Liquid effluents are classified according to the following:
- Chemical Oxygen Demand (COD)
- Biological Oxygen Demand (BOD)
- Dissolved Oxygen (DO)
- Toxic chemicals such as phenols, cyanides etc.
- Total Dissolved Solids (TDS)
CHEMICAL OXYGEN DEMAND (COD):
The amount of oxygen needed for oxidising organic matter (including some inorganic solids) by chemical oxidation is known as chemical oxygen demand (COD).  OR The COD is the amount of oxygen expressed in parts per million (ppm) or milligrams per liter (mg/l) consumed under specific conditions in the oxidation of organic and oxidizable inorganic material. 
Description and Procedure:
In the COD test, the oxidizing bacteria of the BOD test are replaced by a strong oxidizing agent under acidic conditions. A sample of the water-water containing organic material is mixed with an excess of potassium dichromate and sulphuric acid and the mixture is heated under total reflux conditions for a period of 2 hours. During digestion, the chemically oxidizable organic material reduces a stoichiometrically equivalent amount of dichromate, the remaining dichromate is titrated with standard ferrous ammonium sulphate solution. The amount of oxidizable organic material reduced gives a measure of the amount of oxidizable material. Dichromate has advantages over other oxidants in oxidizing power and applicability to a wide variety of samples. The COD does not distinguish between organic materials that are biodegradable and those that are not, and, hence, gives a measure of the total oxidizable organic material in the sample. Due to this the COD test results are higher than those of BOD tests carried out on the same sample. 
The COD test is much more useful than the BOD test for estimating oxygen requirements of certain industrial wastewaters. It is valuable for wastes where BOD test is not applicable due to presence of toxic substances, low rate of oxidation, or other similar factors. Ratios of BOD to COD can be employed to get an indication of the degree of biotreatability of the waste. Ratios of 0.8 or higher indicate wastes that are highly amenable to biological treatment, while lower ratios indicate that the wastes are not favourable to biological treatment. 
BIOCHEMICAL OXYGEN DEMAND (BOD):
It is a measure of the oxygen utilized by micro-organisms during the oxidation of organic materials.  OR A measure of the amount of biodegradable organic substances in water. It is expressed as the number of milligrams of oxygen required by the micro-organisms to oxidise the organics in a litre of the water. 
Description and Procedure:
It is the most widely known measure for assessing the water pollution potential of a given organic waste. On an average, the demand for oxygen is directly proportional to the amount of organic waste which has to be broken down. Hence, BOD is a direct measure of oxygen requirements and an indirect measure of biodegradable organic matter. 
The most widely used and accepted measure of biodegradable organic content of wastewater is the 5-day, 20˚C BOD value. The brief analytical procedure is outlined below:
- Two standard 300-ml BOD bottles are filled completely with the wastewater of which the BOD is to be measured and the bottles are sealed.
- Oxygen content of one bottle is determined immediately.
- The other bottle is incubated at 20˚C for 5 days in total darkness, after which its oxygen content is measured.
- The difference between the two DO (Dissolved Oxygen) values is the amount of oxygen that is consumed by microorganisms during the 5 days and is reported as the BOD5 (5-day BOD) value of the sample. 
Drinking water usually has a BOD of less than 1mg/l, and water is considered fairly pure with a BOD of 3mg/l. But, when the BOD value reaches 5mg/l, the water is of doubtful purity. A comparison of these BOD levels with the range of values encountered in industrial water-waters indicates the seriousness of the problem. 
Table 1: Range of BOD levels of some industrial effluents 
BOD range (mg/l)
|Food processing wastes||500-4000|
|Mixed chrome tanning||500-5000|
Description and Procedure:
There are four processes which actually affect the DO content in the water: reaeration, photosynthesis, respiration and the oxidation of wastes.
Figure 1: Saturation concentrations of dissolved oxygen is fresh water 
Surface waters of good quality should be saturated with dissolved oxygen. A fall in DO level is one of the first indications that a water body is polluted by organic matter. It is usually determined by Winkler’s method, which is based on the reaction of dissolved oxygen with manganese ions to form a precipitate of manganese dioxide.
Mn2+ + O2 ———–> MnO2
The manganese dioxide is then treated with iodide ions when iodine is liberated in an amount chemically equivalent to the original dissolved oxygen.
MnO2 + 2I– + 4H+ ———–> Mn2+ + I2 + 2H2O
The liberated iodine is determined, usually by titrating it with sodium thiosulphate.
2S2O32- + I2 ———–>S4O62- + 2I– 
Dissolved oxygen (DO) is essential for sustaining the plant and animal life in any aquatic system. For example, warm-water fish requires a minimum DO level of at least 5 mg/l (5 ppm). If the DO level drops below the level necessary to sustain normal life, then the aquatic system is classified as polluted.  The amount of dissolved oxygen in receiving water is the single most important factor determining the waste assimilation capacity of a body of water.  In steam boilers dissolved oxygen causes corrosion and must be removed from feed water before it enters the feed pump. The damage done by oxygen can be especially serious in boilers operating at pressures above about 17 atm. 
TOTAL DISSOLVED SOLIDS:
The amount of dissolved substances in a water sample is referred to as the total dissolved solids (TDS). 
Analytically, the term total solids in wastewater refers to the matter than remains as residue after evaporation and drying at 103 to 105˚C. The total solids are classified as shown in Figure 2.
Figure 2: Solids content of medium strength domestic wastewater. 
The higher the level of TDS the more contaminated a water body may be, whether that be from natural or anthropogenic sources.
Description and Significance:
Colour is an important parameter from the standpoint of aesthetics. Color may be organic or mineral origin. Organic sources include algae, tannins, humic compounds, etc.; inorganic sources are iron and manganese compounds, chemicals and dyes from various industries. Color is measured by comparison with known standards. 
WATER QUALITY STANDARDS:
Table 2: ISI standards for inland surface waters (mg/l) 
Raw water supplied for public use
Tolerance limits for effluents discharged
|Oil and Grease||–||10|
Table 3 summarizes several quality criteria and their standards for drinking water as suggested by the following agencies:
- Indian Council of Medical Research (ICMR)
- World Health Organization (WHO)
- United States Public Health Service (USPHS)
Table 3: Standards for drinking water
|Turbidity (units)Colour (units)Odour||55Nothing disagreeable||2525Nothing disagreeable||55Un-objectionable||2550Un-objectionable||515TO=3||—|
|pH, units||7-8.5||6.5 or 9.2||7-8.5||6.5 or 9.2||–||–|
|Bacteriological:||–||1 coliform per 100ml||–||1 coliform per 100ml||–||1 coliform per 100ml|
aConstituents should not be present in excess of listed concentrations where other more suitable supplies are or can be made available.
bConstituents in excess of the concentrations listed shall constitute grounds for rejection of the supply.
A=Recommended maximum concentrationa (mg/l except where shown otherwise)
B=Maximum permissible concentrationb (mg/l except where shown otherwise) 
- Textbook of Environmental Engineering by P. Venugopala Rao, Eastern Economy Edition
- Environmental Pollution Control Engineering by C.S. Rao, Wiley Eastern
- Encyclopaedia of Environmental Science and Engineering, Fifth Edition
- Dictionary of Water and Waste Management, Elsevier
- Fundamentals of Hydrology by Tim Davie
- Drinking Water Quality : Problems and Solutions by N. F. Gray
Some more terms you can learn: GLOSSARY OF COMMONLY USED TERMS IN WASTEWATER TREATMENT AND SEWAGE POLLUTION CONTROL
Last Edited: January 9 2018