**5. Disposal systems for polyethylene**

Polyethylene usage poses a significant threat to public hygiene and the aquatic environment. PE gets accumulated at dumping sites, waterways, gutters, drains, agricultural fields, residences, and roadsides, resulting in a refuse stockpile. The littering of the environment with polyethylene has made the disposal system very difficult. Governments, corporate organizations, and individuals are looking for a cleaner environment by developing different ways of disposing of the waste generated through polyethylene. Some of the proposed disposal systems include (1) recycling, (2) Composting, (3) Landfills, and (5) incineration [36].

#### **5.1 Recycling**

Recycling is one of the methods of reducing polyethylene waste littering in our environment. Polyethylene recycling is a process by which polyethylene waste is

collected, recovered, and converted into valuable products. These materials can be found in agriculture, packaging, food, pipes, beverage bags, toys, electronics construction, electronic and allied sectors [37, 38]. The most straightforward plastic recycling processes involve collecting, sorting, washing, shredding, melting, and pelletizing. The actual process varies, and this is based on the type of recycled product to be formed. The waste materials collection involves picking all the plastic bags and bottles from dumpsites, waste bins, riversides, homes, businesses, and institutions and gathering them at collection points, recycling facilities for reuse. These collected plastics are sorted into each polyethylene group, separated manually or with machines, and washed to remove impurities. Washing removes the impurities such as adhesives, labels, oil that affect operation. Then they are shredded into fine particles and are fed into various component machines for use. Also, shredded plastic pieces can be used for other applications, such as a binder for paving stones and block making and additive within the asphalt. Recycling involves three processes, namely mechanical, chemical and energy recovery processes [39].

The mechanical recycling process involves using cleaned, sorted, and shredded plastic granules or pellets to form new products like trash cans, toys, bottles, bags, and other reusable products. The primary recycling process involves the conversion of the cleaned shredded plastic waste into similar products using injection molding and extrusion principles. The quality of the manufactured product can be improved by the introduction of virgin raw materials, reinforcers, stabilizers and master batches to the scrap or plastic waste. The difference between the primary recycled product and that gotten from the virgin raw materials is in the quality of the manufactured products. Severino et al. [40] studied the effect of nanofillers and compatibilizers on the mechanical properties of extruded low-density polyethylene waste and the results showed enhanced properties when compared to ordinary LDPE waste. The primary and secondary types of recycling involve the reuse of the products in their original form and structure. **Figures 2**–**5** show the mechanical processing of used polyethylene films taking place at the University of Nigeria Nsukka, Enugu State, Nigeria. These films are used water sachets were used to make paving stones for the construction industry. Virgin polyethylene granules can be added here to improve the desired mechanical properties. This mechanical processing technique also involves the collection of plastic waste, sorting, cleaning, drying and reusing of plastic waste. The cleaned waste plastic can be cut into size, agglomerated and then extruded into pellets before being used to manufacture the desired products [41]. The pellets or granules can be fed into an injection molding machine, blow molding machine or an extruder to form different products. Also, the melt can be mixed with other materials to form varieties of composites.

It is in the secondary mechanical process that plastic blocks, paving stones and varieties of products are formed for construction and structural purposes. It can be used as partition walls in buildings for non-load bearing applications. Plastic waste has been used as aggregate in asphalt to improve the skid and crack resistance of pavements [42]. Kumi- Larbi and associates studied the effect of particle size of sand on some properties of water sachet/sand composites used as paving stones/ bricks and the results showed that the LDPE (water sachet) bonded sand showed an improvement in the durability, compressive strength, specific heat and thermal diffusivity of the bricks and can be used to as a substitute for cement in some building applications [43].

The third method or tertiary recycling of polyethylene products involves a chemical recycling process in which polymers are chemically converted to monomers or depolymerized to monomers and oligomers through a chemical reaction. The chemical inertness of the polymer and its variable structural nature has limited most studies on polyethylene to pyrolysis. The chemical recycling process is a

**Figure 2.** *Typical dumpsite where waste collection is being carried.*

**Figure 3.** *Sorting of the polyethylene film waste collected.*

process that reduces a polymer to its original monomeric form. The technology uses chemical reactions, heat or both to break down used plastics into raw materials for other chemicals, fuel or new plastics. PE can be processed through thermal or catalytic pyrolysis to produce monomers and small organic molecules. Thermal pyrolysis uses heat under high pressure to break down the PE to different smaller monomeric molecules while catalytic pyrolysis uses a catalyst to reduce both the reaction time and temperature required to break down the PE into its component
