The role of NiFe_(2)O_(4)nanoparticle in the anaerobic digestion(AD)of waste activated sludge(WAS)

Anaerobic digestion(AD)is a promising technology for the treatment of waste activated sludge(WAS)with energy recovery.However,the low methane yield and slow methanogenesis limit its broad application.In this study,the NiFe_(2)O_(4)nanoparticles(NPs)were fabricated and applied as a conductive material to enhance the AD via promoting the direct interspecies electron transfer(DIET).The crystal structure,specific surface area,morphology and elemental composition of the as-prepared NiFe_(2)O_(4)NPs were characterized by X-ray diffraction(XRD),Brunauer-Emmett-Teller(BET),scanning electron microscopy(SEM)and energy dispersive spectroscopy(EDS).The biochemical methane potential(BMP)test was performed(lasting for 35 days)to evaluate the energy recovery in AD with the addition of the NiFe_(2)O_(4)NPs.The results illustrate that NiFe_(2)O_(4)NPs could accelerate both the hydrolysis,acidogenesis and methanogenesis,i.e.,the cumulative methane production and daily methane yield increased from 96.76±1.70 mL/gVS and 8.24±1.26 mL gVS^(-1)d^(-1)in the absence of NiFe_(2)O_(4)NPs(Group A)to 123.69±3.20 mL/gVS and 9.71±0.77 mL gVS^(-1)d^(-1)in the presence of NiFe_(2)O_(4)NPs(Group B).The model simulation results showed that both the first-order kinetic model and the modified Gompertz model can well simulate the experimental results.The hydrolysis rate constant k increased from 0.04±0.01 d^(-1)in Group A to 0.06±0.01 d^(-1)in Group B.And the maximum methane production potential and activity were both improved after adding NiFe_(2)O_(4).The microbial community analysis revealed that the microorganisms associated with hydrolysis and acidogenesis were more abundant in the presence of NiFe_(2)O_(4).And the methanogenic archaea were enriched to a larger extent,resulted in the higher methanogenesis activities via dosing NiFe_(2)O_(4).

High rate production of concentrated sulfides from metal bearing wastewater in an expanded bed hydrogenotrophic sulfate reducing bioreactor

Metallurgical wastewaters contain high concentrations of sulfate,up to 15 g L^(-1).Sulfate-reducing bioreactors are employed to treat these wastewaters,reducing sulfates to sulfides which subsequently coprecipitate metals.Sulfate loading and reduction rates are typically restricted by the total H2S concentration.Sulfide stripping,sulfide precipitation and dilution are the main strategies employed to minimize inhibition by H2S,but can be adversely compromised by suboptimal sulfate reduction,clogging and additional energy costs.Here,metallurgical wastewater was treated for over 250 days using two hydrogenotrophic granular activated carbon expanded bed bioreactors without additional removal of sulfides.H2S toxicity was minimized by operating at pH 8±0.15,resulting in an average sulfate removal of 7.08±0.08 g L^(-1),sulfide concentrations of 2.1±0.2 g L^(-1) and peaks up to 2.3±0.2 g L^(-1).A sulfate reduction rate of 20.6±0.9 g L^(-1)d^(-1) was achieved,with maxima up to 27.2 g L^(-1)d^(-1),which is among the highest reported considering a literature review of 39 studies.The rates reported here are 6e8 times higher than those reported for other reactors without active sulfide removal and the only reported for expanded bed sulfate-reducing bioreactors using H2.By increasing the influent sulfate concentration and maintaining high sulfide concentrations,sulfate reducers were promoted while fermenters and methanogens were suppressed.Industrial wastewater containing 4.4 g L^(-1) sulfate,0.036 g L^(-1) nitrate and various metals(As,Fe,Tl,Zn,Ni,Sb,Co and Cd)was successfully treated with all metal(loid)s,nitrates and sulfates removed below discharge limits.