Characterization of biomass from a sulfidogenic, volatile fatty acid-degrading granular sludge reactor

Abstract

This paper describes the properties of sulfidogenic aggregates developed in a granular sludge bed reactor operating under different upward liquid velocities (1–6 m h −1), influent composition (acetate, propionate, and butyrate ratios of 1:1:1, 5:3:2, or 1:2:2) and pH (8 and 7). The limiting factor for the treatment performance was the acetate removal rate with maximum sulfidogenic degradation rates of 0.38 g C 2-COD g −1 VSS d −1. Although the reactor operated with an excess of sulfate ( COD SO 4 2− ratio 0.5), methanogenic bacteria still contributed partially to the COD removal. Sludge activity tests were used to quantify the partitioning of methanogenic and sulfidogenic COD utilization. The use of high superficial upward liquid velocities (4 and 6 m h −1) in the reactor increased the methanogenic contribution to the total activity to 40% and 70%, respectively, during the degradation of a volatile fatty acid mixture and with acetate as the sole substrate. An increase in the reactor free hydrogen sulfide concentration from 100 to 240 mg l −1 decreased the methanogenic contribution to less than 10% and 20% for the conversion of a volatile fatty acid mixture and acetate, respectively. 13C-NMR spectroscopy showed that propionate conversion proceeded via the randomizing pathway. In the absence of sulfate, only acetate was converted by methanogenic activity. The poor propionate and butyrate removal in the absence of sulfate suggests a lack of hydrogenotrophic methanogenic activity in the sulfidogenic sludge. Granulation occurred under sulfidogenic conditions as demonstrated by the continuous increase in the average granular size. The obtained sulfidogenic granules had a low granular strength (13–25 kN m −2), high settling velocities (14–37 m h −1 for 95% of the biomass), and a density of 1,022 kg m −3.