| Abstract | Lab-scale and pilot-scale experiments were undertaken to assess the fate of sludge borne metals in relation to organic matter decomposition in vertical flow reed beds for sludge dewatering. Three pilot-scale reed beds were loaded with septage, a highly organic sludge exhibiting low metal contamination. The sludge loading rate and moisture management regime were variable. Septage loading and monitoring were undertaken for three years. Four lab-scale reed beds were loaded with a highly contaminated industrial sewage sludge at sludge loading rates ranging between 50 kg TS.m-2.yea{1 and 200 kg TS.m-2.yea{1• Sludge loading was undertaken for two consecutive months, followed by an ageing period of one year. Metal fraction distribution (Cu, Zn, Mn, Fe, Ni, Pb and Cr) in the raw sludges was assessed by means of a sequential extraction procedure. The raw industrial sludge was characterised by high metal mobility. It was found that substantial amounts of Mn (445 mg.kg-1 DW or 70% of total Mn), Ni (632 mg.kg-1 DW or 42% of total Ni), Fe (11,246 mg.kg-1 DW or 32% of total Fe), Zn (2,315 mg.kg-1 DW or 19% of total Zn and Pb (258 mg.kg-1 DW or 14% of total Pb) in the raw industrial sewage sludge were present in potentially bioavailable forms, being associated with the exchangeable and acid extractable phases. Copper and chromium were less mobile, with 24 mg.kg-1 DW Cu (1.2% of total Cu) and 7 mg.kg-1 DW Cr (0.8% of total Cr) occurring in these fractions. In septage, for which case only Cu, Zn, Mn and Fe were investigated, a similar metal distribution was observed. Upon sludge loading and ageing, metals were mostly transferred to less available metal phases. This was the case for Pb, Cr, Fe, Mn and Ni. Yet, more than 41 % and 19% of total Mn and Ni respectively remained in the mobile metal exchangeable and acid extractable phases. Copper and Zn became more available upon sludge ageing. Following organic matter decomposition, concentrations of metals in the residual sludge increased. Consequently, a relative decrease of a particular metal fraction with respect to time, did not necessarily result in lower absolute amounts of the particular metal in that fraction. Similar observations were made following heat treatment. The loss of oxidisable and, to a lesser degree, reducible fractions following heating, was accompanied by both a higher metal mobility (increased exchangeable and/or acid extractable fractions), and a metal immobilisation process (increased residual :fractions). The observed organic matter decomposition in the pilot-scale reed beds was only half that observed in the lab-scale reed beds. This was due to the relatively wetter and cooler conditions prevailing in the dewatered septage layer. Consequently, metals remained bound to the organic matter for much longer. This inhibited their mobilisation. Following heat treatment, Fe was the only element that immobilised fully, being completely retrieved in the reducible and residual fractions. Copper, Zn and Mn significantly mobilised upon heat treatment. In order to assess metal leach.ability from the dewatered sludges following final disposal, two standardised leaching tests were performed on each sludge. The potential leach.ability test assessed the maximum amount of metals that could possibly be released under adverse environmental conditions. Between 20% and 33% of total Zn (1,740 mg.kg-1 DW), Mn (236 mg.kg-1 DW) and Ni (1 ,756 mg.kg-1 DW) in dewatered industrial sewage sludge were found to be potentially leach.able. Metal leach.ability from dewatered septage was similar in relative figures, but due to the lower total metal concentrations in septage, the potential leach.ability in absolute amounts was much lower. A maximum of 417 mg.kg-1 Zn and of 117 mg.kg-1 DW Mn were found to be leach.able. Copper, Fe, Pb and Cr were found to be much
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less leachable from the dewatered sludges. Generally, less than 1 % of their total contents were released during the leaching procedure. The cascade leaching test or the actual leachability test assessed long-term actual metal leachability. Leaching test results can only be understood in terms of environmental effects when they are applied to situations in the field. Migration of metals from a hypothetical landfill was calculated based on simple mass balances. The time scale of the leaching process in the field was related to the cumulative LIS ratio of the leaching test. Based on the results of the leaching test and on the sorption characteristics of the soil underlying the landfill, concentrations and flows of metals from the sludges were estimated with respect to time. The exercise was carried out for Zn, Cu and Pb. Zn was the most mobile element of the three considered, as was also confirmed by the sequential extraction data. In the case of industrial sewage sludge, this mobility resulted in deep migration, extending over 4 metres depth in sandy loam soils in the long-term (160 years) at concentrations of 160 mg.kg-1 DW. In clay soils, Zn migrated only a few centimetres deep, but top soil concentrations reached 26,000 mg.kg-1 DW, which is highly in excess of the acceptable limits in any circumstances. Lead and copper were less mobile. Even in the long-term, migration would be limited. However, concentrations in the top soil would reach levels between 7,000 and 8,000 mg.kg-1 DW in clay soils and between 400 and 600 mg.kg-1 DW in sandy loam soils. These levels were also higher than the Dutch trigger values for soil remediation. It was concluded that sewage sludge disposal to an unconfined landfill would lead to severe environmental effects. In the case of septage, it was found that metal migration would be limited and that its reuse as a soil conditioner would be more appropriate than its disposal to landfill. |