Impacts of Free Nitrous Acid on Problematic Bacteria During Food Waste Storage for Anaerobic Digestion

Biosolids generated in wastewater treatment facilities require further treatment, and improper management can greatly contribute to greenhouse gas emission and cause other environmental issues. When organic-rich biosolids can be converted to generate value-added products like biogas, feedstocks, and fertilizers through different technologies, such as anaerobic digestion, it also provides a favorite niche for problematic microorganisms, increasing the health risk, especially to facility operators and process workers. Pathogenic bacteria and odor caused by microbial activities of sulfate-reducing bacteria (SRB) can hinder the treatment of biosolids (and other organic wastes diverted from other sources) in resource recovery facilities due to operator health concerns and low-quality end products. Free nitrous acids (FNA) pre-treatments have been reported to enhance anaerobic digestibility of biosolids from wastewater. FNA's biocidal effects on microorganisms indicate its potential to control problematic microorganisms. In this work, the effectiveness of FNA control of pathogen and SRB during the pretreatment of biosolid was assessed by applying FNA (0-5.0 mg-FNA/L) to biosolids (75 g VS/L) for 72 hours. Pathogen numbers and hydrogen sulfide production was monitored. As shown in Fig. 1, the highest reduction in Total Coliform and E. coli number was found in the group with 5.0 mg-FNA/L. showing 99.7% and 97% reduction, respectively, in the first 4 hours, compared with the 68.8% and 55.7% in the control group without FNA addition. Similar result was observed for sulfide production. As shown in Fig. 2, a complete reduction of H2S was found in the FNA treatment group at 4 hours except 0.3 mg FNA L-1. However, longer treatment time (>24 hours) showed accumulation of H2S in the FNA treatment groups, which might be caused by decreasing effective FNA concentration due to consumption of nitrite by remaining active N-associated microorganisms (e.g., denitrifiers). Further experiments are in process to determine the optimal FNA pre-treatment conditions, including pH, nitrite dosing amount, and mixture of biosolids with other organic waste (i.e., post-consumer food waste), while the impacts of added FNA on downstream resource recovery processes (i.e., anaerobic digestion) will be also evaluated. Results from this work are expected to benefit the practitioners in the field of biosolids management for optimizing the design of FNA pre-treatment for simultaneously controlling problematic microorganisms and enhancing efficacy of resource recovery processes. Figure 1. Concentration (expressed as most probable number (MPN) in log scale) of total coliform (A) and E. coli (B) during 72 hours pre-treatment of biosolids with different FNA concentration (0, 0.3, 0.9, 1.52, 2.13, and 5 mg FNA L-1) Error bar represents the standard deviation of triplicate. Figure 2. Concentration of hydrogen sulfide (H2S) during 72 hours pre-treatment of biosolids with different FNA concentration (0, 0.3, 0.9, 1.52, 2.13, and 5 mg FNA L-1) Error bar represents the standard deviation of triplicate.