ENVIRONMENTAL SUSTAINABILITY OF THE ENHANCEMENT OF PLASTIC MATERIAL RECOVERED FROM INDUSTRIAL LANDFILL WITH LANDFILL MINING

Abstract

Global material use, both renewable and non-renewable sources origin, has tripled since the 80s, thanks to the improved living standards. As a consequence, an ever-growing waste production has needed to be managed, trying at the same time to minimize environmental damages. 
The easiest way for waste disposal has always been represented by landfilling, even before landfill regulations have been enacted, with catastrophic consequences on the environment.
The growing difficulty in recovering raw materials has led to explore unconventional ways to obtain materials, leading to consider landfills as mines full of potential resources.
Landfill mining is a set of technologies that operates on landfills in order to recover the largest possible amount of matter and treat the waste contained in it.
Moreover, if applied to old thinking landfills, where containment barriers to pollutants are absent, landfill mining allows, together with the recovery of materials, also to restore polluted areas through decontamination and to secure the site, thanks to the realization of adequate containment barriers, capture and treatment systems for any leachate and biogas produced, also whenever the barriers or the environmental equipment are not long efficient. 
The purpose of the present study is to analyze, through the Life Cycle Assessment (LCA) methodology, the environmental impacts and benefits deriving from the valorization of plastic materials recovered with this type of intervention. In particular, the boundaries of the studied system include the treatment of waste recovered from an industrial landfill and the subsequent transformation of the separated plastic into secondary material through appropriate mechanical treatments.
The LCA analysis involved the landfill mining operation carried out by IREOS S.p.A. on the industrial landfill “Alumix” area of Portovesme, a hamlet of Portoscuso (CI), a site of about 8 hectares located in the southern part of Sardinia. In this landfill, in the 80s, industrial waste from the  metallurgical activities of the area were buried uncontrollably. 
The waste treatment processes consist in a 36-month processing period and consist mainly of the following steps: 
• excavation of waste from the landfill body; 
• mechanical sorting to remove bulky waste such as tires, wood, coarse iron; 
• pre-treatment to reduce the size of the material; 
• primary screening to separate the coarse materials from the fine ones;
• volumetric reduction, magnetic and gravimetric separation of fine fractions (wet treatments).
The water used in the wet treatments is treated in a clarification plant, chemical-physical treatment and sludge-pressing filter and, then, recirculated into the treatment plant.
Fine components such as plastic, aluminum, iron and coal are sent for material and energy recovery, while inert fractions that are not sold, such as sand, filter-pressed sludge, cathodes and anodes, are sent for disposal. In the study emissions from treatment are considered and calculated from concentration values measured in air. The plastic fraction, recovered at the end of the treatment and made up of 95% of HDPE, is sent to a processing plant, which, after grinding and extrusion, returns 20 the second material in the form of granules. Each phase of processing of the recovered plastic is accompanied by dust suction systems, whose magnitude has been hypothesized considering a percentage of the treated mass. Finally, a comparative LCA analysis was carried out between the production of the secondary granulate and a granulate material obtained from virgin HDPE described in a database process. The analysis of the comparison shows that damage of the production scenario of the HDPE granulate recovered from landfill (1,65E-4 Pt) is 81% lower compared to the damage of production of virgin HDPE granulate (9,1E-4 Pt).
In particular, the category Resources presents the highest reduction, equal to about 89%. In fact, in the case of virgin granulate production, the damage in the Resources category is mainly due to the crude oil resource (54,42%) which is the main resource for the production of virgin HDPE of the database process (in fact to obtain 1 kg of virgin HDPE 0,91 kg of crude oil are required).
Analyzing the production of the granulate with recycled plastic, it emerged that the greatest contribution to the damage is due to the energy used in the extrusion process (36,84%).
In conclusion, the environmental advantage of the production of secondary HDPE compared to virgin material consists precisely in the avoided extraction of crude oil for the production of the primary resource.