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Assessment of partial treatment of polyethylene glycol wastewaters by wet air oxidation
PII: S0043-1354(99)00320-6
Wat. Res. Vol. 34, No. 5, pp. 1620±1628, 2000 # 2000 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0043-1354/00/$ - see front matter
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ASSESSMENT OF PARTIAL TREATMENT OF POLYETHYLENE GLYCOL WASTEWATERS BY WET AIR OXIDATION DIONISSIOS MANTZAVINOSM, ERIK LAUER, MORTAZA SAHIBZADA, ANDREW G. LIVINGSTONM and IAN S. METCALFE* Department of Chemical Engineering and Chemical Technology, Imperial College of Science, Technology and Medicine, Prince Consort Road, London SW7 2BY, UK (First received 1 February 1999; accepted in revised form 1 July 1999) AbstractÐThe pretreatment of a model, polyethylene glycol-containing wastewater by wet air oxidation was studied in the context of integrated chemical and biological treatment processes. Both semibatch and continuous uncatalysed experiments were carried out at temperatures from 383 to 473 K, residence times up to 60 min and an oxygen partial pressure of 2.8 MPa. The concentration of total organic carbon (TOC) and chemical oxygen demand (COD) was followed throughout the reaction. Combined parameters and simple analytical techniques were used and provided a straightforward assessment of the chemical pretreatment eciency with respect to partial oxidation and polymer degradation. It was found that at the conditions under consideration, partial oxidation dominates over total oxidation and is accompanied by substantial fragmentation of the original polymer to lower molecular weight compounds. Semibatch operation favours higher TOC and COD removal rates than continuous operation, while continuous operation generally favours higher partial oxidation eciencies than semibatch operation. The bene®ts arising from the assessment of the extent of chemical pretreatment for the rational design of integrated treatment processes are also discussed. # 2000 Elsevier Science Ltd. All rights reserved Key wordsÐpolymers, pretreatment, process integration, wastewaters, wet oxidation
NOMENCLATURE
AOSC COD MW PEG TOC X Y WAO
INTRODUCTION
average oxidation state of carbon (±) chemical oxygen demand (mg lÿ1) molecular weight (g gmolÿ1) polyethylene glycol total organic carbon (mg lÿ1) extent of COD removal by partial oxidation (±) degree of polymerisation (±) wet air oxidation
Greek letters a number of scissions per molecule (±) E eciency of COD removal by partial oxidation (±) Subscripts o
initial value
*Author to whom all correspondence should be addressed. Present address: School of Chemical Engineering, The University of Edinburgh, May®eld Road, Edinburgh EH9 3JL, UK. Tel.: +44-131-650-8553; fax: +44-131650-6551; e-mail: [email protected]
Wet air oxidation (WAO) is an emerging technology for treating wastewaters that are either too dilute to incinerate or too concentrated or toxic to biologically degrade (Debellefontaine et al., 1996; Gulyas, 1997). Although the main application of WAO is still the conditioning and/or destruction of waste activated sludge (Dietrich et al., 1985), over the last several years, increased interest has been shown in the potential capability of WAO for partial or total treatment of various classes of organiccontaining wastewaters (Mishra et al., 1995). Our previous work has shown that WAO can be eectively employed as either a single process or part of a process combination to remove various organic compounds. Polyphenols can be completely removed through a WAO process (Mantzavinos et al., 1996a, b), while high molecular weight (MW) water-soluble polymers can be eectively pretreated by WAO prior to membrane separation and biological treatment processes (Mantzavinos et al., 1996c, d, 1998; Hellenbrand et al., 1997; Otal et al., 1997). In the context of integrated chemical and biological treatment processes partial WAO may be
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Partial wet oxidation of polyethylene glycols
employed as an ecient pretreatment step to improve the biodegradability of compounds that are initially biorecalcitrant. Ideally, the role of chemical pretreatment would be to convert all the biorecalcitrant compounds to biodegradable molecules while avoiding excessive total oxidation to carbon dioxide and water. The latter is particularly important when pure oxygen is used as an oxidant since minimisation of total oxidation will result in reduced oxidant consumption. However, the bene®cial synergy between chemical pretreatment and subsequent biological treatment will be lost if the pretreatment is too short to introduce signi®cant changes in the biorecalcitrant molecules, or too long resulting in the processes being decoupled. Therefore, information regarding the properties of the main reaction intermediates formed and mechanisms involved during chemical oxidation is necessary in deciding the degree of oxidation required prior to subsequent biological treatment. However, determination of reaction intermediates and their concentration±time pro®les usually requires state of the art analytical techniques which (a) are generally complex and time consuming and (b) may not be applicable when actual wastewaters or more complex model wastewaters are considered. Therefore, it is bene®cial to investigate the use of simple analytical techniques for the determination of lumped parameters such as TOC, COD, total solids etc. to characterise an oxidised reaction mixture. Most of the studies into WAO reported in the literature are focused on semibatch operation (Mishra et al., 1995) of the WAO reactor (i.e. the gaseous oxidant is continuously fed, while the liquid feedstock is fed in batchwise mode). However, for the rational design of an integrated treatment process information regarding continuous WAO operation (i.e. both oxidant and liquid feedstock are continuously fed) is necessary since an integrated process is likely to operate continuously rather than in batchwise mode. In this work the semibatch and continuous partial wet air oxidation of a model wastewater containing polyethylene glycol of 10 000 MW is investigated. Lumped parameters such as TOC and COD are followed throughout the reaction and simple analytical methods are employed in order to assess the oxidation state of the partially oxidised reaction mixture as well as the extent of polymer degradation.
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commonly found in industrial and domestic wastewaters. For this work, PEG of 10 000 nominal MW was provided by Fluka Chemicals, U.K. and used without further puri®cation to prepare aqueous solutions. PEG 10 000 was dissolved in deionised water at an initial concentration of 1 g lÿ1 resulting in a solution with the following properties: TOCo: 550 mg lÿ1, CODo: 1850 mg lÿ1. Wet oxidation reactor A 300 ml stainless steel high pressure reactor (Baskerville, U.K.) capable of performing semibatch or continuous experiments at pressures up to 10 MPa and temperatures up to 573 K was used. Experiments were carried out at temperatures of 383 , 403 , 423 and 473 K and an oxygen partial pressure of 2.8 MPa. PEG 10 000 feedstock was loaded into the reactor either continuously or in batchwise mode and heated up to the operating temperature under nitrogen, while stirring at 18.3 rps. As soon as the desired temperature was reached pure oxygen was continuously fed to the reactor to start the reaction. Therefore, the reactor operating mode was either semibatch (when liquid phase was fed in batchwise mode) or continuous (when liquid phase was continuously fed). For continuous experiments, since dynamic state operation was expected to occur before the system would reach steady state, the reaction was allowed to proceed for 6±8 residence times during which the concentration of the liquid euent (usually in terms of TOC) was periodically monitored. As soon as the concentration reached a constant value steady state operation was assumed. The experimental setup and procedures followed for semibatch (Mantzavinos et al., 1996c) and continuous (Otal et al., 1997) operation are described in detail elsewhere. Liquid samples periodically drawn from the reactor were analysed with respect to their TOC and COD contents as well as concentration of acid end-groups. Total organic carbon (TOC) TOC was measured with a Shimadzu 5050 TOC analyser which utilises combustion with subsequent nondispersive infrared (NDIR) gas analysis. Total carbon (TC) was ®rst measured and then the inorganic carbon (IC) was measured. TOC was determined by subtracting IC from TC. The uncertainty in this assay, quoted as the relative standard deviation of three separate measurements, was never larger than 1% for the range of TOC concentrations measured. Chemical oxygen demand (COD) COD was determined by the dichromate method. The appropriate amount of sample was introduced into commercially available digestion solution (Hach Europe, Belgium) containing sulphuric acid, mercuric sulphate and chromic acid. The mixture was then incubated for 120 min at 423 K in a COD reactor (Model 45600, Hach Company, U.S.A.), and the COD concentration was measured colorimetrically using a DR/700 colorimeter (Camlab, U.K.). The average value of six separate readings was taken, and the relative standard deviation was always less than 1.5% for the range of COD concentrations measured. Acid number analysis
MATERIALS AND METHODS
Materials Polyethylene glycols (PEG: HO(CH2CH2O)yH) are an important group of nonionic synthetic water-soluble polymers of ethylene oxide. These compounds have widespread use in the manufacture of nonionic surfactants, lubricants, pharmaceuticals, cosmetics, antifreezes etc. and can be
Acid number analysis (according to a method provided by Inspec, U.K.) was used to determine the concentration of acid end-groups present in wet oxidised solutions. A sample of the wet oxidised solution was titrated with a 0.1 M solution of KOH in an isopropanol phase using anapthylamine as indicator. The accuracy of the titration method was tested by titrating aqueous solutions of diglycolic acid and PEG 600 di-acid. The discrepancy between the theoretical and experimental concentrations of acid
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Dionissios Mantzavinos et al.
Fig. 1. TOC±time pro®le during semibatch oxidation of PEG 10 000 at various temperatures. (w) 383 K, (r) 403 K, (q) 423 K, () 473 K.
Fig. 3. TOC±residence time pro®le during continuous oxidation of PEG 10 000 at various temperatures. (w) 383 K, (r) 403 K, (q) 423 K, () 473 K.
end-groups present in these compounds was less than 1.5%.
Figs 1 and 2 show the concentration±time pro®le for TOC and COD respectively during the semibatch oxidation of PEG 10 000 at various temperatures. It can be seen that the rate of TOC removal is slow throughout the reaction at any temperature with only about 10 and 20% of organics being totally oxidised after 30 min of oxidation at 403 and 473 K respectively. Conversely, about 20 and 45% of COD removal was recorded after 30 min of oxidation at 403 and 473 K respectively. These results imply the presence in the reaction mixture of compounds that are resistant to further oxidation since both TOC and COD remain in relatively high concentrations even after 60 min of oxidation at 473 K. In a previous study (Mantzavinos et al., 1996c) the semibatch wet oxidation of various MW
PEGs was investigated with respect to reaction mechanisms and pathways occurring during WAO. It was found that under mild operating conditions (i.e. temperatures up to 423 K and times up to 60 min), little total oxidation occurred. However, such conditions were sucient to convert the original polymer to oligomers and short-chain organic acids through the formation of lower MW polymeric compounds. It was also suggested that polymer degradation occurs mainly through random scission reactions governed by a free radical autoxidation mechanism. Figs 3 and 4 show the concentration±residence time pro®le for TOC and COD respectively during the continuous, steady state oxidation of PEG 10 000 at various temperatures. After 30 min of oxidation at 403 and 473 K, respectively, 5 and 15% TOC removal was recorded, while TOC remained nearly unchanged after 60 min of oxidation at 383 K. Conversely, about 20% and 40% COD removal was recorded after 30 min of oxidation at 403 and 473 K respectively, while 10% COD
Fig. 2. COD±time pro®le during semibatch oxidation of PEG 10 000 at various temperatures. (w) 383 K, (r) 403 K, (q) 423 K, () 473 K.
Fig. 4. COD±residence time pro®le during continuous oxidation of PEG 10 000 at various temperatures. (w) 383 K, (r) 403 K, (q) 423 K, () 473 K.
RESULTS AND DISCUSSION
Removal of TOC and COD
Partial wet oxidation of polyethylene glycols
removal was recorded after 60 min of oxidation at 383 K. It can also be seen that the TOC and COD removal rates during continuous operation are generally lower than those during semibatch operation indicating that the extent of oxidation during continuous operation is less than that during semibatch operation. This may be attributed to the fact that (a) a semibatch reactor would favour higher concentrations of any intermediate whereas a continuous, well-mixed reactor would have more of a mixture of original PEG 10 000 and intermediates because of its eective distribution of residence times, (b) dead zones and short cut ¯ows may occur during continuous operation and (c) continuous operation would alter the reaction pathways of the corresponding semibatch operation due to reactions occurring between fresh feed and the already oxidised compounds present in the reaction mixture. Such reactions could possibly include condensation and repolymerisation and form intermediates that are more stable than those encountered in semibatch operation. However, the possibility that already existing free radical species may react with fresh feed as well as other compounds present in the reaction mixture thus enhancing propagation and degradation reactions cannot be completely disregarded. It can also be seen that even at low temperatures and short times some total oxidation occurs (i.e. 3% TOC removal was recorded after 5 min of semibatch oxidation at 403 K). This is unlikely to be attributed to the oxidation of stable intermediates formed during the wet oxidation of PEG 10 000 since compounds such as short-chain organic acids and oligomers are known to be resistant to total oxidation even under more severe operating conditions (Imamura et al., 1982; Ruelle et al., 1986; Nikolaou et al., 1994; Mantzavinos et al., 1996c). Therefore, it can be hypothesised that some of the early intermediates formed during the oxidation of PEG 10 000 may be directly oxidised to total oxidation end-products (i.e. carbon dioxide and water) without passing through more stable intermediates. This is consistent with a generalised WAO reaction scheme proposed by other investigators (Li et al., 1991; Lin et al., 1996). According to this scheme the original organics as well as some relatively unstable intermediates present in the reaction mixture are either partially oxidised to form stable intermediates (usually represented by acetic acid) or directly oxidised to total oxidation end-products. Stable intermediates are eventually oxidised to endproducts. Assessment of partial oxidation To assess the oxidation state of the reaction mixture during WAO, TOC and COD can be combined in various ways and provide parameters such as the average oxidation state of carbon atoms in the reaction mixture (Scott and Ollis, 1995) and the e-
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Fig. 5. Comparison of average oxidation state of carbon (AOSC) between (a) semibatch oxidation, (b) continuous oxidation of PEG 10 000 at various temperatures. (w) 383 K, (r) 403 K, (q) 423 K, () 473 K.
ciency of COD removal through partial oxidation (Jochimsen and Jekel, 1997). The average oxidation state of carbon atoms (AOSC) is de®ned as follows: AOSC 4 ÿ 1:5 COD=TOC
1
where AOSC takes values between ÿ4 (i.e. for methane) and +4 (i.e. for carbon dioxide) and TOC and COD are expressed in mass concentrations. As carbon dioxide does not remain dissolved in the liquid phase and is therefore not taken into account when COD and TOC are measured, AOSC does not increase if only total oxidation occurs. Higher AOSC values consequently indicate a higher oxidation state of the organics in the mixture. Fig. 5 shows a comparison of the AOSC values during semibatch and continuous oxidation of PEG 10 000 at various temperatures. It can be seen that AOSC increases with increasing temperature and time for the whole range of temperatures employed during semibatch operation as the organics present in the reaction mixture become more oxidised. However, during continuous operation AOSC increases as temperature increases from 383
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Dionissios Mantzavinos et al.
Fig. 6. Comparison of extent of partial COD removal (X ) between (a) semibatch oxidation, (b) continuous oxidation of PEG 10 000 at various temperatures. (w) 383 K, (r) 403 K, (q) 423 K, () 473 K.
Fig. 7. Comparison of eciency of partial COD removal (E ) between (a) semibatch oxidation, (b) continuous oxidation of PEG 10 000 at various temperatures. (w) 383 K, (r) 403 K, (q) 423 K, () 473 K.
to 423 K, while a further temperature increase from 423 to 473 K does not seem to signi®cantly aect the AOSC values in the reaction mixture. The main drawback associated with the use of AOSC is that it cannot indicate how much partial and total oxidation has taken place since the start of the reaction. Clearly this parameter only re¯ects the oxidation state at one given time and it cannot be used for assessing the progress of partial oxidation reactions. To dierentiate between partial and total oxidation reactions, COD removal by partial oxidation (CODpartial) is expressed by comparing COD/TOC ratios at dierent reaction times to the original ratio as follows (Jochimsen and Jekel, 1997):
information with respect to the ratio between partial and total oxidation that had occurred. This is important when the chemical pretreatment objective is not only to maximise the extent of partial oxidation but also minimise the extent of total oxidation. Therefore, the eciency of COD removal through partial oxidation (E ) can be de®ned as the ratio of COD removal by partial oxidation and overall COD removal:
CODpartial CODo TOC=TOCo ÿ COD
2
The extent of COD removal by partial oxidation (X ) can be de®ned as follows: X CODpartial =CODo
3
Eqs. (2) and/or (3) can be used to follow the progress of partial oxidation without, however, giving
e CODpartial = CODo ÿ COD
4
where E reaches the value of 0 when only total oxidation occurs and 1 for the ideal case that only partial oxidation occurs. Fig. 6 shows a comparison of the extent of COD removal by partial oxidation (X ) during semibatch and continuous operation at various temperatures. As temperature increases from 383 to 473 K the extent of partial oxidation consistently increases with increasing temperature and time during semibatch operation. It can also be seen that for the semibatch run performed at 383 K, the extent of partial COD removal for short times (i.e. less than 60 min) may take negative (and
Partial wet oxidation of polyethylene glycols
consequently meaningless) values since the experimental error associated with COD and TOC measurements was comparable to the extent of COD and TOC removal that had occurred. Conversely, a somewhat dierent behaviour is observed during continuous operation where the extent of partial oxidation increases as temperature increases from 383 to 423 K and then decreases as temperature increases further from 423 to 473 K. Fig. 7 shows a comparison of the eciency of COD removal through partial oxidation (E ) during semibatch and continuous oxidation of PEG 10 000 at various temperatures. (For the semibatch run performed at 383 K, E becomes negative for reaction times less than 60 min due to experimental error. Therefore, these values are not shown in Fig. 7 for clarity). It can be seen that for most of the range of experimental conditions under consideration for both semibatch and continuous operation, partial oxidation seems to be more important than total oxidation since E is always greater than 0.5 (i.e. more than 50% of the overall COD removal is due to partial oxidation reactions). The only exception where total oxidation seems to be more important than partial oxidation is after 60 min of continuous oxidation at 473 K with E being equal to 0.43. It can also be seen that at temperatures up to 423 K the eciency (E ) during continuous operation is considerably higher than that during semibatch operation. However, as temperature increases from 423 to 473 K the eciency (E ) signi®cantly decreases during continuous operation, but remains nearly unchanged during semibatch operation. These results suggest that at relatively low or moderate temperatures continuous operation gives more partial oxidation than semibatch operation. It can also be seen that the eciency (E ) generally decreases with increasing residence time during continuous operation. For instance, as the residence time increases from 15 to 60 min during continuous operation at 423 K the eciency (E ) decreases from 0.95 to 0.65, while the extent of partial oxidation (as can be seen in Fig. 6(b)) remains nearly unchanged; this implies that the extent of total oxidation signi®cantly increases with increasing residence time. Assessment of polymer degradation Information regarding partial COD removal can prove useful in assessing the number of polymer chain scissions and consequently the decrease in average MW of the organics in the reaction mixture. However, to establish a relationship between COD removal and polymer degradation several assumptions have to be made as follows: (a) At relatively low temperatures, polymers undergo only oxidative degradation, while thermal degradation reactions (which may occur at higher temperatures) are neglected, (b) oxidative degradation eventually leads to the formation of compounds that have acid
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Fig. 8. Assessment of Mn,COD during oxidation of PEG 10 000 at various temperatures. (w) 383 K, semibatch, (.) 383 K, continuous, (r) 403 K, semibatch, (R) 403 K, continuous, (q) 423 K, semibatch, (Q) 423 K, continuous.
end-groups (i.e. di-acids), while compounds such as hydroperoxides, alcohols and aldehydes are reactive and easily oxidised to form acids, (c) at relatively low temperatures and reaction times, the concentration of speci®c compounds (such as short-chain acids with only one acid group per molecule (i.e. acetic acid)) is negligible. In this case, random scission of the bond between two repeating units of the polymer and subsequent oxidation of the fragments to form two new acids requires two oxygen molecules and can be described as follows: 0CH2 CH2 OCH2 CH2 O0 2 O2 4 0 CH2 COOH HOOCCH2 O0 H2 O
5
The total number of bond scissions (atotal) that has occurred during the fragmentation and oxidation of polymer can then be linked to partial COD removal and calculated as follows: atotal 0:5 CODpartial =320006:023 1023
6
where 0.5 is derived from the oxygen stoichiometry in Eq. (5), 32 000 mg gmolÿ1 is the MW of oxygen and 6.023 1023 molecules gmolÿ1 is the Avogadro's number. The total number of bond scissions may be converted into the number of scissions per initial molecule (a ) if the initial number of PEG molecules (NPEG,o) in the reaction volume is known: a atotal =NPEG,o
7
The degree of polymerisation (i.e. the number of repeating molecules joined together) of a polymer can be calculated from the known MW values of the polymer, the repeating molecule (i.e. monomer) and the polymer end-group. For instance, in the case of PEG 10 000 (HO(CH2CH2O)yH) the degree of polymerisation (Y ) can be calculated as follows:
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Dionissios Mantzavinos et al.
Y 10000 ÿ 18=44110000=44
8
where 18 corresponds to the MW of PEG endgroup (OH and H) and 44 corresponds to the MW of ethylene oxide. If PEG 10 000 undergoes random scission degradation, the number average degree of polymerisation (Yn) and consequently the number average MW (Mn,COD) of the oxidised solution decreases with increasing extent of degradation as follows: Yn =Yn,o 1Mn,COD =Mn,o 1 aÿ1
9
Therefore, the number average MW of the oxidised solution (Mn,COD) can be assessed from Eq. (9) if the MW of the original solution (Mn,o) and the number of bond scissions per molecule (a ) are known. Fig. 8 shows the calculated Mn,COD values during semibatch and continuous oxidation of PEG 10 000 at temperatures up to 423 K. It has to be pointed out that Mn,COD was not assessed for the runs performed at 473 K since at this relatively high temperature (a) thermal degradation may become important and (b) speci®c compounds such as short-chain acids may be present at relatively high concentrations (Mantzavinos et al., 1996c), thus invalidating the assumptions associated with the use of this method. (For the runs performed at 383 K, Mn,COD is not shown for short times (i.e. less than 60 min) due to the experimental error associated with COD and TOC measurements.) It can be seen that Mn,COD, as would be expected, decreases with increasing temperature during both semibatch and continuous operation. It should also be pointed out that Mn,COD provides an assessment rather than a precise determination of the average number MW due to the assumptions associated with the use of this method. However, it may prove useful in, for instance, evaluating the dierences between semibatch and continuous operation. It can be seen that at residence times up to 30 min the Mn,COD values during continuous operation are considerably lower than those during semibatch operation; this can be explained due to dierent reaction mechanisms and pathways involved. During continuous operation free radicals are continuously generated and remain at relatively constant concentrations due to the addition of fresh feed that results in high rates of initiation reactions. Therefore, free radicals are always available to react with fresh feed as well as with other compounds present in the reaction mixture, thus leading to enhanced propagation and degradation reaction rates. Conversely, during the early stages of semibatch operation initiation reactions are the rate-limiting step due to the initial absence of free radicals from the reaction mixture. As the reaction proceeds the concentration of free radicals also decreases due to termination reactions and the lack of compounds (i.e. fresh feed) that can form new free radicals. Alternatively, the number average MW of a reac-
Fig. 9. Assessment of molecular weight during semibatch oxidation of PEG 10 000 at 383 K. (w) Mp, (r) Mn,acid.
tion mixture of known TOC content can be assessed from the concentration of acid end-groups (which is measured by acid number analysis) that are contained in di-acids as follows. The number of moles of hydroxyl groups (NOH) used for the titration are equal to the number of moles of acid groups (NCOOH) present in the sample. Since one molecule of di-acid contains two acid groups, the number of polymer molecules (Npolymer) should be equal to: Npolymer 0:5NCOOH 0:5NOH
10
The number of carbon atoms (Nc) that are contained in a sample volume (V ) of known TOC content is given by: Nc TOC=12V
11
The number average degree of polymerisation can then be calculated as follows: Yn Nc =Npolymer 2Nc =NOH
12
The number average MW of the oxidised solution (Mn,acid) can then be assessed from the known Yn, Yn,o and Mn,o values according to Eq. (9). Fig. 9 shows the number average MW of the reaction mixture during the semibatch oxidation at 383 K as assessed by acid number analysis. Peak molecular weight (Mp) values determined by gel permeation chromatography (GPC) coupled with conventional high performance liquid chromatography are also given. (To determine Mp, a calibration curve relating peak elution time from the chromatographic column to Mp was established using standard calibration PEG samples of known, narrow MW distributions. The analytical procedures followed are described in detail elsewhere (Mantzavinos et al., 1996c).) It can be seen that as would be expected, the MW decreases with increasing time regardless of the method used to assess it. However, the values given by GPC and acid num-
Partial wet oxidation of polyethylene glycols
ber analysis cannot be directly compared to each other since these methods use dierent physical and chemical properties to determine MW. It should also be pointed out that Mn,acid provides an assessment rather than a precise determination of the average number MW due to the assumptions associated with the use of this method. However, in the context of integrated treatment processes, assessment of polymer degradation could prove useful in providing information regarding the extent of chemical pretreatment required prior to subsequent treatment. This can be demonstrated by considering the following example: Since the biodegradation rate of polymers generally decreases with increasing MW the role of WAO would ideally be to fragment PEG 10 000 to polymeric fractions of biodegradable MW. In a previous study (Otal et al., 1997), it was found that PEGs with MW more than 6000 were less readily degradable aerobically than those with MW between 6000 and 200. It was also found that the biodegradation rate of PEGs with a MW of 6000 or less was nearly constant regardless of the MW. As can be seen from Fig. 9 an average MW of 6000 or less can be achieved after 30 min of WAO at 383 K with Mn,acid and Mp being about 1300 and 2300 respectively. Although these values are signi®cantly dierent from each other, use of any of the aforementioned methods to assess MW would lead to a common conclusion with respect to the extent of WAO required prior to subsequent biological treatment. In this case, acid number analysis may prove a useful analytical tool that is certainly less complex and costly than a chromatographic method. CONCLUSIONS
The overall eciency of an integrated wastewater treatment process comprising partial WAO followed by membrane separation and/or biological treatment is strongly aected by the performance of chemical pretreatment. The eciency and objectives of chemical pretreatment are determined by the physical, chemical and biological properties of each individual wastewater under consideration, the requirements of the subsequent treatment process and the reactor operating mode employed. For instance, in the case of polymer-containing wastewaters the target of WAO pretreatment should be to fragment all high MW, nonbiodegradable polymers to lower MW, more readily biodegradable compounds, while maximising process eciency towards partial oxidation. Lumped parameters such as TOC and COD as well as simple analytical techniques provide time- and cost-eective tools to obtain useful quantitative information in this prospect. Assessment of chemical pretreatment ecacy followed by process modelling and optimisation will then allow a systematic and rational approach to be used in designing an eective wastewater treatment
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process. Future research will focus on the treatment of actual, polymer processing wastewaters with respect to the eect of various operating conditions (i.e. reactor mode of operation, use of catalysts) on process ecacy. AcknowledgementsÐThe authors wish to thank the Commission of the European Communities for the ®nancial support of this work under Grant No. EV5V-CT930249.
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Wat. Res. Vol. 34, No. 5, pp. 1620±1628, 2000 # 2000 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0043-1354/00/$ - see front matter
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ASSESSMENT OF PARTIAL TREATMENT OF POLYETHYLENE GLYCOL WASTEWATERS BY WET AIR OXIDATION DIONISSIOS MANTZAVINOSM, ERIK LAUER, MORTAZA SAHIBZADA, ANDREW G. LIVINGSTONM and IAN S. METCALFE* Department of Chemical Engineering and Chemical Technology, Imperial College of Science, Technology and Medicine, Prince Consort Road, London SW7 2BY, UK (First received 1 February 1999; accepted in revised form 1 July 1999) AbstractÐThe pretreatment of a model, polyethylene glycol-containing wastewater by wet air oxidation was studied in the context of integrated chemical and biological treatment processes. Both semibatch and continuous uncatalysed experiments were carried out at temperatures from 383 to 473 K, residence times up to 60 min and an oxygen partial pressure of 2.8 MPa. The concentration of total organic carbon (TOC) and chemical oxygen demand (COD) was followed throughout the reaction. Combined parameters and simple analytical techniques were used and provided a straightforward assessment of the chemical pretreatment eciency with respect to partial oxidation and polymer degradation. It was found that at the conditions under consideration, partial oxidation dominates over total oxidation and is accompanied by substantial fragmentation of the original polymer to lower molecular weight compounds. Semibatch operation favours higher TOC and COD removal rates than continuous operation, while continuous operation generally favours higher partial oxidation eciencies than semibatch operation. The bene®ts arising from the assessment of the extent of chemical pretreatment for the rational design of integrated treatment processes are also discussed. # 2000 Elsevier Science Ltd. All rights reserved Key wordsÐpolymers, pretreatment, process integration, wastewaters, wet oxidation
NOMENCLATURE
AOSC COD MW PEG TOC X Y WAO
INTRODUCTION
average oxidation state of carbon (±) chemical oxygen demand (mg lÿ1) molecular weight (g gmolÿ1) polyethylene glycol total organic carbon (mg lÿ1) extent of COD removal by partial oxidation (±) degree of polymerisation (±) wet air oxidation
Greek letters a number of scissions per molecule (±) E eciency of COD removal by partial oxidation (±) Subscripts o
initial value
*Author to whom all correspondence should be addressed. Present address: School of Chemical Engineering, The University of Edinburgh, May®eld Road, Edinburgh EH9 3JL, UK. Tel.: +44-131-650-8553; fax: +44-131650-6551; e-mail: [email protected]
Wet air oxidation (WAO) is an emerging technology for treating wastewaters that are either too dilute to incinerate or too concentrated or toxic to biologically degrade (Debellefontaine et al., 1996; Gulyas, 1997). Although the main application of WAO is still the conditioning and/or destruction of waste activated sludge (Dietrich et al., 1985), over the last several years, increased interest has been shown in the potential capability of WAO for partial or total treatment of various classes of organiccontaining wastewaters (Mishra et al., 1995). Our previous work has shown that WAO can be eectively employed as either a single process or part of a process combination to remove various organic compounds. Polyphenols can be completely removed through a WAO process (Mantzavinos et al., 1996a, b), while high molecular weight (MW) water-soluble polymers can be eectively pretreated by WAO prior to membrane separation and biological treatment processes (Mantzavinos et al., 1996c, d, 1998; Hellenbrand et al., 1997; Otal et al., 1997). In the context of integrated chemical and biological treatment processes partial WAO may be
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Partial wet oxidation of polyethylene glycols
employed as an ecient pretreatment step to improve the biodegradability of compounds that are initially biorecalcitrant. Ideally, the role of chemical pretreatment would be to convert all the biorecalcitrant compounds to biodegradable molecules while avoiding excessive total oxidation to carbon dioxide and water. The latter is particularly important when pure oxygen is used as an oxidant since minimisation of total oxidation will result in reduced oxidant consumption. However, the bene®cial synergy between chemical pretreatment and subsequent biological treatment will be lost if the pretreatment is too short to introduce signi®cant changes in the biorecalcitrant molecules, or too long resulting in the processes being decoupled. Therefore, information regarding the properties of the main reaction intermediates formed and mechanisms involved during chemical oxidation is necessary in deciding the degree of oxidation required prior to subsequent biological treatment. However, determination of reaction intermediates and their concentration±time pro®les usually requires state of the art analytical techniques which (a) are generally complex and time consuming and (b) may not be applicable when actual wastewaters or more complex model wastewaters are considered. Therefore, it is bene®cial to investigate the use of simple analytical techniques for the determination of lumped parameters such as TOC, COD, total solids etc. to characterise an oxidised reaction mixture. Most of the studies into WAO reported in the literature are focused on semibatch operation (Mishra et al., 1995) of the WAO reactor (i.e. the gaseous oxidant is continuously fed, while the liquid feedstock is fed in batchwise mode). However, for the rational design of an integrated treatment process information regarding continuous WAO operation (i.e. both oxidant and liquid feedstock are continuously fed) is necessary since an integrated process is likely to operate continuously rather than in batchwise mode. In this work the semibatch and continuous partial wet air oxidation of a model wastewater containing polyethylene glycol of 10 000 MW is investigated. Lumped parameters such as TOC and COD are followed throughout the reaction and simple analytical methods are employed in order to assess the oxidation state of the partially oxidised reaction mixture as well as the extent of polymer degradation.
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commonly found in industrial and domestic wastewaters. For this work, PEG of 10 000 nominal MW was provided by Fluka Chemicals, U.K. and used without further puri®cation to prepare aqueous solutions. PEG 10 000 was dissolved in deionised water at an initial concentration of 1 g lÿ1 resulting in a solution with the following properties: TOCo: 550 mg lÿ1, CODo: 1850 mg lÿ1. Wet oxidation reactor A 300 ml stainless steel high pressure reactor (Baskerville, U.K.) capable of performing semibatch or continuous experiments at pressures up to 10 MPa and temperatures up to 573 K was used. Experiments were carried out at temperatures of 383 , 403 , 423 and 473 K and an oxygen partial pressure of 2.8 MPa. PEG 10 000 feedstock was loaded into the reactor either continuously or in batchwise mode and heated up to the operating temperature under nitrogen, while stirring at 18.3 rps. As soon as the desired temperature was reached pure oxygen was continuously fed to the reactor to start the reaction. Therefore, the reactor operating mode was either semibatch (when liquid phase was fed in batchwise mode) or continuous (when liquid phase was continuously fed). For continuous experiments, since dynamic state operation was expected to occur before the system would reach steady state, the reaction was allowed to proceed for 6±8 residence times during which the concentration of the liquid euent (usually in terms of TOC) was periodically monitored. As soon as the concentration reached a constant value steady state operation was assumed. The experimental setup and procedures followed for semibatch (Mantzavinos et al., 1996c) and continuous (Otal et al., 1997) operation are described in detail elsewhere. Liquid samples periodically drawn from the reactor were analysed with respect to their TOC and COD contents as well as concentration of acid end-groups. Total organic carbon (TOC) TOC was measured with a Shimadzu 5050 TOC analyser which utilises combustion with subsequent nondispersive infrared (NDIR) gas analysis. Total carbon (TC) was ®rst measured and then the inorganic carbon (IC) was measured. TOC was determined by subtracting IC from TC. The uncertainty in this assay, quoted as the relative standard deviation of three separate measurements, was never larger than 1% for the range of TOC concentrations measured. Chemical oxygen demand (COD) COD was determined by the dichromate method. The appropriate amount of sample was introduced into commercially available digestion solution (Hach Europe, Belgium) containing sulphuric acid, mercuric sulphate and chromic acid. The mixture was then incubated for 120 min at 423 K in a COD reactor (Model 45600, Hach Company, U.S.A.), and the COD concentration was measured colorimetrically using a DR/700 colorimeter (Camlab, U.K.). The average value of six separate readings was taken, and the relative standard deviation was always less than 1.5% for the range of COD concentrations measured. Acid number analysis
MATERIALS AND METHODS
Materials Polyethylene glycols (PEG: HO(CH2CH2O)yH) are an important group of nonionic synthetic water-soluble polymers of ethylene oxide. These compounds have widespread use in the manufacture of nonionic surfactants, lubricants, pharmaceuticals, cosmetics, antifreezes etc. and can be
Acid number analysis (according to a method provided by Inspec, U.K.) was used to determine the concentration of acid end-groups present in wet oxidised solutions. A sample of the wet oxidised solution was titrated with a 0.1 M solution of KOH in an isopropanol phase using anapthylamine as indicator. The accuracy of the titration method was tested by titrating aqueous solutions of diglycolic acid and PEG 600 di-acid. The discrepancy between the theoretical and experimental concentrations of acid
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Dionissios Mantzavinos et al.
Fig. 1. TOC±time pro®le during semibatch oxidation of PEG 10 000 at various temperatures. (w) 383 K, (r) 403 K, (q) 423 K, () 473 K.
Fig. 3. TOC±residence time pro®le during continuous oxidation of PEG 10 000 at various temperatures. (w) 383 K, (r) 403 K, (q) 423 K, () 473 K.
end-groups present in these compounds was less than 1.5%.
Figs 1 and 2 show the concentration±time pro®le for TOC and COD respectively during the semibatch oxidation of PEG 10 000 at various temperatures. It can be seen that the rate of TOC removal is slow throughout the reaction at any temperature with only about 10 and 20% of organics being totally oxidised after 30 min of oxidation at 403 and 473 K respectively. Conversely, about 20 and 45% of COD removal was recorded after 30 min of oxidation at 403 and 473 K respectively. These results imply the presence in the reaction mixture of compounds that are resistant to further oxidation since both TOC and COD remain in relatively high concentrations even after 60 min of oxidation at 473 K. In a previous study (Mantzavinos et al., 1996c) the semibatch wet oxidation of various MW
PEGs was investigated with respect to reaction mechanisms and pathways occurring during WAO. It was found that under mild operating conditions (i.e. temperatures up to 423 K and times up to 60 min), little total oxidation occurred. However, such conditions were sucient to convert the original polymer to oligomers and short-chain organic acids through the formation of lower MW polymeric compounds. It was also suggested that polymer degradation occurs mainly through random scission reactions governed by a free radical autoxidation mechanism. Figs 3 and 4 show the concentration±residence time pro®le for TOC and COD respectively during the continuous, steady state oxidation of PEG 10 000 at various temperatures. After 30 min of oxidation at 403 and 473 K, respectively, 5 and 15% TOC removal was recorded, while TOC remained nearly unchanged after 60 min of oxidation at 383 K. Conversely, about 20% and 40% COD removal was recorded after 30 min of oxidation at 403 and 473 K respectively, while 10% COD
Fig. 2. COD±time pro®le during semibatch oxidation of PEG 10 000 at various temperatures. (w) 383 K, (r) 403 K, (q) 423 K, () 473 K.
Fig. 4. COD±residence time pro®le during continuous oxidation of PEG 10 000 at various temperatures. (w) 383 K, (r) 403 K, (q) 423 K, () 473 K.
RESULTS AND DISCUSSION
Removal of TOC and COD
Partial wet oxidation of polyethylene glycols
removal was recorded after 60 min of oxidation at 383 K. It can also be seen that the TOC and COD removal rates during continuous operation are generally lower than those during semibatch operation indicating that the extent of oxidation during continuous operation is less than that during semibatch operation. This may be attributed to the fact that (a) a semibatch reactor would favour higher concentrations of any intermediate whereas a continuous, well-mixed reactor would have more of a mixture of original PEG 10 000 and intermediates because of its eective distribution of residence times, (b) dead zones and short cut ¯ows may occur during continuous operation and (c) continuous operation would alter the reaction pathways of the corresponding semibatch operation due to reactions occurring between fresh feed and the already oxidised compounds present in the reaction mixture. Such reactions could possibly include condensation and repolymerisation and form intermediates that are more stable than those encountered in semibatch operation. However, the possibility that already existing free radical species may react with fresh feed as well as other compounds present in the reaction mixture thus enhancing propagation and degradation reactions cannot be completely disregarded. It can also be seen that even at low temperatures and short times some total oxidation occurs (i.e. 3% TOC removal was recorded after 5 min of semibatch oxidation at 403 K). This is unlikely to be attributed to the oxidation of stable intermediates formed during the wet oxidation of PEG 10 000 since compounds such as short-chain organic acids and oligomers are known to be resistant to total oxidation even under more severe operating conditions (Imamura et al., 1982; Ruelle et al., 1986; Nikolaou et al., 1994; Mantzavinos et al., 1996c). Therefore, it can be hypothesised that some of the early intermediates formed during the oxidation of PEG 10 000 may be directly oxidised to total oxidation end-products (i.e. carbon dioxide and water) without passing through more stable intermediates. This is consistent with a generalised WAO reaction scheme proposed by other investigators (Li et al., 1991; Lin et al., 1996). According to this scheme the original organics as well as some relatively unstable intermediates present in the reaction mixture are either partially oxidised to form stable intermediates (usually represented by acetic acid) or directly oxidised to total oxidation end-products. Stable intermediates are eventually oxidised to endproducts. Assessment of partial oxidation To assess the oxidation state of the reaction mixture during WAO, TOC and COD can be combined in various ways and provide parameters such as the average oxidation state of carbon atoms in the reaction mixture (Scott and Ollis, 1995) and the e-
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Fig. 5. Comparison of average oxidation state of carbon (AOSC) between (a) semibatch oxidation, (b) continuous oxidation of PEG 10 000 at various temperatures. (w) 383 K, (r) 403 K, (q) 423 K, () 473 K.
ciency of COD removal through partial oxidation (Jochimsen and Jekel, 1997). The average oxidation state of carbon atoms (AOSC) is de®ned as follows: AOSC 4 ÿ 1:5 COD=TOC
1
where AOSC takes values between ÿ4 (i.e. for methane) and +4 (i.e. for carbon dioxide) and TOC and COD are expressed in mass concentrations. As carbon dioxide does not remain dissolved in the liquid phase and is therefore not taken into account when COD and TOC are measured, AOSC does not increase if only total oxidation occurs. Higher AOSC values consequently indicate a higher oxidation state of the organics in the mixture. Fig. 5 shows a comparison of the AOSC values during semibatch and continuous oxidation of PEG 10 000 at various temperatures. It can be seen that AOSC increases with increasing temperature and time for the whole range of temperatures employed during semibatch operation as the organics present in the reaction mixture become more oxidised. However, during continuous operation AOSC increases as temperature increases from 383
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Dionissios Mantzavinos et al.
Fig. 6. Comparison of extent of partial COD removal (X ) between (a) semibatch oxidation, (b) continuous oxidation of PEG 10 000 at various temperatures. (w) 383 K, (r) 403 K, (q) 423 K, () 473 K.
Fig. 7. Comparison of eciency of partial COD removal (E ) between (a) semibatch oxidation, (b) continuous oxidation of PEG 10 000 at various temperatures. (w) 383 K, (r) 403 K, (q) 423 K, () 473 K.
to 423 K, while a further temperature increase from 423 to 473 K does not seem to signi®cantly aect the AOSC values in the reaction mixture. The main drawback associated with the use of AOSC is that it cannot indicate how much partial and total oxidation has taken place since the start of the reaction. Clearly this parameter only re¯ects the oxidation state at one given time and it cannot be used for assessing the progress of partial oxidation reactions. To dierentiate between partial and total oxidation reactions, COD removal by partial oxidation (CODpartial) is expressed by comparing COD/TOC ratios at dierent reaction times to the original ratio as follows (Jochimsen and Jekel, 1997):
information with respect to the ratio between partial and total oxidation that had occurred. This is important when the chemical pretreatment objective is not only to maximise the extent of partial oxidation but also minimise the extent of total oxidation. Therefore, the eciency of COD removal through partial oxidation (E ) can be de®ned as the ratio of COD removal by partial oxidation and overall COD removal:
CODpartial CODo TOC=TOCo ÿ COD
2
The extent of COD removal by partial oxidation (X ) can be de®ned as follows: X CODpartial =CODo
3
Eqs. (2) and/or (3) can be used to follow the progress of partial oxidation without, however, giving
e CODpartial = CODo ÿ COD
4
where E reaches the value of 0 when only total oxidation occurs and 1 for the ideal case that only partial oxidation occurs. Fig. 6 shows a comparison of the extent of COD removal by partial oxidation (X ) during semibatch and continuous operation at various temperatures. As temperature increases from 383 to 473 K the extent of partial oxidation consistently increases with increasing temperature and time during semibatch operation. It can also be seen that for the semibatch run performed at 383 K, the extent of partial COD removal for short times (i.e. less than 60 min) may take negative (and
Partial wet oxidation of polyethylene glycols
consequently meaningless) values since the experimental error associated with COD and TOC measurements was comparable to the extent of COD and TOC removal that had occurred. Conversely, a somewhat dierent behaviour is observed during continuous operation where the extent of partial oxidation increases as temperature increases from 383 to 423 K and then decreases as temperature increases further from 423 to 473 K. Fig. 7 shows a comparison of the eciency of COD removal through partial oxidation (E ) during semibatch and continuous oxidation of PEG 10 000 at various temperatures. (For the semibatch run performed at 383 K, E becomes negative for reaction times less than 60 min due to experimental error. Therefore, these values are not shown in Fig. 7 for clarity). It can be seen that for most of the range of experimental conditions under consideration for both semibatch and continuous operation, partial oxidation seems to be more important than total oxidation since E is always greater than 0.5 (i.e. more than 50% of the overall COD removal is due to partial oxidation reactions). The only exception where total oxidation seems to be more important than partial oxidation is after 60 min of continuous oxidation at 473 K with E being equal to 0.43. It can also be seen that at temperatures up to 423 K the eciency (E ) during continuous operation is considerably higher than that during semibatch operation. However, as temperature increases from 423 to 473 K the eciency (E ) signi®cantly decreases during continuous operation, but remains nearly unchanged during semibatch operation. These results suggest that at relatively low or moderate temperatures continuous operation gives more partial oxidation than semibatch operation. It can also be seen that the eciency (E ) generally decreases with increasing residence time during continuous operation. For instance, as the residence time increases from 15 to 60 min during continuous operation at 423 K the eciency (E ) decreases from 0.95 to 0.65, while the extent of partial oxidation (as can be seen in Fig. 6(b)) remains nearly unchanged; this implies that the extent of total oxidation signi®cantly increases with increasing residence time. Assessment of polymer degradation Information regarding partial COD removal can prove useful in assessing the number of polymer chain scissions and consequently the decrease in average MW of the organics in the reaction mixture. However, to establish a relationship between COD removal and polymer degradation several assumptions have to be made as follows: (a) At relatively low temperatures, polymers undergo only oxidative degradation, while thermal degradation reactions (which may occur at higher temperatures) are neglected, (b) oxidative degradation eventually leads to the formation of compounds that have acid
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Fig. 8. Assessment of Mn,COD during oxidation of PEG 10 000 at various temperatures. (w) 383 K, semibatch, (.) 383 K, continuous, (r) 403 K, semibatch, (R) 403 K, continuous, (q) 423 K, semibatch, (Q) 423 K, continuous.
end-groups (i.e. di-acids), while compounds such as hydroperoxides, alcohols and aldehydes are reactive and easily oxidised to form acids, (c) at relatively low temperatures and reaction times, the concentration of speci®c compounds (such as short-chain acids with only one acid group per molecule (i.e. acetic acid)) is negligible. In this case, random scission of the bond between two repeating units of the polymer and subsequent oxidation of the fragments to form two new acids requires two oxygen molecules and can be described as follows: 0CH2 CH2 OCH2 CH2 O0 2 O2 4 0 CH2 COOH HOOCCH2 O0 H2 O
5
The total number of bond scissions (atotal) that has occurred during the fragmentation and oxidation of polymer can then be linked to partial COD removal and calculated as follows: atotal 0:5 CODpartial =320006:023 1023
6
where 0.5 is derived from the oxygen stoichiometry in Eq. (5), 32 000 mg gmolÿ1 is the MW of oxygen and 6.023 1023 molecules gmolÿ1 is the Avogadro's number. The total number of bond scissions may be converted into the number of scissions per initial molecule (a ) if the initial number of PEG molecules (NPEG,o) in the reaction volume is known: a atotal =NPEG,o
7
The degree of polymerisation (i.e. the number of repeating molecules joined together) of a polymer can be calculated from the known MW values of the polymer, the repeating molecule (i.e. monomer) and the polymer end-group. For instance, in the case of PEG 10 000 (HO(CH2CH2O)yH) the degree of polymerisation (Y ) can be calculated as follows:
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Dionissios Mantzavinos et al.
Y 10000 ÿ 18=44110000=44
8
where 18 corresponds to the MW of PEG endgroup (OH and H) and 44 corresponds to the MW of ethylene oxide. If PEG 10 000 undergoes random scission degradation, the number average degree of polymerisation (Yn) and consequently the number average MW (Mn,COD) of the oxidised solution decreases with increasing extent of degradation as follows: Yn =Yn,o 1Mn,COD =Mn,o 1 aÿ1
9
Therefore, the number average MW of the oxidised solution (Mn,COD) can be assessed from Eq. (9) if the MW of the original solution (Mn,o) and the number of bond scissions per molecule (a ) are known. Fig. 8 shows the calculated Mn,COD values during semibatch and continuous oxidation of PEG 10 000 at temperatures up to 423 K. It has to be pointed out that Mn,COD was not assessed for the runs performed at 473 K since at this relatively high temperature (a) thermal degradation may become important and (b) speci®c compounds such as short-chain acids may be present at relatively high concentrations (Mantzavinos et al., 1996c), thus invalidating the assumptions associated with the use of this method. (For the runs performed at 383 K, Mn,COD is not shown for short times (i.e. less than 60 min) due to the experimental error associated with COD and TOC measurements.) It can be seen that Mn,COD, as would be expected, decreases with increasing temperature during both semibatch and continuous operation. It should also be pointed out that Mn,COD provides an assessment rather than a precise determination of the average number MW due to the assumptions associated with the use of this method. However, it may prove useful in, for instance, evaluating the dierences between semibatch and continuous operation. It can be seen that at residence times up to 30 min the Mn,COD values during continuous operation are considerably lower than those during semibatch operation; this can be explained due to dierent reaction mechanisms and pathways involved. During continuous operation free radicals are continuously generated and remain at relatively constant concentrations due to the addition of fresh feed that results in high rates of initiation reactions. Therefore, free radicals are always available to react with fresh feed as well as with other compounds present in the reaction mixture, thus leading to enhanced propagation and degradation reaction rates. Conversely, during the early stages of semibatch operation initiation reactions are the rate-limiting step due to the initial absence of free radicals from the reaction mixture. As the reaction proceeds the concentration of free radicals also decreases due to termination reactions and the lack of compounds (i.e. fresh feed) that can form new free radicals. Alternatively, the number average MW of a reac-
Fig. 9. Assessment of molecular weight during semibatch oxidation of PEG 10 000 at 383 K. (w) Mp, (r) Mn,acid.
tion mixture of known TOC content can be assessed from the concentration of acid end-groups (which is measured by acid number analysis) that are contained in di-acids as follows. The number of moles of hydroxyl groups (NOH) used for the titration are equal to the number of moles of acid groups (NCOOH) present in the sample. Since one molecule of di-acid contains two acid groups, the number of polymer molecules (Npolymer) should be equal to: Npolymer 0:5NCOOH 0:5NOH
10
The number of carbon atoms (Nc) that are contained in a sample volume (V ) of known TOC content is given by: Nc TOC=12V
11
The number average degree of polymerisation can then be calculated as follows: Yn Nc =Npolymer 2Nc =NOH
12
The number average MW of the oxidised solution (Mn,acid) can then be assessed from the known Yn, Yn,o and Mn,o values according to Eq. (9). Fig. 9 shows the number average MW of the reaction mixture during the semibatch oxidation at 383 K as assessed by acid number analysis. Peak molecular weight (Mp) values determined by gel permeation chromatography (GPC) coupled with conventional high performance liquid chromatography are also given. (To determine Mp, a calibration curve relating peak elution time from the chromatographic column to Mp was established using standard calibration PEG samples of known, narrow MW distributions. The analytical procedures followed are described in detail elsewhere (Mantzavinos et al., 1996c).) It can be seen that as would be expected, the MW decreases with increasing time regardless of the method used to assess it. However, the values given by GPC and acid num-
Partial wet oxidation of polyethylene glycols
ber analysis cannot be directly compared to each other since these methods use dierent physical and chemical properties to determine MW. It should also be pointed out that Mn,acid provides an assessment rather than a precise determination of the average number MW due to the assumptions associated with the use of this method. However, in the context of integrated treatment processes, assessment of polymer degradation could prove useful in providing information regarding the extent of chemical pretreatment required prior to subsequent treatment. This can be demonstrated by considering the following example: Since the biodegradation rate of polymers generally decreases with increasing MW the role of WAO would ideally be to fragment PEG 10 000 to polymeric fractions of biodegradable MW. In a previous study (Otal et al., 1997), it was found that PEGs with MW more than 6000 were less readily degradable aerobically than those with MW between 6000 and 200. It was also found that the biodegradation rate of PEGs with a MW of 6000 or less was nearly constant regardless of the MW. As can be seen from Fig. 9 an average MW of 6000 or less can be achieved after 30 min of WAO at 383 K with Mn,acid and Mp being about 1300 and 2300 respectively. Although these values are signi®cantly dierent from each other, use of any of the aforementioned methods to assess MW would lead to a common conclusion with respect to the extent of WAO required prior to subsequent biological treatment. In this case, acid number analysis may prove a useful analytical tool that is certainly less complex and costly than a chromatographic method. CONCLUSIONS
The overall eciency of an integrated wastewater treatment process comprising partial WAO followed by membrane separation and/or biological treatment is strongly aected by the performance of chemical pretreatment. The eciency and objectives of chemical pretreatment are determined by the physical, chemical and biological properties of each individual wastewater under consideration, the requirements of the subsequent treatment process and the reactor operating mode employed. For instance, in the case of polymer-containing wastewaters the target of WAO pretreatment should be to fragment all high MW, nonbiodegradable polymers to lower MW, more readily biodegradable compounds, while maximising process eciency towards partial oxidation. Lumped parameters such as TOC and COD as well as simple analytical techniques provide time- and cost-eective tools to obtain useful quantitative information in this prospect. Assessment of chemical pretreatment ecacy followed by process modelling and optimisation will then allow a systematic and rational approach to be used in designing an eective wastewater treatment
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process. Future research will focus on the treatment of actual, polymer processing wastewaters with respect to the eect of various operating conditions (i.e. reactor mode of operation, use of catalysts) on process ecacy. AcknowledgementsÐThe authors wish to thank the Commission of the European Communities for the ®nancial support of this work under Grant No. EV5V-CT930249.
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