“Circular management of beach litter: densimetric separation of natural and anthropic fractions”

Introduction Accumulation of waste on coastlines is a common problem in the Mediterranean Sea with significant environmental and economic consequences. These accumulations, mostly consisting of biomass (e.g., Posidonia oceanica) and anthropogenic debris (e.g., plastics, wood, metals), are classified as urban waste (Legislative Decree 152/2006) and are not serviced by waste management consortia, leading to disposal primarily in authorized landfills. Although it is clear that maintaining the natural material is essential for preserving the coastal ecosystem, local authorities frequently face pressure to remove these deposits to improve beach aesthetics. Moreover, biomass accumulations are notably rich in anthropogenic materials. The collection and treatment of beach litter is crucial in safeguarding marine-coastal environments and recovering Secondary Raw Materials (SRM). This article presents the first results of an experimental study aimed at densimetric separation of organic fractions (Posidonia oceanica) from natural inorganic fractions (sand) and plastic anthropogenic wastes. Materials and methods The research activities involved sampling waste from the beaches of Mola of Bari and Fasano (Apulia, Italy). Samples were analyzed to determine the composition of organic natural fractions, mineral natural fractions, and anthropogenic waste. Each fraction was then chemically and physically characterized (e.g., granulometric analysis, ecological classification of plant species, sink-float for microplastic selection). Moreover, densimetric separation tests were conducted to separate Oceanic Posidonia from sand or plastic waste. These tests were performed by varying the three parameters of the densimetric table: air flow, vibration, and inclination. Results Mola di Bari exhibited higher moisture content (65.08%) compared to Fasano (28.25%), resulting in lower dry matter in Mola (34.92%) versus Fasano (71.75%). Fasano also showed higher ash content (47.65%) relative to dry matter, compared to Mola (19.54%). pH was slightly alkaline in both samples (Fasano: 8.48, Mola: 8.15), with Mola exhibiting significantly higher conductivity (7952.27 μS/cm) and lower redox potential (-60.93 mV) than Fasano (-80.43 mV), indicating higher salinity in Mola. The raw waste was separated into natural organic, natural mineral, and anthropogenic fractions. At Fasano, the natural mineral fraction predominated (86.36%), while Mola's sample had a significantly higher organic fraction (78.13%). The anthropogenic fraction was minimal at Fasano (0.39%) but represented 5.1% in Mola. Granulometric analysis revealed Fasano's Posidonia was composed mainly of particles larger than 5 mm (D50 = 4.90 mm), while Mola’s fraction consisted of smaller particles (2 mm to 0.063 mm, D50 = 0.57 mm), falling within the sand class. Bioassay results showed a germination index of 100% for Mola and 90% for Fasano, both exceeding the inhibition threshold, indicating good vitality, significant germination capacity, and low toxicity, confirming the compost’s suitability for agricultural use. Sediments from Fasano were predominantly sandy (92%), with 8% gravel, D50 = 1.35 mm, while Mola’s sediments were almost entirely sand (99.99%), with a D50 of 0.35 mm. Germination indices for Fasano (96%) and Mola (85%) exceeded the inhibition threshold, confirming minimal toxicity. The plastic fraction in Fasano was dominated by PET (65.1%), followed by PS (21.7%) and POM (7.6%), with minimal presence of other plastics (PE, PES, PP, PA, PO, PVC), highlighting a potential risk of lightweight plastic pollution. The plastic composition in Mola di Bari differs significantly from Fasano, with PP (polypropylene) making up 43.41% of the total, followed by PET (polyethylene terephthalate) at 24.46% and POM (polyoxymethylene) at 14.01%. Other plastics like PA, PS, PES, and PE are present in smaller amounts. PVC represents just 0.14%. For the separation of 20% Posidonia – 80% Sand, the following results were observed by varying the vibration frequency (P) and air flow rate (V) while maintaining an inclination angle of 2°: • At P = 50 Hz, V = 55 Hz, Purity Posidonia was 54.41%, Purity of Sand was 100%, and the Efficiency was 89.78%. • At P = 50 Hz, V = 60 Hz, Purity of Posidonia reached 100%, Purity of Sand was 99.91%, and Efficiency was 99.81%. • At P = 45 Hz, V = 60 Hz, Purity of Posidonia remained 100%, Purity of Sand was 98.52%, and Efficiency was 87.04%. For the separation of 20% Plastic – 80% Sand, the separation results were as follows with V = 60 Hz and varying P: • At P = 50 Hz, Purity of Plastic was 33.0%, Purity of Sand was 99.0%, and Efficiency was 82.0%. • At P = 45 Hz, Purity of Plastic was 33.2%, Purity of Sand was 98.2%, and E was 84.0%. The best separation efficiency (greater than 85%) was achieved with an inclination of 2°, a vibration frequency of 45 Hz, and an air flow rate of 60 Hz, highlighting the critical role of optimizing operational parameters for effective separation. Conclusion This study provides a comprehensive analysis of the separation of organic (Posidonia oceanica) and anthropogenic waste from sand using densimetric methods. The results indicate significant differences in the waste composition between the two sampling sites, Mola di Bari and Fasano, with Mola exhibiting a higher proportion of organic fractions, especially Posidonia, and a greater plastic content compared to Fasano. The higher moisture content and salinity in Mola suggest different environmental conditions, which may influence the effectiveness of waste management strategies. The densimetric separation tests demonstrated that optimal separation efficiency for both Posidonia-sand and plastic-sand mixtures was achieved under specific operational parameters. The best results, with efficiencies exceeding 85%, were obtained with an inclination angle of 2°, a vibration frequency of 45 Hz, and an air flow rate of 60 Hz, highlighting the importance of fine-tuning these parameters for effective separation. These findings suggest that densimetric separation could be a promising technique for efficiently sorting organic and plastic fractions from beach waste, contributing to both the reduction of pollution and the recovery of secondary raw materials. Further studies are needed to refine these methods and assess their scalability and economic feasibility for large-scale waste management applications. Additionally, exploring the environmental implications of recycling beach waste, particularly in coastal ecosystems, could offer valuable insights into sustainable waste management practices in the Mediterranean region.