Learn more about ConSepts, Soakaways, Sewage Treatment Plant, and Laws and Regulations

Septic Tanks


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. Also read this blog post about septic tank problems.


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 here.


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?


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 immitate 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 here for more details.

Q: But what if I use a drainage rod with a 3 inch rubber disc?
A: 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.


Q: Is bleach bad for a septic tank?

A: Only if used excessively.


Q: Can you drive over a plastic septic tank?

A: 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.


Q: How much does a septic tank cost to replace?

A: A septic tank costs about £2500 to replace but it is likely that it is the soakawy is what needs replacing. This can cost £2000. Ring Allerton.


Q: What happens if you never pump your septic tank?

A: You ruin the soakaway.


Q: Is a plastic septic tank better than concrete?

A: Just as good but often easier to install. A lot depends on ground conditions. 


Q: How do I clean my septic tank naturally?

A: 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!


Q: How often does a 1000 gallon septic tank need to be pumped?

A: Every year as a rule. Perhaps every two years if only two or three people are using it.

Q: Should I buy a house with a septic tank?

A: Yes if the house is right in other regards. But, have it surveyed first. That is most essential.


Q: Does all waste water go to a septic tank?

A: 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. 


Q: Should I empty a Klargester septic tank?
A: Yes, every year to protect the soakaway.


Q: How does a BioDisc septic tank work? 

A: 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 blog post for more information.




Q: What is a ConSept?

A: A conversion of a septic tank, a cesspool, a well, or some other underground tank to a sewage treatment plant.


Q: Can I outfall ConSept treated water to a ditch?

A: YES, but only if you obtain a permit from the EA. Though Allerton will help you with it. Contact us for more details.


Q: If my septic tank goes into a soakaway, do I need permission to install a ConSept?

A: NO. But you do if you decide to outfall it into a ditch.


Q: What does the engineer do in a service of a ConSept, what does it entail?

A: Check pumps, insitu, takeout, strip and clean, check distributor head/wiring junction box/panel and timer if applicable.


Q: Why does our unit trip?

A: Too much back pressure for the air blower. Needs desludging. Discuss this with the Service engineer.


Q: How long is it before we can use toilets while the ConSept is being installed?

A: Immediately, and you can use both shower and washing machine later in the day.


Q: What happens with wet wipes?

A: Not normally a problem.


Q: How to reset unit after power cut/ pumps tripping electrics?

A: Turn off. Turn on.


Q: When will my unit need emptying?

A: ConSept in a GRP tank, once every 2 years but in a brick tank better to empty every year. Discuss with the Service Engineer. He is in a better place to make judgement.


Q: Can we use bleach?

A: Bleach can be used in moderate amounts. The unit should not smell like a ST and bleach in excess is not needed. A cap full per week should be enough, preferably not all on one day.


Q: Can I use a cesspool?

A: Yes.



Q: Can I construct a new Soakaway?

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: 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 plasticene 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!

Q: How much water is needed for a percolation test?
A: If the ground is bone dry then a lot of water is needed, maybe bucketsfull for each test hole. If there is water in the bottom of the hole as you dig it then the percolation test will be pointless. See BS6297 2007: Code of Practice for the Design and installation of Drainage. Fields for use in Water Treatment. Please read our blog post for more information.
Q: What is a rainwater soakaway?
A: A soakaway to take water from roofs and hard surfaces, yards and patios for example.
Q: Is my house connected to surface water drainage?
A: Probably. The roof water could be led to a ditch however. In some areas, like Lincoln, both roof water and foul water are taken away in the same pipes.
Q: How do you fix poor drainage in clay soil?
A: Farmers farm clay soils very profitably providing they are properly drained. The pipes are about 700 mm deep, are laid with a fall as little as 1 in 1000. They need gravel over the pipe in order to collect water from the surface. The farmer helps this process by deep cultivations such as mole ploughing and subsoiling.
Q: How many crates do I need for a soakaway?
A: Crates are very good for rainwater but not permitted for septic tank water or treated effluent from a sewage treatment plant. The number depends on the percolation test results and the roof area and the annual rainfall in that area.
Q: What to do if the sewer backs up?
A: Call for the desludger, rod the drains, stop using water where possible, panic and then ring Allerton!

Sewage Treatment


Q: What are the challenges in sewage treatment?

A: Converting horrible sewage into relatively clean water and using nature to do it seems like a miracle at times. To do it is not so easy though. That is the constant challenge.


Q: Is planning permission required for a sewage treatment plant?

A: Yes. For links and more information, check out our page on binding rules.


Q: What is the best domestic sewage treatment plant?

A: They are all good. They have to satisfy rigoruos testing with human sewage for a long period. Personally I prefer the WPL Diamond


Q: What is the biggest problem with conventional sewage treatment?

A: If there is excessive use of chemicals, then the bacteria are killed. We need the aerobic bacteria to break down not just the organic matter, but the chemicals as well.

We use bleach and other agents for cleaning the house and chemicals on our hair and bodies. We use chemicals to sterilise our hands. We use all sorts of powerful agents in the kitchen sink. All this has to be neutralise by our friendly bacteria. If too much chemical is used, then they can't do the job properly.
Oil and grease suffocate the aerobic bacteria, allowing the anaerobic bacteria to make the whole lot septic again. Putting spent oil into a bottle is better than down the sink.
So, everything is permitted but in moderation!


Q: How does a small sewage treatment plant work?

A: All sewage systems work by adding oxygen to the sewage which passes through.

 These days it is mainly done by passing fine air bubbles through the water, allowing the bacteria air as well as food and water. They convert the sewage to harmless gases in the air. 

Q: Does a sewage treatment plant need a soakaway?

A: Not necessarily. It can quite easily go to a ditch or river or other running water.

Soakaways are fine though. Effluent from a septic tank has 200 ppm. Water from a sewage treatment plant has less than 20ppm.
Rainwater is quite clean and about 5 ppm. Tap water is zero of course

Q: What size sewage treatment plant do I need?A: The Environment Agency and British Water reckon on a 4 bedroomed house being 6 PE (population equivelent).

This generally needs a plant about 2 metres in diameter by just less than 3 metres deep. Much bigger than a septic tank.

Q: How much does it cost to install a sewage treatment plant UK?

A: To simply install a sewage treatment plant it would cost between £3300 and £4000 in normal conditions

Rock makes it more expensive but running sand can double the cost. Try digging a hole on the beach where the tide has just gone out!

Q: What is the difference between a sewage treatment plant and a septic tank? (refer to blog post)


Q: How long do sewage treatment plants last?

A: Allertons have units that we still service and they are 25 years old and going strong.

Klargester are even older!
They are not made for a limited life as white furniture is. They do need to be maintained though. Fortunately there are no moving parts in the WPL units.  The diamond Sewage Treatment Plant relies on a blower and when we service the unit we make sure the blower is good for another year at least. We are constantly refurbishingf and replacing them so as to keep the m working constantly  I cannot think of anything else that lasts  as long 

Q: What is a BioDisc sewage treatment?

A: Read our blog post for more information about BioDiscs


Q:What is package sewage treatment plant?

A: This is a peice of kit which converts raw sewage to clean water. Not clean enough to drink of course as it can still carry E. coli and other pathogens.

It is high in nitrates and phosphate. I should not be sprayed onto plants but can be used as underground irrigation. Roots have the ability to take what they want and to leave the pathogens in the ground.

Q: How do you make a sewage treatment plant?

A: People over the centuries have tried very hard to make an effective sewage treatment Plant. But you need chemists amd engineers working together to get the right balance of air, water volumes and flows. 


Q: What is the purpose of a sewage treatment plant?

A: A sewage treatment plant coverts foul water into clean water, not perfect but good enough to go into rivers and streams.


Q: How do you size a sewage treatment plant?

A: To Size a sewage treatment plant go here.


Q: Are sewage treatment plants noisy?

A: The blower makes a noise and bubbling makes a noise. How noisy depends on your hearing and expectations.

 If you are going to sit in the garden near one it would be good to vist someone with the same model. The biodisc is relativly quiet.

Q: How many types of STP plants are there?

A: Dozens. Plenty to choose from, but best get to see one and see the quality of effluent coming form the unit. Ask questions. Contact us for more information.


Q: Do we drink sewage water?

A: Never. Treated water does go into rivers. The Trent and Thames for example. But while water is abstracted from the rivers and reservoirs, it is chemically treated before we drink it.


Q: What is sewage and its treatment?

A: Sewage is waste water from the lavatory, kitchen sink, wash basins, baths, showers and washing machines.


Q: What is the first step in the sewage treatment process?

A: The first thing is to settle out all the solids, organic and inorganic. The dirty water then is treated with air. Just air. The aerobic bacteria need oxygen and food so they feed on the organic matter, converting it to gases we find naturally in the air.


Q: Why is wastewater a problem?

A: Wastewater poisons the ditches streams and rivers.  So little can survive in it.


Q:What does sludge mean?

A:Sludge is the solids that collect in a septic tank or sewage treatment plant and it needs to be removed to be dealt with by the Water Authorites


Q: Does a sewage treatment plant smell?

A: A properly service sewage treatment plant smells a little. It reminds me of the smell walking in a wood, scuffling your feet through the leaves.


Q: Is breathing in sewage harmful?

A: Genarally no, but if the sewage gases get too strong then it can kill. We use confined spaces staff and certified equipment when we go into a sewage treatment plant or st or ps.


Q: How do you reduce STP smell?

A: Get it serviced properly. Check that it is not being overloaded above what it was designed for. Prevent grease getting into the system by checking the grease trap. Check that rainwater is not getting into the system


Q: What chemicals are used to treat sewage?

A: None.


Q: What contaminants are in sewage?

A: All that you bring into the house. Without the lavatory waste, the plant would stuggle to break down all those chemicals that we buy in bottles!



Q: What is the difference between wastewater and sewage?

A: Generally sewage from a house can be called waste water or sewage wastewater. Commercial waste water covers a whole lot of chemical processes. Food production, factory waste, abatoirs, irrigation, and swimming pool water.

The humble sewage treatment plant can not deal with this and special treatment systems are employed.


Q: How is BOD removed from wastewater?

A: By encouraging aerobic bacteria to digest the organic matter.



Q: Is raw sewage dangerous?

A: All sewage is dangerous and needs to be dealt with carefully, by people properly trained by British Water.


Q: Is STP water good for plants?

A: Yes as it has nitrogen and phoshates in solution.


Q: What is STP capacity?

A: This depends on the loadings.  Packaged sewage treatment plant come in sizes such as 6pe, 10 pe, and so on. 


Q: What do we call sewage water after it has been treated in a sewage plant?

A: We call it “treated effluent" It looks clean but it still carries chemicals and pathogens. 


Q: Why is sewage treatment important?

A: The rivers would become polluted and poisonous, not just to fish and plants, but would wipe us all out faster than Covid!


Q: What does raw sewage mean?

A: Raw sewage is all the waste from the house. Lavatories, baths, showers, wash basins, kithcen sinks, washing machines, dishwashers, etc.


Q: What is a Klargester sewage treatment plant?

A: Klargester was probably the first company to sell packed Sewage Treatment Plant in large numbers.

 I guess the price then was similar to today's prices but allowing for inflation, very much cheaper today than then. And much improved too. 



Q: When will my unit need emptying?

A: For Diamond units, every 3 to 5 years.


Laws and Regulations


Q: Binding Rules. How do I know I am not breaking the rules?

A: Read the General Binding rules. Click here or ring us. Also see official rules here


Q: Do I need planning permission if I change from a Septic Tank to a Diamond unit (or any other Sewage Treatment Plant)?




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