An adequate sewage sludge was collected at the Rovereto waste water purification plant near Trento (Italy). The mainly civil dwellings guaranted a relatively low metal concentration. Several hundred kilograms of sludge material were collected from a drying oven at the waste water treatment plant (175-180°C). The material was passed through a 2mm sieve discarding the fraction > 2 mm and eliminating extraneous particles such as stones, roots, fragments of plastic and metal. The fraction < 2mm was devided into 10 batches of 10 kg which all underwent gamma irradiation at a dose of 25kGy (sufficient to sterilize the material). The batches were pooled together for further processing which consisted of crushing with a Retsch SR3 mill with an inner 0.75 mm sieve. After crushing the material was further sieved mechanically to obtain the fraction < 90 µm. This material was inserted in a rigid mixing drum with out-of-axis fins. The drum was kept rotating for 170 hours at 40 rpm until a sufficiently homogenous material with particle size < 90 µm was obtained; the optimal homogenization time was based on previous experience with this type of material. The final material was bottled in 50 g portions in clean brown bottles closed with a plastic insert and screw cap; the bottling procedure was carried out in a stepwise manner: twenty bottles were filled while the drum was stationary, then the drum was operated again for 3 minutes and 20 other bottles were filled etc. keywords: environmental material , biosolids , trace elements , heavy metals , fractionation analysis , aqua regia soluble fraction
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