Effect of hydraulic retention time on metal precipitation in sulfate reducing inverse fluidized bed reactors

Abstract

BACKGROUND: Metal sulfide recovery in sulfate reducing bioreactors is a challenge due to the formation of small precipitates with poor settling properties. The size of the metal sulfide precipitates with the change in operational parameters such as pH, sulfide concentration and reactor configuration has been previously studied. The effect of the hydraulic retention time (HRT) on the metal precipitate characteristics such as particle size for settling has not yet been addressed. RESULTS:The change in size of the metal (Cu, Zn, Pb and Cd) sulfide precipitates as a function of the HRT was studied in two sulfate reducing inversed fluidized bed (IFB) reactors operating at different chemical oxygen demand concentrations to produce high and low sulfide concentrations. The decrease of the HRT from 24 to 9 h in both IFB reactors affected the contact time of the precipitates formed, thus making differences in aggregation and particle growth regardless of the differences in sulfide concentration. Further HRT decrease to 4.5 h affected the sulfate reducing activity for sulfide production and hence, the supersaturation level and solid phase speciation. Metal sulfide precipitates affected the sulfate reducing activity and community in the biofilm, probably because of the stronger local supersaturation causing metal sulfides accumulation in the biofilm.CONCLUSIONS: This study shows that the HRT is an important factor determining the size and thus the settling rate of the metal sulfides formed in bioreactors.

Example study for granular bioreactor stratification: Three-dimensional evaluation of a sulfate-reducing granular bioreactor

“…Luteococcus is mesophilic lactic streptococci that can produce lactic acid via fermentation 39 . Lactate has been shown to be a favorable substrate for SRB metabolism in terms of energy and biomass production 40 .…”

Simultaneous Sulfate Reduction and Metal Precipitation in an Inverse Fluidized Bed Reactor

“…Zn-phosphate (Villa-Gomez et al 2012b) also occurred (Table 4.2). Therefore, the decrease of the HRT should be aligned to the sulfate reduction rate in order to maintain…”

“…Although the time scales of particle growth in metal sulfide precipitation has only been reported for a retention time of 200 min in chemical precipitator reactors (Frédéric et al 2009), in contrast with a change of retention time from 24 to 9 h applied in this study (Figure 4.4), there is evidence that the growth of the precipitates continues exponential at the time length studied in chemical precipitator reactors (Frédéric et al 2009;Couto and Mesquita 1994). In addition, substances commonly present in bioreactors such as phosphate (Villa-Gomez et al 2012b) and bioproducts such as extracellular proteins ), which are not present in chemical precipitator reactors, could enhance the agglomeration process. The presence of extracellular proteins produced by SRB indeed promotes the aggregation of zinc sulfide precipitates in natural environments ).…”

Metal Removal and Recovery from Mining Wastewater and E-Waste Leachate

“…It has been reported that an IFB has the potential to be used for high organic rich wastewater like 79 distillery effluent, achieving ~84% COD removal at an organic loading rate (OLR) of 35 kg COD/m 3 /d (Sowmeyan et al, 2008a;Sowmeyan et al, 2008b). In recent studies focusing on the use of IFB for metals (Cd, Cu, Pb and Zn) removal, it was shown that the IFB can achieve >90% metal removal efficiency (Villa-Gomez et al, 2011;Villa-Gomez et al, 2015). In those studies, the sulfide concentration and HRT were found to be important factors determining the location and size of the precipitate.…”