There are so many types of septic tank systems that we can only really scratch the surface of the subject at this stage, the type and construction of your tank will be dependant on age, location and who installed it. Many of these systems were installed prior to any building regulations being in place and even after they came into force tanks were installed quietly without any involvement from building control.
Even though drainmen and sanitary engineers have been around for many years a lot of these systems were installed by builders, this is not to say that a good builder does not know how to install drainage systems but like any trade there are the good and the bad, unlike a dodgy roof or window frame a poorly constructed septic tank can go on for years without being noticed because they tend to be out of the way at the bottom of the garden.
Tanks can be brick/block built or concrete sectional and i have worked on tanks that were basically a 2mtr deep vitrified clay ware Gully, modern tanks are constructed from fibre glass or plastic and again it all depends on the position of the proposed tank and how accessible it is for an excavator as to what type of tank you will find. I have also worked on septic tank systems constructed out of old oil tanks and once we excavated a system that was partially constructed out of a large oak barrel.
The image below shows a basic brick built three chamber tank, the first two chambers are holding tanks where solids and waste float to the top of the tank while the heavier sediment sinks to the base, the installation of Dip Pipes or H Pipes prevents the solids from passing through the system. The last chamber is a filtration tank where the waste water passes through a filter media of some description before exiting the tank through the outlet pipe.
It is just as likely that your brick built tank will have one holding chamber which discharges directly into the filter media or there may well be two holding chambers and no filter chamber. We still find properties with no chambers and no filter media, they just discharge directly into rivers and brooks which as you can understand the rivers authority get a little upset about.
The system separates the solids and waste from the water, the build up of waste or the crust is then broken down by bacteria and enzyme to some degree, every so often the tank requires emptying to remove an excessive build up of waste matter and sediment from the base of the tank.
Storm Water And Septic Tanks Systems
Storm water should be kept separate from the septic tank system as it can dilute and hinder the bacteria and enzymes as they do their thing in the tank, the constant and heavy flow from prolonged rainfall will also carry more solids. grease and fibres through the system potentially clogging the filter media or soak-away system. On older properties where they discharge into rivers, brooks or water courses you often find the storm water system connected onto the outlet pipe from a tank in order to dilute the final discharge water.
Septic Tank Emptying
Every tank is different as is the use it gets, a family of four will obviously test a tank more than the little old lady down the lane, i have worked on modern systems that require a yearly empty and i have excavated tanks that have been buried for forty years and have never been emptied
Modern detergents used in the home every day can effect the workings of the tank, you can buy enzyme and other products that will help maintain the workings of the tank or you can as the farmers used to do throw the odd dead cat or chicken in to activate the bacteria. Your contractor will be able to advise as to how often your tank requires emptying and it is good practice to leave a certain amount of the solids and crust in the tank after every empty to give the tank a good start on the next batch.
Dip Pipes & H Pipes In Septic Tanks
These are placed within the dividing walls of a chamber to isolate the solids and waste matter but they also allow water to freely move through the system, they are no great mystery and are easily constructed out of standard bends and square junctions. The base of the dip pipe is extended down towards the base of the chamber so that the deeper sediment free waste water passes up through the pipe and into the next stage of the system.
Unfortunately the older vitrified clay pipe work was both heavy and brittle and prone to damage when the tank is emptied, so you often find the dip pipes to be broken off and lay at the base of the tank.
Clogged filter beds or soak-away's are generally the result of solids moving freely through the septic tank system due to a lack of dip pipes, H pipes and baffle walls.
Baffle Walls In Septic Tanks
There are alternative methods used on brick built chambers for separating the solids such as baffle walls, Image A below shows how the water passes below the central baffle wall before rising and flowing through the outlet pipe, the solids are retained in the first chamber as they literally hit a brick wall. Image B shows how this is easily achieved by missing out alternate bricks in the base course of the baffle wall. This method works well as long as the tank is emptied and maintained, as a build up of sediment in the base of the first chamber will seal the wall preventing waste water from passing through the system.
Septic Tank Filter Systems
The waste water that discharges from the holding tanks would normally pass through a filter tank prior to running to a soak-away or outfall, this could simply be another tank full of clinker which the water trickles through on its way to the outlet.
These tanks do over the years become clogged with silt and sediment and require attention, this can be due to a lack of dip pipes on the main tanks, the storm water from the property passing through the tank or the beams or slabs that cover the tank allowing soil and silt to be washed into the filter.
When this happens it is necessary to dig out the tank and either wash off the existing filter media before replacing it or removing it from site and replacing it., with clinker not being as freely available as it once was washed lime stone or similar can be used.
Other defects found on these systems are the wrong sized filter media being used as a small grade stone or clinker will clog up easily, or the outlet from the last holding tank discharging into the small section of the filter bed instead of being dispersed evenly over the whole surface area using a filtration sheet.
Another problem found on these types of system is that if the outlet pipe from the filter system blocks due to something like root ingress there is seldom an access point to clear the line and you have to either excavate the filter tank or excavate on the outlet pipe.
Modern Septic Tanks
Modern tanks constructed from fibre glass and plastics work in much the same way as the older systems as the image to the right shows, the central baffles divide the tank into three sections whilst allowing waste water to move through the system.
The inlet pipe discharges directly into section (a) where some solids float and others settle, clarified effluent passes up through the slotted baffle into area (b) where finer particles of scum and waste are again separated before the waste water passes into section (c) and into the outlet pipe work.
The central ball rises to the top of section (a) forming a seal when the tank initially fills, when the tank is emptied the ball lowers allowing access to the sludge at the bottom of section (a)
Septic Tank Soak-Away Systems & Leachfields
Any septic tank system is only as good as its outlet or soak-away and again the construction of the soak-away will depend on the location of the tank, the ground type and just as importantly who installed it.
A tank soak-away could be anything from a 100mtrs of perforated pipe work laid in a herring bone system or as is more common on some of the older systems a small length of pipe leading to a pit full of tile and bricks and in the right ground conditions it can literally be a length of solid pipe with a brick over the end.
These soak-away`s can fail due to the sub-soil around the pipe work becoming sealed due to grease, fat and fibres passing through the tank system or they can become saturated or water logged, it is then necessary to sonar trace the system and excavate to confirm its extent and construction.
Unfortunately many contractors use materials designed for land drainage when installing a leach field, there seems little point installing 75mtrs of fully perforated pipe work surrounded in granular fill when all the water leaves the pipe work within the first 3 meters downstream of the tank.
You will note the image above shows a system on the outlet of the tank that is basically a loop of drainage , the pipe work used is a solid wall pipe with slots or holes evenly spaced on each side of the pipe, the idea being that if you install the pipe work as a level unit it will always be half full and when water enters the loop it is dispersed evenly into the entire system through the holes in the side of the pipes.
This of course will never happen unless you employ laser levels and satellites but you get the general idea, but by using this method of installation you allow the waste water to soak into a larger area which in turn helps with the breakdown of the waste water and prolongs the life of the system
Septic tanks installed local to brooks, rivers and water courses are potentially a pollution threat there are also occasions when it is not possible to install a soak-away on the outlet of a septic tank system because of the ground type or not having the required area of land in which to drain, it is therefore necessary to install a Sewage Treatment Plant.
Whilst septic tanks clean the water water sufficiently to discharge into a subterranean soak-away the second stage of sewage treatment is required if the tank outlet is to run directly to a water course of some description. This involves installing a unit fitted with a compressor that provides continuous aeration to the tank which encourages the bacteria, the use of such systems also does away with the need for a filtration tank.
Septic Tank Conversion To Treatment Plants
There are units that can be applied to an existing septic tank which will convert it into a Treatment Plant cleaning the waste sufficiently enough to be discharged into a water course.
Which ever system you choose thorough checks should me made with the manufacturer and any relevant bodies such as the environment agency before the system is installed, as hefty fines can be handed out for polluting a water course.