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 recycling
Article
New Terahertz Wave Sorting Technology to Improve Plastic
Containers and Packaging Waste Recycling in Japan
Jeongsoo Yu 1, * , Xiaoyue Liu 1, *, Gaku Manago 1 , Tadao Tanabe 2 , Shiori Osanai 1 and Kazuaki Okubo 1

1 Graduate School of International Cultural Studies, Tohoku University, 41 kawauchi, Aoba-ku,

Sendai 9808576, Miyagi-ken, Japan
2 Department of Engineering and Design, Shibaura Institute of Technology, 3-7-5, Toyosu, Koto-ku,
Tokyo 1358548, Japan
* Correspondence: jeongsoo.yu.d7@tohoku.ac.jp (J.Y.); liu.xiaoyue.p4@alumni.tohoku.ac.jp (X.L.)

Abstract: Plastic product consumption and disposal are widespread. Given that these products are
derived from crude oil, it is critical to reduce their consumption and effectively recycle plastic waste
as recycled resources to achieve a low-carbon society. Japan enacted the “Containers and Packaging
Recycling Law” in 2000, encouraging the recycling of plastic packaging and containers. However,
material recycling of plastic waste has stalled due to recycling costs and technical challenges. This
paper examines and evaluates the transition process and operational status of Japan’s recycling policy
for plastic containers and packaging, as well as clarifies the limits of resources recycling. The limits
and issues of current identification technology are discussed by analyzing the characteristics of plastic
marks and detecting the material compositions of waste samples. The paper also discusses a new
sorting technology that uses terahertz waves to improve plastic recycling. This analysis revealed
that plastic containers are typically made of two or more plastic materials, which makes resources
recycling more difficult. Terahertz waves are safer than other high-accuracy sorting technologies
currently in use. Thus, material recycling can be expanded by accurately analyzing the composition
Citation: Yu, J.; Liu, X.; Manago, G.;
of plastic waste and introducing sorting devices appropriate for achieving the circular economy with
Tanabe, T.; Osanai, S.; Okubo, K. New
sustainable resource recycling.
Terahertz Wave Sorting Technology
to Improve Plastic Containers and
Keywords: plastic waste; resources recycling; sustainability; recycling policy; sorting technology
Packaging Waste Recycling in Japan.
Recycling 2022, 7, 66. https://
doi.org/10.3390/recycling7050066

Academic Editors: Michele John and 1. Introduction

Wan-Ting (Grace) Chen
Due to climate change caused by global warming, it has become evident that large-
Received: 31 July 2022 scale earthquakes, super typhoons, and wildfires occur frequently each year in various
Accepted: 7 September 2022 countries around the world, and to resolve these problems, countries around the world
Published: 9 September 2022 must immediately reduce their emissions of greenhouse gases. The Paris Agreement, which
Publisher’s Note: MDPI stays neutral
took effect in November 2016, requires all countries to submit and renew their greenhouse
with regard to jurisdictional claims in
gas emission reduction targets every five years. Japan announced in 2021 that it would
published maps and institutional affil- aim to achieve carbon neutrality by 2050, reduce its greenhouse gas emissions by 46% in
iations. the fiscal year 2030 from its fiscal year 2013 levels, and continue strenuous efforts in its
challenge to meet the lofty goal of cutting its emissions by 50% [1].
In the meantime, Japan adopted the Ocean Plastics Charter at the G7 conference held in
Canada in June 2018 and called on countries around the world to tackle the plastic problem
Copyright: © 2022 by the authors. in earnest with the goal of “Working with industry towards 100% reusable, recyclable, or,
Licensee MDPI, Basel, Switzerland. where viable alternatives do not exist, recoverable, plastics by 2030” [2].
This article is an open access article In addition, according to the Ministry of the Environment (MOE), the rising demands
distributed under the terms and for resources and energy, as well as the amount of waste generated, are becoming increas-
conditions of the Creative Commons ingly problematic on a global scale. The transition to a “circular economy” that aims
Attribution (CC BY) license (https://
for the sustainable circulation of resources becomes a global trend. Under the various
creativecommons.org/licenses/by/
recycling systems established for the formation of a recycling-oriented society, Japan aims
4.0/).

Recycling 2022, 7, 66. https://doi.org/10.3390/recycling7050066 https://www.mdpi.com/journal/recycling

Recycling 2022, 7, 66 2 of 16

to promote reduce, reuse, recycle (the 3Rs) and the circular economy through cooperation
among different stakeholders. In particular, Japan’s Annual Report on the Environment
emphasizes “materialization of the plastic resource recycling strategy” and “international
cooperation for solving the marine plastic waste problem”, demonstrating a high level of
concern for the waste plastic problem [3].
Inexpensive, easy-to-mold, and highly convenient plastic products are being consumed
and disposed of in large quantities, mainly in developing countries. Given that these
products are derived from crude oil, it is crucial to reduce the consumption of plastic
products and effectively recycle waste plastics as recycled resources to achieve a low-
carbon society and the circular economy. However, the COVID-19 pandemic has led to
an increase in the use of single-use plastic containers, home delivery packaging, plastic
shopping bags, etc. In 2021, when Sapporo City declared a state of emergency, changes in
environmental consciousness began to be seen, such as an increase of approximately 12%
in plastic containers and packaging and other plastic waste [4].
Japan’s plastic containers and packaging recycling policy is based on EU policy (Euro-
pean Parliament and Council Directive 94/62/EC of 20 December 1994 on packaging and
packaging waste), and Asian countries often refer to Japan’s recycling policy. However, it
is a unique and complex recycling system, and to achieve the goal of sustainable resources
recycling, many issues must be resolved, including improvements and developments in
policy, recycling technology, resources recycling efficiency, and the enhancement of en-
vironmental awareness. Japan then finally signed the agreement at the G20 meeting in
Osaka. Furthermore, looking ahead beyond those initiatives and existing actions by each
member, it shares and calls on other members of the international community to also
share, as a common global vision, the “Osaka Blue Ocean Vision” that we aim to reduce
additional pollution by marine plastic litter to zero by 2050 through a comprehensive
life-cycle approach that includes reducing the discharge of mismanaged plastic litter by
improved waste management and innovative solutions while recognizing the important
role of plastics for society. They also endorsed the “G20 Implementation Framework for
Actions on Marine Plastic Litter” [5].
In this way, the problem of plastic waste is an important issue for solving the carbon-
neutral and circular economies that are attracting global attention. Japan has established
recycling policies since the mid-1990s to create a recycling-based society. In 2000, Japan
enacted the “Law for Promotion of Sorted Collection and Recycling of Containers and
Packaging”, which encouraged the recycling of plastic containers and packaging. However,
material recycling of waste plastics has stalled due to recycling costs and technical obstacles.
There are numerous studies on the proper treatment and recycling of waste plastics,
most of them are comparative analyses of plastic types and treatment methods with the
specific case. It is noteworthy that Maga, D., Hiebel, M., and Aryan, V. (2019) conducted
an environmental impact assessment based on differences in tray materials and waste
management policies in EU countries [6]. Changes in recycling policies influence the
selection of materials for containers and packaging by businesses, and as a result, the
environmental impact of material changes will also shift. Alhazmi, H., Almansour, F., and
Aldhafeeri, Z. (2021) conducted a comprehensive review of LCA studies on plastics and
highlighted the need for environmental impact assessment from a long-term perspective as
an issue with existing LCA studies on plastics [7]. The study by Jang, Y.C., Lee, G., Kwon, Y.,
Lim, J.-h., and Jeong, J.-h. (2020) and other LCA studies of plastic recycling (MFA: material
flow analysis case studies) that have been conducted on waste treatment methods and
carbon dioxide emissions of recycling technologies are frequently compared and analyzed,
or scenarios are analyzed under various conditions of materials and waste management [8].
However, it is difficult for the results of environmental impact assessments based on limited
conditions to provide clear evidence for policy recommendations or the development of
new technologies.
A study by Meert, J., Izzo, A., and Atkinson, J.D. (2021) analyzed the effects of
municipal recycling policies on the impact of plastic bag bans on retail return polyethylene
Recycling 2022, 7, 66 3 of 16

in New York City. This paper examines the effects of plastic bag recycling before and after
the implementation of plastic bag bans. It is commendable that the policy effects were
analyzed by collecting, sorting, and classifying plastic waste. However, it is difficult to
assert that this analysis and evaluation of plastic materials are accurate [9].
On the other hand, there are many research results on comparative analysis of recycling
technologies for waste plastics; however, no revolutionary recycling technologies have
been developed in the last few decades, and the majority of these studies compare material
sorting technologies and material recycling versus chemical recycling. Ragaert, K., Delva,
L., and Van Geem, K. (2017) presented a structured analysis of the characteristics of chemical
recycling and mechanical recycling. Particularly, for mechanical recycling, they discussed
the limitations and significance of sorting technology, whereas for chemical recycling, they
presented the most recent technology and outlined future possibilities [10]. Similar to this
paper, Beghetto, V., Sole, R., Buranello, C., Al-Abkal, M., and Facchin, M. (2021) examined
the current status and recycling technologies for plastic containers and packaging waste.
They provide an overview of the latest trends in polyethylene (PE), polypropylene (PP),
polyethylene terephthalate (PET), and polystyrene (PS) [11]. Furthermore, Khalid, M.Y.,
Arif, Z.U., Ahmed, W., and Arshad, H. (2022) present specific recycling technologies and
case studies based on a vast amount of the most recent data. Not only does the paper
provide a comparative analysis of recycling technologies, but it also presents various
applications of recycled plastics and considers recycling as well as reuse perspectives [12].
Thus, sorting technology for waste plastics is crucial not only for material recycling, but
also for chemical recycling, and is effective in energy recovery for controlling air pollutants.
According to previous research, the majority of studies addressing the issue of waste
plastic focused on recycling policies and technologies, analyzing the relationship between
policies and technologies and environmental impacts. However, no studies have proposed
a comprehensive waste recycling policy based on the analysis of changes in recycling
policies and actual conditions, the examination of the characteristics of plastic contain-
ers and packaging waste, the discussion of recycling issues, and the proposal of new
sorting technologies.
This paper examines and analyzes the transition process and operational status of
Japan’s plastic container and packaging recycling policy, and clarifies the limits of sustain-
able resource recycling. By analyzing the characteristics of plastic marks of 1439 household
plastic containers and packaging samples, and detecting material compositions of 3.22 kg
sorted container and packaging waste, it identifies obstacles to waste plastic recycling and
corrective measures in recent recycling economy trends. In addition, by verifying and
comparing technologies for sorting plastic wastes, this study introduces and discusses a
new sorting technology using terahertz waves to improve plastic resource recycling, and
presents the possibility of sustainable resource recycling by proposing a new plastic sorting
technology. Through discussions and research on recycling policies, waste characteristics,
and sorting technologies, new possibilities for building the circular economy with sustain-
able resource recycling are presented. The originality of this study lies in the use of plastic
waste samples from households and recycling spots to evaluate local characteristics and the
current status of plastic waste disposal and recycling, as well as to validate the application
of the terahertz technology in plastic waste recycling.

2. Plastic Recycling Policy and Its Actual Performance

2.1. Characteristics of Plastic Container and Packaging Waste Recycling System in Japan
2.1.1. Circular Economic Trends in the EU
The idea of the circular economy appeared in the 6th Environment Action Programme
(6EAP) in 2002. Resource-Efficient Europe in 2011, which embodies this plan, introduced
the concept of “waste to resources”. Strategies for 2020 include expanding demand for
recycled resources and revitalizing the market by giving economic incentives, clarifying
the criteria for “final disposal as waste”, transitioning from landfill to reuse and recycling,
Recycling 2022, 7, 66 4 of 16

considering standards for recycled resources contained in products, establishing standards

for durability and reusability, and expanding producer responsibility [13,14].
Closing the loop—An EU action plan for the Circular Economy, in 2015, emphasized
the “life cycle approach” and cited plastics as one of its top priorities. Plastics-related
life cycle approaches include eco-design (durability, recyclability, etc.), extended producer
responsibility (EPR) in product design, acceleration of the recycled materials market, and
establishment of quality standards [15]. Following the Circular Economy Plan in 2015, A
European Strategy for Plastics in a Circular Economy was announced in 2018 [16]. The
objectives are to reduce disposable plastics through regulatory measures and to recycle
more than half of plastic containers and packaging. The numerical target is to commercialize
10 million tons of recycled plastic (nearly 40% of EU plastic waste) annually in the EU
market by 2025. As one of the methods to realize this strategy, the idea of environmental
regulation (eco-modulation) in EPR charges is drawing attention. It allows producers to
pay different amounts depending on how easily the product can be reused or recycled [17].
Furthermore, the new Circular Economy Action Plan, revised in 2020, outlines action
plans for creating value from key product value chains and waste. It is encouraged that
plastics-related key products, such as plastic containers and packaging, construction and
buildings, and automobiles, should be designed to be recyclable and manufactured with
recycled materials [17,18].

2.1.2. Individual Recycling Laws and Container and Packaging Recycling in Japan
The Containers and Packaging Recycling Law in Japan was promulgated in 1995,
partially enforced for cans, bottles, and PET bottles in 1997, and fully implemented in 2000,
including plastic containers and packaging waste. In particular, plastics accounted for
39.4% by volume (8.2% by weight) of the containers and packaging waste at the beginning
of revision (2006) [19]. Furthermore, in 2021 after the COVID-19 pandemic, the proportion
increased to 50.4% (12.8% by weight) [20].
In 1971, during the period of high economic growth, and since the “Waste Management
and Public Cleansing Act” was enacted to reduce the volume of waste generated in large
quantities aiming for hygiene and non-pollution, the general waste administration has
maintained this policy for more than 20 years. However, since the above-mentioned
container and packaging recycling policies were fully enforced in 2000, individual recycling
laws have been introduced one after another to form a recycling-based society. Starting
with the “Basic Act on Establishing a Sound Material-Cycle Society” in 2001, the “Law for
the Promotion of Effective Utilization of Resources”, “Home Appliance Recycling Law”,
“Food Recycling Law”, and the “Act on Promoting Green Procurement” were enacted. To
build a recycling-based society where the consumption of natural resources is suppressed
and environmental load is vigorously reduced, the 3R Initiative has been comprehensively
promoted based on the individual recycling systems that incorporate the concept of the
3Rs. The “Construction Material Recycling Law” was enforced in 2002, the “Automobile
Recycling Law” in 2005, and the “Small Home Appliance Recycling Law” in 2013 [21].
It can be said that the Containers and Packaging Recycling Law was a catalyst for
promoting 3R policies, but the relationship between consumers, local governments, and
producers is complicated, and a reexamination is necessary to achieve a circular economy.
Although it was revised once in 2006, the same recycling policies and methods have been
maintained for many years despite the rapid increase in and diversification of plastic waste.
To study the revisions of the container and packaging recycling system, Yu (2006) suggests
that by thoroughly analyzing the characteristics, environmental impact, and recycling
costs of each container, and accumulating and disclosing the information, clearer policy
decisions and fairer responsibility-sharing can be made [22]. With the enactment of the “Act
on the Promotion of Resource Circulation Related to Plastics” in April 2022, sustainable
resource circulation has ushered in a major turnaround. However, to improve recycling
efficiency and determine the priority of recycling technologies, a lot of data collection and
Recycling 2022, 7, 66 5 of 16

data analyses are needed, and the issues to be discussed based on the analysis results
(evidence) have piled up.

2.1.3. Transition of Container and Packaging Waste Recycling System

Like Germany’s “Packaging Waste Decree (1991)” and France’s “Packaging Waste
Decree (1992)”, after EU countries started to recycle containers and packaging waste, the
EU adopted the “EU Containers and Packaging Recycling Directive” in 1994. After that,
Japan and South Korea developed their container and packaging recycling systems by
referring to the systems of Germany and France, with the former formulating a “French-
type” system that emphasizes partnerships between industry and local governments,
and the latter formulating a “German-type” system that clarifies the responsibilities and
roles of industry [21]. That is, in Japan, producers bear part of the recycling costs based
on the extended producer responsibility, while the actual collection, transportation, and
intermediate treatment (sorting, compressing, packaging, and storage) of containers and
packaging waste are performed by local governments. In 1997, there was a rapid increase in
the amount of PET bottles, which were not only light and easy to form, but also convenient
to keep and carry. On the other hand, however, the generation of containers and packaging
waste and the increase in disposal costs became a social problem. Later, in 2000, with the
implementation of “Basic Act on Establishing a Sound Material-Cycle Society”, the scope
of containers and packaging recycling was expanded to include plastic containers and
packaging. According to a survey by the Ministry of Economy, Trade, and Industry, about
76.3% of municipalities in Japan are implementing the classification of plastic containers
and packaging [23].
In the case of Sendai City, in response to the full enforcement of the Containers
and Packaging Recycling Law in 2000, a model project for sorted collection of plastic
containers and packaging other than PET bottles was implemented, and comprehensive
sorted collection has been carried out throughout the city since 2002. Most of the large cities
in Japan have built cleansing plants (waste incineration plants), aiming for reducing the
volume of a large amount of general waste and for sanitary treatment. Of course, instead
of simply incinerating, energy recovery is implemented by using the waste heat as a heat
source in the factory, generating electricity, or operating a heated swimming pool. Further,
to improve the energy recovery efficiency, the higher the energy potential of general waste
as the heat source of waste incineration, the better. In other words, lots of waste plastic
having a high calorific value should be contained. However, it is estimated that the start
of the sorted collection of plastic containers and packaging waste will reduce the calorific
value of waste by about 12% and the income from electric power selling by about 10% in
the case of Sendai City [24]. On the other hand, if the amount of recycling (materials and
chemical recycling) of plastic containers and packaging increases, it can be expected to
reduce air pollutants such as carbon dioxide and hydrochloric gas. Thus, if the relationship
between existing waste treatment facilities and recycling is not fully studied in terms of
environmental impact and economic evaluation, inefficiencies would be likely to occur in
waste management administration.
In 2006, the “Revised Containers and Packaging Recycling Law” was enacted. In
this amendment, the Containers and Packaging Recycling Law was revised from the
aspects of promotion of reduce, reuse, recycle (the 3Rs) for containers and packaging waste,
improvement of the efficiency of recycling costs for the whole society, and cooperation
among the state, local governments, producers, consumers, and other stakeholders. Due
to the soaring crude oil prices at that time, it was believed that the PET bottles would
flow overseas. To recycle containers and packaging waste in Japan, content so that the
containers and packaging waste can be smoothly delivered from municipalities to the
designated corporations has been added to the law. In addition, the fine for “free riders”
was raised. In April 2007, efforts to curb emissions were promoted and fuel conversion was
recognized as a recycling method of plastic containers and packaging. The following year,
a mechanism was created to contribute funds to businesses or municipalities, to improve
Recycling 2022, 7, 66 6 of 16

the efficiency of recycling costs of containers and packaging waste, support the sorted
collection in municipalities, and ensure high-quality renewable resources [25].
In the “Resource Circulation Strategy for Plastics” formulated in May 2019, to promote
plastic resource recycling, thorough implementation of reduction was positioned as one of
the key strategies. In the strategy, the necessity of mandatory charging for plastic shopping
bags (prohibition of free distribution, etc.) was presented. To achieve it, the “Method
of Charging for Plastic Shopping Bags (Draft)” was summarized. In addition, based on
this, the “Ministerial Orders related to the Act on the Promotion of Sorted Collection and
Recycling of Containers and Packaging” was revised [26].
In June 2021, to promote the resource recovery of plastics, stakeholders in the whole
process from the design of plastic-based products to the disposal of plastic-based product
waste established the “Act on the Promotion of Resource Circulation Related to Plastics
(commonly known as the New Plastic Law)”, and put it into effect on 1 April 2022. The law
aims to accelerate the transition to a circular economy through “3Rs + Renewable” through-
out the life cycle of plastics. Therefore, at the design and manufacturing stages, the design
of plastic products would be converted to eco-friendly types, and the government would
certify and support related products. Then, it would promote the reduction in disposable
products at the sales and provision stages, and admit the collection routes not only by
local governments but also by manufacturers and dischargers at the discharge/collection
and recycling stages. On this basis, including existing plastic containers and packaging,
the efficient collection and recycling of all types of plastic have been promoted [27]. The
transitions of the Containers and Packaging Recycling Law are shown in Table 1.

Table 1. The transition of Containers and Packaging Recycling Law [28].

Year Action and Contents of Laws

1995 Container and Packaging Recycling Law enacted
1997 Containers and Packaging Recycling Law partially enforced (bottles, cans, PET bottles, etc.)
2000 Containers and Packaging Recycling Law fully enforced (paper containers and packaging, plastic containers and packaging)
2006 Revised Containers and Packaging Recycling Law enacted
2018 Revised Containers and Packaging Recycling Law fully enforced (mechanism for business entities to disburse funds to municipalities)
2019 Resource Circulation Strategy for Plastics enacted
Ministerial Orders related to the Act on the Promotion of Sorted Collection and Recycling of Containers and Packaging revised
2019
(introduction of fee-incurring plastic checkout bags)
Act on the Promotion of Resource Circulation Related to Plastics enacted (increase collection items, charge for single-use products,
2022
diversify collection routes)

2.2. Analysis of Current Status and Issues of Plastic Containers and Packaging Recycling
2.2.1. Impediments to Sustainable Resource Circulation
With the implementation and revision of the Containers and Packaging Recycling
Law in 2000, municipalities increased their public relations and education efforts, and
citizens’ environmental awareness improved, so the containers and packaging recycling
system contributed to the reduction in waste to a certain extent. In Japan, mostly material
recycling, energy recovery, and chemical recycling are used for the treatment of plastic
waste. Collected plastic wastes are washed, sorted, pelletized, and utilized as a raw material
for material recycling. Considering the priority of the 3Rs, material recycling should have
been prioritized originally. However, as shown in Figure 1, energy recycling shows the
highest recycling rate, whereas material recycling is on a declining trend and has remained
flat at about 21% in recent years (2015–2020) [29].
Easy-to-sort plastic containers and packaging, such as PET bottles and trays, are
relatively advanced in terms of material recycling. In the case of other plastics, numerous
dirty or composite materials are difficult to sort out, making them challenging to recycle.
Since composite materials are a feature of containers and packaging waste in Japan, it is
difficult to recycle them as single materials.
On the other hand, under the Containers and Packaging Recycling Law, the recycling
system is composed of consumers (sorted disposal), municipalities (sorted collection), and
 mained flat at about 21% in recent years (2015–2020) [29].
Easy-to-sort plastic containers and packaging, such as PET bottles and trays, are rel-
atively advanced in terms of material recycling. In the case of other plastics, numerous
dirty or composite materials are difficult to sort out, making them challenging to recycle.
Since composite materials are a feature of containers and packaging waste in Japan, it is
Recycling 2022, 7, 66 7 of 16
difficult to recycle them as single materials.
On the other hand, under the Containers and Packaging Recycling Law, the recycling
system is composed of consumers (sorted disposal), municipalities (sorted collection), and
business
business entities
entities (recycling) [30]. Municipalities
(recycling) [30]. Municipalitiesbear
bearthe
the collection
collection cost,
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entities should
tities should paypay
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recycling. Currently,
Currently, over over 70%
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ing and separating plasticplastic containers
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and packaging wastewaste
[31]. [31].

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waste. Additionally, compared
compared to 2004, the cost
to 2004, of collection
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containers waste has
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increased 2) [32].
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municipal costs
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the most important process
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Recycling 2022, 7, x FOR PEER REVIEW
In other words, since the burden on municipalities has reached its limit, it is 8 of to
desirable 17
waste resources. In other words, since the burden on municipalities has reached its limit,
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it is desirable torecycling
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the
recycling system because of the current cost burden on the municipalities. These factors
are considered the impediments to sustainable resource recycling of plastic packaging and
containers.
Recycling 2022, 7, 66 8 of 16

recycling system because of the current cost burden on the municipalities. These factors
are considered the impediments to sustainable resource recycling of plastic packaging
and containers.

2.2.2. Significance and Limits of the Plastic Resource Circulation Act

The impact of marine litter on the ecosystem is becoming more serious, so it is essential
to restrict the use of plastic products and promote their collection and recycling on a
global scale. To address climate change issues and realize a genuinely zero-carbon society,
the conversion of fossil resource-derived materials to alternative materials is necessary.
Furthermore, restrictions on importing plastic waste have been tightened in China and
other countries, so the domestic circulation of plastic resources is necessary. Based on
this background, Japan enacted the New Plastic Law to promote the resource recycling of
plastics further.
This new law imposes an eco-friendly design in the manufacturing process that
contributes to reducing the generation of plastic waste and promoting recycling of materials.
That is, developing guidelines for designing plastic products and promoting the use of
single materials and easy recycling design will allow for the material recycling of plastic
containers and packaging waste [34].
Collected household waste currently varies according to the population, the age range,
and the environmental efforts of municipalities. Therefore, obtaining a nationwide unified
standard (such as type of resources and collection methods) for implementing sorted
collection is challenging. The new law allows municipalities to take advantage of their
autonomy and permits them to utilize different collection routes for plastic containers and
packaging. Furthermore, it allows multiple municipalities to collaborate and encourage
recycling. This kind of mechanism aims to increase the number of household plastic
resources recovered.
On the other hand, this law has established a certification system for manufactur-
ers, distributors, and service providers to voluntarily collect used plastic products. By
increasing collection routes, more plastic products and single materials can be collected
while shopping, and the recycling rate can be improved. It is anticipated that promoting
the voluntary collection will diversify collection routes and encourage the collection and
recycling of plastic products. The processing costs for plastic containers and packaging
have increased as a result of import regulations, and the utilization of voluntary collection
routes may reduce the cost burden on municipalities.
Although this new recycling law encouraged individual stakeholders to take part in
the recycling system, such as the Small Home Appliance Recycling Law, it is not mandatory.
At the same time, it can be supposed that municipalities will expand the number of waste
items to increase the amount of collected plastic waste. However, as plastic products
become more complex and diverse, how to sort the recyclable materials that can be recycled
is considered an important issue.

3. Local Characteristics of Plastic Containers and Packaging Waste

3.1. Collection Area and Sample Size of Plastic Containers and Packaging Waste
Under the support of a major recycling company in the Tohoku region of Japan, plastic
containers and packaging waste samples (1439 items) were randomly collected from household
plastic waste in five areas based on the population: Aomori City, Goshogawara City, Hiranai
Town in Aomori Prefecture, Koriyama City, Aizu Wakamatsu City in Fukushima Prefecture.
Regarding the characteristics of the five areas, Aomori City is the prefectural capital
with a population of about 270,000 and has developed into an important transportation
and logistics hub connecting Hokkaido. Goshogawara City is located in the northwestern
portion of Aomori Prefecture with a population of 50,000, and its agriculture and fishing in-
dustries are thriving. Hiranai Town is town with a scallop aquaculture industry. Koriyama
City is located in the center of Fukushima Prefecture and is the third-largest city with a pop-
ulation of 320,000. Aizu Wakamatsu City is situated in the western portion of Fukushima
Recycling 2022, 7, 66 9 of 16

Prefecture and is home to numerous well-known tourist destinations. Populations and the
number of samples in the five areas are shown in Table 2.

Table 2. Population and sample size in studied areas.

Area Population Sample Size

Aomori City 272,752 [35] 247
Goshogawara City 52,057 [36] 346
Hiranai Town 10,308 [37] 263
Koriyama City 164,332 [38] 264
Aizu Wakamatsu City 115,009 [39] 319

3.2. Types and Regional Differences of Plastic Containers and Packaging Waste in Five Areas
To analyze the material compositions of the samples, Fourier transform infrared
spectroscopy (FTIR), which can read and analyze the absorption spectrum of infrared
rays specific to the material being measured, was used to separate single materials and
composite material. Samples of single materials were separated into PP, PE, PS, polyvinyl
chloride (PVC), and PET.
The results are shown in Table 3. It can be seen that PP, usually used for packaging
of bread and sweets, shares about 30% of the total, indicating that it is widely used as a
material for plastic containers and packaging. There are regional differences in PP emissions,
with Hiranai Town emitting the most emissions compared to other cities. Assumably, this
is because sashimi and sushi trays are frequently discarded in Hiranai Town, which has a
thriving fishing industry. PVC material was not detected in all regions, which means that
PVC is not commonly used in household containers and packaging. Even though the lids
and containers of lunch boxes are usually made of PET material, the proportion of PET
does not exceed 10%. Composite materials often have different materials on the front and
back of the packaging, such as PP on the front side and PE on the back side. The ratio of
composite material containers and packaging waste is about 25% of the total. This indicates
that composite materials are commonly used. The arterial industry, which is the producer
of plastic containers and packaging, uses composite materials to ensure the quality and
designs of products to increase consumers’ purchasing motivation, and recycling of the
venous industry has not been considered.

Table 3. The proportion of sample size by different material compositions in studied areas.

Area PP PE PS PVC PET Composite Material and Unknown

Aomori City 29% 23% 21% 0% 4% 23%
Goshogawara City 34% 15% 17% 0% 9% 25%
Hiranai Town 31% 13% 27% 0% 6% 23%
Koriyama City 31% 19% 22% 0% 8% 19%
Aizu Wakamatsu City 24% 23% 22% 0% 4% 26%
Total 30% 19% 22% 0% 6% 23%

3.3. Plastic Marks and Material Labels of Plastic Containers and Packaging Waste
In Japan, the law (“Act on the Promotion of Effective Utilization of Resources”) requires
that containers and packaging be labeled with an identification mark. However, some of
the samples did not have any plastic marks. This was usually the case with plastic bags
and packaging materials used for international online shopping. In addition, it was not
possible to analyze all the plastic marks on all of the samples because some of them were
broken when they were collected. As shown in Table 4, 688 samples (about 48% of total
samples) were able to confirm the plastic marks. Material labels on samples were checked as
well. Material labels are a way to inform consumers about the material composition of the
product and to encourage recycling. Of the 688 samples, 298 (43%) had material labels. In
other words, less than 50% of the remaining samples have only the plastic mark, indicating
Recycling 2022, 7, 66 10 of 16

that the material labeling is left to the discretion of each manufacturer. This highlights the
ambiguity of this Law for Promotion of Effective Utilization of Resources Recycling.

Table 4. Sample size and proportion of plastic marks and material labels.

Area Sample Size with Plastic Marks Sample Size with Material Labels Proportion of Material Labels
Aomori City 138 53 38.4%
Goshogawara City 196 104 53.1%
Hiranai Town 121 61 50.4%
Koriyama City 126 31 24.6%
Aizu Wakamatsu City 107 49 45.8%
Total 688 298 43.3%

3.4. Evaluation of Plastic Containers and Packaging Waste and Near Infrared Sorting Technology
On the technical side, existing near-infrared devices are utilized to sort plastic contain-
ers and packaging waste. These near-infrared devices are widely used by all of the recyclers
in Japan to separate waste plastics. Measuring the waste plastic’s absorption spectrum by
reflection can test the material. The identified materials are sorted and recycled as pellets
or ingots based on their composition. The recycled material is reportedly traded at a lower
price than virgin material due to its high impurity. Therefore, it is a problem that impurities
are mistakenly mixed in with the sorted materials using near-infrared devices. In this study,
a group of sorted PP material (1.36 kg) and a group of sorted PE material (1.86 kg) were
randomly picked out from the recycling process to estimate the near-infrared technology.
To detect the composition of plastic materials, all sample materials are identified using the
FTIR device.
The results (Table 5) showed that the sample identified as PP contained approximately
40% impurities, with a near-infrared device accuracy of approximately 60%. The PE group
was found to consist of approximately 70% PE and 5% PP. These results indicate that the
near-infrared device can sort plastic containers and packaging waste with an accuracy of
60–70%. On the other hand, 30–40% of different materials are mixed in, and this is one of
the reasons why recycled pellets are less expensive. It is considered that this near-infrared
method can be used to produce high-quality recycled pellets by enhancing the accuracy.

Table 5. The weight and its proportion of materials sorted by near-infrared device [40].

Group The Content Rate of PP The Content Rate of PE The Content Rate of Other Materials
PP group 58.8% (0.8 kg) 8.8% (0.12 kg) 32.4% (0.44 kg)
PE group 5.4% (0.1 kg) 69.9% (1.3 kg) 24.7% (0.46 kg)

3.5. Proposal for Advanced Sorting of Plastic Containers and Packaging Waste
3.5.1. Evaluation of Identification of Terahertz Waves
It has been confirmed that 0.12 kg (78 items) of PE and 0.1 kg (28 items) of PP are
misclassified by near-infrared devices. The inclusion of these misclassified different materi-
als diminishes the quality of recycled products made from plastic wastes. In addition to
near-infrared sorting equipment, the authors propose terahertz sorting. Misclassified PE
and PP were exposed to two terahertz waves and their transmittance was measured.
The results are shown in Figure 3. Since 95% of PP items are plastic films, the transmit-
tance rate approaches 90% for both terahertz waves. Even for PP trays, the transmittance
exceeds 80%. For PE items, the transmittance rate of PE films is over 90%. However, unlike
PP items, PE bottle caps and trays are detected with transmission rates below 80%. If
the discriminating threshold between PP and PE by terahertz waves is set below 80%,
approximately 37% of PE samples (29 items) misclassified as PP by the near-infrared device
can be recovered.
 The results are shown in Figure 3. Since 95% of PP items are plastic films, the trans-
mittance rate approaches 90% for both terahertz waves. Even for PP trays, the transmit-
tance exceeds 80%. For PE items, the transmittance rate of PE films is over 90%. However,
unlike PP items, PE bottle caps and trays are detected with transmission rates below 80%.
If the discriminating threshold between PP and PE by terahertz waves is set below 80%,
Recycling 2022, 7, 66 11 of 16
approximately 37% of PE samples (29 items) misclassified as PP by the near-infrared de-
vice can be recovered.

Figure3.3.Identification
Figure Identificationresults
resultsof
ofmisclassified
misclassifiedPP
PPand
andPE
PEsamples
samplesusing
usingterahertz
terahertzwaves.
waves.

3.5.2.
3.5.2.Possibility
PossibilityofofAdvanced
AdvancedIdentification
Identification
These
These results have shownthat
results have shown thatplastic
plasticcontainers
containersand
andpackaging
packagingwaste
wasteare
arecomposed
composed
of
of different
differentmaterials.
materials. It
It was
was also
also confirmed
confirmed that
that the
the existing
existing near-infrared
near-infrared device
device was
was
incapable
incapableofofidentifying
identifyingsome
some waste
wastecontainers so so
containers those wastes
those areare
wastes burned as energy.
burned On the
as energy. On
other hand, the limitations of the existing near-infrared sorting device can be compensated
the other hand, the limitations of the existing near-infrared sorting device can be compen-
for with
sated forterahertz waves. waves.
with terahertz To perform more advanced
To perform identification
more advanced and classification,
identification the
and classifica-
authors are developing an identification technology using terahertz waves.
tion, the authors are developing an identification technology using terahertz waves.
Terahertz waves are both linear and transmissive, and the transmittance differs de-
pending on the material by irradiating the plastic. Terahertz waves are suitable for the
identification of colored plastic containers and packaging waste because they are not af-
fected by the color of the waste, which has been a problem in the past. In this study,
previously collected information on plastic containers and packaging waste has been con-
verted into big data, and machine learning has also been developed. It is considered that
new technologies will aid in the identification and separation of plastic containers and
packaging waste.

4. Potential of New Sorting Technology

4.1. What Is a Terahertz Wave?
Terahertz waves have been difficult to generate and detect, but devices and equipment
in this frequency range have recently been developed for practical use, including 5G com-
munications and automatic car driving (0.076 THz). A terahertz wave is an electromagnetic
wave in the frequency range of 0.1 to 10 THz and wavelength range of 30 to 3000 µm.
Since this region is located between radio waves and light waves, terahertz waves have
both the transparency of a radio wave and the straightness of a light wave. Straightness
enables optical designs that would be impossible for radio waves. Its energy is equivalent
to room temperature, so it does not ionize materials and is safe for the human body. In a
pioneering study of terahertz identification, Koch carried out a study on wood and food
products in 1998 [41]. For black plastics, which are difficult to discriminate by NIR, sub-
terahertz discrimination in the 300–350 GHz frequency band has been demonstrated [42].
In 2019, the industrial application of terahertz non-destructive testing, covering measure-
ment principles, evaluation of internal structures, material identification, and coating film
evaluation, was widely explained, including not only actual examples but also diagrams of
the equipment [43].
The terahertz region corresponds to the energy of vibrations between molecular
chains in plastics, rather than local vibrational modes with small effective masses, which
are detected by conventional infrared spectroscopy. Therefore, based on the dielectric
constant at the terahertz frequency, it is possible to identify plastics by material based
on the unique spectrum revealed by the plastic. The transmission and reflectance of
terahertz waves are based on plastic material and additives, which enables sorting based
on the discrimination of materials. Terahertz waves can be used to identify colored and
Recycling 2022, 7, 66 12 of 16

flame retardant-containing plastics, which are currently difficult to sort. Furthermore, this
dielectric constant is sensitive to strain and degradation of plastic materials.
As an example, Figure 4 shows a single-frequency measurement system in which
Recycling 2022, 7, x FOR PEER reflectance can be monitored at the same time. THz light sources are GUNN, IMPATT,
REVIEW and
13 of
of 17
Recycling 2022, 7, x FOR PEER REVIEW 13 17
TUNNETT diodes, and the detector is a Schottky barrier diode. These solid-state devices
are compact and portable, and are expected to be used as portable instruments.

Figure 4.
Figure 4. Terahertz
Terahertz measurement
measurement system
system with
with reflection
reflection configuration.
configuration.
Figure 4. Terahertz measurement system with reflection configuration.
The identification
The identificationalgorithms
identification algorithmsfor
algorithms forfour
for fourtypical
four typical
typical container
container
container andand
and packaging
packaging
packaging plastics
plastics
plastics (PP,
(PP,(PP,
PE,
PE,
PE, PS,
PS, and
and PET)
PET) are
are shown
shown in
in Figure
Figure 5.
5. The
The circled
circled symbols
symbols in
in
PS, and PET) are shown in Figure 5. The circled symbols in the figure indicate the transmit- the
the figure
figure indicate
indicate the
the
transmittance
transmittance
tance correspondingcorresponding
corresponding to the plastic
to thesample
to the plastic plasticat sample
sample at each frequency.
at each frequency.
each frequency. Olefin-basedOlefin-based
Olefin-based
PP and PE PP and
PPhave
and
PE have
PE
highhave high terahertz
high
terahertz terahertz
transparency transparency
transparency
and lowand and low reflectance
low
reflectance reflectance
over a wideover aterahertz
over a wide
wide terahertz
terahertz
frequency frequency
frequency
range,
range, whereas
range,
whereas whereas
PS has lowPS transparency
PS has low
has low transparency
transparency and high
and
and high reflectance. high reflectance.
reflectance.
In contrast, InIn contrast,
PS contrast, PS has
PS has low
has low transparency low
transparency and
and high reflectance,
transparency high
and high reflectance,
while while
PET haswhile
reflectance, PET has
no transmittance no transmittance
in the frequency
PET has no transmittance in the frequency
range
in the range
above 1range
frequency THz
above
and
aboveits11reflectance
THz and
THz and its
itsisreflectance
reflectance
even higher is even
is even PP,
than higher
higher than
PE, than
and PS.PP, PE
PP, PE,has
PE, andaPS.
and PS. PE has
has aa characteristic
characteristic
PE characteristic
absorption
absorption
at 2.2 THz, at
absorption at 2.2 THz,
which
2.2 THz,
causes which
which causes an
an causes
increase aninincrease
increase
reflectancein reflectance
in reflectance in that
that frequency
in that frequency
in frequency
range. The range. The
unique
range. The
unique transmittance/reflectance
transmittance/reflectance
unique transmittance/reflectance of
of terahertz terahertz waves
waves waves
of terahertz for each for each
forplastic plastic allows
allowsallows
each plastic for discrimi-
for discrimination,
for discrimi-
nation,
and
nation, and measurements
and measurements
measurements at 1.0
at 1.0 THz,
at 1.0 THz,
1.8THz,
THz, 1.8
and
1.8 THz,
THz, and
2.2and
THz2.2 2.2 THz
areTHz are effective
effective
are effective for sorting
for sorting
for sorting
PP, PE, PP,PP,
PS,
PE, PS,
andPS,
PE, PET. and PET.
and PET.

Figure 5. Identification algorithms for PP, PE, PS, and PET.

Figure 5.
Figure 5. Identification
Identification algorithms
algorithms for
for PP,
PP, PE,
PE, PS,
PS, and
and PET.
PET.

4.2. Comparison
4.2. Comparison ofof Sorting
Sorting Technologies
Technologies for
for Plastic
Plastic Wastes
Wastes
Table 66 shows
Table shows the
the material
material sorting
sorting technologies
technologies for
for waste
waste plastics,
plastics, including
including specific
specific
gravity sorting, near-infrared sorting, and X-ray fluorescence (XRF) identification
gravity sorting, near-infrared sorting, and X-ray fluorescence (XRF) identification in in ad-
ad-
Recycling 2022, 7, 66 13 of 16

4.2. Comparison of Sorting Technologies for Plastic Wastes

Table 6 shows the material sorting technologies for waste plastics, including specific
gravity sorting, near-infrared sorting, and X-ray fluorescence (XRF) identification in ad-
dition to terahertz technology. “+” means yes and good. “#” means neutral. “-” means
no and bad. The specific gravity of plastics increases due to the presence of additives,
so the accuracy of specific gravity sorting is poor. Near-infrared is absorbed by colors
and printing inks on plastics, and reflectance is reduced, so the identification accuracy
is not high. X-ray fluorescence analysis has a low intensity of irradiating X-rays for safe
use on site, which reduces the accuracy of sorting, especially if metal powder adheres
to the surface. In addition, electromagnetic sorting used in conventional recycling can
detect magnetic materials such as iron, but it cannot sort non-magnetic materials such as
aluminum vapor-deposited film used for plastic interior film and lithium-ion batteries.

Table 6. Comparison of plastic identification methods.

Terahertz Near-Infrared Specific Gravity XRF

Material + # # #
Plastic Type Additive + - - -
Aluminum coating + - - +
Accuracy + # - #
Safety + + + -

Terahertz waves can identify even colored plastics, which dominate the general waste
stream, and can also detect additives that are included in the material. Furthermore, they
can also detect aluminum film in multi-layered plastic (laminated film), which is used to
avoid freeze-drying, including for frozen foods. Thus, the feature of terahertz identification
is the ability to identify multi-layered plastics that contain coloring and additives, which
are a large portion of general wastes. Industrial waste plastics are often single-layered
plastics that do not contain additives or contain only plastic materials and information on
additives can be obtained.

5. Conclusions and Challenges for the Future

Even though Japan has been recycling plastic containers and packaging waste for
more than two decades, single-use plastics emissions remain high. Although the charging
for plastic bags and restrictions on single-use containers have been enacted, the consump-
tion of plastic products has increased due to the COVID-19 pandemic. In addition, the
material recycling of plastic waste, which should be emphasized, has stalled due to the
high dependence on energy recovery, the diversification of plastic products, recycling costs,
and technical obstacles for waste sorting. Instead of focusing solely on the convenience
and functionality of consumers and designing products that are easy to recycle, avoiding
the use of plastic products and packaging with composite materials or that are difficult
to recycle is considered a fundamental solution, however, it is also true that altering the
existing arterial and venous industrial systems simultaneously is difficult.
In other words, there is a limit to the development of recycling policies based on
the EPR, and it is necessary to provide incentives to motivate producers, consumers, and
recyclers to actively promote material recycling. The characteristics analysis of plastic
waste from households reveals that they are usually composed of two or more plastic
materials, or with aluminum-embedded materials, flame retardants, or additives. Actually,
approximately 70% of the recycled materials produced at recycling facilities are single
materials, and 30% of them are other complex materials. Possessing sophisticated plastic
waste sorting and quality control becomes an essential issue. To produce recycled plas-
tic materials of higher quality, it is necessary to increase the precision of identification
and sorting.
Recycling 2022, 7, 66 14 of 16

This study examines and evaluates local characteristics of plastic containers and pack-
aging waste and clarifies the limits and issues of current identification technology, and then
introduces and discusses a new sorting technology using terahertz waves. Plastic sorting
utilizing terahertz waves can differentiate the material of black plastic, as well as plastic
with aluminum deposition, compared to the weak points of near-infrared. Additionally, ter-
ahertz waves are safer than specific gravity sorting and XRP sorting with accuracy. Plastic
containers and packaging waste are diverse and intricate, and their composition varies by
area. Existing near-infrared devices are insufficient to precisely and rapidly identify their
materials. Thus, the development of an identification and sorting device that combines
near-infrared and terahertz waves is proposed. Material recycling can be expanded by
accurately analyzing the composition of plastic containers, as well as by introducing sorting
equipment suitable for each local recycling facility. It is also essential to categorize and
sort those materials that are unsuitable for material recycling into chemical recycling and
energy recovery. Ultimately, if such a recycling process can be completed, it will be possible
to build a sustainable resource circulation network that promotes a low-carbon society and
a circular economy.
The results of this study are based on literature reviews, data analysis, and demon-
stration experiments, and it is necessary to collect and analyze huge amounts of big data
and improve the accuracy of sorting in the future. Although it has been confirmed that the
use of higher terahertz waves can improve the discrimination accuracy for each material,
diverse shapes, stains, the presence or absence of additives, and the degree of deterioration
have not been adequately tested. It will be necessary to collect, analyze, and compile big
data from various recycling facilities and areas.

Author Contributions: Conceptualization, J.Y. and X.L.; methodology, J.Y. and X.L.; validation, J.Y.,
X.L., G.M. and T.T.; formal analysis, J.Y., X.L., G.M., T.T. and K.O.; investigation, J.Y., X.L., G.M.,
T.T., S.O. and K.O.; resources, J.Y. and X.L.; data curation, J.Y.; writing—original draft preparation,
J.Y., X.L., G.M. and T.T.; writing—review and editing, J.Y., X.L. and S.O.; visualization, J.Y. and X.L.;
supervision, J.Y.; project administration, J.Y.; funding acquisition, J.Y. All authors have read and
agreed to the published version of the manuscript.
Funding: This work was supported by the program for Creating Start-ups from Advanced Research
and Technology (2021) by the Japan Science and Technology Agency (JST).
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Not applicable.
Conflicts of Interest: The funders had no role in the design of the study; in the collection, analyses,
or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

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