UV / H 2 O 2 process performance i provement by ultrafiltration and physicochemical clarification systems for industrial effluent pretreatment

The present study evaluated the removal of TOC from an effluent with high organic load resulted from the treatment of oil-water emulsion by thermal process. Hollow Fiber Ultrafiltration membrane (HF-UF) and physicochemical clarification process were used as pretreatment options to assess the influence of feed effluent quality on the UV/H2O2 oxidation process. Results for TOC removals showed HF-UF and physicochemical clarification processes can significantly improve the efficiency of UV/H2O2 oxidation process, when compared with the direct effluent oxidation. Reaction time for obtaining a TOC removal higher than 90% was reduced to approximately half of the time needed when no pretreatment was applied. Considering both pretreatment processes it was not possible to notice any significant difference on the UV/H2O2 oxidation process performance. However, the complexity of physicochemical process due to the use of three different chemicals and sludge production made the HF-UF process the best pretreatment alternative, without increasing the Total Dissolved Solids of the effluent, a very important issue when water reuse is considered.


INTRODUCTION
The industrial development is one of main causes for environmental degradation of urbanized areas in Brazil, mainly of water bodies.Over the past two decades, environmental regulatory requirements have become more stringent because of increased awareness of the human health and ecological risks associated with environmental contaminants.
Metal working fluids are used in many small companies for metal forming and machining process, like cutting, grinding, and milling, however considering the complexity of the effluents produced, most of those companies has no capacity to treat it, then they hire a specialized company to do this job.Companies that provide such kind of service collect effluents from many sources and them treat it by specific ways, charging their service according the volume processed.One of the technologies used for treating such king of effluent is the thermal separation process (Gutiérrez et al., 2011;2010;Fakhru'l-Razi et al., 2009).The use of thermal separation processes, depending on the inflow effluent characteristics can result in a condensate with a high organic load, because of stripping of volatile and semi-volatile compounds, besides a small fraction of non-volatile substances (Mao et al., 2010), which not allows releasing it to the environment or reusing it.Therefore, a complementary treatment step is necessary in order to cost-effectively meet environmental standards.
Among these technologies the Advanced Photochemical Oxidation Processes (APOP) are very promising, because it could be considered a green technology, involving the generation and use of powerful but relatively nonselective transient oxidizing species, primarily the hydroxyl radical (•OH) for contaminant destruction.The •OH can be generated by both photochemical and non-photochemical processes to oxidize many contaminants (USEPA, 1998).Among the different available AOP, the one which uses UV light and hydrogen peroxide (UV/H 2 O 2 ) was the first one to be used for wastewater treatment (Litter, 2005).This process proceeds in two main steps: hydroxyl radicals production by direct H 2 O 2 photolysis and oxidation of organic compounds by these radicals (Braun e Oliveros, 1997).In some cases the photon energy is enough to break chemical bounds of organic molecules, resulting on its degradation.The UV/H 2 O 2 process has been shown very efficient on the degradation of various organic pollutants.Many successful application of UV/H 2 O 2 could be found in the literature, for instance, the works of Beltrán et al. (1997), for phenol oxidation, Cater et al. (2000), for gasoline contaminated groundwater treatment, Arslan et al. (2000), for the treatment of dyes contaminated effluents, Einschlag et al. (2002), for oxidation of nitroaromatic compounds, and more recently the work of Gao et al. (2009), for the treatment of ametryn contaminated waters.The advantage of UV/H 2 O 2 processes is that the H 2 O 2 is the only chemical that needs to be added, and is easily converted to oxygen and water.
Even with the spread use of UV/H 2 O 2 process, most of them are applied for degradation of specific contaminants, present in relatively clean matrices, which is not the case for industrial wastewaters.In a recent review developed by Diya'uddeen et al. (2011), it is pointed out that wastewater composition could reduce the UV activated efficiency and even make its application unfeasible.The presence of discoloring substances or suspended solids can reduce the process efficiency by adsorbing or reducing UV light transmittance, which makes necessary the use of pretreatment systems, in order to improve contaminant degradation.Many technologies could be used as a pretreatment for the application of UV/H 2 O 2 oxidation process.However the best option should consider not only the improvement on the oxidation process efficiency, but the overall system performance.In some cases AOP are used as a pretreatment for improving the efficiency of biological process for treating complex wastewater focusing water reuse (Oller et al., 2011), where they concluded that is necessary to improve the knowledge about organics degradation kinetics and reactor modeling considering the combined process.Considering the contaminants presents in condensates from thermal separation process will probably be partially emulsified, presenting high color and turbidity, the efficiency of UV/H 2 O 2 oxidation process will be reduced.To overcome this problem a clarification pretreatment process will be necessary, and for this purpose, physicochemical clarification or membrane filtration processes could be efficiently applied (Yahiaoui et al., 2011;Fakhru'l-Razi et al., 2009).
Focusing the overall performance of a treatment system for industrial effluent treatment, the aim of this study was to evaluate the influence of two pretreatment technologies: (i) physicochemical clarification (ii) Hollow Fiber Ultrafiltration Membraneon the performance of UV/H 2 O 2 oxidation process for treating an industrial effluent with a high organic load.The pretreatment technologies were chosen based on the effluent characteristics.

Effluent Composition
In the experiments an effluent arising from a thermal separation process (vapor compression evaporator), treating oil-water emulsions with a high TOC load was used.Because of evaporation process characteristics, a high level of organic contaminants is still present in the condensate, which not allows its releasing to the environment or even its reuse for industrial purposes.The main composition of raw effluent generated in the thermal separation process is given in Table 1.According to the data presented in Table 1, most appropriated pretreatment technologies should be able to remove effluent color and turbidity, because it could significantly affect the UV/H 2 O 2 performance, justifying the use of ultrafiltration and physicochemical clarification as pretreatment processes.

Membrane Separation Process -HF-UF
The membrane separation process was carried out using a laboratory scale ultrafiltration unit (Demofilter -Koch Membrane Systems).Prior to the experiment, the effluent was filtered through a 100 µm bag filter in order to remove coarse suspended solids, if present.Experiments were carried out using a hollow fiber membrane module from Koch Membrane Systems, 1" HF 1.0-45-XM50, with a Molecular Weight Cut-off (MWCO) of 50,000 Daltons.The temperature in all experiments was 20±0.9 °C and the transmembrane pressure (TPM) was kept at 1.26 bar.Membrane system was operated in batch, with full concentrate recirculation to the feed tank.Permeate from UF system was used in the oxidation experiments.

Physicochemical Clarification Process
Effluent clarification was carried out in Jar-test equipment (Milan JT 203/6) with the addition of three different chemicals from Procytek Industria e Comercio Ltda.The chemical doses are presented in Table 2.Each chemical was added separately according to the data presented in Table 3. Thirty liters of effluent were treated using chemical clarification.The clarified effluent was used in oxidation experiments with UV/H 2 O 2 .

UV/H 2 O 2 Process
Effluents from the ultrafiltration and physicochemical clarification processes, besides the raw effluent were then treated using the UV/H 2 O 2 oxidation process in order to obtain the TOC degradation kinetic constants.The photochemical reactor from Germetec UV/IR Technology Ltd. was equipped with a 1,600 watts medium pressure UV lamp.All tests were performed in batch with full effluent recirculation (Figure 1).Samples from the photochemical reactor effluent were taken each hour during the tests for the analysis of TOC and at the end of test for the analysis of the same parameters presented in Table 1.For the evaluation of TOC degradation kinetic constants a pseudo first order reaction was considered according equations 1 and 2, derived from a mass balance based on the diagram of Figure 1.Since H 2 O 2 was added in excess a pseudo first order kinetics can be assumed (USEPA, 1998).Equation 1 refers to the TOC degradation in the UV reactor and equation 2 refers to the TOC concentration leaving the storage tank, where no TOC degradation is expected.Oxidation kinetic constant was obtained by plotting the natural logarithm of TOC concentration with time, according the linearization of Equation (2).Operational conditions for the oxidation assays are presented in Table 4.

H 2 O 2 dosage
Commercial hydrogen peroxide dosage was determined based on the concentration of TOC fed to the UV reactor according to alpha (α) correlation stated in Table 4, using Equation 3. [3] where: : hydrogen peroxide volume (mL); : total organic carbon in the effluent fed to the UV reactor (mg C/L); : effluent volume (L); : density of the hydrogen peroxide solution (g/cm 3 ) : mass content (m/m) of hydrogen peroxide solution (%).0.1: correction factor.

Pretreatment Performance
As it was mentioned, two types of pretreatment were applied prior oxidation experiments were initiated: (i) physicochemical clarification using 3 different chemicals and (ii) Hollow Fiber Ultrafiltration Membrane with MWCO of 50,000 Da.Those pretreatments were studied in order to verify the effect of feed water quality on TOC kinetics removal by UV/H 2 O 2 oxidation process, mainly with respect of turbidity and color removal which may prevent the penetration of UV light through wastewater during the oxidation experiments.
Thirty eight liters of effluent were treated by ultrafiltration, with a water recovery of 84%.In this experiment, the permeate stream was collected separately for subsequent oxidation experiment.Mean permeate flow rate was 21.6 ± 9.0 L/h, which could be considered adequate for effluent treatment.The clarification process resulted in a production of 670 mL of sludge resulting in a production rate of 22 liters of sludge per cubic meter of treated effluent.Compared to the concentrate produced by the ultrafiltration system, the sludge from the physicochemical process is a drawback because it will be necessary to implement a sludge conditioning process before its final disposal, while the concentrate from UF system could be sent back to the evaporation system.
Table 5 presents the results for contaminant removal obtained by each pretreatment process.Comparing these results it could be observed that HF-UF pretreatment system showed a best performance, except for oil & grease and N-organic removal.Another point that should be mentioned is the increasing of electrical conductivity (34%), and Total Dissolved Solids (191%), in the effluent from clarification process, mainly because chemicals added during treatment.With regard the TOC removals both processes were inefficient, indicating the necessity of one additional treatment step, before its final disposal or reuse.Figure 2 presents the results for turbidity, color, COD, and TOC removal efficiencies.3.2.TOC removal using UV/H 2 O 2 oxidation process, and kinetics constants TOC removals by UV/H 2 O 2 oxidation process were close to 90% in all experiments (Figure 3).However, when the pretreatment processes were applied the rate of TOC degradation increased, showing that effluent pretreatment is necessary for improving the UV/H 2 O 2 oxidation performance (Figure 4 and Table 6).These results are most probably related to the effluent clarification, which resulted in a better UV light transmission in the photochemical reactor, since color and turbidity can absorb UV light.Reaction time for obtaining a TOC removal of 90% was reduced to approximately 4 hours, half of the time needed when no pretreatment was applied.It is in accordance to the work developed by Benítez et al. (2008).
The results presented in this study are not in agreement the ones obtained by Saquib, Vinckier, and Van der Bruggen (2010), where they concluded that ultrafiltration process has no influence on the performance of an oxidation process using O 3 /H 2 O 2 oxidation process.This could most be associated to the differences on inflow streams characteristics and experimental procedures.It should be mentioned that Saquib et al. (2010), used previously filtered natural water in the experiments, and no mention about water turbidity or color were made.Besides the analyses of the contaminants presented in Table 5, samples of all streams were sent for a private laboratory Bioagri Ambiental, in order to analyze the concentration of volatile and semi-volatile contaminants by gas chromatography.The results of gas chromatography analyses are presented in Figure 5, where it could be seen that the physicochemical clarification process was slightly more efficient than the UF one, and that UV/H 2 O 2 oxidation process was very efficient for all conditions evaluated, because the concentration of these contaminants in all samples analyzed were below the equipment detection limit.

CONCLUSIONS
In this work a physicochemical clarification process and an ultrafiltration separation system were evaluated as a pretreatment option for industrial wastewater treatment by UV/H 2 O 2 oxidation process, in comparison to its direct treatment.The following conclusions can be drawn:  Results obtained from the treatment of an effluent arising from an evaporation process of oil and water emulsion showed that ultrafiltration and physicochemical process, present no significant differences on contaminants removal, but that ultrafiltration system presents a better performance because there is no need for chemicals use. A considerable improvement on the efficiency of TOC removal from the effluents of UF and physicochemical clarification process, compared with the raw effluent, was obtained;  Considering both pretreatment processes it was not possible to notice any significant difference on the performance of UV/H 2 O 2 oxidation process.However, the complexity of physicochemical process, because of the use of three different chemicals, and because of the sludge produced, makes the UF process the best pretreatment option.

Figure 2 :
Figure 2: Efficiency of pretreatment systems for turbidity, color, COD, and TOC removal.

Figure 5 .
Figure 5. Gas analyses results for volatile and semivolatile chemicals

Table 2 -
Chemical doses used in clarification experiment.

Table 3 -
Operation condition during clarification step.

Table 5 .
Characteristics of HF-UF and Physicochemical effluent.