The inhibitory effects of heavy metals and organic compounds on the net maximum specific growth rate of the autotrophic biomass in activated sludge

A respirometry technique can be applied as an effective method to determine the net maximum specific growth rate of autotrophic biomass under both normal conditions and when inhibition occurs. The net maximum specific growth rate of uninhibited autotrophic biomass, expressed as ((μ) over cap (A) - b(A)), is approximately 0.8 per day [Proceeding of the International Congress on CHISA, Prague, 2002, p. 1]. Several heavy metals and organic compounds have inhibitory effects. Copper (Cu2+) has stronger inhibitory effects than zinc (Zn2+), and inhibits the nitrification process by 50% at 0.08 mg/l [((μ) over cap (A) - b(A)) = 0.4 per day], while the same concentration of Zn2+ establishes 12% inhibition only [((μ) over cap (A)-b(A)) = 0.75 per day]. Inhibition with Cu2+ starts at concentrations above 0.05 mg/l, while this is above 0.3 mg/l for Zn2+. The inhibition of the nitrification process is complete at 1.2 mg/l for both Cu2+ and Zn2+. Among the selected organic compounds tested in the experiments, the degree of inhibition decreases as follows: chlorobenzene > trichloroethylene (TCE) > phenol > ethylbenzene. Chlorobenzene already inhibits the autotrophic biomass at 0.25 mg/l. The nitrification process is totally inhibited by adding 0.75 mg/l of chlorobenzene. TCE has a less inhibitory effect on the nitrification process and 50% inhibition is noticed at 0.75 mg/l TCE. The nitrification process is totally inhibited at 1 mg/l TCE. Phenol inhibits the nitrification for 50% at 3 mg/l. The inhibitory effect of phenol is almost constant in the range 4-10 mg/l and complete inhibition is reached at 50 mg/l. The inhibitory effect of ethylbenzene is 50% at 8 mg/l and the autotrophic biomass is totally inhibited at 50 mg/l.