how septic tanks work

How Septic Tanks Work

A septic tank system would normally comprise of one/two/three chambers which allow the solids in the waste water to separate, the solids either float to the surface or settle at the bottom of the chamber/s whilst the micro-organisms do their thing. Most of the solids in suspension are composed of nitrogen, carbon, oxygen and hydrogen, the bacteria liquify parts of the suspended matter and release the remainder as a gas such as nitrogen, carbon dioxide and methane.

The remaining waste water would then either run directly to a river or brook (obviously not acceptable today) or to a soak-away or leach field. Many septic tanks have a filtration tank where the waste water filters through a media such as furnace clinker, hard coal, coke breeze and broken granite before leaving the system.

Typical Two Tank System

The image above shows a basic two chamber brick built tank, it is essential that barriers or baffles prevent solids and waste from passing through the system and into the outfall. In this case Dip Pipes and a H Pipe are used to isolate each chamber.

The waste water enters the 1st holding chamber through a dip pipe, the lighter waste floats to the top of the water and the heavier matter settles at the base of the tank. Waste water then has to pass up through the H Pipes and into the 2nd chamber where the process is repeated so as to allow any remaining matter to separate before the water leaves the tank system through the final dip pipe.

As part of a regular maintenance programme the tanks should be emptied with the crust and sediment mostly removed, it is however good practice to leave a small amount of the debris within the tanks to kick start the bacteria process on the next batch of waste.


System With Filter Chamber  

The waste water passing through the filter bed is cleaned further before finding the outlet pipe and running to its final outfall or soak-away, the fine particles of waste remaining in the filter media are again dealt with by micro-organisms.

A well installed filter bed will have a filtration sheet on top of it to ensure that the waste water passes evenly through the media.


Dip Pipes

An essential part of any septic tank system are the Dip Pipes and H Pipes that isolate each chamber and prevent solids from moving through the system.

They are no great mystery and can be easily assembled using standard drainage square junctions, the image to the right shows how waste water moves freely between two holding chambers whilst the solid waste and matter are retained in the first section of the system.

www.draindomain.com_septic_tank_H Pipes

Baffle Walls

Another method commonly used to devide chambers in a septic tank would be by installing baffle walls, again this is simply acheived by missing alternate bricks within the lower courses of the cental deviding wall, regular emtying of the system would be required to prevent a build up of sediment from cealing the said holes.
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