Understanding Septic Tanks
Did you ever wonder what a septic tank (ST) is and how it works? Well we answer your burning questions on this page.
Inefficiency of Septic Tanks
lf we consider STs as a means of treating sewage, accepted measurements show how inefficient they are. Like lead pipes, if they were offered now as a new idea, they would not find acceptance except in certain conditions. Keep reading to learn more!
What are our customers saying?
Consider the Following: Organic Solids in Sewage
A Septic Tank is a tank in the ground which partially treats raw sewage before the dirty water goes into a soakaway (drainage field).
Raw sewage, i.e., that which comes from the bath, shower, lavatory, dishwasher and washing machine is a complex set of chemicals together with the obvious organic matter. In fact, it is the organic waste which provides food for the bacteria to help break down the chemicals: washing powders, cleaning materials and shampoos etc. Basically all that we buy in bottles and eventually flush away.
How well does a ST cope with all this?
The answer is not very well.
Raw sewage from a typical household is valued as 300 BOD or Biological Oxygen Demand. This is the same as 300 ppm (parts per million).
What comes from a modern GRP ST and some of the bigger brick tanks is no better than 200ppm. Clearly the ST retains a third of the solids which have to be removed and treated at a sewage works. – A very inefficient system indeed.
The ground is expected to take this dirty grey-black effluent and deal with it.
The fact is it can’t. Over the years the gravel below the soakaway becomes black with all the voids filled with the dead bodies of anaerobic bacteria. The water can no longer soak away below the pipe so it rises up above the pipe.
This process starts from day one. Over several years nothing appears amiss, not until the water can only get away at a level above the pipe. And as it is so doing, blocks this breed of soil. This can go on for many years… slowly blocking the soil above, below and around the soakaway. A lot depends on the nature of the soil, the water regime and time and the number of people using the system.
The Septic Tank Has Three Zones
The bottom is for storage of the sinking solids, the top will form a crust often, but not always. This is the FOG i.e. fats oils and grease.
The volume of water below the floating grease and above the solids at the bottom is the Settlement Zone. Here the mixture of water, organic solids and chemically laden water is allowed to settle. A family of four would use 600 litres each day. The volume of the tank after being emptied, is 2800 litres for the smallest ST.
So the water has four or five days to move slowly through the tank, allowing solids in suspension to settle out. This volume is reduced after a year or so, now the water is taking just three days to pass through. lf the ST is not emptied, then the bottom solids increase in depth and the settlement zone is reduced… and so the speed increases.
If it is halved, then the speed is doubled and not all the solids settle out and so rather than 200 ppm going into the ground more like 230 to 250 ppm goes through, hastening the demise of the soakaway.
Now the water rises higher and higher above the perforated pipe. Till in fact it is above the tee pipes in a brick ST or FOG controlling baffles in a modern GRP tank. Now we have the grease escaping, accelerating the breakdown of the soakaway.
So many think it is because of heavy rainfall that winter. This obviously saturates the ground and makes matters worse but regularly de-sludging will help the life of the soakaway whatever the rainfall.
Eventually the water level in the solid affects not just the ST but the water in the manholes, so much so that you worry when you flush the lavatory or the kitchen waste pipe floods onto the footpath… Your investigations show the manholes to be full. Using less water helps but eventually you have to have the tank emptied more and more frequently.
This is where Allerton gets called in to solve the problem. Installing a pipe into a ditch is not the answer as the Binding Rules make clear.
If there is a ditch, by treating the water so that the final effluent is not 200 ppm but 20 ppm then there is a very good chance that a permit from the Environment Agency can be obtained.
But what if there is no ditch? What if the soakaway area is limited to a small garden. What if access to the garden is by a metre wide gate preventing easy digging and movement of gravel and spoil to the tip??
What Then?
Then Allerton will survey the situation, taking measurements, asking details questions and come up with an answer. The result is often the installing of the ConSept.
It is a piece of kit, made to measure, installed in the ST via the lid. The anaerobic bacteria are blown away by the powerful air blower which creates an aerobic condition and encourages the good aerobic bacteria to live, breed and multiply, living on the organic material and breaking down much of the chemicals too.
This is good news, sure, but how to get rid of the effluent when the soakaway has failed?
Allerton simply dig down to the outlet pipe and put an expanding plug in the pipe as it leaves the tank, preventing any water in the ground getting back into the ST.
Now the clever bit, we use the air syphonically to lift the treated water from the tank up to ground level, into a sample chamber just 200 mm deep.
Now we connect this chamber back to the existing soakaway pipe.
What happens next is that we now have cleaned water, akin to rain water, travelling along the perforated pipe, rising up above the pipe till it can move freely just below the topsoil. Once away from the contaminated soil, it finds its way down into the ground. The big difference is that the water is not backing up into the ST or any of the manholes between it and the house.
Eventually the clean water will have cleaned the contaminated soil and so the ground slowly has a chance to recover.
Sometimes this ideal doesn't work! What to do then?
The sample chamber is only 200 mmm deep and an additional pipe can easily be dug by hand at a very shallow depth, in the topsoil. So a digger is not needed, just a Saturday afternoon of hard labour!
Think about this: Five people use 150 litres each so each day produce 750 litres. A roof 6 m by 10 m sheds its water to the down pipe and its soakaway, 60 sm x say 10 mm = 600 litres. Where does it go to?… Into the ground without you even knowing that there is a soakaway.
This is because it can soak away at a shallow depth and also because it is clean water, said to be about 5 ppm.
So our cleaned water is now almost on a par with roof water.
We have installed well over 1500 of the ConSepts, mainly putting the effluent back to the soakaway, but some of the tanks, with a permit from the EA, outfall into ditches.
We have twenty years of experience with these units and nearly 50 years’ experience in drainage of all kinds.
The units, like all we sell, need proper servicing and our British Water trained engineers are well qualified to do this.
Learn more about Septic Tanks
Every 12 months, with a ConSept. Two years is possible, depending on the size of the Septic Tank.
No. All septic tanks should be emptied to protect the soakaway! Read the section about septic tank emptying.
NO, heavy rain can wash all the stored solids into the Soakaway or ditch or block the ConSept.
1. You will need to establish what type of water is flowing through each manhole on the property.
2. You will need to work out where the water from the stormwater manholes flows to e.g. into a ditch or soakaway.
Lift the manhole covers and observe. Have someone flush the toilets, then run the taps for the shower, then bath, etc. The water coming from each of these sources is foul water. And if you see it flowing through the manhole, it is called a foul manhole.
Once you establish that it is a foul manhole, then check to see that no stormwater goes into any foul manhole. To check that, on a rainy day stand in the rain with nobody in the house and see if any water goes through each foul manhole. On a summer’s day you can squirt water into the gutter to imitate rainfall.
Afterwards, draw a plan of the property and show the foul manholes and pipes in red and the pipes and manholes for the stormwater in blue. This is also useful when you sell the house.
Yes. All lavatory, showers, wash basins, sinks etc are classed as foul. These must go to the septic tank. Rain water must NOT go into the septic tank.
The best ones are made from GRP, glass reinforced plastic. Or Polyetheylene. Concrete ring septic tanks are good but need two chambers.
Polyethene and GRP septic tanks should last 20 years at least, providing they are emptied every year to protect the soakaway and the baffle.
A sewage treatment plant, a cesspool, or a reed bed. If there is no room for a soakaway then a cesspool is used. These are huge tanks and expensive to install and expensive to have emptied.
The smallest septic tank is good for a four bedroomed house. Costs £600 to £750 plus vat.
It is proportionate to the number of people in the house, so it doesn’t matter. However, a septic tank for just laundry is not a good fit. It needs anaerobic bacteria to break down the organic waste and to help break up the chemicals in washing products. Speak to Allerton for advice on this before installing!
It’s difficult to gauge, so have it emptied every year. This will help lengthen the life of a soakaway. Learn more about soakaways.
Carefully feel to see where the heavier sludge is. Deduct this from the depth of the tank and when there is a foot of sludge it is full. Any deeper and the settlement zone above the sludge level is compromised and the flow of water increases, carrying fine solids into the soakway. Not good.
Only if used excessively.
Septic tanks are not meant to be driven over. However, if a reinforced raft is constructed and there is a small gap between the surface concrete and the tank, then it should be fine. Take professional advice.
A septic tank costs about £2500 to replace but it is likely that it is the soakaway is what needs replacing. This can cost £2000. Ring Allerton.
You ruin the soakaway.
Just as good but often easier to install. A lot depends on ground conditions.
It should emptied regularly by a registered waste contractor. The mess inside the walls of the septic tank is actually full of bacteria and should be left alone. Do not hose it down!
Every year as a rule.
Perhaps every two years if only two or three people are using it.
Yes if the house is right in other regards. But, have it surveyed first. That is most essential.
All waste water does not have to go to a septic tank. It could go to a cesspool, a sewage treatment plant, or a septic tank… or better still to the mains. 95% of sewage in the UK goes to the mains.
Yes, every year to protect the soakaway.
The term BioDisc refers to a Sewage Treatment Plant made by Klargester. It is not a septic tank.
It can be discharged to the ditch without EA permission as it is an Exempt unit. It can also discharge to a soakaway if there is no ditch. Read this Klargester blog post for more information.
Why Allerton?
Qualified
Having Allerton service your unit each year means that the problem is recognised quickly and the drainage system saved. We are specialists in off-mains drainage and have 50 years of experience and are knowledgeable in servicing all types of sewage treatment plant installed in the UK.
Value for Money
We offer a free initial consultation for installations, even if we have to travel a distance to do the site visit. We keep our customers in mind when it comes to pricing. We have saved our customers hundreds of pounds over the years. We can do the same for you! Speak to us for a quote!
Friendly
From the office staff to the engineers in the field we are extremely friendly and have received great reviews to prove it. You know you're in good hands with Allerton
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