Shortlisted for Best Overseas Project

How do you remediate a site polluted with recalcitrant contamination, which can’t be disturbed?

 

At this former industrial site in Nyköping, Sweden, the answer was a truly international approach, bringing together remediation experts from Canada, Italy, Belgium, and Sweden, to implement world-class in-situ thermal and dual phase extraction technologies, which:

 

• Had a 75% lower carbon footprint than other viable remediation options, with zero onsite carbon emissions

• Cut programme time by approx. 35% through be- spoke application of one-of-kind ET-DSP™ technology

• Recovered a contaminant mass of 34,889kg – 20,000kg more than the mass estimate

• Enabled creation of a new travel centre for the local community 

 

The challenge

 

Jernhusen AB, the company responsible for railway station buildings on Sweden’s rail network, identified Nyköping as a site with significant contamination. The site had a long history of hydrocarbon storage, associated with the railway. A WSP survey revealed the extent of the contamination left behind, including:

 

• Free-phase hydrocarbons (NAPL) running at 7mbgl

• Extensive metals in soils

• Aliphatic and aromatic hydrocarbons, PAHs and BTEX hydrocarbons in both soil and groundwater

 

The thickness of free-phase product varied across the site with a maximum of 1.7m in places.

 

Results showed widespread exceedances of both the Swedish Environmental Protection Agency’s general guideline values, and Waste Sweden’s limit values for hazardous waste.

 

The site’s geology was also highly variable, with:

 

• Uppermost subsoils between 0.5-2.0mbgl com- prising sand and gravel

• 2-4m of silty clay running below this, overlining 3.0m of sand before bedrock was encountered

• Groundwater occurring as a perched layer approx. 3.0mbgl, with groundwater wells experiencing a slow recharge rate

 

An opportunity

 

If contamination could be treated, there was real social value to be derived from this site.

 

Jernhusen AB could create a new travel centre, offering office space, service functions for travellers and visitors, and opportunities for trade and cafes. This would improve local infrastructure and create jobs within the local community.

 

A long-term local polluter would be replaced by something truly useful.

 

But there was a hurdle to overcome: railway lines bisecting the site could not be disturbed, which meant immediate remediation via. traditional methods was not possible.

 

One-of-a-kind technology

 

A sustainability analysis ruled out ISCO and Bioremediation due to ineffectiveness with the high concentrations at the site. This left excavation and thermal as the only viable options, with nearby railways preventing excavation.

 

Faced with recalcitrant contamination spread across a 2,100m² area, consultants deemed shallow surface excavation of metals-impacted soils, followed by in-situ thermal remediation, as the best treatment option.

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But not all in-situ thermal technology is created equal.

 

Canadian thermal remediation specialists, McMillan-McGee (Mc²), brought their patented Electro-Thermal Dynamic Stripping Process (ET-DSP™) to the project – which is designed to be more effective than other forms of electro-thermal heating.

 

Mc² partnered with Geostream Group – an Italian manufacturer of state-of-the-art groundwater treatment plant – to create the most effective and robust methodology for treating contamination. The team worked under main contractor, Geoserve AB, and brought in Mc²’s Belgian arm, Euremtech, to lead on in-situ thermal process design.

 

ET-DSP™ involves the introduction of electrical current into contaminated soil via electrodes strategically placed throughout a contaminated zone. Using three-phase power, it transfers current to each individual electrode and heats the subsurface.

 

The heat transfer mechanism volatilises contaminants in-situ or, as in this case, drives them to extraction wells via. a guided steam front. Electrical heating increases the temperature of the soil and groundwater by conducting current through the interstitial water that fills the porosity voids in soil.

 

ET-DSP™ benefits from the three dominant mechanisms of heat transfer: conduction, convection and electrical heating. The application of convective heating, accomplished by injecting water at controlled rates into the ends of the electrodes where the electrical field has the highest density, makes ET-DSP™ far more effective than rival technologies.

 

Without convective heat transfer, conventional remediation processes are limited by retardation of the contaminant in-situ, especially in fine sediments and clays. This means primary technologies must be operated for long periods and may not achieve clean-up standards in low-permeability soils or where the vapor pressure of the contaminant is low.

 

The process can take years, whereas ET-DSP™ achieves a significant reduction in contaminant within 3 to 8 months. So far, it has been used successfully on >140 sites worldwide.

 

Application in the field

 

ET-DSP™ thermal technology

 

With the depth to bedrock varying across the site, the team carried out a pre-design investigation which tagged the depth at each electrode and extraction well location.

 

Mc² used this data to design an electrode configuration unique to each borehole, providing effective heating at the soil interval and the top of the underlying bedrock below. A similar approach was used in the design of extraction wells. This meant having to conduct real-time engineering in the field.

 

On average, each electrode had an energy input of 6kW and – with multiple electrodes in each well across the site – total site energy was 978kW. To cool the electrodes, and allow a supply of water for steam production, a liquid injection rate of 22l/min was used on the site.

 

The system ran for 281 days and achieved 92% uptime (including planned maintenance).

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Dual phase extraction at high temperatures

 

Geostream designed and built a dual phase extraction system to remove contaminants from the gas flow. The dual phase system removed both liquids (groundwater/oils) and vapours (air/steam/hydrocarbon vapours) from the same wells.

 

The process was complicated by the high temperature of the gasses entering the plant and the fact that, as the project evolved, less liquid and more vapour was recovered as the groundwater turned to steam.

 

To minimise energy inputs, a heating exchange system was used to condense the effluent gases whilst pre- heating the injection water.

 

The entire system was developed with year-round operations in mind, and was able to successfully operate through the cold Swedish winter.

 

Once the gasses entering the treatment plant had been condensed to below 8oC, the liquid and remaining gasses were separated. Liquid phases were then further separated, with hydrocarbons removed before the water was sent to an air stripper. There, hydrocarbons were removed from water that had been passed to activated carbon filters, then on to a consented discharge.

 

Off-gasses from dual phase extraction and air strip- ping were passed to an electric catalytic oxidizer and activated carbon filters, before a monitored discharge to the atmosphere. A redundant carbon bed system was employed for treatment of vapours during maintenance of the catalytic system.

 

Sustainability

 

Sustainability was at the heart of the project, with zero on-site carbon emissions.

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A carbon footprint analysis showed that ET-DSP™ would be ~25% of the only other viable remediation option – excavation. ET-DSP™ made use of an electrical grid comprised of renewables and nuclear energy.

 

All equipment used on-site was re-used – including copper leads, power supplies, and the MPE system – and sonic drilling technology was used to minimise waste from the site.

 

The team used convective heating to ensure efficient use of electricity, and electrodes were designed to need the minimum amount of conductive material in the ground.

 

Compliance and SHEQ

 

With the aid of local partners, the team worked to overcome multiple complex regulatory hurdles. Many were linked to delivering remediation safely, with high voltages, high temperatures, and process plant, each having their own implications.

 

Specialist measures included: earth bonding to prevent stray currents, PID monitoring of the atmosphere, using a sealed surface to isolate the contamination, and uprated PPE for staff.

 

Zero health and safety incidents occurred at the site.

 

Engaging with local stakeholders

 

While not within the team’s remit, this landmark project attracted a lot of local interest, and they were keen to provide support.

 

A highlight included being interviewed by journalists, explaining technical elements of what was happening on site. This made the front page of the local paper.

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Results

 

Within the core treatment area the system reached temperatures of >100oC within 3 months, with an average temperature across the whole monitored area >88oC.

 

This provides an efficient balance to recover as much product as possible via enhanced mobilisation through lower viscosity, and without the energy penalty of boiling off all water in the subsurface.

 

Throughout operations, LNAPL mass recovery was logged intermittently as the operators collected and transferred the product from primary separator tanks. The mass in this liquid phase totalled 32,476kg.

 

Mass recovered in the vapour phase, measured by PID and correlated to laboratory samples, totalled 1,480kg. This mass was destroyed by the catalytic oxidizer onsite or captured in the GACs. Mass recovered from the GAC vessels (liquid & vapour phases) was approximately 933kg. The cumulative contaminant mass recovery, following 281 days of operations, was 34,889kg.

 

This hugely exceeded the initial mass estimate of 14,000kg, removing a significant amount of pollution without disturbing the rail tracks, and paving the way for a new transport centre in the local community.

 

RESULTS SUMMARY

 

• Had a 75% lower carbon footprint than the other viable remediation option, excavation, with zero onsite carbon emissions

 

• Cut programme time by approx. 35% (reducing associated energy use and cost) through bespoke application of one-of-kind ET-DSP™ technology

 

• Recovered a contaminant mass of 34,889kg – 20,000kg more than the mass estimate

 

• Enabled creation of a new travel centre for the local community

 

• A truly international collaboration, bringing in specialists from Canada, Italy, Sweden and Belgium