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13. Treatment of furfural waste water
92
13. Treatment of Furfural Waste Water The waste water of all furfural plants contains some carboxylic acids, with acetic acid being the principal load, ranging from 1 to 5 percent by weight, as well as some furfural, in concentrations up to 600 ppm. The furfural concentration depends on the quality of the first distillation column, with good columns going down to 50 ppm. If the concentration of acetic acid is too low to justify an acid recovery plant, it is common practice, wherever possible, to simply discharge the waste water into a river or the sea. However, with an increasing awareness of environmental issues, and with a tightening legislation in this regard, it is unlikely that such discharges will be permitted for much longer. Against this background, anaerobic digestion of the waste water has become the procedure of choice for such cases. Application of this process to furfural waste water was studied in detail by Wirtz and Dague [44]. Contrary to widespread erroneous belief, these authors established beyond any doubt that at the concentrations occurring in furfural waste water, the toxicity of furfural for some microorganisms does not apply to methane bacteria. They not only thrive on acetic acid but eat up furfural as well. For the low concentrations of acetic acid in furfural waste water, these bacteria, known to decompose acetic acid to methane and carbon dioxide according to the reaction CH3COOH ---->--->CH4 + CO2, are employed as "filters" defined as plastic packings on which the methane bacteria are retained. The study by Wirtz and Dague was made on upflow "filters" of fully and partially packed design as shown in Figure 46. The packing consisted of 1.6 cm FLEXIRINGS manufactured by KOCH ENGINEERING of Wichita, Kansas. The void space of these rings is 94 %. While the right-hand "filter" was fully packed, the left-hand "filter" was packed only in the upper two thirds of the reactor volume. As in start-up both reactors were filled completely with biomass (microorganisms from a municipal waste water treatment plant), the left-hand reactor had only a fluid bed of biomass (but no packing) in the lower third of its volume. For this reason, it was called "Upflow Blanket Filter" (UBF) to indicate that it was a
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Figure 46. The Experimental Setup used by Wirtz and Dague. 1 - Partially packed reactor (Upflow Blanket Filter) 2 - Fully packed reactor 3 - Separation tank 4 - Bubble flask 5 - Scrubber 6 - Sampling port 7 - Gasometer
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94
hybrid between the normal fully packed "filter" (right-hand reactor) and a now obsolete "Upflow Anaerobic Sludge Blanket" (UASB) reactor, which had no packing at all, only a "blanket" (fluid bed) of microorganisms. The furfural waste water (from a QUAKER OATS plant in Cedar Rapids, Iowa) had a load in the order of 13 000 mg COD/liter, an acetic acid content of 1.2 % by weight, a pH of approximately 2.6, and a furfural content of 600 mg/liter. As shown in Figure 46, some of the waste water, after having passed the "filter", was branched off and added to the incoming stream. The ratio of incoming waste water to recycled waste water was chosen to be 1:1, which means that the stream having passed the "filter" was split in two halves, one half being recycled, and the other half exiting. Due to this recycling, the raw waste water was partially neutralized by the relatively high pH (7.0 to 7.6) of the recycled water, but this was not enough as methanogenic microorganisms work only at pH values above 4.5, so that sodium bicarbonate was fed in to satisfy this condition. Furthermore, essential nutrients and trace minerals had to be added to comply with the requirements of anaerobic processing. The digestion was carried out at 35 ~ to obtain a reasonable reaction rate. The formation of new microorganisms ("biomass") was extremely small, amounting to less than 0.05 grams per gram of COD removed. The exiting stream entered a separation tank where the biogas (CH4 + CO2) was removed from the liquid phase. The biogas then passed a bubble flask for visual observation, and finally a hydrogen sulfide scrubber before being measured in a gasometer. Ahead of the gasometer, samples could be withdrawn for gas analysis. Results obtained by Wirtz and Dague with a "systemic residence time" of 12 hours are shown in Figure 47, where their "systemic residence time", also called "hydraulic retention time" (HRT), is defined as the quotient of the reactor volume and the input rate of raw waste water. Due to the 1:1 recycle scheme, the stream entering the reactor is twice as large as the input rate of raw waste water, so that the "true residence time in the reactor" is only 50 percent of the "systemic residence time". In Figure 47, the percentage of dissolved COD removed is plotted versus the "specific COD load", also called "organic loading rate" (OLR), defined as the COD input (in grams of COD per day) per unit of reactor volume (in liters). As the reactor volume was 9.5 liters (both the residence time and the specific COD load are referred to the empty reactor), a
95
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Figure 47. Results of Anaerobic Digestion of Furfural Waste Water as reported by Wirtz and Dague for a Systemic Residence Time of 12 Hours. Circles: Partially Packed Reactor Squares" Fully Packed Reactor
systemic residence time of 12 hours = 0.5 days corresponds to a raw waste water feed rate F = 9.5 liter/0.5 days = 19 liter/day. With the raw waste water carrying 13 g of COD per liter, this introduced 13 g of COD/liter x 19 liter/day = 247 g of COD/day so that the specific COD load was 247 g/day per 9.5 liters of reactor volume, i.e. 26 g day -1 liter -s, as represented by the points on the right-hand side. The points further on the left-hand side were obtained with correspondingly diluted waste water. In view of the break of the curves in Figure 47, Wirtz and Dague concluded that for a removal of 90 % of the soluble COD, the maximum permissible specific COD load is 23 g day l liter -~.
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In steady state operation, both reactors tested performed equally well, but the fully packed reactor was found to be superior in handling shut-down periods. The methane production was found to be 0.347 liter (STP) per gram of COD destroyed. Based on these laboratory results, an industrial furfural waste water plant, designed by PROSERPOL of France and UHDE of South Africa, is shown schematically in Figure 48. After removal of 90 percem of the COD by such anaerobic processing, the remaining 10 percent, down to 100 ppm, can be taken out by a follow-up aerobic treatment. It must be noted, however, that the aerobic removal of the last 10 percent of the COD requires a larger plant than the anaerobic removal of the first 90 percem. Noteworthily, Figure 47 shows that it is not possible to remove the last 10 percent of the COD by anaerobic processing, not even with a hugely increased residence time. This indicates that these last l0 percent are due to substances which cannot be assimilated by methanogenic bacteria. As furfural waste water is known to contain formic acid at a level amounting to roughly 10 percent of the acetic acid load, it can be concluded that methanogenic bacteria are unable to consume formic acid. Apart from being painfully large, a follow-up aerobic treatmem has two other disadvantages: (a) A high growth rate of microorganisms, thus posing a sludge problem. (b) A high consumption of electric energy to get oxygen into the water. Per unit of COD removal, the power requirement of an aerobic plato exceeds that of an anaerobic plant by a factor of 7.7.
Reference [44] R. A. Wirtz and R. R. Dague, Waste Management 13 (1993) 309-315.
13. Treatment of Furfural Waste Water The waste water of all furfural plants contains some carboxylic acids, with acetic acid being the principal load, ranging from 1 to 5 percent by weight, as well as some furfural, in concentrations up to 600 ppm. The furfural concentration depends on the quality of the first distillation column, with good columns going down to 50 ppm. If the concentration of acetic acid is too low to justify an acid recovery plant, it is common practice, wherever possible, to simply discharge the waste water into a river or the sea. However, with an increasing awareness of environmental issues, and with a tightening legislation in this regard, it is unlikely that such discharges will be permitted for much longer. Against this background, anaerobic digestion of the waste water has become the procedure of choice for such cases. Application of this process to furfural waste water was studied in detail by Wirtz and Dague [44]. Contrary to widespread erroneous belief, these authors established beyond any doubt that at the concentrations occurring in furfural waste water, the toxicity of furfural for some microorganisms does not apply to methane bacteria. They not only thrive on acetic acid but eat up furfural as well. For the low concentrations of acetic acid in furfural waste water, these bacteria, known to decompose acetic acid to methane and carbon dioxide according to the reaction CH3COOH ---->--->CH4 + CO2, are employed as "filters" defined as plastic packings on which the methane bacteria are retained. The study by Wirtz and Dague was made on upflow "filters" of fully and partially packed design as shown in Figure 46. The packing consisted of 1.6 cm FLEXIRINGS manufactured by KOCH ENGINEERING of Wichita, Kansas. The void space of these rings is 94 %. While the right-hand "filter" was fully packed, the left-hand "filter" was packed only in the upper two thirds of the reactor volume. As in start-up both reactors were filled completely with biomass (microorganisms from a municipal waste water treatment plant), the left-hand reactor had only a fluid bed of biomass (but no packing) in the lower third of its volume. For this reason, it was called "Upflow Blanket Filter" (UBF) to indicate that it was a
93
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.
.
.
.
.
.
.
.
.
.
Figure 46. The Experimental Setup used by Wirtz and Dague. 1 - Partially packed reactor (Upflow Blanket Filter) 2 - Fully packed reactor 3 - Separation tank 4 - Bubble flask 5 - Scrubber 6 - Sampling port 7 - Gasometer
,5"
94
hybrid between the normal fully packed "filter" (right-hand reactor) and a now obsolete "Upflow Anaerobic Sludge Blanket" (UASB) reactor, which had no packing at all, only a "blanket" (fluid bed) of microorganisms. The furfural waste water (from a QUAKER OATS plant in Cedar Rapids, Iowa) had a load in the order of 13 000 mg COD/liter, an acetic acid content of 1.2 % by weight, a pH of approximately 2.6, and a furfural content of 600 mg/liter. As shown in Figure 46, some of the waste water, after having passed the "filter", was branched off and added to the incoming stream. The ratio of incoming waste water to recycled waste water was chosen to be 1:1, which means that the stream having passed the "filter" was split in two halves, one half being recycled, and the other half exiting. Due to this recycling, the raw waste water was partially neutralized by the relatively high pH (7.0 to 7.6) of the recycled water, but this was not enough as methanogenic microorganisms work only at pH values above 4.5, so that sodium bicarbonate was fed in to satisfy this condition. Furthermore, essential nutrients and trace minerals had to be added to comply with the requirements of anaerobic processing. The digestion was carried out at 35 ~ to obtain a reasonable reaction rate. The formation of new microorganisms ("biomass") was extremely small, amounting to less than 0.05 grams per gram of COD removed. The exiting stream entered a separation tank where the biogas (CH4 + CO2) was removed from the liquid phase. The biogas then passed a bubble flask for visual observation, and finally a hydrogen sulfide scrubber before being measured in a gasometer. Ahead of the gasometer, samples could be withdrawn for gas analysis. Results obtained by Wirtz and Dague with a "systemic residence time" of 12 hours are shown in Figure 47, where their "systemic residence time", also called "hydraulic retention time" (HRT), is defined as the quotient of the reactor volume and the input rate of raw waste water. Due to the 1:1 recycle scheme, the stream entering the reactor is twice as large as the input rate of raw waste water, so that the "true residence time in the reactor" is only 50 percent of the "systemic residence time". In Figure 47, the percentage of dissolved COD removed is plotted versus the "specific COD load", also called "organic loading rate" (OLR), defined as the COD input (in grams of COD per day) per unit of reactor volume (in liters). As the reactor volume was 9.5 liters (both the residence time and the specific COD load are referred to the empty reactor), a
95
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Figure 47. Results of Anaerobic Digestion of Furfural Waste Water as reported by Wirtz and Dague for a Systemic Residence Time of 12 Hours. Circles: Partially Packed Reactor Squares" Fully Packed Reactor
systemic residence time of 12 hours = 0.5 days corresponds to a raw waste water feed rate F = 9.5 liter/0.5 days = 19 liter/day. With the raw waste water carrying 13 g of COD per liter, this introduced 13 g of COD/liter x 19 liter/day = 247 g of COD/day so that the specific COD load was 247 g/day per 9.5 liters of reactor volume, i.e. 26 g day -1 liter -s, as represented by the points on the right-hand side. The points further on the left-hand side were obtained with correspondingly diluted waste water. In view of the break of the curves in Figure 47, Wirtz and Dague concluded that for a removal of 90 % of the soluble COD, the maximum permissible specific COD load is 23 g day l liter -~.
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-
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. . . . .
I
B
%
0
,.Q 0
97
In steady state operation, both reactors tested performed equally well, but the fully packed reactor was found to be superior in handling shut-down periods. The methane production was found to be 0.347 liter (STP) per gram of COD destroyed. Based on these laboratory results, an industrial furfural waste water plant, designed by PROSERPOL of France and UHDE of South Africa, is shown schematically in Figure 48. After removal of 90 percem of the COD by such anaerobic processing, the remaining 10 percent, down to 100 ppm, can be taken out by a follow-up aerobic treatment. It must be noted, however, that the aerobic removal of the last 10 percent of the COD requires a larger plant than the anaerobic removal of the first 90 percem. Noteworthily, Figure 47 shows that it is not possible to remove the last 10 percent of the COD by anaerobic processing, not even with a hugely increased residence time. This indicates that these last l0 percent are due to substances which cannot be assimilated by methanogenic bacteria. As furfural waste water is known to contain formic acid at a level amounting to roughly 10 percent of the acetic acid load, it can be concluded that methanogenic bacteria are unable to consume formic acid. Apart from being painfully large, a follow-up aerobic treatmem has two other disadvantages: (a) A high growth rate of microorganisms, thus posing a sludge problem. (b) A high consumption of electric energy to get oxygen into the water. Per unit of COD removal, the power requirement of an aerobic plato exceeds that of an anaerobic plant by a factor of 7.7.
Reference [44] R. A. Wirtz and R. R. Dague, Waste Management 13 (1993) 309-315.