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Ammonium chloride (NHCl) was usually used as a model ammonia source to simulate ammonia inhibition during anaerobic digestion (AD) of nitrogen-rich feedstocks. However, ammonia in AD originates mainly from degradation of proteins, urea and nucleic acids, which is distinct from NHCl. Thus, in this study, the inhibitory effect of a "natural" ammonia source (urea) and NHCl, on four pure methanogenic strains (aceticlastic: Methanosarcina thermophila, Methanosarcina barkeri; hydrogenotrophic: Methanoculleus bourgensis, Methanoculleus thermophilus), was assessed under mesophilic (37 °C) and thermophilic (55 °C) conditions. The results showed that urea hydrolysis increased pH significantly to unsuitable levels for methanogenic growth, while NHCl had a negligible effect on pH. After adjusting initial pH to 7 and 8, urea was significantly stronger inhibitor with longer lag phases to methanogenesis compared to NHCl. Overall, urea seems to be more toxic on both aceticlastic and hydrogenotrophic methanogens compared to NHCl under the same total and free ammonia levels.

The increasing worldwide population and rapid urbanization have led to huge amount of fossil fuels consumption and waste generation. The awareness of living in a sustainable society is pushing people to target a low-carbon energy structure. Hydrogen, a carbonfree energy source, draws more and more attention. Particularly, biohydrogen from organic waste calls great interest by generating hydrogen and disposing waste simultaneously. Therefore, the three main technologies converting waste to biohydrogen: biological fermentation, thermochemical gasification and microbial electrolysis cell, were reviewed in this study from both technological and environmental perspective. The results showed that a variety of waste streams have been tested to produce hydrogen and different production efficiency were reported. The most favourable waste material for fermentation and microbial electrolysis cell were different types of wastewater, and agricultural lignocellulosic waste was also intensively studied in fermentation. Whereas wooden waste and municipal solid waste were the two wastes investigated the most in gasification.Optimization of the operational parameters was proved to improve the hydrogen production. However, researches focusing on scale-up of these technologies are still needed. On the other hand, life cycle assessment demonstrated that waste gasification had a better environmental profile compared to other technologies. However, the majority of the reviewed life cycle assessment studies failed to further explain the robustness due to the lack of sensitivity and uncertainty analysis, indicating high quality life cycle assessment studies are needed in the future.

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