Using bioleaching to remove metals from sewage sludge intended for land application

Removal of heavy metal contaminants from sewage sludge is a necessity before it is used as an agricultural fertilizer (biosolid), due to environmental concerns and municipal, provincial and federal regulations. The bioleaching method is recommended as an economical and effective process for the removal of heavy metals from the Ashbridges Bay Treatment Plant (ABTP) sludge, some of them with concentrations exceeding the recommended level by the Ontario Ministry of Agriculture, Food, and Rural Affairs guidelines. The Gram-negative thiobacilli is a group of organisms with physiological and morphological similarity and grows by oxidizing ferrous ion and reduced sulphur compounds. One species of thiobacillus, T. ferrooxidan , was recommended as an effective bacterium for the heavy metal removal from sewage sludge. This research involved the incubation of adapted sludge using fresh raw digested sludge and activated sludge of ABTP. Using adapted sludge for the bioleaching process, the method was tested in a continuously stirred tank reactor (CSTR) in combination with a series of jar tests. Results showed that the metal removal efficiency increases with decreasing pH, and the solids content does not affect the removal efficiency of cupper and zinc very much during short term jar test. The results from the long-term (20-day) CSTR test demonstrated that the high T ferrooxians-contained adapted sludge could remove copper from the sewage sludge of ABTP very effectively, by as much as 79.2%. In comparison, the simultaneous removal efficiency of zinc and cadmium were also studied for the same process and, they are 82.0% and 83.9% respectively. The TSS degradation constant rate during the 20 days' bioleaching was found to be 0.0522 day -1. It is concluded that Ontario should continue to apply sludge to agricultural land, as sludge is an economic alternative, promotes recycling of resources, and is a valuable fertilizer. However, the toxic metals in sludge should be removed from sewage sludge using the bioleaching process to recommended level before it is disposed as a fertilizer for land application.