François René

Separation efficiency of a vacuum gas lift for microalgae harvesting.

Low-energy and low-cost separation of microalgae from water is important to the economics of microalgae harvesting and processing. Flotation under vacuum using a vacuum gas lift for microalgae harvesting was investigated for different airflow rates, bubble sizes, salinities and harvest volumes. Harvesting efficiency (HE) and concentration factor (CF) of the vacuum gas lift increased by around 50% when the airflow rate was reduced from 20 to 10 L min(-1). Reduced bubble size multiplied HE and CF 10 times when specific microbubble diffusers were used or when the salinity of the water was increased from 0‰ to 40‰. The reduction in harvest volume from 100 to 1L increased the CF from 10 to 130. An optimized vacuum gas lift could allow partial microalgae harvesting using less than 0.2 kWh kg(-1) DW, thus reducing energy costs 10-100 times compared to complete harvesting processes, albeit at the expense of a less concentrated biomass harvest.

Assessing aquaculture sustainability: a comparative methodology

Little work dealing with the evaluation of aquaculture system sustainability has so far been undertaken on a global and comparative basis. Moreover, such work is mostly based on very unbalanced approaches in terms of the dimensions of sustainable development that are taken into account. The approach adopted in this article is designed to encompass all the dimensions of sustainability including the institutional one (governance). The taking into account of this latter, in particular, together with the role played by aquaculture in sustainability at the territorial level gives the approach its original and innovative nature. The process of establishing the checklist of sustainability indicators in aquaculture relies on a hierarchical nesting approach which makes it possible to link indicators with general sustainability criteria and principles. At once multidisciplinary and participatory, the approach compares several countries with highly differentiated types of aquaculture system. An original finding from this work is that the technically most intensive farming model scores better than more extensive systems, which might have been thought to be closer to natural systems in their environmental dimension and therefore intuitively more ‘sustainable’. This result suggests relating sustainability outcomes to the level of control and of devolved responsibilities.

Evaluation of Aquaculture System Sustainability: A Methodology and Comparative Approaches

Over the last 30 years, aquaculture has experienced an unprecedented development in global animal production with an average yearly growth rate of over 10% between 1980 and 2000 (FAO, 2009). During the same period, capture fisheries saw their progression gradually grind to a standstill and growth stopped from 1995 (total catch fluctuating between 90 and 95 Mt/year according to the year). The growth of aquaculture, despite its benefits and the fact that it is the only way to meet the increase in demand for sea products, evaluated at 270Mt in 2050 (Chevassus au Louis et Lazard, 2009; Wijkstrom, 2003), raises a certain number of issues directly related to its sustainable development. Amongst these are issues related to feed for the farmed organisms, to their biological diversity, to the farms’ economic sustainability, to the impact of aquaculture development on social equity and to the set of arrangements constituting the sector’s governance. Feed, for example, is currently the subject of significant controversy as shown by the emblematic article of Naylor et al. (2000) that exposes the impact on catches of the massive use of fish meal and fish oil in fish and prawn aquaculture and advocates the return to sparser aquaculture systems, directly inspired by traditional Asian systems which use more extensive techniques based on polyculture and fertilisation and where artificial feed is only seen as a potential supplement. This diagnosis, although interesting as it generated much debate, was, however, incomplete and, in fact, inaccurate: by focusing on a single criterion and a single dimension (environmental) of sustainability, the authors were led to make proposals that had no chance of being adopted by the actors. De facto, farming systems have continued to intensify and this has led to a sustained increase in the use of