Understanding Septic Tanks


lf we consider septic tanks 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.


 Let us consider the organic solids in sewage

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.

Detergent image for sewage



How well does a Septic Tank 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 septic tank and some of the bigger brick tanks is no better than 200ppm. Clearly the septic tank 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 septic tank.

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 septic tank 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 septic tank 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.


Manhole Cover Allerton Sewage Treatment

Eventually the water level in the solid affects not just the septic tank 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.

Allerton Septic tank conversion It is a piece of kit, made to measure, installed in the septic tank 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 septic tank.

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 septic tank 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.