Enhanced submerged membrane bioreactor combined with biosurfactant rhamnolipids: Performance for frying oil degradation and membrane fouling reduction
Graphical abstract
Highlights
► Pilot scale SMBR enhanced by rhamnolipids was developed for oily wastewater treatment. ► The removal efficiency of oil and grease can be increased up to 90% at short HRT. ► RSMBR exhibited 10 times higher membrane permeability than conventional SMBR.
Introduction
As is well known, the increasing presence of fat, oil, and grease (FOG) in wastewater are posing more and more serious challenges to the environment. Especially in recent years, urbanization promotes the dramatic broadening of restaurant. Million tons of frying oil containing wastewater from food industry and kitchens has been discharged into rivers around the world every year, which represents a significant toxicity hazard to our daily life. Therefore, the development of new methods for degradation of oily wastewater has attracted considerable attention. Physical and chemical processes have been investigated in the previous papers, like membrane separation (Asatekin and Mayes, 2009), kaolinite (Lebedeva and Fogden, 2010) or activated carbon adsorption, H2O2/UV processes, and Fenton processes (Bressan et al., 2004), but those treatments are often costly and less effective. Alternatively, the biological treatments are regarded as efficient options to deal with oil-containing wastewater (Guo et al., 2009a, Guo et al., 2009b, Ma et al., 2009). Membrane bioreactor (MBR), as one of most promising technologies in water treatment, has big potential for the stable and efficient biological degradation in oily wastewater treatment (Scholz and Fuchs, 2000, Viero et al., 2008).
However, further wide application of MBR for the treatment of oily wastewater is usually subject to numerous limitations. For example, the oil and grease in wastewater are usually clumped together, which are resistant to the biodegradation of many microorganisms (Li and Wrenn, 2004). More importantly, the biological techniques are mostly incapable of complete elimination of emulsified oil from wastewater due to the fact that they prove to be poorly biodegradable (Scholz and Fuchs, 2000). The use of surfactants in the biological process is proposed to be a promising strategy for enhancing the biotreatment of high-strength oil and grease. Surfactants, both chemical and biological, are amphiphilic compounds which can increase hydrophobic or insoluble organic substrates dispersion in aqueous phase or changes the property of cell surfaces, thereby improve their bioavailability to the microorganism. Chemical surfactants have been effectively used in different kinds of biotreatment systems (Liu et al., 2004a, Liu et al., 2004b, Liwarska-Bizukojc et al., 2008). Nevertheless, the addition of chemically synthesized surfactants to biological process is not an optimal solution because of the low biodegradability and inevitable toxicity towards sludge microbes. In comparison to synthetic surfactants, the application of biologically produced surfactants (biosurfactants) in bioremediation process may be more acceptable from a social point of view due to the biodegradability and low toxicities of biosurfactants (Singh et al., 2007). Zhang et al. (2009) investigated the effect of rhamnolipids addition on an aerobic activated sludge treating frying oil wastewater. Results showed that higher removal efficiency (95%) was observed in the presence of rhamnolipids at a short time of 12 h. Similar applications have been reported, including biodegradation of diesel in shaking flasks (Whang et al., 2009), and aerobic treatment of petrochemical industry wastewater in a completely stirred tank reactor (CSTR) (Sponza and Gök, 2010). So far the biosurfactants applied in MBR process for frying oil wastewater treatment has not been mentioned yet.
Moreover, membrane fouling is always addressed as a major problem limiting the wide application of MBR. In general, membrane fouling is attributed to particle and organic matters deposited on membrane surface or internal clogging of membranes (Bani-Melhem and Elektorowicz, 2010, Tian et al., 2011). In order to seek for distinct strategies to control membrane fouling in MBR application, most previous researches focused on membrane module cleaning (physical and chemical methods) (Lim and Bai, 2003), operating parameters optimization (such as solid retention time (SRT), hydraulic retention time (HRT), dissolved oxygen (DO) concentration, and temperature) (Chang et al., 2002) and characteristic improvement of activated sludge via addition of adsorbents and coagulants (Ji et al., 2010), etc.
In this paper, the attempt to apply biosurfactants into the SMBR has been first made for frying oil wastewater treatment. The main objective is to explore the effect of biosurfactant rhamnolipids on degradation of frying oil and the characteristics of membrane fouling in SMBR system. For comparison, a hybrid SMBR with additional rhamnolipids (RSMBR) and a controlled SMBR (CSMBR) were operated in parallel under the same condition.
Section snippets
Biosurfactant
The surfactants used in this study are rhamnolipids, which are kind of anionics biosurfactants. Rhamnolipids were produced by Pseudomonas sp. zju.um1 which was previously isolated in our laboratory, and waste frying oil was used for the fermentation as sole carbon source (Zhu et al., 2007). The cost of the free-cell culture broth containing rhamnolipids was only 2800 USD/m3. By analysis, this clear culture broth was presented to contain 35 g/L of rhamnolipids. The critical micelle concentration
Comparison of oil degradation in the control and test reactor
The COD of influent and effluent from RSMBR and CSMBR process and the corresponding removal efficiency at short hydraulic time of 1 day (24 h) are shown in Fig. 2. The results indicated that high residual oil grease, maximum COD concentration in permeate of 248.8 and 980.03 mg/L, and minimum COD concentration of 84.60 and 476.12 mg/L, were detected respectively in the CSMBR without addition of rhamnolipids under two different conditions. As the initial COD was detected to be above 500 mg/L in CSMBR,
Conclusions
In current study, a pilot scale setup of enhanced SMBR combined with rhamnolipids (RSMBR) was first applied for frying oil wastewater treatment. The results demonstrated that the novel combined process was not only reliable for oily wastewater treatment, but also reduced the membrane fouling remarkably. Approximately 90% COD removal efficiency can be achieved by using RSMBR at a 1 day short HRT, even higher than that in CSMBR treatment at a 5 days HRT. In the last stage of RSMBR operation, the
Acknowledgements
Financial support for this work was provided by the National Natural Science Foundation of China (Grants No. 21236008, 21176226) and Zhejiang Provincial Bureau of Science and Technology (Grant No. 2008C13014-2).
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Co-corresponding author. Address: Department of Chemical and Biological Engineering, Zhejiang University, 38 Zheda Road, Hangzhou, Zhejiang 310027, PR China. Tel.: +86 571 87953193; fax: 86 571 87951227.