Asif uz zaman(Lecturer)

Civil Engineering DepartmentFaculty of Engineering

RabighKing Abdulaziz University

Measurement of oxygen

demanding wastes � There are several measures of oxygen demand.

1.Theoretical oxygen demand (ThOD) 2.Chemical oxygen demand (COD) 3.Biochemical oxygen demand (BOD)demand (BOD)

Estimation of organic content of

the wastewater� The organic matter present in the water body can be

analyzed in laboratory by determining:

� Bio-chemical Oxygen Demand (BOD),� Bio-chemical Oxygen Demand (BOD),

� Chemical Oxygen Demand (COD), and by

� The determination of Total Organic Carbon (TOC) interms of Theoretical Oxygen Demand(ThOD).

Theoretical oxygen demand� If the chemical composition of the substance is known

then the amount of O2 required to completely oxidize aparticular organic substance may be calculated fromstoichiometry. This amount of oxygen is known as thestoichiometry. This amount of oxygen is known as thetheoretical oxygen demand (ThOD)

Biochemical oxygen demand� The amount of oxygen consumed during microbial

utilization of organics is called the Biochemical Oxygendemand (BOD):

� In simple words, the BOD is the amount of oxygen in mg/Lrequired to stabilize polluted water completely by means ofrequired to stabilize polluted water completely by means ofan aerobic process.

� They are of two types as Carbonaceous oxygen demand(CBOD) and nitrogenous oxygen demand(NBOD).

� BOD test is the most widely used method for measuringorganic matter because of the direct conceptualrelationship between BOD and oxygen depletion inreceiving waters.

Biochemical Oxygen Demand (BOD)

� The BOD test is an indirect measure of organic matterbecause we actually measure only the change in dissolvedoxygen concentration caused by the microorganisms asthey degrade the organic matter.they degrade the organic matter.

� The BOD test is an indirect measurement of organic matterin terms of the oxygen requirement to convert them intostable end products.

� The biochemical oxygen demand is represented as BOD5

200C, which indicates the total amount of oxygenconsumed for biochemical oxidation of organic matter forfirst five days at 200C incubation temperature.

BOD test � The BOD is measured by determining the oxygen consumed from a

sample placed in an air tight container and kept in a controlled environment for a preselected period of time.

� During the BOD test the organic matter will be converted into stable end product such as CO2,sulphate (SO4), orthophosphate (PO4) and nitrate (NO3).

The simple representation of carbonaceous BOD can be explained as below:

Contd..� In the standard test, a 300 ml BOD bottle is used and

the sample is incubated at 200C for 5 days.

� Since, the saturation value of DO for water at 20oC is only 9.1 mg/L, it is usually necessary to dilute the only 9.1 mg/L, it is usually necessary to dilute the samples with BOD-free, oxygen saturated water to measure BOD values greater than just a few milligrams per liter.

� Hence, according to BOD values expected for that wastewater appropriate dilution should be carried out. Thus, the actual BOD of the unseeded sample can be worked out as

BOD test

BOD bottle in actual

Suitability of BOD test� For COD/BOD ratio less than 2 the wastewater is most

suitable for biological treatment;

� for COD/BOD ratio between 2 to 6 acclimation of theinoculum is required for effective biological treatment;inoculum is required for effective biological treatment;and

� for higher COD/BOD ratio biological treatment ofsuch wastewater may not be suitable option.

BOD of seeded sample� When the dilution water is seeded with 1 to 2 ml of

sewage per litre of water to provide necessary bacterial population for biodegradation of waste, it becomes necessary to subtract the oxygen demand of the seed necessary to subtract the oxygen demand of the seed from the mixed sample.

� Thus, the BOD of the wastewater with seeded sample can be worked out as below. The DO drop in blank is multiplied by (1- p) because this volume of blank is only present in the sample.

Contd...

Worked out examples on bod