Biodégradation in wastewater treatment plans is of environmental importance, because we need to know how much the concentration of organic matter is reduced by the treatment. In this context, it is often used to indicate the percentage of the theoretical oxygen demand (ThOD), which is the amount of oxygen used by a complete oxidation. BOD is an abbreviation for biological oxygen demand. It is often used with indication of the considered time, for instance, BOD5, which is the biological oxygen demand for period of 5 days. BOD5 is often used to indicate the concentration of organic matter, because the environmental interest in water is the oxygen depletion for a reasonable period. Five days was chosen, when BOD was introduced in UK more than hundred years ago, because all British rivers reach the sea within 5 days. The stoichiometric equation organic matter + oxygen = carbon dioxide + water shows that for typical municipal wastewater, 1 g of organic matter will require approximately 1.4 g of oxygen. The usual biodegradation rate, k, for the organic matter in municipal wastewater, which is relatively readily biodegraded, is 0.35 1/24h. It means that if we indicate the loading of organic matter expressed by oxygen demand by L, we have
If BOD5 is for instance 150 mgl_1, Lo is 181 mgl _1. Notice that L0 = BODinfinite = ThOD.
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