Abstract
Contamination of wastewater with organic-limited nitrates has become an urgent problem in wastewater treatment. The cooperating heterotrophic with sulfur autotrophic denitrification is an alternative process and the efficiency has been assessed in many studies treating simulated wastewater under different operating conditions. However, due to the complex and diverse nature of actual wastewater, more studies treating actual wastewater are still needed to evaluate the feasibility of collaborative denitrification. In this study, lab-scale experiments were performed with actual nitrate polluted water of two different concentrations, with glucose and sodium thiosulfate introduced as mixed electron donors in the coupling sulfur-based autotrophic and heterotrophic denitrification. Results showed that the optimum denitrification performance was exhibited when the influent substrate mass ratio of C/N/S was 1.3/1/1.9, with a maximum denitrification rate of 3.52 kg NO3−-N/(m3 day) and nitrate removal efficiency of 93% in the coupled systems. Illumina high-throughput sequencing analysis revealed that autotrophic, facultative, and heterotrophic bacteria jointly contributed to high nitrogen removal efficiency. The autotrophic denitrification maintained as the predominant process, while the second most prevalent denitrification process gradually changed from heterotrophic to facultative with the increase of influent concentration at optimum C/N/S ratio conditions. Furthermore, the initiation of dissimilatory nitrate reduction to ammonium (DNRA) was very pivotal in promoting the entire denitrification process. These results suggested that sulfur-based autotrophic coupled with heterotrophic denitrifying process is an alternative and promising method to treat nitrate containing wastewater.
Highlights
Several researchers have attempted the participation of various electron donors by cooperating heterotrophic with sulfur autotrophic denitrification in specific reactors [11,12], and found that collaborative denitrification achieved the acid-base balance of the reaction system [13], and improved the rate of denitrification and reduced the sludge yield [14] (Equations (1) and (2))
The theoretical amounts of glucose and sodium thiosulfate were added at a C/N/S ratio of 2.4/1/1.9, which were computed based on the stoichiometry of Equations (1) and (2), in case the two denitrification processes contributed
The co-existence and complementation of heterotrophic and autotrophic denitrification was successfully achieved with a shorter start-up time by introducing mixed electron donors, such as glucose and sodium thiosulfate, resulting in improved denitrification performance
Summary
The denitrification rate is higher during heterotrophic conditions, the associated problems, such as secondary pollution, high sludge production rates, and increased operating costs are difficult to control [5,6]. Sulfur-based autotrophic denitrification has been considered as an alternative process owing to its stability, non-toxicity, cost-effectiveness, and eco-friendly operation under normal conditions [7,8]. The main disadvantages of sulfur-based autotrophic denitrification, including sulfate and acid generation, have limited the largescale promotion of this process [9,10]. The composition of the actual wastewater is more complex and diverse than simulated wastewater, studies with actual wastewater will be more representative and better to evaluate the feasibility of coupling sulfur-based autotrophic and heterotrophic denitrification
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