Rumyana Djingova

1. Introduction The increasing production of wastewater as a result of intensive anthropogenic activity leads to a reciprocal increase in sewage sludge production in the wastewater treatment plants (WWTP). Sludges can contain various pollutants, in particular, potentially toxic elements (PTEs) [1,2,3], polyaromatic hydrocarbons [1,2], dioxins [1], polychlorinated biphenyls [1], and pathogenic organisms [1].