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The addition of zero-valent iron (ZVI) and ZVI & persulfate (PS) were efficient approaches to enhance the production of volatile fatty acids (VFAs), especially butyric acid, from food wastes (FW) during anaerobic fermentation. The maximal concentration of VFAs was increased from 1256 mg COD/L in the control reactor to 8245 mg COD/L with ZVI addition, and it was further improved to 9800 mg COD/L with the PS/ZVI treatment. An investigation of the mechanisms revealed that both the ZVI and PS/ZVI treatments improved the bioavailable substrates in FW and enhanced the bioconversion efficiency of fermentation substrates, especially proteins and lipids. The provision of a sufficient amount of bioavailable substrates was advantageous to the enrichment of the functional bacteria that are responsible for the production of VFAs. Additionally, the microbial activity and key metabolic enzymes involved in the biological VFAs generation processes were stimulated in the ZVI and PS/ZVI-added reactors, which jointly contributed to high-rate VFAs yields.

As cost-effective and high-efficient oxidants, the hypochlorite chemicals have been widely utilized for bleaching and disinfection. However, its potential applications in wastewater treatment and sludge disposal were less concerned. This paper mainly summarized the state-of-the-art applications of hypochlorite technology in wastewater and sludge treatment based on the main influencing factors and potential mechanisms of hypochlorite treatment. The results indicated that the hypochlorite approaches were not only effective in pollutants removal and membrane fouling mitigation for wastewater treatment, but also contributed to sludge dewatering and resource recovery for sludge disposal. The ClO- and large generated free active radicals (i.e., reactive chlorine species and reactive oxygen species), which possessed strong oxidative ability, were the primary contributors to the pollutants decomposition, and colloids/microbes flocs disintegration during the hypochlorite treatment process. The performance of hypochlorite treatment was highly associated with various factors (i.e., pH, temperature, hypochlorite types and dosage). In combination with the reasonable activators (i.e., Fe2+ and ultraviolet), auxiliary agents, and innovative processes (i.e., hydrothermal and electro-oxidation), the operational performance of hypochlorite technology could be further enhanced. Finally, the feasibility and benefits of hypochlorite application for wastewater and sludge treatment were analyzed, and the existing challenges and future research efforts that need to be made have also prospected. The review can hopefully provide a theoretical basis and technical guidance to extend the application of hypochlorite technology for wastewater treatment and sludge disposal on large scale.

The effective removal of refractory antidepressant in wastewater is challenging. In this study, a novel strategy of cysteine-assisted Fe2+/persulfate system (Fe2+/Cys/PS) was applied for the venlafaxine (Ven, as a typical antidepressant) degradation. The obtained results revealed that the Ven removal was evidently accelerated and enhanced in Fe2+/Cys/PS process, and achieved complete degradation in 5 min with optimal dosage. Further analysis indicated that the Ven degradation efficiency was associated with the chemical concentrations (i.e. Fe2+, Cys and PS) and operational conditions (i.e. pH and temperature). Moreover, the reactions were not impacted by the co-occurring organic matters (i.e. fulvic acid) and inorganic ions (i.e. Cl-) potentially existing in real wastewater matrices. Mechanistic explorations demonstrated that the presence of Cys promoted the Fe3+/Fe2+ redox cycle, and thus enhanced the reactive oxygen species yields (ROS). The OH was considered as the primary ROS in Fe2+/Cys/PS process for Ven degradation via the radical scavenger verification. Also, the main intermediates of Ven degradation were identified, and the possible transformation pathway was proposed, in which the hydroxylation attacked by the OH was the main reaction. Moreover, the active reaction sites in Ven were calculated with the density function theory (DFT), which was consistent with the observed metabolic routes.

Model simulation is an effective approach to optimize the operational performance of wastewater treatment plants (WWTPs). This study presents a novel strategy to enhance the total nitrogen (TN) removal in WWTPs by GPS-X integrated with response surface methodology. The sensitivities of 61 parameters were screened and analyzed, and 6 critical parameters (i.e., μmax A, KA/a, μmax H, KH/ss, YH and μmaxPAO) were selected for further adjustment. The accuracy of GPS-X for WWTPs modeling was validated by static and dynamic simulations with actual operational data. The results showed that the DO concentration diffused in different biological compartments exhibited significant effects on the denitrification rate. The TN removal is also associated with SRT. The significance and optimization orders of key parameters were analyzed. With the optimization of DO in biological units and SRT, the nitrification and denitrification rates were improved to 97.1 and 85.3% respectively, saving 17.9% energy consumption.

The treatments of peroxydisulfate (PDS) activated with different iron forms (zero-valent iron (ZVI), ferrous iron (Fe2+) and nano zero-valent iron (NZVI)) all contributed to the generation of volatile fatty acids (VFAs) during waste activated sludge (WAS) anaerobic fermentation. The maximal VFAs generation was 3036, 5537 and 3533 mg COD/L in the PDS/ZVI, PDS/Fe2+ and PDS/NZVI reactors, respectively, while it was only 702 mg COD/L in the control. The enhancing effects followed the order of PDS/Fe2+ > PDS/NZVI > PDS/ZVI. ZVI and NZVI showed no dual promoting effects with PDS on the VFAs production. Mechanisms exploration indicated that the simultaneous improvement of WAS solubilization and hydrolysis (high concentrations of soluble proteins and carbohydrates) and enrichment of fermentative bacteria (i.e. Bacteroides, Clostridium, Fonticella, and etc.) involved in VFAs generation were the main causes of VFAs promotion in the PDS treated systems. However, the reductive ZVI and NZVI partially consumed the generated free radicals (i.e. SO4− and/or OH), which possess strong oxidative potentials and are the main contributors to extracellular polymeric substances disintegration in WAS. This consumption of free radicals accounted for the lower efficiency of solubilization and hydrolysis and consequently reduced VFAs production in the PDS/NZVI and PDS/ZVI reactors compared with that in PDS/Fe2+ reactor. Moreover, the treatment of PDS activated by different forms of iron improved the VSS reduction extent and dewaterability of fermented sludge compared with that of the control, which is advantageous to the ultimate disposal of WAS.