Wastewaters from fossil fuel refining, pharmaceuticals, and pesticides are the main sources of
 phenolic compounds. Those with more complex structures are often more toxic than the simple
 phenol, and yet little is known about the treatment of wastewater containing a mixture of
 phenolic pollutants. . The present study was aimed at assessing the efficacy of UASB and SBR
 for the treatment of mixtures of phenolics compounds.
 The experiments were conducted in a laboratory scale UASB reactor, which had a volume of
 1.414 l. A gas-liquid solid separator, GLSS was provided at the top of the reactor. The reactor
 was operated at constant HRT of 24 h throughout the study. The reactor was seeded with
 digested sewage sludge obtained from Okhla Sewage Treatment Plant, New Delhi, India.
 Initially the microbial culture was acclimatized to phenol concentration of 600 mg l-1 in UASB.
 Thereafter, different phenol: m-cresols ratio were introduced, 10:1, 4.5:1, 2.7:1, 1.75:1 and
 1.2:1 and the performance of the reactor was evaluated under each case. The second
 experiment was conducted in a laboratory scale SBR reactor with a working volume of 1.4 l.
 The effluent was drawn from a volumetric exchange ratio of 50%. A fine bubble aerator in the
 bottom of the column was used to introduce air. The reactor was seeded with aerobic digested
 sludge obtained from Star Paper Mill, Saharanpur, UP, India. A constant HRT of 12h was kept
 throughout the study. Reactor was operated sequentially with fill, react, settle and draw periods
 for a cycle of 6h. In order to establish a viable biomass and minimize any potential toxic effects
 due to presence of phenolic compounds, the sludge was fed with phenol as batch culture up to
 1000 mg l-1 concentration. After acclimatization, different phenol: m cresol ratio were
 introduced 6.5:1, 2:1, 1.1:1, 0.5:1 and the performance of the reactor was evaluated under
 each case. A start up period of 40 days was required to acclimatize the anaerobic bacterial at
 HRT of 24 hrs for phenol concentration of 200 mg l-1. UASB reactor successfully biodegraded
 phenol and m-cresol up to a maximum ratio (1.25:1) (300: 250 mg l-1) with 80% efficiency. A start up period of 30 days was required to acclimatize aerobic bacteria with phenol to
 concentration up to 1000 mg l-1 as batch culture. The maximum phenol and m-cresol up to (1.1: 1)
 (800: 700 mg l-1) was successfully treated with efficiency of 95% in SBR.The results indicates
 that anaerobic treatment by UASB and aerobic treatment by SBR can be successfully used for
 phenol/cresol mixture, representative of major substrates in chemical and petrochemical
 wastewater and the results shows proper acclimatization period is essential for the degradation
 of m - cresol and phenol. Moreover, SBR was found as a better alternative than UASB reactor
 as it is more efficient and higher concentration of m cresols can be successfully degraded.