Characterization of primary and secondary wastewater treatment sludge from a pulp and board mill complex to evaluate the feasibility of utilization as a soil amendment agent and a fertilizer product

In this study, we have determined the main important physical and chemical properties as well as the heavy metal concentrations of the primary and secondary wastewater treatment sludge from a pulp and board mill complex located in Finland in order to evaluate the utilization of these by-products as a soil amendment agent or a fertilizer product. Easily soluble Ca, Mg, K, Na, P and S concentrations in the sludges were extracted by ammonium acetate (CH₃COONH₄) and easily soluble Cu, Mn and Zn concentrations by CH₃COONH₄ + Na₂EDTA. For the determination of total nutrient and total heavy metal concentrations in sludges, they were digested using aqua regia (3 mL HCl + 9 mL HNO₃). The total heavy concentrations in the primary sludge were lower than the Finnish heavy metal limit values for fertilizer products. In the secondary sludge, all other total heavy metals than the total Cd concentration (4.8 mg/kg; d.w.) were lower than the heavy metal limit values for fertilizer products. Due to the low total heavy metal concentrations, which were lower than the Finnish limit values for fertilizer products, the primary sludge is a potential fertilizer. Although the total Cd concentration (4.8 mg/kg; d.w.) in the secondary sludge exceed the limit value of 1.5 mg/kg (d.w.) for fertilizer products, this residue may be used as a soil improver, a growing media or as a fertilizer product in landfill sites or in other closed industrial areas, because the Finnish limit values for fertilizer products are not applied at these sites.