Formatted Title
Sustainable Cyclic Remediation of a Heavily Impacted Site with Xylene: 100% Biological Treatment of Soil, Groundwater, and Soil Vapor
Background/Objectives
A former triacetate production site caused heavy xylene contamination of up to 27,850 mg/kg in soil and 89,000 µg/L in groundwater, to a depth of 10 m-bgl, resulting in a total contaminant mass of 100 ton of xylene. A 100% on-site and in situ biological remedial approach was proposed to remediate soil and groundwater heavily impacted with BTEX, mainly xylene, being both more sustainable and economic compared to conventional (off-site) techniques. The project was conducted in a lump sum contract with a guaranteed outcome.
The present high mass of volatile contaminants presented a high financial risk in this project mainly related to treatment of highly contaminated air from the biopiles and water treatment system. With this project different biological air treatment systems were developed to mitigate this financial risk and present a most sustainable remediation technique compared to active carbon.
Approach/Activities
Remediation consisted of 100% on-site and in situ biological techniques including on-site biopiling and in situ biosparging for groundwater. The excavated soil was placed in biopiles within the excavation pit, where dosing of nutrients and bioventing stimulated biodegradation of the contaminants. In this way, transport of contaminated soil to a different location was avoided, thus no trucks were needed, and nuisance to the neighborhoods was limited.
Contaminated air was extracted from the biopiles and treated by means of activated carbons (GAC). However, working under a lump sum contract and with GAC not being environmental friendly, an innovative biological air treatment (BAT) technique was developed during the project to reduce cost and use of GAC. The BAT consisted of redesigning the biopiles, after the soil treatment was completed, to treat soil vapors (500-2,000 ppm) originating from new biopiles, as they still contain nutrients and active bacteria.
After soil treatment, the site was levelled and an enhanced in situ biosparging system was installed to treat residual soil and groundwater contamination.
Results/Lessons Learned
All 25,000 m3 of heavily impacted soil have been cleaned far below the remedial target values in 1.5 year. Starting from concentration levels up to 10,000 mg/kg, biopiles concentrations were reduced within 60 to 80 days to concentrations levels below 50 mg/kg.
The BAT process worked with an average and constant efficiency of 80% which resulted in a strong reduction of the GAC consumption.
The impacted groundwater (up to 89,000 µg/L of xylene) was treated by in situ biosparging resulting in a structural 70-99% decrease of concentration levels within one year.
The biological water treatment system, used to treat contaminated groundwater extracted to lower the groundwater table, worked with high efficiencies, especially given the high influent concentrations (efficiency rate up to 99-99.9%) and removed the bulk of the incoming groundwater mass, leading to a noticeable reduction of activated carbon as well.
The project showed the power of circular bioremediation and has evidenced that innovation, sustainability, and cost can go hand in hand. 99% of the contamination was degraded biologically either in soil, groundwater, or air. In addition, 0 m3 of soil left the site seriously reducing transport movements around the site. Cleaned-soil was used as air treatment. Another remarkable outcome is that the active carbon consumption was reduced with 98% reducing waste and costs.
The project shows how lump sum contracting drives innovation and cost efficiency with the development of innovative biological air treatment techniques. Circularity in essence : contaminated soil is treated biological on site to harmless end products and subsequently used as biological filter to treat contaminated air, and finally used as backfill material at the site.