Fragile and eternal at the same time, glass is one of the most interesting materials from the point view of recycling. Thanks to its physical and mechanical properties, it is a material that is not degraded during the recycling process and can be reused an almost infinite number of times. A bottle that is remelted in the furnace of a glassworks gives rise to another bottle, with the same qualities as the previous one; this total recyclability allows considerable energy savings in the melting phase. The collected glass undergoes an initial sorting to removal any polluting materials, to crush and remove any metal bodies, before moving on to manual sorting that removes any ceramic and metal traces still present. The material obtained is referred to as furnace-ready scrap glass or “cullet”. At this point the stages of glass production are followed including the mixing of sand (silica) with limestone, soda and additives plus a variable percentage of cullet (up to 90%). The materials are cooked in special furnaces up to 1,500°C to reach melting point. The vitreous mass obtained is sent to machines where it is blown into special moulds to be transformed into a new container. The use of cullet requires a lower melting temperature than virgin material and produces fewer atmospheric emissions. 1 kg of cullet produces 1 kg of new product. As a result, there are significant energy and material savings, equal to 25-30% and 100% respectively, while CO₂eq emissions are reduced by 40%.

If we look around, we can see that many of our everyday objects are made of an extremely versatile, light and also economic material: plastic. Toothbrushes, mobile phone covers, pens and markers, many computer and television components, all these objects and many others contain at least a little plastic. But there is not just one type of plastic. If we think about it, objects made of this material differ greatly- just compare a shopping bag with a detergent bottle and you will immediately notice many differences. In actual fact, the term plastic is an umbrella term for several families of polymers, which are long chains of molecules of high molecular weight and made up of a large number of molecular groups derived from the refining of oil and containing carbon, hydrogen, oxygen and chlorine. Each type of plastic corresponds to a different material with specific physical, chemical and mechanical characteristics. This heterogeneity calls for different recycling processes depending on the polymer or the family of polymers, so in this case it is not possible to speak generically of plastic recycling, because in reality there are so many different kinds of plastic. The most common and most widespread kinds used in our daily lives can be split into two large groups: thermoplastics, which soften with heat and then harden when cooled, and thermosets that solidify irreversibly if exposed to heat. Thermoplastic resins are the simplest to recycle and among these the most common categories in our daily life are:

PE, polyethylene, with which bags, bottles, containers, film, etc. are generally made, according to the type of processing it undergoes;
PP, polypropylene, used for a large number of different objects, from food trays to garden furniture;
PVC, polyvinyl chloride, for trays, film, pipes;
PET, polyethylene terephthalate, used for soft drink and mineral water bottles, synthetic fibres;
PS, polystyrene, mainly used for caps, plates, cutlery and food trays.

The recycling process can be either mechanical (more common) or chemical. First of all, in the case of mechanical recycling, the material collected during separate collection must be sorted, in order to identify and eliminate foreign substances and divide the different types of packaging both by polymer and by colour (where possible). It is essential to sort different plastic materials in order to increase the effectiveness of the recycling. The sorted material is then sent to the recycling line where it is shredded, washed, ground, dried and the final step where it is turned into granules or flakes which can be used in processing plants. Chemical recycling, on the other hand, is implemented on an industrial scale and looks to break the polymer macromolecule into its simplest building blocks (monomers), to be used as new raw materials. Granules and flakes can be used for different purposes depending on the starting polymer: for example from PET bottles it is possible to make fibres and textile materials (such as fleece blankets), from PE it is possible to make bottles and containers, while from PVC come drain pipes and electrical equipment. In Italy, it is only possible to recycle plastic packaging, but the rate is still high. Furthermore, unlike paper and glass, energy recovery can also be achieved with plastic (let’s not forget that plastic comes from oil!), given that its lower calorific value, i.e. the amount of heat released during combustion, is such as to justify this option (30-35 MJ/kg), which accounts for about 33% of the recovered packaging in Italy.
How much do we save by properly recycling plastic?
Energy savings are high, from 40 to 90%, with an average of 50%; while material savings can reach 100%! If we avoid sending plastic to landfills and recycle it correctly, we can avoid emissions to the sum of 1.39 kg CO₂eq per kg of plastic, thus helping to counteract the increase in carbon dioxide emissions into the atmosphere.

Wood is not one of the most common materials found in our sorted waste, but it is just as important as the others. Wood, like plastic, does not always have the same characteristics: there are indeed many types, which are used in different ways depending on its features. In any case, recycling wood is fundamental for two reasons: first of all, natural resources are conserved, as by recovering such material we cut down fewer trees; secondly, by avoiding sending wood to landfills, it prevents the emission of greenhouse gases such as methane and carbon dioxide into the atmosphere. As regards household separate waste collection, wood waste mainly concerns furniture, furnishings, doors, fixtures and various bulky items, while wooden packaging is only present in negligible quantities and includes crates for fruit and vegetables, valuable boxes for wines, liqueurs and spirits, small cheese boxes and corks, occasionally pallets. For this reason, wooden materials follow a separate path and are collected locally by agreement with the relevant organization, Rilegno in Italy, or taken personally to local collection centres provided for citizens. All wood can be recycled, and the resulting material is good quality. The wood waste collected mainly undergoes mechanical recycling: the material coming from various sources is sorted and cleaned of foreign bodies (metals, paper, various plastics, inert substances), then it is shredded into small flakes (chips) ready for use. These chips, after a drying process to lower moisture content levels, are then pressed, together with very low formaldehyde content glues, to make chipboard panels, with the same characteristics as the new ones used for producing furniture, home furnishings and interior and exterior cladding for homes and offices. 95% of wood waste ends up as chipboard. The rest is used for the production of cellulose pulp for paper mills or is treated to make it suitable for use as a raw material for the production of wood-cement blocks to be used in green building. A small part can be used in composting plants for the production of compost or fertilizer destined for large-scale retail. Finally, the waste can be transformed through various processes into solid fuel for incineration plants or for plants dedicated to the combustion of biomass suitable for the production of heat and energy. A further regeneration path is that of pallets, which can be distributed and reintroduced into the market.

Light and versatile, durable and easy to work with, aluminium is a metal with exceptional characteristics that make it particularly suitable not only for the production of cans, but also for car components and building products. Recycling aluminium is essential because its production from virgin material is a particularly painstaking process from a material and energy point of view: it is actually obtained from bauxite, a sedimentary rock, and 4 ton of bauxite and 14 MWh of electricity are required to get only 1 ton of aluminium. But what happens to our cans after we have thrown them into the same recycling bag/bin as plastic or glass? Aluminium is generally collected with other materials, such as plastic and glass, to reduce the costs of the collection. The first step in recovery is separating the cans from the other packaging, then these are crushed and separated from any ferrous waste. Afterwards the cans undergo a treatment at 500°C in order to remove paints and other substances on them, before being melted at 800°C to produce new materials. The advantages of aluminium recycling include (as in the case of glass) the fact the quality does not deteriorate during the process, a feature that allows this material to be recycled over and over, therefore providing a considerable energy saving (electricity savings are about 95%), given the particularly energy-intensive process of processing bauxite, and also material. Energy recovery is also possible: in fact, aluminium dust and sheets can be seen as fuels and, if heated to 850°C, 1 kg of aluminium releases 31 MJ of energy, the same energy released by 1 kg of coal. In this way, energy and resource savings are very high: 95% of energy saved and 100% of material.

What happens to a banana skin after we throw it away? If we wanted to conduct an experiment and leave it in the garden, we would notice that in a short time the skin would change and disappear bit by bit, leaving in its place a new organic substance that will be gradually absorbed by the soil. This happens because a banana is organic waste and biodegradable, just like kitchen food scraps and plant/tree cuttings and is therefore easily decomposed and transformed by saprophytic bacteria. As you can see, organic waste can also be recovered. How? Organic waste can be transformed into biogas through the anaerobic digestion process to produce fuel and/or electrical and thermal energy, or organic waste is transformed through biological treatment, composting, in order to recover the organic material present in it and obtain a new material called compost. Compost is not a fertilizer, but is defined as an organic soil conditioner, because it provides the soil with organic matter and nutrients (nitrogen, phosphorus and potassium), meaning fewer chemical fertilizers are needed. The process involves the decomposition of the organic substance under aerobic conditions, by microorganisms; or in the presence of oxygen, obtaining compost, CO2, water and heat as the main products of the reaction. It is a natural phenomenon that takes place through the insufflation of air and the regular turning of the material to speed up the process. Times for the production of compost vary according to the material or the period of the year but range from approximately 2 to 6 months. Microorganisms are the main proponents of the process, they come in many different strains (bacteria, fungi, algae, protozoa, etc.) and are usually naturally present in sufficient quantities in organic waste; it is important to remember that these microorganisms must have the optimal conditions so that they can perform their job correctly. Therefore, during the production of compost, it is important to pay attention to a few parameters: oxygen and sufficient porosity of the material to ensure circulation, the right amount of moisture and the correct carbon-to-nitrogen ratio. The starting materials that must be used, according to legislation, are the organic fraction of MSW, collected separately; vegetable waste from agricultural crops; sawdust, shavings, wood fragments; livestock waste; paper and cardboard (in small quantities); sewage sludge, municipal wastewater and unused, untreated wood waste. Hazardous waste and materials that have undergone chemical treatments are strictly forbidden, as well as inert substances, which would hinder the degradation process. In fact, it is very important that the compost does not contain pollutants, heavy metals and pathogens. During the composting process, the materials are properly mixed to obtain an optimal C/N (Carbon-to-Nitrogen) ratio, for example wet materials contain a low C/N ratio, while dry materials that create the structure have a high ratio. Composting consists of two main phases: a first phase of accelerated bio-oxidation (ACT, active composting time), where waste is highly putrescible, the metabolic process is very fast and consumes a lot of oxygen; a second phase of maturation, where the metabolic process is slowed down and oxygen consumption is reduced and, finally, any refining pre-treatments or post-treatments. Usage depends on the quality of the material: fertilizing the soil with a layer of compost (mixed with manure), mulching, landfill covering, etc. Composting can be done both on a domestic scale, with the small volumes of one’s own kitchen waste plus other selected materials, or on an industrial scale where large volumes are used and all physical and chemical parameters are properly monitored in order to obtain a quality compost that can be resold on the market. Domestic composting can be easily done with composters which come in different capacities (from 30 to 60 L usually) and which are readily available. The treatment of the organic fraction of waste produced in large areas is usually carried out within private and/or public centralized facilities.