How do septic tanks work, and do I need to use additives?

A septic tank should cost £200-300. Off the shelf models are simpler, cheaper and more reliable than DIY. The size of a tank in litres can be estimated in litres using: 2000 + (180 x the number of of people). If the tank is too small there will be no time for settlement.

The tank should always be filled to a constant level - if the level fluctuates, sewage must be leaking out and/or ground water leaking in. If it is being installed on flat land or if there is a need to raise the effluent then the tank can have a third chamber from which effluent can be pumped.

The tank will fill up until it reaches the equilibrium point, after which incoming sewage will displace effluent to the outlet. The crust will gradually form on the top of the tank and sludge will build up on the bottom.

For septic tanks there's really no need to add bacteria. In our temperate climate the anaerobic digestion rate is so slow that septic tanks function much more as a sedimentation tank than a bio-digester, i.e. physical processes are much more important than biological, so there is no need to try to optimise the biological processes with additives. A healthy septic tank should already have all the micro-organisms it needs. One would be better advised to help an ailing septic tank by reducing water use in the house and ensuring that all the right bits are inside the tank ('dip pipes', baffles etc), and getting it emptied from time to time. The only time we recommend additives for any sewage system is the addition of freeze dried bacteria or sewage to package treatment plants that are not used in winter and need booting up in the spring. The only exception is systems connected to commercial kitchens without grease traps in (when fat breaks down it makes it really acid, and this can slow down the bugs).

Some people have septic tanks that they say have never needed emptying. This may be because they have a very old tank built of brick, or maybe a pit with a very large capacity. Old systems can leak liquids and solids through cracks or through the cement joints, leading to groundwater contamination.

In hot countries you can sometimes do without them as the composting rate is so much quicker in higher temperatures.

Unblocking drains is a different matter. Drain cleaning products may help, but we haven't done actual studies on this. They are the sort of thing that should be used as a one off. If pipes regularly block then there is a problem to address (blocked pipes being just the symptom). This could be a poorly performing septic tank, or excessive grease (in which case install a grease trap and stop grease entering sewer at source), or a poorly designed or broken stretch of pipework (in which case you may need to excavate and rebuild).

There may be no crust on the surface of the water in the tank if just toilets flow into the tank, and not waste water (from baths / showers / kitchen). The crust is made up mostly of floating fats, oils and grease (and food). These come from the kitchen sink mainly, but also some grease from the skin is found in bath/shower water. So if all these sources are absent there may be no crust. It's all to the good, and you may not have to empty your tank for a good while.

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