Comparison of heavy metal content in two sludge drying reed beds of different age

摘要

The fate of heavy metals (HMs) and nutrients along the vertical profile within the sludge layer and their accumulation into reed plant (Phragmites australis) biomass from SDRBs of different running times or ages has not been well documented until now. In this study, dried biosolid samples along the vertical profile of the sludge layer (0-2, 2-10,10-20, 20-30 cm) and plant biomass samples (roots, rhizomes and aerial parts) were collected from two full-scale SDRBs of two different running times (12 years in SDRB I and 6 years in SDRB II) and their concentrations of eight HMs (Fe, Mn, Cu, Zn, Pb, Cr, Ni, Mo), organic matter and nutrient contents were analyzed. In general, total organic carbon (TOC) concentrations were found to be higher (within a range of 15-30%) in the samples of the older SDRB I (12 years) than the younger SDRB II (6 years). HMs like Fe, Mn, Cu, Zn, Pb, and Ni were predominantly accumulated within the biosolids of both SDRBs but comparatively higher concentrations were observed in the samples collected from the younger SDRB II than the samples from the older SDRB I. This clearly suggested that the longer the treatment or mineralization time, the lower might be the accumulation of HMs within the biosolids. In general, the concentrations of the HMs were increased with sludge depth, probably due to the effect of higher organic matter mineralization and dewatering. The TOC content in the biosolids from the upper layer (0-2 cm) with 367.6 and 305.1 g kg(-1) was found to be lower in the bottom layer (2030 cm) with a concentration of 128.83 and 99.85 g kg(-1), which was resulted to a decreasing of concentrations by 64.9% and 673% in the SDRB I and SDRB II, respectively. This result suggested that the degree of mineralization process is higher or more stabilization occurs in the deeper layers as compared to the upper layer of the accumulated biosolids. Similarly, the nutrient (N, P, K, Ca, Mg, S) concentrations were found to be comparatively higher (within a range of 5-62%) within the biosolids of the younger SDRB II than the older SDRB I but decreased along the sludge depth in both SDRBs. Exceptions were the concentrations of potassium (K) and magnesium (Mg), which were increasing along the deeper layers of the biosolids from both the SDRBs. The concentrations of Fe (4760 mg kg(-1)), Zn (688 mg kg(-1)) and Cr (29 mg kg(-1)) were remarkably higher in the plant matter samples collected from the older SDRB I. Comparatively much higher concentration of HMs was adsorbed or accumulated in the roots and rhizomes than translocated to the above-ground plant biomass in both SDRBs. Our results suggested that the concentrations of HMs within the accumulated biosolids of both the SDRBs were well-below the maximum permitted legal limits for agricultural land application according to the EU Sewage Sludge Directive but a higher HM uptake by the plant biomass and probable effect on their growth might be necessary to consider for future investigations. (C) 2014 Elsevier B.V. All rights reserved.