Many heavily contaminated sites have free products within the soils and water table. Removal of this free product is of primary importance in the remediation of any contaminated site, as free phase contaminants provide the biggest reservoir for contaminants to migrate and cause further pollution.
There are two types of free product:
Light non-aqueous phase liquids (LNAPL): these contaminants are less dense than water and float on the water table surface.
Dense non-aqueous phase liquid (DNAPL):, these contaminants are denser than water and sink to the bottom of water table and rest on impermeable surfaces such as clays and bedrock.
Free phase product recovery is often applied in combination with other treatment processes such as soil vapour extraction (SVE) , dual phase extraction, ex-situ treatments and bioremediation. It is rarely undertaken alone except in the least sensitive environmental settings. Depending on physical properties, product can be recovered as a liquid or vapour and the recovery could be enhanced by steam injection or surfactant flushing.
Free product recovery is applicable to all chemicals, common free product contaminants are:
- Light non-aqueous Phase liquids (LNAPL)
- Diesel range organics (DRO)
- Petroleum range organics (PRO)
- Dense non-aqueous phase liquid (DNAPL)
- Chlorinated solvents
- Coal tars
- Polychlorinated biphenyls (PCBs)
Free phase product recovery can be used as an ex-situ method from open excavation using floating or sinking skimmers or via in-situ remediation methods. Where wells or trenches are excavated or installed to allow access to the free product. Then skimmer or top loading total fluid pumps are used to collect LNAPL free product or product only for a low intensity recovery where time may not be the driver. Bottom loading total fluid pumps can be used for DNAPL. Free product is collected and filtered and then stored onsite ready for off site recycling at a suitability licensed facility. Volatile free product (e.g. petrol or xylene) can be volatilised by air sparging and recovered as a vapour by dual phase or high vacuum systems. Free phase product recovery equipment includes:
- Abstraction system e.g. high vacuum system
- Abstraction pumps (pneumatic pumps skimmer pumps etc)
- Free product storage tank
- Secure storage area for free product
- Control systems
- Compressor (for pneumatic pumps)
- Oil water separator
Vertase FLI have designed and operated many free phase product recovery systems for both recovery from open excavation and via in-situ systems from beneath development and operational sites. One such system designed and operated by Vertase FLI was to remove LNAPL free product from a disused aluminium casting foundry in preparation for the sites redevelopment.
The free product recovery system comprised of drilling of 32 abstraction and monitoring wells, pneumatic top loading total fluid and product only pumps for removal of free product. Abstracted free product was filtered through a tilted plate oil water separator and stored on site ready for off-site recycling.
The unit ran automatically with periodical maintenance check for 68 weeks, a total of 16,800 litres of free product was removed, the majority during the first 28 weeks of operations. Despite the duration of the remedial works, due to the compact design and nature of in-situ remediation techniques used by Vertase FLI the remediation of the site did not impact on the redevelopment schedule.
Advantages and Limitations
- Large contaminant mass recovered in a short time period
- Free product often constitutes over 99% of plume contaminant mass
- Proven track record on UK contaminated sites
- In-situ possibilities (only required a small area for unit setup)
- Significant volume reduction of contaminants possible
- Often further treatment to remove residual contaminant concentrations required (e.g. in-situ bioremediation)
- Removal of DNAPL often difficult due to nature of contaminants