It is crucial to have a review on the role of iron in water treatment for the guidance towards the selection of appropriate processes, content of iron, and application conditions, as there are few reviews available at present and the systematic information is lacking for both researchers and engineers. The objectives of this review are to summarize the state of arts with respect to iron applied in nitrogen removal, discuss chemical and biological or bio-chemical combined nitrogen removal pathways and processes coupled with iron, and to reveal reaction mechanisms as well as providing references or even solutions to pertinent the practical engineering application of nitrate removal coupling with iron. The following information have been summarized and discussed in details: (1) iron based materials with varieties of preparations and forms, (2) major coupling ways of nitrogen removal methods or processes with iron application, (3) chemical reaction equations about a variety of chemical and biological or bio-chemical combined processes and the main mechanisms. In addition, challenges and/or drawbacks during the nitrogen removal processes will also be discussed in this paper, which is aimed to seek better practical engineering applications of nitrate removal coupling with iron.

Granular activated carbon (GAC) could promote methane production from organic wastes, but a wide range of dosages has been reported. In present study, different GAC dosages of 0, 0.5, 5 and 25 g/L were supplemented into anaerobic digesters and the methanogenic degradation kinetics of acetate, propionate and butyrate were characterized, respectively. At high organic load of 5 g/L, the degradation rates of propionate and butyrate increased by 1.5-4.7 and 2.5-7.0 times at varied GAC dosages. The methane production rates (Rmax) from propionate and butyrate were significantly elevated when increasing GAC dosage up to 5 g/L. However, only a minor increment was found for acetate degradation either at 1 g/L or 5 g/L. The stimulatory mechanism of GAC for accelerated syntrophic degradation of propionate and butyrate can be primarily attributed to the triggering effect on acetogenesis, as evidenced by the enrichment of syntrophic bacteria e.g. Thermovirga, Synergistaceae, and Syntrophomonas etc.