Element

Remarks

Temperature

Probably the most important factor; longer survival at lower temperatures; freezing kills bacteria and protozoan parasites, but prolongs virus survival.

Moisture

Low moisture content in soil can reduce bacterial populations.

Light

UV in sunlight is harmful.

pH

Most are stable at pH values of natural waters. Enteric bacteria are less stable at pH > 9 and pH < 6.

Salts

Some viruses are protected against heat inactivation by the presence of certain cations.

Organic matter

The presence of sewage usually results in longer survival.

Suspended solids or sediments

Association with solids prolongs survival of enteric bacteria and virus.

Biological factors

Native microflora is usually antagonistic.