A comparative study of sulfidogenic and methanogenic activities during the treatment of landfill leachate: Part I

A comprehensive anaerobic leachate treatment in batch reactors was conducted for 9 months. Activities of two main bacterial groups, sulfidogens and methanogens were observed with monitoring parameters as chemical oxygen demand (COD), total gas production, pH, alkalinity, dissolved sulfide and volatile suspended solids/total suspended solids (VSS/TSS). Leachate contained high concentrations of volatile fatty acids. Some competition between microbial species was observed. COD was applied at a range of 1100-8200 mg/L at a slowly increasing rate over time. COD removal was mostly above 80%, dissolved sulfide was produced at a range of 100-450 mg/L in the first weeks, proving high sulfidogenic activity and making 40-50% of COD removal. Methanogenesis seemed under stress and improved after several weeks with increasing COD load whereas hydrogen sulfide (HS) production leveled off around 100 mg/L, making 4-8% COD removal by sulfidogens. Then, 40-70% of overall COD removal was achieved by methanogens. Volatile fatty acids (VFA) except acetic acid (propionic, isobutyric, butyric, isovaleric, valeric, caproic and heptanoic acids) were readily degraded and found near detection limit on gas chromatographic runs, accordingly VFA-oxidizing bacteria were assumed to be highly active and tolerant to sulfide levels encountered in our study, also with hydrogenotrophic groups providing favorable conditions for the process. pH was the key parameter to determine the degree of inhibition from sulfurous species. Alkalinity was produced proportionally with COD applied as a result of VFA degradation. Overall system performance was good and optimum pH was 7.8-8.2 at which inhibition due to unionized VFA was eliminated.