Pure Plastic vs Biodegradable Plastic

Question

I've been reading around (here, here, here and here) and I can't really find an answer to this question: is it better to throw, for example, dog poop, inside a pure plastic bag or use biodegradable plastic?

I read that biodegradable plastic is not really a "holy" material and that they are actually bad. But is it worse than pure plastic?. Let's assume that the following events:

1. Dog Poops
2. Owner collects the poop in a bag
3. The bag goes in the trash bin
4. The trash bin is transported to the landfill
5. The poop and the bag are both in the landfill which is exposed to normal ambient condition and gradually, filled over.

Which would impact the environment more: Pure Plastic or Biodegradable Plastic?

Any answer or source would be highly appreciated!

Answer 1

It doesn't appear that there are any specific studies comparing the environmental impact of dog waste disposed of in biodegradable vs non-biodegradable bags, however there's quite a bit of work that's been done to compare these two types of plastic, considering production, use, and disposal.

Bio-based and petrol-based plastic production comparison

One of the best studies I could find looking at the production phase is "Sustainability Metrics: Life Cycle Assessment and Green Design in Polymers" (pdf).

In this study the authors perform an LCA on the production processes for 12 different polymers, comparing both life cycle environmental impacts and adherence to green design principles. The major findings are summarized in the chart below:

The environmental impacts analyzed are: acidification, carcinogenic human health hazards, ecotoxicity, eutrophication, global warming potential, noncarcinogenic human health hazards, ozone depletion, respiratory effects, smog, and nonrenewable energy use.

Green design principles look at such things as: avoided waste, material efficiency, avoided pollution, energy efficiency, use of renewable resources, use of local resources, design for recycling, design for biodegradability, and cost effectiveness.

Based on their findings, the authors conclude:

Results from this study show a qualified positive correlation between adherence to green design principles and a reduction of the environmental impacts of production. The qualification results from a disparity between biopolymers and petroleum polymers. While biopolymers rank highly in terms of green design, they exhibit relatively large environmental impacts from production.

One of the main reasons that some petrol-based plastics perform better than bio-based plastics is that the plants used as feedstocks for bio-plastics often may involve petroleum-derived fertilizers and pesticides, and the chemical processes for bio-plastic synthesis can be energy and/or chemical intensive.

The authors note that their study does not include LCA for use and end-of-life, due to the fact that the plastics produced are raw materials for a plethora of different products.

Application to dog waste bags

Based on this result, it would seem clear that bio-based plastics are preferable to petrol-based in any situation where the product would be re-used several times. However, in the case of dog waste bags, it is obvious that the product will only be used once. Therefore, it's harder to say whether a bio-based bag will be preferable to a petrol-based bag.

At least one study ("Bioplastics and Petroleum-based Plastics: Strengths and Weaknesses") indicates that petroleum based polyethylene (PE or PET) bags outperform one type of bio-based bags (Mater-bi, a patented biodegradable plastic product) in end of life when comparing recycling of PET to composting of the bio-bags. So, considering that dog waste bags will not be recycled, this could indicate that bio-bags will perform better in that use case.

Answer 2

If the bag ends up in a landfill as most waste does it's not really gonna biodegrade. Doesn't matter which type of bag you use. Biodegradability requires a air rich environment to feed microbial activity, not found in a anaerobic landfill.