Effects Of Nanoscale Zero-valent Iron Particles On Biological Nitrogen And Phosphorus Removal And Microorganisms In Activated Sludge

The issue of eutrophication grows more severely in recent years, the sewage treatment industry in China is facing the problem of nitrogen and phosphorus removal from wastewater, economical and efficient removal of nitrogen and phosphorus has been a focus of wastewater treatment research. Because of the contradictions between nitrogen and phosphorus removal process, it is often difficult to achieve synchronization high efficient of nitrogen and phosphorus removal for conventional biological nitrogen and phosphorus removal process.Nanoscale zero-valent iron (NZVI) particles are widely applied in the remediation of wastewaters in recent years because of their large surface areas and high surface reactivity. In order to solve the problem of nitrogen and phosphorus removal from wastewater, this study investigated the simultaneous adsorption and chemical precipitation of inorganic phosphate anions from aqueous solutions by NZVI, the effect of NZVI particles on biological nitrogen and phosphorus removal was investigated, the effects of NZVI on microbial activity and microbial community structure of activated sludge were also investigated. The results were concluded as follow:1. Study on phosphate removal from aqueous solutions by NZVI. Batch experiments were conducted using different NZVI particle dosages, initial phosphate concentrations, pH values and dispersing agent conditions to investigate these factors’ influences on phosphate removal with NZVI. The mechanism of phosphate removal by the NZVI particles was also studied. The following conclusions were drawn based on the results. The phosphate removal efficiency increased with an increasing dosage of NZVI particles. A maximum phosphate removal of87.01%was obtained when using600mg/L of NZVI particles. The phosphate removal efficiency decreased from72.89%to51.39%as the initial PO43-concentration increased from10to90mg/L. Acidic conditions facilitated better phosphate removal. The maximum phosphate removal efficiency of99.41%was obtained at a pH of2. When NZVI was synthesized in a CMC-water solution, it removed phosphate at a rate of81.25%, which was better than the removal rate of NZVI synthesized in an ethanol-water solution (66.00%). NZVI particles showed a remarkably stronger phosphate removal ability than did microscale particles. The phosphate removal was achieved through simultaneous adsorption and chemical precipitation. An XRD analysis revealed that the phosphate was precipitated as crystalline ferrous phosphate with Fe2+2. Study on the effect of NZVI particles on biological nitrogen and phosphorus removal. During the exposure experiments, four test concentrations (0,20,50, and200mg/L) of NZVI particles were administered. The phosphate removal efficiencies were improved with the addition of NZVI. The phosphate removal efficiency increased with an increasing dosage of NZVI particles. A maximum phosphate removal of97.69%was obtained when using400mg/L of NZVI particles. The addition of lower concentration of NZVI particles (20and50mg/L) showed no measurable effect on nitrogen removal, however, when the NZVI concentration was200mg/L, the NH4-N removal rate was inhibited.3. Study on the effect of NZVI particles on microbial activity in activated sludge. The cytotoxicity of NZVI to activated sludge was characterized based on the generation of intracellular reactive oxygen species (ROS) and release of lactate dehydrogenase (LDH). The ATP content and dehydrogenase activity were used as indicators to evaluate the effects of NZVI on microbial activity and biomass in activated sludge. The microbial activities were inhibited upon exposure to NZVI according to the ATP and ROS results. LDH release assay suggested that lower concentrations of NZVI (20and50mg/L) boosted the dehydrogenase activity.4. Study on the effect of NZVI particles on microbial community structure of activated sludge. High throughput16S rRNA gene pyrosequencing was used to sequence the16SrDNA of microorganisms in activated sludge deeply, evaluating the effect of NZVI particles on microbial community structure of activated sludge and determining the sensitive bacteria with the increasing concentrations of NZVI. The results show that the addition of NZVI particles showed no measurable effect on bacterial community compositions, uncultured bacterial genera Sinobacteraceae, Xanthomonadaceae, Alcaligenaceae and Propionivibrio were sensitive to NZVI particles.