Fig. 1

Effect of hydraulic retention time (HRT) on sulfate concentration during 125-day operation under the neutral (a) and acid (b) conditions. The HRT was changed from 25 to 12 h, 8 h, 6 h, and 5 h under the neutral condition, and to 20 h, 12 h, 8 h, and 7 h under the acid condition. Symbols influent, open circles; effluent, gray circles. Arrows indicate the sampling points for depth direction analysis in respective HRTs