Elsevier

Bioresource Technology

Volume 150, December 2013, Pages 513-522
Bioresource Technology

The environmental sustainability of microalgae as feed for aquaculture: A life cycle perspective

https://doi.org/10.1016/j.biortech.2013.08.044Get rights and content

Highlights

  • Resource and carbon footprint of an operative microalgae pilot plant (LCA).

  • Upscaling scenarios established in cooperation with industry.

  • Comparing sustainability results of microalgae production to traditional fish feed.

  • Promising results for an upscaled algae production in terms of resource footprint.

Abstract

The environmental sustainability of microalgae production for aquaculture purposes was analyzed using exergy analysis (EA) and life cycle assessment (LCA). A production process (pilot 2012, 240 m2) was assessed and compared with two upscaling scenarios (pilot 2013, 1320 m2 and first production scale 2015, 2.5 ha). The EA at process level revealed that drying and cultivation had the lowest efficiencies. The LCA showed an improvement in resource efficiency after upscaling: 55.5 MJex,CEENE/MJex DW biomass was extracted from nature in 2012, which was reduced to 21.6 and 2.46 MJex,CEENE/MJex DW in the hypothetical 2013 and 2015 scenarios, respectively. Upscaling caused the carbon footprint to decline by factor 20 (0.09 kg CO2,eq/MJex DW in 2015). In the upscaling scenarios, microalgae production for aquaculture purposes appeared to be more sustainable in resource use than a reference fish feed (7.70 MJex,CEENE and 0.05 kg CO2,eq per MJex DW).

Introduction

Microalgae are some of the oldest life forms on earth, but they have only recently been recognized as a very promising (but challenging) source of biomass. Algae produce more biomass than terrestrial plants per unit area due to a higher photosynthetic efficiency (Lardon et al., 2009). Furthermore, their production does not compete directly with food crops because they can be cultivated on marginal land using fresh, brackish or salt water, depending on the algae species. They take up CO2 as a carbon source during photosynthesis and they use nutrients that are often found in excess, such as nitrogen and phosphorus. Because of these properties, algae are capable of both sequestering CO2 and purifying nutrient-rich waste streams (Clarens et al., 2010 and Pittman et al., 2011).

Initially, microalgae research focused mainly on the production of biodiesel, bioethanol and biogas from algae as a response to the growing demand for fuel and the depletion of fossil resources (Lardon et al., 2009, Chisti, 2008). A few studies have been published on bio-hydrogen generated from algae-utilizing bacteria and the production of syngas or bio-oil. Several environmental assessments have already been done to evaluate the use of algae as a sustainable source of bioenergy (Lardon et al., 2009, Stephenson et al., 2010, Udom et al., 2013). That research shows that in most cases, producing fuel from algae has a negative energy balance, meaning that the energy demand to produce the biodiesel exceeds the energy it generates. In addition, CO2 is emitted throughout the life cycle rather than being sequestered. The negative environmental balance and the relatively low market price for biodiesel clearly make microalgae unsustainable for energy purposes in the short term. But biofuel production from algae is a relatively new technology. If one accounts for co-products, recycling of nutrients and absorption of CO2 from flue gases, biodiesel production from algae actually shows greater long-term potential than terrestrial biofuels such as corn ethanol (Liu et al., 2012). Algae could be used for many other applications as well, but further research and development will be required to render this technology both commercially viable and environmentally sustainable. The only way to offset the commercial and environmental costs of cultivating and processing algae would be to guarantee production of high quality end-products.

Microalgae contain large amounts of useful carbohydrates, proteins, lipids and antioxidants. This makes them an essential food source in the rearing of all stages of marine bivalve molluscs (clams, oysters, scallops), the larval stages of some marine gastropods (abalone, conch), larvae of several marine fish species and penaeid shrimp, and zooplankton (Muller-Feuga, 2000). Microalgae also produce omega-3 and omega-6 (long-chain) poly-unsaturated fatty acids that are recognized as being essential in human nutrition. These beneficial fatty acids are currently extracted almost exclusively from fish. The feeding fish to fish principle used in aquaculture is unsustainable because more than 1 kg fish is needed to produce 1 kg of carnivorous farmed fish. Small pelagic species, such as Peruvian anchovy, are co-captured and subjected to several processes to produce fish oil and fish meal, which are mainly used as feed in aquaculture systems. Researchers around the globe are concerned because aquaculture production is expected to continue to rise in the short term. This will result in increased fishing pressure on wild stocks to supply both fish meal and fish oil, which threatens the sustainability of the species in question. Prices for these fish-based products are already increasing, which has sparked a search for alternatives to these sources of fatty acids (Shepherd, 2013). Commercial-scale production of algae biomass could be a workable alternative to fish meal and fish oil in aquaculture. This would reduce both the cost and ecological impact of intensive fish farming (Muller-Feuga, 2000). Because of the large short term market potential of aquaculture, cultivating algae for fish feed applications has the potential to become profitable in the near future.

Most algae cultivation systems can be described as either open or closed photobioreactors. Closed systems (e.g., tubular photobioreactors (PBR), flat panel photobioreactors and column photobioreactors) are the most well-known. They generally consume more energy and require a large capital investment but deliver a relatively high biomass yield. In contrast, open algae growth ponds have lower energy requirements but produce less biomass for the same area. Open ponds can also be affected by contamination and evaporation losses (Jorquera et al., 2010, Lardon et al., 2009, Stephenson et al., 2010).

In 2009, Michiels unveiled the patented ProviAPT (Proviron Advanced Photobioreactor Technology). This system has the advantages of closed reactors yet avoids the drawbacks of open systems (e.g., contamination, evaporation) and closed systems (e.g., scaling up). It is a plastic bag (12 m2) filled with water. Each bag, which rests on the ground, contains 35 embedded plastic panels where the algae grow. This yields a reactive surface of 7 m2 (Michiels, 2009). Water and nutrients are pumped into one side of the panels and algae are harvested on the other side via an overflow system. This system is relatively inexpensive to build (<€10/m2) because the structure is constructed entirely of plastic and the production can be automated.

Various life cycle assessment (LCA) studies on the sustainability of microalgae production have been published (e.g., Lardon et al., 2009, Jorquera et al., 2010, Soratana and Landis, 2011). The data for these studies were generally collected from literature or were based on small-scale lab trials. Little research has been done that scrutinizes realistic algae production scenarios in an industrial setup. The aim of this study was to overcome these limitations by examining the entire production process in an industrial setup and, via two hypothetical upscaling scenarios, to optimize the configuration for a more sustainable algae cultivation. Three algal production scenarios were studied: (1) the pilot setup of Nannochloropsis sp. cultivated in 20 ProviAPT reactors was based on actual production runs (the pilot 2012 scenario covers 240 m2), (2) a realistic but hypothetical upscaling of pilot 2012 including recycling and more efficient use of equipment (the pilot 2013 scenario covers 1320 m2) and (3) a further hypothetical upscaling using waste streams and a more efficient processing equipment in a warmer climate such as in Spain (first production scenario 2015 covers 2.5 ha). For all three scenarios, an exergy analysis (EA) and life cycle analysis (LCA) was performed. The exergy concept was applied because exergy quantifies the ability to cause change and is not conserved, in contrast to energy, which exposes the inefficient processes (Dewulf et al., 2007b). During the exergy analysis, the exergy consumption as well as the exergy efficiency of the different subprocesses (process level) and of the entire foreground production system (gate-to-gate) was determined. Furthermore, a cradle-to-gate life cycle analysis (LCA) has been performed because of the increasing awareness of the possible impact associated with the full chain of products, processes and services. LCA is a powerful tool to detect the different environmental aspects and potential environmental impact of a product or service throughout its life cycle from raw materials to production, use, collection and end-of-life treatment including any recycling and disposal (European Commission, 2009). The resource consumption of the entire product life cycle was determined using the CEENE method (Dewulf et al., 2007a). In addition, the impact on climate change was assessed using the IPCC 2007 method (IPCC, 2007). This study had two main objectives: (1) to examine different aspects of the ecological sustainability of the production of microalgae in the ProviAPT cultivation system and (2) to determine the potential of microalgae production as fish feed application and to make a comparative study of the environmental impact of algae based fish feed versus traditional fish feed.

Section snippets

Description of the process

In 2012, Nannochloropsis sp. was cultivated in 20 ProviAPT photobioreactors (240 m2) at a production site near Antwerp, Belgium. The photobioreactor consisted of a large transparent bag (surface area 12 m2) containing embedded plastic panels. The surface area under the panels measured a total of 7 m2 (‘productive area’). Nutrients and CO2-enriched air were injected (semi-)continuously into the panels and the microalgae were harvested via an overflow system. The 5000 l of water surrounding the

Exergy analysis at process level

An exergy analysis was made of the algae pilot scale facility of 2012 and the forecast scenarios for 2013 and 2015. The chemical, physical, electrical and radiation exergy of all material and energy flows of each subprocess were calculated. Table 3 gives the exergy efficiencies at process level for each scenario.

Conclusions

Based on the algae production scenarios, the least exergy efficient processes (cultivation and drying) could be improved by a factor of 5 and 7, respectively, in only three years. Recycling of nutrients and savings on energy use were identified as important ways to increase the sustainability of algae production. Upscaling, reactor design improvements, enhancement of photosynthetic yield and a good choice of location also contributed to a lower resource and carbon footprint. Although additional

Acknowledgements

The authors thank Bert Lemmens, project manager at VITO, for his assistance in data collection. This work was financially supported by EnAlgae, a 4-year Strategic Initiative of the INTERREG IVB North West Europe programme.

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