Non-combustion remediation strategies

The remediation strategy was focused on removing contamination or breaking the pollutant pathway/linkage between the identified source and the receptor. Pre-remediation investigation identified areas where concentrations of hydrocarbons were in excess of the remedial targets. The soils from the former steelworks, contaminated by hydrocarbons, required treatment or disposal in order to remove the source of contamination and allow further development of the area. Estimates based on available ground investigation data suggested that 20 000 m3 of material would require treatment across the whole site. The initial strategy for dealing with these materials was to maximise the amount treated on site through bioremediation. The variation in the nature of the contaminating hydrocarbons, however, made it difficult to use only one remediation strategy. More volatile petroleum fractions were more likely to be suitable for bioremediation, but viscous tarry materials required disposal to the off-site landfill or alternative treatment processes. The soil contaminated areas were excavated and treated to remove concentrations of hydrocarbons before being re-used in the overall works as fill. The excavation areas were validated to ensure that no significant contaminants remained at the site. Samples were taken from the base and sides of excavations at the rate of 1 sample per 200m3 of floor space and 1 sample per 20m of side wall. Remediated materials was tested and only released once treatment targets had been achieved.

Bioremediation was performed using windrows, utilising natural soil bacteria to break down hydrocarbons, leaving a soil or engineering fill suitable for re-use. To prevent rainwater mobilising contaminants and migrating into the underlying soil the windrows were constructed on a concrete pad, securing the site. All water leakages from the concrete pad were collected and disposed off-site.

The more viscous hydrocarbons required either thermal treatment or disposal to landfill. To remove tarry hydrocarbons form the solid matrix, thermal desorption was used which is an environmental remediation technology that utilizes heat to increase the volatility of contaminants i.e. hydrocarbons, collecting them as an off-gas and finally cooling and condensing. Extracted hydrocarbons were collected and the remediated soil was then deemed suitable for re-use as a fill.

The remediation strategy, including bioremediation and thermal desorption of the hydrocarbon affected soils, was chosen because of its ability to achieve the remediation objectives, well-proven technology and capability for use on-site with a mobile plant licence. Off-site landfill disposals would be less sustainable because of increased traffic flow and lack of suitable facilities within the region. Similarly, on-site disposal was rejected due to development constraints associated with the waste management facilities and long time scales involved in obtaining and surrendering waste permits.

Environmental monitoring was carried out on-site to monitor water quality within a local culvert (Ebbw Fawr culvert) and groundwater within the glacial till on site.

Use of non-combustion technologies on-site had positive social feedback. The former steelworks area is surrounded by residential areas and possible use of a mobile incineration plant on-site, with the possibility of producing/creating combustion products like PCDDs, PCDFs and other by-product POPs, could have had a negative impact on the population.
 
One of the Stockholm Convention’s objectives is to eliminate all processes in which POPs are formed, including all combustion technologies of waste destruction. The destruction technologies should not, themselves cause hazards, generate POPs or otherwise threaten or injure health and/or the environment.