Anoxic oscillating MBR for photosynthetic bacteria harvesting and high salinity wastewater treatment
Graphical abstract
Introduction
Recently, with the fast development of economy and industry, such as food-processing, electroplate manufacture, textile and dyeing industry, million tons of salinity wastewater has been discharged into rivers and oceans (Lefebvre and Moletta, 2006). As reported, high-salinity food-processing and industrial wastewater has always been regarded as a kind of refractory wastewater, and the presence of soluble salts and organic containments represents more significant hazard to our environment (Jang et al., 2013). Therefore, to develop the environmentally friendly and effective technology for high-salinity wastewater treatment is essential (Jeong et al., 2014).
In recent years, some traditional biological processes such as activated sludge technology, sequencing batch reactor (SBR) process and anaerobic sludge method have been used for the saline organic wastewater treatment (Campos et al., 2002, Gebauer, 2004, Rene et al., 2008). However, the existence of large numbers of soluble inorganic salts can cause the unbalance of osmotic stress across the cell wall, and consequently had deleterious impacts on the growth of microorganisms and their activity (Kargi and Dincer, 1996). To overcome these problems, the screening and cultivation of salt-adapted microbes involving an adaptation to high salt concentrations have attracted considerable attention for the saline effluents treatment, which has proved feasible to salt removal by physical and chemical treatment. Alternatively, photosynthetic bacteria (PSB) due to its high salt-tolerance and activity have big potential in treating various high-organics-load wastewaters. Meanwhile, the cultivated PSB biomass is supposed to be a good source of other high-value products, like health products, nutraceutical and animal feeds (Kuo et al., 2012, Munoz and Guieysse, 2006), and often used as feedstock for the sustainable and renewable biofuel production (Keskin et al., 2011). Therefore, it may be one interesting alternative to integrate PSB cultivation with high-salinity wastewater treatment. However, the cultivation of PSB in a continuous-flow (Kim et al., 2012) and on a large-scale still remains a big challenge. The slow growth of biomass, poor settling properties and difficulty in harvesting restricts the widespread application of PSB in wastewater treatment and renewable energy production (Liu et al., 2007).
To deal with the aforementioned problem, reported studies suggested that membrane bioreactor can be proposed as a promising strategy for the high-density cultivation of a diverse community of microorganisms because of its excellent solid/liquid separation performance. In recent years, combining PSB with MBR has been constructed for the wastewater treatment and cultivation of biomass (Lu et al., 2013). Compared with the traditional methods, PSB-MBR combined system takes advantages of easy operation, high biomass recovery and excellent degradation performance. However, finding effective ways to eliminate membrane fouling, as well as improve the biomass production and reduce operational cost in the meantime, is still an urgent task for large scale application. Although many efforts have been devoted on developing various methods to eliminate fouling of MBR (Kola et al., 2014, Pradhan et al., 2015, Qin et al., 2012, Wang et al., 2014), fouling is still inevitable. For the sake of alleviating membrane fouling, many researches are now being focused on the modification of membrane surface (Abednejad et al., 2014, Maximous et al., 2009), design of dynamic membrane module (Qin et al., 2015), improvement of membrane module cleaning methods (Lim and Renbi, 2003), optimization of operating parameters (such as SRT, HRT, DO and T) (Chang et al., 2002) and development of novel MBR combined systems (Deng et al., 2014, Kim et al., 2013, Zhang et al., 2013), etc.
In this paper, we aim to develop a novel pendulum type oscillation (PTO) hollow fiber membrane combined with PSB bioreactor for non-biodegradable high-salinity food processing and industrial wastewater treatment, in which the harvesting of PSB was easily performed in succession and on a large scale and the membrane fouling can be effectively reduced. To our knowledge, such MBR system applied for microbe’s cultivation together with wastewater treatment has not been reported so far. Based on this new idea, the objective of the present study is mainly concentrated on assessing the effectiveness and performance of PTO-MBR in terms of biomass production, organics degradation and membrane fouling control. In order to further improve biomass production and reduce operating cost, the main attention will be paid on exploring the effect of cycle of light/dark and aeration on PSB growth and high-salinity wastewater treatment.
Section snippets
Photosynthetic bacteria and synthetic high salinity organic wastewater
Photosynthetic bacteria (PSB) used in this study was obtained from the company of Zhejiang Dinglong, China. By analysis, the dominant bacteria belonged to Rhodopseudomonas palustris, which was consistent with the previous studies reported by Kuo et al. (2012). In each stage, the initial PSB concentration was approximately 0.45 g/L dry weight. The photosynthetic bacteria were grown under the shaking culture in RCVBN medium.
High-salinity wastewater such as food processing wastewater was used in
Effect of the module oscillating on PSB biomass production
Biomass production of PSB along with the photo-bioreactor operation time was monitored and calculated by profiling in term of suspended solids, and the results of volumetric biomass concentration and biomass daily productivity for PTO-MBR and CMBR systems are shown in Fig. 2. As apparent from figure, the quantity of PSB biomass represented a more rapid increase trend in the initial stages and the PSB biomass harvested in the bioreactor increased from 0.75 to 2.75 g/L for CMBR system (Fig. 2A).
Conclusion
In current study, PTO-MBR was first developed as a promisingly and economical alternative for high-salinity wastewater treatment as well as PSB biomass harvest in success and on large scale. Results show clear advantages of PTO-MBR over CMBR in terms of biomass production, organics degradation and membrane fouling reduction. The importance of the study lies not just in their excellent performance, but as a general procedure for integrating the harvesting of various microbes (such as microalgae)
Acknowledgements
We thank for financial support the National Natural Science Foundation of China (Grant Nos. 21236008 and 21506193), the Zhejiang Provincial Bureau of Science and Technology (Grant No. 2016C33007), China Postdoctoral Science Foundation (Grant No. 2015M581958), Zhejiang Provincial Postdoctoral Science Foundation and the Minjiang Scholarship from Fujian Provincial Government.
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