what is biological oxygen demand and chemical oxygen demand ?
Answers
Biologocal Oxygen Demand) means the amount of
(in mg/l) microorganisms like bacteria need to
e organic polution (sugars, fat, proteins, ...). Note
t all polution can be removed (eaten) by bacteria.
lue in polluted water is normally higher than the
ater. Increased BOD can be resulted due to tic sewage, petroleum residues and wastes of s and crops.
hemical Oxygen Demand) is the amount of
required to degenerate all poution in a chemical
y adding oxidising agents and heating). In general
Cemical destruction you can remove more polution
ith the biological way.
is only a measurement of consumed oxygen by
microorganisms to decompose or oxidize
matter and COD refers the requirement of ed oxygen for the oxidation of organic and ic constituents both. Hence COD must be greater
OD.
the recombination of oxygen atoms and isotopic exchange with molecular oxygen. The simplest heterogeneous catalytic process involving molecular oxygen is the dissociation of O2 into atoms together with the reverse reaction, the recombination of O-atoms. As the dissociation reaction is highly endothermic, temperatures greater than 1000° C are required before appreciable rates of reaction are observed and this restricts the range of catalysts that can be studied. At low pressures and at temperatures of 1000°-1500° C, the reaction proceeds on platinum via the dissociative oxygen chemisorption of oxygen and the desorption of O-atoms. The surface coverage with atomic oxygen is assumed to be low on account of the high temperature. The interaction of atoms or radicals is known to proceed rapidly even without a catalyst, but the large amount of heat evolved in reaction favors decomposition of the product. Solid catalysts, as third bodies, adsorb the energy released and dissipate it, stabilizing the O2 molecule formed. At the same time, a catalyst takes part in intermediate chemical reactions with the O-atoms and, as a result, the specificity of different surfaces is manifested.