J. Chem. T echnol. Biotechnol. 1997, 70, 147È156

Integrated Wet Air Oxidation and BiologicalTreatment of Polyethylene Glycol-ContainingWastewatersEmilia Otal,1 Dionissios Mantzavinos,2 Maria V. Delgado,1 Rolf Hellenbrand,2Julian Lebrato,1 Ian S. Metcalfe2 & Andrew G. Livingston2*1 Grupo TAR, Escuela Universitaria Politecnica, University of Seville, Virgen de Africa 7, Seville 41011,Spain2 Department of Chemical Engineering and Chemical Technology, Imperial College of Science, Technologyand Medicine, Prince Consort Rd, London SW7 2BY, UK

(Received 13 December 1996 ; revised version received 15 March 1997 ; accepted 24 April 1997)

Abstract : The treatment of a model wastewater containing polyethylene glycol(PEG) of molecular weight 10 000 in an integrated wet air oxidationÈaerobicoxidation process was investigated. Partial wet air oxidation under mild oper-ating conditions was capable of converting the original polymer to lower molec-ular weight compounds, such as oligomers and short-chain organic acids. Thee†ect of molecular weight on the aerobic biodegradability of polyethylene glycolwas assessed by performing shake Ñasks experiments with various polyethyleneglycols in a molecular weight range from 200È35 000 and it was found that bio-degradability generally decreased with increasing molecular weight. Aqueoussolutions of PEG 10 000 were subjected to continuous wet air oxidation at atemperature of 423 K and a residence time of 30 min and the oxidised effluentswere then subjected to subsequent continuous aerobic oxidation at residencetimes varying between 0É5 and 4 days. Simultaneously, continuous aerobic oxida-tion experiments on solutions of PEG 10 000 were performed and used tocompare the efficiency of the integrated treatment process with that of the directbiological treatment. Partial pretreatment by wet air oxidation under mild oper-ating conditions resulted in effluents whose biodegradation rates were signiÐ-cantly higher than those of the original, unoxidised wastewater, as assessed bytotal organic carbon (TOC) removal rates. The original wastewater was practi-cally non-biodegradable at a 0É5-day residence time with direct biological treat-ment, while an 80% TOC removal was achieved when biological treatment wascoupled with a preceding wet air oxidation step. Conversely, with a 4-dayresidence time for the direct biological treatment the original wastewater wasonly partially degradable with 60È70% TOC removal recorded ; with the inte-grated treatment process at a 4-day residence time in the bioreactor the overallTOC removal was greater than 90%.

J. Chem. T echnol. Biotechnol. 70, 147È156 (1997)No. of Figures : 11. No. of Tables : 1. No. of Refs : 19

Key words : polyethylene glycols, wastewaters, wet air oxidation, integrated treat-ment

* To whom correspondence should be addressed.Contract grant sponsor : Commission of the European Communities.Contract grant number : EV5V-CT93-0249.

1471997 SCI. J. Chem. T echnol. Biotechnol. 0268-2575/97/$17.50. Printed in Great Britain(

148 E. Otal, D. Mantzavinos, M. V . Delgado, R. Hellenbrand, J. L ebrato, I. S. Metcalfe, A. G. L ivingston

NOTATION

BOD Biological oxygen demand (ppm)COD Chemical oxygen demand (ppm)GPC Gel permeation cChromatographyMW Molecular weight (g mol~1)PEG Polyethylene glycolsTOC Total organic carbon (ppm)UV/VIS UV and visible light detectorWAO Wet air oxidation

1 INTRODUCTION

Wastewaters produced in many industrial processesoften contain organic compounds that are not amenableto direct biological treatment. Although such waste-waters may be treated by means of incineration andadvanced chemical oxidation processes (i.e. ozonation,wet air oxidation, oxidation), the latter areUV/H2O2considered to be generally less environmentally friendlyand more costly than biological treatment processes.

Polyethylene glycols (PEG: areHO(CH2CH2O)nH)

an important group of non-ionic synthetic water-solublepolymers of ethylene oxide. These compounds are com-monly used in the production of lubricants, phar-maceuticals, cosmetics and antifreeze for automobileradiators, in the conservation treatment of ancientwaterlogged wood, and in the manufacture of non-ionicsurfactants. With this widespread industrial and domes-tic use of PEGs, concern has been expressed about thefate of these polymers in the environment and severalstudies have been performed on their biodegradability.Haines and Alexander1 studied the aerobic degradationof PEGs using soil microorganisms and reported thateven PEGs up to 20 000 molecular weight (MW) werebiodegradable. Dwyer and Tiedje2,3 studied the anaero-bic degradation of PEGs and found that PEG 20 000could be metabolised by anaerobic bacteria, while therate of degradation was found to be inversely pro-portional to the number of ethylene glycol units presentin the molecule. In further studies, Kawai and co-workers4 studied the metabolism of PEGs using a syn-ergistic, mixed culture which was able to utilise PEGsup to 20 000 MW. Obradors and Aguilar5 studied theaerobic degradation of PEGs using a bacterial isolatederived from river sediments and reported biodegrada-tion of PEGs up to 14 000 MW. Sewage bacteria havebeen used by Watson and Jones6 to degrade PEGs upto 1500 MW, while Cox and Conway7 were able todegrade aerobically PEGs up to 4000 MW. Althoughthe above studies report that even high MW PEGs areamenable to biodegradation, quantitative assessment ofthe rate of biodegradation is not given. However, it isgenerally accepted1,2,6 that the rate of biodegradationdecreases with increasing MW, making high MW poly-mers practically non-biodegradable. Conversely, therate of chemical oxidation of PEGs by means of wet air

oxidation (WAO) was found to decrease with decreasingMW.8,9 Short-chain organic acids (e.g. acetic acid)which are formed during WAO of various classes oforganic compounds are known to be refractory tofurther oxidation ; total oxidation of these acids tocarbon dioxide is usually the rate-limiting step in WAOprocesses.10

Combining these two observations regarding therates of biological and chemical CwC bond scissionleads to an attractive potential alternative to completeoxidation of high MW non-biodegradable PEGsthrough chemical means. An integrated chemicalÈbiological treatment process comprising a short WAOpretreatment step to convert initially bioresistant highMW polymers to more readily biodegradable lowerMW intermediates, followed by biological degradationof these compounds (see Fig. 1) should be more efficientthan using any of the single processes on their own.Therefore, a great deal of research over the last decadehas focused on the use of integrated chemicalÈbiologicalprocesses to treat organic-containing wastewaters.11However, in most of the studies reported in the liter-ature, chemical pretreatment comprises processes suchas ozonation, UV irradiation, use of FentonÏs reagentand photocatalysis as well as various combinations ofthese processes, while relatively few studies report theuse of WAO as a pretreatment step.

As early as the 1970s, Wilhelmi and Ely12 demon-strated the efficiency of a two-step WAO and biologicalprocess to successfully treat wastewaters from pesticideand acrylonitrile manufacturing. They reported thatWAO increased the ratio of a pesticideBOD5/CODmanufacturing effluent from 0É27 (before WAO) to 0É85(after WAO), thus enhancing the biotreatability of theeffluent. They also reported that an integrated processcould completely mineralise wastewaters from acrylo-nitrile manufacturing with more than 99% COD, BODand cyanide removal achieved. Copa and coworkers13studied the treatment of various acrylonitrile manufac-turing wastewaters in a three-step process comprisingWAO, evaporation/crystallisation (to remove ammon-

Fig. 1. Concept of integrated WAO and biological wastewatertreatment.

T reatment of polyethylene glycol-containing wastewaters 149

ium sulphate) and biological activated carbon treatment(carbon adsorption combined with biologicalassimilation) and found that the integrated processcould produce a Ðnal effluent suitable for discharge intosurface waters. Mishra and coworkers14 also investi-gated the treatment of an acrylonitrile manufacturingeffluent by means of WAO followed by aerobic activat-ed sludge treatment. Wastewater from an acrylonitrileplant with an initial COD of 120 000 ppm was sub-jected to WAO for 8 h at 498 K after which 65% CODremoval occurred. The wet oxidised effluent was thendiluted by a factor of four and the resulting effluent(with a COD of 10 000 ppm) could be further degradedaerobically with a COD removal from 72% to 95É6%(at residence times in the biological reactor from 5 to 25days respectively). However, after biological treatmentthe effluent was still highly coloured and an additionalpowdered activated carbon treatment step was neededto remove colour.

Chakchouk and coworkers,15 who studied the WAO(promoted by hydrogen peroxide) and subsequentaerobic biodegradation of olive mill wastewaters,reported that the poor biodegradability of the originalwastewater (mainly due to the presence of polyphenols,phenolic acids and tannins) was signiÐcantly improvedafter WAO pretreatment. This was thought to be due tothe oxidation of the original compounds to lowerorganic acids such as formic, acetic, oxalic and succinicacids. Hao and coworkers16 investigated the impact ofWAO on the biotreatability of TNT red water usingthree di†erent microbial cultures and found that, ingeneral, WAO decreased the biodegradability of theeffluent by introducing toxic and/or inhibitory e†ects.They also reported that, in some cases, the biodegrad-ability of the wet oxidised effluents decreased as theWAO treatment temperature increased (from 533 K to613 K). The treatment of a model, phenol-containingwastewater by means of an integrated WAO and acti-vated sludge process was demonstrated by Lin andChuang.17 Wastewater containing 3 g dm~3 of phenolwas subjected to WAO at 473 K for 15 min and 30 minafter which about 75% and 85% COD removal wasrespectively achieved. The wet oxidised effluents werethen aerobically oxidised with acclimatised activatedsludge and it was found that 70% and 89% CODremoval occurred (for the effluents after 15 min and30 min of WAO respectively) after 240 min of aerobicoxidation, while only 3É6% COD removal was observedat 360 min of aerobic oxidation (using the same sludge)for the original, unoxidised phenol solution. Neverthe-less, the biodegradability of a model, multicomponentphenolic wastewater (containing phenol, m-chlorophenol, m-cresol and p-ethylphenol at a concen-tration of 3 g dm~3 each) was found to increase onlymarginally after 30 or 60 min of WAO at 473 K. It wasthought that WAO conditions were not severe enoughto remove the phenolic compounds, due to the higher

initial concentration of the multicomponent wastewater(compared with that of the single phenol wastewater),and it was suggested that extended WAO would beneeded prior to further biological treatment.

The above studies reported in the literature show thatpretreatment by WAO generally results in effluentswhose biodegradability is higher than that of the orig-inal wastewater. However, in some cases the oppositee†ect has been observed since WAO may have led tothe formation of intermediates which would have beenmore toxic or inhibitory than the original compounds.To design an efficient integrated process on a rationalbasis, knowledge of the properties of the major reactionintermediates and their susceptibility to degradation byeach process is required. The resistance of these inter-mediates to further chemical oxidation as well as theirbiodegradability and toxicity will directly inÑuence thee†ectiveness of each step and determine the optimumdegree of oxidation at which it would be optimal toswitch processes.11

It is also notable that relatively few studies report thee†ect of chemical pretreatment on the biodegradabilityof PEGs and other polymers. Suzuki and coworkers18studied the ozonation and subsequent fragmentation ofPEG 8000 and found that a soil microorganism whichwas not capable of utilising PEG 8000 could easilydegrade PEG 300. In further studies, Suzuki andcoworkers19 studied the e†ect of ozonation on the bio-degradability of various water-soluble polymers andreported that a signiÐcant improvement in biodegrad-ability occurred after ozonation pretreatment. Imamuraand coworkers8 studied the WAO of various MWPEGs between 200 and 20 000 and found that the bio-degradability of the oxidised compounds, as assessed bythe ratio, was improved compared to thatBOD5/CODof the original polymers.

The aim of this work was to study the integratedtreatment of a model wastewater containing PEG10 000 by means of a continuous WAO process fol-lowed by continuous aerobic oxidation, while the effi-ciency of the integrated treatment process wascompared with that of a direct biological treatment ofPEG-containing wastewaters. To establish an inte-grated process on a rational basis, information regard-ing the e†ect of WAO on PEG 10 000 was obtainedfrom a previous study,9 and shake Ñasks experimentswere performed to assess the aerobic biodegradability ofvarious MW PEGs.

2 EXPERIMENTAL AND ANALYTICAL

2.1 Continuous WAO

A high pressure autoclave (Baskerville ScientiÐc Ltd,UK) capable of performing semibatch or continuousexperiments at pressures up to 10 MPa and tem-peratures up to 573 K was used for WAO experiments

150 E. Otal, D. Mantzavinos, M. V . Delgado, R. Hellenbrand, J. L ebrato, I. S. Metcalfe, A. G. L ivingston

and is described in detail elsewhere.9 Continuous WAOof 1 g dm~3 solutions of PEG 10 000 was performed at423 K and at a residence time of 30 min, with anoxygen partial pressure of 3 MPa. The solution wascontinuously fed from a 5 dm~3 feed tank to the reac-tion vessel through a high pressure double diaphragmpump (Lewa FC1, Germany) at a Ñowrate of9É33 cm3 min~1, while oxygen was continuouslysparged at a Ñowrate of 1 dm3 min~1. The reactor con-tents were stirred by means of a gas-inducing typeimpeller at a stirring speed of 1100 rpm, while a 2 kWelectric heater was used to heat the reaction vessel tothe desired temperature. A liquid level conductivityprobe maintained the liquid holdup of the reactor at280 cm3 by discontinuously opening and closing an airactuated liquid outlet valve, and the discharged liquidwas cooled and collected in a storage tank. The pH ofthe solution was adjusted to 3É5 using TheH2SO4 .experiment was allowed to proceed for four to sixresidence times during which TOC removal rate wasmeasured. It was found that, at the operating conditionsunder consideration, a steady-state TOC of the reactoreffluent was reached after less than 120 min (fourresidence times) of operation.

2.2 Continuous aerobic oxidation

Biological oxidation experiments were performed in twoidentical fermentation systems manufactured by New

Brunswick ScientiÐc Ltd, UK. Each system comprised a2 dm3 Ñat-bottomed stirred glass vessel. An electricheating jacket was wrapped around the vessel to heat itto the desired temperature. Air was introduced via asparge tube situated at the bottom of the vessel, and gasÑow could be adjusted up to a maximum Ñowrate of2 dm3 min~1. pH was measured by an Ingold pH elec-trode and controlled through two peristaltic pumpscapable of delivering acid or alkali (as necessary) to thevessel. Dissolved oxygen level was monitored by aPhoenix galvanic probe.

Figure 2 shows a schematic diagram of the integratedWAO and biological treatment process. Continuousaerobic biological oxidation experiments using bothunoxidised PEG 10 000 and the wet oxidised solutionswere performed simultaneously in the two identical fer-mentation systems, at residence times of 0É5, 1, 2 and 4days. Temperature in the bioreactors was held at 308 K,the pH at 7 ^ 0É15 units (using 1 mol dm~3 solutions of

or KOH), the liquid holdup at 1800 ^ 50 cm3,H2SO4while laboratory air was continuously fed at a Ñowrateof 0É9 dm3 min~1 and the stirring speed was set to210 rpm. A multichannel peristaltic pump (WM 205S,Watson Marlow Ltd, UK) was used to continuouslyfeed and discharge the vessels, while organic feedstockand nutrient medium were fed to the bioreactorsseparately at equal Ñowrates. Initially, batch experi-ments were performed to allow the culture to acclima-tise to the substrate (either PEG 10 000 or wet oxidised

Fig. 2. Schematic diagram of integrated WAO and biological treatment process.

T reatment of polyethylene glycol-containing wastewaters 151

solution). After 2 weeks of acclimation, 710 cm3 ofliquid (containing culture, biomedium and residualorganics) taken from the batch runs were mixed with1É09 dm3 of the corresponding fresh feed and contin-uous operation commenced. Samples of about 10 cm3were periodically withdrawn from the fermenters, Ðl-tered with a 0É45 km disposable Ðlter and analysed withrespect to their TOC content. TOC of the fermentationfeedstocks was also measured periodically.

2.3 Inoculum

A facultative, mixed culture which was isolated from adomestic treatment plant in Seville, Spain and adaptedto PEG 10 000 for more than 2 years, was used in thisstudy. The culture was inoculated with a carbon-freenutrient medium, the composition of which was asfollows : 2 mg dm~3 8É8 mg dm~3Na2EDTA,

8 mg dm~3ZnSO4 . 7H2O, FeCl3 . 6H2O,1É232 mg dm~3 0É3136 mg dm~3MnSO4 . H2O,

587É2 mg dm~3CoCl3 . 7H2O, MgSO4 . 7H2O,133É6 mg dm~3 0É38 mg dm~3CaCl2 . H2O,

0É4 mg dm~3Na2MoO4 . 2H2O, CuSO4 . 5H2O,2655 mg dm~3 2080 mg dm~3KH2PO4 , K2HPO4and 2000 mg dm~3 (NH4)2SO4 .

2.4 Shake Ñasks experiments

Shake Ñasks experiments were performed with1 g dm~3 solutions of PEGs with MW varying from200 to 35 000. Flasks containing 190 cm3 of the corre-sponding aqueous solution of PEG and 10 cm3 of inoc-ulum taken from both the fermenters during continuousoperation (5 cm3 from each) were placed in a thermo-statically controlled waterbath at 308 ^ 1 K andshaken at 180 rpm. Samples periodically drawn fromthe Ñasks were analysed with respect to their TOCcontent and biomass growth. The latter was assessed bymeasuring the absorbance of the sample at 660 nm witha Philips UV/VIS spectrophotometer (PU 8625, UK).

2.5 Total organic carbon (TOC) assay

The degree of total oxidation to carbon dioxide thathad occurred during WAO and biological oxidationwas assessed by measuring the liquid phase totalorganic carbon content. TOC was measured with a Shi-madzu 5050 TOC Analyser whose operation is basedon combustion/non-dispersive infrared (NDIR) gasanalysis. The uncertainty in this assay, quoted as therelative standard deviation of three separate measure-ments, was never larger than 2% for the range of TOCconcentrations measured.

3 RESULTS AND DISCUSSION

3.1 E†ect of MW on aerobic biodegradability of PEGs

The results from the shake Ñasks experiments per-formed to assess the e†ect of MW on the aerobic biode-gradability of PEGs are shown in Figs 3 and 4. Figure 3shows the change in biomass concentration over theexperiment where the normalised absorbance is(I/I0)plotted against time (the initial absorbance wasI0about 0É1 absorbance units in all cases), while Fig. 4shows the TOCÈtime proÐle. It can be seen that thebiomass concentration for PEG 200 increased withinthe Ðrst 16 h, while a lag phase of 16È22 h was observedfor PEGs 1000 and 6000 before biomass increased. Thebiomass concentration for all of the PEGs up to6000 MW reached a maximum at about 90È100 h, afterwhich it began to decrease. These results are in verygood agreement with the TOC removal rates observedduring the shake Ñasks experiments. Within the Ðrst22 h, 15% TOC removal was recorded for PEG 200,while only 4% and 2% removal was recorded for PEGs1000 and 6000 respectively. However after about 100 h,90% TOC removal occurred for all of the PEGs up to

Fig. 3. Biomass growth during shake Ñasks experiments forvarious MW PEGs. (a) PEG 200, PEG 1000,È|È ÈKÈ

PEG 6000. (b) PEG 10 000, ÈÂÈ PEGÈ=È ÈLÈ20 000, PEG 35 000.È…È

152 E. Otal, D. Mantzavinos, M. V . Delgado, R. Hellenbrand, J. L ebrato, I. S. Metcalfe, A. G. L ivingston

Fig. 4. TOC proÐle during shake Ñasks experiments forvarious MW PEGs. (a) PEG 200, PEG 1000,È|È ÈKÈ

PEG 6000. (b) PEG 10 000, ÈÂÈ PEGÈ=È ÈLÈ20 000, PEG 35 000.È…È

6000 MW. This also explains why the biomass concen-tration decreased after this time.

Growth on PEG 10 000 exhibited a lag phase ofabout 90 h during which neither biomass concentrationnor TOC removal were found to increase signiÐcantly.However within the next 50 h, TOC rapidly decreasedto about 7% of its initial value. Under the conditionsused in this study, the culture which had been enrichedon PEG 10 000 was found to be incapable of utilisingeither PEG 20 000 or PEG 35 000. The shake Ñasksexperiments for PEGs 20 000 and 35 000 were furtherextended to 720 h, after which time biodegradation hadstill not taken place.

It is also notable that although the length of the lagphase seems to increase with increasing MW, the TOCremoval rate during the growth period seems to beindependent of the MW. Comparing the slopes of TOCremoval curves (Fig. 4) subsequent to the lag phase, itcan be seen that the biodegradation rates for variousMW PEGs up to 10 000 are comparable. A possibleexplanation for this behaviour would involve themechanism through which microorganisms utilisePEGs. It could be hypothesised that after the micro-organisms adapt to the original PEG (i.e. after the lagphase) they do not directly convert the organic sub-

strate to carbon dioxide but rather fragment the orig-inal polymer to lower MW metabolic intermediateswhich are then assimilated and oxidised further tocarbon dioxide. These metabolic intermediates couldpossibly be common no matter how large the originalpolymer is, and this would explain why the TOCremoval rates for various MW PEGs were comparableto each other.

In previous studies, the mechanisms involved in theaerobic biodegradation of PEGs were investigated.Haines and Alexander1 suggested that the aerobic deg-radation of PEGs involved the participation of a PEG-hydrolysing extracellular enzyme and reported that cellsdid not directly degrade the original polymer but ratherdegraded the fragments generated extracellularly, suchfragments mainly being ethylene glycol and diethyleneglycol. Obradors and Aguilar,5 who also studied theaerobic degradation of PEGs, suggested that PEGswould cross the outer cellular membrane, enter the peri-plasm and be subsequently metabolised in the peri-plasmic space, while they ascribed the inability of theirculture to metabolise PEGs with MW above 14 000 tothe existence of a permeability barrier. They also foundthat, in contrast to the extracellular enzymatic activityreported by Haines and Alexander,1 an intracellularPEG-dehydrogenase was responsible for catalysingPEG oxidation. Although a thorough investigation onthe biodegradation mechanisms of PEGs was notattempted in the present study, the results obtainedseem to be consistent with those reported by Hainesand Alexander.1

3.2 E†ect of WAO pretreatment of PEG 10 000

Continuous uncatalysed WAO of PEG 10 000 was per-formed at 423 K at a residence time of 30 min. Theseconditions were chosen since a previous study9 showedthat mild WAO conditions suffice to fragment theoriginal PEG 10 000 to lower MW compounds. Figure5 shows the MW distribution during the semibatch oxi-dation of PEG 10 000 at 423 K, 3 MPa of oxygenpartial pressure and various reaction times. Resultswere obtained by gel permeation chromatography(GPC), and the refractive index response is plottedagainst elution time (analytical conditions described indetail elsewhere9). Peak A corresponds to the originalsolution of PEG 10 000, while peak 1 corresponds toMW D300 and peak 2 to MW 62 (ethylene glycol). Itcan be seen that even after 5 min of semibatch oxida-tion at 423 K (curve B) nearly all of the originalpolymer has been converted to lower MW compounds,while after 30 min at 423 K (curve D) only polymericfractions of MW less than about 300 are present in thereaction mixture. Although the wet oxidised effluentreported in the present study was not analysed withrespect to its composition, these earlier results indicate

T reatment of polyethylene glycol-containing wastewaters 153

Fig. 5. The MW distribution during semibatch WAO of PEG10 000 at 423 K and various reaction times as given by GPCanalysis. A : Original PEG 10 000, B : 5 min, C : 15 min, D:30 min, E : 60 min. Peak 1 corresponds to MW D300 and

peak 2 to MW 62 (ethylene glycol).

that continuous WAO at 423 K for 30 min wouldsuffice to fragment the original polymer to lower MWcompounds. Under these conditions only about 7%TOC reduction (\oxidation to carbon dioxide) wasobserved.

3.3 Integrated WAO and aerobic treatment ofPEG 10 000

Both the wet oxidised and unoxidised solutions of PEG10 000 were inoculated and subjected to batch aerobicoxidation. After inoculation, the TOC content of bothsolutions increased by up to 20%, possibly due to someorganic carbon introduced/released by the inoculum.Figure 6 shows the TOCÈtime proÐle during the batchaerobic oxidation of both solutions. It can be seen thatafter a lag phase of 4 days during which no TOCremoval was observed, TOC began to decrease. Thiswas more pronounced for the wet oxidised solution inwhich a 26% TOC removed occurred between days 4and 5, while the TOC removal of the unoxidised PEG10 000 solution was signiÐcantly lower than that of theoxidised solution throughout the experiment. Only afterday 11 did the TOC removal rate of the original PEGsolution signiÐcantly increase, and at the end of thisexperiment 70% and 45% TOC removal were recordedfor the oxidised and unoxidised solutions respectively.

Fig. 6. TOC proÐle during batch aerobic oxidation. È…ÈUnoxidised PEG 10 000, after WAO.ÈLÈ

After this acclimation period, the operation wasswitched from batch to continuous operation at aresidence time of 2 days, the results of which are shownin Fig. 7. It can be seen that the TOC removal in Fer-menter 1 containing the oxidised solution increased to90% within the Ðrst 5 days of operation after which itremained remarkably stable. Conversely, the per-formance of Fermenter 2 containing the unoxidisedPEG was found to be quite unstable throughout theexperiment with TOC removal varying from as low as20% to as high as 90%. However after day 6 the TOCremoval in Fermenter 2 consistently decreased, while anincreasing wash-out of the culture was observed. At thispoint, 50 cm3 of liquid was swapped between Fer-menters 1 and 2 in an attempt to restore the per-formance of Fermenter 2 and to ensure that bothfermenters had equivalent opportunities to develop anenriched culture. This did not improve the performanceof Fermenter 2 and the TOC removal rate decreasedfurther. At day 10, Fermenter 2 was switched from con-tinuous to batch operation and 50 cm3 of liquid wastransferred from Fermenter 1 to Fermenter 2. After 2

Fig. 7. TOC removal during continuous aerobic oxidation.Residence time\ 2 days. Unoxidised PEG 10 000,È…È

after WAO. ` : 50 cm3 swapped between fermenters, B :ÈLÈ50 cm3 transferred from Fermenter 1 to Fermenter 2, È È È È :

batch operation.

154 E. Otal, D. Mantzavinos, M. V . Delgado, R. Hellenbrand, J. L ebrato, I. S. Metcalfe, A. G. L ivingston

Fig. 8. TOC removal during continuous aerobic oxidation.Residence time\ 4 days. Unoxidised PEG 10 000,È…È

after WAO.ÈLÈ

days of further batch operation, the TOC removalindeed increased from about 20% to about 70%. At theend of this experiment, Fermenter 2 was switched againto continuous operation and the residence time wasincreased to 4 days in both fermenters.

Fig. 9. TOC removal during continuous aerobic oxidation.Residence time\ 1 day. Unoxidised PEG 10 000,È…È

after WAO. ` : 50 cm3 swapped between fermenters,ÈLÈÈ È È È : batch operation.

Fig. 10. TOC removal during continuous aerobic oxidation.Residence time\ 0É5 day. Unoxidised PEG 10 000,È…È

after WAO.ÈLÈ

Figure 8 shows the subsequent TOC removal rateduring continuous operation at a residence time of 4days. A TOC removal of 93% was achieved in Fer-menter 1 with its performance remaining stablethroughout the experiment. The TOC removal rate inFermenter 2 still Ñuctuated between 60% and 70%,although its performance was clearly more stable thanthat during the 2-day residence time experiment.

At the end of this experiment, the residence time wasdecreased from 4 days to 1 day ; the results are shown inFig. 9. It can be seen that both fermenters exhibitedrather unstable behaviour, although the TOC removalin Fermenter 1 was generally higher than that in Fer-menter 2. After day 8, the TOC removal in Fermenter 2rapidly decreased, and an increasing wash-out of theculture was observed. At day 11, Fermenter 2 wasswitched to batch operation and 50 cm3 of liquid wasswapped between Fermenters 1 and 2 (50 cm3 of liquidwas also swapped at days 7 and 13). After 2 days offurther batch operation, the TOC removal in Fermenter2 increased only slightly from 16% to 28%. At the endof this run, Fermenter 2 was switched back to contin-uous operation.

In a Ðnal experiment, the residence time wasdecreased further to 0É5 day and the results are shownin Fig. 10. An 80% TOC removal was achieved in Fer-menter 1 and this value remained remarkably stablethroughout the length of the experiment (this was 5days for Fermenter 1, while the experiment wasextended to 7 days for Fermenter 2). Conversely, theTOC removal in Fermenter 2 was as low as about 5%during the Ðrst 3 days of operation. Then, a suddenTOC removal increase was observed between day 3 and4 after which TOC removal dropped again to about 5%and remained at this level for the rest of the experiment.At the end of this experiment, Fermenter 2 was oper-ated in batch mode for seven more days during whichthe TOC removal remained unchanged at the level of5% (not shown).

The results from Figs 7È10 clearly show that WAO ofPEG 10 000 at mild operating conditions (30 min at423 K) dramatically improves the aerobic bio-treatability of the original solution. At residence timesas low as half a day, an 80% TOC removal could beachieved during the aerobic oxidation of the wet oxi-dised solution, while the unoxidised solution could notbe degraded to a signiÐcant extent under these condi-tions. Even at residence times as high as 4 days, thebiodegradation rate of the oxidised solution was clearlyhigher than that of the unoxidised solution.

4 CONCLUSIONS

In this work, a continuous integrated process compris-ing WAO and subsequent aerobic oxidation to treat a

T reatment of polyethylene glycol-containing wastewaters 155

TABLE 1Mean TOC Removal During Integrated WAO and Biological Treatment of PEG 10 000

Residence time, days T OC after T OC after biological Overall T OC removal(biological oxidation) W AO, ppm oxidation, ppm for integrated treatment,

%

0 514É5 È 6É80É5 512 109^ 5 80É3 ^ 0É80É5a È 526 ^ 8 4É8 ^ 1É42b 511É6 48 ^ 7 91É2 ^ 1É32b È 353 ^ 70 35É3 ^ 12É94 518É7 36 ^ 5 93É5 ^ 0É84c È 179 ^ 28 67É9 ^ 5

a Values from day 3È5 are not included.b Values averaged after day 6.c Value at day 2 is not included.Initial TOC\ 550 ^ 7 ppm.WAO conditions : 30 min at 423 K and 3 MPa of oxygen partial pressure.

Fig. 11. Efficiency of integrated process and comparison withdirect biological treatment.

model wastewater containing PEG 10 000 was studied.Table 1 and Fig. 11 summarise the results. While theresults of the biological work are the results of singletrials which due to the nature of these experiments showconsiderable internal variability, it can be seen that ashort WAO pretreatment step may reduce the bio-reactor space time required by at least an order of mag-nitude. For instance, an overall 80% TOC removalcould be achieved when WAO was coupled with a0É5-day residence time of subsequent biological treat-ment, while only about 60È70% TOC removal could beachieved after a 4-day residence time of direct biologicaltreatment.

Process integration seems conceptually advantageousto treat wastewaters containing high MW polymerssince their biodegradation is usually restricted bymolecular size, and chemical pretreatment can convertthese polymers to lower MW, more readily biodegrad-able compounds. For the optimisation of the integratedprocess, knowledge of the properties of the main reac-tion intermediates, reaction mechanisms and pathwaysplay an important role, as they can provide the neces-sary insights into the observed synergism. This allows asystematic approach to be used in designing an efficientintegrated treatment process.

ACKNOWLEDGEMENTS

The authors would like to thank the Commission of theEuropean Communities for the Ðnancial support of thiswork under grant no. EV5V-CT93-0249.

REFERENCES

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