD and for chemical oxidation is COD. Other tests
for the organic material are the Permanganate value (PV) and the
Total O2 Demand (TOD). Definition of BOD Quantity of oxygen
utilised by a mixed population f microorganisms in the aerobic
oxidation of organic matter in a sample of wastewater at a
temperature of 20oC. It is the most commonly used parameter to:
to define the strength of municipal or organic industrial
wastewater. to measure the waste loadings to treatment plants to
determine the relative oxygen requirement of treated effluents and
polluted
waters to determine compliance with wastewater discharge
permits
BOD test Reaction takes place in closed bottle so that the O2
utilised can be measured. A typical BOD bottle is 300ml (Fig.
2.2).
The wastewater sample is mixed with a seed comprising a large
number of microorganisms which are capable of oxidising the
wastewater being tested. Seeding of the wastewater many not be
necessary if the wastewater contains enough microorganisms like
domestic wastewater, unlike industrial wastewaters which require an
acclimated seed.
a dilution solution containing MgSO4, CaCl2 and FeCl3 aerated to
saturation with oxygen is required.
the saturated dilution is added into the BOD bottle and the
bottle is filled to the top
the bottle is then tightly closed to ensure that no oxygen
enters from the atmosphere
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the closed bottle is placed in an incubator, in the dark at
20oC. a blank is also required which contains a seed and dilution
water only changes in the dissolved oxygen content are measured by
suitable
electrodes at intervals rapid oxidation of BOD occurs with the
first 7 days
Fig. 2.2 Typical BOD bottle Process taking place in a BOD
bottle
Catabolism This is the first oxidation of the carbonaceous
material into smaller molecules
to provide energy which is used in the synthesis of new cellular
material.
energymoleculessmallOHCOOCHON ++++ 222 organic material
Anabolism It is the second stage and is the resynthesis of the
small molecules into the cell components required by the growing
organisms, utilising the energy generated in the first
reaction:
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OHNOHCEnergyOmoleculesSmall 22752 +++
Metabolism The stage at the end of the incubation period when
the amount of biodegradable organic material is very low. The
starving microorganisms are forced to oxidise their cellular
carbonaceous material in order to provide the energy for
viability.
energyOHNHCOONOHC ++++ 2322275
The oxidation continues for approximate 28 days to give the
ultimate BOD (BODu). At this stage all the biological oxidation has
ceased and any organic material left remaining in the wastewater is
resistant to biological oxidation and may be safely discharged into
the water course without fear of exerting any further oxygen
demand. The BOD test is performed in the presence of a nitrifying
inhibitor to stop the following reaction, which takes place in the
presence of ammonia:
++ +++ HOHNOONH 22 2324 To minimise errors, a shorter period of
incubation instead of 28 days is employed. Normally 5 days at 20oC.
The hypothetical carbonaceous and nitrification demand is
illustrated in Fig. 2.3.
BOD removal
4Fig. 2.3 Hypothetical carbonaceous and nitrification demand
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Computation of BOD for unseeded wastewater
PDDBOD 21 = , mg/l
Where; D1 =initial DO of diluted buffer wastewater (phosphate
buffer,
MgSO4, CaCl2, FeCl3 and wastewater), mg/l D2 =final DO of
diluted wastewater after incubation for 5 days, mg/l P =decimal
fraction of wastewater sample
= ( )( ) bottleBODofvolumemlsamplewastewaterofvolumeml
Computation of BOD for seeded wastewater ( ) ( )
PfBBDDBOD 2121 =
Where; D1 =DO of diluted wastewater sample about 15 after
preparation
D2 =DO of diluted wastewater sample after incubation B1 =DO of
seeded diluted sample after 15 preparation
B2 =DO of seeded diluted sample after incubation f =ratio of
seed volume in seeded wastewater test to seed
volume in BOD test on seed
f =1
1
%
%
Binseedofmlor
Dinseedofmlor
P =decimal fraction of wastewater sample used
=bottleBODofvolumeml
samplewastewaterofml
BOD TEST limitations
test provide measure of the oxygen consumed in the biological
oxidation of carbonaceous material in a sample over 5 days at
20oC
at the end of the 5 days, some of the biological carbonaceous
material originally present in the sample remains unbiodegradable,
and therefore remain not assessed.
not possible to perform a mass balance a high concentration of
active acclimated seed bacteria is required pre-treatment is needed
when dealing with toxic wastes nitrifying organisms must be reduce
because nitrification may begin during
the test
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long period of time needed to obtain results the 5 day may not
correspond to the point where the soluble organic mater
that is present has been used The 5 day BOD has now been the
standard reference used internationally by engineers and regulatory
agencies. It originated from the United Kingdom conditions. It was
recommend in the UK at the beginning of the century that 5 day BOD
at 18.3oC value be used as a reference in Britain (Tchobanoglous et
al, 1985). The reason being that British rivers do not have flow
time to the sea greater than 5 days and average summer temperature
do not exceed 18.3oC. The temperature has been rounded upwards to
20oC and this has remained the universal scientific and legal
reference. Therefore it can be concluded that the BOD5 has no a
clear theoretical meaning and the basis of its adoption is also
inappropriate, nevertheless it has remained unchanged. Some of the
following relationships can be used to estimated BOD or COD.
( 0.35.1 = )BODCOD for raw domestic wastewater (Metcalf and
Eddy)
( 0.40.3 =BODCOD ) for industrial wastewater (Metcalf and
Eddy)
Chemical oxygen demand (COD) It is one of the parameters like
BOD which is widely used to characterise the organic strength of
the wastewater and pollution of natural waters. Definition It is
the amount off oxygen required for chemical oxidation of organic
matter to carbon dioxide. The value of COD in wastewater is greater
than BOD, because chemical oxidation decomposes more compounds than
biological, and even the non-biodegradable organic matter is
oxidised. Also COD is greater because BOD5 is not the ultimate BOD.
COD TEST
A strong oxidising chemical agent is use and normally potassium
dichromate in an acid medium.
known diluted volumes of wastewater are put into a graduated
flask and diluted with pure water
a known quantity of standard potassium dichromate solution,
sulphuric acid reagent containing silver sulphate, are added to the
sample
the mixture is vaporised and condensed for 2 hours and organics
are mostly destroyed:
+++ ++++ CrOHCOHOCrorganics heat 22272 after cooling, the
mixture is diluted with distilled water
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the dichromate remaining in the specimen is titrated with
standard ferrous ammonium sulphate using ferroin indicator to
determine the COD
ferrous ion reacts with dichromate ion with an end point colour
change from blue-green to reddish brown
( ) 800424
= SONHenormalityFtitrantsamplemlblankmlCOD Characterisation of
inorganic compounds
Unlike organic material, there is no test like oxygen demand
inorganic compounds which pose a threat or serous pollution are
limited and
it is not feasible to perform all the tests individual tests are
conducted for those compounds most likely to prove
trouble some e.g. nitrogen, phosphorous, heavy metals and
pesticides. Characterisation of microorganisms These are the
diseases causing organisms which are tested and should satisfy the
disposal standards for health reasons and the most common are the
Ecoli. Table: 2.2 Comparison of tests for organic material
7
Test Advantage Disadvantage BOD simple and popular
used in majority of design equation familiar to most engineers
produces information on both
carbonaceous and nitrogenous oxygen demand
long period of incubation reproducibility poor ( 15%)
susceptible to inhibition by
many industrial wastes
COD simple and inexpensive comparatively rapid (data
available
within 3hr) gives an accurate indication of the
fraction of the wastewater amenable to biodegradation
good reproducibility ( 5-10%)
does not oxidise ammonia many non-biodegradable
organic compounds exert an oxygen demand
interference from high concentrations of chloride ions
PV simple inexpensive apparatus required
rapid (results available within 40 min)
ideal for field testing good reproducibility ( 6%)
many organic compounds are not oxidised by the mild
conditions.
certain inorganic compounds may contribute a high oxygen
demand
TOC very rapid (data available in minutes)
may be readily automated reproducibility excellent ( 3-6%)
expensive apparatus and skilled technicians needed
little comparative data available
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cmutsvangwa:, Wastewater Engineering, Dept. of Civil and Water
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Characterisation of wastewater solids content They are either
soluble or insoluble:
suspended solids dissolved solids volatile suspended solids
The characterisation of solids content is illustrated in Fig.
2.4 and 2.5. Definitions of solids
Total solids: all matter that remains as residue after
evaporation at 103-1050C. They exclude settleable solids that will
settle to the bottom of a con-shaped container called the Imhoff
cone after 60 minutes if settlement. Settleable solids expresses as
ml/l are an approximate measure of the quantity of sludge that can
be removed by primary sedimentation.
Total solids =suspended (non-filterable) +filterable
(filtrate)
Filtrate
Colloidal particles and dissolved solids that will pass through
the filter paper called the Whatman (GF/C), with a nominal pore
size of 1.2m. The colloidal particles range from 0.001-2 m. The
dissolved solids include both the organic and inorganic ions.
Volatile solids This is the part of the organic matter that is
destroyed at 550-600oC, and what remains as ash is the non-volatile
solids or fixed solids (FS). The VS and FS are found in both the
filtrated and suspended solids (Fig. 2.4).
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Chapter 3A: Wastewater characterisation
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Fig. 2.4: Wastewater characterisation of solid content
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Table2.3: Physical, chemical and biological characteristics of
wastewater
Source: Metcalf and Eddy, (1995), Wastewater engineering,
treatment, disposal and
reuse, McGraw Hill, New York, USA
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Table 2.4
Source: Metcalf and Eddy, (1995), Wastewater engineering,
treatment, disposal and
reuse, McGraw Hill, New York, USA
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Table 2.5
Source: Metcalf and Eddy, (1995), Wastewater engineering,
treatment, disposal and
reuse, McGraw Hill, New York, USA
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Table 2.6
Source: Metcalf and Eddy, (1995), Wastewater engineering,
treatment, disposal and
reuse, McGraw Hill, New York, USA
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Table 2.6
Source: Metcalf and Eddy, (1995), Wastewater engineering,
treatment, disposal and
reuse, McGraw Hill, New York, USA
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Table 2.8
2 Source: Metcalf and Eddy, (1995), Wastewater engineering,
treatment, disposal and
reuse, McGraw Hill, New York, USA
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Fig. 2.4: Wastewater characterisation of solid content Source:
Metcalf and Eddy, (1995), Wastewater engineering, treatment,
disposal and
reuse, McGraw Hill, New York, USA 1. Mara D., (1976), Sewage
Treatment in Hot Climates, John Wiley, UK 2. Mara D., (1997),
Design of Waste Stabilisation Ponds in India, Lagoon
Technology, UK 3. Hammer M. J., (1986), Water and wastewater
technology, Prentice Hall, USA 4. Mara D, D, Sewage treatment in
hot climates 5. Metcalf and Eddy, (1995), Wastewater engineering,
treatment, disposal and
reuse, McGraw Hill, New York, USA 6. Schroeder E.D., (1971),
Water and wastewater treatment, McGraw Hill, New
York, USA 7. Tchobanoglous and Schroeder Water Quality
Chapter 3A: Wastewater characterisation
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