Influence of COD/[SO_{4 2-}] ratio and Na+ concentration on organic matter and sulfate removal in UASB reactor

ABSTRACT

The anaerobic treatment of industrial wastewater presenting high concentrations of sulfate and sodium may be limited by several factors, including the ratio COD / [SO₄ ^2-], the concentration of SO₄ ^2- and the concentration of Na⁺. The competition between sulfate reducing bacteria and methanogenic archaea for substrate, the concentration of sulfide generated in sulfidogenesis, and the inhibition by cations are among the main phenomena that have been investigated. This paper presented and discussed the results of a UASB reactor with a volume of 10.5 L and flowrate of 16.0 L.day^-1, subjected to increasing COD/sulfate ratio whereas influent COD was kept constant. Glucose, acetate and methanol (≈2,000 COD mg.L^-1) were the carbon sources, and the COD/sulfate ratio ranged from 0.20 to 6.15 for sulfate concentrations of 0.3 to 10.0 g.L^-1, Na + concentrations of 0.70 g.L^-1 to 5.40 g.L^-1. The COD removal efficiency was maintained above 80%, and the sulfate removal was limited to 800 mg.L^-1 throughout the experimental period. The specific methanogenic activity (SMA) was 0.630 gCH₄-DQO.gSSV^-1 for 0.300 gSO₄ ^2-.L^-1 (COD/sulfate of 6.15), decreasing to 0.168 (70% decrease) in the last phase (COD /sulfate of 0.24) at concentrations of 10.000 gSO₄ ^2-.L^-1 and of Na⁺ 5.41±0.10 g.L^-1. The progressive increase of sulfate and sodium concentrations at constant influent COD resulted in methanogenesis inhibition by sodium and sulfide, even at concentrations bellow the inhibition limits, if individually considered. The effect of sodium in the granule structure was determinant for methanogenesis inhibition.

Keywords:
methanogenesis; sulfidogenesis; COD/sulfate; cation inhibition Na⁺