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Global warming along with energy demand and rising prices of natural energy resources have motivated studies to find some renewable and clean energy. The use of algae as the third-generation biofuel can avoid the competition for farmland, and algae can be considered as a potential future source of renewable energy. Algae can be used for biogas production through anaerobic digestion (AD). Fucus vesiculosus and Fucus serratus are the two dominating species of brown seaweed growing in the Baltic Sea in the southwest of Sweden. Pretreatment can significantly affect the biogas production because hydrolysis of the algae cell wall structure is a rate-limiting step in the AD process. In this study, four different pretreatments: mechanical, microwave (600 W, 2 min), ultrasonic (110 V, 15 min), and microwave combined with ultrasonic (600 W, 2 min; 110 V, 15 min) were applied to the seaweed and then codigested with a biogas plant leachate. The aim of this study was to investigate methane yields from AD after these pretreatments. The results showed that when compared with only mechanical pretreatment, the ultrasonic, ultrasonic combined with microwave, and microwave pretreatments could obtain increased cumulative methane yields of 167, 185, and 156%, respectively. The combined pretreatment showed a maximum methane yield of 260 mL/g·of volatile solids after 20 days of digestion. The ultrasonic combined with microwave pretreatment showed a significant improvement in methane yield when compared with the mechanical pretreatment.

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The differentiations in nitrogen-converting activity and microbial community structure between granular size fractions in a continuous completely autotrophic nitrogen removal over nitrite (CANON) reactor, having a superior specific nitrogen removal rate of 0.24 g/(g VSS·d), were investigated by batch tests and high-throughput pyrosequencing analysis, respectively. Results revealed that a high dissolved oxygen concentration (>1.8 mg/l) could result in efficient nitrite accumulation with small granules (0.2-0.6 mm in diameter), because aerobic ammonium-oxidizing bacteria (genus ) predominated therein. Meanwhile, intermediate size granules (1.4-2.0 mm in diameter) showed the highest nitrogen removal activity of 40.4 mg/(g VSS·h) under sufficient oxygen supply, corresponding to the relative abundance ratio of aerobic to anaerobic ammonium-oxidizing bacteria (genus Kuenenia) of 5.7. Additionally, a dual substrate competition for oxygen and nitrite would be considered as the main mechanism for repression of nitrite-oxidizing bacteria, and the few spp. did not remarkably affect the overall performance of the reactor. Because all the granular size fractions could accomplish the CANON process independently under oxygen limiting conditions, maintaining a diversity of granular size would facilitate the stability of the suspended growth CANON system.

Effects of different C/N (NO2–N) ratios on nitrogen removal and microbial community structure were investigated using an anaerobic baffled reactor (ABR). Results indicated that the C/N ratio exerted an important effect on nitrogen removal in the anammox-coupling-denitrification process associated with the ABR. When the C/N ratio was 1.29, the ABR could achieve the highest total nitrogen (TN) removal efficiency of 99.9%. Most of TN was removed in the 1st and 2nd compartment, accounting for about 81.0–97.6% of total TN removal. The nitrogen removal resulted from the interaction among anammox, heterotrophic denitrificans, and other microbes within the ABR. The contribution of anammox to nitrogen removal varied from 6.8% to 32.4%. High-throughput MiSeq sequencing analyses revealed that the C/N ratio was one of the most important factors regulating the microbial community structure, and the predominant phylum changed from Proteobacteria to Chloroflexi with the elevated C/N ratio. In addition, the Candidatus Brocadia was the major anammox bacterium, and its percentage varied from 1.0–2.9% at day 9 to 2.8–9.1% at day 46.