Reaction water, a high-strength COD (chemical oxygen demand) petrochemical effluent,
is generated during the Fischer-Tropsch reaction in the SASOL Synthol process at
SASOL SynFuels, Secunda, South Africa. Distillation of the reaction water to remove
non- and oxygenated hydrocarbons yields approximately 25 - 30 ML/d of an organic
(carboxylic) acid-enriched stream (average COD of 16 000 mg/L) containing primarily
C2 – C5 organic acids, light oils, aldehydes, ketones, cresols and phenols. Together with
the Oily sewer water (API) and Stripped Gas Liquor (SGL) process streams, this process
effluent is currently treated in ten dedicated activated sludge basins. However, the
successful operation of these activated sludge systems has proven to be difficult with low
organic loading rates (3.5 kg COD/m3.d) low COD removal efficiencies (<80 %) and
high specific air requirements (60 - 75 m3 air/kg CODrem). It is hypothesised that these
operational difficulties can be attributed to organic shock loadings, variation in
volumetric and hydraulic loadings, as well as variations in the composition of the various
process streams being treated. Due to the fact that the Fischer-Tropsch (Synthol) reaction
water constitutes 70 % of the COD load on the activated sludge systems, alternative
processes to improve the treatment cost and efficiency of the Fischer-Tropsch acid stream
are being investigated. Various studies evaluating the aerobic and anaerobic treatment of
Fischer-Tropsch reaction water alone in suspended growth wastewater treatment systems
have proven unsuccessful. High rate fixed-film processes or biofilm reactors, of which
the fluidised-bed reactors are considered to he one of the most effective and promising
processes for the treatment of high-strength industrial wastewaters, could he a suitable
alternative. The primary aim of this study was to evaluate the suitability of biological
fluidised-bed reactors (BFBRs) for the treatment of Fischer-Tropsch reaction water.
During this study, the use of aerobic and anaerobic biological fluidised-bed reactors
(BFBR), using sand and granular activated carbon (GAC) as support matrices, were
evaluated for the treatment of a synthetic effluent analogous to the Fischer-Tropsch
reaction water stream. After inoculation, the reactors were operated in batch mode for 10
days at a bed height expansion of 30% and a temperature of 30 ºC to facilitate biofilm
formation on the various support matrices. This was followed by continuous operation of
the reactors at hydraulic retention times (HRTs) of 2 days. While the COD of the
influent and subsequent organic loading rate (OLR) was incrementally increased from 1
600 mg/L to a maximum of 20 000 mg/L and 18 000 mg/L for the aerobic and anaerobic
reactors, respectively. Once the maximum influent COD concentration had been
achieved the OLR was further increased by decreasing the HRTs of the aerobic and
anaerobic reactors to 24h and 8h, and 36h, 24h and 19h, respectively. The dissolved
O2 concentration in the main reactor columns of the aerobic reactors was constantly
maintained at 0.50 mg/L.
Chemical Oxygen Demand (COD) removal efficiencies in excess of 80 % at OLR of up
to 30 kg COD/m3.d were achieved in the aerobic BFBRs using both sand and GAC as
support matrices. Specific air requirements were calculated to be approximately 35 and
41 m3 air/kg CODrem for the BFBRs using sand and GAC as support matrices,
respectively. The oxygen transfer efficiency was calculated to be approximately 5.4 %.
At high OLR (> 15 kg COD/m3.d) significant problems were experienced with plugging
and subsequent channelling in the BFBR using GAC as support matrix and the reactor had
to be backwashed frequently in order to remove excess biomass. Despite these backwash
procedures, COD removal efficiencies recovered to previous levels within 24 hours. In
contrast, no significant problems were encountered with plug formation and channelling
in the BFBR using sand as support matrix. In general the overall reactor performance
and COD removal efficiency of the aerobic BFBR using sand as support matrix was more
stable and consistent than the BFBR using GAC as support matrix. This BFBR was also
more resilient to variations in operational conditions, such as the lowering of the
hydraulic retention times and changes in the influent pH. Both aerobic reactors displayed
high resilience and COD removal efficiencies in excess of 80 % were achieved during
shock loadings. However, both reactors were highly sensitive to changes in pH and any
decrease in pH below the pKa values of the volatile fatty acids in the influent (pKa of
acetic acid = 4.76) resulted in significant reductions in COD removal efficiencies.
Maintenance of reactor pH above 5.0 was thus an essential facet of reactor operation.
It has been reported that the VFA/alkalinity ratio can be used to assess the stability of
biological reactors. The VFA/alkalinity ratios of the aerobic BFBRs containing sand and
GAC as support matrices were stable (VFNalkalinity ratios of < 0.3 - 0.4) until the OLR
increased above 10 kg/m3.d. At OLRs higher than 10 kg/m3.d the VFA/alkalinity ratios
in the BFBR using sand support matrix increased to 4, above the failure limit value of 0.3
- 0.4. In contrast the VFA/alkalinity ratios of the BFBR using GAC support matrix
remained stable until an OLR of 15 kg/m3.d was obtained, where the VFA/alkalinity
ratios then increased to > 3. Towards the end of the study when an OLR of
approximately 25 kg/m3.d was obtained the VFA/alkalinity ratios of both the BFBRs
using sand and GAC as support matrices increased to 9 and 6 respectively, indicating the
decrease in reactor stability and acidification of the process. Total solid (TS) and volatile
solid (VS) concentrations in the aerobic BFBRs were initially high and decreased over
time. While the total suspended solids (TSS) and volatile suspended solids (VSS)
concentrations were initially low and increased over time as the OLR was increased, this
is thought to be as a result of decreased HRT leading to biomass washout.
The anaerobic BFBR using sand as support matrix never stabilised and COD removal
efficiency remained very low (< 30 %), possibly due to the high levels of shear forces.
Further studies concerning the use of sand as support matrix were subsequently
terminated. An average COD removal efficiency of approximately 60 % was achieved in
the anaerobic BFBR using GAC as a support matrix at organic loading rates lower than
10 kg COD/m3.d. The removal efficiency gradually decreased to 50 % as organic loading
rates were increased to 20 kg COD/m3.d. At OLRs of 20 kg COD/m3.d, the biogas and
methane yields of the anaerobic BFBR using GAC as support matrix were determined to
be approximately 0.38 m3 biogas/kg CODrem (0.3 m3 biogas/m3reactor vol.d), and 0.20 m3
CH4/kg CODrem (0.23 m3 CH4/m3reactor vol.d), respectively. This value is 57 % of the
theoretical maximum methane yield attainable (3.5 m3 CH4/kg CODrem). The methane
yield increased as the OLR increased, however, when the OLR reached 8 kg/m3.d the
methane yield levelled off and remained constant at approximately 2 m3 CH4/m3reactor vol.d.
Although the methane content of the biogas was initially very low (< 30 %), the methane
content gradually increased to 60 % at OLRs of 20 kg COD/m3.d. The anaerobic BFBR
using GAC as support matrix determined that as the OLR increased (>12 kg/m3.d), the
VFA/alkalinity ratio increased to approximately 5, this is indicative of the decrease in
stability and acidification of the process. The anaerobic BFBR using GAC as support
matrix experienced no problems with plug formation and channelling. This is due to the
lower biomass production by anaerobic microorganisms than in the aerobic reactors. The
TS and VS concentrations were lower than the aerobic concentrations but followed the
same trend of decreasing over time, while the TSS and VSS concentrations increased due
to decreased HRTs. The anaerobic BFBR was sensitive to dramatic variations in organic
loading rates, pH and COD removal efficiencies decreased significantly after any shock
loadings.
Compared to the activated sludge systems currently being used for the biological
treatment of Fischer-Tropsch reaction water at SASOL SynFuels, Secunda, South Africa,
a seven-fold increase in OLR and a 55 % reduction in the specific air requirement was
achieved using the aerobic BFBRs. The methane produced could also be used as an
alternative source of energy. It is, however, evident that the support matrix has a
significant influence on reactor performance. Excellent results were achieved using sand
and GAC as support matrices in the aerobic and anaerobic BFBRs, respectively. It is
thus recommended that future research be conducted on the optimisation of the use of
aerobic and anaerobic BFBRs using these support matrices.
Based on the results obtained from this study, it can be concluded that both aerobic and
anaerobic treatment of a synthetic effluent analogous to the Fischer-Tropsch reaction
water as generated by SASOL in the Fischer-Tropsch Synthol process were successful
and that the application of fluidised-bed reactors (attached growth systems) could serve
as a feasible alternative technology when compared to the current activated sludge
treatment systems (suspended growth) currently used.
Keywords: aerobic treatment, anaerobic treatment, biological fluidised-bed reactors,
petrochemical effluent, Fischer-Tropsch reaction water, industrial wastewater. / Thesis (M. Omgewingswetenskappe)--North-West University, Potchefstroom Campus, 2004.
| Identifer | oai:union.ndltd.org:NWUBOLOKA1/oai:dspace.nwu.ac.za:10394/452 |
| Date | January 2004 |
| Creators | Swabey, Katharine Gaenor Aske |
| Publisher | North-West University |
| Source Sets | North-West University |
| Detected Language | English |
| Type | Thesis |
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