A drainage field, often referred to as a soakaway, follows a septic tank or a package sewage treatment plant. It fulfils three functions:
To that end, a drainage field is more accurately called a treatment field. It’s absolutely not just a device for getting the partially clarified effluent from the septic tank to soak away as quickly as possible.
Designing an effective and long-lasting treatment field is straightforward. There’s a British Standard Code of Practice - BS 6297:2007+A1:2008 Code of practice for the design and installation of drainage fields for use in wastewater treatment - that explains everything that must be done, from site evaluation through percolation testing through trial holes to work out where the water table sits to methods of construction. Very briefly, the size of the treatment field is a function of the flow of wastewater from the property served and the rate at which the ground can accept that wastewater – its porosity – to finish treating it before the now-clean rejoins the groundwater.
Why’s it important to clean the wastewater before it flows back into the groundwater? It’s because a lot of drinking water is abstracted from groundwater held in underground aquifers. Any pollution of the aquifer means the company supplying your mains water has to treat it more intensively, which means it costs more – a cost which is always passed back to you, the consumer. And it tastes worse because of the chemicals used to treat it.