Waste waters are characterized on the basis of various

1) Physical characteristics- color, odour, dissolved oxygen, etc.

2) Chemical characteristics-COD, pH, acidity, alkalinity,total C,etc

3) Biochemical characteristics-BOD, presence of pathogenic bacteria, toxicity to aquatic organisms, plants & other life forms, etc.

1) BOD(Biochemical oxygen demand)-

It is defined as the biochemical oxygen demand of water is a measure of amount of free oxygen required by the bacteria & other micro-organism for biological oxidation of organic matter under aerobic conditions at $20^{\circ}c$ for period of 5 days.

Micro-organisms

Organic matter+$O_{2}-------------\rightarrow CO_{2}+H_{2}O$

$[CH_{2}O]$ Bacteria at $20^{\circ}c$ & 5 days

Significance-

1. It is useful in designing of treatment plants & calculation of waster load
2. The higher the BOD of a sample the higher will be pollution
3. It helps in pollution control

BOD=$(Do_{b}-Do_{i})$X Dilution factor

ml of sample after dilution

=$(Do_{b}-Do_{i})$X-------------mg/lit

ml of sample before dilution

Where,

$Do_{b}$ = Dissolve oxygen present in the effluent sample before incubation(period between infection starts & appearance of first symptoms)

$Do_{i}$ - Dissolve oxygen present in the effluent sample after incubation

2)COD(chemical oxygen demand)-

It is defined as the amount of oxygen required by organic matter in sample water for its oxidation by strong oxidizing agents $(K_{2}Cr_{2}O_{7})$ in the hot conditions. It is a measure of both the biologically oxidisable & biologically inert organic matter.

As a result COD values are always greater than BOD values

$(V_{1}-V_{2})$ NX 8000

COD=---------mg/lit

Where,

$V_{1}$- Volume of ferrous ammonium sulphate in blank expt.(for calculation unreacted dichromate)

$V_{2}$- Volume of ferrous ammonium sulphate in test expt

Y-Volume of water sample taken for test

Significance-

1. It measure the effect of pollutants on dissolve oxygen.
2. It helps in designing & calculation of efficiency of the water treatment plants.
3. It helps in deciding the disposal of domestic & industrial effluents in various types of water streams