How Much Do Septic Tanks Cost?

People who live in towns and cities often take it for granted that their waste solids and liquids from sinks, washing machines, baths and toilets are neatly flushed away down the drain, never to be seen again. For many people who live in the country, this is a luxury they can only dream about. For those of us who are not on mains drainage, the most common alternative is the use of a septic tank sewage system to get rid of our waste.

How do septic tanks work?

Water engineers and plumbers classify the water that comes out of a house into two types:

  • Grey Water. This is the waste that comes from sinks, showers, baths, washing machines and dishwashers.
  • Black Water. This is the waste that comes from a normal flush toilet.

Both these types of wastewater end up in a septic tank where they are stored and allowed to settle. The water on top of the sludge isn’t highly contaminated after settling so is eventually allowed to decant through a filter into a soakaway in the surrounding ground where it can naturally filter through the soil until it reaches the water table. The remaining sludge decomposes over time using the natural bacteria present in the sewage. This takes a long time and if there is a lot of sewage produced, eventually the solids build up and need emptying and that can only be done by a registered septic tank drainage company.

Large or small septic tank?

Obviously, the size of a septic tank will directly affect the cost as will the material of construction and whether it is an above or below the ground tank.

Let’s first look at the size: The first instinct is to go for a small tank. Not only are they cheaper but they will cost less to install. What people don’t consider is that they will need emptying far more often and this can be expensive if it’s every month or so. Don’t forget that septic tanks aren’t just for receiving sewage, they will also accept all the grey water produced by your home. Maybe if there is only one person living in the house then there won’t be much waste to get rid of. If however, the residents are a typical family there will be plenty of effluent being discharged every day. It makes sense therefore to choose a tank that is large enough to accept everything produced by an average family. In fact, there is a case for over estimating the size or choosing the largest tank you have room for and can afford. The extra costs to install the septic system will in the long term, be far less compared to those of frequent emptying.

Types of septic tanks

There are two types of septic systems:

  • Above ground. These are obviously cheaper to install than an underground one and they tend to be smaller so will cost less to buy as well. Above ground septic tanks can only be used where there is enough space in your garden and you don’t mind having a barbeque or letting the kids play next to a septic tank.
  • Below ground. These are the commonest type of tank and tend to be larger than above ground tanks. You have to excavate a large amount of earth, far more than the size of the tank because there are not only foundations to build but also you need space for pea gravel to surround the tank to act as a soakaway for the overflow water.

Before you choose either one of these, you must contact your local authority because there may be restrictions stating which type of septic tank you are allowed to install in your area.

Obviously the restrictions and the decision about which type to choose is only relevant if you are putting in a new septic tank installation. For a replacement tank, you just have to install the same type as before.

Why does a septic tank need replacing?

Sometimes when you are in your garden or indoors in the bathroom or kitchen you may start to smell a terrible odour or see sewage backing up into the lowest drain in the house. This could be the sink in the kitchen or if you have a downstairs bathroom it might be the shower tray. If this happens it might be time for a septic tank replacement. But how do you know what is actually wrong with the system? If it’s just something like a broken or collapsed pipe then it isn’t a major job to fix it. If it is something else then it could cost many hundreds of pounds to put right.

The first thing to do is to try to work out what is the cause.

  • Blocked pipe. If sewage is backing up in the house, try to find out why. Lift the lid of the septic tank and have a look inside. If the liquid level is lower than the inlet pipe then you have a blocked drain pipe. Call a plumber to find out if it a simple blockage or if the pipe has collapsed.
  • Blocked Septic Tank filter. If the level in the tank is higher than the inlet pipe, call a tank pumping contractor to come and empty your tank. This will allow you to look at the filter and see if it is clogged. If that is the case then it is relatively simple for a specialist to unblock.
  • If you have had flooding in the ground where the tank is installed, do not empty the tank because it may cause the tank to float, breaking the pipe joints. Leave this problem to the specialist to sort out and try to clean up the mess in the house as much as you can.
  • Stinking drainfield and soggy ground. If the ground around where the tank is installed is soggy and stinking you may have problems with your tank. Keep everyone away as this is a health hazard. Put up a temporary fence around the affected area if you have kids or pets. Pump the tank and reduce water use until you can sort out what is actually happening.

If the ground around the tank and the soakaway (the drainfield) has failed it might be because the tank hasn’t been pumped enough, resulting in the sludge layer becoming so thick there isn’t anywhere for the water to pool and separate from the solids. Grease and sewage get through the filter and soak into the ground, clogging up the soakaway. Eventually, this bubbles up to the surface and it is too far gone to easily fix. The drainfield needs replacing. The only way to fix this is to dig another excavation and install a replacement tank. Eventually, the bacteria at the old site will run out of food, die and decompose. If the new site clogs up someday, you can abandon that site and return to the original tank.

There are certain circumstances when you either need a replacement tank of the same size or you need to install a larger tank.

  1. Raw sewage is a very corrosive environment as the hydrogen sulphide given off by the decomposing sewage quickly turns into sulphuric acid. The acid affects any metal struts and bolts inside the tank and if the tank is concrete built then the acid affects the concrete too. When the tank starts to corrode it is time to have a replacement installed before anything serious occurs.
  2. If another house joins the private drainage system then the tank will need to be replaced by a larger one. The original tank was chosen specifically for a certain volume of water and if this increases dramatically then both the tank and the soakaway need to be increased in size to cope.
  3. If you, your spouse and three kids have just bought a house with private drainage and it was previously occupied by one or two people, you might find that the drainfield soakaway may not be able to manage with the increased daily flow. Even if the previous occupants never had problems, it doesn’t mean that you won’t.

There are other alternatives that you can choose from but these are outside the scope of this article. If you have problems with your septic tank, do not hesitate to contact a specialist company who will be able to advise you on the alternatives.

Septic Tank Replacement Cost

The difference between the cost of replacing a septic tank into an existing system and completely installing a new system will obviously be the extra cost of pipework from the house to tank as well as the cost for installing the foundations into the excavation. There will also be costs associated with disposing of the soil that you have dug out to make room for the tank and soakaway. Although there will be some soil used as backfill, there will be a lot left over that needs to be disposed of. If you have room and enjoy gardening maybe you can use the soil to landscape the garden. This will certainly save some money, but the amount saved will depend on how much soil you have managed to incorporate back into your garden.

The first task, when replacing a tank is to have the existing tank emptied of all sludge and rinsed out so it is safe to manhandle and take to the waste disposal site.

Unless you have used a mini excavator and have some basic plumbing skills, it is wise to contact a specialist contractor to do this job for you even though it will cost money. The removal of the old tank and the plumbing and positioning of the new septic tank are critical f you don’t want an uphill slope in the pipes and a tank that slowly floats to the surface.  Although you can buy cheaper concrete septic tanks, they don’t have such a long life as a more expensive high-density polyethene septic tank. The costs to buy and install a polyethene tank will vary depending on

  • The size and brand available from your local building supplies merchant or specialist supplier.
  • The contractor you hire to complete the installation.
Size Cost of materials Average installation cost Project duration
Small tank £850 £1800 3 to 4 days
Medium tank £1000 £2200 4 to 5 days
Large tank £1300 £3500 5 to 7 days

The cost to hire a specialist contractor will be in the region of £150 to £450 per day, but usually, the contractor and any labourers will quote for an overall price regardless of time spent on the job and this will include the use of an excavator and dumper truck too. The figures quoted here are average indicative prices only that will vary depending on where you are in the country, whether there are any access problems getting the equipment from the road to the proposed tank installation site and whether the plumbing or foundations need repair. Remember that you will have to buy soakaway material and dispose of any soil left over as well as the original septic tank. These tasks will add considerably to the septic tank installation cost.

Maintenance Costs

Once the septic tank is in place and buried, there will be very few maintenance jobs to do apart from having the sludge safely removed from the tank once or twice a year (depending on the size of the tank). The cost of emptying a septic tank will vary depending on:

  • The tank volume.
  • How much sludge has to be removed.
  • The overall time needed to clean the tank.

If the size of the tank has been estimated correctly depending on the number of people living in the house then it should just need emptying approximately once a year. Keep an eye on it though because you don’t want it to overflow.

Size of tank Average cost to empty tank Approx. duration
Small £120 45 minutes
Medium £180 1 hour
Large £250 1 to 2 hours
Extra large £350 2 to 3 hours

To Finish

For many houses in the UK, a septic tank sewage system is the only way that sewage and wastewater can be handled. Although the initial cost of installing a tank or even replacing one that has severely deteriorated can be quite large, if the tank is emptied regularly and periodically visually checked inside, it should give you many years service. All you have to do is make sure the size of the tank is chosen to suit the expected water flow and learn to do the inspections recommended by the manufacturer.

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