I’m sure this has bothered most of us at some point. It is about the use of plastic bags. Plastic consumption and its environmental effect due to poor biodegradability frequently come under the public discourse. And here is an interesting research report based on the Life Cycle Analysis (LCA) of a few commonly used carrier bags in the UK, published by the Environment Agency (2011).
LCA is a standard methodology to estimate the material, energy usage, and environmental impact throughout a product’s lifecycle (‘cradle to grave’). The study covered only the carriers available from the UK. The material in focus was conventional HDPE, HDPE with pro-degradant additive, starch-polyester (biopolymer), paper, heavy-duty LDPE, non-woven PP and cotton bag.
The end-of-life processes for the different materials included landfill and incineration (for all) and mechanical recycling and composting, where applicable.
The following table contains the energy use and waste generation from 1000 bags of each material.
| Bag Type | Electricity (kWh) | Heat (from NG) (kWh) | Heat (from Fuel oil) (kWh) | Waste (g) |
| Conventional HDPE | 6.151 | 418.4 | ||
| HDPE + additive | 6.392 | 426.1 | ||
| biopolymer | 17.24 | 94.8 | ||
| LDPE | 32.58 | 13.953 | 171.2 | |
| Non-woven PP | 87.75 | 5,850 | ||
| Cotton | 11 | 1,800 |
The study assumed the reuse of about 40% of all lightweight carrier bags as bin liners.
The next one up was the global warming potential. GWP (excluding primary reuse) for the cotton bag (250 kg CO2 eq.) was more than ten times that of any other bag! It was followed by PP (22), LDPE (7), paper (6), biopolymer (5), HDPE (2.1) with additive and HDPE (2), in descending order. In other words, a cotton bag requires to be used 173 times to match HDPE and PP 14 times.
Here is a summary:
| Bag type | Sensitivity | GWP (kg CO2 eq) |
| HDPE | Baseline | 1.578 |
| HDPE | Recycling | 1.400 |
| HDPE | Recycling (no reuse) | 1.785 |
| HDPE prodegradant | Baseline | 1.750 |
| biopolymer | Baseline | 4.184 |
| biopolymer | Composting | 2.895 |
| biopolymer | Composting (no reuse) | 3.329 |
| Paper bag (4 uses) | Baseline | 1.381 |
| Paper bag (4 uses) | Recycling | 1.090 |
| Paper bag (4 uses) | Composting | 1.256 |
| LDPE (5 uses) | Baseline | 1.385 |
| LDPE (5 uses) | 100% Recycling | 1.196 |
| PP (14 uses) | Baseline | 1.536 |
| PP (14 uses) | 100% Recycling | 1.292 |
| Cotton bag (172 uses) | Baseline | 1.579 |
To conclude
HDPE bags have the lowest environmental impact among lightweight bags on 8 out of 9 counts.
The starch-polyester (biopolymer) bag has the highest impact in 7 out of 9.
The paper bag needs to be used four times to match HDPE’s global warming potential.
The cotton bag has a greater impact than the HDPE bag in 7 out of 9 categories, even when used 173 times.
The key to reducing the impact is to reuse as much as possible.
Reference
Life cycle assessment of supermarket carrier bags: a review of the bags available in 2006
