The seasonal changes of heavy metal content as limiting factor of sewage sludge re-use as fertilizer ingredient, based on data from WWTPs in Poland

In Europe, half of the amount of sewage sludge from wastewater treatment plants (WWTPs) is used in agriculture, in Poland about 27%, respectively. Fertilizing products (compost or organic-mineral fertilizers) produced from sewage sludge are promise alternative for directly use of no-treated waste. The paper presents seasonal changes of Cu, Pb, Ni, Zn, Cr, Cd and Hg content in sewage sludge from different regions of Poland collected during one year. It was noted wide range of the metal amount in dependence on season andWWTPs capacity. The Cu concentration was from 0.4 to 784 mgkg–1 d.m., Ni from 0.25 to 1281 mgkg–1 d.m., Cd from 0.005 to 14.85 mgkg–1 d.m., Pb from 0.11 to 306.2 mgkg��1 d.m., Hg from<0.001 to 2.3 mgkg–1, Cr from 0.23 to 854 mgkg–1 and Zn from 11 to 4669 mgkg–1, respectively. It has been shown that Cu, Cd, Pb, Hg and Cr content did not exceed the permissible levels for agricultural purposes according to Polish law. There was no clear correlation between the heavy metal amount and the WWTP capacity, expressed as population equivalent (PE). However some increase trend in the case of Cu, Ni, Hg and Cr concentration at the higher PE value was found. It has been concluded that in spite relatively low level of heavy metals in Polish WWTPs, it may be limiting factor for production high quality fertilizers on the base of sewage sludge.