National Grid, Atkins and Bilfinger Berger won Most sustainable remediation project for their remediation of a former gasworks
National Grid owns a small part of a former gasworks site, an area of about 70m by 50m. Within this boundary there is a buried tar tank, a legacy of former operations, which is brick lined and about 20m in diameter and 4.5m deep. It is infilled with demolition debris, soil, silt and is covered with tarmac. Coal tar is present in the tank, originally at depths averaging 3.4m, but in some parts as shallow as 1.5m below ground level (mbgl).
The tar tank was breached on one side by the historic construction of a 24" cast iron gas main. The conceptual site model showed that the breach had potential to allow slow migration of tar along the path of the gas main and off-site, so the remedial objective was to remove the opportunity for tar to mobilise off site. Regulatory acceptance of the objectives and the potential to use an in-situ remedial solution was obtained by Atkins early on in the project.
Long-term environmental benefits
A detailed remediation performance specification was drafted and the Bilfinger Berger Environmental tender was both in line with the key elements of the specification and offered notable innovation, especially relating to sustainability principles.
Over a five month programme to date the mean level of tar in the ground has reduced from 3.4mbgl to 4.3mbgl, and in total 18,500l of neat tar has been recovered. Best consideration of all environmental aspects was demonstrated, including:
- Bio-diesel was used instead of red-diesel
- Surplus energy generated on the site re-routed to the national grid - 15,000 kWh to date
- All recovered tar recycled into blended RFO - no tar sent to landfill
- All non-hazardous soil arisings re-used on site
- General refuse segregated and paper/cardboard recycled
- All abstracted water from tar tank returned to ground following treatment, so minimal disposals to foul sewer
- Process plant incorporated headspace extraction to control odours
- Avoidance of excavation and disposal as a remedial option reduced HGV traffic movements to plant and equipment deliveries with only occasional transfer of recovered tar for recycling required and limited soil disposal requirements from drill arisings.
The system has a very high efficiency. The CHP captures 97% of the fuel energy, of which approximately 85% is used after process losses in the remediation. Capital expenditure for the contractor was relatively high, but the system was designed with longer term in mind. Lower energy costs will give lower remediation costs after 2-3 years payback compared to conventionally powered schemes of similar specification.
There was no increase in energy consumption over a pump and treat system powered by a normal site generator - so all the heating is ‘free' in energy terms.
"Atkins made a demonstrable effort in all possible aspects of sustainable development with this project." Duncan Sanders