Brick Built Septic Tanks
- 21 November, 2019
What are Septic Tanks?
Septic tanks (ST) are used to deal with sewage water from lavatories, kitchens, bath, showers, and wash basins. They are not meant to take rainwater as large volumes of rain water can wash all the carefully stored solids out into the soakaway! This can contribute to a failed soakaway, and potential contamination.
How Brick Build Septic Tanks Are Designed
The old septic tank design has often been modified to suit ground conditions at the time of construction. Their depths often not as planned due to hard rock or wet running sand, or too much water ingress creating excavation problems. Once circumstances affect design, the purpose of the ST is compromised. Ideally, a ST should have two chambers, the first twice as big as the second. Often, they are constructed with one chamber only. Another compromise based on cost!
Consequences
The consequence, generally not understood by all, is that the amount of solids carried out into the soakaway is far more than it should have been. The ST is designed to have about 600mmm (2ft) of solids in the bottom. More than this and the tank is full! Once it’s full of solids it should be emptied immediately.
The height of the inlet pipe should be 1.4m above the tank bottom. This allows for a settlement zone, 800mm deep and with a volume of water which allows the incoming effluent to gently sink to the bottom storage zone. The speed of water through the tank, if allowed to increase, increases the amount of solids the water can carry.
The solids that do accumulate on the bottom increase in depth and so the speed of water increases, as the settlement zone volume decreases, carrying most of the solids, including fine solids scoured from the bottom into the soakaway or drainage field. This is why regular sewage system maintenance and septic tank emptying are essential.
Rule of Thumb
- Each household is different, but it is generally accepted that raw sewage has 300 ppm, solids to water.
- The effluent from a good ST is considered to be about 200ppm.
- Treated water, using a small packaged treatment plant is 20ppm
- Roof water is about 5ppm with tap water zero of course!!
So, only a third of solids are retained, two thirds goes into the soakaway.
Life of a Soakaway
The life of such a soakaway varies depending on ground conditions. Typically, a soakaway will last 10 to 20 years, but 60 years is possible too! While I have described the effect of not de-sludging on the bottom solids, the top crust, or floating solids can be an even bigger menace.
Floating solids are fat, Oil and grease (FOG). It is essential that floating solids do not enter the soakaway. Grease will live in the soil for years and bacteria there do not readily break it down. To prevent this happening, a Tee pipe should be installed on the outlet pipe. This way, floating solids will not simply float out! However, if there is no Tee pipe or it has been broken off, the Fog goes into surrounding drainage field, causing a blocked soakaway.
If the ST has a tee pipe but is not de-sludged each year, then the floating solids layer gets thicker, and so goes over the top of the tee. So again, the soakaway is under attack. Instead of a decent 20 year life, it can have less than say five years.
However, nothing appears to happen on the surface, as it is all hidden from view, until that is, the level of dirty water in the soakaway is well above the outlet pipe. Gradually the depth of water increases until all of the floating solids go straight over the outlet tee pipe. A failed soakaway like this can cause contamination and terrible smells.
De-sludging
Now we have the sludging up of the soakaway speeding up. We may have a few years to go, but eventually the ST and manholes uphill of the tank get filled to the top (or close to it).
Solids do not flow properly in water filled pipes. This manifests its self by septic tank overflowing at manholes and or ST. After a little time the kitchen waste overflows onto the path. Or the downstairs lavatory will not flush! This is not the time to panic.
The time to panic was years ago when the water in the ST is at the same level as the inlet pipe indicating that the Soakaway was failing. Septic tank de-sludging solves the problem in the short term but three weeks later, it all starts to happen again, as the soakaway is already compromised.
What can be done? There are many options but none of them cheap.
Much depends on the state of the garden, its size and accessibility for medium sized diggers. If you get advice quickly you will save real money as emptying it is a very short term solution, and not a cheap option either.
Grease and oil should be poured into old bottles and sent to land waste. Fats cling to fats, whether in London’s huge sewers or in household drains and Septic tanks and even Sewage Treatment Plants (STPs) are affected.
Rainwater down pipes should be checked, ideally when it is raining, to see if any water is getting into the sewage system.
You can also monitor the health of your drainage field by checking the ground water levels within it – if levels are increasing this could be an indicator of septic tank issues, or a blocked drainage field.
We developed the Allerton Ground Water Tube (AGWT) to make this task easier! Find out more about the AGWT.
Conclusion
In a few years’ time, very few septic tanks will be installed in heavy clay soils. However, the biggest merit of the septic tank is that it has very low maintenance cost since it just needs de-sluding once a year. With a sewage treatment plant there comes the cost of electricity as well as servicing.
FAQ
Q: Are soakaway worms useful?
A: We have seen them in Sewage Treament Plants (as well as flies). If there are in there it is fine, but we don’t suggest introducing them into the system.
Q: Can I still make use of the septic tank if the soakaway has failed?
A: If it is GRP constructed you can. If it is brick, it will depend on sizes. You can still make use of the septic tank by installing a ConSept if it’s big enough. In order to fit a ConSept, the water level below the inlet level needs to be more than 1.1m. Also the volume of water needs to be more than 2 cubic metres.
Read our page about Septic Tank Soakaway Problems for more info.
Q: How often does a septic tank need emptying?
A: As a rule, once a year for the average household. It depends on the size of the septic tank and how many people are using it. Two years is possible, depending on the size of the septic tank. If the frequency is more than once a year, it is usually because a soakaway has failed and that shouldn’t be happening.
Check out this table about the pros and cons of septic tanks. Frequent emptying can be quite expensive.
Q: Are there septic tanks that don’t need emptying?
A. No. All septic tanks should be emptied to protect the soakaway!
Read the section about septic tank emptying.
Q: Can rain water go into a septic tank?
A: NO, heavy rain can wash all the stored solids into the soakaway or ditch or block the ConSept.
Q: How do I test to see if rain water gets into the septic tank?
A: 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 summers 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.
Q: Does shower water go into a septic tank?
A: 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.
Q: Which type of septic tank is best?
A: The best ones are made from GRP, glass reinforced plastic. Or Polyetheylene. Concrete ring septic tanks are good but need two chambers.
Q: How long do plastic septic tanks last?
A: Polyethene and GRP septic tanks should last 20 years at least, providing they are emptied every year to protect the soakaway and the baffle.
Q: What is the alternative to a septic tank?
A: 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.
Q: How much does a 3 bedroom septic tank cost?
A: The smallest septic tank is good for a four bedroomed house. Costs £600 to £750 plus vat.
Q: How many loads of laundry can I do with a septic tank?
A: 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!
Q: How do I know when my septic tank is full?
A: It’s difficult to gauge, so have it emptied every year. This will help lengthen the life of a soakaway.
Read our Septic Tank Soakaway Problems page for more details.
A: Soakaways from Septic tanks are bound to get blocked. Raw sewage has 300 parts per million (ppm). Water from a septic tank is not much better at 200 ppm. So those fine particles start to block the soakaway and the ground below and around the soakaway pipes from day one!
Q: Do soakaways need maintenance?
A: It is very hard to maintain a soakaway. Like all filters, they block!
Q: How long do soakaways last?
A: The more permeable the soil, the longer the soakaway lasts. It is important to not let the floating solids escape into the soakaway by having the septic tank emptied annually.
Q: How do I know if the soakaway is blocked?
A: The water in the outlet pipe is covering the bottom of the pipe. It should be free of water!
Q: Why do soakaways fail?
A: Soakaways have a limited life, even if the septic tank is emptied annually. It is a filter in effect. All filters get blocked eventually.
Q: How deep is a soakaway?
A: The soakaway is as deep as the outlet pipe as it leaves the septic tank. If it is in the ground too deep it may not work at all! Clay at depth is much more like Plasticine and clay is used to line reservoirs and canals! If sandy, the soakaway will work much better, but not if the table rises in a wet winter to a level near the soakaway pipe. A wet flannel is useless!
Q: Do soakaway worms work?
A: The ground is full of earthworms and other worms. They don’t like anaerobic conditions.
Q: How do you use soakaway worms?
A: I do not think that they work and so I would not use them!
Q: Are soakaways covered by insurance?
A: Not as a rule. They are limited in their life. Sometimes if they’re too deep they have a very short life!
Q: How long does a septic tank soakaway last?
A: Generally 5-25 years.
Q: How do I install a soakaway in my garden?
A: The best soakaway for sewage effluent is gravel. The Environmental Agency and British Water say the plastic soakaway bales should only be used for stormwater, i.e. water that is not foul.
- The way to install a foul soakaway is to lay perforated pipe 100 mm dia as flat as you can. This prevents all the water rushing to the far end, having missed the clean gravel in the first part!
- The gravel should be single size, so as to have the biggest voids.
- The bed should be 600 mm wide and about 300 deep.
- For septic tank water, you will need to replace the soakaway in another part of the garden some 5 to 25 years later. NO one knows!
Q: How far should a soakaway be from the house?
A: 7 metres or more.
Q: How do you unblock a rainwater drain pipe?
A: Rainwater drain pipes and gutters should be checked in December and cleared of grit from the roof, bird droppings, dirty leaves, any grass and sycamore seeds in particular.
At the foot of the down pipe, the back inlet gulley and its grid should be cleared of all debris. This does mean plunging your hand into the gulley to remove all the leaves, etc.
Any accumulation means that future solids will overflow into the storm soakaway, reducing its life. Rainwater soakaways tend to be shallow and rainwater has about 5 ppm solids to water. Septic tank water is 200 ppm or more!
Q: How do you do a soakaway test?
A: See BS6297 2007: Code of Practice for the design and installation of drainage. Fields for use in Wastewater Treatment.
Q: Does a septic tank need a soakaway?
A: I’m afraid it does. Septic tanks seldom fail!
Victorian septic tanks were designed for far less water usage than today’s profligate use. But it is generally the soakaway that needs replacing. This will need to go in another part of the garden if the existing ones position is known and if there is space. The garden may have many old systems. No one knows!
Q: What is the difference between a soakaway and a drainage field?
A: Both terms are used for the same thing. A system of pipes used to disperse water underground!
Q: What to fill a soakaway with?
A: It is not good to dig a hole and fill it with stones, bricks, or other hard material. It’s far better to have a long pipe with clean, single size gravel under the pipe. This way the load is spread over a large area, not all in one place.
Q: Does my house have a soakaway?
A: Many houses have soakaways for the downpipes from the roofs. Because rainwater is clean, the soakaway works faultlessly. That is providing the gutters are cleaned and the back inlet gulleys emptied.
Q: How deep should a soakaway be in clay soil?
A: All soakaways should be as shallow as possible.
- If you are having to pump, keep the pipe to within 100 mm from the surface.
- Water travels quickly in topsoil, both laterally and downwards.
- Dig a hole 3 feet deep in clay soil and dig a hole 1 foot deep nearby.
- Put a bucket of water in each hole and observe.
Q: Who is responsible for unblocking drains?
A: The householder. The tenant or the landlord if it is in the agreement. The local council will not maintain drains on private land.
Q: What size should a soakaway be?
A: That depends on the nature of the water percolation test. Rainwater at 5 ppm? Treated effluent from a sewage unit at 20 ppm? Or effluent from a septic tank at 100 ppm?
Q: Do I need building regs for a soakaway?
A: Yes
Q: Can rain water go into foul?
A: No! Rain can be so heavy and in large volumes rainwater can flush away all the carefully settled solids in one go! All into the poor old soakaway, clogging it with yet more solids and worse, floating solids, i.e. fats and oils!
A: YES, if there is room.
Q: Can a soakaway get blocked?
A: Soakaways from Septic tanks are bound to get blocked. Raw sewage has 300 parts per million (ppm). Water from a septic tank is not much better at 200 ppm. So those fine particles start to block the soakaway and the ground below and around the soakaway pipes from day one!
Q: Do soakaways need maintenance?
A: It is very hard to maintain a soakaway. Like all filters, they block!
Q: How long do soakaways last?
A: The more permeable the soil, the longer the soakaway lasts. It is important to not let the floating solids escape into the soakaway by having the septic tank emptied annually.
Q: Who is responsible for unblocking drains?
A: The householder. The tenant or the landlord if it is in the agreement. The local council will not maintain drains on private land.