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Cesspools & Silage Tanks: The Differences Explained

Cesspools and silage tanks are similar in appearance and both are underground effluent storage tanks, but there are key differences in what they are used for.

To look at them, cesspools seem to be almost identical products with very similar uses, but they are in fact quite different. In this short blog post, we look at the differences between them to show why they shouldn’t be confused with each other.

Cesspool vs. Silage Tank = Home vs. Farm

While both are used to store effluent, a cesspool is primarily used in domestic situations where it stores wastewater and human effluent. (A quick aside: this isn’t the same as a septic tank or sewage treatment plant, both of which process the wastewater and effluent; a cesspool merely contains it until it is emptied.)

On the other hand, a silage tank is used exclusively in agriculture to store silage effluent. This is the byproduct of ensiling – a method of preserving the nutrients in moist forage crops, using a lactic acid solid-state fermentation process. Reduced to a liquid form, silage effluent makes a highly potent fertiliser. In a silage tank, the effluent is similarly held until extracted

Cesspools Explained

Cesspools are now a less popular means of domestic wastewater management as they require more frequent emptying than a septic tank or sewage treatment plant, but there are a few situations where they are the preferred or only option.

If you are considering installing a cesspool, there are a few points to bear in mind. First of all, they are now illegal in Scotland, so if you live north of the border you’ll need to look at a septic tank or sewage treatment plant instead.

Most of the rules and regulations surrounding cesspools depend on whether you already have a cesspool on your property or want to install one.

If you have a cesspool on your property…

If you already have a cesspool you don’t need to register it, apply for a permit or confirm it complies with General Binding Rules. However, the Environment Agency or your local authority are empowered to force you to repair or replace it if it is in poor condition, and to issue large fines if it is found to not comply with the law.

You should also:

  • regularly check the effluent level in the cesspool, using a high-level alarm system, as allowing a cesspool to overfill is an offence under the 1936 Public Health Act
  • never open the cesspool manhole cover to check the level, as effluent produces harmful toxic fumes
  • ensure it is properly maintained and checked periodically for leaks or damage
  • have it regularly emptied by a registered waste carrier who will give you certificates for each emptying to confirm the effluent has been disposed of at a specialised site
  • keep the area around the manhole clear so the contractor can easily access it for emptying
  • use biologically friendly household cleaning products, as this will prevent hazardous or flammable liquids from being added to the effluent.

If you want to install a new cesspool on your property…

Before installing a new cesspool, you will need to ensure the proposed location of the cesspool complies with Building Regulations by being at least 7 metres from any buildings and 2 metres from any boundary.

You will also need to obtain planning permission from your local authority.

It’s also important that you get a cesspool large enough for your needs. For domestic situations, this equates to a minimum capacity of 18,000 litres for two occupants and a further 6,800 litres for each additional occupant.

Silage Tanks Explained

While silage effluent is a potent fertiliser, it is also highly toxic; one estimate suggests that 500 tonnes of unwilted silage could potentially cause pollution on the same scale as the daily untreated sewage output of a town or city with a population of 200,000.

This explains one key difference between a silage tank and a cesspool. To prevent leakages that could contaminate soil or watercourses, a silage tank always features an inner protective lining of bituminous paint, butyl rubber, glass fibre or resin.

Sizing a silage tank

The size of the silage tank required is proportionate to the size of the silo capacity. If the silo capacity is 1500 cubic metres or less, the silage tank capacity must be at least 3 cubic metres for every 150 cubic metres of silo capacity.

For silos with a capacity larger than 1500 cubic metres, the minimum silage tank capacity must be 30 cubic metres plus one additional cubic metre for every 150 cubic metres above 1500 cubic metres of silo capacity.

If you want to install a silage tank on your property…

Regulations vary depending on where your farm is located. If you are in England you must notify the Environment Agency at least 14 days before you intend to install the tank. The same 14-day period applies in Wales, where you must notify Natural Resources Wales (NRW).

In Scotland and Northern Ireland, the notification period is 28 days. Scottish farmers should notify the Scottish Environment Protection Agency (SEPA), while in Northern Ireland it is the Department of Agriculture, Environment and Rural Affairs (DAERA).

In general terms, a silage tank must be installed at least 10 meters away from inland or coastal waters although, under the instruction of a local or national authority, a wider safety zone may be required in certain areas.

Installing a silage tank below ground or partly below ground provides a degree of protection and this is possibly the most common option. If you are installing a silage tank either partly or fully submerged, ensure it comes with a minimum life expectancy of 20 years without maintenance.

If the silage tank is to be installed wholly aboveground, it must have an expected lifespan of 20 years including maintenance.

Silage and slurry: do they mix?

In theory, providing the tank is large enough to contain both types of effluent and is built to withstand the stresses that both types might place on it, there’s no reason why silage and slurry shouldn’t be contained in a single tank.

Bear in mind, however, that mixing slurry can produce gases that can be lethal to humans and livestock, so it may be better to play it safe. It’s also important that silage is never added to an under-floor slurry store.

Can a cesspool replace a silage tank?

As they are almost identical in design and function, and a cesspool is considerably cheaper, it might be tempting to save money by using a cesspool in place of a silage tank. However, without the protective lining of a silage tank, a cesspool would have comparatively little resistance to the highly toxic, acidic contents. The chances of leakages and environmental pollution would therefore be much higher and if this does occur, substantial fines are likely to be levied. As the damaged cesspool would then need to be replaced by a proper silage tank, it’s probably cheaper and wiser to make this investment at the beginning.


We hope you have found this information helpful and interesting. If you have any further questions you are always welcome to call our friendly team of experts on 01420 555600 or email [email protected]

Also, look out for more articles in our ongoing series of blog posts, bringing you useful information, insights, guides and tips on all things drainage!

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