If a substance is biodegradable, it means it can be broken down into its basic chemical building blocks in soil or water.
The prefix “bio” means that the job is done by living things — primarily microorganisms such as bacteria. Everything natural — from rocks to paper — will biodegrade eventually. Crude oil, for example, from inside Earth will biodegrade in its natural state. Oil spills are devastating not because oil doesn’t biodegrade, but because the amount of oil is much greater than the microorganisms available to degrade it.
And once it’s turned into plastic, the chemical structure of oil has been changed so much that microorganisms can’t break it down. So most plastics aren’t biodegradeable. The time it takes for something to biodegrade also varies — anywhere from a few days to many years. Food waste biodegrades quickly if you bury a little, but much more slowly in landfills. That’s because in landfills, there’s not enough air, sunlight and bacteria to get the decay process started.
And that’s why researchers digging in landfills have unearthed hot dogs and grapes — 25 years old — and still recognizable.
Different materials can degrade in different ways with different end results. Ideally, a biodegradable substance breaks down into molecules plants can use, such as carbon dioxide, water, and naturally occurring minerals.
Enzymes are proteins produced by biological organisms that allow chemical reactions to occur in nature.
The stronger the chemical bonds between elements in a molecule, the more difficult the molecule is to degrade.
Detergents might break down in a natural freshwater “aerobic” (having oxygen) environment, but not in an “anaerobic” (lacking oxygen) environment such as sewage treatment plant digestors, or natural ecosystems such as swamps, flooded soils, or surface water sediments.
[from https://www.greengood.com/terms_to_know/biodegradable_definitions.htm Soap is a natural, organic product that is inherently biodegradable. The soapy greywater from a single household may biodegrade easily in a backyard. But if that same soap went down a sewage line that fed into a waterway along with the soap used by a million or more residents that live along that waterway, there may be waves of soapsuds on the beaches, because there’s more soap going into the waterway than it has microorganisms to biodegrade. Oil spills are devastating not because oil doesn’t biodegrade, but rather because the amount of oil is much greater than the number of microorganisms available to degrade it. It has been estimated that it will take 50 years for the oil spilled in 1989 by the Exxon Valdez to degrade. Lakes and streams have become polluted because the amount of sewage dumped into them has been overwhelming. As much as we need to consider the biodegradability of the product, we need to consider the capacity of the system the biodegradable substance or material is being placed into.
Terms like “biodegradable” as they are used on consumer products are defined and enforced by the Federal Trade Commission (FTC). This and similar terms (compostable, eco-friendly, etc.) are considered “general claims” that usually are not independently verified – but placed on the product by the manufacturer to increase sales. If qualified on the product, they may not be deceptive, but most are.
The FTC does enforce truth in advertising laws, and has this to say about “biodegradable” at:
(b) Degradable/biodegradable/photodegradable: It is deceptive to misrepresent, directly or by implication, that a product or package is degradable, biodegradable or photodegradable. An unqualified claim that a product or package is degradable, biodegradable or photodegradable should be substantiated by competent and reliable scientific evidence that the entire product or package will completely break down and return to nature, i.e., decompose into elements found in nature within a reasonably short period of time after customary disposal.
Claims of degradability, biodegradability or photodegradability should be qualified to the extent necessary to avoid consumer deception about: (1) the product or package’s ability to degrade in the environment where it is customarily disposed; and (2) the rate and extent of degradation. *most personal and laundry cleaning products that claim to be biodegradable probably are, since these products are diluted during use/disposal and end up in municipal wastewater treatment systems where activation processes do in fact render the chemicals to their “natural” constituent forms. Most soluble chemicals do so, with the notable exception of many long chain polymers (including organichlorides), which are extremely stable and of course notorious in their estrogenic activity.
Beyond that, though, even relatively benign chemical breakdown products can have deleterious effects on the environment into which the treated wastewater is discharged. Bottom line is that true biodegradability is far from good for the environment. Take laundry detergent, for one example. Most are labeled as biodegradable, and most in fact are. But they degrade into mostly sodium salts that are harmful to floral and faunal growth in both aquatic and terrestrial systems. That’s why you can’t use biodegradable detergents directly on plants in greywater systems – these detergents must be disposed of in treatment plants or septic tanks to make the effluent harmless.
As to other products such as those made of paper, these may be biodegradable, but only if they are disposed of unconventionally. You send a paper plate or cup to a landfill, and it won’t biodegrade. You litter it out the window of your car on the freeway and it will biodegrade. Thus most products that become solid waste are don’t get not biodegraded, regardless of content.