Urban Environmental Quality and Human Health: Conceiving a Sustainable Future

Death and Afterlife: Recycling?

In this section we will discuss the ultimate fate of the plastic bottles. This will also involve a project requiring that the students come up with creative ways to keep bottles out of the waste stream. At the moment I am thinking that might include everything from not buying the bottles to creative uses/recycled art projects for the bottles of a more permanent nature. This stage should also include a summary of the costs (primarily economic and environmental) of the production and disposal of the bottles. Our art teacher has been very successful in having her students do projects using found materials, which might be a good application. However, I would like to see the students do a project showing how to keep plastic out of the waste stream.

Recycling polyethylene terephthalate

An amazing array of technologies have evolved for the separation and processing of plastics. After the issue of finding a market for recycled plastics, the major issue with recycling plastics is sorting and separating plastic resins so that they may be reprocessed into new forms. The technology that has evolved for this purpose is a fascinating subject all by itself. The process of recycling can really be said to start in the birth and adolescent phases of the bottle's life. The more parts and materials that are added to the bottle, the harder it is to recycle the bottle at the end of its useful life. Paper or Plastic is a very good resource for looking at how design decisions can influence the ultimate end of a packaging material. Daniel Imhoff has included a number of very interesting case studies illustrating the benefits of considering the entire life cycle of a product in the design phases of packaging production. The PET industry has put together a very interesting collection of best practices for recycling PET, which describes much of the information found below. ¹⁶

Recycled PET has a number of uses. Since 1991 100% recycled PET has been used in quart and pint sized plastic containers for fresh fruits and vegetables. Since 1992 recycled PET has been used in the clamshell packages for bakery and deli goods with a layer of virgin PET coating laminated on to the food contact surfaces. Since 1993 recycled PET has sometimes formed the center of a "sandwich" of laminated layers used to make soft drink bottles with a layer of virgin PET lining the food contact surface of the bottle. In 1994 a soft drink bottle was allowed which contains 100% recycled PET. Additionally, recycled PET is used to make thermoforming sheets and films for blister packs, and hard plastic laundry scoops; shipping strapping for boxes; engineered resins for use in cars; and fiber for fleece, carpeting, and quilted material applications. ¹⁷ However, in order to be reused, PET must first be collects, sorted, and cleaned.

Collection

Once plastics have been used to the extent of their useful life, some plastics will be recycled. As discussed earlier, some plastics are more easily recycled than others. However, any plastic that is going to be recycled must be collected and brought to a central processing facility. In the case of PET this is done in several ways. PET is unusual in that many states have deposit laws which require stores to reclaim some or all PET soft drink bottles that they sell. The PET bottles, like soda bottles, which go into the redemption machines found in many states, yield a high purity recyclable material stream for the plastic recycling industry. However, not all states have mandatory redemption laws and not all PET bottles go through the grocery story redemption process. This means that the bottle must be collected in another way. The other two most common collection methods are curbside recycling and drop-off centers or transfer stations. The PET best practices document also mentions buy-back centers, but I have never seen one available to ordinary consumers, although they may be more common in other parts of the country. ¹⁸ Once the plastic is collected, it is generally sold to a plastics recovery facility of some sort; there are various types with minor variations in the work that they do, which are not largely relevant to this unit. However, if you are interested, I recommend reading the best practices document mentioned above.

Sorting ¹⁹

Sorting plastics can be done either manually, automatically, or manually and automatically. The method used depends on the source of plastic coming into the plastics facility, the cost of running different types of sorting machinery, and the needs or requirements of the used plastics consumer. In nearly all cases, incoming plastics will pass over a screen where loose debris is separated from the plastics. From there, the plastic containers will generally pass to a conveyer belt system and through the sorting process.

Manual sorting may be divided into two categories. Positive sorting is when the desired plastics, PET in this case, are taken off the conveyer belt and sorted onto another belt or into a hopper. This is like taking your own luggage off the baggage carousel at the airport. Negative sorting is when the desired bottle is left on the belt and undesirables are taken off the belt. Positive sorting is most commonly done when the input stream contains many types of plastics and typically results in a fairly pure stream of the desired plastics. Negative sorting is more common when the incoming stream is more homogenous and can result in a less pure stream when more varied plastics are used as the input. Occasionally, ultraviolet light is used to help separate PET plastics from PVC plastics because PET bottle tend to fluoresce blue in UV light while PVC bottles tend to fluoresce yellow or green in UV light. However, as this does not always hold true, for various reasons, it is not a foolproof technique, and it exposes workers to ultraviolet light, which sunscreen manufacturers will remind you causes sunburn.

Automated sorting systems almost always involve light and typically fall into three categories: optical sorting by color, optical sorting by electromagnetic transmission, and optical sorting by electromagnetic reflection or fluorescence. The electromagnetic energy used in the automated sorting machines is generally either near-infrared light, visible light, or x-rays. Visible light optical sorting machines are frequently used to sort out colored plastics from clear ones. X-ray transmission mechanisms pass x-rays through the bottle material to a sensor on the other side. A computer processes the amount of light that comes through the bottle to see if any radiation was absorbed by chlorine atoms, which would indicated that the plastic is PVC. X-ray reflection machines measure the amount of x-ray radiation bouncing back from the plastic, again to determine if any chlorine atoms are present, which would indicate that the plastic is PVC. Near infrared light transmission mechanisms shine a beam of light through the bottle and analyze the light that passes through the bottle. Every substance absorbs light slightly differently, so the computer is able to tell which type of bottle is being examined by the light which is able to pass through, or not be absorbed, by the bottle. The machine, then sorts the bottle according to its composition. The bottles are then either bailed or ground into small pieces called dirty flake.

Cleaning

Sometimes bottles will come to a processing facility already ground into pieces called dirty flakes. This is more common in cases where the bottles were fairly homogenous to begin with, like soda bottles coming out of a redemption machine at the grocery store. In other cases, the bottles have come from an initial sorting facility and have been ground into dirty flake for sale. In either case, the flakes must be cleaned before they can be reused.

Cleaning generally starts with a blast of fresh air. In the same way that wheat and chaff may be separated by a breeze blowing through the threshed grain, lighter bits of paper and undesirable tiny bits of plastic (fines), dust, dirt and other impurities can be separated from the desired flaked of PET. The PET flakes, then, take a bath in a detergent which removes further dirt, glue, labels and other nasty things stuck to the bottle pieces. The cleaned pieces are allowed to settle to the bottom while other plastics and bits of labels float on the surface. This step is sometimes done in a centrifuge instead. The flakes are allowed to dry and sometimes are passes through an electrostatic separator to remove bits of metal which are sometimes present in recycled tennis ball containers. If the flakes were not very pure coming into the facility or if the customer requires extremely pure product, the cleaned flakes are sometimes passed through one of the automated sorting machines described above. ²⁰ The clean, dried flakes are turned into the pellets described in the birth section above and can be used alone or with virgin PET to make new materials.

Class Activity

There are a number of possible classroom activities which could be used with this section of the unit. In my classroom, I will have my students do a project on keeping plastics out of the waste stream. As mentioned above, our art teacher is very good at having her students do projects using found pieces. I plan to have my students do some art work in a similar fashion using found plastics, either to show how plastics can be recycled and kept out of the waste stream or to show how plastics can be kept out of the waste stream by making them into pieces of art.

However, there are a number of other approaches to the topic. For example, students could do an interdisciplinary project lobbying a legislature to reduce the toxins in plastics used for foods. Students could do a project exploring what happens to plastic waste in their own community. Students could do a project aimed at increasing the recycling rate in their community or simply starting a recycling program in their own school. There are a number of resources in the next section which may be helpful for implementing these types of classroom activities. Be Creative!