Short CommunicationThe influences of the recycle process on the bacterial community in a pilot scale microalgae raceway pond
Introduction
Improvements in the flocculation methods to increase the efficiencies of harvesting microalgae have been well researched and has been demonstrated with dissolved air flocculation (DAF)(Chu et al., 2011, Haarhoff and Edzwald, 2013), electroflocculation (Lee et al., 2013) and centrifugal force (Pahl et al., 2013). Combinations of these technologies has been shown to shorten the time required for recovering microalgae; for example by combining electroflocculation with dissolved air flotation, the flocculation time was reduced from 30 min to 14 min (Xu et al., 2010). Flocculation methods have been shown to be both effective for low concentrations of microalgae and inexpensive, this makes it suitable for the initial concentration of the biomass (Molina Grima et al., 2003). The wastewater industry has long made use of centrifugation for the dewatering of solids. Similarly, the recovery of microalgal cells via centrifugation results in the rapid harvesting of up to 94% of the algal biomass (Molina Grima et al., 2003).
Reduction and reuse of waste is a key part of the environmental and economic sustainability for the production of microalgal biofuels (Cho et al., 2011). One aspect of this is the use of recycled water from the harvest process. The large volumes of water which are found in open systems together with the low density of microalgae provide challenges in efficiencies and cost effectiveness; therefore it is essential to reclaim the water from the harvest process. An example of why recycling water is essential was shown by Yang et al. (2011); their life cycle analysis of biodiesel production from microalgae showed that the recycling of water from harvest reduces the water and nutrient usage by 84% and 55% respectively. These authors also showed that by reclaiming the water following microalgal harvest no further additions of potassium, magnesium and sulphur to the open pond system were required (Yang et al., 2011). Furthermore the use of recycled water collected from harvesting stages prevents new input of water from external sources (which may also contain undesired organisms), significantly reducing the costs associated with the acquisition of water.
Previous research has focused on the ability to use recycled water, with a main focus on nutrient recycling. To date minimal research has been conducted on the bacterial community dynamics within the recycled water (Cho et al., 2011). When developing a harvesting system it is important to influence the growth towards the desired organism; previous studies have shown that recycled water can enhance the growth of unwanted microorganisms during the flocculation process, which is not desired when growing organisms of interest (Guo et al., 2011). Bacteria have shown to have a varying effect on microalgal growth: the symbiosis between microalgae and bacteria has shown to be beneficial due to bacterial ability to produce B12, an essential vitamin for microalgae (Goecke et al., 2013, Kazamia et al., 2012); particular negative aspects of enhanced bacterial growth are the introduction of competition for nutrients and loss of nitrogen through denitrification processes (Christenson and Sims, 2011). To maintain a large scale open pond in a sterile condition is impractical as it is exposed to the environment; however ensuring that the conditions are selective towards desired microorganisms is achievable. Monitoring the bacterial population in terms of biomass and diversity within the recycled water from a harvesting system is essential to prevent an increase in the bacterial load with each harvest cycle.
The aim of this investigation was to determine the effects on the bacterial community dynamics during the harvest of Tetraselmis sp. This study also assessed the effectiveness of using molecular biological methods such as polymerase chain reaction (PCR) combined with denaturing gradient gel electrophoresis (DGGE) and real time PCR (RT-PCR) as tools to effectively determine the overall efficiency of a pilot scale microalgal biofuels harvest system. These tools have been commonly used to evaluate microbial communities and effects of treatment in other systems (Erkelens et al., 2012).
Section snippets
Process description and plant design
Harvesting of Tetraselmis sp. in hyper saline water was conducted on a daily basis from an open pond system (120,000 L) (Muradel, Australia). The harvest of the microalgae was conducted over a two stage system involving firstly electroflocculation and secondly continuous centrifugation. The electroflocculation unit consisted of aluminium sheets with a separation of 0.15 m between each electrode and a DC power supply. The electroflocculator unit was 2.4 m × 1.2 m × 0.15 m with a linear flow velocity
Bacterial cell counts with real time PCR
The results of the QPCR analyses showed that the microalgal harvesting stage had a significant impact on the number of bacteria present in the recycled water (Table. 1). The use of real time PCR was shown to be a highly sensitive tool for the detection of organisms. A standard was formed with a known bacterial cell count; the standard used gave an R2 value of 0.98. The overall bacterial cell concentration was higher in the final pond sample (34,983 ± 8,798 gene copies/mL) than in the initial pond
Conclusion
It was observed the bacterial community from the water recycled at the electroflocculation stage still consisted of bacteria, while the centrifuge stage greatly reduced the bacterial community. It is recommended that total removal of the bacteria is not essential but can be controlled much better with the centrifuge process, though the removal of organisms which are not of interest from the recycled water may not be beneficial as the removal of symbiotic partners with the microalgae may occur.
Acknowledgements
The authors would like to thank Marissa Miller for obtaining the samples. The authors would also like to thank the South Australian Regional Facility for Molecular Evolution and Ecology for the use of their facilities. This research was supported under Australian Research Council’s Linkage Projects funding scheme (Project LP100200616) with industry partner SQC Pty Ltd. The views expressed here are those of the authors and are not necessarily those of the Australian Research Council.
References (20)
- et al.
Reuse of effluent water from a municipal wastewater treatment plant in microalgae cultivation for biofuel production
Bioresour. Technol.
(2011) - et al.
Production and harvesting of microalgae for wastewater treatment, biofuels, and bioproducts
Biotechnol. Adv.
(2011) - et al.
Comparison of inclined plate sedimentation and dissolved air flotation for the minimisation of subsequent nitrogenous disinfection by-product formation
Chemosphere
(2011) - et al.
Sustainable remediation – the application of bioremediated soil for use in the degradation of TNT chips
J. Environ. Manage.
(2012) - et al.
Growth and repair potential of three species of bacteria in reclaimed wastewater after uv disinfection
Biomed. Environ. Sci.
(2011) - et al.
Adapting dissolved air flotation for the clarification of seawater
Desalination
(2013) - et al.
Harvesting of marine microalgae by electroflocculation: the energetics, plant design, and economics
Appl. Energy
(2013) - et al.
Recovery of microalgal biomass and metabolites: process options and economics
Biotechnol. Adv.
(2003) - et al.
Life-cycle analysis on biodiesel production from microalgae: water footprint and nutrients balance
Bioresour. Technol.
(2011) - et al.
Carbohydrate-degrading bacteria closely associated with Tetraselmis indica: influence on algal growth
Aquatic Biol.
(2012)
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