Traditional methods of treating groundwater impacted by NAPL often involves pumping and treating groundwater this is a costly and indiscriminate technique which often entails pumping significant volumes of groundwater with little of no recovery of free phase oil. A number of systems are available that target the NAPL directly.
The Non-aqueous Extraction Technique (NET) utilizes a fabric consisting of thousands of oleophilic/hydrophobic fibres bound into a strip with a Kevlar core, which is conveyed in a continuous loop into a groundwater well to intersect the oil-water interface. The strip is supplied in a range of widths from ranging from 5cm to 90cm to suit the diameter of the extraction well, and can adsorb the product with a 99% recovery efficiency compared to around 10 % for conventional methods. The fabric loop circulates into the well and contaminated groundwater, returning to the surface where the adsorbed product is removed in a specially designed desorption unit, and the recovered oil collected in a storage drum or tank for disposal or recycling. It can be deployed in wells up to 60m deep to recover a range of NAPLs including aviation fuel, kerosene, diesel, petrol, fuel oils, solvents, & weathered products, and has achieved recovery rates of up to 6,500 litres of product per hour from a single well.
The fabric is held taut in the well by a weighted tail pulley which is installed near the bottom of the well for DNAPL recovery. For LNAPL recovery, the pulley is set approximately 1 to 2 metres below the water level which ensures product recovery continues even if the groundwater level fluctuates. This method of operation ensures that any groundwater fluctuations will have little impact on product recovery and the system rarely needs level adjustments.
The NET system is operated via a waterproof control panel with a variable speed switch controlling the rate of product recovery, a timer to operate the system intermittently and an overflow switch to turn the system off if the oil drum is full. Optional accessories include telemetry for remote controlling the system and a transfer pump to move the product to larger storage containers.
The NET system has a small foot print, with equipment and collection barrels contained within an area as little as 3m2, and can even be deployed below ground enabling your site to be fully operational. This means groundwater remediation operations can be carried out with minimal impact on other site activities, making it suitable for brownfield sites being redeveloped or fully operational sites with high levels of traffic.
The NET can use as little as 1% of the power requirements for conventional groundwater treatment systems, enabling it to run using green energy if required e.g. Solar and Wind power. This allows NET to be used in remote areas independently.
Remote sites can be operated and/or monitored using a telemetry system. Options include secure web-enabled applications for world-wide access.
NET is very simple to install with a minimal workforce and can be easily moved around the site. A new system can be operational in 2 hours.
The simple and robust design of NET means failures are unlikely resulting in very low downtime. The parts for NET are simple to replace and do not require an experienced technician.
Unlike pump & treat where you have to dispose of water, carbon & sand, you only have to dispose of the product and the fabric. As the NAPL is collected in a concentrated form, free from water, in most cases it can be recycled, often meaning it can be disposed of free of charge.
Cognition can conduct recharge transmissivity tests on site. Once the recharge capability has been measured, the NET system can be optimised to align the recovery of NAPL with the recharge rate to maintain the NAPL hydraulic connection, which pulls NAPL into the well, increasing the recharge and thus product recovery. This can easily be done using the variable speed or the intermittent control switch.
To assess the viability of using the NET system, and optimising installation and operation, a review of site investigation information needs to be undertaken. The site location, physical setting, site geology, information on NAPL plumes and redevelopment plans will be considered in the selection of extraction wells. Cognition is also able to discuss site remedial targets with the Regulators on behalf of the clients and agree end points. NET will recover product quicker than other commonly used conventional groundwater treatment technologies, reaching the agreed endpoint sooner. Typically, NET units will be deployed for several weeks to months, with regular assessment of NAPL recovery and monitoring of performance using monitoring wells. NET units can easily be moved from well to well to ensure the maximum recovery of free product is achieved.
The NET system is proven technology used worldwide including the USA, Europe, Middle East, Asia & Africa. In the UK, Cognition has deployed the NET system at a Nuclear site and former gas works sites contaminated with both DNAPL and LNAPL. This technique has been widely accepted by regulatory bodies as a means of remediating groundwater. Cognition is promoting the technique for use in countries where there is widespread gross contamination such as Kuwait. The system is also been configurated to treat surface water systems impacted by oil sludge.