What is an Invert Level?

An invert level is the measurement or depth of the drainage pipe from ground level to the base of the pipe as it enters the inlet.
It is important that the sewage enters the tank through the drainage pipe via gravity. A deeper invert may be needed to allow for this.
The invert level is different on every site, and the Bio-Pure can be adapted to meet any individual requirement.


The correct gradient must be established to ensure the sewage is flushed efficiently through the pipe with no build up of solids.
The recommended gradient level is between 1 in 40 (2.5%) and 1 in 110 (0.90%). This means that for every 40 feet horizontally there will be a 1 foot vertical drop (or rise). A gradient of this level will provide sufficient flow velocity.
The gradient level considered ideal is 1 in 80 (1.25%).
Having too steep a gradient will separate the liquids from the solids as the sewage is flowing at a much quicker rate. This will then cause a build up of solids, leading to a blockage within the pipe.
However having a gradient that is not steep enough can also cause similar problems.
If you cannot discharge via gravity or if the outlet pipe is above the inlet, then a pumped outlet will be required to pump the effluent over the gradient and into a watercourse or soakaway.

How to measure a gradient

The easiest way to measure a gradient is to divide the vertical rise (or drop) by the horizontal distance.
For example, if you have a 2 feet drop or rise on a horizontal distance of 105 feet you divide the 2 together and multiply by 100 to get the percentage gradient of the slope.
2 / 105 = 0.01904761904762 then multiply that by 100 which would give you a 1.9% gradient.