New in situ remediation technology cuts costs and time
What do you do if you are responsible for contaminated land but have neither the budget for disposing of soil to landfill, nor the time for biological treatment? Jim Delves, Research and Development Director of engineering firm Dynamic Processing Solutions PLC, explains the ins and outs of the company’s new technology for in situ treatment of soil contaminated with hydrocarbons, heavy metals and even radioactive waste (see Fig 1).
During the first stage of the process the contaminated soil is loaded into a hopper from where it is transported using a ‘patent pending’ HydroTrans system which allows the material to be processed as a slurry.
The tubular sonic processor is normally the same diameter as the slurry discharge pipe to avoid pressure drops or velocity variations.
The sonic processor turns electrical energy into kinetic energy, in the form of sound waves which, when generated in a liquid, travel at up to 500 km/hour. This process attempts to create a vacuum – which nature abhors – in the liquid phase, creating tiny cavitation bubbles up to 100 microns in diameter. These bubbles collapse to cause an implosion/explosion, generating localised heat up to 500°C for about a nanosecond, which increases the rate of the reaction.
The bubbles and the enzyme contact hydrocarbons and heavy metals in the slurry and move them from the pores and crevices within the solids into the liquid phase. Once removed the hydrocarbons remain in the water phase leaving the solids clean.
The sonic process also breaks down and de-conglomerates the solids to expose greater surface area to remove contaminates. It also prepares the slurry for the next process stage: de-watering solids using a solid/liquid cyclone.
Cyclones are well known in the mineral industry and work well if the slurry is fed in at a stable flow, pressure and concentration. This is where the combination of HydroTrans and the sonic processor excel. Any fine particles or hydrocarbons left within the pores of the solids are removed by the cyclone’s enhanced gravity forces, further aided by attrition generated within the cyclone.
Clean solids report to the underflow of the cyclone and most contaminates, heavy metals, hydrocarbons and enzymes report to the overflow. The solids can be dried using a rotary screen and backfilled on site either by dumper or conveyor.
The water phase is then processed using an Econova system to which DPS has an exclusive license for use outside of the offshore oil and gas industry.
Dynamic Processing Solutions PLC, through their Industrial and Environmental Division are using technology developed for the offshore oil and gas industry to treat contaminated ground water at source.
The process involves several patented operations to allow the removal of hydrocarbons and heavy metals in a single pass system that consists of contaminated water in, clean water out, with a separate concentrated waste stream.
The system is based on a novel centrifugal oil/water separator, electro-coagulation, and a centrifugal clarifier followed by a clean on-line filter.
Dirty water is screened before the oil/water separator. The centrifugation separator is novel in that the separation is controlled by pressure to allow the interface of oil and water within the centrifuge to be determined by a simple pressure differential between the water out phase and oil out phase.
The system will handle oil concentrations from 0% to 100%, so hydrocarbon slugs do not give problems.
Fig 3. Oil/water separator
This first stage process can deal with 0-99% oil/water ratios with an oil density up to 0.97 and deliver water purity down to 10 ppm, whilst solids are not a problem as the centrifuge has a facility to remove solids ‘on-line’. The recovered oil can also be recovered as a product and sold for its calorific value rather than pay for disposal or rely on bio treatment.
Fig. 4 – Internals of oil/water separator showing solids fluidisation nozzles
Should no hydrocarbons be present at a particular site, then this unit can be omitted. The system is completely modular, allowing a plug and play philosophy.
The electro-coagulation system consists of aluminium and iron cathodes and anodes. A direct current is passed through the aluminium and iron bars to cause sacrificial electrode ions to move into an electrolyte. The contaminants are removed either by chemical reaction and precipitation or by causing colloidal material to coalesce. Different anode and cathode materials are selected for specific contaminants.
Fig. 6 – Electro-coagulator
Oxygen is evolved at the anodes, whilst hydrogen is evolved at the cathodes, both in the form of gas bubble nucleates. The system is designed to manage the very low levels of hydrogen to ensure H2S does not become a problem.
The bubbles grow at the electrode surface until they reach a size for buoyancy and shear forces to dislodge them. In some cases this process can be used to generate a biocide such as chlorine for use in the system.
The water phase from the oil/water separation is passed through the electro-coagulation process which performs three important functions to allow the cleaning of waste water:
- the first is the disruption of colloidal suspended solids;
- the second is the electrocution of micro-organisms in the water; and
- the third is the generation of charged nucleation particles which cause flocculation to occur.
The treated water is then passed from the electro-coagulator into a centrifugal clarifier.
Fig. 7 – Clarifier
The clarification process is used to remove all the contaminants that have been caused to coagulate and drop out of solution (precipitation). The centrifugal clarifier forces the water stream at high ‘G’ forces to cause minute gravity affected particles to concentrate out of the water phase. Although electro-coagulation has been known for some time, a small footprint method of removing and compacting these weak solid flocs has not been available prior to this new centrifugal clarifier.
Fig. 8 – Electro-coagulation, showing electro-coagulation supernatant influent on left and waste solids on right
The clarified liquid is then passed on to a filter and the concentrated solids exit the process through a bag filter as a semi-dry product.
See fig. 9 for typical water analyses from the treatment process.
The system can typically can handle 0-230 m³/hr continuous flow, or variable flow. Handles variable oil/water ratio of various viscosities and removes solids ‘on-line’ without the need to stop the process.
Jim Delves, R & D Director
Dynamic Processing Solutions PLC
Tel: 0870 774 1144