Aerobic Microbes in water feed on organic matter and dissolved oxygen to form additional cells. Carbon dioxide and water are the end products.

One -third of the organic matter is oxidised. This yields energy to synthesise remaining organics into additional living cells.

Pollution occurs when excess wastes enter the water-stream system. the key factor is dissolved oxygen (DO).

Organic wastes stimulate bacteria growth which uses up oxygen.
As the dissolved oxygen in the water is depleted, the fish are the first to die, then the lower forms of life including protozoa. When oxygen disappears the water becomes septic. anaerobic Bacteria then produce odours and dark colours in the water.

If no more wastes enter the environment, eventually the bacteria will convert all waste to gases and trace elements and the dissolved oxygen in the water will begin to rise to the point where the water will be safe for fish again.

Almost all toxic waste can be removed from water using bacteria.
Some synthetic toxins are exceptions to this rule.

Controlling waste entering an aquatic environment from all sources and maximising the bacterial degradation of this waste will reduce the oxygen demand and make it available to other life forms whilst maintaining a safe environment for aerobic bacteria.

The rate at which organic waste is consumed depends upon the size and variety of bacterial population. As metabolism proceeds the number of microbes increase and the food supply is consumed at a faster rate,
( more efficient bacteria =faster conversion of waste )

To begin with the bacteria double their mass through cell division at set intervals and have a plentiful supply of food and is called LOG GROWTH or EXPONENTIAL GROWTH PHASE The interval varies according to different species and strains.

Food then becomes more scarce and the growth rate slows until finally the food supply is depleted to the point where Bacteria begin to absorb their own cellular mass this is called the ENDOGENOUS phase.

In an aquatic environment Algae use carbon dioxide and release oxygen whilst aerobic bacteria need oxygen but release carbon dioxide and so assist each other to survive.