A large percentage of sewage consists of complex organic-chemical conglomerates that are broken down (degraded) into their simple constituent compounds by bacteria. This is a natural process that takes time. The more complex the compounds (like trans fats), the longer it takes. Human sewage generally has a high fat content (see Changes in modern effluent). As a result, COD (sewage scum) typically has a specific gravity lighter than water, so it floats. That is why there should always be a 'crust' in the primary chamber of your septic tank.

Bacteria know what’s good for them, so they rise to the food source and become concentrated in the liquid in the top third of the primary chamber. The microbe population is at its most dense under the crust, gnawing away at it. Partially degraded COD slowly descends and is attacked by lower bacterial orders in a continuously refining process. Inorganic radicals (minerals, sulphur compounds and metals), which form less than two percent of human sewage, drift to the floor where they settle as a fine sludge that will need to be removed periodically (usually every 15-20 years or so).

Where do the bacteria come from?

Initially from the tools, earth, dust and air that enters the tank during installation or construction. Then, when the first few loo pans full of sewage arrive they bring with them new colonic bacteria from the human gut. Gradually the bacterial populations coalesce and adapt to their new sewage substrate and build up a biomass that reproduces and feeds off the sewage, degrading it as it does so. For more on bacteria in action go to How ‘bugs’ actually work.

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