Performance and mechanism of free nitrous acid on the solubilization of waste activated sludge

Insights into the multi-targeted effects of free nitrous acid on the microalgae Chlorella sorokiniana in wastewater

Microalgal-bacterial consortium process (MBCP) proposed as an alternative to the activated sludge process contains free nitrous acid (FNA). FNA antimicrobial influences on nitrifiers have been demonstrated. However, its influence on microalgae is largely unknown, limiting the system stability of MBCP. This study revealed the multi-targeted responses of a model wastewater microalgae, Chlorella sorokiniana, to FNA exposure through physiological and transcriptomic analyses. Results showed a concentration-dependent FNA-influence as both microalgal growth and photosynthesis (Fv/Fm, rETR, Y(II), NPQ) inversely correlated with FNA doses. Increased ROS, MDA content (5.0-fold), SOD (2.7-fold), and LDH (12.0-fold) activities in the treatments revealed FNA-induced oxidative pressure. Moreover, RNA-sequencing results revealed significantly downregulated genes related to photosynthesis, respiration, nitrogen metabolism, and tricarboxylic acid cycle. Comparatively, peroxisome, chlorophyll, and carotenoid genes were upregulated. These findings elucidate the inhibitory mechanisms of FNA on microalgae and contribute towards the prospective practical application of the MBCP system for sustainable wastewater treatment.

Robust Nitritation of Anaerobic Digester Centrate Using Dual Stressors and Timed Alkali Additions

Nitrogen is commonly removed from wastewater by nitrification to nitrate followed by nitrate reduction to N₂. Shortcut N removal saves energy by limiting ammonia oxidation to nitrite, but nitrite accumulation can be unstable. We hypothesized that repeated short-term exposures of ammonia-oxidizing communities to free ammonia (FA) and free nitrous acid (FNA) would stabilize nitritation by selecting against nitrite-oxidizing bacteria (NOB). Accordingly, we evaluated ammonium oxidation of anaerobic digester centrate in two bench-scale sequencing batch reactors (SBRs), seeded with the same inoculum and operated identically but with differing pH-control strategies. A single stressor SBR (SS/SBR) using pH set-point control produced HNO₃, while a dual stressor SBR (DS/SBR) using timed alkalinity addition (TAA) produced HNO₂ (ammonium removal efficiency of 97 ± 2%; nitrite accumulation ratio of 98 ± 1%). The TAA protocol was developed during an adaptation period with continuous pH monitoring. After adaptation, automated TAA enabled stable nitritation without set-point control. In the SS/SBR, repeatedly exposing the community to FA (8-10 h/exposure, one exposure/cycle) selected for FA-tolerant ammonia-oxidizing bacteria (Nitrosomonas sp. NM107) and NOB (Nitrobacter sp.). In the DS/SBR, repeatedly exposing the community to FA (2-4 h/exposure, three exposures/cycle) and FNA (4-6 h/exposure, two exposures/cycle) selected for FA- and FNA-resistant AOB (Nitrosomonas IWT514) and against NOB, stabilizing nitritation.

Synergy of combined free nitrous acid and Fenton technology in enhancing anaerobic digestion of actual sewage waste activated sludge

In this study, actual swage waste activated sludge in batch reactors was employed to assess the synergistic effect of free nitrous acid and Fenton pre-treatments on enhancing methane production in the anaerobic digestion process. In addition to methane enhancement, the mechanisms driving the enhancement were also investigated via measuring enzymes activity and solubilisation of organic matter. This study revealed that the combined pre-treatments solubilised organic matter significantly more than the bioreactors pre-treated with individual FNA and Fenton. For understanding the influence of pre-treatments on solubilisation of organic matter, soluble protein, soluble polysaccharide and soluble chemical oxygen demand (SCOD) were measured before and after the treatments and it was shown that they respectively increased by 973%, 33% and 353% after the treatments. Protease and cellulase activity, as the key constituents of the microbial community in activated sludge, decreased considerably after the combined pre-treatments 42% and 32% respectively, which resulted in considerable methane enhancement. The results corroborate the synergy of the combined FNA and Fenton pre-treatment in degrading the organic and microbial constituents in waste activated sludge, paving the way for the big-scale implementation of these technologies.

Enhanced solubilization and biochemical methane potential of waste activated sludge by combined free nitrous acid and potassium ferrate pretreatment

The increasing production of waste activated sludge (WAS) from wastewater treatment plants presents an inherent environmental burden. In this study, Free nitrous acid combined with potassium ferrate (FNA + PF) pretreatment was used to enhance solubilization and biochemical methane potential of WAS. Results indicated that the maximum removal rates of total suspended solid by PF, FNA, and PF + FNA pretreatment were 21.84%, 38.09%, and 56.17%, respectively. The biochemical methane potential of WAS without pretreatment reached 61.22 L CH₄/kg VSS added while this value increased to 147.07 L CH₄/kg VSS added after FNA + PF pretreatment (0.06 g/g TSS NaNO₂ and 0.25 g/g TSS K₂FeO₄). Shotgun metagenomic analysis revealed that FNA + PF pretreatment could increase the diversity and stability of microbial communities by shifting methanogenic pathways from strictly acetoclastic to acetoclastic/hydrogenotrophic, thereby enhancing methane production. This study suggested that FNA + PF pretreatment is a promising technology to reduce WAS and enhance methane production by pretreated WAS during anaerobic digestion.