Biodegradability of Substances Discharged to the Aquatic Environment

The term "biodegradability" is much used today, when most people, due to media exposure, are aware of the "environment" and "pollution".  To say that a product or substance is "biodegradable" infers to the layman that it is ecologically "safe".  Certainly it is safer than substances that contain persistent toxins (e.g. some pesticides, chlorinated hydrocarbons, tributyl tin, heavy metals etc). 

However, many substances are biodegradable, but would not be considered "ecologically safe" by the layman.  A good example of this would be cyanide which can be broken down through the cyanate stage to free carbon dioxide and nitrogen by bacteria.  So cyanide is readily biodegradable.  A responsible person would not, however, discharge high levels of cyanide or its compounds to a river or foul sewer.

When any product or substance is disposed of to a drain it will eventually, in most cases reach a watercourse (in the case of a surface water drain) or a sewage treatment works (in the case of a foul drain or foul sewer).  Both of these ultimate destinations is a finely balanced biological system.  The watercourse is a natural one, having a delicate construction of bacteria, algae, macro and micro fauna and flora, insects, larvae, fish, birds and plants all inter-related to result in a balanced eco-system.  

At a municipal sewage treatment works, a similar but artificial (man-made) system exists, since bacterial treatment has evolved historically as the most effective and cheapest method of treating the complex mixture of dilute wastes present in sewage.  Anything which disturbs this delicately balanced eco-system either by heavy "shock loads" or by long term  accumulation effects, can have devastating results.  More directly, poisonous compounds, oxygen depletion or nutrient enrichment can also kill or inhibit the organisms responsible for a balanced river system or an effective sewage treatment plant.

In cases of discharge to drain or sewer biodegradability is the ultimate fate of a compound in the aquatic environment, and its impact on that environment depends upon several factors.  These include the amount discharged, the rate and extent of degradation by biological and physico-chemical processes, its resulting concentration relative to the receiving water, the uses to which that water may subsequently be put (i.e. fishery, potable supply, crop spraying etc), the toxicity of the compound to aquatic plants, micro-organisms and animals (particularly man).

Physico-chemical degradation processes are thought to play only a subsidiary role in the aquatic environment, compared to their roles in air or soil, except that adsorption onto solids and assimilation into micro and macro organisms (a process called bio-accumulation) may play a part.  These latter processes, together with biodegradation are termed "bio-elimination" and may have the effect of transferring the problem to another medium within the environment.

Even substances which are inherently "ecologically safe", or non-toxic, but are biodegradable, can produce damaging or inhibiting effects to ecosystems.  Such biodegradable substances, when discharged to a sewage treatment plant or a water course can cause total failure of the former, rendering sewage treatment impossible, or can cause the death of flora and fauna and massive fish kills when discharged to a river.  

An example would be pollution by spillages of sugar solutions or molasses.  Such compounds are highly biodegradable and create a massive oxygen demand, stripping the receiving  waters of all dissolved oxygen and causing the death of aquatic life.  This is a devastating short term effect.  Another example of an adverse biodegradable effect would be the occurrence of a biodegradable substance causing a gross enrichment of organic and inorganic nutrients in an aquatic environment such as high nitrate or/and phosphate levels.  

Such effects are known as 'eutrophication'.  The initial effect is that of causing lavish growths of micro-algae.  The action of the algae results in oxygen depletion of the water with secondary effects of smell and taste as anaerobic reactions take over.  This in turn causes the generation of sulphides, extensive fish death and destruction of non-resistant micro organisms.  The final result is the almost total colonization of the water by a small number of resistant species, which in some waters can cause  eventual "death" of the lake or river as these species take over.

In conclusion, it will be seen from the above that not all biodegradable substances are safe to discharge to the aquatic environment.  Products labelled "biodegradable" are therefore not necessarily as benign as one would think.

David F Tilley
07/07/2011

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