Biochemical Oxygen Demand

1. Sanjivani Rural Education Society’s
Sanjivani College of Engineering, Kopargaon – 423601
(An Autonomous Institute, Affiliated to SPPU, Pune)
ISO 9001:2015 Certified, Approved by AICTE, Accredited by NBA and NAAC (A Grade)
Department of Civil Engineering
Course Title: Environmental Engineering – II (401001)
Biochemical Oxygen Demand
Instructor
Mr. Chaudhari V. S (Assistant Professor)
Contact id: chaudharivishalcivil@sanjivani.org.in
Environmental Engineering– II: BOD

2. Environmental Engineering– II: BOD

3. Environmental Engineering– II: BOD
 Water is “polluted” by many organic matter in its course of flow
When organic matter is present in a water supply, the bacteria present in
water will begin the process of breaking down this waste.
With this much of the available dissolved oxygen is consumed by aerobic
bacteria, robbing other aquatic organisms of the oxygen they need to live.
Biological Oxygen Demand (BOD) is a measure of the oxygen used by
microorganisms to decompose this waste.
A large quantity of organic waste in the water supply, guarantees a large
number bacteria present to decompose this waste.
In this case, the demand for oxygen will be high (due to all the bacteria) so the
BOD level will be high.
As the waste is consumed or dispersed through the water, BOD levels will
begin to decline.

4. Environmental Engineering– II: BOD
Is Biochemical oxygen demand different from
Biological oxygen demand….???
Biochemical oxygen demand measures the molecular
oxygen utilized for the biochemical degradation of organic material (carbonaceous demand)
and the oxygen used to oxidize inorganic material
such as sulfides and ferrous ion.
It also may measure the amount of oxygen used to oxidize reduced forms of nitrogen
(nitrogenous demand).
Biological oxygen demand only addresses the oxygen used up by the bacteria to degrade
organic
substances.

5. Environmental Engineering– II: BOD
Environmental
significance
 BOD tests are used for determining the pollution strength of organic waste,
domestic or industrial.
 They used for designing suitable treatment methods, organic loading of
treatment plants and also for evaluating efficiencies of unit operation and
treatment systems.
 They are used for determining suitability of treated efficiencies for disposal
with reference to recommended standards.
 BOD tests are also used in carrying out stream sanitation studies and for
enforcing water pollution control measures.

6. Environmental Engineering– II: BOD
 Any effluent to be discharged into natural bodies of water should have BOD less than
30 mg/L.
 Drinking water usually has a BOD of less than 1 mg/L.
 But, when BOD value reaches 5 mg/L, the water is doubtful in purity.

7. Environmental Engineering– II: BOD
Apparatus required Reagents required

8. Environmental Engineering– II: BOD
Necessity of Dilution: -
 A known volume of a sample of waste water, diluted using specially prepared distilled
water, is incubated at 20°C for 5 days. DO depletion in test bottles is a measure of fair
amount of biodegradable organic matter present in the sample. To avoid getting zero 5-
days ‘DO’, a dilution technique is adopted.
 DO depletion is the difference between immediate ‘DO’ of the diluted sample and 5-days
‘DO’ of diluted and incubated sample.
 Immediate ‘DO’ should be determined within 15 minutes of dilution of sample. Immediate
‘DO’ should be quite high near the saturation value. A value of around 7 mg/l however is
considered satisfactory.
 5 days DO of diluted and incubated sample should be at least 2 mg/l and also ‘DO’
depletion should not be less than 2 mg/l. So, empirically, the maximum & minimum limits
of ‘DO’ depletion are ‘(7-2) = 5 mg/l’ and ‘(7-5) = 2 mg/l’
Dilution techniques:
This is based on the expected ‘BOD’ value of the test sample considering the equation
‘BOD5’ (at 20°C) = (immediate ‘DO’ of diluted sample – 5 day ‘DO’ of diluted and incubated
sample) × dilution factor
This may be = (7 mg/l – 5 mg/l) DF ---------- (min)
or = (7 mg/l – 2 mg/l) DF ---------- (max)

9. Environmental Engineering– II: BOD
Seeding:
• Seeding is inoculating organic matter with live microorganisms that can initiate
biodegradation of organic matter under favourable conditions.
• Samples of domestic sewage & industrial effluents mixed with sanitary sewage, in the pH
range of 6 to 8.5, themselves contain enough living microorganisms and do not requires
seeding.
• Samples of industrial organic wastes; such as sugar factory waste, paper and pulp mill
waste, textile waste, distillery waste etc require seeding particularly when their pH
correction and seeding at time of test.
• The seed may be bacteria from organically rich soul or the supernatant of aerated
domestic sewage or an extract from the aerator mixed liquor of a successfully working
ASP.
Necessity of Blank: -
• The added seed may contain organic matter. Thus due to seeding there will be increase in
the amount of organic matter. Therefore correction is applied by deducting the DO
depletion in blank.Conditions of BOD test: -
1. Temperature should be constant throughout the test.
2. Sufficient nutrients should be present for the growth of microorganisms.
3. pH should be maintained in the range of 6.5 to 8.5.
4. Sufficient quantity of bacteria should be present in sample under incubation.

10. Environmental Engineering– II: BOD
Procedure: -
(A)Preparation of dilution Water: -
1. Aerate one liter distilled water to get DO above 7 mg/lit.
2. Add 1ml each of phosphate buffer (to maintain pH), magnesium sulphate, calcium
chloride, ferric chloride (to provide micronutrients and maintain osmotic condition for
favorable growth of microorganisms)
3. Add 2ml settled sludge (for seeding)
Dilution Water

11. Environmental Engineering– II: BOD
(B) BOD Test: -
1. Fill two BOD bottles with dilution water (Blank Solution).
2. Keep one bottle for incubation and find initial DO of another bottle (B1)
3. Fill two BOD bottles with sample diluted with dilution water.
4. Keep one bottle for incubation and find initial DO of another bottle (D1)
5. Find DO of incubated bottles (D2 and B2) after 5 days incubation.
D. O Determination by Winkler’s Method

12. Environmental Engineering– II: BOD
Step 1. Fill two BOD bottles with dilution
water (Blank Solution).
B1 (Find Initial DO) B2 (Find D.O after 5 days incubation at
200)
Dilution Water

13. Environmental Engineering– II: BOD
Step 2. Fill two BOD bottles with sample diluted with dilution water.
Dilution factor (F) =
𝑣𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑑𝑖𝑙𝑢𝑡𝑒𝑑 𝑠𝑎𝑚𝑝𝑙𝑒
𝑉𝑜𝑙𝑢𝑚𝑒 𝑜𝑓 𝑢𝑛𝑑𝑖𝑙𝑢𝑡𝑒𝑑 𝑠𝑒𝑤𝑎𝑔𝑒 𝑠𝑎𝑚𝑝𝑙𝑒
EX.- 1% diluted sample means 1 ml of sewage diluted to make 100 ml of test sample
Hence D.F = 100
If we take 250 ml w/w sample and dilute it to 1000 ml then dilution factor will be = 4
Means, 250 ml + 750 ml = 1000 ml
(w/w sample) (Dilution water) (Total)
D1
D2

14. Environmental Engineering– II: BOD
Observation Table
Sr. No. Sample Dilution Factor DO of Sample DO of Blank B.O.D. mg/l
D1 D2 B1 B2
1
Sample 1
2
Sample 2
3
Sample 3
BOD5 mg/lit. = [(D1 – D2) – (B1 – B2) x D.F]
D1 = D.O of diluted sample on 0th day
D2 = D.O of diluted sample at the end of 5th day.
B1 = D.O of the dilution water (Blank Solution) on 0th day.
B2 = D.O of dilution water (Blank Solution) at the end of 5th day.

15. Environmental Engineering– II: BOD