Coke oven wastewater treatment by two activated sludge systems

In this study two bench scale activated sludge systems were used, a CSTR and an SBR for
the treatment of coke – oven wastewater. Both reactors were inoculated with activated sludge
from a municipal wastewater treatment plant. At the first stages of operation, reactors were
feed by a mixture of municipal wastewater and synthetic wastewater. Full acclimatization of
the microorganisms to synthetic wastewater was achieved in 60 days. The operation of the
reactors was divided into three distinct periods. The first period was characterized by the
treatment of high organic but non-toxic synthetic wastewater. During this period COD and
BOD5 removal efficiencies reached 95 and 98% respectively, in both reactors. Nutrient
removal was better in the SBR reactor rather than in the CSTR. In the second period phenol
was added in concentrations up to 300 mg l-1. Degradation of phenol started about the 20th
day after its introduction to the reactors. In this period no effects of phenol to nutrient removal
were observed, whereas the removal efficiency of organic matter in both reactors was slightly
decreased. During the third period phenol concentrations of the influent were gradually
increased to 1000 mg l-1, while cyanide and thiocyanite were added to the influent
composition to concentrations reaching concentrations of 20 and 250 mg l-1 respectively. The
composition of the influent of this period was a full assimilation of coke oven wastewater.
Introduction of increased phenol concentrations along with cyanide compounds initiated
irreversible effects on the activated sludge microfauna of the CSTR causing inherent
problems to the treatment process, while SBR showed greater capacity to withstand and
degrade toxic compounds. The beginning of this period was characterized by decreased
settleability of the suspended solids as well as decrease of organic matter and nutrient
removal efficiencies. Monitoring of the effluent characteristics during this period reported over
90% for organic load, 85% of nutrient removal and over 90% of phenol and cyanide removal
in SBR, while the removal efficiencies for the CSTR were 75, 65 and 80% respectively.