The concentration of trace elements in sewage sludge from wastewater treatment plant in Gniewino

Abstract

The sewage sludge originating from wastewater treatment plants (WWTP) serving rural areas is suggested for agricultural or natural usage. However, sewage sludge is beforehand subjected to the several pre-treatments, which involve stabilization, hygienisation and pre-composting. These methods mainly decrease the amount of organic substances and the presence of microorganisms, but hardly affect the concentrations of heavy metals. The advantages of using sludges as fertilizer for improving and sustaining soil fertility and crop production are numerous. The addition of sewage sludge to soils could affect the potential availability of heavy metals. Trace elements are distributed in the soil in various forms: solid phases, free ions in soil solution, soluble organicmineral complexes, or adsorbed on colloidal particles. The most undesirable heavy metals in sewage sludge that are toxic for the living organisms include: cadmium, chromium, nickel, lead and mercury. In the study, the concentrations of trace elements (Pb, Cd, Cr, Hg, Ni, Zn, Al, As, Se, B, Ba, Br, Ca, Cu, Fe, Mn, Na, Ga, Li, Mo, Sr, Mg, K, Ru, Tl, V, U) were tested in the sewage sludge obtained from a WWTP serving rural areas (PE < 9 000). In each case, the tested sewage sludge was meeting the criteria of stabilization and was used for agriculture and land reclamation purpose. All the samples were collected in 2016 and subjected to microwave mineralization in a closed system in aqua regia. The total amounts of macro and microelements were determined with a spectrophotometer Coupled Plasma emission ICP-OES. It was found that the total concentrations of trace metals in all of sewage sludges are the same as the Polish regulation limit of pollutants for sludge to be used in agriculture. European legislation is less restrictive and permits higher contents of heavy metals in sludge used for agriculture than Asia. The trace elements (cadmium: 1.16 mg·kg-1/d.m. in thePolish sewage sludge, are much higher than those in the other countries. Copper and zinc were the most prevalent elements observed (111.28 mg·kg-1/d.m. and 282.94 mg·kg-1/d.m., respectively). The concentrations of copper in the Polish sewage sludge are much lower (49-130 mg·kg-1/d.m.) than european sewage sludge (522-562 mg·kg-1/d.m.). The two of the tested heavy metals (beryllium, bismuth) were under the detection limit, while gallium, molybdenum, thallium, vanadium and silver were detected in the concentrations lower than 0.005 mg·kg-1/d.m. According to the obtained results, in all the tested samples, the total amount of trace elements, did not exceed the limit values in sewage sludge for their use in agriculture and land reclamation.