Monday, April 13, 2020

CHEMICAL CHARACTERISTICS OF SEWAGE - BOD, COD, Nutrients, DO - In sanitary sewage about 75 % of suspended solids and 40% of filterable solids are organic. These solids are derived from both animals, plant and humans. Organic compounds usually consist of C; H; O; N along with S; P and Iron. The organic substances found in sewage are Protein (40-60%); Carbohydrates (25-50%), fats and oils (10%). Along with these organic compounds small amount of synthetic organic compounds like VOCs, pesticides, insecticides, Organic Priority Pollutants are also present in sewage. Sewage also contain inorganic substances. Tests like BOD, COD, Nitrogen, phosphorus, alkalinity etc. give the chemical characteristics of sewage. When biodegradable organic matter is released into a water body, microorganisms feed on the wastes, breaking them into simpler organic and inorganic substances. When this decomposition occurs in aerobic environment the process produces non-objectionable, stable end products like CO2, SO4, PO4 and NO3. When sufficient O2 is not available Anaerobic decomposition occurs by different microorganisms. They produce end products that can be highly objectionable, including H2S; NH3 and CH4. Such products are usually unstable. Bacteria placed in contact with organic matter will utilize it as food source.

Chemical Characteristics of Sewage
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Chemical Characteristics of Sewage
BOD, COD, Nutrients, DO
By: Haseeb Jamal 




Sewerage characteristics can be divided into three broad categories:
·      Physical (Temperature, colour, smell, solids)
·      Chemical (BOD, COD, Nutrients and dissolved solids; and
·         Biological
Chemical Characteristics of Sewage (Waste Water)
·      In sanitary sewage about 75 % of suspended solids and 40% of filterable solids are organic.
·      These solids are derived from both animals, plant and humans. Organic compounds usually consist of C; H; O; N along with S; P and Iron.
·      The organic substances found in sewage are Protein (40-60%); Carbohydrates (25-50%), fats and oils (10%).
·      Along with these organic compounds small amount of synthetic organic compounds like VOCs, pesticides, insecticides, Organic Priority Pollutants are also present in sewage.
·      Sewage also contain inorganic substances.
·      Tests like BOD, COD, Nitrogen, phosphorus, alkalinity etc. give the chemical characteristics of sewage.
BOD (Biochemical Oxygen Demand):
When biodegradable organic matter is released into a water body, microorganisms feed on the wastes, breaking them into simpler organic and inorganic substances.
When this decomposition occurs in aerobic environment the process produces non-objectionable, stable end products like CO2, SO4, PO4 and NO3. 
A simplified form of Aerobic decomposition is 
O.M + O2 + Microorganisms
CO2 + H2O + C5 H7 NO2 (New Cells) = stable products like NO3; PO4; NO3)
When sufficient O2 is not available Anaerobic decomposition occurs by different microorganisms.
They produce end products that can be highly objectionable, including H2S; NH3 and CH4.
The reaction is O.M + Microorganisms
CO2 + H2O + C5 H7 NO2 (New Cells) = Unstable products (NH3; H2S; CH4
·      Such products are usually unstable.
·      Bacteria placed in contact with organic matter will utilize it as food source.
·      In the utilization of the organic material it will eventually be oxidized to stable end products such as CO2 and H2O etc.
·      The amount of oxygen required by the bacteria to oxidize the organic matter present in sewage to stable end products is known as biochemical oxygen demand.
·      BODu is the maximum amount of oxygen usage by microorganisms over a long period of time. A good measure of maximum bioavailability.
·      BOD5 is the amount of oxygen consumed (in mg/L) over a 5-day period at 20 oC (in the dark). BOD5 is a measure of the bioavailability over a 5-day period under controlled conditions.
Carbonaceous biochemical oxygen demand or CBOD is a method defined test measured by the depletion of dissolved oxygen by biological organisms in a body of water in which the contribution from nitrogenous bacteria has been suppressed.
CBOD is a method defined parameter is widely used as an indication of the pollutant removal from wastewater. It is listed as a conventional pollutant in the U.S. Clean Water Act.
Chemical Oxygen Demand
·      In addition to CBOD and NBOD measured, there are two other indicators to describe the oxygen demands of wastewater. They are Chemical oxygen demand and theoretical oxygen demand.
·      The biodegradable organic matters are degraded completely by microorganisms either by CBOD or NBOD.
·      There are some organic matters like cellulose, phenols, benzene and tannic acid which are resistant to biodegradation. Similarly, other organic matters like pesticides, insecticides and various industrial chemicals are non-biodegradable and they are toxic to microorganisms.
·      The COD is a measured quantity that does not depend on microorganisms. To calculate the concentration of oxygen for non-biodegradable materials a strong oxidizing agent known as potassium dichromate will be used.
·     The reaction is Organic matter (CaHbOc) + Cr2O7-2+ H2O –à Cr +3 + CO2 + H2O
·      The COD test is much quicker than BOD test, but it does not distinguish between the biodegradable and non-biodegradable organic matter.
The measured COD is usually more than BOD if there is non-biodegradable impurity present. If all are the biodegradable organic matter, then COD remains the same as that of BOD. Roughly the BOD/COD is 0.4 to 0.8.
Theoretical Oxygen Demand (TheoD):
Organic matter of animal or vegetable origin in wastewater is generally a combination of carbon, hydrogen, oxygen, nitrogen and other elements.
If the chemical composition of an organic matter is known then the amount of oxygen required to oxidize it to carbon dioxide and water can be calculated using stoichiometry.
This amount of oxygen is known as Theoretical Oxygen Demand.
If that oxidation is carried out by bacteria then it is BOD, if by chemical process then it is COD. If a combination of both then it is TheoD.

Haseeb Jamal. I am a Civil Engineer, graduated from University of Engineering and Technology, PeshawarPakistan in 2010. I also have a PG-Diploma in Disaster Management and MS in Urban Infrastructure Engineering (In Progress). My expertise include civil related softwares like AutoCAD, SAP2000, MS Project, Primavera, MS Office and GIS. My technical skills include project management, monitoring and evaluation, structural assessment, disaster risk management, Quantity survey, land survey, material testing, site management and technical writing. I am trained in writing project progress reports as well as proposals and concept papers. I have also received advanced training on surveying, proposal writing, Monitoring and Evaluation of projects as well as organizations.
I have worked as Project Engineer at National Research and Development Foundation, Peshawar and CENCON Associates. I also worked with Spectra Engineering Solutions as Senior Civil Engineer in monitoring of World Bank and UNDP funded projects all over Khyber Pakhtunkhwa and FATA. Currently, I am working as Deputy Manager Development at NayaTel, Peshawar.
Chemical Characteristics of Sewage

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