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Energy from Waste

Incinerator Bottom Ash (IBA)

What is Incinerator Bottom Ash (IBA)?

IBA is the ash that is left over after waste is burnt in an incinerator. Municipal energy from waste plants that use incineration burn a wide range of municipal wastes and therefore the term ‘ash’ is slightly misleading because it is not all powdery but contains glass, brick, rubble, sand, grit, metal, stone, concrete, ceramics and fused clinker as well as combusted

 products such as ash and slag.

IBA is different from Air Pollution Control (APC) residue, which is the by product of cleaning up flue gases from the combustion process and is a mixture of fly ash, and the reagents (mainly lime) used in the flue gas treatment APC residues are classified as hazardous waste and account for approximately 2 % by weight of the waste inputs.

How much IBA does a typical municipal incinerator produce?  


IBA produced from a typical municipal incinerator represents about 20-30% of the input waste. 

Approximately 1 million tonnes of IBA are produced in England and Wales each year. In 2011, 86% of IBA produced was reused.
Source: EA Pollution Inventory Returns & Waste Returns

What happens to IBA?

The IBA is removed from the furnace, and large objects, such as stones, bricks and metals are screened out and are recycled/reused where possible. The remaining ash must then be characterised by the plant operator as either hazardous or non-hazardous and taken off site. 

If the IBA is non-hazardous it can be  processed into a secondary aggregate and used in road sub base, a bulk filler for construction and in cement bound materials.

If the IBA is hazardous it must either be disposed of in a hazardous waste landfill or go for further treatment.

Incinerator Bottom Ash – hydrogen gas release potential

Elemental hydrogen is a highly flammable gas which is considerably lighter than air and is one of the most abundant elements on earth.

There is the potential for hydrogen gas to be released from IBA, particularly from fresh IBA which can be wet and highly alkaline after quenching. In the open air or well ventilated conditions hydrogen gas quickly dissipates into the atmosphere. However, any generated hydrogen gas could potentially become a hazard, if it accumulates in confined and/or poorly ventilated spaces due to its highly flammable nature.

Although quantities of hydrogen produced are likely to be less than the trigger for classification as ‘hazardous’ under HP3, there is a residual risk and it is therefore considered best practice for IBA risk assessments to consider hydrogen gas evolution and precautions to be undertaken as appropriate to minimise the risk of fire or explosion. These could include buildings, transport vessels or containers where IBA is stored having adequate ventilation to ensure that any gas can be dispersed safely. Communication of the residual risk and appropriate control measures is normally done through a Material Safety Data Sheet (MSDS).

IBA aggregate (IBAA) has been through an additional metal removal process, so the risk of hydrogen release is much reduced.


Reuse and Recycling of IBA

How is non-hazardous IBA reused / recycled?
  • Use of IBA as a secondary Aggregate 
IBA can be recycled in a number of construction applications to replace primary aggregates extracted from quarries.

Recycling IBA in this way also avoids landfill disposal. IBA used in this way
is regulated by the Environment Agency and must also conform to relevant publicly available civil engineering standards. During 2011, 86% of IBA produced was reused in this way.

  • Recovery of metals from IBA 
Approximately 6% or 100,000 tonnes of ferrous and non-ferrous metals 
are also recovered from IBA  to be reprocessed into new metal products.

Quality Protocols

The ESA is also supporting the Environment Agency and WRAP to develop
an End of Waste Quality protocol for IBA.

A key element of this process will be demonstration that the processed
IBA (IBAA) is in compliance with a recognised UK or European aggregate specification. Well processed IBA can meet the requirements of many
applications specified by the Highways Agency – Specification for Highways Works, covering use in drainage, specialist bulk fill, unbound mixtures, hydraulically bound mixtures and a component of bituminous mixtures.                 Source: Ballast Phoenix Ltd

Compliance with a recognised aggregate specification and a health and environmental risk assessment  is considered sufficient to ensure that the fully recovered product may be used safely in the environment without the requirement for further waste regulatory control.

Further information can be found here

Sampling and Testing of IBA

In 2010 ESA published a ‘Sampling and Testing Protocol’ which was approved by the Environment Agency.  It sets out in detail how plants should sample and analyse IBA.

Operators of municipal energy from waste (EfW) facilities have been collecting IBA according to the protocol since January 2011.