Former Gasworks

Tewkesbury

The sites history

The former Tewkesbury gasworks operated from the 1832 through to the 1960s and left a significant legacy of gasworks contamination in the made aground and below ground structures at the site. The site was sold in several parcels meaning that several of the below ground structures crossed the boundary with third party properties, meaning a unique solution would be needed to address potential environmental risks.

Technologies used

Thermally enhanced DNAPL recovery was completed to facilitate recovery of viscous tars within two below ground gasholder bases (GH4, GH5). In addition, an area of historic DNAPL presence adjacent to GH4 was also targeted with heating to enhance recovery.

Heating in GH4 was completed with the introduction hot water in an open loop system that recirculated the injected waters to elevate the temperatures. This water was heated by an external boiler with the temperature of the recirculated waters reaching over 65 degrees centigrade. Based on on-site monitoring the temperatures recorded within GH4 on average 60 degrees centigrade indicating a minimal loss of heat between the boiler and the tank. The heating of GH4, significantly reduced the viscosity of the DNAPL allowing more than 3,000 litres to be recovered prior to the sealing of the void spaces in the tank by injection cement/bentonite grouting.

Based on laboratory and site data the thermal enhancement increase the recovery of the DNAPL by 300%, and as such it was considered a success and justified its selection as an additional technology for deployment.

In the locations outside of the below ground structures heating was used through a close loop system which locally heated DANPL immediately surrounding the boreholes. The site works demonstrated that due to heat loss to groundwater and the soil, thermal enchantment in the open aquifer only marginally increased recovery.

Based on the works completed, the thermal enhancement was a significant benefit within the identified historic gasholder bases and in particular in GH4 where DNAPL viscosity through investigation was confirmed as very low and initially such that recovery would not have been possible.

Our observations

Based on the observations made the following key conclusions were drawn:

Advanced heating was confirmed to have the ability to introduce sufficient heat (65 degrees and above) into a confined structure to enhance DNAPL recovery, which would have not been possible in the absence of heating.
Advanced heating was optimal in areas of mixed ground and where it was possible to position locations within approximately 3m of each other to ensure optimal heat transfer.
Heating within a confined structure allowed heat retention for a period of up to 3 days without further heating and as such continued ability to pump previously non- recoverable DNAPL.
Heat loss between treatment and recirculation is critical to maintaining temperature without the need to significantly increase the initial water heating requirement.
The technology, based on requirement for use of boiler requires significant fuel use for generators which needs to be considered. In this instance, based on a fixed programme, heating within a structure and the ability to site equipment in close proximity was still sustainable and allowed for an enhanced overall improvement.
Heated systems require additional safety controls based on the use of hot water and pressurised systems.
Heated systems require increased recovery well sizes with increased drilling and installations costs to allow the placement of heating elements and return lines.