potential markets and market values of final products. However, there is increasing
recognition of the role of plastics in organic fertilisers which can be a vehicle for
entry of plastics into the environment (Weithmann et al. 2018; Ng et al. 2018;
Browne et al. 2011) and which, in turn, may pose risks to biota and public health
through their entry into the human food chain (van Cauwenberghe and Janssen
2014). Ng et al. (2018) pose that plastic particle loading in agroecosystems due to the
application of fertilisers through to the application of plastic mulching is increasing.
Most plastics (except for degradable and biodegradable) have been purposely
designed to resist degradation. Research on the degradation of polyethylene
(Albertson 1989) concluded that the degradation of polyethylene (PE) occurs at a
rate of approximately 0.5% of weight per annum. The biodegradation of PE is
primarily affected by irradiation from an ultraviolet source and other factors such
as molecular weight, additives and surface area (Kawai et al. 1999). For example,
increased branching of polymers reduces the rate of degradation, and additives can
be used to weaken the carbon–carbon backbone of a polymer, whilst microorganism
attack on PE is a secondary process (Scott 1975).
The composting of PE in some European countries, such as Germany, is
prohibited as PE is unable to pass their standards governing compostability.
Germany’s standard DIN V 54900 (testing the compostability of polymeric materials
(German Standards Agency 1998)) defines methods for testing the compostability of
a polymeric material. The test indicates if a polymer will be disintegrated and
converted into constituents of the compost under controlled composting conditions.
This standard also determines if the composting process or quality of the final
product is affected by the polymer or its degradation products. The word ‘degradation’ implies a loss of properties. The trigger for degradation could be a ‘microbially,
hydrolytically or oxidatively susceptible linkage built into the backbone of the
polymer’ or, alternatively, ‘additives that catalyse breakdown of the polymer’
(Narayan 2000). This ‘trigger’ can be specifically designed to ensure degradation
does not occur within the ‘in-use lifetime’ but will begin upon disposal within a
given environment. Degradation of plastics will ultimately depend on several factors
such as the microbial activity of the disposal or treatment environment. Within a
composting environment, thermo-oxidation plays the dominant role in degradation,
since temperatures can easily exceed 60–70
C for a prolonged period of time.
Plastics can also degrade in a variety of ways through reacting with sunlight (photooxidation), bacteria (microorganisms), chemicals and macro-organisms (invertebrates and insects). However, compost windrows containing plastic contamination
typically experience lower average temperatures (Davis 2005) which may lead to
retardation of the composting process and poor pasteurisation outcomes. Whilst
visual contamination of plastics in composts and soil ameliorants is worrying, of
most concern is the prevalence of nano- and microplastics as well as the range of
additives applied to plastics (inks through metals designed to facilitate the degradation process); and what happens to all of these materials in the environment long
term as they degrade into various intermediately products?
Whilst organic wastes (food and garden) from households and commercial
enterprises are a valuable source of materials for manufacture into nutrient-rich
128
G. Davis
recognition of the role of plastics in organic fertilisers which can be a vehicle for
entry of plastics into the environment (Weithmann et al. 2018; Ng et al. 2018;
Browne et al. 2011) and which, in turn, may pose risks to biota and public health
through their entry into the human food chain (van Cauwenberghe and Janssen
2014). Ng et al. (2018) pose that plastic particle loading in agroecosystems due to the
application of fertilisers through to the application of plastic mulching is increasing.
Most plastics (except for degradable and biodegradable) have been purposely
designed to resist degradation. Research on the degradation of polyethylene
(Albertson 1989) concluded that the degradation of polyethylene (PE) occurs at a
rate of approximately 0.5% of weight per annum. The biodegradation of PE is
primarily affected by irradiation from an ultraviolet source and other factors such
as molecular weight, additives and surface area (Kawai et al. 1999). For example,
increased branching of polymers reduces the rate of degradation, and additives can
be used to weaken the carbon–carbon backbone of a polymer, whilst microorganism
attack on PE is a secondary process (Scott 1975).
The composting of PE in some European countries, such as Germany, is
prohibited as PE is unable to pass their standards governing compostability.
Germany’s standard DIN V 54900 (testing the compostability of polymeric materials
(German Standards Agency 1998)) defines methods for testing the compostability of
a polymeric material. The test indicates if a polymer will be disintegrated and
converted into constituents of the compost under controlled composting conditions.
This standard also determines if the composting process or quality of the final
product is affected by the polymer or its degradation products. The word ‘degradation’ implies a loss of properties. The trigger for degradation could be a ‘microbially,
hydrolytically or oxidatively susceptible linkage built into the backbone of the
polymer’ or, alternatively, ‘additives that catalyse breakdown of the polymer’
(Narayan 2000). This ‘trigger’ can be specifically designed to ensure degradation
does not occur within the ‘in-use lifetime’ but will begin upon disposal within a
given environment. Degradation of plastics will ultimately depend on several factors
such as the microbial activity of the disposal or treatment environment. Within a
composting environment, thermo-oxidation plays the dominant role in degradation,
since temperatures can easily exceed 60–70
C for a prolonged period of time.
Plastics can also degrade in a variety of ways through reacting with sunlight (photooxidation), bacteria (microorganisms), chemicals and macro-organisms (invertebrates and insects). However, compost windrows containing plastic contamination
typically experience lower average temperatures (Davis 2005) which may lead to
retardation of the composting process and poor pasteurisation outcomes. Whilst
visual contamination of plastics in composts and soil ameliorants is worrying, of
most concern is the prevalence of nano- and microplastics as well as the range of
additives applied to plastics (inks through metals designed to facilitate the degradation process); and what happens to all of these materials in the environment long
term as they degrade into various intermediately products?
Whilst organic wastes (food and garden) from households and commercial
enterprises are a valuable source of materials for manufacture into nutrient-rich
128
G. Davis