Case studies: Dewatering
Dewatering of sewage sludge
Treatment of raw sewage produces sludge with solids content in the range 1-3%. The requirement to dewater and re-use or dispose of these materials is a fundamental issue and persistent and complex problem for all water companies in any country.
Conventional dewatering of sewage sludge involves the use of belt filter presses or centrifuges to reduce the water content to permit disposal.
Disposal is best effected if the sludge material has a dry solids content greater than 25-30%. In general terms conventional belt press treatment systems produce a sludge cake with a dry solids contents significantly below this level.
A step change in belt press dewatering performance has been achieved by combining conventional belt press technology with EKG. This breakthrough adds the dewatering effect of electroosmosis to that of conventional hydraulic dewatering.
The form of the EKG is an Electrokinetic patented enhancement of standard filtration belts.
An example of using EKG technology in sewage dewatering by Thames Water Company (London) is shown below.
Left: Conventional treatment, 19% dry solids content. Right: EKG enhanced treatment - 31% dry solids yielding 39% volume reduction and improved stacking
This shows that, at a dry solids content of 19%, the material is still in a semi-liquid form and difficult to transport, this can require mixing with straw to provide mechanical stability. If disposal is by incineration then fuel oil has to be added to increase the thermal content of the sludge.
At the 31% dry solids content produced by the EKG treatment, the sludge is now a solid and can be handled without the addition of bulking material and that the volume of material to be disposed of is reduced by 39%.
In addition, at 30+% dry solids many sludge cakes are auto-thermic and can be used as a fuel.
The economics and cost savings of using EKG technology to dewater sewage sludge has been identified by McLoughlin (2005) as tabulated below.
Disposal cost comparison (after McLoughlin 2005)
Based on typical sewage treatment site with six dewatering machines, this will give a potential cost saving of c£800,000 per annum.
Dewatering of diamond mine tailings
The process of diamond extraction is reliant on large quantities of water to concentrate the mineral; this produces large quantities of waste.
The process results in two broad types of discard material, which are differentiated according to their dominant grain size: grits (>75 μm) and slimes (<75 μm).
With the increasing importance of sustainable use of water in the mining industry practices such as Paste and Thickened Tailings Disposal (P&TTD) are being adopted increasingly in the diamond mining sector.
Water recovery prior to disposal offers a number of advantages including reuse of water, reduction in size of the disposal facility and an increased lifespan for a given facility.
Whilst P&TTD processes reduce disposal volumes and recover water, the lack of a dewatering stage means that thickened tailings or pastes must be pumped in a liquid state to the disposal site using high pressure, high volume positive displacement pumps.
Full scale trials at Kimberley, South Africa, using an EKG belt press have demonstrated that major savings can be obtained with regard to energy costs associated with disposal, reduction in waste volumes requiring disposal, recovery of water and a significant reduction in the carbon footprint (CO2) of the mining process.
The dewatered tailings are suitable for transportation to the disposal site by conveyor, hence potentially saving substantially on the current cost of tailings disposal by large displacement pumps and pipelines.
The trial showed the following potential benefits of the EKG belt system to the Kimberley site when compared pumping slimes to a tailings dam:
- 55% reduction in carbon dioxide emissions (5700 TCO2 /yr)
- 55% reduction in power consumption (5800 MWhr/yr)
- 67% reduction in water discarded (water recovery of 1.75 MT/yr).
- 49% reduction in volume of tailings
- The potential elimination of the need for a tailings dam.
Case studies: Ground Engineering
Electrokinetic stabilisation of a railway embankment
Electrokinetic geosynthetic (EKG) technology has been used to successfully stabilise a failing clay embankment in London resulting in a 26% cost reduction and a 47% reduction in carbon footprint over conventional methods.
There are 20,000 km of earth structures (cuttings and embankments) on the UK highway and rail networks. Few were built to modern geotechnical engineering standards. The ongoing maintenance and remediation that these structures now require has become a major engineering issue for many UK infrastructure owners.
Toe weighting and/or slope regrading is commonly used to tackle the problem, but these do not address the problem of shrink-swell or pore water pressure changes and typically delay failure rather than prevent it. In addition, these methods can consume large quantities of primary aggregate and energy and are becoming less environmentally and economically viable.
Network Rail identified EKG ground treatment as a novel slope treatment method which could:
- stabilise the slope
- require only modest access owing to the absence large plant
- involve low relative energy consumption
- reduce cost
A trial was conducted on a 22m stretch of a 9m high Victorian embankment. The embankment had been constructed by end tipping a mixture of weathered London Clay and other material such as brick and stone fragments onto underlying alluvium and terrace gravels. An assessment of the embankment identified several sections as unstable. Inclinometer readings indicated a slip surface at approximately 2.5m depth, which could either be a shallow translational slide or a deeper circular failure. Stability calculations indicated a factor of safety (FoS) for the slope of only 1.0.
Cross section of the slope showing the stratigraphy and postulated failure planes
EKG treatment
EKG treatment was designed to accommodate either of the identified failure mechanisms. The treatment was based around an array of EKG electrodes installed at 2m centres in the form of tessellating hexagonal cells, with the hexagon being defined by anode stations and a central cathode.
Upon application of a DC potential (60-80V) electroosmosis forced water to flow from the soil adjacent to the anodes to the cathodes. The treatment took only six weeks and resulted in:
- dewatering from the cathodes >25 times that from control drains.
- a reduction in plasticity and shrinkage characteristics.
- an increase in groundwater temperature from 10OC to 20OC.
- a modest DC power consumption of only 11.5kWhrs/m3 of soil treated.
- improvements in shear strength parameters (c’ and Φ’)
- a 263% improvement in the bond strength of the anodes acting as nails
- a cessation of slope movement.
Following EKG treatment the anodes have been retained as permanent soil nails and the horizontal cathodes retained to act as permanent drainage.
Installation of EKG materials into a railway embankment
Comparison of pull out strength for electrokinetic and control reinforcement
Slope stability analyses were undertaken pre and post treatment. The analytical results are shown in the table below.
Longevity of treatment
The use of EKG to stabilise slopes is a long term solution because:
- Soft weak embankment materials consolidate and improve in shear strength with EKG treatment. This consolidation is permanent.
- Additionally, EKG treatment works best on these soft materials, which are critical to the stability, and in this way the treatment can be considered ‘self selecting’
- Modifications in soil clay chemistry such as cementation and plasticity occur under conditions induced by electroosmotic flow. Given the fine grained nature and very low transmissivity of the soil, the probability of the reversal of these changes is negligible and hence the effects are considered permanent.
- Enhancement of soil/reinforcement bond is a long term effect.
- Passive drainage (de-activated cathodes) is retained in the slope.
Costs
A cost analysis comparing slope stabilisation using the EKG method with the lowest cost alternative of gabion baskets and slope slackening, indicated that the EKG treatment produced total project cost savings of 26%.
Carbon Footprint
A carbon footprint comparison of the EKG and conventional treatment options showed 47% lower emissions by using EKG.
Induced currents
Issues have been raised regarding the possibility of ‘stray’ currents. For clarification, this term is used to denote electric currents which do not flow where intended and are caused by two mechanisms:
- Direct conduction
- Induced currents
An analysis of the EKG treatment indicated that such currents are negligible.
Benefits of EKG treatment
In summary, the benefits of EKG treatment include:
- Effective method for slope stabilisation
- Reduced cost
- Reduced access requirements for labour and plant and materials
- Reduced health and safety risk
- Rapid deployment and low labour requirements
- The treatment can proceed whilst maintaining the railway in service (as occurred during the trial)
- Long term drainage of the slope can be provided for by the filtration and drainage functions of the EKGs in the passive mode.
- Sustainability benefits including reduced carbon footprint and elimination of the use of primary aggregates.
Other benefits include:
- The treatment is gradual, only acts when current is on and does not induce rapid changes in ground conditions especially settlement. This therefore provides the option to cease treatment immediately if ever it were deemed necessary.
- The treatment can be flexible in approach by varying voltage, electrode spacing and duration of treatment.
Further flexibility will be possible by manipulating the electrode array and angle of electrode installation to accommodate in situ obstacles such as trees.
More details of the above work may be found in Lamont-Black et al 2009 in the downloads page.
EKG reinforced soil wall, County Durham
The objective of the trial was to construct a reinforced soil wall constructed with super soft clay fill. In this application the EKG materials acted both as drainage paths and reinforcement.
Most codes of practice do not permit the use of cohesive soils in the construction of reinforced soil due to potential problems of short term stability and its influence on the durability of metallic reinforcement. Short term stability results from low shear strength and poor bond between the reinforcement and the cohesive soil which is aggravated by the development of positive pore water pressures at the soil/reinforcement interface. To illustrate the power of EKG to permit the use of material usually considered to be totally unsuitable, a reinforced-soil wall was constructed with fill in the form of a clay slurry (defined as a disturbed cohesive soil with a water content higher than the liquid limit).
The wall was constructed using a 'wraparound' design, utilising sandbags for the front face to temporarily retain the liquid fill. The ends of the trial wall were retained using conventional reinforced soil blocks, and the wall was raised using a staged construction technique. Clay slurry was prepared in a pit adjacent to the wall and poured in 300mm layers.
Preparation and pouring of super soft fill
Each lift was constructed and dewatered vertically by electro-osmosis applied via horizontally placed EKG electrodes. Once a lift had been successfully treated then the next lift was constructed, the original cathode now becoming the anode, and the original anode being turned off and reverting to a reinforcing role.
Construction sequence of the reinforced soil wall
The result of the trial showed that the shear strength of fill in the form of a wet slurry could be increased to permit safe construction of a vertical reinforced soil wall. Another finding was that the reinforcement/soil bond increases in proportion to the increase in shear strength.
EKG reinforced wall after the construction of lift No. 3
This use of EKG technology offers the potential for the use of very poor quality materials which are ubiquitous and otherwise represent a liability rather than an asset.
Consolidation of sewage lagoon material
EKG materials in the form of modular drains were used in a trial to dewater lagooned sewage sludge
Once deposited in a lagoon sewage sludge in the form of a very thick liquid or cake is a difficult deposit to handle and most known methods to remediate or stabilise them are very costly or environmentally intrusive or both. The rationale behind this trial was to dewater the material in situ using EKGs and thus avoid costly double handling and/or mixing with cementitious stabilisers
A field pilot trial using material excavated from a sewage lagoon and placed in large skips demonstrated the effectiveness and efficiency of using ePVDs to dewater lagooned sludge by an in situ process. The trial demonstrated that the technology could dewater sewage sludge thus raising the solids content from 10.5% to 27% whilst at the same time producing a clean discharge filtrate water with BOD< 3.0 mg/l and a COD of 250mg/l. The power consumption was 43kWhr/m3. These data represent the first field trial of this technique and indicate that it can provide a viable economic solution to treating in situ lagoon sludges. The trial also indicated that significant further improvements in solids contents and reductions in power consumption are achievable at full scale.
Improvement of sludge through gradual dewatering using EKG
Further details of this trial are available in Lamont-Black, Glendinning, Jones, Huntley & Smith 2005 in the downloads page.
Consolidation of super soft soil
EKG has been used in the form of an electrically conductive prefabricated vertical drain (e-PVD), to consolidate super soft kaolin clay. A large test pit was filled with kaolin clay with a moisture content of 85% to a depth of 2.4m with EKG vertical drains installed as shown in. The clay was left to settle for 100 hours, resulting in a consolidation of 20mm.
Electro-osmosis was then applied for a period of 500 hours which produced further consolidation of 340mm and an increase in shear strength from < 1kN/m² to 15–30kN/m². To produce an equivalent result using conventional means would have required a surcharge loading of 10m of fill, which would have been impossible to place on the super soft soil.
During electro-osmosis water flows from the anode to the cathode; as a result the area around the anodes experiences the greatest reductions in moisture content and improvement in shear strength. In order to minimise moisture content anisotropy, the trial was completed with a phase of polarity reversal in order to draw water away from the electrodes which were acting as anodes in the first phase. Polarity reversal resulted in a more even distribution of shear strength in the soil.
Large lab scale consolidation of super soft soil using EKG
The trial demonstrated that dewatering/consolidation using EKG prefabricated drains is rapid, effective and, energy efficient. For full scale applications, consolidation by EKG has a number of potential advantages over conventional wick drain technologies:
- it is faster
- no surcharge is required
- the effectiveness of the e-PVD is unaffected by kinking or smearing as consolidation proceeds
Sports Turf Conditioning
Natural turf trial
EKG was installed in 130m2 of an unused section of a multipurpose sports pitch at Newcastle University. A common cathode array (lower electrodes) was installed by mole plough at approximately 0.5m depth and 0.5m spacing. As the upper electrodes (anodes) will have a much greater effect on the turf grass and soil condition, they were arranged in four alternative arrays located within the root zone at approximately 90mm below the surface.
EKG electrodes installed into natural turf
The four arrays were operated alongside control zones. Data were gathered over several months from June to September. The characteristics of the site were established prior to activation. Electrokinetic treatment comprised 3 and 6 hour activations of the system using a DC voltage gradient of approximately 0.25V/m, supplied by lead acid batteries.
Data were gathered on water content, shear strength, ball bounce, root zone CO2 production, chlorophyll fluorescence ratio, root strength, thatch thickness, shoot growth and current. Evaluations were made as time-series of gradual changes, and as more discrete ‘events’. The former generally related to plant health whereas the latter related to physical condition of the soil.
Summary of physical data: clay based pitch
EKG was activated after simulated heavy rainfall. The physical data showed that in all cases (except the 'strips' anode array) the activation of EKG improved ball bounce and shear strength and reduced the water content compared to the control.
Biological data collected showed that over the course of the four month trial, EKG treated turf showed improvements in: root weight, root pullout strength, shade tolerance and reductions in thatch development.
Suspended water table trial
The next phase of the research involved the application of electrokinetic treatment to a uniform sand based, 'suspended water table' pitch.
The turf comprised an 80:20 mixture of perennial ryegrass (L. prenne) and smooth-stalked meadowgrass (P. pratensis), grown on a root zone of medium sand with 10% fine sand. The profile was similar to that used in high quality constructions of winter sports pitches and golf greens. The turf used was chosen to closely resemble the turf used on modern high quality winter pitches. The pitch was constructed with two identical halves each of 6.25m2. One side was activated, the other acted as a control.
The data collected showed a distinct difference between the two halves of the constructed suspended water table pitch. The activated EKG half showed higher shear strength, ball bounce, CO2 production, root mass, root strength, soil microbial activity and chlorophyll fluorescence. The unactivated half showed higher levels of thatch thickness, pH and water content. There were no significant differences in temperature or yield.
Summary of physical data: suspended water table pitch
Root weight in suspended water table pitch
The microbiological results were of particular interest and showed a much more rapid degradation of agar on ‘bait sticks’ which indicated increased general microbial activity in the activated area.
These results, taken together with the increases in CO2 production, root weight and root pull out strength were interpreted as system responses to the electrolytic production of oxygen in the root zone. Such activity offers clear health benefits for natural sports turf.
Consumption of organic bait in suspended water table pitch
Summary
EKG provides a means by which to improve the physical performance of grass during the growing season but at the same time provide a greater root mass for the storage of carbohydrates to maintain health and the ability to repair wear and damage during the cold winter months. An advantage of the electrokinetic system is that it may be used much more frequently than traditional aeration methods, to provide oxygen where and when it is needed.
More details of this work can be found in Lamont-Black, Jones, Glendinning & Huntley, 2006 in the downloads page
Electrokinetic thankfully acknowledge the Institute of Groundsmanship for their support of this work.
