If your brick or block built tank has two or three chambers there is a good chance that the last one will be a filter chamber, even if you can only see one or two access covers there may well be a buried filter chamber on the outlet of your tank. It is not uncommon to discover buried filters as they are seldom inspected and never emptied unlike the holding chambers and therefore become buried or concealed over the years

How Septic Tank Filtration Systems Work

As the name suggests the waste water passes or filters through a filter media before leaving the tank, the process itself cleans the waste to some degree but a second process occurs as a biomass forms in the media which aided by the oxygen within the granular fill cleans the debris left by the filtering process.

As with the septic tanks themselves there are many variations on the design of these tanks, some are merely a third chamber below ground or they can be above ground and open to the elements with elaborate pipe work designed to disperse the waste water from a rotating arm onto the filter media as shown in the image above.

Filter Media

Traditionally the filter media would be furnace clinker, hard coal or coke breeze but with coal fired power stations being out of vogue other materials have also been used such as broken granite, slate or lime stone

There are now synthetic media's available including plastic (see image to right), cut foam and recycled crushed glass

Filtration Sheets

There is little point having a 2mtr square chamber full of clinker or lime stone if the waste water discharges out of the end of a 100mm diameter pipe and into the same spot on top of the media, we therefore install a filtration sheet in order to disperse the water evenly through the tank which both helps the biomass do its thing and generally prolongs the life of the process.

Waste water discharging into one small section on the surface of the media does not capitalise on the full capacity of the chamber, the cleaning process works when the whole area is used over a period of time. The use of a sheet makes the cleaning of the chamber easier as the sheet can be removed and scraped off as opposed to having to excavate the media itself.

A filtration sheet would normally consist of a corrugated sheet (no asbestos please) with holes or slots evenly placed on the lower level of the troughs. Running at a right angle to the sheet, directly beneath the inlet pipe at one end of the tank you place a length of guttering or half pipe, again with slots or holes in unison with the corrugated troughs below. The idea being that the water disperses evenly through the gutter and onto the sheet which again disperses the water evenly over the whole surface area of the media.

Side View	Overhead View

When Filtration Tanks Fail

When filter chambers clog and fail it is usually time to get the shovels out, as you can imagine this is where your drainman or sanitary engineer really earns his money as the filter media has to be excavated usually by hand.

A decision then has to be made as to whether the original media can be washed off and replaced or if it has to be removed from site and replaced. Things to be considered are, can you wash it off without causing any pollution or making a mess of the garden and is the excavated media suitable for going back in the tank , if the media requires removing from site then the job becomes labour intensive if everything has to be barrowed to a skip before all your new media has to be barrowed back to the tank.

There is little point in washing and replacing the media, or importing a couple of tonne of new stone if you don`t install a filter sheet, little point in installing a filter sheet if there are no dip pipes, and so on. It therefore makes sense to give the whole system an overhaul and as the tank would normally require emptying prior to the works starting now would be the time to do it.