This research introduces an alternative mixed culture fermentation technology for anaerobic digestion to recover valuable products from low grade biomass. In this mixed culture fermentation, organic waste streams are converted to caproate and caprylate as precursors for biodiesel or chemicals. It was found that acetate, as the main intermediate of anaerobic digestion, can be elongated to medium chain fatty acids with six and eight carbon atoms. Mixed microbial communities were able to produce 8.17 g l À1 caproate and 0.32 g l À1 caprylate under methanogenesis-suppressed conditions in a stable batch reactor run. The highest production rate was 25.6 mM C caproate per day with a product yield of 0.6 mol C per mol C. This elongation process occurred with both ethanol and hydrogen as electron donors, demonstrating the flexibility of the process. Microbial characterization revealed that the microbial populations were stable and dominated by relatives of Clostridium kluyveri.

SUMMARYThe world needs sustainable, efficient, and renewable energy production. We present the plant microbial fuel cell (plant-MFC), a concept that exploits a bioenergy source in situ. In the plant-MFC, plants and bacteria were present to convert solar energy into green electricity. The principal idea is that plants produce rhizodeposits, mostly in the form of carbohydrates, and the bacteria convert these rhizodeposits into electrical energy via the fuel cell. Here, we demonstrated the proof of principle using Reed mannagrass. We achieved a maximal electrical power production of 67 mW m À2 anode surface. This system was characterized by: (1) nondestructive, in situ harvesting of bioenergy; (2) potential implementation in wetlands and poor soils without competition to food or conventional bioenergy production, which makes it an additional bioenergy supply; (3) an estimated potential electricity production of 21 GJ ha À1 year À1 ð5800 kWh ha À1 year À1 Þ in Europe; and (4) carbon neutral and combustion emission-free operation.

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A biotechnological process is described to remove hydrogen sulfide (H(2)S) from high-pressure natural gas and sour gases produced in the petrochemical industry. The process operates at halo-alkaline conditions and combines an aerobic sulfide-oxidizing reactor with an anaerobic sulfate (SO(4) (2-)) and thiosulfate (S(2)O(3) (2-)) reducing reactor. The feasibility of biological H(2)S oxidation at pH around 10 and total sodium concentration of 2 mol L(-1) was studied in gas-lift bioreactors, using halo-alkaliphilic sulfur-oxidizing bacteria (HA-SOB). Reactor operation at different oxygen to sulfide (O(2):H(2)S) supply ratios resulted in a stable low redox potential that was directly related with the polysulfide (S(x) (2-)) and total sulfide concentration in the bioreactor. Selectivity for SO(4) (2-) formation decreased with increasing S(x) (2-) and total sulfide concentrations. At total sulfide concentrations above 0.25 mmol L(-1), selectivity for SO(4) (2-) formation approached zero and the end products of H(2)S oxidation were elemental sulfur (S(0)) and S(2)O(3) (2-). Maximum selectivity for S(0) formation (83.3+/-0.7%) during stable reactor operation was obtained at a molar O(2):H(2)S supply ratio of 0.65. Under these conditions, intermediary S(x) (2-) plays a major role in the process. Instead of dissolved sulfide (HS(-)), S(x) (2-) seemed to be the most important electron donor for HA-SOB under S(0) producing conditions. In addition, abiotic oxidation of S(x) (2-) was the main cause of undesirable formation of S(2)O(3) (2-). The observed biomass growth yield under SO(4) (2-) producing conditions was 0.86 g N mol(-1) H(2)S. When selectivity for SO(4) (2-) formation was below 5%, almost no biomass growth was observed.

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Anaerobic oxidation of methane (AOM) in marine sediments is coupled to sulfate reduction (SR). AOM is mediated by distinct groups of archaea, called anaerobic methanotrophs (ANME). ANME co-exist with sulfate-reducing bacteria, which are also involved in AOM coupled SR. The microorganisms involved in AOM coupled to SR are extremely difficult to grow in vitro. Here, a novel well-mixed submerged-membrane bioreactor system is used to grow and enrich the microorganisms mediating AOM coupled to SR. Four reactors were inoculated with sediment sampled in the Eckernförde Bay (Baltic Sea) and operated at a methane and sulfate loading rate of 4.8 L L(-1) day(-1) (196 mmol L(-1) day(-1)) and 3.0 mmol L(-1) day(-1). Two bioreactors were controlled at 15 degrees C and two at 30 degrees C, one reactor at 30 degrees C contained also anaerobic granular sludge. At 15 degrees C, the volumetric AOM and SR rates doubled approximately every 3.8 months. After 884 days, an enrichment culture was obtained with an AOM and SR rate of 1.0 mmol g(volatile suspended solids) (-1) day(-1) (286 micromol g(dry weight) (-1) day(-1)). No increase in AOM and SR was observed in the two bioreactors operated at 30 degrees C. The microbial community of one of the 15 degrees C reactors was analyzed. ANME-2a became the dominant archaea. This study showed that sulfate reduction with methane as electron donor is possible in well-mixed bioreactors and that the submerged-membrane bioreactor system is an excellent system to enrich slow-growing microorganisms, like methanotrophic archaea.

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