In contrast to simple biodegradable substances which decompose quickly, there are some compounds which have a remarkable degree of resistance to the natural agencies of decay and decomposition.
These are often harmful substances which persist in the environment for long duration of time during which they are taken up in the biosphere, accumulated and bio-magnified to concentration potentially toxic to organisms at higher trophic levels in the food chain. Many of these chemicals are carcinogenic, teratogenic and mutagenic in nature.
Pollutants grouped together under the category of wastes resistant to degradation, are organic substances produced naturally or are synthesised by man. Many of these are such chemicals which have caused much concern due to their wide-spread use and dissemination. Broadly speaking these chemicals can be grouped into the following three categories:
(A) Pesticides and allied chemicals.
(B) Crude petroleum and its derivatives.
(C) Polymers, plastics, plasticizers and other wastes.
Though most of these chemicals are referred to as being persistent, the versatile and efficient machinary which Nature employs for decay and decomposition of pollutants is capable of decomposing even these recalcitrant and refractory chemicals.
But to do so, suitable conditions and abiotic as well as biotic processes have to be put together for which Nature requires time on its own scales and not on scales dictated by human needs. These chemicals, if left to Nature’s devices alone are degraded rather slowly, slower than the rate of their entry into the environment. They tend to accumulate within the system causing toxicity to those living organisms which happen to get exposures. Grave ecological problems appear where the magnitude of pollution is considerably large.