Monitoring toxic impact of free nitrous acid for hydrogen sulfide reduction applications - Case Study
SENTRY technology was deployed as a real-time sensor to control the dosing of free nitrous acid (FNA) in a domestic wastewater treatment stream. The sensor response was evaluated as a solution for optimized dosing of the toxic cleaning agent. Key findings demonstrated:
- Free nitrous acid dosing caused a major (92% drop in MET), rapid and repeatable response on the SENTRY sensors.
- Output from SENTRY sensors recovered after dosing events.
- This SENTRY bio-electrode information could be tied to understanding when and how often to dose the FNA for optimized hydrogen sulfide inhibition.
There are a number of applications in which free nitrous acid (FNA), the protonated form of nitrite (HN02), is used as an antimicrobial agent due to its bacteriostatic and bactericidal effects on a range of microbes. FNA has been shown to be the factor in inhibiting denitrifying and phosphorus uptake processes. The impact of FNA on the nitrite oxidizing bacteria has been characterized and demonstrated more than 700 days of inhibition during SBR operation (Svehla et al., 2014). Besides, nitrous acid acts as a mutagen by deamination of the NH2 group of adenine and/or cytosine to an ether group, thus altering their base pairing. FNA has been thus used to induce mutagenesis and this is lethal to cells without repair / correction
In relation to the inhibition of hydrogen sulfide generation FNA has been demonstrated as a suitable inhibitory compound at bench-scale and more recently in commercial demonstration applications. For bench-scale studies FNA levels succeeded in suppressing sulfide production and methane production. After stopping FNA dosage, SRB recovered to about 70-80% in 2 months. Key conclusions of this study indicated FNA as low as 0.18 mg-N/L could suppress both sulfide and methane production after 24-hour of exposure. The suppression is followed by a slow recovery after stopping the FNA addition. FNA is toxic to the sewer biofilm cells and the biocidal effect is enormous under the experimental conditions (Guangming Jiang et al., 2010).
Commercial FNA applications for the inhibition of hydrogen sulfide production are now available through the technology developed by the University of Queensland (Australia-USP technologies). Case Studies have been completed using FNA to inhibit H2S production in a sewer system in Florida.