This report is the result of research commissioned and funded by the Environment Agency. The Environment Agency is the leading public body protecting and improving the environment in England and Wales.
The following types of carrier bag were studied:
- a conventional, lightweight carrier made from high-density polyethylene (HDPE);
- a lightweight HDPE carrier with a prodegradant additive designed to break the down the plastic into smaller pieces;
- a biodegradable carrier made from a starch-polyester (biopolymer) blend;
- a paper carrier;
- a “bag for life” made from low-density polyethylene (LDPE);
- a heavier more durable bag, often with stiffening inserts made from non woven polypropylene (PP); and
- a cotton bag.
Carrier bag are each designed for a different number of uses. Those intended to last longer need more resources in their production and are therefore likely to produce greater environmental impacts if compared on a bag for bag basis.
Extraction/production of raw materials
The weight and raw material composition of carrier bags vary depending on the requirements set by the supermarkets and the processing methods used by the producer. The bag weight used here for each carrier bag type is an average based on individual supermarket bag weights and market share.
The film is assumed to be composed of two-thirds polyethylene (PE) and one third polyamide (PA).
The weights of the corrugated boxes reported by bag producers fluctuated widely, with some producers reporting the box to be heavier than its content.
For the starch-polyester blend bag, the corrugated box weight reported by the producer was used, although this was heavier than conventional carrier bag packaging.
Bag production processes
All plastic bags are produced from plastic melt. This is generally blown and sealed to form a bag, except for the non-woven PP bag which is produced from a molten filament using a spun bonded process.
The energy demand for these processes is mainly met by grid electricity and this energy consumption depends on the polymer type, density, production equipment and capacity.
The energy consumption and waste generated by the production of 1000 bags is shown in table 4.2.
90 per cent of LDPE bags are produced in Turkey and Germany and 10 per cent in China and Malaysia. All conventional HDPE, HDPE prodegradant and PP bags are imported from the Far East. Data on the production of starch-polyester blend into carrier bags was provided by a bag producer in Norway. All grid electricity use was modelled according to the relevant country (China, Turkey and Norway).
None of the cotton bag producers contacted provided any data on cotton bag production and data on conversion of cotton fabric into carrier bags were estimated. We assumed that the bags were produced in China, using electric sewing machines.
For this study, we assumed that all HDPE and PP bags were produced in the Far East and 90 per cent of the LDPE bags were produced in Turkey and the remainder in China. Transportation by lorry was based on a 16-32 tonne vehicle.
Reuse, recycling & end-of-life
The secondary use of lightweight plastic carrier bags (i.e. the conventional HDPE bag, the prodegradant HDPE bag and the starch-polyester bag) was modelled using the avoided production of bin liners.
Overall it was estimated that 76 per cent of single use carrier bags were reused.
We therefore calculated that 40.3 per cent (53 per cent of 76 per cent) of all lightweight carrier bags avoided the use of bin liners.
The avoided production of virgin materials through recycling during production was also included in the study, adjusted for any loss in material performance due to the recycling process.
In practice, performance loss is often compensated for by the use of an extra amount of recycled material in a product, making it heavier than one produced only from virgin materials.
At the end-of-life 86 per cent of all bags were assumed to be landfilled and 14 per cent incinerated (DEFRA 2008). Statistics for paper recycling in England (DEFRA 2007) were also used to model the recovery of primary packaging cardboard in supermarkets with 77.3 per cent of cardboard assumed to be recycled.
When bag recycling at end-of-life was included, it was assumed that all the plastic carrier bags collected at end-of-life for recycling were exported for recycling to China. In the UK in 2005, 65 per cent of plastic film collected for recycling was exported overseas, mainly to China and other Far East countries (BPI 2007).
The inclusion of HDPE bags with prodegradant additive in the HDPE recycling stream is recognised by industry as potentially reducing recyclate quality.
Although prodegradant additives were a small proportion of the polyethylene film being recycled, their separation from conventional HDPE is viewed as highly desirable and the recycling of HDPE prodegradant bags at end-of-life has been excluded from the study.
Global warming potential (GWP)
The GWP of all of the carrier bags studied is dominated by raw material extraction and production which ranges from 57 per cent of the impact for the starch polyester bag to 99 per cent for the cotton bag.
This impact is normally due to the production of the most prevalent material with 64 per cent of the HDPE bag impact generated directly from the extraction and production of HDPE
The avoided production of virgin material due to the recycling of post-production waste and primary packaging has a relatively small net effect due to the low proportion of scrap material reprocessed and due to the impacts of cardboard recycling being of similar size to the benefits of avoided production.
Packaging materials generally contribute between 0.4 per cent and 4 per cent of the overall global warming impact for each type of carrier.
The GWP from grid electricity used to produce carrier bags varies from 38 per cent of the overall impact for HDPE bags to 0.4 per cent for the starch polyester bag, although the proportion was influenced by the impact of other lifecycle stages such as raw material extraction and production as well as the electricity mix in the country of origin: the HDPE bag is assumed to be produced in China, which relies heavily on electricity generated from burning coal, whereas the starch polyester blend bag is assumed to be produced in Norway where 99 per cent of the grid electricity is generated through hydropower.
The impact of transportation on the total GWP is generally between 0.8 per cent and 14 per cent and is heavily dependent on the road transport distance.
The transportation of the starch polyester bag has the highest impact of all carrier transport and transport is also more significant in its lifecycle (21 per cent of total impact) because the starchpolyester blend is carried by road from Italy to Norway and the finished product by road/sea to the UK.
In the case of the HDPE, HDPE prodegradant, PP and cotton bags, where bags are shipped from the Far East, the impact of that shipping is between 60-70 per cent of the transport impact.
The end-of-life impacts of all bags contribute between 0.2 per cent and 33 per cent to overall GWPs. The end-of-life of the plastic carrier bags (the conventional HDPE, HDPE prodegradant, LDPE and PP bags) is generally between 5 per cent and 7 per cent and is dominated by the GWP of plastic incineration.
However, the end-of-life of the paper bag and the starch polyester bag is dominated by landfill which contributes over 18 per cent and 29 per cent respectively to the overall impact. Incineration does provide a 5 per cent reduction in the GWP of the paper bag due to the energy from waste incineration which offsets the direct global warming impact .
The influence of the secondary reuse of 40.3 per cent of the lightweight bags is shown in the large reduction created by the avoided products lifecycle stage in both figures.
This reuse generates a reduction of 12 per cent for the starch polyester blend bag, 29 per cent for the HDPE prodegradant bag and 32 per cent for the conventional HDPE bag.
The exclusion of any primary reuse from figure 5.1 unsurprisingly shows that reusable carrier bags, without primary reuse, have a higher global warming potential than conventional HDPE carrier bags.
However, the required reuse shown in figure 5.2 shows that this level is practicable for reusable plastic bags, although for paper bags it remains hypothetical.
Starch-polyester blend carrier bag
Raw material production is the highest contribution in all of the eight impact categories. However, no specific material or process can be identified other than the production of the starch-polyester due to the aggregated nature of the data provided by Novamont.
The influence of the transport of raw materials is very similar to that for the conventional HDPE bag. Although the distance is not as great, the materials are transported by lorry from Italy to the north of Norway and this produces a larger impact than sea transport in many categories.
The end-of-life of the starch polyester bag only significantly influences its global warming potential and photochemical oxidation due to its degradation in landfill to release methane.
This contributes approximately 29 per cent to the GWP impact. Although bag production requires more energy than the conventional HDPE bag, the production has lower impacts because of the use of Norwegian grid electricity, which has very low impacts.
The reuse of carrier bags as bin liners and the benefit of waste to energy at end of life reduce the overall environmental impacts of the starch-polyester blend bags by a similar amount to the other lightweight plastic bags.
Secondary use of lightweight bags
An increase in recycling and composting at end-of-life
We investigated the effect of increased recycling and composting at end-of-life on all the impact categories considered. All of the lightweight carrier bags were considered with and without secondary reuse.
The 40 per cent of lightweight carrier bags that are reused as bin liners are therefore managed as residual municipal waste, leaving almost 60 per cent to go to recycling or composting. When secondary reuse is excluded we have assumed all bags are recycled or composted.
The inclusion of HDPE bags with prodegradant additive in the HDPE recycling stream is recognised by industry as a potential problem for recyclate quality.and the recycling of HDPE prodegradant bags at end-of-life has not been considered.
The production of compost has been excluded because the amount of compost produced would have little or no effect on the results.
Therefore, there is no reduction in resource use and a slight increase in abiotic depletion. Global warming potential and photochemical oxidation are both substantially reduced because composting avoids the impact of landfill, which has a considerable effect on these categories.
When no starch-polyester bags are reused as bin liners and all bags are composted, seven of the nine impacts are increased. This is more significant for abiotic depletion and photochemical oxidation where the avoided production of bin liners was particularly important.
However, recycling increases fresh water ecotoxicity due to the release of copper to the water during recycling and terrestrial ecotoxicity from composting due to the release of metallic contaminants to soil and water.
Generally, when secondary reuse is reduced and replaced by recycling, impacts such as GWP and abiotic depletion are increased.
Impact catagoires such as human toxicity are also affected by increases in recycling because of reduced incineration and the energy recovered resulting in an increase in electricity generated by coal and gas combustion.
The composting of the starch-polyester and paper bags also increases many of the impacts of these carrier bags, although the recycling and composting of the paper bag and the composting of the starch polyester bag reduced GWP by avoiding the generation of methane associated with landfill.
Impact Assessment Method
The inclusion of biogenic carbon dioxide in the eco-indicator results increases the impact of the starch-polyester blend bag and the paper bag in comparison to the other carrier bag options.
The amount of biogenic carbon dioxide equivalents emitted at the end-of-life of these bag lifecycle is greater than the biogenic carbon dioxide equivalents absorbed during production, therefore providing a marginal net increase in the GWP impact.
For the starch-polyester bags it was estimated that 0.59 kilograms of carbon dioxide is absorbed from the atmosphere for every kilogram of mater-bi produced.
The starch-polyester bag degrades fully to methane and carbon dioxide in landfill, producing a higher global warming impact from the end-of-life than the paper bag, which does not fully degrade in landfill.
The cotton, starch-polyester blend and paper bags have the highest land use due to the land required for the growth of raw materials, although the impact of land use on starchpolyester bag is dominated by the use of corrugated board for packaging with only 20 per cent of the land use impact from the production of the starch polyester.
The starch-polyester carrier bags considered were based on manufacturer’s data and weighed almost twice as much as conventional HDPE carrier bags.
They had the highest impacts of the lightweight carrier bags in every category apart from abiotic resource depletion. Since the reference period, the weight of starch polyester bags has been reduced by manufacturers to similar to that of the conventional HDPE bag.
On a weight for weight basis this suggests that the global warming potential, acidification and photochemical oxidation impacts of the starch-polyester bag would be similar to conventional HDPE carrier bags, as indicated in other reports on the subject.
However, the impacts of global warming potential, eutrophication, toxicity and ecotoxicity for the starch-polyester blend bag studied would still be worse than conventional plastic bags due to the high impacts of raw material production, transport and landfill on those categories.
The secondary reuse of lightweight carrier bags was fundamental to their environmental performance, particularly in terms of abiotic depletion, global warming potential, toxicity, ecotoxicity and photochemical oxidation.
Recycling and composting reduced the global warming potential of the paper bag by 21 per cent and nine per cent respectively, but could also cause significant rises in aquatic and terrestrial ecotoxicity.
- The environmental impact of carrier bags is dominated by resource use and production. Transport, secondary packaging and end-of-life processing generally have a minimal influence on their environmental performance.
- The key to reducing the impact of all carrier bags is to reuse them as much as possible and where reuse for shopping is not practical, secondary reuse in application such as bin liners is beneficial.
- The reuse of conventional HDPE and other lightweight carrier bags for shopping and/or as bin-liners can substantially improve their environmental performance.
- Reusing lightweight carrier bags as bin liners produces greater benefits than recycling bags due to the benefits of avoiding the production of the bin liners they replace.
- For the impacts categories considered, the HDPE bag with prodegradant additives increased the environmental impacts from those of the conventional HDPE bag.
- Starch-polyester blend bags have a higher global warming potential than conventional polymer bags, due to the increased weight of material in a bag, higher material production impacts and a higher end-of-life impact in landfill.
- Recycling or composting generally produces only a small reduction in global warming potential and abiotic depletion. The reduction is greatest for the biodegradable bags – paper and starch-polyester. Composting of starch-polyester bags significantly reduces the contribution of the end-of-life stage to global warming.
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