Sulfur-based denitrification of organic deficient, acidic, low temperature and heavy metal contaminated waters in fluidized-bed reactors
Autotrophic denitrification driven by reduced sulfur compounds is a promising and cost-effective biological nitrogen removal process, being recommended for the treatment of organic-deficient water, e.g. groundwater and several industrial wastewaters. Autotrophic denitrifiers utilize inorganic compounds as sources of energy and carbon. The lack of organics eliminates the need of post-treatments to remove excess organic carbon and limits the formation of harmful organic byproducts (e.g. trihalomethanes, THM), resulting in a clean and safe treatment also for drinking water. Wastewaters from mining and metal-finishing industry commonly feature low pH and temperatures as well as high heavy metal concentrations. Nitrogen removal from these waters is a technical challenge, since denitrifying bacteria usually thrives at circumneutral pH and ambient temperatures (20-30°C).
The aim of this study was to develop a robust and efficient sulfur-based denitrification bioreactor process able to tolerate acidic pH, psychrophilic temperatures (< 20°C) and high nickel concentrations. The process was preliminary optimized in batch bioassays investigating the influence of sulfur source, i.e. thiosulfate (S2O32-) and biogenic and chemically synthesized elemental sulfur (S0), S0 particle size (powder and lentils), denitrifying culture (pure and mixed cultures of Thiobacillus) and temperature (6-30°C) on denitrification kinetics. The use of S2O32- and a pure culture of Thiobacillus denitrificans resulted in the highest denitrification rates. Biogenic S0 was tested for the first time as electron donor for autotrophic denitrification, showing 1.7-fold faster NO3- removal than that achieved with chemically synthesized S0 powder. The rates of thiosulfate-driven denitrification exponentially increased with temperature, being modeled according to the Arrhenius equation with an apparent activation energy Ea of 76.6 kJ/mol and a temperature coefficient Q10 of 3.0.
Fluidized-bed reactors (FBRs) were used to investigate continuous thiosulfate-driven denitrification under decreasing feed pH (5.25-7.00) and temperatures (3-20°C). Denitrification efficiency > 99% was observed at feed and effluent pH as low as 5.75 and 5.30, respectively. At lower feed pH values, the denitrification activity rapidly decreased due to an inorganic carbon deficiency. The addition of a carbonation unit providing CO2 as supplemental carbon source to the FBR biofilm allowed complete denitrification even at a pH of 4.75. In the same FBR, high-rate (up to 3.3 kg N-NO3-/m3 d) thiosulfate-driven denitrification was maintained at temperatures as low as 3°C. The impact of two Ni compounds, i.e. NiEDTA2- and NiCl2, commonly used in metallurgical industry, on sulfur-based denitrification was investigated in a parallel FBR at 20 (±2)°C and nickel concentrations in the range of 5-200 mg Ni/L. Preliminary batch bioassays were carried out to assess Ni and free EDTA toxicity on sulfur-based denitrification. In batch bioassays, 25-100 mg Ni/L of NiCl2 inhibited NO3- removal by 7-16%, whereas no inhibition was observed with NiEDTA2-. Free EDTA inhibited sulfur-based denitrification at concentrations exceeding 100 mg/L. Both Ni compounds showed no significant detrimental effects on sulfur-based denitrification in FBR at the tested concentrations. Nickel mass balance, solid-phase characterization and thermodynamic modeling revealed that nickel precipitates were mostly washed out with the effluent, due to the slow Ni precipitation kinetics and high upflow velocities in the FBR. Nickel phosphate, sulfide and oxide were indicated as the main nickel precipitates and were mostly amorphous.
FBRs were shown to be powerful and robust biofilm systems for nitrogen removal under acidic pH, psychrophilic temperatures and high nickel concentrations. The results of this study are of great interest for the treatment of NO3- contaminated ground and mining waters in cold regions (e.g. Canadian and Scandinavian regions) as well as acidic and heavy-metal-laden industrial wastewaters.