Development and simulation of a struvite crystallization fluidized bed reactor with enhanced external recirculation for phosphorous and ammonium recovery.

Recovering nitrogen and phosphorus from waste water in the form of struvite is an effective way to recycle resources. The insufficient purity of the resulting struvite and the large loss of nitrogen and phosphorus are the challenges at present. Therefore, it is urgent to develop innovative method in struvite crystallization process for efficient nitrogen and phosphorus recovery. This study proposed a crystallization method to reduce the loss of nitrogen and phosphorus by a struvite fluidized bed reactor (FBR) with optimized structure and operation conditions. The properties of struvite obtained under various conditions in the reactor were studied, and the internal operating conditions of the reactor were simulated with COMSOL Multiphysics to verify the effectiveness of the reactor optimization. This reactor achieved stable operation under the conditions of N/P = 1:1 and pH = 9.0. The purity of struvite obtained reached 98.5%, the conversion rate of ammonia nitrogen reached 97.2%, and struvite crystals could grow to 84 µm within 24 h. The simulation results showed that the Venturi tubes installed at multiple locations increased the turbulent energy to 4 × 10-4 m2/s2, which greatly improved the mass transfer efficiency. The trajectory of the crystal particles was consistent with the fluid flow field, which promoted the purification and growth of the crystal. In general, the new FBR with enhanced external recirculation would be a very feasible way to improve crystal growth and crystal purification of struvite, and it could enhance the recovery efficiency of nitrogen and phosphorus with reduced cost.

Performance, microbial community and inhibition kinetics of long-term Cu2+ stress on an air-lift nitritation reactor with self-recirculation.

Biological nitrogen removal process could be affected due to the presence of heavy metals owing to their toxicity and accumulation in the sludge. In this study, the impact of Cu2+ shock on a long-term nitritation operation was investigated in an air-lift reactor with self-recirculation. Both the dynamics of microbial community and inhibition kinetics under Cu2+ stress were ascertained. The results showed that Cu2+ exerted severe inhibition on nitritation performance of an air-lift reactor (ALR) at 25 mg/L. The corresponding NH4+-N removal efficiency decreased to below 50%, which was mainly due to the variation of microbial community structure, especially the inhibition of nitrifiers like Nitrosomonas (the relative abundance decreased from 30% to 1% after Cu2+ inhibition). Kinetic parameters were obtained and compared after fitting the Haldane model. The long-term Cu2+ stress on the ALR aggravated the ammonium affinity and the resistance to substrate self-inhibition of the nitritation sludge, but reduced the resistance to Cu2+ inhibition. Furthermore, Cu2+ acted as uncompetitive inhibitor on nitritation process. Our results provide new insights into the nitritation characteristics under long-term Cu2+ stress.

The long-term effects of hexavalent chromium on anaerobic ammonium oxidation process: Performance inhibition, hexavalent chromium reduction and unexpected nitrite oxidation.

The toxicity of hexavalent chromium (Cr(VI)) is one of the challenges in implementing Anammox process to ammonium-rich wastewater treatment. However, the response of Anammox process to Cr(VI) stress and the inhibition mechanism remain unclear. Here, two Anammox UASB reactors were operated for 285 days under different Cr(VI) stresses. The results showed Anammox performance was not affected at low Cr(VI) concentration (i.e., 0-0.5 mg L-1), but was severely inhibited at 0.8 mg L-1. Attempts to domesticate Anammox process to higher Cr(VI) by lowering nitrogen loading rate were failed. Examination of Cr(VI) fate showed the occurrence of extracellular and intracellular Cr(VI) reduction to Cr(III). The inhibition was ascribed to the significant intracellular Cr(VI) reduction, accounting for 99.78% of the total Cr(VI) reduction. Moreover, under long-term Cr(VI) exposure, most nitrite was oxidized to nitrate. But microbial community showed no enrichment of Cr(VI) reducing bacteria and other nitrogen transformation-related bacteria.

The application of aged refuse in nitrification biofilter: Process performance and characterization.

High adsorption capacity, good biocompatibility and low cost are highly demanded for biofilter used in ammonium-rich wastewater treatment. In this study, we used SEM, BET, XRD and 16S rRNA to document the evidence for good performance in adsorption and biodegradation in aged refuse. Parallel experiment between raw and inert refuse showed ammonium adsorption occurred at the initial week, with the highest ammonium removal efficiency of 90.36%, but saturated during the subsequent long-term operation. Meanwhile, over 6months' operation of an aged refuse biofilter was conducted to confirm that nitrification was the main pathway of ammonium conversion. The maximum nitrogen loading rate could reach up to as high as 1.28kg/m3/d, with ammonium removal efficiency at 99%. Further, high nitrifier biodiversity were detected with 'Nitrosomonas' and 'Nitrospira' in domination in the refuse. However, Nitrospira would outcompete Nitrosomonas under the oxygen limiting condition and resulted in the failure of partial nitrification. The physicochemical and biological analysis show that biodegradation is the main ammonium conversion pathway, which is the critical finding of this work. This investigation would help to accelerate the application of the aged refuse process in ammonium-rich wastewater treatment.

Insights into the role of extracellular polymeric substances in Zn2+ adsorption in different biological sludge systems.

The adsorption behavior of Zn2+ in four different biological sludge systems, i.e. activated sludge, denitrification sludge, short-cut nitrification sludge, and anammox granules, was investigated. The results indicated that all sludge samples possessed considerable potential for Zn2+ adsorption. Short-cut nitrification sludge possessed the highest Zn2+ maximum adsorption capacity (qm) of 36.4 mg g SS-1, which was much higher than other sludges applied (12.8-14.7 mg g SS-1). Besides, the adsorption rate for short-cut nitrification sludge was fastest among the four types of sludge after fitting with a pseudo-second-order rate equation. Comparing with the physicochemical properties of the four sludges, the soluble extracellular polymeric substances (EPS), especially soluble polysaccharide (PS), played a prior role in binding metal cations (i.e., Zn). The present study also showed that with less than 30% of Zn2+ trapped by EPS, 61.6-71.9% of Zn2+could be harvested directly by cells, indicating that the protecting capability by EPS was limited. Therefore, it is important to remove metal ions as early as possible if the activated sludge processes encountered high stress of heavy metal. Graphical abstract ᅟ.