Heavy metals in municipal sewage sludge – a brief characteristic of potential threats and methods used to assess the ecological risk

Population growth and increasing effectiveness of biological wastewater treatment processes are reasons for a systematic increase in the amount of municipal sewage sludge. Beyond the soil forming values and fertilizer properties, sludge is highly enriched with heavy metals, including these particularly toxic to the environment (Cd, Cr, Cu, Ni, Pb, Zn, Hg), which contributes to an increase in the level of ecological risk. A variety of methods (indices) have been proposed in order to assess the ecological risk of heavy metals in sewage sludge. Among them we can distinguish the total content and speciation indices. It is important to remember that total concentration of heavy metals in sewage sludge does not provide complete information about the level of ecological risk that these elements pose to the environment and living organisms, including humans. Metals bound to the sludge structure may undergo transformation while being subjected to the processes conducted in the wastewater treatment plants (WWTPs). Therefore, it is necessary to determine also their chemical forms. The common method used to identify the chemical fractions of metals in sludge, is the three – step sequential extraction by the Community Bureau of Reference procedure (BCR; now Standards, Measurements and Testing Programme).