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Treatment plant for dredged sediments from the Arno river drainage channel in Pisa.
The Arno floodway is the most important strategic work for mitigating flood risk in the lower Arno Valley and the city of Pisa. The main objective of the designed works is to restore the full functionality of the floodway and to reduce the flood risk.
Location: Pisa - Livorno, Italy
Client: Province of Pisa – Soil Defense Service
Cost of total Works: 8,879,300.00 €
Services: Detailed design and Environmental Impact Assessment
Period: 10/2012 – 01/2013
The dredging operations foreseen for the restoration of the original hydraulic conditions of the Arno floodway, required to preserve the surrounding areas from flooding, generate a flux of sediments of 600,000 m³. The grain size of the sediments, and the presence of pollutants change along the floodway; a treatment plant is necessary for maximizing reuse of the dredged material. In particular, the purposes of the treatment plant are mainly directed to dewatering of the incoming dredged material, in order to reuse it for dike re-profiling works and beach nourishment, to minimization of the disposal of the contaminated fraction and to allow discharge of the treated water into the floodway, according to national legal standards.
The dredged material will be transported by barges and then pumped to the treatment plant; it can be divided into two main fractions: the fine-pelitic fraction (silt-clay) is widely spread along the inland part (465,000 m³), while the sand fraction (135,000 m³) comes mainly from the estuary of the floodway. The main pollutants are HCs, PCBs and PAHs; the concentration varies along the floodway, with the higher values in the deeper layers of the riverbed, in close proximity to the estuary and the surrounding industrial areas such as refineries and port terminals.
The following foreseen sediment treatment processes depend on the nature of the incoming material; two different approaches have been considered, both based on physical methods such as natural desiccation, for the fine-pelitic fraction and soil separation, classification for sandy material. The first process is based on the creation of 4 desiccation ponds in which the fine material is laid in thin layers, in order to ensure a natural evaporation, in a time period of 3 weeks. This solution, primarily characterized by a very low energy impact, has been adopted in view of the significant quantities of material to be treated, the high availability of surfaces and “medium-long” process times. At the same time, a volume reduction of the contaminated fractions that will be identified and quantified by soil analysis during dredging operations and the treatment process is ensured. If the degree of contamination for these fractions results higher than national legal standards for reuse also after dilution, the contaminated material will be disposed to landfill. For the sand fraction a sand recovery unit has been foreseen: the unit, based on the separation of the material using hydrocyclones, ensures high values of classification, concentrating the contaminated parts in the overflow that will be treated in a chemical-physical water treatment plant: the treated water is stored and reused in the treatment processes. The sludge resulting from the clariflocculation process is mechanically dewatered and disposed to landfill.