IV. Wastewater Treatment
Topic IV. 4. Pollution Loads on the Receiving
Water. Wastewater Quality Parameters - BOD,
COD, TOC, PE. Oxygen Balance in the Receiving
Water
Pollution Load
Definition: Mass of a definite pollutant, released in definite period (day,
month, year) into the receiving water with the wastewater emissions.
Kinds of pollution loads:
 Suspended solids - SS
 Organic substances containing carbon - BOD, COD, TOC
 Substances containing nitrogen - Ntot, Norg, NH4-N, NO3-N
 Substances, containing phosphorus - Ptot, Porg , PO4-P, P2O5-P
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Wastewater Quality Parameters - BOD, COD, PE
Biochemical Oxygen Demand - BOD, g/m3
Definition: Mass of dissolved oxygen utilised in definite time by the aerobic
micro-organisms during processes of dissimilation of organic substances,
containing carbon.
Particularities:
 BOD is a complex measure, indicating indirectly availability of
biodegradable pollutants mass concentration
 BOD reflects only biodegradable part of carbonaceous organic
substances
 BOD reflects only such a part of carbonaceous organic substances,
which is utilised in processes of dissimilation (breathing)
 Biodegradable organic substances utilised by the aerobic microorganisms in the processes of assimilation (cell mass building) is not
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reflected by BOD
Biochemical Oxygen Demand - BOD
Fazes of aerobic oxidation of organic substances
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Biochemical Oxygen Demand - BOD
BOD Kinetics
dLt / dt  k1' .Lt; Lt  La .10k .t
1
;
k1  k1' / 2,3
Yt  Lt  La  Lt  La .(1  10k1.t )
k1 (T2 )  k1 (T1 ).1(T1 T2 )
1  1,048
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Chemical Oxygen Demand - COD, g/m3
Definition: Mass of the oxidiser utilised in oxidation of
(almost) all the organic substances in water.
Particularities:
 COD is associated with potassium dichromate K2Cr2O7 as an oxidiser in presence of silver sulphate AgSO4 as a catalyzer
 Similar parameter, called oxidability is associated with
potassium permanganate - KMnO4 as an oxidiser
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Total Organic Carbon - TOC, g/m3
Definition: Carbon mass of the organic substances in water/wastewater
Analytical determination:
 Procedure as for COD determination
Corg  K 2Cr2O7  HCl  CO2  ....
 Procedure for C-fixing in BaCO3
CO2  Ba(OH ) 2  BaCO3 
Particularities:
 TOC is a general measure for organic pollutants mass in
water/wastewater (similar to COD function, but more precise)
 TOC reflects both biodegradable and biorefractory organic
substances in water/wastewater
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People Equivalent - PE
Definition: Number of population releasing the same pollution load as
the one released with definite industrial wastewater emission
Qind .Cind
PE 
a.Qdom
Qind - industrial water quantity, m3/d
Qdom - domestic water quantity, m3/d
Cind - pollutant concentration in industrial wastewater, g/m3
a - pollutant daily mass per citizen (pollutant norm), g/d.ca
Particularities:
 PE differ according to the kind and norm of the pollutant
 PE is a general and common measure of industrial pollution load
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People Equivalent - PE
Pollutants norms
 Suspended solids


Non-settled wastewater- a = 65 g/d.ca
Settled wastewater - a = 40 g/d.ca
 BOD5


Non-settled wastewater- a = 60 g/d.ca
Settled wastewater - a = 40 g/d.ca
 Nitrogen - a = 8 - 12 g/d.ca
 Phosphorus (as PO4+) - a = 4,4 g/d.ca
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Oxygen Balance in the Receiving Water
 Rate of dissolved oxygen consumption for biodegradable organic
pollutants dissimilation in the receiving water (BOD kinetics)
dLt / dt  k1' .Lt
Lt  L0 .10k1 .t
 Rate of dissolved oxygen recovering in the receiving water
(reaeration)
dOt / dt  km' .(Os  Ot )  km' .Dt
(T T )
 2  1,018
Dt  D0 .10k2 .t k 2  k m' / 2,3 k 2 (T2 )  k 2 (T1 ). 2
2
2
 Net rate of dissolved oxygen concentration change in the receiving
water
dOnet / dt  dOt / dt  dLt dt  k m' .Dt  k1' .Lt
Onet  Os  Dt
Os  const
dDt / dt  k1' .Lt  k m' .Dt
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Oxygen Balance in the Receiving Water
Streeter - Phelps Equation
(integral of the last differential equation)
k1 .L0
Dt 
.(10k1 .t  10k2 .t )  D0 .10k2 .t
k 2  k1
t  0  L0 , D0
t crit  Dt ,max
t crit

1
 k 2  D0 .(k 2  k1)  


. lg .1 

k 2  k1 
k1 .L0

 k1 

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Oxygen Balance in the Receiving Water
Graphical Interpretation of the Streeter-Phelps Equation
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