Reed bed systems for sludge treatment: case studies in Italy.

In recent years, reed bed systems (RBSs) have been widely considered as a valid technology for sludge treatment. In this study are presented results about sludge stabilization occurring within beds in four RBSs, situated in Tuscany (Italy). The results showed that stabilization of the sludge over time occurred in all RBSs, as shown by the low content of water-soluble carbon and dehydrogenase activity, which measures indirectly the overall microbial metabolism, and by the re-synthesis of humic-like matter highlighted by the pyrolytic indices of mineralization and humification. Results about heavy metal fractionation, an appropriate technique to estimate the heavy metal bioavailability and sludge biotoxicity, showed that the process of sludge stabilization occurring in RBSs retains metals in fractions related to the stabilized organic matter, making metals less bioavailable. Moreover, the concentrations of various toxic organic compounds were below the limit of concentration suggested by the European Union's Working Document on Sludge, for land application. The effectiveness of the stabilization processes in RBs was hence clearly proven by the results that measured mineralization and humification processes, and by the low levels of bioavailable heavy metals and toxic organic compounds in stabilized sludges.

Phytoremediation of dredged marine sediment: monitoring of chemical and biochemical processes contributing to sediment reclamation.

In this study, a pilot phytoremediation experiment was performed to treat about 80 m(3) of silty saline sediments contaminated by heavy metals and organic compounds. After preliminary mixing with a sandy soil and green compost application, three different plant treatments [Paspalum vaginatum (P); P. vaginatum + Spartium junceum (P + S); P. vaginatum + Tamarix gallica (P + T)] were compared to each other and to an unplanted control (C) in order to evaluate the plant efficiency in remediating and ameliorating agronomical and functional sediment properties. The experiment was monitored for one year after planting by taking sediment samples at two depths and performing several chemical and biochemical analyses. After one year, the increase in hydrolytic enzyme and dehydrogenase activities indicated the stimulation of sediment functionality. Additionally, the availability of energy sources derived from organic matter application and plant-root activity promoted the formation of a stable organic matter fraction. Finally, P + S and P + T were also effective in decontaminating polluted marine sediments from both organic (total petroleum hydrocarbons, TPH) and inorganic (heavy metal) pollutants.

Decontamination and functional reclamation of dredged brackish sediments.

The continuous stream of sediments, dredged from harbors and waterways for keeping shipping traffic efficiency, is a considerable ongoing problem recognized worldwide. This problem gets worse as most of the sediments dredged from commercial ports and waterways turn out to be polluted by a wide range of organic and inorganic contaminants. In this study, phytoremediation was explored as a sustainable reclamation technology for turning slightly-polluted brackish dredged sediments into a matrix feasible for productive use. To test this possibility, a phytoremediation experimentation was carried out in containers of about 0.7 m(3) each, filled with brackish dredged sediments contaminated by heavy metals and hydrocarbons. The sediments were pre-conditioned by adding an agronomic soil (30 % v/v) to improve their clayey granulometric composition, and by topping the mixture with high quality compost (4 kg m(-2)) to favour the initial adaptation of the selected vegetal species. The following plant treatments were tested: (1) Paspalum vaginatum, (2) Phragmites australis, (3) Spartium junceum + P. vaginatum, (4) Nerium oleander + P. vaginatum, (5) Tamarix gallica + P. vaginatum, and (6) unplanted control. Eighteen months after the beginning of the experimentation, all the plant species were found in healthy condition and well developed. Throughout the whole experiment, the monitored biological parameters (total microbial population and dehydrogenase activity) were generally observed as constantly increasing in all the planted sediments more than in the control, pointing out an improvement of the chemico-physical conditions of both microorganisms and plants. The concentration decrease of organic and inorganic contaminants (>35 and 20 %, respectively) in the treatments with plants, particularly in the T. gallica + P. vaginatum, confirmed the importance of the root-microorganism interaction in activating the decontamination processes. Finally, the healthy state of the plants and the sediment characteristics, approaching those of an uncontaminated natural soil (technosoil), indicated the efficiency and success of this technology for brackish sediments reclamation.

Phytotreatment of sludges (Phragmites australis) for their reuse in the environment.

The aim of this study is the evaluation of the agronomic characteristics acquired by a phytotreated sludge coming from a wastewater treatment plant (WWTP) located in Tuscany (central Italy). The chemical characterization showed values which are within the Italian legislation limits for mixed composts. From an agronomic point of view, the sludge did not present a level of phytotoxicity, as shown by the germination index (GI% = 77). Furthermore, pathogen compounds are inexistent (Escherichia coli < 1,000 CFU/g). Different substrates (obtained by mixing the sludge with sandy agronomic soil - 0.5% w/w, 1% w/w, 2.5% w/w and 5% w/w) were prepared in order to evaluate the best mixture performance in terms of water retention capacity and plant growth. No significant differences were observed for all sludge mixtures. Different plants were tested in plots (Lepidium sativum, Cucumis sativus and Avena sativa). The best plant adaptation, measured as dry biomass production, was observed for Avena sativa. The results obtained underlined that the phytotreatment of sludge can bring about the transformation of sewage sludges into organic products that are reusable in agriculture, if previously mixed with other appropriate materials and taking into account their heavy metal content.

Hydraulic and biochemical analyses on full-scale sludge consolidation reed beds in Tuscany (Italy).

The management of sewage sludge has recently become one of the most significant challenges in wastewater management. Reed bed systems appear to be an efficient and economical solution for sludge management in small wastewater treatment plants. Four years ago, one of the holding companies for water and wastewater in central Italy adopted this technology in 6 wastewater treatment plants. Hydraulic and biochemical analyses were performed on the most representative site to asses the behaviour of reed beds with regard to dewatering, mineralization and humification of disposed sludge. Moreover, daily water content analysis were performed in the interval between subsequent sludge loadings. Results indicated a decrease of sludge volume by about 93% on a yearly basis. Biochemical analysis highlighted that mineralization processes decrease over time due to a rapid decrease of microbial activity and labile substrates, such as DHase enzyme and water-soluble carbon and ammonium, respectively. Moreover, a significant interrelationship between the parameters linked with mineralization was found: after two years of operation, the process of mineralization of organic matter is still predominant in the humification of organic matter. Daily water content data were used to define a semi empirical equation describing the dynamics of the dewatering process. Overall, the use of sludge reed beds resulted feasible, ecologically sustainable and cost-effective.