ELECTROKINETIC DEWATERING
Part of our Low Impact, Innovative Solutions for Ground Engineering, Mining & Waste Management
EK dewatering of slurries, sludges and mine tailings, which is offered by Electrokinetic, is a requirement of a large number of processes and activities. The science and engineering of this activity recognises four broad stages.
Each stage of the process has a variety of methods that can be adopted. Removal of water from fine-grained material is most challenging, as solid particles settle very slowly (sedimentation and thickening), or water flows very slowly through a matrix (dewatering). EK electroosmosis combined with filtration can yield enhanced performance with increased solid content or increased speed in the dewatering process to the extent that filtration of challenging material can be viable. Aimed at mixtures of water and fine-grained solids, there are multiple distinct applications for EK dewatering methods.
The Four Broad Stages of EK Dewatering
Sedimentation or Settling
Usually achieved by some manner of active or passive settling of solids through a liquid column.
Thickening
Taking a liquid mixture such as a sludge and removing water to produce a higher density liquid or paste.
Dewatering
Taking a sludge or paste and removing water to effectively create a phase transition wherein the material has been dewatered.
Drying
Cake (dewatered sludge) is dried to produce a friable granular or free flowing material.
How Dewatering Bags Work
EK dewatering bags enhance conventional pressure-driven flow with electroosmosis, creating dewatering rates up to four orders of magnitude greater than conventional hydraulic flow in fine-grained materials.
Application of direct current via inserted anode rods transports water electroosmotically towards the cathode, which is integral to the bag fabric, where it can then drain.
By arranging the geometry of the EK bag into a bisected torus, the flow-inducing gradients are doubled (voltage gradient for electroosmosis and head for hydraulic), and the flow path length is halved, thus quadrupling the effectiveness of both flow mechanisms. Combining both approaches, the overall effect on the dewatering rate is greater still.
EK dewatering bags are easily installed, suspended from a simple framework or supported on a drainage platform.
Combine electroosmosis and hydraulic filtration to permit dewatering in hanging bags – previously impractical or effectively impossible.
Enabling dewatering of small volumes of industrial waste such as may be associated with food production.
Enable removal of water from arisings from borehole drilling.
Enable dewatering arisings from roadside gulley operations.
EK In Situ Dewatering
Some materials are placed in open voids in the ground (owing to space availability and the difficulty in dewatering slurry).
Where materials are very soft and fine grained, removal of water is challenging when material is too thin to shovel, but too thick to pump.
Electrokinetic methods combine electroosmosis with conventional well-point technology to permit effective in situ water removal.
How EK In Situ Dewatering Works
EKG materials formed as prefabricated vertical drains can increase the rate of consolidation through electroosmotic flow. This may be employed when dewatering materials for subsequent excavation or in situ ground improvement prior to site development.
In fine-grained soils, the electroosmosis-induced flow rate can be 3-4 orders of magnitude greater than the hydraulic flow rate.
Vertical drains can be inserted into the ground by lance or mandrill. Treatment reduces consolidation time by 50-80% compared to conventional wick drains and does not require the addition of surcharge.
Typical applications of in situ EKG dewatering are lagooned materials such as mine wastes or sewage sludges, materials that tend to have high water contents and low hydraulic conductivity.
EK In Situ Dewatering Applications
The technique is applicable in situations where conventional approaches are impractical or uneconomic (such as wick drains + surcharge, cementitious stabilisation, dilute and dewater).
Each application requires individual consideration relating to target strength, water content, speed of treatment and cost.
Treatment can accommodate these objectives by varying the number of installed electrodes, the applied voltage and the duration of treatment.
Examples of the use of this technique are the remediation of historical lagoons, e.g. tailings or industrial treatment sludges.
Performance & Capacity
The EK bag dewatering process involves topping up the content as water is drained therefore, the overall volume of material that can be removed in one process can range from 2 m³ to over 10 m³ depending on the solids content and the dewaterability of the waste material.
EK dewatering bags will operate on a slurry of approximately 0.5% to 10% dry solids, and will raise the solids content over a period of one to three days, depending on operating voltage and slurry characteristics.
Typically, the final solids contents for an organic slurry are in the range of 15 to 35%, and for a mineral slurry, 30 to 50% plus. For multipurpose applications, EK dewatering bags can be emptied by attaching lifting ropes to the two toe loops at the bottom of the bag and rotating and emptying the bag.
The anode comprises a circular mounting ring and a series of 16 mild steel rods. The rods are up to 25mm in diameter and are gradually consumed, the rate of which will be dependent on the characteristics of the slurry.
EK dewatering bags can be operated with or without the use of flocculent and/or coagulant. The use of these chemical additives may enhance dewatering and reduce the time required to turn a slurry into a solid.
Ancillary Equipment
The operation of EK dewatering bags requires the following ancillary equipment:
We can supply the EK bags alone, or including the ancillary equipment.
A simple framework or a drainage platform
Slurry delivery method e.g. pumping
A DC supply 0 to 60V up to 2kw per bag depending upon slurry characteristics
Electrical cables for connection
An anode ring and one set of sacrificial rods