Biodegradable is always best!
When we read the marketing blurb on the side of a consumer package we often see the claim that the product is biodegradable. Does that mean it is harmless to the environment? Not necessarily.
Let imagine the following incident has occurred:
Two road tankers collide while on a bridge puncturing their tanks, which then leak into the nearby river. One tanker is carrying milk, the other diesel.
Which of these products is considered a bigger environmental threat?
The answer is milk. Milk is soluble in water and has a high BOD (biological oxygen demand).
This is a key parameter for the pollution potential of the milk. BOD is used to measure the impact as the micro-organisms in the water use the milk as food. As the micro-organisms consume the milk they deplete oxygen levels in the water.
Diesel has a much lower initial impact and can be cleaned up, thus having a lower overall impact.
We tend to think that if a substance is harmless to us than it is harmless to the environment. This is not always true. Most fish kills that occur are due to lack of oxygen in the water. This often occurs after a rainfall event where organic matter such as leaves are washed into a watercourse..
A real example:
When conducting an environmental site survey of a large bakery, I discovered a pile of dough weighing 2 tonnes oozing through a stormwater grate. When I asked the staff about this, they explained that they were disposing of the off-specification batch of dough to stormwater. They felt that this was okay because bread dough is biodegradable!
When we are disposing of waste to the industrial or municipal waste system biodegradability is a good thing as biodegradable waste can be composted or converted to biogas. Both processes produce saleable products and are essentially greenhouse gas neutral.
Just be sure that your waste biodegradable materials are ending up in the right place.