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Performance of a commercial industrial-scale UF-based process for treatment of oily wastewaters
Journal of Environmental Management 128 (2013) 413e420
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Journal of Environmental Management journal homepage: www.elsevier.com/locate/jenvman
Performance of a commercial industrial-scale UF-based process for treatment of oily wastewaters M. Karhu a, *, T. Kuokkanen b, J. Rämö c, M. Mikola a, J. Tanskanen a a
Chemical Process Engineering Laboratory, Department of Process and Environmental Engineering, University of Oulu, P.O. Box 4300, 90014 Oulu, Finland Department of Chemistry, University of Oulu, P.O. Box 3000, 90014 Oulu, Finland c Thule Institute, University of Oulu, P.O. Box 7300, 90014 Oulu, Finland b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 February 2013 Received in revised form 8 May 2013 Accepted 25 May 2013 Available online
An evaluation was made of the performance of a commercial industrial-scale ultrafiltration (UF)-based process for treatment of highly concentrated oily wastewaters. Wastewater samples were gathered from two plants treating industrial wastewaters in 2008, and in 2011 (only from one of the plants), from three points of a UF-based treatment train. The wastewater samples were analyzed by measuring the BOD7, COD, TOC and total surface charge (TSC). The inorganic content and zeta potentials of the samples were analyzed and GC-FID/MS analyses were performed. The removal performances of BOD7, COD, TOC and TSC in 2008 and 2011 for both plants were very high. Initial concentrations of contaminants in 2011 were lower than in 2008, therefore the COD and TSC reductions were also lower in 2011 than three years before. Regardless of the high performance of UF-based processes in both plants, at times the residual concentrations were considerable. This could be explained by the high initial concentrations and also by the presence of the dissolved compounds that were characterized. Linear correlation was observed between COD and TOC, and between COD and TSC. The correlation between COD and TSC could be utilized for process control purposes. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Characterization of wastewater Oil emulsion Oily wastewater Ultrafiltration
1. Introduction Oil is one of the major contaminants in industrial wastewaters. Oily industrial wastewaters are extremely heterogenous, containing various types of hydrocarbons, surfactants, metals, acids etc. Stable oily wastewaters consisting of highly chemically and physically emulsified oils are the most challenging ones in terms of effective treatment. Ultrafiltration (UF) is a widely accepted and commonly used method for the treatment of oily wastewaters. Ceramic UF membranes have high thermal and chemical stability, therefore they are broadly used in treating industrial wastewaters. However, UF membrane fouling is one of the major drawbacks of this treatment method, slowing down the entire treatment process, and being uneconomical because of the need for washing. UF also presents difficulties in removing very small-sized oil droplets or dissolved hydrocarbons from oily wastewaters. In essence, a single treatment operation for oily wastewaters is not enough to decrease oil concentration to the required level; instead a combination of
* Corresponding author. Tel.: þ358 29448 2357; fax: þ358 29448 2304. E-mail address: mirjam.karhu@oulu.fi (M. Karhu). 0301-4797/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jenvman.2013.05.053
various treatment operations is needed to treat heterogenous oily wastewaters. The majority of studies on the filtration of industrial oily wastewaters were performed in the 1990s, and research has continued in recent decades to a lesser degree. The studies usually deal with the suitability and fouling of polymeric or ceramic microfiltration (MF) and UF membranes for treatment of synthetic oil-in-water emulsions (Chakrabarty et al., 2008; Chen et al., 2009; Janknecht et al., 2004; Mittal et al., 2011; Srijaroonrat et al., 1999) or real oily wastewaters (Abadi et al., 2011; Ebrahimi et al., 2010; Ghidossi et al., 2009; Qiao et al., 2008; Tomaszewska et al., 2005; Yliwati and Ismail, 2011), or with the combining of MF or UF with other treatment methods such as a photocatalytic process (Karakulski et al., 1998) and reverse osmosis (Tomaszewska et al., 2005). UF processes for oily wastewater treatment have usually been studied in laboratory scale (e.g. Ebrahimi et al., 2010) but also a few pilot-scale studies can be found, as follows. Tomaszewska et al. (2005) investigated the possibility of bilge water treatment in the integrated UF/RO (reverse osmosis) system. Study was carried out with the use of two pilot plants: the UF process with tubular module and the RO process with spiral wound modules. Qiao et al. (2008) designed a pilot-scale plant involving aeration tank, air flotation, sand filter and UF (a hollow fiber membrane) for
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treatment of oilfield-produced wastewater. Ghidossi et al. (2009) developed an industrial UF-based (with a 300-kDa ceramic membrane) process for treatment of bilge and ballast water on board. In addition to the performance of UF-based processes fouling of the membranes have been under intensive investigation. Studies on fouling have usually been focused on understanding the phenomenon of fouling and concentration polarization for various types of membranes, the effects of operating conditions (e.g. fluxes, pH, temperature, solution chemistry such as salt concentration, surface properties of the membrane and transmembrane pressure) (Benito et al., 2001; Hesampour et al., 2008; Rezvanpour et al., 2009) and tools to reduce fouling (Chen et al., 2012; Krsti c et al., 2007; Peng and Tremblay, 2008). The main purpose of this work was to study the performance of a commercial industrial-scale UF-based process for treatment of real and concentrated oily wastewater mixtures from the metal industry, oil separation wells, and ships in the form of bilge water etc. The study focused on two wastewater treatment plants that treat oily wastewaters with the same type of UF-based process. The object was a) to explore the current situation regarding treatment of oily wastewaters, b) to evaluate the performance of unit operations and the entire process, c) to study the problems when dealing with especially highly oil contaminated wastewaters and d) to establish possible correlations between the parameters to be determined, such as biological oxygen demand (BOD), chemical oxygen demand (COD), total organic carbon (TOC), total surface charge (TSC) and zeta potential for process regulation purposes. To the best of our knowledge, there have been no other scientific articles on such a comprehensive evaluation and characterization study of highly oily industrial wastewaters treated with a commercial industrial-scale UF-based process. 2. Material and methods 2.1. Oily wastewater samples Oily wastewater samples were taken from two wastewater treatment plants (WWTP1 and WWTP2), owned by one of the biggest environmental companies in Finland, which treats various types of wastewaters such as oily wastewaters (UF-based process), and waters containing heavy metals as well as acids and bases. The UF-based process (Fig. 1) is similar in both plants; however, the process of WWPT2 includes one extra sand filter and ion exchanger after the UF unit (ceramic, 50 nm pore size and water flow about
700 L hour1). The sand filter contains quartz sand of three different granule sizes. The ion exchangers are cation changers containing weakly acidic resins with chelating iminodiacetate groups. The oily wastewaters entering WWTP1 and WWTP2 are routed through a drum filter before a belt filter and then homogenized by mixing different types of oily wastewaters, targeting neutral pH values. Effluents from both plants are then piped to municipal wastewater treatment plants. The oily wastewater samples were taken separately from sampling points before UF, after UF, as well as after the first sand filter and ion exchanger (Fig. 1). The series of samples were taken in 2008 during nine days from WWTP1 (From May to June) and seven days from WWTP2 (July to September). The samples in 2011 were taken during six days (June) from WWTP1 to study any possible change in quality in the sample wastewaters. Table 1 presents all of the studied series of samples where each series contained samples from before UF, after UF, as well as after the first sand filter and ion exchanger. Fig. 2 presents the wastewater samples before UF and after UF treatment. 2.2. Analyses The inorganic content of the wastewater samples was determined by inductively coupled plasma e optical emission spectrometry (ICP-OES) at Suomen Ympäristöpalvelu Oy, a FINAS accredited test laboratory (T231). The wastewater samples were first digested with nitric acid in a CEM MARS 5 microprocessor controlled microwave oven with CEM HP 500 teflon vessels (CEM Corp., Matthews, USA) (EPA3015A). The ICP-OES analyses were then performed with Thermo Electron IRIS Intrepid II XDL inductively coupled plasma optical emission spectrometer (Franklin, USA). The inorganic content was analyzed for two series of samples from WWTP1 and for one series from WWTP2 in 2008. The organic compounds of the oily wastewater samples were analyzed by gas chromatography using Hewlett Packard 6890 Series GC system equipped with a FID detector in 2008 or with a mass selective detector (Hewlett Packard 5973 Mass Selective Detector) in 2011, an autosampler (Hewlett Packard 6890 Series Injector) and a fused silica Supelco Equity-1 capillary column (15 m 0.25 mm) with a nominal film thickness of 0.25 mm in 2008 and with a J&W Scientific DB-624 column by Agilent Technologies (30 m 0.32 mm) with a nominal film thickness of 1.8 mm in 2011. The GC oven program in 2008 was started at 50 C, was held for 1 min and then increased to 300 C at 10 C min1 and then held at 300 C for 5 min.
Fig. 1. Unit operations of UF-based process in WWTP1 and WWTP2 as well as samples 1) before UF, 2) after UF, and 3a and 3b) after the first sand filter and ion exchanger in WWTP1 and WWTP2.
M. Karhu et al. / Journal of Environmental Management 128 (2013) 413e420 Table 1 Series of oily wastewater samples. Series
WWTP
Year
1e9 10e15 16e22
1 1 2
2008 2011 2008
The GC program in 2011 was started at 40 C, was held for 1 min and then increased to 270 C at 15 C min1 and then held at 270 C for 5 min. The most turbid oily wastewaters were filtered before the GC-FID/MS analyses. The Spectrum library was used for identification of the compounds in the GCeMS analyses. The GC-FID/MS analyses were performed for three series of samples from WWTP1 in 2008, for two series of samples for WWTP2 and for five series of samples from WWTP1 in 2011. WTW inoLab pH 720 was used for measuring the pH. The COD was measured using the Hach Lange photometric cuvette test in different measuring ranges. The COD measurement was based on oxidizable substances in sample reacting with potassium dichromate in sulfuric acid solution in the presence of silver sulfate as a catalyst. Chloride was masked by mercury sulfate. Cr3þ ions were formed in the reaction and the green coloration of Cr3þ was evaluated. The sample was first added into a cuvette and then heated in 148 C for 2 h using Hach-Lange HT 200S thermostat system (Hach Lange for Water Quality). After digestion the cuvette was allowed to cool down into room temperature and the absorbance of sample was measured with Hach-Lange DR 2800 spectrophotometer at wavelength of 605 nm. The TOC was measured by using a Sievers 900 Portable TOC Analyzer. However, the samples had to be diluted significantly due to the limit value of 50 ppm. The TSC of particles in the colloidal solution was measured with a Mütek PCD 03 pH. The COD and TSC values were determined for all of the samples and TOC only for the samples from 2008. The zeta potentials of the wastewater samples were determined with a CoulterÒ DelsaÔ 440SX (Beckman Coulter, Miami, FL, USA). The theory of zeta potential measurement can be found in detail from (Leiviskä et al., 2005). Zeta potentials were determined for the wastewater samples after UF and after the first sand filter and ion exchanger of the UF-based processes in 2008 for seven series of samples from WWTP1 and WWTP2. The zeta potentials for samples before UF could not be analyzed because of the high concentration of particles. The apparatus used for BOD measurements in solution was the OxiTopÒ Control system (WTW). The theory behind BOD measurement and its calculation can be found in more detail in (Karhu et al., 2009; Roppola, 2009). The OxiTopÒ Control system (WTW) gives 360 measuring points per measuring period, and thus it is
Fig. 2. Oily wastewater samples A) before UF and B) after the UF unit.
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possible to obtain data about the proceeding of biodegradation. The oily wastewaters were diluted with nutrient solution containing nutrients and inoculum (SFS-EN 1899-1). Depending on the measuring range proper amount of diluted wastewater was added directly to the BOD OxiTopÒ bottle. 20 drops L1 of nitrification inhibitor as n-allylthiourea was added into the BOD OxiTopÒ bottle. BOD measurements were carried out under a constant temperature (20 0.2 C) for 7 days. The effect of inhibition on the BOD7 value of the wastewaters was also analyzed by diluting samples with various dilution factors. The biodegradation degree [%] was calculated based on the TOC value of the sample from Equation (1):
Biodegradation degree½% ¼
BOD7 MC 100% TOC MO2
(1)
where BOD7 is the biological oxygen demand during seven days [mg L1], TOC is the total organic carbon [mg L1], MC is the molecular mass of carbon and MO2 is the molecular mass of the oxygen molecule. BOD7 was determined for the three series of samples from both plants taken in 2008. 3. Results and discussion 3.1. Inorganic content of oily wastewaters Appendixes A and B contains data about inorganic content of one series of samples for WWTP1 and WWTP2, respectively. The two series of samples for WWTP1 varied slightly in their initial inorganic content, although there was no significant difference between the initial inorganic content of samples from WWTP1 and WWTP2. All the inorganic species had already been mainly removed by UF, probably due to the attachment of inorganic species to the oil emulsions. Only a minor decrease in the inorganic content was due to the sand filter and ion exchanger as a result of the low initial concentrations entering the sand filter and ion exchanger. The inorganic content of the effluents was low; however, the worst reductions (below 90%) with the UF-based process were typically gained for calcium, potassium, magnesium, sodium, nickel, boron, phosphorus and sulfur (from sulfuric acid for pH adjustment before ion exchangers). Also, on some days, the concentrations of arsenic, molybdenum and vanadium were not significantly reduced, although concentrations in effluents were low. The most interesting inorganic species was boron, which was present in high concentrations in the wastewaters of both plants. The amount of boron had even increased in WWTP1 during the UF-based process, whereas the reduction of boron for WWTP2 was low. The discussion concerning boron continues in Section 3.2. 3.2. Organic compounds in oily wastewaters The GC spectra were very complicated, containing various hydrocarbons, alcohols, acids etc. Fig. 3 presents an example of a gas chromatogram for one of the samples. It was not possible to identify all of the organic compounds of the samples by GC-FID/MS because of the overlapping of the peaks. However, some important conclusions can be drawn. The most interesting compounds in the samples from WWTP1 (2008 and 2011) were phenol, 2-phenoxyethanol (not so often present in the 2011 samples), and triethanolamine borate (also known as boron 2,20 ,200 -nitrilotriethoxide). In addition, the WWTP2 samples typically contained triethanolamine borate. The 2011 WWTP1 samples often contained tripropylene glycol. Whenever this was present, it was not removed effectively. Phenol is present in wastewaters for example from refineries, and is toxic even at low concentrations. Phenol is toxic to most
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Fig. 3. A typical gas chromatogram for one of the oily wastewater samples.
types of microorganisms at high concentrations and can be a growth rate inhibitory to even those species that are capable of degrading it (Busca et al., 2008). 2-phenyoxyethanol in wastewaters is released to the environment from various chemical plastics, photographic and mechanical product industries. It is expected to degrade through biodegradation if released into soil or water (The National Center for Biotechnology Information). Boric acid (biocide) has been used among other things for the protection of wood from fungal decay. However, it can leach from wood, therefore many attempts have been made to limit the leaching rate of boron from treated wood by coordinating the boron atom to nitrogen with an electron-pair dative such as in boratranes, for instance. Triethanolamine borate with five-membered rings has higher fungicidal effectiveness compared to boratranes that have six-membered rings (Franich, 2001; Franich et al., 2011). Tripropylene glycol is used in the manufacture of polyester polyols for urethane applications, in the manufacture of hydraulic and brake fluids, inks and some paints, and as a precursor of low molecular mass esters and polyesters. Tripropylene glycol is expected to be resistant to biodegradation in both soil and water environments by analogy with dipropylene glycol (The National Center for Biotechnology Information). In this study, it appeared that phenol, 2-phenoxyethanol, triethanolamine borate and tripropylene glycol exhibited only slight reductions. Furthermore the amount of triethanolamine borate was reduced the least. This boron compound could explain the high boron content of the wastewaters and its enrichment during the UF-based process. In conclusion, the amounts of dissolved compounds were reduced even though some compounds passed through the UFbased process, causing high values of BOD, COD and TOC. These compounds could also cause problems in post-treatment in municipal wastewater treatment processes. GC-FID/MS is not an adequate analyzing method for oily wastewaters with high concentrations of contaminants as such, but used with fractionation of oily wastewaters by extraction it could give more information without the overlapping of peaks.
Fig. 4. COD values of oily wastewater samples.
sample. The average COD reduction of the entire treatment process in 2008 was 90% for WWTP1 (varying between 86 and 98%) and 95% for WWTP2 (varying between 82 and 99%). In 2011, the corresponding COD reduction was 80% (varying between 70 and 88%). The TOC values were reduced significantly by the treatment process. The average TOC reduction of the entire treatment process was 81% for WWTP1 (varying between 68 and 94%) and 90% for WWTP2 (varying between 71 and 98%). However, the repeatability of TOC measurements was low because significant dilutions had to be performed on the oily wastewater samples. The TSC values of untreated wastewater samples were extremely negative (up to 22.9 meq L1) and varied significantly on a daily basis. The average TSC reduction in 2008 was 97% for WWTP1 and WWTP2 (varying between 95 and 99.5% for WWTP1 and 85e99% for WWTP2), and in 2011 about 93% for WWTP1 (varying between 80 and 99%). The TSC values of samples taken after the first sand filter and ion exchanger were always less negative than 0.50 meq L1. To conclude, the samples from WWTP1 and WWTP2 had on average similar initial values of COD (49.6e654 g L1) and TOC (5.70e97.3 g L1). In 2011, the initial values of COD (27.6e73.0 g L1) for WWTP1 had decreased on average by as much as five times from those in 2008. Initial TSC values were higher for WWTP2 (between 0.84 and 22.9 meq L1) than those for WWTP1 (between 1.20 and 16.8 meq L1). In 2011, the initial TSC values
3.3. COD, TOC and TSC values The COD, TOC, and TSC values (Figs. 4e6) that were measured demonstrated that the quality of oily wastewaters to be treated in the two wastewater treatment plants varied substantially between the sampling days, particularly for WWTP1. The COD values of untreated wastewater samples were very high. The WWTP1 samples (in 2008) had the highest values of COD and TOC, up to 654 g L1 and 97.3 g L1, respectively. The COD values of samples after UF and after the first sand filter and ion exchanger were significantly reduced compared to the COD values of the untreated
Fig. 5. TOC values of oily wastewater samples.
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adjustment before the sand filter. The decrease in pH explained the increase in zeta potentials. 3.5. BOD7 and biodegradation degrees
Fig. 6. TSC values of oily wastewater samples.
(between 1.60 and 3.80 meq L1) were on average three times less for WWTP1 than in 2008. The main reductions of COD, TOC and TSC were achieved with UF and only minor reductions were gained with the sand filter and ion exchangers. The treatment process in both plants exhibited high removal performance. However, the residuals of COD (5.30e 17.5 g L1) and TOC (1.40e5.70 g L1) were at times significantly high, due to dissolved contaminants passing through the UF membrane, potentially causing problems in the post-treatment of oily wastewaters by biological methods. Nevertheless, COD residuals in 2011 had decreased from 2008 on average by 4.00 g L1.
3.4. Zeta potentials and pH values Fig. 7 presents the zeta potentials of the samples taken after the UF unit and those taken after the first sand filter and ion exchanger. Slightly negative TSC values and zeta potentials indicated that the samples studied contained a small amount of low-charged droplets. Clear reductions of zeta potentials were recorded for WWTP1, while for WWTP2 the zeta potentials of samples after UF and after the first sand filter and ion exchanger were almost the same. The pH values of samples taken after the first sand filter and ion exchanger from WWTP1 in 2008 were around 5 because of the pH
Fig. 8 presents the BOD7 values determined for samples from both plants. The average BOD7 reductions were 81% for the entire process (varying between 79 and 86%) for WWTP1 and 74% (varying between 67 and 86%) for WWTP2. These reductions were less than the reductions for COD, TOC and TSC. The initial values of BOD7, on average, were twice as high for WWTP1 and the BOD7 residuals were slightly higher for WWTP1 than for WWTP2. The residuals were between 2.20 and 5.50 g L1. For untreated and treated samples, a lag phase of one day was observed at the beginning of the biodegradation process. Microbes responsible for biodegradation adapt to a new carbon source during the lag phase. After the lag phase, the biodegradation of the untreated samples proceeded steadily throughout the entire measuring period without leveling off. For the treated samples, biodegradation was the most vigorous on the second day and leveled off significantly after that. The main reductions of BOD7, were achieved with UF and only minor reductions were gained with the sand filter and ion exchangers, as was also observed with COD, TOC and TSC reductions. The effect of inhibition on BOD7 values was studied by measuring the BOD7 values for the same sample with various dilution factors. In this study, it was observed that for an untreated sample the BOD7 value increased significantly with sufficiently high dilution factors. This was not, however, the case for treated samples. The increase in BOD7 with an increasing dilution factor value could be explained by the existence of substances in untreated samples that inhibited bacterial activity. The effect of these inhibitive substances appears to decrease when diluting samples, thus increasing the BOD7 value. Roppola et al. (2009) discovered the effect of inhibition on the BOD7 values of pulp and paper mill wastewater samples diluted with a low dilution factor. Roppola et al. (2006) also gave another explanation, which was the lack of nutrients since nutrients were provided only by the dilution solution. In conclusion, according to these results, potentially inhibitive substances may be removed by UF, because no inhibition was observed in the treated samples. However, no universal conclusions can be drawn from this study due to the limited number of samples studied.
Fig. 7. Zeta potentials of oily wastewater samples. The pH values of the samples are marked above the columns.
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Fig. 8. BOD7 values of oily wastewater samples.
Fig. 10. Correlation of COD and TOC for studied samples.
Fig. 9 presents the biodegradation degrees of the oily wastewater samples. The biodegradation degree for untreated samples was 12e58% and for treated samples 41e73%. The existence of inhibitive substances could explain the lower biodegradation degree of untreated samples compared to treated samples. For the two series of samples analyzed by the OxiTopÒ method with the lowest TOC values, there was no significant difference between the biodegradation degrees of untreated and treated samples. Thus, on the basis of the results, it was concluded that inhibition occurred only for the samples with the highest oil content.
for WWTP2. The organic load entering WWTP1 were lower in 2011 than in 2008, therefore COD and TSC reductions were also lower in 2011 than three years before. It seemed that some development had thus been made in the pre-treatment of oily wastewaters entering WWTP1 as shown in the COD, TSC values of the influents. According to the results, the performance of the UF-based process was high overall and there was no significant difference between WWTP1 and WWTP2. The most significant BOD7, COD, TOC and TSC reductions were gained with UF treatment and only a minor decrease in these parameters was gained with the sand filter and ion exchangers. The sand filter and ion exchanger operated as back-up operations for the UF- unit. However, BOD7, COD and TOC residuals were occasionally considerable. Findings of the present study are in accordance with Tomaszewska et al. (2005) and Qiao et al. (2008) where a high oil removal but a low degree of COD or TOC removal was observed. High BOD7, COD and TOC residuals were probably due to small-sized organic substances passing through the membrane. GC-FID/MS studies showed that some organic compounds, such as phenol, 2-phenoxyethanol, triethanolamine borate and tripropylene glycol, were not totally removed with the UF-based process, and could have caused inhibition in the active sludge process at the municipal wastewater treatment plant, although no such inhibitive effect was discovered for the studied effluents in BOD measurements. For organic compounds passing through the UF-based process, advanced oxidation processes (Karakulski et al., 1998) could be used to limit the organic load entering the municipal wastewater treatment plant. Concerning inorganic content, the highest reductions were gained using the UF unit. Tomaszewska et al. (2005) observed a low retention of the same inorganic
3.6. Correlation between parameters The correlations between different parameters were determined. The best linear correlations were obtained for COD and TOC, COD and TSC, as well as for TOC and BOD7 (linear only at low concentrations) (Figs. 10 and 11). The linear correlation between COD and TSC could be used in the control of a wastewater treatment process because the COD of the wastewater to be treated could be estimated easily by measuring the TSC. 3.7. The performance of the UF-based process The oily wastewaters to be treated in the two wastewater treatment plants contained a very high organic load. The reductions of the entire UF-based process were the highest when the initial concentrations were the highest. The COD and TOC reductions in 2008 were five and nine percentage units higher for WWTP1 than
Fig. 9. Biodegradation degrees of oily wastewater samples in seven days.
Fig. 11. Correlation of COD and TSC for studied samples.
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substances (Ca, K, Na, Mg, Mn, SO4 and P2O5) with which the lowest reductions were obtained in this study. Flux decline, due to fouling of the membrane when treating highly oily wastewaters, is a serious problem when dealing with a UF process. The oily wastewaters studied here contained highly emulsified oils (physically and chemically emulsified), thus the oil droplets easily blocked the pores of the membrane and formed an oil layer on the surface of the UF membrane. Also, the varying quality and composition of oily wastewaters to be treated by the UF-based process increased the probability of UF membrane fouling. Therefore, in the future, more attention should be paid to the specific treatment of different types of oily wastewaters. Chemical backflushing of the UF membrane had to be performed frequently (weekly or even daily). Continual washing of the UF membrane then caused the slowing down of the entire treatment process. Chemical cleaning methods also generate new waste solutions as well as higher operating costs. The operation time of the UF membrane could be lengthened by pre-treatment before UF by breaking down the oil emulsions and increasing droplet sizes, using for example electrocoagulation (Karhu et al., 2012) or coagulationflocculation, where the dosage of coagulant could be controlled by means of the linear dependency of the COD and TSC values. Decreasing the flux (pressure) in the membrane system could decrease the fouling of the UF membrane. The critical flux is defined as the flux below which the decline of flux with time does not occur but above which flux decline and fouling is observed (Falahati and Tremblay, 2011). In emulsion separations by membranes, the critical flux could be specified as the flux above which oil passes through the membrane pores in great amounts (Nazzal and Wiesner, 1996). 4. Conclusions In this study, an evaluation was made of the performance of UFbased processes used in two wastewater treatment plants on the treatment of highly oily wastewaters. The oily wastewaters treated in the two wastewater treatment plants possessed a very high organic load. However, by 2011, the initial concentrations of contaminants at WWTP1 had decreased significantly since 2008. The performance of UF-based processes proved overall to be high and there was no significant difference between the performance of the two plants. Residual concentrations were occasionally very high due to a) the high initial concentrations of the untreated wastewaters and b) the presence of organic compounds not totally removed by the UF-based process. The main reductions in COD, TOC and TSC and inorganic content were achieved by the UF unit and only minor reductions were gained with the subsequent sand filter and ion exchangers. Inhibition of bacterial activity in untreated wastewaters was observed, although no such inhibitive effect was seen in the treated waters. A clear linear correlation between COD and TOC and between COD and TSC was observed. The correlation of COD and TSC values could be utilized for process control purposes. Acknowledgments The authors would like to thank the Graduate School in Chemical Engineering (GSCE), the VALOKATA e project (University of Oulu, the Finnish Funding Agency for Technology and Innovation (Tekes) from the European Regional Development Fund) for
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funding the research, Suomen Ympäristöpalvelu Oy for the inorganic analyses and Päivi Joensuu from Mass Spectrometry Laboratory (University of Oulu) for the analyses of organic compounds. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jenvman.2013.05.053. References Abadi, S.R.H., Sebzari, M.R., Hemati, M., Rekabdar, F., Mohammadi, T., 2011. Ceramic membrane performance in microfiltration of oily wastewater. Desalination 265, 222e228. Benito, J.M., Ebel, S., Gutiérrez, B., Pazos, C., Coca, J., 2001. Ultrafiltration of a waste emulsified cutting oil using organic membranes. Water Air. Soil. Pollut. 128, 181e195. Busca, G., Berardinelli, S., Resini, C., Arrighi, L., 2008. Technologies for the removal of phenols from fluid streams: a short review of recent developments. J. Hazard. Mater. 160, 265e288. Chakrabarty, B., Ghoshal, A.K., Purkait, M.K., 2008. Ultrafiltration of stable oil-inwater emulsion by polysulfone membrane. J. Memb. Sci. 325, 427e437. Chen, W., Su, Y., Wang, L., Jiang, Z., 2009. The improved oil/water separation performance of cellulose acetate-graft-polyacrylonitrile membranes. J. Memb. Sci. 337, 98e105. Chen, X., Hong, L., Xu, Y., Ong, Z.W., 2012. Ceramic pore channels with inducted carbon nanotubes for removing oil from water. ACS Appl. Mater. Interfaces 4, 1909e1918. Ebrahimi, M., Willershausen, D., Ashaghi, K.S., Engel, L., Placido, L., Mund, P., Bolduan, P., Czermak, P., 2010. Investigations on the use of different ceramic membranes for efficient oil-field produced water treatment. Desalination 250, 991e996. EPA 3015A (revision 1), 2007. Microwave Assisted Acid Digestion of Aqueous Samples and Extracts. US Environmental Protection Agency, p. 25. Falahati, H., Tremblay, A.Y., 2011. Flux dependent oil permeation in the ultrafiltration of highly concentrated and unstable oil-in-water emulsions. J. Memb. Sci. 371, 239e247. Franich, R.A., 2001. Preservative Compounds and their Use, NZ Patent 514356. Franich, R.A., Nicholson, B.K., Kroese, H.W., Gallagher, S.S., Meder, R., Lane, J.R., Kelly, B.D., 2011. Synthesis, characterization and crystal structures of the boratranes: 2,10,11-trioxa-6-aza-1-boratricyclo[4.4.4.01,6] tetradecane (tri-n-propanolamine borate), and 3-(4-methoxy)phenoxymethyl-7,10-dimethyl-2,8,9trioxa-5-aza-1-boratricyclo[3.3.3.01,5]-undecane. Polyhedron 30, 2884e2889. Ghidossi, R., Veyret, D., Scotto, J.L., Jalabert, T., Moulin, P., 2009. Ferry oily wastewater treatment. Sep. Purif. Technol. 64, 296e303. Hach Lange for Water Quality, 2004. Working Procedure LCK 914 COD Cuvette Test Measuring Range 5-60 g/L, p. 4. Hesampour, M., Krzyzaniak, A., Nyström, M., 2008. Treatment of waste water from metal working by ultrafiltration, considering the effects of operating conditions. Desalination 222, 212e221. Janknecht, P., Lopes, A.D., Mendes, A.M., 2004. Removal of industrial cutting oil from oil emulsions by polymeric ultra- and microfiltration membranes. Environ. Sci. Technol. 38, 4878e4883. Karakulski, K., Morawski, W.A., Grzechulska, J., 1998. Purification of bilge water by hybrid ultrafiltration and photocatalytic processes. Sep. Purif. Technol. 14, 163e173. Karhu, M., Kaakinen, J., Kuokkanen, T., Rämö, J., 2009. Biodegradation of light fuel oils in water and soil as determined by the manometric respirometric method. Water Air. Soil. Pollut. 197, 3e14. Karhu, M., Kuokkanen, V., Kuokkanen, T., Rämö, J., 2012. Bench scale electrocoagulation studies of bio oil-in-water and synthetic oil-in-water emulsions. Sep. Purif. Technol. 96, 296e305. Krsti c, D.M., Höflinger, W., Koris, A.K., Vatais, G.N., 2007. Energy-saving potential of cross-flow ultrafiltration with inserted static mixer: application to an oil-inwater emulsion. Sep. Purif. Technol. 57, 134e139. Leiviskä, T., Rämö, J., Sillanpää, M., Vuorenpalo, V.-M., 2005. Multimodal zeta potential distribution in chemical pulp bleaching filtrates. J. Pulp Paper Sci. 31, 167e174. Mittal, P., Jana, S., Mohanty, K., 2011. Synthesis of low-cost hydrophilic ceramicpolymeric composite membrane for treatment of oily wastewater. Desalination 282, 54e62. Nazzal, F.F., Wiesner, M.R., 1996. Microfiltration of oil-in-water emulsions. Water Environ. Res. 68, 1187e1191. Peng, H., Tremblay, A.Y., 2008. Membrane regeneration and filtration modeling in treating oily wastewaters. J. Memb. Sci. 324, 59e66. Qiao, X., Zhang, Z., Yu, J., Ye, X., 2008. Performance characteristics of a hybrid membrane pilot-scale plant for oilfield/produced wastewater. Desalination 225, 113e122. Rezvanpour, A., Roostaazad, R., Hesampour, M., Nyström, M., Ghotbi, C., 2009. Effective factors in the treatment of kerosene-water emulsion by using UF membranes. J. Hazard. Mater. 161, 1216e1224.
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Roppola, K., 2009. Environmental Applications of Manometric Respirometric Methods. Doctoral thesis. Faculty of science, Department of the Chemistry, University of Oulu, Finland. http://herkules.oulu.fi/isbn9789514290794/index. html?lang¼en (accessed 06.07.12.). Roppola, K., Kuokkanen, T., Nurmesniemi, H., Rämö, J., Pöykiö, R., Prokkola, H., 2006. Comparison study of manometric respirometric test and common chemical methods in the determination of BOD7 in a pulp and paper mill’s wastewaters. J. Autom. Methods Manage. Chem.. http://dx.doi.org/10.1155/JAMMC/2006/90384. Roppola, K., Kuokkanen, T., Rämö, J., Prokkola, H., Ruotsalainen, J., 2009. Characterisation of organic fractions of pulp and paper mill wastewater with a manometric respirometric biochemical oxygen demand method and automatic chemical oxygen demand analyses. Chem. Speciation Bioavailability 21, 121e130.
SFS-EN 1899-1, 1998. Water Quality. Determination of Biochemical Oxygen Demand after N Days (BODn). Part 1: Dilution and Seeding Method with Allylthiourea Addition. Helsinki, Finland, p. 24. Srijaroonrat, P., Julien, E., Aurelle, Y., 1999. Unstable secondary oil/water emulsion treatment ultrafiltration: fouling control by backflushing. J. Memb. Sci. 159, 11e20. The National Center for Biotechnology Information, PubChem Database. http:// pubchem.ncbi.nlm.nih.gov/. (accessed 16.08.12.). Tomaszewska, M., Orecki, A., Karakulski, K., 2005. Treatment of bilge water using a combination of ultrafiltration and reverse osmosis. Desalination 185, 203e212. Yliwati, E., Ismail, A.F., 2011. Effect of additives concentration on the surface properties and performance of PVDF ultrafiltration membranes for refinery produced wastewater treatment. Desalination 273, 226e234.
Contents lists available at SciVerse ScienceDirect
Journal of Environmental Management journal homepage: www.elsevier.com/locate/jenvman
Performance of a commercial industrial-scale UF-based process for treatment of oily wastewaters M. Karhu a, *, T. Kuokkanen b, J. Rämö c, M. Mikola a, J. Tanskanen a a
Chemical Process Engineering Laboratory, Department of Process and Environmental Engineering, University of Oulu, P.O. Box 4300, 90014 Oulu, Finland Department of Chemistry, University of Oulu, P.O. Box 3000, 90014 Oulu, Finland c Thule Institute, University of Oulu, P.O. Box 7300, 90014 Oulu, Finland b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 February 2013 Received in revised form 8 May 2013 Accepted 25 May 2013 Available online
An evaluation was made of the performance of a commercial industrial-scale ultrafiltration (UF)-based process for treatment of highly concentrated oily wastewaters. Wastewater samples were gathered from two plants treating industrial wastewaters in 2008, and in 2011 (only from one of the plants), from three points of a UF-based treatment train. The wastewater samples were analyzed by measuring the BOD7, COD, TOC and total surface charge (TSC). The inorganic content and zeta potentials of the samples were analyzed and GC-FID/MS analyses were performed. The removal performances of BOD7, COD, TOC and TSC in 2008 and 2011 for both plants were very high. Initial concentrations of contaminants in 2011 were lower than in 2008, therefore the COD and TSC reductions were also lower in 2011 than three years before. Regardless of the high performance of UF-based processes in both plants, at times the residual concentrations were considerable. This could be explained by the high initial concentrations and also by the presence of the dissolved compounds that were characterized. Linear correlation was observed between COD and TOC, and between COD and TSC. The correlation between COD and TSC could be utilized for process control purposes. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: Characterization of wastewater Oil emulsion Oily wastewater Ultrafiltration
1. Introduction Oil is one of the major contaminants in industrial wastewaters. Oily industrial wastewaters are extremely heterogenous, containing various types of hydrocarbons, surfactants, metals, acids etc. Stable oily wastewaters consisting of highly chemically and physically emulsified oils are the most challenging ones in terms of effective treatment. Ultrafiltration (UF) is a widely accepted and commonly used method for the treatment of oily wastewaters. Ceramic UF membranes have high thermal and chemical stability, therefore they are broadly used in treating industrial wastewaters. However, UF membrane fouling is one of the major drawbacks of this treatment method, slowing down the entire treatment process, and being uneconomical because of the need for washing. UF also presents difficulties in removing very small-sized oil droplets or dissolved hydrocarbons from oily wastewaters. In essence, a single treatment operation for oily wastewaters is not enough to decrease oil concentration to the required level; instead a combination of
* Corresponding author. Tel.: þ358 29448 2357; fax: þ358 29448 2304. E-mail address: mirjam.karhu@oulu.fi (M. Karhu). 0301-4797/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jenvman.2013.05.053
various treatment operations is needed to treat heterogenous oily wastewaters. The majority of studies on the filtration of industrial oily wastewaters were performed in the 1990s, and research has continued in recent decades to a lesser degree. The studies usually deal with the suitability and fouling of polymeric or ceramic microfiltration (MF) and UF membranes for treatment of synthetic oil-in-water emulsions (Chakrabarty et al., 2008; Chen et al., 2009; Janknecht et al., 2004; Mittal et al., 2011; Srijaroonrat et al., 1999) or real oily wastewaters (Abadi et al., 2011; Ebrahimi et al., 2010; Ghidossi et al., 2009; Qiao et al., 2008; Tomaszewska et al., 2005; Yliwati and Ismail, 2011), or with the combining of MF or UF with other treatment methods such as a photocatalytic process (Karakulski et al., 1998) and reverse osmosis (Tomaszewska et al., 2005). UF processes for oily wastewater treatment have usually been studied in laboratory scale (e.g. Ebrahimi et al., 2010) but also a few pilot-scale studies can be found, as follows. Tomaszewska et al. (2005) investigated the possibility of bilge water treatment in the integrated UF/RO (reverse osmosis) system. Study was carried out with the use of two pilot plants: the UF process with tubular module and the RO process with spiral wound modules. Qiao et al. (2008) designed a pilot-scale plant involving aeration tank, air flotation, sand filter and UF (a hollow fiber membrane) for
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treatment of oilfield-produced wastewater. Ghidossi et al. (2009) developed an industrial UF-based (with a 300-kDa ceramic membrane) process for treatment of bilge and ballast water on board. In addition to the performance of UF-based processes fouling of the membranes have been under intensive investigation. Studies on fouling have usually been focused on understanding the phenomenon of fouling and concentration polarization for various types of membranes, the effects of operating conditions (e.g. fluxes, pH, temperature, solution chemistry such as salt concentration, surface properties of the membrane and transmembrane pressure) (Benito et al., 2001; Hesampour et al., 2008; Rezvanpour et al., 2009) and tools to reduce fouling (Chen et al., 2012; Krsti c et al., 2007; Peng and Tremblay, 2008). The main purpose of this work was to study the performance of a commercial industrial-scale UF-based process for treatment of real and concentrated oily wastewater mixtures from the metal industry, oil separation wells, and ships in the form of bilge water etc. The study focused on two wastewater treatment plants that treat oily wastewaters with the same type of UF-based process. The object was a) to explore the current situation regarding treatment of oily wastewaters, b) to evaluate the performance of unit operations and the entire process, c) to study the problems when dealing with especially highly oil contaminated wastewaters and d) to establish possible correlations between the parameters to be determined, such as biological oxygen demand (BOD), chemical oxygen demand (COD), total organic carbon (TOC), total surface charge (TSC) and zeta potential for process regulation purposes. To the best of our knowledge, there have been no other scientific articles on such a comprehensive evaluation and characterization study of highly oily industrial wastewaters treated with a commercial industrial-scale UF-based process. 2. Material and methods 2.1. Oily wastewater samples Oily wastewater samples were taken from two wastewater treatment plants (WWTP1 and WWTP2), owned by one of the biggest environmental companies in Finland, which treats various types of wastewaters such as oily wastewaters (UF-based process), and waters containing heavy metals as well as acids and bases. The UF-based process (Fig. 1) is similar in both plants; however, the process of WWPT2 includes one extra sand filter and ion exchanger after the UF unit (ceramic, 50 nm pore size and water flow about
700 L hour1). The sand filter contains quartz sand of three different granule sizes. The ion exchangers are cation changers containing weakly acidic resins with chelating iminodiacetate groups. The oily wastewaters entering WWTP1 and WWTP2 are routed through a drum filter before a belt filter and then homogenized by mixing different types of oily wastewaters, targeting neutral pH values. Effluents from both plants are then piped to municipal wastewater treatment plants. The oily wastewater samples were taken separately from sampling points before UF, after UF, as well as after the first sand filter and ion exchanger (Fig. 1). The series of samples were taken in 2008 during nine days from WWTP1 (From May to June) and seven days from WWTP2 (July to September). The samples in 2011 were taken during six days (June) from WWTP1 to study any possible change in quality in the sample wastewaters. Table 1 presents all of the studied series of samples where each series contained samples from before UF, after UF, as well as after the first sand filter and ion exchanger. Fig. 2 presents the wastewater samples before UF and after UF treatment. 2.2. Analyses The inorganic content of the wastewater samples was determined by inductively coupled plasma e optical emission spectrometry (ICP-OES) at Suomen Ympäristöpalvelu Oy, a FINAS accredited test laboratory (T231). The wastewater samples were first digested with nitric acid in a CEM MARS 5 microprocessor controlled microwave oven with CEM HP 500 teflon vessels (CEM Corp., Matthews, USA) (EPA3015A). The ICP-OES analyses were then performed with Thermo Electron IRIS Intrepid II XDL inductively coupled plasma optical emission spectrometer (Franklin, USA). The inorganic content was analyzed for two series of samples from WWTP1 and for one series from WWTP2 in 2008. The organic compounds of the oily wastewater samples were analyzed by gas chromatography using Hewlett Packard 6890 Series GC system equipped with a FID detector in 2008 or with a mass selective detector (Hewlett Packard 5973 Mass Selective Detector) in 2011, an autosampler (Hewlett Packard 6890 Series Injector) and a fused silica Supelco Equity-1 capillary column (15 m 0.25 mm) with a nominal film thickness of 0.25 mm in 2008 and with a J&W Scientific DB-624 column by Agilent Technologies (30 m 0.32 mm) with a nominal film thickness of 1.8 mm in 2011. The GC oven program in 2008 was started at 50 C, was held for 1 min and then increased to 300 C at 10 C min1 and then held at 300 C for 5 min.
Fig. 1. Unit operations of UF-based process in WWTP1 and WWTP2 as well as samples 1) before UF, 2) after UF, and 3a and 3b) after the first sand filter and ion exchanger in WWTP1 and WWTP2.
M. Karhu et al. / Journal of Environmental Management 128 (2013) 413e420 Table 1 Series of oily wastewater samples. Series
WWTP
Year
1e9 10e15 16e22
1 1 2
2008 2011 2008
The GC program in 2011 was started at 40 C, was held for 1 min and then increased to 270 C at 15 C min1 and then held at 270 C for 5 min. The most turbid oily wastewaters were filtered before the GC-FID/MS analyses. The Spectrum library was used for identification of the compounds in the GCeMS analyses. The GC-FID/MS analyses were performed for three series of samples from WWTP1 in 2008, for two series of samples for WWTP2 and for five series of samples from WWTP1 in 2011. WTW inoLab pH 720 was used for measuring the pH. The COD was measured using the Hach Lange photometric cuvette test in different measuring ranges. The COD measurement was based on oxidizable substances in sample reacting with potassium dichromate in sulfuric acid solution in the presence of silver sulfate as a catalyst. Chloride was masked by mercury sulfate. Cr3þ ions were formed in the reaction and the green coloration of Cr3þ was evaluated. The sample was first added into a cuvette and then heated in 148 C for 2 h using Hach-Lange HT 200S thermostat system (Hach Lange for Water Quality). After digestion the cuvette was allowed to cool down into room temperature and the absorbance of sample was measured with Hach-Lange DR 2800 spectrophotometer at wavelength of 605 nm. The TOC was measured by using a Sievers 900 Portable TOC Analyzer. However, the samples had to be diluted significantly due to the limit value of 50 ppm. The TSC of particles in the colloidal solution was measured with a Mütek PCD 03 pH. The COD and TSC values were determined for all of the samples and TOC only for the samples from 2008. The zeta potentials of the wastewater samples were determined with a CoulterÒ DelsaÔ 440SX (Beckman Coulter, Miami, FL, USA). The theory of zeta potential measurement can be found in detail from (Leiviskä et al., 2005). Zeta potentials were determined for the wastewater samples after UF and after the first sand filter and ion exchanger of the UF-based processes in 2008 for seven series of samples from WWTP1 and WWTP2. The zeta potentials for samples before UF could not be analyzed because of the high concentration of particles. The apparatus used for BOD measurements in solution was the OxiTopÒ Control system (WTW). The theory behind BOD measurement and its calculation can be found in more detail in (Karhu et al., 2009; Roppola, 2009). The OxiTopÒ Control system (WTW) gives 360 measuring points per measuring period, and thus it is
Fig. 2. Oily wastewater samples A) before UF and B) after the UF unit.
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possible to obtain data about the proceeding of biodegradation. The oily wastewaters were diluted with nutrient solution containing nutrients and inoculum (SFS-EN 1899-1). Depending on the measuring range proper amount of diluted wastewater was added directly to the BOD OxiTopÒ bottle. 20 drops L1 of nitrification inhibitor as n-allylthiourea was added into the BOD OxiTopÒ bottle. BOD measurements were carried out under a constant temperature (20 0.2 C) for 7 days. The effect of inhibition on the BOD7 value of the wastewaters was also analyzed by diluting samples with various dilution factors. The biodegradation degree [%] was calculated based on the TOC value of the sample from Equation (1):
Biodegradation degree½% ¼
BOD7 MC 100% TOC MO2
(1)
where BOD7 is the biological oxygen demand during seven days [mg L1], TOC is the total organic carbon [mg L1], MC is the molecular mass of carbon and MO2 is the molecular mass of the oxygen molecule. BOD7 was determined for the three series of samples from both plants taken in 2008. 3. Results and discussion 3.1. Inorganic content of oily wastewaters Appendixes A and B contains data about inorganic content of one series of samples for WWTP1 and WWTP2, respectively. The two series of samples for WWTP1 varied slightly in their initial inorganic content, although there was no significant difference between the initial inorganic content of samples from WWTP1 and WWTP2. All the inorganic species had already been mainly removed by UF, probably due to the attachment of inorganic species to the oil emulsions. Only a minor decrease in the inorganic content was due to the sand filter and ion exchanger as a result of the low initial concentrations entering the sand filter and ion exchanger. The inorganic content of the effluents was low; however, the worst reductions (below 90%) with the UF-based process were typically gained for calcium, potassium, magnesium, sodium, nickel, boron, phosphorus and sulfur (from sulfuric acid for pH adjustment before ion exchangers). Also, on some days, the concentrations of arsenic, molybdenum and vanadium were not significantly reduced, although concentrations in effluents were low. The most interesting inorganic species was boron, which was present in high concentrations in the wastewaters of both plants. The amount of boron had even increased in WWTP1 during the UF-based process, whereas the reduction of boron for WWTP2 was low. The discussion concerning boron continues in Section 3.2. 3.2. Organic compounds in oily wastewaters The GC spectra were very complicated, containing various hydrocarbons, alcohols, acids etc. Fig. 3 presents an example of a gas chromatogram for one of the samples. It was not possible to identify all of the organic compounds of the samples by GC-FID/MS because of the overlapping of the peaks. However, some important conclusions can be drawn. The most interesting compounds in the samples from WWTP1 (2008 and 2011) were phenol, 2-phenoxyethanol (not so often present in the 2011 samples), and triethanolamine borate (also known as boron 2,20 ,200 -nitrilotriethoxide). In addition, the WWTP2 samples typically contained triethanolamine borate. The 2011 WWTP1 samples often contained tripropylene glycol. Whenever this was present, it was not removed effectively. Phenol is present in wastewaters for example from refineries, and is toxic even at low concentrations. Phenol is toxic to most
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Fig. 3. A typical gas chromatogram for one of the oily wastewater samples.
types of microorganisms at high concentrations and can be a growth rate inhibitory to even those species that are capable of degrading it (Busca et al., 2008). 2-phenyoxyethanol in wastewaters is released to the environment from various chemical plastics, photographic and mechanical product industries. It is expected to degrade through biodegradation if released into soil or water (The National Center for Biotechnology Information). Boric acid (biocide) has been used among other things for the protection of wood from fungal decay. However, it can leach from wood, therefore many attempts have been made to limit the leaching rate of boron from treated wood by coordinating the boron atom to nitrogen with an electron-pair dative such as in boratranes, for instance. Triethanolamine borate with five-membered rings has higher fungicidal effectiveness compared to boratranes that have six-membered rings (Franich, 2001; Franich et al., 2011). Tripropylene glycol is used in the manufacture of polyester polyols for urethane applications, in the manufacture of hydraulic and brake fluids, inks and some paints, and as a precursor of low molecular mass esters and polyesters. Tripropylene glycol is expected to be resistant to biodegradation in both soil and water environments by analogy with dipropylene glycol (The National Center for Biotechnology Information). In this study, it appeared that phenol, 2-phenoxyethanol, triethanolamine borate and tripropylene glycol exhibited only slight reductions. Furthermore the amount of triethanolamine borate was reduced the least. This boron compound could explain the high boron content of the wastewaters and its enrichment during the UF-based process. In conclusion, the amounts of dissolved compounds were reduced even though some compounds passed through the UFbased process, causing high values of BOD, COD and TOC. These compounds could also cause problems in post-treatment in municipal wastewater treatment processes. GC-FID/MS is not an adequate analyzing method for oily wastewaters with high concentrations of contaminants as such, but used with fractionation of oily wastewaters by extraction it could give more information without the overlapping of peaks.
Fig. 4. COD values of oily wastewater samples.
sample. The average COD reduction of the entire treatment process in 2008 was 90% for WWTP1 (varying between 86 and 98%) and 95% for WWTP2 (varying between 82 and 99%). In 2011, the corresponding COD reduction was 80% (varying between 70 and 88%). The TOC values were reduced significantly by the treatment process. The average TOC reduction of the entire treatment process was 81% for WWTP1 (varying between 68 and 94%) and 90% for WWTP2 (varying between 71 and 98%). However, the repeatability of TOC measurements was low because significant dilutions had to be performed on the oily wastewater samples. The TSC values of untreated wastewater samples were extremely negative (up to 22.9 meq L1) and varied significantly on a daily basis. The average TSC reduction in 2008 was 97% for WWTP1 and WWTP2 (varying between 95 and 99.5% for WWTP1 and 85e99% for WWTP2), and in 2011 about 93% for WWTP1 (varying between 80 and 99%). The TSC values of samples taken after the first sand filter and ion exchanger were always less negative than 0.50 meq L1. To conclude, the samples from WWTP1 and WWTP2 had on average similar initial values of COD (49.6e654 g L1) and TOC (5.70e97.3 g L1). In 2011, the initial values of COD (27.6e73.0 g L1) for WWTP1 had decreased on average by as much as five times from those in 2008. Initial TSC values were higher for WWTP2 (between 0.84 and 22.9 meq L1) than those for WWTP1 (between 1.20 and 16.8 meq L1). In 2011, the initial TSC values
3.3. COD, TOC and TSC values The COD, TOC, and TSC values (Figs. 4e6) that were measured demonstrated that the quality of oily wastewaters to be treated in the two wastewater treatment plants varied substantially between the sampling days, particularly for WWTP1. The COD values of untreated wastewater samples were very high. The WWTP1 samples (in 2008) had the highest values of COD and TOC, up to 654 g L1 and 97.3 g L1, respectively. The COD values of samples after UF and after the first sand filter and ion exchanger were significantly reduced compared to the COD values of the untreated
Fig. 5. TOC values of oily wastewater samples.
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adjustment before the sand filter. The decrease in pH explained the increase in zeta potentials. 3.5. BOD7 and biodegradation degrees
Fig. 6. TSC values of oily wastewater samples.
(between 1.60 and 3.80 meq L1) were on average three times less for WWTP1 than in 2008. The main reductions of COD, TOC and TSC were achieved with UF and only minor reductions were gained with the sand filter and ion exchangers. The treatment process in both plants exhibited high removal performance. However, the residuals of COD (5.30e 17.5 g L1) and TOC (1.40e5.70 g L1) were at times significantly high, due to dissolved contaminants passing through the UF membrane, potentially causing problems in the post-treatment of oily wastewaters by biological methods. Nevertheless, COD residuals in 2011 had decreased from 2008 on average by 4.00 g L1.
3.4. Zeta potentials and pH values Fig. 7 presents the zeta potentials of the samples taken after the UF unit and those taken after the first sand filter and ion exchanger. Slightly negative TSC values and zeta potentials indicated that the samples studied contained a small amount of low-charged droplets. Clear reductions of zeta potentials were recorded for WWTP1, while for WWTP2 the zeta potentials of samples after UF and after the first sand filter and ion exchanger were almost the same. The pH values of samples taken after the first sand filter and ion exchanger from WWTP1 in 2008 were around 5 because of the pH
Fig. 8 presents the BOD7 values determined for samples from both plants. The average BOD7 reductions were 81% for the entire process (varying between 79 and 86%) for WWTP1 and 74% (varying between 67 and 86%) for WWTP2. These reductions were less than the reductions for COD, TOC and TSC. The initial values of BOD7, on average, were twice as high for WWTP1 and the BOD7 residuals were slightly higher for WWTP1 than for WWTP2. The residuals were between 2.20 and 5.50 g L1. For untreated and treated samples, a lag phase of one day was observed at the beginning of the biodegradation process. Microbes responsible for biodegradation adapt to a new carbon source during the lag phase. After the lag phase, the biodegradation of the untreated samples proceeded steadily throughout the entire measuring period without leveling off. For the treated samples, biodegradation was the most vigorous on the second day and leveled off significantly after that. The main reductions of BOD7, were achieved with UF and only minor reductions were gained with the sand filter and ion exchangers, as was also observed with COD, TOC and TSC reductions. The effect of inhibition on BOD7 values was studied by measuring the BOD7 values for the same sample with various dilution factors. In this study, it was observed that for an untreated sample the BOD7 value increased significantly with sufficiently high dilution factors. This was not, however, the case for treated samples. The increase in BOD7 with an increasing dilution factor value could be explained by the existence of substances in untreated samples that inhibited bacterial activity. The effect of these inhibitive substances appears to decrease when diluting samples, thus increasing the BOD7 value. Roppola et al. (2009) discovered the effect of inhibition on the BOD7 values of pulp and paper mill wastewater samples diluted with a low dilution factor. Roppola et al. (2006) also gave another explanation, which was the lack of nutrients since nutrients were provided only by the dilution solution. In conclusion, according to these results, potentially inhibitive substances may be removed by UF, because no inhibition was observed in the treated samples. However, no universal conclusions can be drawn from this study due to the limited number of samples studied.
Fig. 7. Zeta potentials of oily wastewater samples. The pH values of the samples are marked above the columns.
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Fig. 8. BOD7 values of oily wastewater samples.
Fig. 10. Correlation of COD and TOC for studied samples.
Fig. 9 presents the biodegradation degrees of the oily wastewater samples. The biodegradation degree for untreated samples was 12e58% and for treated samples 41e73%. The existence of inhibitive substances could explain the lower biodegradation degree of untreated samples compared to treated samples. For the two series of samples analyzed by the OxiTopÒ method with the lowest TOC values, there was no significant difference between the biodegradation degrees of untreated and treated samples. Thus, on the basis of the results, it was concluded that inhibition occurred only for the samples with the highest oil content.
for WWTP2. The organic load entering WWTP1 were lower in 2011 than in 2008, therefore COD and TSC reductions were also lower in 2011 than three years before. It seemed that some development had thus been made in the pre-treatment of oily wastewaters entering WWTP1 as shown in the COD, TSC values of the influents. According to the results, the performance of the UF-based process was high overall and there was no significant difference between WWTP1 and WWTP2. The most significant BOD7, COD, TOC and TSC reductions were gained with UF treatment and only a minor decrease in these parameters was gained with the sand filter and ion exchangers. The sand filter and ion exchanger operated as back-up operations for the UF- unit. However, BOD7, COD and TOC residuals were occasionally considerable. Findings of the present study are in accordance with Tomaszewska et al. (2005) and Qiao et al. (2008) where a high oil removal but a low degree of COD or TOC removal was observed. High BOD7, COD and TOC residuals were probably due to small-sized organic substances passing through the membrane. GC-FID/MS studies showed that some organic compounds, such as phenol, 2-phenoxyethanol, triethanolamine borate and tripropylene glycol, were not totally removed with the UF-based process, and could have caused inhibition in the active sludge process at the municipal wastewater treatment plant, although no such inhibitive effect was discovered for the studied effluents in BOD measurements. For organic compounds passing through the UF-based process, advanced oxidation processes (Karakulski et al., 1998) could be used to limit the organic load entering the municipal wastewater treatment plant. Concerning inorganic content, the highest reductions were gained using the UF unit. Tomaszewska et al. (2005) observed a low retention of the same inorganic
3.6. Correlation between parameters The correlations between different parameters were determined. The best linear correlations were obtained for COD and TOC, COD and TSC, as well as for TOC and BOD7 (linear only at low concentrations) (Figs. 10 and 11). The linear correlation between COD and TSC could be used in the control of a wastewater treatment process because the COD of the wastewater to be treated could be estimated easily by measuring the TSC. 3.7. The performance of the UF-based process The oily wastewaters to be treated in the two wastewater treatment plants contained a very high organic load. The reductions of the entire UF-based process were the highest when the initial concentrations were the highest. The COD and TOC reductions in 2008 were five and nine percentage units higher for WWTP1 than
Fig. 9. Biodegradation degrees of oily wastewater samples in seven days.
Fig. 11. Correlation of COD and TSC for studied samples.
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substances (Ca, K, Na, Mg, Mn, SO4 and P2O5) with which the lowest reductions were obtained in this study. Flux decline, due to fouling of the membrane when treating highly oily wastewaters, is a serious problem when dealing with a UF process. The oily wastewaters studied here contained highly emulsified oils (physically and chemically emulsified), thus the oil droplets easily blocked the pores of the membrane and formed an oil layer on the surface of the UF membrane. Also, the varying quality and composition of oily wastewaters to be treated by the UF-based process increased the probability of UF membrane fouling. Therefore, in the future, more attention should be paid to the specific treatment of different types of oily wastewaters. Chemical backflushing of the UF membrane had to be performed frequently (weekly or even daily). Continual washing of the UF membrane then caused the slowing down of the entire treatment process. Chemical cleaning methods also generate new waste solutions as well as higher operating costs. The operation time of the UF membrane could be lengthened by pre-treatment before UF by breaking down the oil emulsions and increasing droplet sizes, using for example electrocoagulation (Karhu et al., 2012) or coagulationflocculation, where the dosage of coagulant could be controlled by means of the linear dependency of the COD and TSC values. Decreasing the flux (pressure) in the membrane system could decrease the fouling of the UF membrane. The critical flux is defined as the flux below which the decline of flux with time does not occur but above which flux decline and fouling is observed (Falahati and Tremblay, 2011). In emulsion separations by membranes, the critical flux could be specified as the flux above which oil passes through the membrane pores in great amounts (Nazzal and Wiesner, 1996). 4. Conclusions In this study, an evaluation was made of the performance of UFbased processes used in two wastewater treatment plants on the treatment of highly oily wastewaters. The oily wastewaters treated in the two wastewater treatment plants possessed a very high organic load. However, by 2011, the initial concentrations of contaminants at WWTP1 had decreased significantly since 2008. The performance of UF-based processes proved overall to be high and there was no significant difference between the performance of the two plants. Residual concentrations were occasionally very high due to a) the high initial concentrations of the untreated wastewaters and b) the presence of organic compounds not totally removed by the UF-based process. The main reductions in COD, TOC and TSC and inorganic content were achieved by the UF unit and only minor reductions were gained with the subsequent sand filter and ion exchangers. Inhibition of bacterial activity in untreated wastewaters was observed, although no such inhibitive effect was seen in the treated waters. A clear linear correlation between COD and TOC and between COD and TSC was observed. The correlation of COD and TSC values could be utilized for process control purposes. Acknowledgments The authors would like to thank the Graduate School in Chemical Engineering (GSCE), the VALOKATA e project (University of Oulu, the Finnish Funding Agency for Technology and Innovation (Tekes) from the European Regional Development Fund) for
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funding the research, Suomen Ympäristöpalvelu Oy for the inorganic analyses and Päivi Joensuu from Mass Spectrometry Laboratory (University of Oulu) for the analyses of organic compounds. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jenvman.2013.05.053. References Abadi, S.R.H., Sebzari, M.R., Hemati, M., Rekabdar, F., Mohammadi, T., 2011. Ceramic membrane performance in microfiltration of oily wastewater. Desalination 265, 222e228. Benito, J.M., Ebel, S., Gutiérrez, B., Pazos, C., Coca, J., 2001. Ultrafiltration of a waste emulsified cutting oil using organic membranes. Water Air. Soil. Pollut. 128, 181e195. Busca, G., Berardinelli, S., Resini, C., Arrighi, L., 2008. Technologies for the removal of phenols from fluid streams: a short review of recent developments. J. Hazard. Mater. 160, 265e288. Chakrabarty, B., Ghoshal, A.K., Purkait, M.K., 2008. Ultrafiltration of stable oil-inwater emulsion by polysulfone membrane. J. Memb. Sci. 325, 427e437. Chen, W., Su, Y., Wang, L., Jiang, Z., 2009. The improved oil/water separation performance of cellulose acetate-graft-polyacrylonitrile membranes. J. Memb. Sci. 337, 98e105. Chen, X., Hong, L., Xu, Y., Ong, Z.W., 2012. Ceramic pore channels with inducted carbon nanotubes for removing oil from water. ACS Appl. Mater. Interfaces 4, 1909e1918. Ebrahimi, M., Willershausen, D., Ashaghi, K.S., Engel, L., Placido, L., Mund, P., Bolduan, P., Czermak, P., 2010. Investigations on the use of different ceramic membranes for efficient oil-field produced water treatment. Desalination 250, 991e996. EPA 3015A (revision 1), 2007. Microwave Assisted Acid Digestion of Aqueous Samples and Extracts. US Environmental Protection Agency, p. 25. Falahati, H., Tremblay, A.Y., 2011. Flux dependent oil permeation in the ultrafiltration of highly concentrated and unstable oil-in-water emulsions. J. Memb. Sci. 371, 239e247. Franich, R.A., 2001. Preservative Compounds and their Use, NZ Patent 514356. Franich, R.A., Nicholson, B.K., Kroese, H.W., Gallagher, S.S., Meder, R., Lane, J.R., Kelly, B.D., 2011. Synthesis, characterization and crystal structures of the boratranes: 2,10,11-trioxa-6-aza-1-boratricyclo[4.4.4.01,6] tetradecane (tri-n-propanolamine borate), and 3-(4-methoxy)phenoxymethyl-7,10-dimethyl-2,8,9trioxa-5-aza-1-boratricyclo[3.3.3.01,5]-undecane. Polyhedron 30, 2884e2889. Ghidossi, R., Veyret, D., Scotto, J.L., Jalabert, T., Moulin, P., 2009. Ferry oily wastewater treatment. Sep. Purif. Technol. 64, 296e303. Hach Lange for Water Quality, 2004. Working Procedure LCK 914 COD Cuvette Test Measuring Range 5-60 g/L, p. 4. Hesampour, M., Krzyzaniak, A., Nyström, M., 2008. Treatment of waste water from metal working by ultrafiltration, considering the effects of operating conditions. Desalination 222, 212e221. Janknecht, P., Lopes, A.D., Mendes, A.M., 2004. Removal of industrial cutting oil from oil emulsions by polymeric ultra- and microfiltration membranes. Environ. Sci. Technol. 38, 4878e4883. Karakulski, K., Morawski, W.A., Grzechulska, J., 1998. Purification of bilge water by hybrid ultrafiltration and photocatalytic processes. Sep. Purif. Technol. 14, 163e173. Karhu, M., Kaakinen, J., Kuokkanen, T., Rämö, J., 2009. Biodegradation of light fuel oils in water and soil as determined by the manometric respirometric method. Water Air. Soil. Pollut. 197, 3e14. Karhu, M., Kuokkanen, V., Kuokkanen, T., Rämö, J., 2012. Bench scale electrocoagulation studies of bio oil-in-water and synthetic oil-in-water emulsions. Sep. Purif. Technol. 96, 296e305. Krsti c, D.M., Höflinger, W., Koris, A.K., Vatais, G.N., 2007. Energy-saving potential of cross-flow ultrafiltration with inserted static mixer: application to an oil-inwater emulsion. Sep. Purif. Technol. 57, 134e139. Leiviskä, T., Rämö, J., Sillanpää, M., Vuorenpalo, V.-M., 2005. Multimodal zeta potential distribution in chemical pulp bleaching filtrates. J. Pulp Paper Sci. 31, 167e174. Mittal, P., Jana, S., Mohanty, K., 2011. Synthesis of low-cost hydrophilic ceramicpolymeric composite membrane for treatment of oily wastewater. Desalination 282, 54e62. Nazzal, F.F., Wiesner, M.R., 1996. Microfiltration of oil-in-water emulsions. Water Environ. Res. 68, 1187e1191. Peng, H., Tremblay, A.Y., 2008. Membrane regeneration and filtration modeling in treating oily wastewaters. J. Memb. Sci. 324, 59e66. Qiao, X., Zhang, Z., Yu, J., Ye, X., 2008. Performance characteristics of a hybrid membrane pilot-scale plant for oilfield/produced wastewater. Desalination 225, 113e122. Rezvanpour, A., Roostaazad, R., Hesampour, M., Nyström, M., Ghotbi, C., 2009. Effective factors in the treatment of kerosene-water emulsion by using UF membranes. J. Hazard. Mater. 161, 1216e1224.
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