POPULAR SOIL REMEDIAL TECHNOLOGIES
In this blog post, we discuss some popular soil remedial technologies which are often used in South Africa to help remediate shallow soil contamination to pre-contamination or acceptable levels
Considerations
When choosing a remedial technology there are some aspects that need to be considered such as:
In or Ex-situ?
When remediating soil, it can either be in-situ (still in the ground) or ex-situ (on the surface). There are advantages and disadvantages of in-situ and ex-situ soil remedial technologies which need to be considered when choosing a remedial technology:
In-Situ Soil Remediation Techniques
Volatilizations/Bioventing
How? | It involves injecting air (and nutrients) into the targeted contaminated zone via vertical injection wells, with the expectation that volatile contaminants will volatilize. A soil venting system (SVE) is usually employed to capture the contaminated/volatilized air. |
Why? | • Oxygenation of soil (and water) enhances in-situ biodegradation + volatiles hydrocarbons which can be extracted from nearby extraction wells. • To treat contaminated soil situated above deep groundwater. |
Pros: | • It stimulates microbial activity (aerobic breakdown) by supplying oxygen, without affecting the soil structure. • Nutrients and moisture can also be added along to improve breakdown. • Bioventing increased the rate of air diffusion • Easy and low cost |
Cons/ limitation: | • Effectiveness may be reduced if a few small preferential pathways are formed – can be overcome by slow air injection rate + several injection wells. • Very sensitive to heterogeneities in the soil • The process can be time consuming • Have to treat the hydrocarbons – GAC |
Targets phase: | Works best for dissolved phase GRO’s |
Video Link |
Enhanced Monitored Natural Attenuation
How? | Actively add nutrients (and oxygen) to the subsurface to serve as electron donors/acceptors for microorganisms to use during breakdown of hydrocarbons. Based on the predominant redox process nutrients such as nitrogen, phosphorus and sulphate can be added via mixing with water. If the system is under aerobic conditions, hydrogen peroxide can be added as a slow releasing source of oxygen. Process can be sped up through bioaugmentation (addition of more bacteria and bacterial cultures). |
Why? | To improve the biodegradation capacity and speed up the breakdown process of hydrocarbons. |
Pro: | • Effective, inexpensive and safe way of degrading hydrocarbons if there are no high-risk receptors present. Can be done ex-situ (such as landfarming – more expensive but can more closely control environmental parameters). |
Con: | • Can be unpredictable and unreliable due the variety of physical, chemical and biological factors if the conditions (pH, temp, nutrient availability) no longer exist which are optimal for the microbes. • lengthy cleanup time and the commitment to monitoring (several years). • Works better in groundwater to help transport nutrients. • Difficult to deliver nutrients uniformly. |
Targets phase: | Dissolved phase hydrocarbon impact (not effective for LNAPL and residual HC globules) |
Video Link |
Ex-situ Soil Remediation Techniques
Excavation
How? | Excavate soil at the known impacted area and place it on the surface with the use of a TLB. Ideally excavated soil should be sorted into contaminated and uncontaminated soil heaps by TLB operator based on visual and odour. Soil samples are collected from soil heaps for waste class purposes. Based on the class the soil can be taken to disposed at a specific landfill site or re-used as backfill. |
Why? | • In case of emergency when lots of product was lost and want to stop product from migrating. • Easiest and fastest way of removing shallow contamination that is localized. |
Pros: | Easy, quick, increases area for additives to help with breakdown (nutrients, oxidisers) |
Cons/ limitation: | • Can become very expensive when lots of impacted and unimpacted soil is generated which needs to be classed and disposed of based on the class (Type 1 – Type 4) at disposal sites that may be far away. • Obstructs normal site operation (may require site shut down). • Can only dispose the excavated soil (limited depth of excavation – mostly vadose zone). • Extreme soil disturbance |
Landfarming
How? | Excavated contaminated soil is placed on the surface and mixed with nutrients to help break down hydrocarbons. The soil can also be periodically tilled to allow oxygen to enter the soil pores which stimulates breakdown and aeration of hydrocarbons. The soil quality is periodically monitored to determine how breakdown is progressing. Once target levels are reached the soil can either be tiled back into the earth, used as backfill or disposed. |
Why? | Landfarming is done to speed up the remediation of contaminated soil through the addition of nutrients and oxygen. |
Pro: | • Possibly the quickest way to remediate hydrocarbon impacted soil. • During tilling aeration of the soil removes most of the hydrocarbons through volitilisation. • Can re-use the same soil after it has been remediated. |
Con: | • The landfarming site must be closely managed to prevent contamination run-off during rain. • Large area is required to farm the soil. • Extreme soil disturbance • Expensive and labor-intensive as leachate collection systems may be required |
Targets phase: | All phases and types hydrocarbon impact (vapour, dissolved, residual, LNAPL) |
Video Link |
Composting
How? | Soil washing involves removing/excavating soil and placing the affected soil into a reactor vessels and vigorously reacted with a washing solution (Octanol or surfactant). Soil is screened after crushing to remove coarse materials (typically less contaminated). Fine material is then mixed well with washing solution. Ideal washing solutions increase solubility and mobility of contaminants with little effect on remaining soil constituents. There may repeated cycles of washing of the soil material. Once cleaned the residual extractant is rinsed out form the cleaned soil with water, and the clean soil is placed back to the ground. |
Why? | It is very effective in removing hardy contaminants from soil such as tar and separates coarse less impacted soil from fine, generally, more impacted soil. |
Pro: | • Drastically reduce contaminant concentrations and can return soil to below-threshold • Can treat a wide range of contaminants • The washing solution can be re-used |
Con: | • Requires a water treatment plant (lots of surface equipment) • Require large volumes of wash water • Extractants need careful selection to minimize environmental impacts • Extreme soil disturbance |
Targets phase: | All phases and wide range of contaminants |
Video Link |
Soil Washing
How? | Soil washing involves removing/excavating soil and placing the affected soil into a reactor vessels and vigorously reacted with a washing solution (Octanol or surfactant). Soil is screened after crushing to remove coarse materials (typically less contaminated). Fine material is then mixed well with washing solution. Ideal washing solutions increase solubility and mobility of contaminants with little effect on remaining soil constituents. There may repeated cycles of washing of the soil material. Once cleaned the residual extractant is rinsed out form the cleaned soil with water, and the clean soil is placed back to the ground. |
Why? | It is very effective in removing hardy contaminants from soil such as tar and separates coarse less impacted soil from fine, generally, more impacted soil. |
Pro: | • Drastically reduce contaminant concentrations and can return soil to below-threshold • Can treat a wide range of contaminants • The washing solution can be re-used |
Con: | • Requires a water treatment plant (lots of surface equipment) • Require large volumes of wash water • Extractants need careful selection to minimize environmental impacts • Extreme soil disturbance |
Targets phase: | All phases and wide range of contaminants |
Video Link |
To request a quote or ask for more information regarding our contaminated land services, please contact Magiel Lourens (CSM Unit Manager) at magiell@gcs-sa.biz | +27 11 803 5726
