Magnetite as an enhancer in methanogenic degradation of volatile fatty acids under ammonia-stressed condition

Anaerobic batch tests with a 2(2) full-factorial design of ammonia (1.5, 6.5 g N/L) and magnetite concentrations (0, 20 mmol/L) were conducted separately for methanogenic degradation of acetate, propionate, and butyrate (volatile fatty acids (VFAs)) to 1) quantify the effect of magnetite as an enhancer in methanogenic degradation of each of the VFAs in conditions without ammonia stress (1.5 g N/L) and with ammonia stress (6.5 g N/L), and 2) identify methanogenic consortia that are related to such enhancement. Among the three VFAs, methanogenic degradation of propionate was the least feasible (57% lower specific methanogenic activity Ram and three times longer lag time lambda than acetate degradation). At low ammonia concentration, only propionate showed improvement in R-CH4 (46%) with supplementation of magnetite. In the ammonia-stressed condition without magnetite, R-CH4 decreased by 38-58% and lambda increased 2.2-8.8 times for all VFAs; magnetite supplementation significantly alleviated these effects. These results demonstrate that magnetite supplementation effectively increases methanogenic degradation of the VFAs even under ammonia-stressed conditions. 16S metagenomic sequencing revealed that distinctive methanogenic consortia were active in the different combinations of substrate, ammonia and magnetite. Alkaliphilus, Hyphomonadaceae SWB02 and Clostridia DTU014, Clostridia D8A-2, ChristenseneUaceae R-7 group and Rikenellaceae DMER64 were identified as potential syntrophic bacteria that can establish magnetite-mediated direct electron transfer with methanogens (Methanosaeta candid, Methanosaeta harundinacea, Methanolinea tarda, Methartoculleus bourgensis and Methanosarcina spp.) during methanogenic degradation of VFAs. The results may be useful as a reference to develop effective strategies using magnetite supplementation to remediate anaerobic digestion processes that have been afflicted by VFA accumulation and ammonia inhibition.