Percolation Areas and Polishing Filters 

Percolation Areas and Polishing Filters

A percolation area is a system of trenches, pipes and gravel which is used to transport septic tank effluent from the tank and transmitting it into the soil for final treatment and disposal. The terminology changes when a percolation area is following a wastewater treatment system, when they are called soil or sand polishing filters.

Polishing filters are not always constructed using soil and can often be sand filters or other tertiary filters. Here we will outline the different options for what comes after your tank.  


Percolation Areas Following Septic Tanks

The septic tank will remove most of the solids from your wastewater, along with some anaerobic digestion. The percolation area is required to further treat your wastewater and distribute your septic tank effluent. The soil or subsoil underneath will act as a filter and a pathway for the effluent to soak into the groundwater. 

The stone should be in the size range 12-32mm, and it should be clean washed stone. The shape is not important, it can be gravel or broken stone aggregate. Each trench is 500mm wide, and the trenches are 2 metres apart, or 2.5 metres from centre to centre.  

Following a septic tank, the maximum trench length is 18 metres, and you should allow 18 metres of trench per person.  


Percolation Areas Following Bio-Crete Wastewater Treatment Systems

The effluent following a wastewater treatment system is much cleaner than that from a septic tank, which means you can reduce the size of your percolation area. If you can achieve a gravity flow from your Bio-Crete tank to your soil polishing filter, you can use a standard percolation trench. These are almost the exact same as the trenches following a septic tank, as described above, except the maximum trench length is 10 metres following a wastewater treatment systemThis can vary depending on your percolation values and you should check the length required for your site with your architect or engineer.  

Soil Polishing Filters

These are commonly used after a Bio-Crete wastewater treatment system when you need to raise or partially raise your soil polishing filter. The most common reasons to raise your soil polishing filter are a high watertable level (watertables can be seasonal) or the presence of rock in the trial hole. The effluent is pumped from the Bio-Crete tank directly into the manifold of 32mm pipes, as shown in the drawings below.  


Sand Polishing Filters

Sand is a highly effective filter medium. It has been used for many years in water and wastewater filtration and now we are using sand as a tertiary filter following wastewater treatment systems. Sand polishing filters following Bio-Crete wastewater treatment systems are a highly effective combination to treat wastewater, while also providing a space saving solution for smaller gardens.  

Similar to soil polishing filters, sand filters use a network of 32mm pipes to distribute the treated effluent into a layer of gravel. From there the effluent will trickle through the sand and into the ground below. Sand filters can be raised or subsurface. They can also be constructed using a single grade of sand, called a mongrade sand filter, or multiple layers of sand with different grain sizes, called a stratified sand filter. 

Soak Holes

Soak holes were quite common following a septic tank, and many of them still exist in working condition to this day. They consist of a deep hole filled with stone. Effluent from the septic tank flows into the stone and down into the ground. Soak holes are no longer recommended because percolation areas offer a much better solution to wastewater treatment and disposal. With a soak hole, the effluent goes into a deep hole, often close to the water table. They can malfunction when the water table is high and can be a source of groundwater pollution. Percolation areas are a better option because the effluent will be spread over a wider area, above the water table and into the soil which will act as a bio-filter to further treat the wastewater as it soaks away.

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