Biodegradability of phenolic compounds as single and mixed substrates by activated sludge

Abstract

The aim of this study was to investigate aerobic biodegradability of phenol, p-cresol,
o-cresol, resorcinol and 5-methylresorcinol by the activated sludge from the Kohtla-Jiirve
wastewater treatment plant (WWTP). Inoculum was acclimated to phenolic compounds, because
the specific WWTP is treating wastewater from the oil shale chemical industry. Biological
treatment of wastewater involves degradation of substrate mixtures rather than degradation of
single compounds. Therefore the biodegradability tests of phenolic compounds as single
substrates as well as mixed substrates at different concentrations were carried out to investigate
their impact on each other's biodegradation.
The respirometric method was used for determination of short-term oxygen demand (BOD,,) and
kinetic parameters characteristic to biodegradation process. The BOD51 of studied phenolic
compounds formed 10-30% of the theoretical oxygen demand, showing the highest and the
lowest values for phenol and o-cresol, respectively. The dependence of oxygen uptake rates on
substrate concentration was investigated and modelled on the basis of Michaelis-Menten and
Haldane kinetics and kinetic parameters, such as the maximum rate of oxygen uptake (V02.max),
the maximum rate of substrate bio-oxidation (Vmax) and half-saturation constant (Ks) were
determined. Among the studied substrates phenol and p-cresol had the highest values of Voi,max,
followed by 5-methylresorcinol. Considering that Ks is related to the affinity of the microbial
community for the given substrate, the activated sludge showed the highest affinity to p-cresol.
The biodegradability of 4 different bi-substrate systems: phenol - p-cresol, phenol - resorcinol,
phenol - 5-methylresorcinol, phenol - o-cresol containing both components at equal
concentrations in the range of 0,005 mM - 0,05 mM, and alternatively containing one substrate at
concentration 0, I mM and the other varied in the above-mentioned range, was also studied, The
V02,max values were found by using Michaelis-Menten kinetics and to take into account the
concentrations of the both substrates, the random sequential mechanism for enzymatic kinetics
was also used for bi-substrate systems. However, for systems containing phenol and p-cresol the
Haldane model appeared to be suitable to describe the biodegradation process due to the substrate
inhibition effect at higher substrate concentration. The results indicated that phenol was more
readily oxidised by the microbial community of activated sludge than cresols or resorcinols.