A bund is a secondary containment system which is designed to capture any leaks or spillages from a primary containment such as a storage tank. A bund consists of an impermeable floor and a set of impermeable walls to create a watertight area around a storage tank, any liquid which escapes the primary containment is retained in the bund and prevented from spilling onto site or escaping to the environment.
It is highly recommended to have secondary containment on all hazardous liquid storage as their uncontrolled release may cause environmental damage and create a hazardous working environment. If you have a pollution incident you can be prosecuted, face an unlimited fine and will be liable for all clean-up costs under the polluter pays principle.
However as with a lot of environmental issues there are few legal requirements for bunding, the Control of Pollution (Oil Storage) Regulations makes it a legal requirement in the UK to have adequate secondary containment on any external oil storage over 200 litres. Other sites e.g. COMAH sites may also have a legal obligation to ensure secondary containment is provided on hazardous liquid storage e.g. chemical tanks.
The first area of bund design we will look at is capacity. The industry standards are the 110% and 25% rules.
The 110% rule is applicable where there is only 1 container stored inside the bund, in this situation the calculation is simple, the bund needs to have a capacity of at least 110% of the primary containment volume. For example if a tank has a capacity of 10,000 litres the bund needs to have a capacity of 11,000 litres.
For bunds which house multiple containers the calculation is slightly more complicated, the bund needs to have a capacity to hold either 110% of the largest primary containment or 25% of the total volume of the primary containers, whichever is greater. If containers are linked these are to be treated as a single container for the purposes of calculations.
In this diagram the 5 containers housed in the bund have a total storage of 60,000 litres with the largest container holding 20,000 litres using the 110% rule the bund would need a capacity of 22,000 litres, using the 25% rule the bund would need a capacity of 15,000 litres therefore the higher volume of 22,000 litres is necessary.
If we reduce the volume of the largest container to 10,000 litres the total volume becomes 50,000 litres with the largest container housing 10,000 litres. Using the 110% rule the bund would need a capacity of 11,000 litres, using the 25% rule the bund would need a capacity of 12,500 litres therefore the higher volume of 12,500 litres is necessary. Where multiple containers are being housed in the same bund it is important to ensure the contents of the tanks are compatible with one another.
The extra 10% capacity is to allow for additional liquids such as rainwater, firefighting media and overfilling of the containers.It also reduces the chance of liquid overtopping the side of the bunds where the contained liquids are a dynamic load meaning the liquid inside the bund is not static, causing a wave effect.
Once you have ascertained the minimum capacity you can design the size and shape of your bund, generally a bund will be designed to have a small footprint so it doesn’t take up too much space on site but there are also other factors you will need to consider.
As a minimum you should have a 750mm separation distance between the primary container and the bund walls to allow for easy tank inspections, however you may wish to increase this distance to protect against spigot flow where a leak in the wall of a tank allows water to jet out of the tank and pass over the bund wall before it reaches the floor. You can also combat spigot flow by constructing taller bund walls.
Taller bund walls will also increase the bund capacity and protect against the tidal wave effect. The tidal wave effect is where a catastrophic failure causes a mass release of liquids which can overtop the walls. However constructing taller walls can cause access issues, limit ventilation and make firefighting difficult. For this reason walls should be constructed no taller than 1.5 meters wherever possible. To help combat the tidal wave effect you can install additional infrastructure to the bunds such as deflector plates which limit the amount of liquid which could overtop the bund in the event of a catastrophic failure.
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Once you have designed the size and shape of the bund you can decide what material the bund should be constructed from. Bunds can be constructed from reinforced concrete or reinforced masonry; however sites may have specific requirements related to construction materials which will need to be adhered to.
Consideration could be given to prefabricated bund walls which are fixed to an impermeable base, these systems are typically cheaper and quicker to install than traditional bund walls and can be dismantled are reassembled if necessary.
In all instances bunds must be designed and constructed to withstand the pressure exerted on them by a catastrophic tank failure this can be up to 6 times the hydrostatic pressure.
Following a new bund install hydrostatic testing should be conducted to ensure the bund is watertight before the bund is put into use.
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Wherever possible pipes and cables should be run over bund walls and not through the bund walls or floor. If penetrations are unavoidable it is essential to ensure they have a watertight seal installed around the pipe/cable, the sealant material must be resistant to the product stored in the bund. On certain sites seals will need to designed in line with specific regulations or guidance.
In order for a bund to be effective and regulatory compliant its base and walls must be impermeable to the liquid it is designed to hold. Where a bund is not resistant to the material, the material can attack the walls and floor leading to serious structural damage to the bund. Substances such as concrete and mortar can also be permeable and if the contaminant can soak into the bund walls it can leach out of the bund. In order to make a bund impermeable a specialist lining can be applied to the bund walls and floor which provides an effective seal ensuring the bund is watertight; different lining materials are available but they should be designed to withstand attack from the material stored inside the bund.
In order to prevent rainwater entering a bund and reducing its capacity it is preferable to install a roof above the bund, however this is often not an option. Where it is not possible to cover a bund it is imperative an appropriate maintenance regime is in place.
Bund maintenance is an extremely important factor which is often overlooked, as with all infrastructure bunds will wear over time and without suitable maintenance can become unfit for purpose. Ensuring a suitable maintenance and inspection regime is in place will help you to extend the lifespan of your bunds as any necessary repairs can be conducted in a reasonable timeframe preventing the damage from becoming worse.
Effective bunds will collect and contain anything which falls inside them, not only the liquid they are designed to capture but also unwanted material such as rain water. If a bund is not maintained properly the unwanted build up could reduce the capacity of the bund and may cause it to fall below the capacity requirements. Rainwater and other debris should be removed regularly. Whenever removing substances from a bund it is essential to test for contamination before they are disposed of. If they are contaminated they will need to be disposed of in line with waste regulations or pumped to a suitable control measure such as an effluent tank or interceptor. Contamination levels can be very low and still require treating as hazardous waste, for oils the threshold can be as little as 0.1%.
For oil bunds automatic bund dewatering units can be installed. The systems discriminate between oil and water to within 5 parts per million and pumps out only the water leaving the oil behind.
As well as conducting regular rainwater removal periodic checks should be undertaken assessing the condition of the bund walls, floor, any penetration seals and bund linings. If any defects are recorded they should be repaired as quickly as possible to prevent the problems from becoming worse, even hairline cracks will provide a pathway for liquids to escape the bund and any structural issues can significantly weaken the strength of the bund walls, which could cause a failure in the event of a dynamic load.
In addition to visual inspections hydrostatic testing can be undertaken to test if a bund is watertight, the bund is filled with water with any loss of liquid recorded by specialist equipment over a set period of time.
If a bund develops problems it is essential to get them fixed as soon as possible, defective bunding can be as bad, if not worse, than no bunding, as it can mask leaks and gives a false sense of security. Over a period of time chemicals/oil leaching out of bunds can go unnoticed and lead to serious ground and groundwater contamination. If bunds have developed cracks they will need to be repaired to ensure the bund is not only watertight but retains the structural integrity to withstand a catastrophic failure.
If the bund lining has failed the bund will need relining or patch repairs to be undertaken.