The mechanism of metals precipitation by biologically generated alkalinity in biofilm reactors

Microbial strains, characterized by increased tolerance and ability to grow in metal bearing wastewaters as well as by effective metal sequestering capability by both active (bioaccumulative) and passive (biosorptive) processes, were tested as inoculum for metal laden wastewater treatment systems. Their capacity to grow in metal bearing wastewater, using an easily available and inexpensive carbon source such as acetate, was studied in batch experiments. Two principal conclusions were drawn: (1) Growth was observed for all the strains examined suggesting that the strains can be acclimated to metals bearing wastewaters. (2) Solution pH increased from neutral to alkaline values during growth (pHinitial=7, pHfinal=10). The later was observed systematically for all strains. Metal precipitation, due to the metabolically generated alkalinity is expected as a result. Supporting evidence for this hypothesis was provided during the operation of two pilot moving bed sand filters treating two different metal bearing wastewaters. Acetate was used as carbon source to support the growth and maintenance of microbial biomass on the sand grains of the filters. The characteristics of the sludge produced from the operation of the pilot plants were subsequently studied in the laboratory. Both sludges were significantly loaded with large amounts of metals. A mechanism of metal precipitation induced by the metabolically generated alkalinity, when acetate is used as carbon source, could be proposed as the main process responsible for the metals sequestering inside the moving bed sandfilter reactor.