Biotecnología del consorcio microbiano formado por bacterias sulfatorreductoras y bacterias oxidadoras de sulfhídrico y reductoras de nitrato

Resumen

The production, accumulation and emission into the atmosphere of hydrogen sulfide in wastewater environments cause corrosion, toxicity and pollution problems that result in increased costs for the water company and health problems for workers. Nitrate treatment is one of the most accepted methods to reduce sulfide concentration in the aqueous and atmospheric phases in sewage systems and other industries where high concentrations of sulfide are reached, like the oil refining industry. However, a consensus has not been yet reached on the mechanisms through which nitrate acts on the microbial community. For this study firstly a pilot scale bioreactor system was designed to be placed at the Wastewater Treatment Plant (WWTP) Guadalete in Jerez de la Frontera. The bioreactor system had an automatic detection and action program to solve pump¿s obstructions and stops of the water flow. After verifying the correct operation of the system, one bioreactor was used as a control and different nitrate concentrations were assayed during 72 hours on the other bioreactor to test the effect on sulfide concentration in both the aqueous phase and the atmosphere. Nitrate caused a rapid effect. A complete cease of sulfide emission to the atmosphere was achieved, but sulfide did not disappear entirely from the aqueous phase at the assayed concentrations. Samples of biofilm from control and treated bioreactors were used to analyze the effect on the microbial community. The great complexity of microbial communities hinders their analysis and it is difficult to detect changes in response to different treatments. The less abundant bacterial groups who may respond to such treatments, like the bacteria involved in the processes of interest for this study, as sulfate reduction and sulfide oxidation, are especially hard to detect. The speed of nitrate response suggests that different groups of nitrate-reducing sulfide-oxidizing bacteria (NR-SOB) already present in the biofilm are activated. This is further supported by the equally fast reversibility of the process to pre-treatment levels of production and emission of sulfur after cessation of the treatment. The activity of some sulfate oxidizing bacterial groups (SRB) decreased but other SRB groups were favored, probably those with increased tolerance to nitrite, so that the sulfate reduction rate was unaltered. To learn more about both the typical microbial composition of the WWTP and the changes in the microbial community associated with nitrate dosage, water and biofilm samples were taken at three time points: pretreatment, after 72 h of treatment with nitrate and 24 h after cessation of treatment. These samples were analyzed using next generation sequencing (pyrosequencing). It was observed that the microbial groups of interest, SRB and NR-SOB are mostly located in biofilms. It was also found that the community of biofilms is more stable than the community of the aqueous phase and the nitrate treatment mainly produces a change in the composition of rare bacterial groups. The spatial location of the processes within the biofilm was studied at different concentrations of sulfate and nitrate with oxygen, sulfur and pH microelectrodes that allowed high resolution vertical microprofiles. Applying biogeochemical modeling techniques to the profiles, sulfate reduction and sulfur oxidation rates within the biofilm were calculated and the system kinetics was analyzed. Sulfide oxidation in presence of nitrate occurs in the upper layer of the biofilm and its extension is determined by the penetration of nitrate from the bulk water into the biofilm. Below this layer there is another where sulfate reduction rate was very high, probably because of the recycling of sulfate within the biofilm by the activity of NR-SOB. The rates of sulfate reduction and sulfide oxidation depend on sulfate and nitrate concentrations in the aqueous phase according to Michaelis-Menten-like saturation kinetics. The values of the half-saturation constant for sulfate and nitrate and the maximum rates are clearly conditioned by the diffusion of substrates within the biofilm. In summary, knowledge of the kinetics of the inhibition process for sulfide net production caused by nitrate and the main microorganisms involved in the same WWTP will allow a suitable system design, which will be effective in sulfide removal from water and avoiding its emission to the atmosphere at the lowest possible cost and without affecting the water treatment process.