Reuse of Hydrocarbon-Contaminated Sludge from Soil Washing Process: Issues and Perspectives

Soil washing is a remediation technology commonly used to treat soil contaminated by total petroleum hydrocarbons (TPHs) (Seyed Razavi et al., 2012). Nowadays in Italy there are about ten plants, mainly placed in the north of the country. After treatment, coarse materials (i.e. gravel and sand) are cleaned and can be reused, e.g. as aggregate for concrete production; contaminants are concentrated in the filter-pressed sludge containing fine grains (i.e silt and clay), which is usually landfilled. This paper analyses different alternatives for sludge reuse at an industrial scale in Italy; the main normative and technical constraints are presented and discussed. In addition, the preliminary results of a lab-scale experimentation aimed at investigating the treatability of soil washing residues by wet oxidation are presented. The reuse and valorisation of soil washing originated sludge is desirable not only from the economic point of view but also because Italian and European legislation put waste recycling as a priority. Several tens of thousands of tons of sludge are produced and landfilled in Italy every year; they represent materials which could be exploited at industrial scale in place of virgin silt and clay to produce stabilised soil for road construction or bricks in brick works. Sludge reuse for cement production is possible, too, but it is probably more expensive. The use of raw sludge produced in soil washing plants implies a potential risk of environmental contamination therefore the basic condition for sludge recovery and reuse is its decontamination, which can be theoretically obtained by thermal desorption, solvent extraction or chemical oxidation. Wet oxidation tests executed by the authors indicate that such a process is potentially applicable. 98 % removal efficiency of TPHs was obtained in a lab-scale reactor setting temperature, treatment time and initial oxygen pressure to 250 °C, 30 min and 20 atm, respectively. Further tests will be carried out in order to identify the most favourable and cheapest treatment conditions.