| Summary: | There is increasing interest in the treatment and reuse of the sewered portion of the
evaporator condensate from krafl pulp mills. The treated evaporator condensate could be
used in brown stock washing, recausticizing and bleaching, instead of clean water. In
addition to reducing the contarninant load to the existing combined mill effluent
treatment system, reducing the raw water requirements and potentially reducing the
impact of discharging the treated condensate to the environment, reusing the condensate
could also result in significant energy savings if the heat content of the evaporator
condensate can be recovered. Also, some legislation proposes a number of incentives for
treating and reusing the condensate as process water.
Methanol and reduced sulphur compounds (RSC) were identified as the primary
contaminants of concern contained in evaporator condensate. These contaminants are of
concern primarily because they are hazardous air pollutants (HAP) and/or foul odorous
compounds. Reusing evaporator condensate in a pulp mill without treatment could result
in the subsequent emission of HAP and odorous compounds and generate unpleasant or
even hazardous working conditions for mill staff. Some trace organic contaminants
contained in evaporator condensate are also of concern primarily because they could
disrupt the pulping process and impact pulp quality. A number of conventional
technologies have been considered for the treatment of evaporator condensate for reuse.
However, the relatively poof treatment efficiencies and/or high costs associated with
these technologies provided incentives to investigate and develop a better treatment
technology. A high temperature membrane bioreactor (MBR) was selected as the most
promising novel technology for the treatment of evaporator condensate for reuse.
A preliminary study indicated that the biological removal of methanol from synthetic
evaporator condensate using a high temperature MBR was feasible. The results
suggested that the specific methanol utilization coefficient was higher during high
temperature biological treatment using an MBR, than in a conventional biological
treatment system.
However, simultaneous biological removal of methanol and RSC from synthetic
condensate using a high temperature MBR was not feasible. A low operating pH was
required for biological oxidation of RSC to occur at elevated temperatures. In addition,
biological removal of methanol was significantly inhibited at the pH required for
biological RSC removal to occur. Therefore, a two stage system, with the first stage
operating at an acidic pH and the second stage operating at a neutral pH, would be
required. This would add significantly to the cost of a biological system to treat
evaporator condensate for reuse. Even at an optimal pH for the growth of sulphuroxidizing
microorganisms, stripping due to the aeration system accounted for
approximately 50 % of the RSC removed from the MBR. The results also indicated that
the stability of a mixed microbial culture at a low pH is questionable. For these reasons,
the biological oxidation of RSC in a high temperature MBR was not considered to be
feasible and simultaneous biological removal of methanol and RSC was not further
investigated.
Further investigations revealed that it was possible to biologically remove methanol from
synthetic evaporator condensate using a high temperature MBR, over the entire expected
range of temperatures for evaporator condensate (55 to 70 °C). However, the operating
temperature exerted a significant impact on methanol removal kinetics. A maximum
specific methanol utilization coefficient and a maximum specific growth coefficient of
approximately 0.84 ± 0.08 /day and 0.11 ± 0.011 /day, respectively, were observed at an
operating temperature of 60 °C. Above 60 °C, both the specific methanol utilization
coefficient and the specific growth coefficient declined sharply, suggesting that at high
operating temperatures, the inactivating effect of temperature on the growth-limiting
enzyme must be considered. A relatively simple model was proposed and used to
accurately estimate the effect of high temperatures on methanol removal kinetics in an
MBR over the temperature range investigated. Based on the model, the optimal
operating temperature for the biological removal of methanol by a mixed microbial
culture was determined to be approximately 60 °C. These results indicated that it is not
only possible to operate an MBR at high temperatures, but also that a higher specific
methanol utilization coefficient can be achieved at a higher operating temperature.
However, care may need to be taken not to exceed the critical operating temperature of
60 °C.
The operating temperature was also observed to have a significant effect on the observed
microbial growth yield in the MBR. At increasing operating temperatures, a larger
fraction of the methanol consumed was converted to energy, reducing the observed
growth yield. These results indicate that at high temperatures, less excess sludge may be
produced, potentially resulting in lower waste sludge handling and disposal costs.
The specific methanol utilization coefficient measured during the treatment of real
evaporator condensate was lower than that observed when treating synthetic evaporator
condensate. The difference was not due to a direct toxic effect from compounds present
in the real evaporator condensate matrix. The reduction was attributed to a shift in the
composition of the microbial community present in the MBR. The shift resulted from
competition between methylotrophic and partial-methylotrophic microorganisms for the
available methanol. Microorganisms that were not capable of growth on methanol as sole
substrate, but were capable of consuming methanol in the presence of other organic
substrates, were defined as partial-methylotrophic microorganisms. The partialmethylotrophic
microorganisms exhibited a lower specific methanol utilization
coefficient (0.29/day) than the methylotrophic microorganisms (0.84/day), resulting in a
lower overall specific methanol utilization coefficient for the mixed microbial culture of
0.59 ± 0.11 /day. Nonetheless, the specific methanol utilization coefficient observed at
60 °C was still more than 30 % higher than previously reported values from other studies
of biological treatment of condensate at much lower temperatures.
High temperature biological treatment using an MBR also successfully removed the nonmethanolic
contaminants of concern contained in evaporator condensate. Over 99 % of
the RSC contained in the evaporator condensate was removed during high temperature
treatment using an MBR. The concentrations of hydrogen sulphide, methyl mercaptan,
dimethyl sulphide and dimethyl sulphide in the evaporator condensate werereduced to
below detection limits (approximately 0.4 mg/L) during high temperature operation using
an MBR. Approximately 93 % of the organic compounds, measured as TOC, contained
in the evaporator condensate could be removed. The concentration of TOC in the
evaporator condensate was reduced from 504 ± 137 mg/L to 52 ± 3.6 mg/L. Over 78 %
of the reduction in TOC was due to the removal of methanol.
Based on assumed removal efficiencies of 99, 90 and 99 % for methanol, TOC and RSC
(as hydrogen sulphide and methyl mercaptan), respectively, as well as the characteristics
of the evaporator condensate from a local kraft pulp mill, a conceptual design for a fullscale,
high temperature MBR to treat an evaporator condensate for reuse was developed.
Capital and operating costs were estimated and compared to the costs for a steam
stripping system. Depending on the type of ultrafiltration membranes used in the MBR
design, the capital cost for the MBR system was 40 to 50 % less than the capital cost of a
steam stripping system capable of achieving comparable contaminant removal
efficiencies. The operating costs for the MBR system were also approximately 50 % less
than the operating costs for a steam stripping system. Therefore, high temperature
biological treatment is not only technically feasible, but is also appears to be
economically more attractive than the currently favored treatment technology (i.e. steam
stripping). === Applied Science, Faculty of === Civil Engineering, Department of === Graduate
|
|---|
