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Anaerobic digestion is widely used as an important source of renewable energy. With the increasing number and capacity of biogas plants also, adequate treatment technologies for whole digestate - the residue from anaerobic digestion - are gaining attention. In this study the state of the art of digestate processing is analysed, and currently used treatment schemes and the various technological processes involved are evaluated. The study combines data and experiences from existing large-scale digestate processing facilities in Austria, Germany, Switzerland and Italy, as well as know-how from technology providers and relevant research projects. However, the field of digestate processing is still quite new and little detailed information about the performance of different technologies at industrial scale is available. Digestate processing is gaining importance since digestate utilisation can become an important bottleneck when increasing biogas production. In addition, the production of renewable fertiliser from digestate is increasingly of interest to replace fossil fertilisers. This study is the first profound attempt to establish an assessment of the state-of-the-art technologies in use.

Cyanobacteria, as photoautotrophic organisms, provide the opportunity to convert CO2 to biomass with light as the sole energy source. Like many other prokaryotes, especially under nutrient deprivation, most cyanobacteria are able to produce polyhydroxyalkanoates (PHAs) as intracellular energy and carbon storage compounds. In contrast to heterotrophic PHA producers, photoautotrophic cyanobacteria do not consume sugars and, therefore, do not depend on agricultural crops, which makes them a green alternative production system. This review summarizes the recent advances in cyanobacterial PHA production. Furthermore, this study reports the working experience with different strains and cultivating conditions in a 200 L pilot plant. The tubular photobioreactor was built at the coal power plant in Dürnrohr, Austria in 2013 for direct utilization of flue gases. The main challenges were the selection of robust production strains, process optimization, and automation, as well as the CO2 availability.

In the last two decades, poly(hydroxyalkanoates) (PHA) were solely produced using heterotrophic bacteria in aerobic cultivation. With respect to the great potential (500 Mt yr -1 ) of raw industrial CO 2 streams and even greater potential of flue gases, the focus on photo-autotrophic biotechnological processes is increasing steadily. Primarily, PHA-gene transfer from heterotrophic bacteria into algae and plant cells was attempted, with the intention to combine the known biosynthesis pathway with autotrophic cultivation. The natural occurrence of PHA in cyanobacteria is known at least since 1966. However, cyanobacteria were never considered for commercial production because the PHA amount based on cell mass and based on volumetric productivity is generally very low. Therefore, strain improvements were suggested, either by gene amplification or by suppression of biochemical pathways competing for the cell's acetate pool. In the late 1990s, the success of genetic modification was confirmed experimentally, elevating the cyanobacteria cell's PHA content. With additional optimization, PHB amounts up to 50 % w/w of biomass dry matter or up to about 2.4 g L -1 bioreactor volume could be produced within 11 days. Considering the land use for agriculture and the competition for plant biomass between food, feed, fuel and energy production, the binding of CO 2 in a biotechnological process using photo-autotrophic microorganisms may become a promising option.

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