Elsevier

Bioresource Technology

Volume 157, April 2014, Pages 364-367
Bioresource Technology

Short Communication
The influences of the recycle process on the bacterial community in a pilot scale microalgae raceway pond

https://doi.org/10.1016/j.biortech.2014.02.056Get rights and content

Highlights

  • Bacteria were present over the whole harvest/recycle process at large scale.

  • Electroflocculation had little effect over the bacterial community.

  • Bacteria found within this study are a symbiotic partner to Tetraselmis sp.

Abstract

The use of recycled media has been shown to be a necessary step within the lifecycle of microalgal biofuels for economic sustainability and reducing the water footprint. However the impact of the harvesting of microalgae on the bacterial load of the recycled water has yet to be investigated. Within this study PCR–DGGE and real-time PCR was used to evaluate the bacterial community dynamics within the recycled water following harvest and concentration steps for a pilot scale open pond system (120,000 L), which was developed for the production of green crude oil from Tetraselmis sp. in hyper saline water. Two stages were used in the harvesting; Stage 1 electroflocculation, and Stage 2 centrifugation. Electroflocculation was shown to have little effect on the bacterial cell concentration. In contrast bacterial diversity and cell concentration within the centrifugation step was greatly reduced.

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.

Cited by (15)

  • Reused cultivation water from a self-inhibiting alga does not inhibit other algae but alters their microbiomes

    2020, Algal Research
    Citation Excerpt :

    Higher initial bacteria diversity may have also led to different results for algae growth responses and for DOC accumulation [79]. Additionally, different harvesting methods, such as filtration, flocculation, and centrifugation, will affect the carryover of bacteria in reused water [80]. In this experimental design, most bacteria were removed from reused water by filtration.

  • Water reuse for sustainable microalgae cultivation: Current knowledge and future directions

    2020, Resources, Conservation and Recycling
    Citation Excerpt :

    Even in a closed-column bioreactor, a high abundance of rod-shaped bacteria were observed in the culture supernatant of Scenedesmus acuminatus (Fig. 1). The percentage of bacteria remaining in the water after algae harvesting varies by harvest method (Erkelens et al., 2014; Gonzalez-Lopez et al., 2013), with essentially all bacteria removed by ultrafiltration systems. Dissolved Organic Matter DOM is potentially responsible for growth inhibition in reused water (Depraetere et al., 2015; Discart et al., 2014; Lu et al., 2019; Rodolfi et al., 2003).

  • Exploring the potency of integrating semi-batch operation into lipid yield performance of Chlamydomonas sp. Tai-03

    2019, Bioresource Technology
    Citation Excerpt :

    Although using 4-Su was cheaper than 5% CO2, the use of OC may escalate the extent of culture contamination by bacteria when scaling up, especially into an open pond system. Upon exposure to air, contamination of microalgal culture with heterotrophic bacteria is unavoidable (Erkelens et al., 2014). Although the bacteria species may not be harmful, this co-existence may lead to a need for additional preventive measures and purification processes depending on the intended application of the microalgal biomass (Chisti, 2016).

View all citing articles on Scopus
View full text