Potential fertilizing properties of sewage sludge treated in the sludge treatment reed beds (STRB)

In previous research the fertilizer value of sludge from reed beds was evaluated based on the sampling sludge on different depths from four Danish sludge treatment reed beds (STRB) after long periods of stabilization. The dewatering efficiency of sewage sludge in the STRB is comparable to mechanical dewatering. The long-term stabilization of sewage sludge progressively decreases the concentration of organic matter due to the process of humification. The aim of the work was to determine the changes of organic matter concentration, nutrients concentrations as well as speciation of selected heavy metals (Cd, Pb, Ni, Cr, Cu and Zn) in vertical profile of the sewage sludge stabilized in the STRB for 7–15 years. The analyzed sewage sludge was collected from the STRB treating sludge from four municipal wastewater treatment plants located in Denmark serving from 9,000 to 40,000 person equivalent. Analyzed heavy metals (except for Zn) were mostly bound with the most stable – residual – fraction. The most stable metals were Pb and Cr, where the share of the residual fraction exceeded 80.0%. The most mobile metal was Zn, where the share of the mobile fractions (calculated as the sum of I, II and III fractions) exceeded 85.0%. The high concentration of nitrogen and phosphorus is the decisive factor in the high valuation of sludge stabilized in the STRB as fertilizer.