Tannery Wastewater Treatment using Alkaliphilic Sediment Inoculum in Anoxic-Oxic Bioprocess
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Date
2011-04
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Addis Ababa University
Abstract
Tanning is one of the oldest industries known by its voluminous wastewater and the use of huge
type and amount of chemicals. In Ethiopia there are over twenty tanning industries. Most of
which release their wastes without pretreatment. TWW is known particularly by its high
organic load, nitrogenous, sulfide and chromium contents and by its salinity and alkalinity. In
the presence of these toxic substances and saline and alkaline conditions, TWW treatment by
biological methods using conventional neutrophilic organisms is hardly efficient. The objective
of this study was to treat TWW by naturally alkaliphilic sludge obtained from Soda Lake Chitu
water and sediment using lab-scale anoxic-oxic activated sludge treatment system and to
evaluate the adaptability of the inoculated sludge to the toxic tannery wastes. The efficacy of
the alkaline sludge and performance of the reactors was evaluated by analyzing COD, TN,
NH4 +, S2-, SO4 2-, NO3 -, Crtot, MLVSS, MLSS and pH of the raw and treated TWW. The system
was operated on continuous basis, but physicochemical data was obtained at specific OLR of
1.92gCOD/L/d and 1.64gCOD/L/d at feed 1 and feed 2 respectively. The system was able to
remove 99% S2-, 93%TN and 89% NO3 - at feed 1 and 96% NH4 +, 93% COD and 92% Crtot at
feed 2 from raw TWW. The result showed that at relatively high OLR there was slightly better
removal of sulfides, TN and nitrate but higher removal of COD, ammonia and chromium was
observed at comparatively lower OLR. In addition, the treatment process was effective in
removing the color and notorious odor of TWW. Among the key factors of the activated sludge
in treating the nitrogenous TWW are denitrifying bacteria. Adaptability of inoculated
microorganisms to the TWW was evaluated by isolating denitrifying bacteria from the
inoculant lake sediment and from the steady state treatment system. Comparative
characterizations of these isolates showed that there was little difference in their pH optima
(9.5-10.5) and range, chromium tolerance (250-2500mg/l) and preferences to energy sources.
Also the differences in their cell wall chemistry (<3%), colony morphology, starch hydrolyzing
ability and catalase reaction were minimal. But remarkable difference was evident in nitrate
reducing ability and salt tolerance. Denitrifying isolates obtained from the treatment system
were found to be complete nitrate reducers, incomplete nitrate reducers, exclusively nitrite
reducers and both nitrite and nitrate reducers, but isolates from the inoculum were only
complete and incomplete nitrate reducers. Presence of exclusively nitrite reducers, both nitrite
and nitrate reducers and tolerance to high salinity by the denitrifying isolates obtained from
the treatment system (DS) showed a shift in denitrification function by the inoculated microbial
community. The shift in denitrification function and tolerance to high salinity conditions by the
DS isolates was supposed to be associated to the adverse nature of TWW and strict anoxic
condition to which the bacteria adapt the new environment than resisting it.
Key words: Tannery wastewater; Lake Chitu, alkaliphiles; denitrifying bacteria; lab scale
activated sludge.
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Keywords
: Tannery wastewater; Lake Chitu, alkaliphiles; denitrifying bacteria; lab scale activated sludge