2.6. Calculation of rate constants
Substrate removal rates after spiking were calculated separately for concentration ranges where degradation was not limited by substrate concentration (zero order), as well as for lower concentration ranges with substrate limiting the degradation process ( first order) (Henze et al., 2002). Data points between 1 and 2 mg N/l were consequently excluded from the calculation of rate constants to avoid inclusion of data points from the transition zone (half order) between zero- and first order ranges in the calculations.
Data points above 2 mg N/l were used to assess the substrate independent removal rates in ammonia and nitrite trials. Zero order removal rates were calculated using least square linear regression in the form of y = ax + b, where y is the substrate concentration in the bulk water, x is the time after spiking, and a (slope) and b are constants. From the slope of the regression line, the total surface area of the biofilter media, and the total water volume of the biofilter system, the zero order removal rate constant k (g/m2 /d) was
0a calculated.