Complementary Processes
Treatment of dredged silts
Soils washing and stabilisation techniques are equally applicable to the treatment of dredged sediments from polluted canal, river and harbours.
The combination of clean material recovery, mechanical dewatering and sludge solidification provides the potential for significant cost savings.
The dredging can be carried out from pontoon mounted excavators and barges or traditional cutter suction equipment.
Chemical Oxidation
Oxidation processes can be used to transform organic contaminants into less toxic and less mobile forms, with complete reactions producing CO2 and water.They are referred to as 'redox reactions', as the oxidising agent gains electrons (reduced valence state-reduction) while the contaminant loses electrons (increased valence state-oxidation). This electron exchange breaks carbon bonds creating new smaller compounds. Oxidants can also be used for iron and chromium precipitation and the destruction of cyanides and sulphides.
The most commonly used oxidising agents are hydrogen peroxide (H2O2), sodium persulphate (S2O8) and potassium permanganate (KMnO4). Many redox reactions require chemical additions, such as pH alterations, or the presence of catalysts (Fentons Reagent combines iron with hydrogen peroxide) to economically facilitate the required reaction. Therefore, for successful applications of oxidation techniques, an understanding of chemical kinetics and the thermodynamic potential of the reactions is essential.
A number of parameters can effect the redox reactions, such as soil organic matter, pH, exisiting redox conditions, nature and extent of contamination, and carbonate/bicarbonate concentrations. Therefore, as an important part of the design process, HBR undertake laboratory treatability studies to establish optimum conditions the reactions to take place and the existing oxidant demand of the soils and groundwater.
HBR apply the process either
in situ, by introducing oxidants into the subsurface with gravity wells or injection methods and ex-situ by soil mixing or groundwater circulation systems. For ex-situ soil applications, HBR's mixing plants incorporate feed systems, which are suitably resistant to the corrosive nature of oxidizing agents and capable of the control and collection of off-gasses produced during reactions.
Contaminants that can be typically treated include:
- petroleum, diesel, MTBE, BTEX
- chlorinated solvents
- pesticides and herbicides
- carbon disulphide
- polychlorinated byphenyls (PCB's)
- cyanides and sulphides
Engineered containment and cut off walls
HBR can design and construct an array of low permeability containment systems to mitigate contaminant migration in the subsurface. These include vertical barriers (walls), basal barriers (floors) and surface barriers (caps).
Vertical techniques provide containment through the use of soil-bentonite and cement-bentonite slurry trench cut-off walls, steel sheet pile walls, puddle clay barrier walls and deep soil mixed barriers. The use of horizontal or complete encapsulation, such as the treatment of redundant landfill sites and other locations with high contamination concentrations affecting groundwater resources, is commonly provided with a basal liner of geotextile and HDPE or bentonite polymer enriched sands. Grouted bottom barriers can also be constructed through existing soils/wastes. These methods promote the use of long term in-situ remediation processes, whilst the groundwater and site end-users are protected. Blackwell have vast experience in the design and construction of landfill cells, which have included engineered basal layers and surface capping systems to prevent infiltration. Geocomposite, geosynthetic and geotextile capping solutions have all been constructed.
Permeable reactive barriers (PRB's)
A PRB is an engineered insitu treatment zone of reactive media. When installed in the flow path of contaminated ground water, PRB's can remove contaminants as it passes through a more permeable 'reactive' section. Lower permeability barrier sections, essentially cut off walls, can be used to direct ground water through the reactive section. These are often termed funnel and gate systems.
A variety of reactive media can be used depending on the contaminants present, which either immobilise contaminants by sorption mechanisms or transform them by biological and chemical processes. Installation can be carried out by soil mixing or traditional slurry walling techniques.
HBR staff have significant experience of the design and construction of PRB's. In order to optimise the design, we undertake laboratory treatability studies, using flow through testing with soils and groundwater from the site of concern, in order to establish sorption capacity, breakthrough, residence time and flow rates with various reactive media. This enables actual barrier performance to be predicted.
Virtual reactive barriers can also be used, consisting of a series of well points, where extraction or injection techniques can be employed to remove or destroy contaminants.
|
