Biodegradation rates can be expressed in several ways. With good approximation, microbiological biodegradation can be expressed as a Monod equation:
where c is the concentration of the considered compound, Y is the yield of biomass B per unit of c, B is the biomass concentration, gmax is the maximum specific growth rate and km is the half-saturation constant. I c << km the expression is reduced to a first order reaction scheme: dc/dt = —K'Bc. K = gmax/kmY. In water, B is highly dependent on the concentration of suspended matter and in soil B is dependent on the porosity of the soil. B may therefore often be considered a constant. It implies that dc/dt = k c. k has the unit 1/h, 1/24 h, 1/w, or 1/y. If the biological half-life is applied, we get the following: ln 2 = 0.7 = kt1/2 , where t1/2 is the half-life.
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