Effect of Fe(III) from Ferric Oxide, Ferric EDTA and Fly Ash on Anaerobic Digestion

Anaerobic systems which treat certain types of wastes sometimes experience a high concentration of volatile fatty acids (VFA) in the effluent. At high concentrations, VFAs are toxic to methanogens and other organisms in anaerobic systems, and they cause a decrease in system pH which can inhibit methanogenesis. Propionic acid is of interest and the possibility of reducing its presence with ferric iron was studied. Ferric iron was examined as an electron acceptor to bring about its reduction to ferrous iron while removing propionate. Ferric iron was supplied to anaerobic systems in the form of ferric oxide (Fe203) and ferric ethylenediaminetetraacetate (FeEDTA). To enhance iron reduction and propionate removal, systems of Fe(III)-reducing enrichment cultures were prepared and, along with a ferric iron compound, used to bioaugment anaerobic systems. These bioaugmentation systems were given an organic loading of2 gCOD/L-day, allowed to achieve quasi-steady state, then increased to 5 gCOD/L-day, allowed to achieve quasi-steady state and then reduced to 2 gCOD/L-day. The influent and effluent chemical oxygen demand (COD) and VFA concentrations were monitored and methane production and pH were recorded over 3 solids retention times (SRT) at each loading. The effect of bioaugmentation on propionate removal was investigated. The effect of ferric iron as a nutrient was also examined. Different doses of ferric oxide were fed to anaerobic systems which were started at an organic loading of 2 gCOD/Lday, allowed to achieve quasi-steady state, then increased to 5 gCOD/L-day, allowed to achieve quasi-steady state and then reduced to 2 gCOD/L-day. The effect of the different doses of ferric oxide on methane production, effluent COD and effluent propionic acid concentration was monitored. Fly ash from a coal-burning power plant was also investigated as nutrient to serve as a potential source of inexpensive iron. Four complete-mix stirred tank reactors were operated using anaerobic sludge from a laboratory digester as seed biomass and waste streams from a paper mill as substrate. Three of these were fed 0 mg/L-day, 110 mg/Lday and 370 mg/L fly ash doses, respectively. The last one received no fly ash but was fed standard laboratory micronutrients. Influent and effluent total and volatile solids concentration, influent and effluent COD and VFA concentrations, methane production, and pH were all measured for each reactor over 45 days to determine the effect of fly ash addition. The bioaugmented systems which received Fe20 3 had lower effluent concentrations of propionic acid, acetic acid and COD when compared with the systems which received Fe20 3 but were not bioaugmented and when compared with the systems which did not receive Fe20 3 and were not bioaugmented. Bioaugmentation with the enrichment culture and adding Fe203 appeared to be beneficial in lowering effluent VFA and COD concentrations. The addition of Fe203 as a nutrient did not have any effects on effluent propionic acid concentration but it decreased average effluent COD concentration at low organic loading. Methane production seemed to increase with increasing doses of Fe293. The addition of fly ash as a nutrient showed no significant effects on effluent COD and VFA concentrations but a stimulatory effect on rate of methane production was observed