The biological treatment of domestic wastewater  and industrial sewage, generates sludge that causes a serious problem of space because it has to use sludge drying beds and be discarded in restricted places like sanitary landfills that are very expensive to maintain.

It is necessary to search for other alternatives. Between the options available like incineration, oceanic and forestall disposal. The agricultural disposal as a fertilizer have been considering promissing, because it proposes a way to transform a residue in a commercial product.

It is important to study each case, performing chemical, physical and biological characterization studies  of the considered sludge, to guarantee that the product have fertilizer properties at a minimal risk to his consumers. During my thesis I worked with a proccess to bioleach heavy metals from sludge using Thiobacillus ferrooxidans bacteria, and this gave pretty good results. The idea of using Thiobacillus to bioleach heavy metals in sludge was initiated by Guy Mercier et al (Universite du Quebec), and they developed at least, two pilot scale bioleaching units treating sludge with high concentration of heavy metals.