Enhancement of Anaerobic Digestibility of Thickened Waste-Activated Sludge Using Combined Free Nitrous Acid Pre-Treatment (Combined FNA-Heat, and FNA-H2O2)

<p>The application of sludge pretreatment strategies to improve solids handling in wastewater treatment plants has gained increasing attention. Free nitrous acid (FNA) sludge pretreatment is a novel approach, as, unlike other chemicals, FNA is a waste-derived, and renewable chemical that can be produced directly from wastewater treatment plants as a by-product, and therefore, FNA-based pretreatment can be determined as an environmentally friendly and economically feasible strategy. The use of FNA as a chemical pretreatment for anaerobic digestion (AD) has indicated the potential to increase the hydrolysis rate by releasing the internal organic matter of thickened waste-activated sludge (TWAS) via its biocidal effects. Furthermore, the effects of combined FNA-based pretreatment on volatile fatty acids production through two-stage AD are unknown. Therefore, this study aimed to investigate the effect of combined FNA-based pretreatment on single- and two-stage AD of TWAS. In the first phase of the study, the influence of different FNA dosage, temperature, and pretreatment time combinations on the anaerobic degradation of TWAS was evaluated through biochemical methane potential tests, and in batch mode. Higher temperature, longer reaction time, and moderate FNA concentration (1.4 mg/L) were favorable for increased solubilization of organic matter and methane production from TWAS. The optimum operating conditions for maximum methane yield were determined to be 60°C, 21 hr, and 1.4 FNA mg HNO2-N /L. The effect of combined FNA-Heat and FNA-H₂O₂ was also studied on single- and two-stage anaerobic digestion of TWAS. Application of combined FNA-Heat and FNA-H₂O₂ pretreatment resulted in up to ~32-43 % total suspended solids reduction, respectively which was more than that of the sole acidogenic fermentation process (~16%). Among all tested scenarios, the implementation of the two-stage AD fed with FNA- H₂O₂ pretreated sludge (1.4 mg HNO₂-N/L and 50 mg H₂O₂ /g TS) revealed the highest ultimate biomethane recovery of 224 mL/g TCOD_added, which was 41% higher than that of the single-stage AD of raw TWAS (159 mL/g TCOD_added). Given the findings of this research, the two-stage AD following combined FNA-based pretreatment can be suggested as an effective strategy for improved bioconversion of TWAS.</p>