.jpg)
Hubble, Bubble Tar from Rubble by Northern Gas Networks
Shortlisted for Brownfield Awards Category 6: Best Remediation Of A Smaller Site
Hubble, Bubble Tar from Rubble – Keswick High Pressure Gas Site Thermal Remediation
A Northern Gas Networks, Sweco and Geo2 Remediation Project
Introduction
NGN own and operate a natural gas pressure reduction station in the centre of Keswick, Cumbria in the Lake District National Park (‘the Site’), a UNESCO World Heritage Site. The Site covers c.0.06 hectares and features a dense array of natural gas distribution infrastructure, most notably a 19 bar gas pressure reduction unit, giving the Site a critical regional role providing gas to over 3000 properties. The Site is prominently located in a dense urban environment within this popular tourist town, with residential and commercial properties within 10m to the north and east and a public car to the south and west. The River Greta (Main River) flows southeast to northwest approximately 15m from the site.
The Site was formerly part of Keswick Gasworks, which operated from 1899 until c.1968 and extended across the land to the south and west, now operated as a Local Authority owned car park. Several town gas production structures were formerly located on Site, including a distinctive semi-circular tar well (tank) as shown on Figure 1.
Figure 1 – Site historical features drawing extract. Former tar well outlined in red.
NGN commissioned Sweco to characterise the site wide environmental conditions and develop a conceptual site model (CSM) and environmental risk assessment. Ground investigation confirmed the presence of the below ground semi-circular tar tank and identified it to be c.15m diameter (covering c.65m2) and 2.3m deep, extending underneath the high pressure gas infrastructure and within 3m of live gas infrastructure in all directions. The tar tank was identified to be infilled with mixed demolition type rubble (including asbestos containing materials (ACMs)) and other typical gasworks waste materials, including spent oxide containing cyanide. The tar tank structure contained water from c.0.4mbgl. Significant hydrocarbon contamination (dissolved and non-aqueous phases) was identified inside the tar tank including up to 0.5m thickness of highly viscous coal tar dense non-aqueous phase liquid (DNAPL) in the tank base. The tar tank area and location relative to gas infrastructure is shown in Figure 2 below.
Figure 2 – Area of former tar tank. Red line indicates extents of tar tank as identified in trial pits (note scars in tarmac) which extends under live high pressure gas infrastructure. Yellow line denotes a live gas main crossing tar tank footprint. Green denotes an unknown utility, suspected to be a redundant gas main. Dense array of other above and below ground utilities out of photograph.
The CSM identified that the DNAPL present within the tank base was substantially contained and hydraulically isolated from surrounding groundwater in the superficial Secondary A Aquifer, with no further significant sources of contamination identified beneath the Site. Whilst being contained under current site conditions, the contamination within the tar tank posed a potentially significant risk to environmental receptors (including surrounding groundwater within the superficial Secondary A Aquifer and the nearby River Greta), which could increase over time in the event of degradation of the tar tank structure.
In 2018 NGN commissioned Sweco to assess the potential to reduce the quantity of DNAPL in the tar tank as far as possible and thereby deliver permanent environmental betterment. A structured, multi-factored remediation options appraisal (ROA) was undertaken by Sweco to identify potentially suitable remediation methods which could meet the remediation objectives under the known severe site constraints as shown in Figure 3.
Figure 3 – Summary of constraints to remediation at Keswick Gas Site
The ROA identified that mechanical pumping alone was unlikely to recover significant DNAPL. The most technically feasible option identified was thermal circulation pump and treat to increase the mobility and rate of recovery of the DNAPL and increase the degradation of any residual dissolved phase contamination.
Demonstration of Applicability of Required Criteria
a) Represents notable innovation or exemplary best practice
NGN appointed Sweco and their specialist remediation sub-contractor Geo2 to complete a remediation feasibility assessment, ensuring delivery of the best feasible outcome, during July 2020. Field testing identified that thermally enhanced down-hole pumped recovery at 50-60oC enabled successful DNAPL recovery from monitoring wells. Bench top analysis identified that the DNAPL had a dynamic viscosity similar to that of treacle at ambient in-ground temperature (5000 cP at 7oC) (see Figure 4). The field and bench testing indicated that at temperatures ranging between 30°C and 40°C the DNAPL would be amenable to thermally enhanced pumped recovery. Pumping tests also identified suitable tank water abstraction and injection rates and confirmed hydraulic connectivity within the tank to enable remediation.
Figure 4 – Keswick Gas Site DNAPL bench top analysis of dynamic viscosity. Treacle has a typical dynamic viscosity of 5,000-10,000 cP and honey 2,000-3000 cP.
Feasibility assessment by Sweco and Geo2 informed the design of a bespoke remediation system. The challenging constraints of the site required a radical innovative solution; operation of a thermally enhanced treatment on a live high pressure gas facility, as summarised in Figures 5 and 6.
To achieve mobile, recoverable DNAPL, the system was designed to heat the entire tar tank to c.40-500c. This was achieved by establishing a closed loop, circulation system to facilitate and distribute heat transfer within the heterogenous tank fill. Using 150mm diameter wells, recovered tank waters were heated to c.60-700c using a 100kW boiler prior to re-injection under low pressure into the former tar tank.
This hot water current, emanating from the injection well, developed hydraulic flow cells with heat exchange through direct water heating, conduction and convection within the substrate and environment.
Parallel to this heating, a dual circulation system was established through two primary abstraction processes, calibrated against the rate of injection:
-
The operation of base loading DNAPL recovery pumps set at the tank base. These pumps targeted the highly mobile, temporarily low viscosity DNAPL which had mobilised along the concrete tank base, enabling recovery for separation and storage prior to disposal; and
-
A high volume ‘mid-well’ abstraction pump, which maintained the water balance within the tank, essential to preserving structural integrity of the tank and minimising settlement of the tank fill materials to avoid potential damage to surrounding gas infrastructure. This recovered both mobilised transitory NAPLs and dissolved phase contaminants.
Prior to heating and recirculation, abstracted water was treated to settle sediments, recover cooling NAPLs within a series of separators and storage vessels, and finally de-contaminated using Granular Activated Carbon (GAC) filtration in readiness for a fresh circulation.
To maximise the efficiency and speed of the operation, the system was computer controlled, with sensor driven feedback and telemetry to permit safe optimised 24-hour operation, water level control and environmental compliance.
Following a two-week installation and commissioning period commencing on 1 March 2021, an intensive eight-week operational period was embarked upon. Weekly visits allowed a highly focused system optimisation plan to be executed, with temperature adjustment and regulation, abstraction pump positioning and flow rate amendment phased to maximise recovery.
The system was demobilised during week commencing 10 May 2021, with monitoring data demonstrating asymptotic conditions within the tank (as discussed in Section e below).
Figure 5 – Overview of principles of operation of Keswick High Pressure Gas Site Thermal Remediation System.
Figure 6 – Aerial image of Keswick High Pressure Gas Site Thermal Remediation during operation. Yellow circles denote abstraction and reinjection wells.
b) Cost effectiveness
The total out turn cost for the project including the feasibility assessment, planning, commissioning and execution of the 11 week remediation project, disposal of all recovered contaminated waste and all COVID-19 control measures was £227,000. Based on the estimated tank volume of 150m3, this represents a highly competitive treatment cost rate of c.£1500 per m3 of contaminated material and delivered an estimated c.50% saving compared to a traditional source excavation and disposal methodology.
Thermally enhanced remediation projects on this size and financial scale are rare and it is considered that this technique provided a highly cost effective methodology to deliver permanent environmental betterment from difficult contamination at a highly constrained site.
c) Compliance with legislation, codes and guidance and adherence to diversity practice
The inherent risks and constraints of this project required safety, sustainability and environmental protection to be placed at the heart of the design. Undertaking these works during the COVID-19 pandemic required frequent revision to the implementation plan to ensure compliance with all relevant prevailing guidance and regulations as they changed throughout 2020 and 2021.
Sweco’s ROA was undertaken with reference to several factors including technical feasibility, sustainability and cost following prevailing guidance including CLR11 (now superseded by LCRM)(1) and SuRF-UK Framework.
The remediation system establishment, positioning and operation required it to be fully compliant with the NGN site control procedures covering this high pressure installation, most notably Safe Controls of Operations(2) and DSEAR hazardous area zoning for flammable gases(3).
Potential safety concerns extended beyond the operational site to the adjacent residential properties, requiring the project team to implement specific control measures including: detailed noise assessment (in accordance with BS4142 and BS5228) and monitoring, and use of super silent generators; volatile emissions monitoring; and traffic planning to minimise our regional and local impact.
The remediation works were undertaken in accordance with Regulatory Position Statement 215(4) and involved proactive engagement with the Environment Agency to ensure full compliance and application of appropriate environmental control measures. This involved preparation of a site specific remediation position statement which established an environmental monitoring protocol with trigger levels and actions which was resolutely adhered to and met throughout the operations.
d) Effective public/stakeholder engagement
The project environmental aspects and impacts assessment identified the potential for adverse impacts on local stakeholders in association with the remediation works, most notably additional noise from the system generator, odours/vapours from recovered DNAPL, and additional traffic movements. A proactive approach to stakeholder engagement was undertaken throughout the project commencing with early engagement with the NGN Maintenance and Safety Teams, Environment Agency, Local Authority and Lake District National Park Planning Authority in preparation for the works. Potential negative impacts were designed out during the project design phase, including:
-
Plant selection and location was carefully designed, working around the explosive zoned areas and the highly restrictive live gas infrastructure, a single point of failure for supply to the whole town.
-
Use of a super silent generator and informed generator positioning on site to minimise noise impacts. A detailed pre-works noise modelling assessment was prepared for the Local Authority confirming no significant noise impact, which was confirmed by check monitoring during works.
-
Encapsulation of the entire tar tank surface using a membrane under negative pressure to capture any vapours for treatment, with check monitoring along the site boundaries throughout the works.
-
Proactive engagement with the Local Authority, County Council Highways team, Keswick Town Council, local Police and local businesses during site mobilisation and demobilisation phases which along with careful logistics planning ensured disruption was kept to a minimum, in particular in the adjacent popular public car park.
NGN completed a pre-works letter drop to inform local residents about the scheme and provide contact details for the project team. This enabled one local resident to proactively contact NGN at the end of the project to state:
“Thank you for sending such a wonderful bunch of men. They were very polite, kept me informed and couldn’t do enough to help me”
Local Keswick resident and permanent car park user
The project team received similar accolades from other stakeholders:
“Thanks for working with us and keeping our car park safe for other users and I’m really pleased you have been able to complete your task.”
Community Services, Allerdale Borough Council
“The project team kept us well informed allowing both ourselves and the project to coordinate our deliveries effectively so no problems were encountered.”
- Keswick Brewing Company, located opposite Keswick AGI
e) Real environmental/economic/social benefit
This project has delivered permanent environmental betterment by the removal of hazardous substances from within an aged in-ground structure which had significant potential to impact the quality of surrounding controlled waters (Secondary A Aquifer and River Greta). A total of 7.5m3 of NAPL and contaminated waters were removed from site during the works, with the remainder of the tank waters circulated c.350 times through the treatment plant resulting in the disposal of c.2m3 of GAC grossly impacted with mobile hydrocarbons.
Project environmental monitoring two weeks after completion of the works identified that:
-
the thickness of DNAPL in the monitoring wells inside the tank had reduced from 0.5m pre-works to smearing only, with no measurable DNAPL; and
-
dissolved concentrations of BTEX, TPH >C5-C35 (aliphatics and aromatics), total cyanide and total phenols in tank waters had reduced by 87%, 94%, 95% and 98% respectively compared to pre-works levels.
The regionally important site gas infrastructure continued to operate throughout the project without hindrance. In addition, no significant impacts were felt by surrounding stakeholders, with several complimenting the project team for their attitudes and behaviours (see above).
The complex setting of this site required design of an innovative remedial solution from the ground up. Successful implementation maximised DNAPL recovery in a short programme duration, with best practice delivery recovering all observable mobile DNAPL and substantially reducing dissolved phase contamination thereby permanently reducing the risk profile of the site to ‘low’ at c.50% of the cost of conventional remediation techniques.