Dissimilatory iron reduction and odor indicator abatement by biofilm communities in swine manure microcosms

Animal waste odors arising from products of anaerobic microbial metabolism create community relations problems for livestock producers. We investigated a novel approach to swine waste odor reduction: the addition of FeCl₃, a commonly used coagulant in municipal wastewater treatment, to stimulate degradation of odorous compounds by dissimilatory iron-reducing bacteria (DIRB). Two hypotheses were tested: (i) FeCl₃ is an effective source of redox-active ferric iron (Fe³⁺) for dissimilatory reduction by bacteria indigenous to swine manure, and (ii) dissimilatory iron reduction results in significant degradation of odorous compounds within 7 days. Our results demonstrated that Fe³⁺ from FeCl₃ was reduced biologically as well as chemically in laboratory microcosms prepared with prefiltered swine manure slurry and limestone gravel, which provided pH buffering and a substrate for microbial biofilm development. Addition of a 1-g liter^-1 equivalent concentration of Fe³⁺ from FeCl ₃, but not from presynthesized ferrihydrite, caused initial, rapid solids flocculation, chemical Fe³⁺ reduction, and E_h increase, followed by a 2-day lag period. Between 2 and 6 days of incubation, increases in Fe²⁺ concentrations were accompanied by significant reductions in concentrations of volatile fatty acids used as odor indicators. Increases in Fe²⁺ concentrations between 2 and 6 days did not occur in FeCl₃-treated microcosms that were sterilized by gamma irradiation or amended with NaN₃, a respiratory inhibitor. DNA sequences obtained from rRNA gene amplicons of bacterial communities in FeCl ₃-treated microcosms were closely related to Desulfitobacterium spp., which are known representatives of DIRB. Use of iron respiration to abate wastewater odors warrants further investigation.