With more fossil fuel under the pumps than in them of late, hydrocarbons have been hitting the headlines. IEM examines a few treatments for the common contaminant.
On-site benzene treatment
In 1999 a consortium of companies, including BP Chemicals, began developing a new E4 process plant on land that had previously been occupied by chemical plants. These plants were demolished in the 1980s and the area had been left vacant ever since. The one-hectare plot required remediation to a level where it was safe to develop it again.
The primary risk driver for the site was benzene, a Class 1 carcinogen with an occupational exposure limit (OEL) of 5ppm. Other contaminants included cumene, a strong smelling class 2 carcinogen with an OEL of 25ppm, together with other BTEX compounds, copper, phosphoric acid and asbestos.
A strategy was developed by IKM Consulting of Falkirk whereby the principles of Best Practicable Environmental Option were followed by recycling the demolition material and bioremediating the sub-soil for re-use on site.
Trial pits were dug on site daily, and grab samples taken to determine the depth and extent of the contamination. Soil was removed and placed within a covered area for bioremediation. The long mounds of soil were kept under ventilated covers and new soil was incorporated into the silty clay to impregnate it with micro-organisms in order to render the benzene and cumene harmless. The material was tested for contamination after a significant time. These tests continued until the soil proved to be safe to return to the original site.
Precision injection the best response
Response Bioremediation was commissioned to remediate the range of contamination within the soils at a development site in Nottingham.
Hot spot areas of hydrocarbon compounds were identified throughout the site. The elevated determinands identified were Polyaromatic Hydrocarbon, Total Petroleum Hydrocarbon and Diesel Range Organics. The bioremediation strategy had to take into account the small timeframe allowed, the industrial end-use and the environmental sensitivity of the aquifer. An in-situ bioremediation strategy, which utilised precision injection technology, was therefore chosen.
Used commonly within the food and pharmaceutical industry, bio-gel is a non-toxic, non-corrosive, organic substance which can be used as a carrier, an insulator and as a source of nourishment for the bacteria used in the bioremediation of soils and water that contain high levels of petroleum hydrocarbons. Site-specific bio-gel contains the correct composite of naturally occurring bacteria, nutrients and dissolved oxygen to maximise the interface of bacteria and contaminant. This promotes the development of biomass and therefore facilitates bioremediation. During the process bio-gel is consumed and converted into CO2 and water.
The bio-gel was manually injected into the target areas of contamination using 19mm diameter hand-held injection lances in a regular grid pattern with injection points at 1.0 metre spacing.
Integrated approach reaches the parts…
The intended site for a new Asda store at Long Eaton near Nottingham was once used as a fuel depot and, consequently, the soils and groundwater were polluted with petroleum hydrocarbons.
There were two main problems facing the site’s contractors, May Gurney; the migration of polluted groundwater into the water table and the decontamination of the existing on-site groundwater.
Therefore, an integrated remediation scheme was adopted. Initially, existing disused buildings had to be demolished, and areas of hard standing broken out. Then, a 550-metre-long soil-mixed reactive containment barrier, comprising some 600 columns, was installed to an average depth of eight metres. These soil-cement columns created a low permeability barrier, which prevented the migration of polluted groundwater which was the main concern at the site.
A major earthworks operation was also undertaken to remove some of the more contaminated soil, which was then replaced with both recycled materials from the site and imported clean fill.
Finally, a pump and treat system was installed and operated. Groundwater was pumped from a series of well points situated across the site, to a treatment area. Here, a series of tanks skimmed off free-phase pollutants from the water which was then filtered through sand and activated carbon chambers to remove any residual contamination.
The treated water was tested to make sure it satisfied specified criteria before being piped back into the ground through an infiltration trench.
Drop me at the station
Treatment Buildings are able to deal with both liquid and vapour hydrocarbon contaminants, either individually or collectively.
The first stage of liquid phase treatment is an oil-water separator unit. By forcing the oil water mixture to flow in a syno-sinoidal direction, the free phase hydrocarbon is separated and automatically skimmed off to storage. The remaining dissolved phase contamination is transferred via a float activated transfer pump to an air stripper. Utilising ‘forced air bubble’ technology the air stripper unit removes the dissolved phase hydrocarbon contamination to a level of 99.99% removal.
In a vapour treatment system contaminated hydrocarbon vapour can be removed via a network of on-site pipework and specifically manufactured wellheads via a Soil Vacuum Extraction unit. By applying a vacuum to the soil the vapour is transferred to the SVE unit housed in the treatment building.
To assist the natural organisms in the break down of the contaminated soil Air Sparge technology can be utilised. This technology introduces a relatively high flow rate at a low pressure to introduce oxygen into the soil via a series of injection boreholes and pipework. Again various optional controls are available to detect equipment failure, temeperature and system over pressurisation.
‘Cleaned’ water and vapour can also be directed through activated carbon beds as a final polishing procedure prior to discharge into sewers or to atmosphere.
A liquid phase treatment building was installed when a supermarket chain discovered that an underground storage tank at one of its filling stations was leaking. More than 10,000 litres of diesel was lost into the surrounding ground, and was seeping rapidly towards the groundwater. Geo Remediation installed a fully integrated, self-contained liquid phase treatment building. Three boreholes were drilled and a dual phase extraction system was used to extract both the contaminated water and hydrocarbon vapour from the contaminated ground.
ISCO: respect for remediation
In-situ chemical oxidation (ISCO) is a technology designed to rapidly break down and remove organic contaminants in groundwater, saturated soils and the capillary fringe. This is achieved through oxidation reactions induced by sequential injection of liquid chemical formulations. The nature and concentration of the chemicals vary, depending on the target contaminants and site-specific conditions.
Heavily contaminated site
In November 1999, QDS was commissioned by an oil company to help facilitate the sale of a former service station site in London. The site was surrounded by historical sources of contamination including a heavily contaminated former gasworks site and an old underground kerosene storage facility. Following a site investigation the presence of diesel range, volatile (BTEX) and MTBE contamination was also identified within the groundwater.
The geology beneath the site comprised River Terrace Deposits overlying alternating sand and clay beds of the Woolwich and Reading Beds and Thanet Sands. The depth to groundwater was approximately two metres across the site. Given the nature of the contaminants, the aquifer conditions and the restricted timeframe, ISCO was selected as the most appropriate remediation technology. A liquid oxidant solution and catalyst were sequentially applied in order to generate hydroxyl free radical ions – one of the most powerful oxidising agents known.
The critical step
It has been discovered that hydroxyl radicals (-OH ions), generated from the breakdown of an oxidant in the presence of a catalyst, are capable of rapidly degrading petroleum hydrocarbons within contaminated groundwater and soils. The initial reaction, known as the critical step, occurs when the hydroxyl radical attacks a hydrogen-carbon bond to form a new breakdown product, and then either combines with the liberated hydrogen atom or the breakdown product to form a new compound. Following the formation of the new compound, subsequent reactions, initiated by the hydroxyl radical, take place until the hydrocarbon compound is broken down into a series of base components.
This process is known as mineralisation. During mineralisation any number of intermediates can be formed. In the case of aromatic nuclei (e.g. benzene/phenol) ring cleavage precedes mineralisation. Complete mineralisation will occur providing there are excess hydroxyl radicals present.
Chemical oxidation is a very powerful process causing a large-scale reduction in organic contaminant loading in extremely short time frames. It is, however, a process to be treated with respect and only operated under stringent health and safety controls. Its strength lies in the ability to rapidly oxidise and remove not only the dissolved phase and adsorbed phase hydrocarbons but also the resultant breakdown products. Any uncatalysed oxidant simply decomposes to oxygen and water, resulting in an increase in dissolved oxygen concentration in groundwater.
In this case, 32 injection wells were installed across the site, each to a depth of approximately 4.5m with gravel packs and bentonite-cement seals to the surface. All well materials, connections and pipework were constructed using chemically resistant materials. The injection lasted three days and was conducted on a continuous 24-hr basis.
The results demonstrated a significant reduction in the concentrations of the contaminant. The percentage reductions ranged from 89% of DRO compounds, 90% of TVH, 95% of BTEX through to almost complete removal of MTBE.
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