Biological Treatment Of Palm Oil Mill Effluent (Pome) Using An Up-Flow Anaerobic Sludge Fixed Film (Uasff) Bioreactor
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Date
2006-12
Authors
Lorestani, Ali Akbar Zinatizadeh
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Abstract
Up-flow anaerobic sludge fixed film (UASFF) bioreactor is a modern
bioreactor and was used for the rapid biotransformation of organic matter to
methane with the help of granulated microbial aggregates. A lab scale UASFF
bioreactor (3.65 lit) with an external settling tank was successfully designed and
operated for palm oil mill effluent (POME) treatment. The bioreactor was
developed in order to shorten the start-up period at low hydraulic retention time
(HRT). The organic loading was gradually increased from 2.67 to 23.15 g
COD/l.d during this period. Granular sludge was found to develop rapidly within
20 days with an increase in size of granules from an initial pinpoint size to about
2 mm. The anaerobic digestion of POME was modeled and analyzed with two
variables i.e. HRT and CODin using response surface methodology (RSM). The
region of exploration for digestion of POME was taken as the area enclosed by
HRT (1 to 6 days) and CODin (5260 to 34725 mgll) boundaries. An increase in
the variables resulted in a decrease in COO removal, SRT and SRF but an
increase in COD removal rate, VFA/Alk, CO2 percentage in biogas and
methane production rate. The proposed kinetic equation and a simplified
Monod's model were successfully employed to describe the kinetics of POME
anaerobic digestion at organic loading rates in the range of 0.88 to 34.73 g
COD/l.d. The methane yields obtained were between 0.287 to 0.348 I CHJg
CODremoved. Biokinetic coefficients i.e. apparent half-velocity constant (A), halfvelocity
constant (Ks), maximum specific microbial growth rate (~m), methane
yield constant (YM), and biomass growth yield constant (Yx) were also evaluated.
The apparent rate constants, K, calculated by simplified Monod model were in
the range of 2.9 to 7.4 I CH4/g COD.d. At different influent COD concentrations,
K values showed a linear relationship with variations in VSS content in the
reactor. In a batch POME digestion, 275 mg CaC03 bicarbonate alkalinity was
produced per 1000 mg CODremoved. About 95 % COD removal was achieved
within 72 h with an initial COO removal rate of 3.5 g COO/l.d. A consecutive
reaction kinetic model employed to simulate the data on sludge activity in batch
experiment showed good fit to the experimental results (R2 > 0.93). The slowest
step was modeled to be the acidification step with rate constants between 0.015
to 0.083 h-1 while those of the methanogenic step were between 0.218 to 0.361
h-1
. Long term performance of the UASFF reactor was investigated with raw
POME as feed at a HRT of 3 days and an influent COD concentration of 44300
mg/l. Physical and chemical pretreatment methods were also conducted.
Experiments on the pretreated POME digestion were conducted based on a
central composite face-centered design (CCFD) and modeled using response
surface methodology (RSM) with two.operating variables i.e. feed flow rate (QF)
and superficial up-flow velocity (Vup). The performance of the reactor fed with
the pre-settled (settling for 3 h) and chemically pretreated (after flocculation)
POME was compared. The optimum conditions for the digestion of the presettled
and chemically pre-treated POME were at QF of 1.65 lid, Vup of 0.6 m/h
and OF of 2.45 lid and Vup of 0.75 m/h, respectively. The experimental findings
were in close agreement with the model prediction. The characterization on the
granular sludge developed in the UASFF bioreactor at various operating
conditions showed that they predominantly consisted of densely packed rod
(Methanosaeta-like microganism) and cocci shaped (Methanosarsina) microorganisms.
Description
Keywords
Up-flow anaerobic sludge , Used for the rapid biotransformation