EDTA disodium salt and sodium formate exhibited a similar cumulative methane production to the control. EDTA disodium salt was expected to enhance methane production by increasing the bioavailability of essential elements (Vintiloiu et al., 2013); however, no additional methane production was observed, possibly because the concentration used in this Doripenem study was significantly higher than that found to be beneficial by Vintiloiu et al. (2013). The methane production of sodium formate was stable, but occurred at a slower rate compared to that of the control. This could be attributed to the high sodium concentration of 1.8 g Na/L, particularly when compared with sodium acetate which contained only 0.7 g Na/L. Additionally, the COD contribution of acetate (1.07 g COD/g) is much larger than formate (0.34 g COD/g) and would have promoted a faster and more consistent rate of methane production (Grobicki and Stuckey, 1989).
Independent of the draw solution and reverse solute flux, elevated salt concentrations would be expected due to the concentration of the natural salinity of wastewater during the FO process. In the case of inorganic draw solutions, further inhibition of methane production and inefficiencies in the anaerobic digestion process could be expected. For the organic draw solutions demonstrated to be beneficial for anaerobic treatment, the salinity of the pre-concentrated wastewater would not be significantly exacerbated by reverse solute flux. Furthermore, significantly higher COD concentrations would be achieved during wastewater pre-concentration as a result of the contribution of reverse solute flux, allowing the opportunity to operate at a lower concentration factor.