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Volatile fatty acids (VFA) are proposed platform molecules for the production of basic chemicals and polymers from organic waste streams. In this study we developed a granular sludge process to produce VFA at high rate, yield and purity while minimizing potential operational costs. A lab-scale anaerobic sequencing batch reactor (ASBR) was fed with 10 g l(-1) glucose as model substrate. Inclusion of a short (2 min) settling phase before effluent discharge enabled effective granulation and very high volumetric conversion rates of 150-300 gCOD l(-1)  d(-1) were observed during glucose conversion. The product spectrum remained similar at the tested pH range with acetate and butyrate as the main products, and a total VFA yield of 60-70% on chemical oxygen demand (COD) basis. The requirement for base addition for pH regulation could be reduced from 1.1 to 0.6 mol OH(-) (mol glucose)(-1) by lowering the pH from 5.5 to 4.5. Solids concentrations in the effluent were 0.6 ± 0.3 g l(-1) but could be reduced to 0.02 ± 0.01 g l(-1) by introduction of an additional settling period of 5 min. The efficient production of VFA at low pH with a virtually solid-free effluent increases the economic feasibility of waste-based chemicals and polymer production. Biotechnol.

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Liquid fuels have excellent properties in terms of storage, logistics and energy density compared to gaseous fuels or electricity. A major disadvantage of liquid fuels is that a vast majority of them is derived from fossil resources. Currently, the consumption rate of fossil fuels by far outcompetes the natural production rate, resulting in elevated atmospheric CO 2 concentrations. Photosynthetic organisms (plants and algae) xate atmospheric CO 2 using solar energy. CO 2 consumption and emission would be balanced if liquid fuels would be derived from plants or algae. However, growing terrestrial plants for biofuel production means less agricultural land and fresh water remains available for food production. Microalgae can grow under marine conditions and outcompete terrestrial plants in terms of areal productivity. On the other hand, cultivation of microalgae introduces new challenges. Species control is, compared to terrestrial plants, much more difficult. Any cultivation system is prone to contamination by undesired algal species threatening stable production. In this study we show that we can overcome this hurdle by creating a selective environment. Our approach allows for large scale, stable production of biofuel precursors and is therefore a substantial step forward in the production of renewable fuels.

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