Comparison of chromium III and VI toxicities in water using sulfur-oxidizing bacterial bioassays.

The toxicities of Cr (III) and Cr (VI) in water were evaluated using sulfur-oxidizing bacterial (SOB) bioassays both in batch and fed-batch conditions. Two days were enough for a quick buildup of SOB consortium in the master culture reactor (MCR). At concentrations up to 100 mg L(-1), Cr (III) was found to be nontoxic in both conditions, while Cr (VI) at very low concentrations (0.1-2 mg L(-1)) was very toxic to the SOB. Literature review suggested that the nontoxic nature of Cr (III) might be due to the absence of the iron uptake pathway in Acidithiobacillus caldus (the predominant bacteria in our reactors), which is required for Cr (III) uptake. The 2-h median effective concentration (EC50) values obtained for Cr (VI) in the batch and fed-batch tests were 2.7 mg L(-1) and 1.5 mg L(-1), respectively.

Assessment of chromium-contaminated groundwater using a thiosulfate-oxidizing bacteria (TOB) biosensor.

The effect of Cr(6+)-contaminated groundwater was assessed using thiosulfate-oxidizing bacteria (TOB). Electrical conductivity (EC), pH, and sulfate production were determined based on thiosulfate oxidation. Final pH values in the different test treatments of Cr(6+)-contaminated groundwater (50-1000 µg Cr(6+)L(-1)) ranged from 2.02 ± 0.09 to 7.76 ± 0.07 and EC ranged from 5.95 ± 0.03 to 3.63 ± 0.03 mS cm(-1). Inhibition of TOB due to Cr(6+) was between 16.7% and 100%, with higher levels of inhibition occurring at higher Cr(6+) concentrations. The median effective concentration (EC50) was 78.96 µg Cr(6+)L(-1). These data demonstrate that TOB can detect less than 100 µg L(-1) of Cr(6+) in the groundwater and can be used as an effective bioassay for toxicity assessment.

Nitrate-contaminated groundwater remediation by combined autotrophic and heterotrophic denitrification for sulfate and pH control: batch tests.

Groundwater remediation was evaluated for combined autotrophic and heterotrophic denitrification under high (154 mg/L as CaCO3) and low (95 mg/L as CaCO3) alkaline conditions. Two levels of acetate (47 and 94 mg/L) and ethanol (24 and 48 mg/L) were added to the reactors. Obtained denitrification rates were 2.89, 2.58, 3.55, 1.96, and 2.0 mg-N/L · h for high alkaline conditions, whereas under low alkaline conditions has given 2.36, 1.94, 2.47, 2.74, and 2.29 mg-N/L · h for control, 47 and 94 mg/L acetate, and 24 and 48 mg/L ethanol, respectively. Nitrite was accumulated for controls but reactors with acetate and ethanol did not accumulate nitrite. Acetate and ethanol addition decreased sulfate to nitrate ratios in the range of 4.5-7.58 for high alkaline conditions (12.77 for control) and 4.43-6.78 for low alkaline conditions (7.90 for control). Acetate was more efficient compared with ethanol in controlling sulfate production and pH maintenance.

Effect of dissolved oxygen tension and agitation rates on sulfur-utilizing autotrophic denitrification: batch tests.

The effect of dissolved oxygen (DO) and agitation rate in open and closed reactors was examined for sulfur-utilizing autotrophic denitrification. The reaction rate constants were determined based on a half-order kinetic model. Declining denitrification rate constants obtained for open reactors those of 8.46, 8.03, and 2.18 for 50 mg NO(3) (-)-N/L, while 11.12, 9.14, and 0.12 mg(1/2)/L(1/2) h were for 100 mg NO(3) (-)-N/L at agitation speeds of 0, 100, and 200 rpm. In closed reactors, the ever-increasing denitrification rates were 10.13, 22.56, and 37.03, whereas for the same nitrate concentrations and speeds the rates were 13.17, 15.63, and 26.67 mg(1/2)/L(1/2) h. The rate constants correlated well (r ( 2 ) = 0.89-0.99) with a half-order kinetic model. In open reactors, high SO(4) (2-)/N ratios (8.02-75.10) while in closed reactors comparatively low SO(4) (2-)/N ratios (6.10-13.39) were obtained. Sulfur oxidation occurred continuously in the presence of DO, resulting in mixed cultures acclimated to sulfur and nitrate. SO(4) (2-) was produced as an end product, which reduced alkalinity and lowered pH over time. Furthermore, DO inhibited sulfur denitrification in open reactors, while agitation in closed reactors increased the rate of denitrification.

Influence of reactive media composition and chemical oxygen demand as methanol on autotrophic sulfur denitrification.

Sulfur-utilizing autotrophic denitrification relies on an inorganic carbon source to reduce the nitrate by producing sulfuric acid as an end product and can be used for the treatment of wastewaters containing high levels of nitrates. In this study, sulfur-denitrifying bacteria were used in anoxic batch tests with sulfur as the electron donor and nitrate as the electron acceptor. Various medium components were tested under different conditions. Sulfur denitrification can drop the medium pH by producing acid, thus stopping the process half way. To control this mechanism, a 2:1 ratio of sulfur to oyster shell powder was used. Oyster shell powder addition to a sulfurdenitrifying reactor completely removed the nitrate. Using 50, 100, and 200 g of sulfur particles, reaction rate constants of 5.33, 6.29, and 7.96 mg(1/2)/l(1/2)·h were obtained, respectively; and using 200 g of sulfur particles showed the highest nitrate removal rates. For different sulfur particle sizes ranging from small (0.85-2.0 mm), medium (2.0-4.0 mm), and large (4.0-4.75 mm), reaction rate constants of 31.56, 10.88, and 6.23 mg(1/2)/l(1/2)·h were calculated. The fastest nitrate removal rate was observed for the smallest particle size. Addition of chemical oxygen demand (COD), methanol as the external carbon source, with the autotrophic denitrification in sufficiently alkaline conditions, created a balance between heterotrophic denitrification (which raises the pH) and sulfur-utilizing autotrophic denitrification, which lowers the pH.