Autotrophic denitrification using inorganic sulfur compounds as electron donor: process optimization and microbial characterization

Abstract

This research aimed at enhancing and optimizing autotrophic denitrification exploiting inorganic sulfur compounds as electron donor and proposing and investigating pyrite as novel substrate. In addition, the simultaneous removal of nitrogen in the form of NH4 + and NO3 - was investigated. In the first part of the PhD research, a screening test of thiosulfate (S2O3 2- ), sulfide (S2- ), elemental sulfur (S0 ) and pyrite (FeS2) as an electron donor was carried out with the principle objective to enrich autotrophic denitrifying bacteria. From the results obtained of this first experiment elemental sulfur and pyrite were chosen to continue with the investigation of autotrophic denitrification in fluidized bed reactors (FBR). Different operative conditions were tested, including nitrate concentration in the influent between 35 to 135 mg N-NO3 - /L, nitrate loading rate varying between 23 and 200 mg NNO3 - /L d and the hydraulic retention time between 4 and 48 h. The FBRs achieved maximum denitrification rates of 142.4 and 184.2 mg N-NO3 - /L d with pyrite and elemental sulfur, respectively. Moreover for the first time the active microbial community developed during pyrite autotrophic denitrification was analyzed and Azospira sp., Ferruginibacter sp., Rhodococcus sp. and Pseudomonas sp. were the predominant genera, while Thiobacillus sp. and Sulfurovum sp. dominated the active community in the sulfur FBR. During the second part of this research a side experiment was conducted that aimed to investigate pyrite-driven autotrophic denitrification performances in the presence of copper (Cu), arsenic (As) and nickel (Ni) as co-contaminant. These metal(loid)s were chosen since they are generally present as impurities in pyritic minerals and no systematic study was previously done on the possibility of denitrification inhibition when natural pyrite is applied. The feasibility of autotrophic denitrification in the presence of such metal(loid)s in concentrations between 2 and 7.5 ppm was established. Ni(II) and As(III) stimulated NO3 - reduction being between 3.3 and 1.6 times faster that of the no-metal control. On the other hand, the presence of Cu(II) negatively influenced the reaction with a kinetic constant decreased of 16, 40 and 28% in the 2, 5 and 7.5 ppm incubations, respectively, but the denitrification activity was never completely inhibited. In the third part of this PhD the simultaneous nitrification and autotrophic denitrification in FBRs with either S 0 or FeS2 was investigated. The response of the reactors was evaluated varying ammonium, nitrate and dissolved oxygen concentrations at HRT of 12 h. A maximum nitrate removal rate of 139.5 mg N/L d was achieved in the pyrite-FBR that resulted to be able to support the two processes of nitrification and autotrophic denitrification simultaneously. The key parameter that limited higher removal rate was, however, the DO concentration. 1.5 mg DO/L resulted to be the maximum applicable concentration in order to not have a nitrate concentration in the effluent above the legal limit of 11 mg NO3 - -N/L. On the other hand, in the elemental sulfur-FBR, the nitrification efficiency was low, while the denitrification was always taking place with a removal efficiency of 97.8% varying in the range of 80.6% and 100% during the trial. In the present research an alternative approach to conventional systems to treat nitrogen contaminated wastewaters (in the forms of ammonium and nitrate) is proposed. Further study and optimization of the system are needed in order to test the proposed configuration with real wastewater and determine the feasibility at pilot/real scale. A life cycle assessment to evaluate the costs and the environmental impacts is therefore required.