Integration of Wet Oxidation and Nanofiltration for Treatment of

Wet oxidation and nanofiltration are employed in an integrated system for the treatment of bioresistant industrial wastewaters. The partial oxidation ...
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Ind. Eng. Chem. Res. 1997, 36, 5054-5062

KINETICS, CATALYSIS, AND REACTION ENGINEERING Integration of Wet Oxidation and Nanofiltration for Treatment of Recalcitrant Organics in Wastewater Rolf Hellenbrand, Dionissios Mantzavinos, Ian S. Metcalfe, and Andrew G. Livingston* Department of Chemical Engineering and Chemical Technology, Imperial College of Science, Technology and Medicine, London SW7 2BY, United Kingdom

Wet oxidation and nanofiltration are employed in an integrated system for the treatment of bioresistant industrial wastewaters. The partial oxidation products formed during a brief period of pretreatment in a low pressure WO reactor are separated by nanofiltration, and larger molecules are recycled into the reactor where they undergo further oxidation. Experiments were carried out with polyethylene glycol as a model compound in aqueous solution, representing polymer manufacture wastewater. The results demonstrate that by using a combination of an oxidative and a separative step, a higher selectivity of the overall process toward partial oxidation can be achieved. The permeate leaving the filtration step is mainly composed of short chain organic acids which could be readily biodegraded in a subsequent biological treatment, or discharged if concentrations are low. Introduction Wastewaters produced in many industrial processes contain organic compounds which are not amenable to conventional biological oxidation. This has led to the development of several alternative oxidation processes ranging from wet oxidation, ozonation, and UV irradiation to electrochemical treatment [Scott and Ollis, 1995]. However, these are often considerably more expensive than biological treatment. Most of the investigations into wet oxidation reported in the literature focus on total destruction of organic compounds to produce water and CO2 [Mishra et al., 1995]. Interestingly, studies of wet oxidation of recalcitrant organics have revealed that the rate-limiting step in complete conversion of organics to carbon dioxide is often the oxidation of short chain carboxylic acids formed as oxidation intermediates from the original compounds [Imamura et al., 1982]. Since many of these short chain acids are readily biodegradable, an obvious alternative to using chemical oxidation for complete conversion of organics to carbon dioxide is to use an integrated chemical-biological process to chemically convert biologically recalcitrant molecules to intermediates (such as short chain acids) which are further amenable to biological degradation. It has been shown that combining partial wet oxidation with subsequent activated sludge treatment was successful in reducing phenolic compound concentrations in wastewater [Lin and Chuang, 1994]. This method was tested further, employing the biological/chemical oxygen demand (BOD/COD) ratio as a parameter to quantify improvements in the biodegradability of wastewaters achieved using wet oxidation as a pretreatment [Kawabata and Urano, 1985]. Improvements in biodegradability of organic compounds subjected to ozonation have also been documented by several authors [Scott and * Author to whom all correspondence should be addressed. Telephone: 00 44 171 594 5582. Fax: +44 171 594 5604. E-mail: [email protected]. S0888-5885(97)00417-X CCC: $14.00

Ollis, 1995; Calvosa et al., 1991; Heinzle et al., 1992; Schmitt and Hempel, 1993]. Although the above studies show the beneficial effects of integrated processes, most of them suffer from a lack of information on how to combine pretreatment and downstream processes. Detailed studies concerning the chemical step are desirable for the design of an efficient integrated process, allowing a systematic approach to be used in deciding on the necessary degree of chemical oxidation in the pretreatment step [Scott and Ollis, 1995]. A major problem of partial wet oxidation is that compounds in the wastewater show differing degrees of resistance to chemical oxidation. This may result in the formation of readily biodegradable intermediates early on during the reaction. If the oxidation is allowed to proceed to an extent where even the more resistant molecules in the mixture are converted into biodegradable intermediates, then biodegradable intermediates formed earlier in the reaction may already have proceeded through total oxidation to carbon dioxide. This is not desirable during the chemical oxidation stage of a chemical-biological treatment, as it constitutes a waste of oxidant, and the oxidation of biodegradable intermediates would be more efficiently achieved in the downstream biological system. We have shown earlier that it is difficult to control the selectivity of wet oxidation toward partial oxidation reactions [Mantzavinos et al., 1996a]. This is not necessarily overcome by the use of catalysts, which often show a tendency to enhance the oxidation of partial oxidation intermediates rather than the original substrate, thereby promoting total oxidation [Mantzavinos et al., 1996b]. Clearly, it would be useful to separate partially oxidized reaction products from unreacted molecules as they are formed, and subject the remaining unoxidized or lightly oxidized molecules to further oxidation. This can be achieved by integrating wet oxidation and nanofiltration. The purpose of the present paper is to demonstrate that by using a combination of an oxidative © 1997 American Chemical Society

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and a separative step, a higher selectivity of the overall process toward partial oxidation can be achieved. Implications for further development of the process are also discussed. Combined Oxidation/Separation Systems There have been numerous attempts to enhance wet oxidation with additional process steps. Levec describes a system where adsorption on carbon is used to concentrate organics in wastewater. After desorption they are wet oxidized in a separate step with the help of catalysts [Levec and Pintar, 1994, 1995]. This system is particularly useful for dilute solutions of organics in water. Asahi Chemical Industries have patented a system in which the effluent of a wet oxidation plant is treated by reverse osmosis, which is claimed to result in a permeate of very low COD, as a large proportion of organics and also inorganics are retained by the membrane system [Asahi Chemical Industries, 1974]. However, the fate of the inorganics and organics, which are resistant to further oxidation and do not permeate RO membranes, is not described. Rautenbach and Mellis describe a system which is based on an intensified biological reactor and a nanofiltration membrane retaining nondegraded organics, with the option of adding an oxidation or a carbon adsorption step in the recycle [Mellis, 1994; Rautenbach and Mellis, 1994, 1995]. In the field of photocatalysis, an integrated membraneoxidation system has been developed in which organics are degraded by catalytic photo-oxidation with the catalyst being immobilized on the surface of a porous ceramic membrane [Bellobono et al., 1994]. Such membranes do not usually have pores that are small enough to separate reaction products from the feed. Full scale wet oxidation systems are often used in conjunction with membrane filtration steps [Luck, 1996]. The major use of membrane filtration is as a preconditioning step for the reactor feed, which may include dilution, concentration, and blending with wastewaters of different chemical composition [Radant et al., 1995]. Whereas blending and dilution, e.g., with reactor effluent, are relatively straightforward, concentration of organics to achieve autothermal operation of the oxidation reactor can only practicably be achieved by membrane processes such as reverse osmosis or nanofiltration, since evaporation is excluded due to its highenergy requirements. Membranes are also used to recover catalyst and unoxidized solids from sludge treatment [Luck, 1996]. Homogeneous catalysts are precipitated in the form of sulfide and then fed back into the reactor as slurry which dissolves due to the acid and oxidative conditions. However, ceramic microfiltration membranes employed in this process are normally not fine enough to retain unoxidized molecules. In this paper, we describe an innovative process (OXYMEM), in which a nanofiltration membrane is used to retain molecules in a wet oxidation reactor until they have been oxidized to the desired extent. To the best of the authors’ knowledge, this is the first report of such a process. Process Development Figure 1 shows the principle of the combined wet oxidation-nanofiltration process (OXYMEM). The wastewater is fed into the oxidation system where a partial conversion of organic molecules into molecules of a higher degree of oxidation takes place. With

Figure 1. Principle of combined wet oxidation-nanofiltration process.

polymer waste, this is often readily achieved at temperatures just above 373 K and oxygen partial pressures below 1 MPa [Mantzavinos et al., 1996a]. Oxidation intermediates typically have molecular weights several times smaller than the original molecules. The partially oxidized mixture is then recirculated over a nanofiltration membrane at a high crossflow velocity. Water, salts, and the smaller partially oxidized organics permeate, whereas unoxidized organics are completely retained. The retentate, which contains unoxidized or lightly oxidized organics in a higher concentration, is fed back to the wet oxidation reactor in addition to the fresh feed. The residence time of organic molecules in the system is variable and depends on their resistance to chemical oxidation; this is in contrast to conventional wet oxidation in which all organics remain in the reactor on average for a single hydraulic residence time. The permeate is finally subjected to further treatment, e.g., by biodegradation, or discharged if it meets consent limits. It should be noted that the OXYMEM process is limited to treating organic compounds whose partial oxidation intermediates have a molecular weight several times smaller than the starting compound. Thus, the process is unlikely to be useful at this stage for dealing with some important categories of wastes, such as phenolic compounds. The steady-state concentration of retained organics which accumulate in the recycle depends on the volumetric recycle ratio and the operating conditions chosen for the reactor, as well as the selectivity of the membrane. The overall rate of oxidation in the loop is a function of the organics concentration. Steady state as defined by a carbon mass balance is reached when the rate at which carbon enters the loop in the feed stream equals the rate at which carbon exits the loop in the form of small oxidation intermediates passing through the membrane (and CO2 in the reactor off-gas, which will be shown to be negligible). Assuming that the total rate of oxidation is proportional to the concentration of organics in the loop, this concentration will adjust until these flows of carbon are equal. Clearly, the chemical composition of the mixture in the loop may still change after a constant carbon concentration is reached. Stable operation of the system is guaranteed as long

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as compounds that do not undergo further oxidation at the chosen conditions can permeate, otherwise a purge stream becomes necessary. In practice, even compounds that have high rejection coefficients will not accumulate indefinitely, as nonidealities in the pore-size distribution of the membrane and leaks lead to a limited permeability, even for very large molecules. It can be seen that the OXYMEM system will behave quite differently from a conventional continuous wet oxidation system. It can also be expected that membranes will show different permeabilities for wet oxidation products than for the original wastewater constituents. Therefore, a systematic strategy was adapted for the investigation and development of the process. Experiments were conducted in three phases. In the first phase, separation characteristics of nanofiltration membranes were determined with solutions of pure compounds. In a second phase, fixed volumes of polymer solution were first treated by continuous wet oxidation at different temperatures and residence times. The reactor effluents were then fractionated in batch membrane experiments in order to determine the dependence of organics rejection on wet oxidation operating parameters. As multicomponent rejection in aqueous solutions of organics by a nanofiltration membrane cannot yet be predicted to a satisfactory extent, an empirical approach was necessary. This phase also served for the determination of suitable operating parameters for the oxidative step of the final phase, in which the integrated process is run continuously by adding fresh feed to the system and continuously withdrawing permeate from the separation step at an identical flow rate. Selection of Model Compound Polyethylene glycols (PEG: HO(CH2CH2O)nH) are an important group of nonionic synthetic water-soluble polymers of ethylene oxide. These compounds are commonly used in the production of lubricants, pharmaceuticals, cosmetics, and antifreeze for automobile radiators, in the conservation treatment of ancient waterlogged wood, and in the manufacture of nonionic surfactants. With this widespread industrial and domestic use of PEGs, concern has been expressed regarding the fate of wastewaters containing these polymers. In addition, PEG can be used as a model compound for many linear water-soluble polymers such as polyalkylene glycols (PAG), another important class of industrial polymers most often manufactured using a mixture of polyethylene oxides and polypropylene oxides. Differences in chemical structures such as methyl side chains make PAGs even more stable and resistant to degradation. Although a range of other compounds are suitable to demonstrate an integrated treatment system, polymers in particular can illustrate the benefits of the proposed system, as their oxidation intermediates cover a large range of molecular weights. Selection of Membranes In order to be capable of treating a wet oxidation reactor effluent, membranes have to fulfill a number of requirements. Apart from retaining the organic compounds above a molecular weight in a range between 200 and 1000 g mol-1, they have to be stable against chemical attack by the effluent and resistant to oxidation by excess oxidant still present in the oxidized process stream. To allow simple and economic interfac-

ing with the wet oxidation process, they should also be stable under elevated temperatures close to the temperatures used in the oxidation step. However, as this present work is concerned with demonstrating the potential of a process still under development, standard polymeric nanofiltration membranes (AFC40 from PCI, Whitchurch, U.K.) which are widely used in research [Linde and Jo¨nnson, 1995] have been used initially. Partial Oxidation Assessment The partial oxidation of polymers cannot be described with a classic reaction engineering approach based on trying to identify intermediates and their concentration-time profiles. Due to the free radical chain mechanism which is generally accepted as governing this type of reaction [Schnabel, 1981], a large number of intermediates result from partial oxidation which are similar in their chemical nature but differ in their degree of oxidation and molecular size. Detailed information about physicochemical properties of partially reacted molecules such as molecular weight distribution and specific intermediates that result from degradation reactions can only be obtained by state-of-the-art analytical methods, which are often limited to the analysis of model compounds and fail to provide similar information in the case of real wastewater samples. Therefore, methods for assessing the degree of partial oxidation will be restricted to using lumped parameters such as TOC and COD and derived parameters. Total Organic Carbon. The parameter used most commonly to describe the performance of wet oxidation is reduction of TOC, which is proportional to the concentration of organics in solution. However, this parameter is limited to describing total oxidation, e.g., the conversion of organic molecules to CO2, and remains unchanged at mild operating conditions where little or no total oxidation occurs. As total oxidation to CO2 is not a desirable reaction in the context of wet oxidation used as a pretreatment step for further reaction or separation, this parameter, if used in an isolated way, must be seen as not providing any useful insight with respect to partial oxidation. However, it becomes more useful in combination with other parameters to describe the selectivity of a reaction or a process toward partial oxidation. Chemical Oxygen Demand. COD data used on its own cannot differentiate between partial and total oxidation, as both types of reaction contribute to a decrease of its value. Nevertheless, many authors use COD for establishing oxidation kinetics or assessing the efficiency of an oxidative wastewater treatment process [Scott and Ollis, 1995]. In order to differentiate between partial and total oxidation, the oxidation state of organic carbon must be measured rather than how much oxygen is still required at a certain reaction time to achieve a given degree of oxidation. This is possible if COD and TOC are combined in a suitable way. Combined Parameters. There are various suggestions in the literature on how to define a partial oxidation parameter [Scott and Ollis, 1995; Jochimsen et al., 1996]. The average oxidation state of carbon atoms (AOSC) in a mixture can be calculated according to the following equation:

AOSC )

4(TOC - COD) TOC

(1)

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using molar concentrations of TOC and COD. The theoretical AOSC is -4 for methane, -1 for EG and PEGs, 0 for acetic acid, +2 for formic acid, +3 for oxalic acid, and finally +4 for carbon dioxide. 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 high oxidation state of the organics in the mixture, but cannot indicate how much partial and total oxidation has taken place since the start of the reaction. Clearly this parameter only reflects the oxidation state at one given reaction time, and it cannot be used for assessing the progress of partial oxidation reaction or comparing the efficiency of two partial oxidation processes. Selectivity toward Partial Oxidation. In order to compare the selectivity of two processes toward partial oxidation, another concept has been proposed [Jochimsen et al., 1996]. COD reduction by partial oxidation is expressed by comparing COD/TOC ratios at different oxidation times to the original ratio as follows:

CODpartox )

(

)

COD0 COD TOC ) TOC0 TOC TOC COD0 - COD (2) TOC0

The efficiency of COD removal through partial preoxidation can then be defined as the ratio of COD removal by partial oxidation and total COD removal:

µCODpartox )

CODpartox COD0 - COD

(3)

µCODpartox reaches the value of 1 for the ideal case of no total oxidation occurring and 0 for only total oxidation occurring. Calculating µCODpartox from experimental data is a useful method for obtaining quantitative data on partial oxidation, as AOSC as well as simple COD/ TOC ratios are specific for the composition of a mixture at one given reaction time. Experimental Section Apparatus. Membrane Filtration System. A modified PROSCALE filtration system manufactured by MILLIPORE (France) was used for membrane separation experiments. PROSCALE is a tangential flow filtration system developed for use in laboratories, with fluid volumes from tens of liters to less than 500 mL. In order to be capable of treating relatively small amounts of wastewater samples at high crossflow velocities, the system has been equipped with a highflow hydraulic pump (1200 L h-1 at 40 bar maximum, Wanner, Minneapolis, MN) and a 1/2 in. tubular membrane module (MIC-RO 240, PCI Membrane Systems, U.K.) accommodating two membrane tubes with a total surface area of 240 cm2. The temperature of the retentate was kept constant at 313 K by means of a water-cooled heat exchanger controlled via an integrated DATAVIS system (Hartmann & Braun, Germany). Pressures at the retentate inlet and outlet of the module as well as the pump speed were monitored continuously. Permeate fluxes were measured gravimetrically using a computer-controlled balance (BALREAD software and series 410 balance, Sartorius, Germany).

Wet Oxidation Reactor. A stainless steel high-pressure reactor (Baskerville Ltd, U.K.) with 300 mL of liquid working volume was used to carry out the wet oxidation experiments. Oxygen was fed continuously to the reactor to maintain an oxygen partial pressure of 3 MPa. The total pressure in the system was the sum of the saturated vapor pressure of water at the operating temperature and the oxygen partial pressure. Wet oxidation product was withdrawn continuously through an air-actuated high-pressure valve which was controlled by a conductivity level probe situated in the reactor. To test if the oxidation was limited by the amount of oxygen dissolved at 3 MPa oxygen partial pressure, experiments have been performed at various oxygen partial pressures and lower stirrer speeds in order to demonstrate that the reactor is free from masstransfer limitations under those conditions. A residence time distribution analysis of the lab reactor showed that the mixing behavior was close to an ideal continuously stirred tank reactor. Analytical Techniques. Total Organic Carbon. Total inorganic carbon was measured with a Shimadzu 5050 TOC Analyser which is based on combustion and subsequent nondispersive infrared gas analysis. Total carbon (TC) was first measured and then the inorganic carbon (IC) was measured. Total organic carbon (TOC) was determined by subtracting IC from TC. The uncertainty in this assay, quoted as the deviation of three separate measurements, was never larger than 1% for the range of TOC concentrations measured. Chemical Oxygen Demand. The chemical oxygen demand was determined by the dichromate microdigester method. The appropriate amount of sample is introduced into commercially available digestion solution (HACH EUROPE, Belgium) containing sulfuric acid, mercuric sulfate, and chromic acid. The mixture is then incubated for 120 min at 423 K in a COD reactor (HACH model 45600, Loveland, CO). After cooling down, the Cr3+ concentration was measured colorimetrically using a colorimeter (CAMLAB Ltd. model DR/ 700, U.K.) that gives a direct readout of COD [mg L-1]. The average value of three separate measurements per vial is taken, and the maximum deviation between three different sample vials did not exceed 1.5%. Experimental Procedures. Membrane Characterization. Separation characteristics of several types of membranes were determined with a range of polyethylene glycols from MW 62 to MW 10 000 by measuring the TOC of retentate and permeate in batch mode at 313 K and 1 MPa transmembrane pressure in order to select a membrane suitable for the OXYMEM process. The axial pressure drop along the membrane module never exceeded 0.04 MPa; therefore the mean of upstream and downstream pressure was used to calculate transmembrane pressure. The absence of leaks at the membrane seals was assured by measuring the rejection of PEG 10 000, which results in a TOC below detection limit in the permeate if membranes are installed correctly. Periodic measurements of water fluxes were compared to the original water flux of the new membrane to verify no membrane degradation had taken place. Fractionation of Wet Oxidation Effluents. Membranes used in fractionation experiments are PCI AFC40 nanofiltration membranes made from a polyamide film. Two liters of effluent continuously drawn from the wet oxidation reactor at temperatures between 383 and 473 K, and at three different residence times, were used to

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Figure 2. Experimental setup and controls for OXYMEM process. S1 ) permeate sampling point; S2 ) retentate sampling point.

fill the recirculation tank of the membrane system. The membrane unit was operated at a crossflow velocity of 1.3 ms-1 and a transmembrane pressure of 1 MPa for 30 min while recycling the permeate to the buffer tank, in order to stabilize the separation conditions. After this period, only 10 mL of permeate were drawn as a sample, to ensure that the concentration of the solution in the buffer tank remains approximately constant. Retentate and permeate samples were analyzed for TOC and COD as described above. Generation of Wet Oxidation Effluents for Membrane Fractionation. Experiments were performed in the reactor at 383, 403, 423, and 473 K, in order to generate effluents for the tests on membrane fractionation of wet oxidation effluents. A polymer solution of concentration 1 g L-1 PEG 10 000 was fed continuously to the reactor at flow rates between 0.25 and 1 L h-1 while stirring at 1100 rpm. A pressurized catchpot was used to avoid evaporation, and liquid product was withdrawn through a water-cooled heat exchanger. Integration of Nanofiltration and Wet Oxidation. Figure 2 shows the laboratory setup used to demonstrate the integrated process. The wet oxidation reactor and nanofiltration system are connected by PVC tubing, and metering pumps are used for the feed and in the reactor recirculation loop. The PEG feed solution had to be pH adjusted in order to not fall below a minimum of pH 4 dictated by the membrane, as the pH falls sharply during wet oxidation due to the formation of organic acids. A simple water-cooled heat exchanger to reduce the WO outlet temperature to 313 K, which is also maintained by a second cooling system in the membrane recirculation tank, was used. Fresh feed is pumped directly into the buffer tank of the PROSCALE system. Reactor feed is drawn by a high-pressure pump from the buffer tank which is well mixed because of the high membrane recirculation rate. The withdrawal of permeate is controlled by varying transmembrane pressure which maintains a stable liquid level in the buffer tank, and liquid holdup in the reactor is directly controlled by the reactor level control. The hydraulic residence time in the whole system under the chosen conditions (2 L of liquid holdup) is 200 min, while the hydraulic residence time in the reactor is 18 min. Actual residence times of the organic compounds in the mixtures cannot be calculated in this way, as reaction products could leave the system in the permeate after one circulation through the reactor as

Figure 3. Rejection curve for PCI AFC40 nanofiltration membrane using aqueous polyethylene glycol solutions of 1 g L-1 at 313 K, 1 MPa transmembrane pressure, and 1.3 ms-1 crossflow velocity. (O) experimental data (s) curve fit.

well as stay several hours if they are refractory to chemical oxidation and are retained by the membrane. Therefore, actual residence times for organic molecules in the system will vary significantly depending on their reactivity. Molecules that have a low reactivity will accumulate in the reactor loop to reach a steady-state concentration at which their removal rate (by chemical oxidation and membrane permeation) matches the rate at which they are produced as stable intermediates in the oxidation reactor. Results and Discussion Membrane Characterization. From the range of AFC membranes, the AFC40 type was found to have the most suitable rejection characteristics for use in this work. The AFC40 rejection curve obtained with polyethylene glycols in a molecular weight range from 62 to 1000 g mol-1 is shown in Figure 3. Rejection is measured in TOC values according to

R)1-

CP TOCP )1CR TOCR

(4)

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Figure 4. Rejection for TOC of continuous WO reactor effluents by PCI AFC40 membrane: influence of wet oxidation temperature and residence time. Wet oxidation effluents generated at (1) 383 K, (0) 403 K, (9) 423 K, and (4) 473 K.

Figure 5. Rejection for COD of continuous WO reactor effluents by PCI AFC40 membrane: influence of wet oxidation temperature and residence time. Wet oxidation effluents generated at (1) 383 K, (0) 403 K, (9) 423 K, and (4) 473 K.

Rejection coefficients given here are observed parameters depending on process parameters throughout, and their values are relevant for the description of membrane separation behavior in the integrated process which is subject to the same process parameters. The calculation of true rejection coefficients independent from mass-transfer limitations and therefore process variables [Pra`danos et al., 1995] is beyond the scope of this work. An empirical function

MW { (MW* ) } -m

R ) exp -

(5)

proposed by Masawaki et al. (1996) is fitted to experimental values, where MW is the molecular weight of the solute, MW* the molecular weight which is rejected by 37% ()exp(-1)), and m a parameter describing the steepness of molecular cutoff.22 Figure 3 shows the above function fitted to the experimental data results in a good correlation of experimental values and the proposed molecular cut-off profile. This curve can therefore conveniently be used for predictions of cut-off behavior, if the limitations of this model are taken into account: First, the rejection profile loses its physical meaning at very low molecular weights. Second, molecules of different chemical nature such as organic acids tend to have rejection coefficients governed by other parameters than molecular weight, and consequently may not obey the above cut-off profile. Unfortunately, knowledge regarding organic multicomponent rejection of nanofiltration membranes is still limited. Empirical models for calculating separation of binary and ternary mixtures of organics in water have been proposed [Laufenberg et al., 1996], but prediction of the separation of more complex mixtures is not yet possible. Separation curves obtained from different types of membranes under comparable conditions can however serve to decide which membrane will be promising for use in the OXYMEM system, if the molecular weight of the organics to be treated is known. Fractionation of Wet Oxidation Effluents. Figures 4 and 5 show the TOC and COD rejections which are obtained when various wet oxidation effluents are fractionated by nanofiltration using an AFC40 membrane under the conditions described above. The trends in the TOC and COD data are quite similar, exhibiting

Figure 6. Assessment of partial oxidation states of retentates and permeates from fractionation experiments (Figures 4 and 5). AOSC of permeates at (O) 403 K oxidation temperature, (4) at 423 K, (0) at 473 K, retentates at (b) 403 K, (2) at 423 K, and (9) at 473 K.

higher rejections of COD and TOC with decreasing temperatures and reaction times. The results confirm the theoretical assumption that oxidation mainly reduces the average chain length present in the mixture, if it is assumed that membrane separation has size exclusion as its major separation mechanism. Interestingly, rejections for COD are slightly higher throughout. This indicates that unreacted molecules which are more likely to be rejected by the membrane do not only have a higher molecular mass, but also contribute more to the oxygen demand of the mixture. Molecules which have already been oxidized further preferentially permeate the membrane, therefore leading to lower TOC rejection coefficients. To show this more clearly, oxidation states of the retentate and permeate mixtures were assessed in more detail. The experimentally determined selectivity of an AFC40 nanofiltration membrane toward more highly oxidized products is described in Figure 6 by comparing AOSC values of retentates and permeates as a function of wet oxidation reaction temperatures and residence times. It can be seen from the oxidation states of the retentates

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Figure 7. Demonstration runs of integrated wet oxidationnanofiltration system: 383 K wet oxidation temperature. (O) retentate TOC and (2) permeate TOC.

Figure 8. Demonstration runs of integrated wet oxidationnanofiltration system: 403 K wet oxidation temperature. (b) retentate TOC and (2) permeate TOC.

(effluents from conventional continuous wet oxidation) that an increase in oxidation time only increases the oxidation state of the remaining organics slightly and a significant increase in reaction temperature is needed to increase the AOSC of the mixture by as little 0.5. This effect can be explained by total oxidation of short chain compounds which becomes more important at higher temperatures and residence times, meaning that carbon dioxide with the highest possible AOSC leaves the reactor while the AOSC value of the remaining organics stays relatively low. On the other hand, the AOSC of the short chain organics in the permeate is much higher and also does not seem to vary with reaction temperature and residence time, implying that regardless of the retentate composition, nanofiltration lets similar molecules permeate. Clearly, the absolute permeate concentration, not shown in the figures above, still depends on oxidation conditions. These are very encouraging results for integrated wet oxidation-membrane treatment, implying that if wet oxidation is coupled with nanofiltration in a continuous process lower temperatures and residence times could be employed in the pretreatment step without a change of permeate partial oxidation state. In a full scale process, this could result in a significant reduction of the wet oxidation reactor size. Integration of Nanofiltration and Wet Oxidation. Results from two runs at different wet oxidation temperatures are shown in Figures 7 and 8. In order to reach steady state quickly, the system is started up with a solution of a higher concentration than that of the continuous feed employed later. Preliminary calculations based on TOC removal rates indicated that a steady-state concentration in the reactor would be 5-10

Figure 9. Dependency of steady state on start-up conditionsintegrated wet oxidation-nanofiltration run at 403 K under different conditions. (b) retentate TOC and (2) permeate TOC.

times higher than that of the original feed. A simple simulation of the system showed steady-state organics concentration in the loop would be reached faster if the reactor is started up with a concentrated polymer solution. Consequently, a PEG solution with a TOC of 10 000 mg L-1 was used to start the system as a closed loop. In the case of the oxidation run at 383K, the addition of fresh feed with a TOC of 540 mg L-1 was started after 1 h of oxidation, while the permeate was withdrawn at the same volumetric rate. Steady state, indicated by the TOC concentration in the outlet matching the TOC of the feed, was nearly reached after 12 h of operation (dotted line in graph). However, the system had to be shut down after this period (vertical line in graph) by cooling the wet oxidation reactor to room temperature and stopping feed and permeate withdrawal. Due to a leak in the buffer tank the permeate TOC concentration of 540 mg L-1 necessary to achieve a closed material balance was not reached during the following 12 h of resumed operation. In the second run, where the wet oxidation reactor temperature was set at 403 K, permeate withdrawal was started only after 2 h which led to a faster stabilization of the system, while a steady organics concentration in the reactor loop was already reached after approximately 10 h. In both cases, the system was shut down after 24 h of operation, which corresponds to a total of 10 L of solution treated in the second run. As the steady state reached by the system might not be unique and could depend on the start-up conditions, the run at the higher oxidation temperature was repeated, halving the initial concentration of the PEG solution and employing a modified start-up procedure. Results are shown in Figure 9. Two liters of a PEG solution containing 5000 mg L-1 of carbon were subjected to wet oxidation in the closed system for 5 h, and the system was shut down after this period. After resuming operation, permeate withdrawal and addition of fresh feed with a TOC of 500 mg L-1 were started. Values of above 1500 mg L-1 of carbon in the first permeate withdrawn show that an accumulation of short chain compounds has taken place in the system during the first 5 h of operation. However, after about 15 h permeate concentration reached the steady-state value dictated by the carbon balance and retentate values also remained stable within measurement errors. Interestingly, the final organics concentration in the loop was practically identical to the steady-state value reached in the previous run, indicating that the steady state of the laboratory system appears independent of start-up condition.

Ind. Eng. Chem. Res., Vol. 36, No. 12, 1997 5061 Table 1. Comparison of Partial Oxidation Data of Conventional Wet Oxidation at 403 and 473 K to OXYMEM at 403 Ka OXYMEM OXYMEM OXYMEM conv. wet conv. wet feed loop permeate oxidation oxidation parameter (403 K) (403 K) (403 K) (403 K) (473 K) COD TOC COD/TOC AOSC CODpartox µCODpartox

1840 540 3.41 -1.11 0 n.d.b

8930 3230 2.69 -0.03 2076 -0.29

1113 540 2.06 +0.91 727 1.0

1527 524 2.91 -0.37 258 0.83

1024 441 2.32 +0.52 479 0.58

a Total average oxidation times 30 min for both systems. b n.d. ) not defined.

The key results for the above integrated experiments are COD and TOC values of the permeate and the reactor loop, as given in Table 1. The accumulation of partially oxidized organic compounds is obviously strongly dependent on temperature, as the steady-state TOC value at 383 K (see Figure 7) is nearly twice as high as the TOC value at 403 K. This is consistent with the assumption made earlier that the total rate of oxidation depends on organic concentration in the loopsat the lower temperature, a higher concentration of organics is required to produce a sufficient oxidation rate to balance feed versus permeate carbon flows. It is also interesting to compare the partial oxidation parameters of the retentate and permeate to those of the effluent of conventional wet oxidation. At 403 K and 30 min residence time, conventional wet oxidation achieves a COD reduction of 313 mg L-1 (17%), 55 mg L-1 (3%) of which is due to total oxidation, and 258 mg L-1 (14%) to partial oxidation. The resulting partial oxidation efficiency is µ ) 0.83. Under comparable total residence times and temperatures, the OXYMEM system obtains 40% COD reduction at negligible total oxidation, indicated by a partial oxidation efficiency of µ ) 1. This is a result of the smaller individual residence time in the recycle reactor, which allows small oxidation intermediates such as formic acid to leave the system after a single pass through the reactor. If conventional wet oxidation is operated at a higher temperature to achieve similar COD reductions, total oxidation becomes much more important. This is illustrated by data from a wet oxidation run at 473 K given in Table 1: a slightly higher COD removal than with the OXYMEM system at 403 K has been achieved at the expense of increasing the reaction temperature by 70 K. However, partial oxidation efficiency has fallen to µ ) 0.58, indicating a high proportion of total oxidation. It is interesting to note the relatively high AOSC value of +0.52 of the remaining organics in the effluent at 473 K, which is likely to be beneficial for a biological aftertreatment, but would be misleading if used as an assessment for partial oxidation. In the loop of the OXYMEM system, an accumulation of organics is taking place which under steady-state conditions leads to COD and TOC values which are several times higher than the corresponding values of the feed. The high CODpartox value shows that, at the same time, significant partial oxidation of these compounds has taken place. The partial oxidation efficiency for the organics in the loop, however, assumes a (technically meaningless) negative value as it cannot take account of any accumulation. If COD/TOC ratios and AOSC values of the organics retained in the loop are compared to conventional wet oxidation data, it becomes clear that these compounds are on average further oxidized than the effluent of the conventional wet

oxidation reactor at the same temperature and total residence time. Comparing the data from conventional partial wet oxidation runs at different temperatures, it can be summarized that high COD reductions can be achieved, but total oxidation of short chain intermediates cannot be avoided at the same time. This indicates that conventional wet oxidation does not have a high selectivity toward oxidising long chain molecules in a mixture of oxidation intermediates. In the case of the OXYMEM system, the retention of long chain molecules in the loop leads to a much higher average oxidation state of the organics in the permeate. The chemical composition and enhanced biodegradability of polymer solutions subjected to wet oxidation has been discussed earlier [9]. Work is currently being undertaken to demonstrate the enhancement of biodegradation rates by the use of the OXYMEM system. Implications for Wastewater Treatment Process A similar process configuration with an adapted volume ratio between reactor and buffer tank could be used for a full scale wastewater treatment system. This would have the additional advantage that the majority of small molecules and salts are withdrawn before they reach the wet oxidation step. Thus, problems with corrosion and overoxidation in the pretreatment step would simultaneously be reduced. A preconcentration of the feed which is often required for autothermal operation of the oxidation reactor could also be achieved by adjusting the membrane and reactor recycle ratios. The laboratory system used for the demonstration of the integrated process still has a relatively low degree of integration. This can be overcome by modifying it in a way to approach the original target of direct integration of membrane separation into the wet oxidation reactor. Further investigation is necessary to determine whether the pressure used to increase the liquid phase oxygen concentration in the reactor can also be used as a driving force for the membrane separation. The potential problem is loss of oxidant across the membrane if the membrane feed is saturated with oxygen above atmospheric pressure, which might also lead to structural problems in the membrane. Furthermore, a thermal integration of the two process parts could be achieved if temperatures of wet oxidation and membrane separation were as close as possible. The operating temperature of wet oxidation might be reduced by the use of a catalyst, the compatibility of which with the membrane step still needs to be addressed. On the other hand, the operating temperature of the membrane filtration step can be brought closer to the wet oxidation temperature. This requirement seems to exclude the use of polymeric membranes in a full scale system. However, as membranes are increasingly used for wastewater treatment, organic and inorganic membranes are being commercially developed that have promising specifications. An example are pH- and temperature-resistant membranes from the SELRO range of MPW Membrane Products Weizmann (Rehovot, Israel). Inorganic KERASEP nanofiltration membranes manufactured by TECHSEP (Miribel, France) are temperature resistant up to levels well above temperatures used in partial wet oxidation. For the treatment of real wastewaters, conditions of the specific case such as wastewater properties, cost of membranes, and energy consumption need to be taken into account to determine the most economical solution.

5062 Ind. Eng. Chem. Res., Vol. 36, No. 12, 1997

At this stage, it is interesting to see that, even with current wet oxidation and membrane separation technology, development of more efficient systems is possible if the synergy between individual process steps is exploited. Conclusions A combined wet oxidation and nanofiltration process for the treatment of bioresistant organic molecules has been demonstrated. Initial experimental results show that the concept of retaining molecules recalcitrant to chemical oxidation in the wet oxidation reactor with a nanofiltration membrane successfully increases the partial oxidation efficiency of the process. At the same time, an effluent containing organics with a higher average oxidation state compared to conventional wet oxidation can be produced at lower temperatures and residence times. The partial wet oxidation of polymer molecules in a model wastewater is therefore significantly enhanced by using a combination of an oxidative and a separative step. Acknowledgment The authors thank the Commission of the European Communities for the financial support of this work under Grant No. EV5V-CT93-0249 and the EPSRC for financial support under Grant No. GR/K 21184. Nomenclature c ) concentration [g L-1] F ) feed MW ) molecular mass [g mol-1] MW* ) critical MW [g mol-1] P ) pressure [MPa] R ) rejection T ) temperature [K] t ) time [min] V ) volume [L] AOSC ) average oxidation state of carbon BOD ) biological oxygen demand [mg of O2 L-1] COD ) chemical oxygen demand [mg of O2 L-1] EG ) ethylene glycol NF ) nanofiltration RO ) reverse osmosis TOC ) total organic carbon [mg L-1] WO ) wet oxidation Greek Letters µ ) efficiency of removal Subscripts 0 ) value at t ) 0 p ) permeate partox ) partial oxidation r ) retentate

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Received for review June 10, 1997 Revised manuscript received September 30, 1997 Accepted October 1, 1997X IE970417M

X Abstract published in Advance ACS Abstracts, November 1, 1997.