Effects of pH and alkalinity on sulfur-denitrification in a biological granular filter

Abstract

Two laboratory-scale biological filters were operated to investigate the effects of alkalinity and pH on removal of nitrate and nitrite in sulfur denitrification filter processes. The concentration of sodium bicarbonate in the feed media was changed from 120 to 240 mg/l during about 3 months in a filter (Run A). The other filter was initially fed with 300 mg/l and then with 240 mg/l (Run B). The performance of the filter was monitored by measuring pH, nitrate, nitrite, sulfate, alkalinity, and thiosulfate. Nitrate concentration in effluent rapidly decreased to lower levels within several days for both filters after inoculation of enrichment culture of sulfur denitrifiers. However there was a large difference in removal of nitrite. When rapid removal of nitrate took place, nitrite accumulation was observed and remained while the bicarbonate concentration was 120 and 150 mg/l. On the other hand the nitrite accumulation disappeared when more bicarbonate (240 and 300 mg/l) was supplied. The experimental results indicated that the nitrite accumulation was closely related to pH condition and alkalinity level in the filter. The stable data of effluent water quality for 5 cases were collected and the relationship discussed between nitrite concentration and pH in effluents. The relationship indicated a strong pH dependency on nitrite accumulation below pH of 7.4. The pH condition around 7 is not so inhibitory to biological activity. Therefore, the pH within the biofilm would be low enough to suppress the nitrite reduction by sulfur denitrifiers, while the pH in effluent was not in the inhibitory range. It was recommended to keep the pH higher than 7.4 to prevent nitrite accumulation in the sulfur denitrification filter.

Origin, causes and effects of increased nitrite concentrations in aquatic environments

“…Similar results were obtained in case of nitrification by Glass and Silverstein (1998), who observed a significantly increased nitrite accumulation (250, 500 or 900 mg NO − 2 -N L −1 ) in sequencing batch reactors when mixed liquid pH was increased during nitrification (pH 7.5, 8.5 or 9.0 respectively). On the other hand, Furumai et al (1996) found an accumulation of nitrite at pH-values lower than 7.4 or at low alkalinity in a denitrifying granular filter.…”

Investigating the kinetics of autotrophic denitrification with thiosulfate: Modeling the denitritation mechanisms and the effect of the acclimation of SO-NR cultures to nitrite

“…3 Development of the kinetic model According to literature [20], nitrate reduction could be represented by a Monod-type equation while nitrite reduction can be described by Haldane-type kinetics. The accumulation of nitrite has also been reported as a kinetic affecting factor which could depend of the pH [21], the S/N ratio used [22][23], the competition for each electron acceptor (nitrite or nitrate) [24], etc. In this work none of these considerations were included in the kinetic model since in a previous study [16] it was observed that the factor affecting the progress of this process for a constant pH was the denitratation and denitritation kinetics, being this fact related with the biomass present in the culture.…”

“…4-7) as well as a Monod-type term to describe thiosulfate oxidation were considered. No elemental sulfur accumulation was included in the model since, as previously referenced [16,21,30], thiosulfate oxidation under anoxic conditions produced directly sulfate as the final product.…”

A review of nitrate reduction using inorganic materials

“…Several studies have investigated thiosulphate as an electron donor [73,74]. The average observed yield was 0.17 g VSS/g NO 3 -N [57].…”