During the last few years, the term sustainability has evolved and developed worldwide, with companies in various industries placing more importance on corporate social responsibilities and focusing on going green with their processes. Saving resources and energy as well as reducing greenhouse gas emissions have gradually climbed their way up companies priority lists.

One such way companies have sought to achieve this is through recycling of material, such as paper, glass and plastics. This last material particularly has been a worldwide and longstanding concern for the environment due to its use. Plastics, used in virtually every industry as material for electrical components, coatings, or even packaging, are generally non-biodegradable, and when burnt in incinerators, emit greenhouse and other gases that are harmful to our environment. As such, more efforts have been made to recycle the material in order to reduce environmental impact.

PET PLASTICS

Every year, more than 500 billion PET bottles are blow-moulded. Logically, each of these is thrown away and the bottle gets recycled to food- or non-food-grade PET or reused for different purposes. Currently on average, about 40 percent of PET bottles are recycled, generally for use as non-food-grade PET such as in China wher they are processed into textile fibres or other utility items.

However, this percentage is slowly growing due to the increasing efforts of governments who ban landfill, disposal sites and waste incineration facilities from accepting PET material. Germany for example, is one such country that has already issued a ban for including plastics in landfill. Statistics show that with this decrease of landfills, the recycling quote increases.

Not only is this increased recycling quote better for the environment, it would also help companies reap additional benefits such as savings in costs and production processes.

Generally, PET provides good chemical resistance. It also has exceedingly high alcohol and essential oil barrier properties, and the degree of its impact resistance and tensile strength is also very good. Using recycled PET (rPET), all of these advantages and characteristics are preserved. In fact, rPET does not lose out in any way to new PET.

Further, according to a study from the Institute for Energy and Environmental Research, beverage manufacturers who use rPET for new bottles are able improve their ecological footprint considerably. Greenhouse gas emissions alone can be reduced by 69.5 percent when rPEt is used compared to crude-oil PET bottle production. This means that PET recycling reduces CO₂ emissions, saves energy, is gentle on renewable resources and has a reduced environmental impact when compared to nonrenewable resources.

FOOD-GRADE RPET

With all the advantages of recycling PET, this opens up myriad opportunities for food and beverage manufacturers to delve into with the use of rPET as packaging material for their products. However, the concern of these manufacturers remains: would rPET be considered safe by food safety standards?

With increasing focus on food safety both by the industry and consumers, ensuring that the rPET is food-grade material is but an essential step. This can be done through including thorough washing and decontamination in a bottle-to-bottle recycling system to produce food-grade flakes (resultant material from PET recycling).

PET WASHING

Washing the PET material is crucial in the recycling of PET for food-grade rPET.

The following is an example of a PET washing process in essence: 1. Pre-washing: Bottles pre-sorted by colour are first ground, and the resultant flakes are then pre-washed at light temperatures. Most of the sand and any entrained dirt particles are also mechanically removed by jets and friction. 2. Intensive Washing: Remaining labels and adhesives are detached in the subsequent caustic treatment zone. 3. Filtration System: The soiled caustic is treated such that it can be used again at a consistent level of quality. 4. Density Separation: A sink-or-swim process wher the lighter polyoefins (PO) from the bottle caps are separated from the heavier PET flakes in a density separator. 5. Multi-Stage Post-Washing: The flakes are then rinsed with hot water. 6. Drying: The washed flakes are dried mechanically and then thermally dried using a hot-air blower. 7. Colour-based Sorting: The flakes are once again sorted by colour and packed in bags once metallic residues (if any) are removed by magnetic separation.

PRE-WASHING

The first step of pre-washing is a particularly important step in the PET washing process, as it ensures the removal of all foreign materials in their entirety, principally labels and films.

Modern-day label materials are typically made of plastic films, often featuring some elaborate printing. While these definitely enhance the visual image of the products, they are categorised as contaminants in the washing process. If labels get as far as the wet washing section, the printing ink can be washed out of them, and come into contact with the PET when dissolved in the washing water. This has to be absolutely avoided if the rPET generated is to be deemed food-grade material. Moreover, another disadvantage of treating labels in the wet washing section is that the substances released there contaminate the wastewater.

Hence, the emphasis of the PET washing process should be placed on this first dry pre-cleaning step in order to ensure that only recyclable constituents of the input material (PET and PO) are processed in the wet washing zone.

A DOUBLE-FUNCTION

The overall washing process also integrates the return of used media. Hot-water rinsing for the flakes is based on a cascade principle, so the continuously treated water can be re-used several times, thus guaranteeing optimum quality of the media employed and rigorous resource-economy. In particular, the combination of intensive washing, caustic cleaning and multistage post-cleaning can lead to high-quality colours in the washed stock.

FILTRATION SYSTEMS FOR DEPENDABLE CLEANING

During wet cleaning of the recyclable constituents (PET and PO), a pollution load is formed. In order to maintain the requisite cleaning performance, this load has to be continuously removed from the process. One way is through filtration systems.

Without adding fresh water in the washing process, the requirements applying to recycling PET for food-grade applications cannot be met. With the aid of the filtration systems, the requisite quantity of fresh water can be utilised to maximum efficiency. In this way, this enables the contents of all the processs water basins to always be replaced, continuously and so rigorously, such that it would prevent contaminated water basins in the system, and keep the loading in the waste water at a very low level.

The contaminants concerned include suspended ink particles and adhesives which would be found as residues in the caustic of the cleaning process. In the event of inadequate separation, these may impair the cleaning effect. If the washing media are excessively contaminated, the PET flakes involved may be re-contaminated by the washing caustic.

In order to eliminate this risk, a membrane-based separation process that involves a cross-flow filtration can be introduced. This would involve the liquid to be cleaned flowing parallel to the membrane, and due to the overpressure inside the system, part of the caustic flows through the membrane. This permeate can then be fed back into the washing caustic.

Additional filtration units can also been integrated for arresting coarse dirt particles in order maximise the PET flake quality in bottle-to-bottle recycling.

With these, the PET washing process can operate cost-efficiently thanks to economical use of caustic soda and fresh water, and together with the minimised heat and energy losses, they ensure dependable cleaning of the PET flakes with the cleaning liquid filtered inline.

DEConTAMINATION PROCESS

once the PET washing process has been completed, the resultant flakes now require treatment so as to ensure its food-grade quality. With a drying reactor, the PET flakes can be adjusted to a uniform temperature level. This thus allows for short process times and a homogeneous temperature distribution in the treatment reactor.

As a result, this process uses a lower level of energy consumption compared to conventional technologies, rendering the decontamination process more cost-efficient. The flakes are decontaminated in a vacuum reactor without any excessive mechanical or thermal stress on the material.