Recovery of anaerobic system treating sulfate-rich wastewater using zero-valent iron

The anaerobic digestion (AD) process often fails in treating wastewater containing high levels of sulfate because sulfate-reducing bacteria (SRB) can reduce sulfate to sulfide and compete with methane-producing archaea (MPA) for their common substrates. The sulfide produced from the sulfate reduction could inhibit both MPA and SRB. In this study, we investigated the application of zero-valent iron (ZVI) to recover the growth and activities of MPA and SRB in an AD process treating wastewater with a relatively low ratio of chemical oxygen demand (COD) to sulfate ratio (SO42-) (9,000 mg COD/L glucose and 4,500 mg/L SO42-). The one-time addition of 210 g ZVI increased the CH4 production rates from near zero to 0.54 ± 0.15 L/L·d. An electron flow analysis (in terms of COD) demonstrated that the CH4-COD was approximately eight times higher than the COD contributed by ZVI electrons. The results indicated that ZVI not only provided extra electrons for use in hydrogenotrophic methanogenesis, but also provided ferrous ions that could co-precipitate with sulfides and reduced the inhibition of acetoclastic methanogens from undissociated H2S. ZVI helped recover the acetoclastic activity of the biomass to approximately the same level as the activity of the inoculum before being inhibited by sulfides and improved the hydrogenotrophic activity by over 72 times. In addition, ZVI had different effects on the acetoclastic activities of the culture attached to ZVI and the suspended culture. The acetoclastic activity of the culture attached to ZVI (0.446 ± 0.003 g COD/g COD·d) was only approximately half of that of the suspended culture (1.056 ± 0.070 g COD/g COD·d).