Electrokinetic revolution looms for sludge dewatering
Electrokinetic sludge dewatering is close to becoming a commercial reality. And, according to David Evington, writing on behalf of EKG, it will dramatically change performanceSince the introduction of the belt filter press, there have been evolutionary developments delivering small but useful improvements in cake dry solids. By contrast, the application of electrokinetics to sludge dewatering now looks set to revolutionise performance.
The concept of electrokinetic sludge dewatering combines electro-osmosis with mechanical dewatering. The idea was explored through research started at Newcastle University in the 1990s and continued by technology company Electrokinetic, a spinout from the university's department of civil engineering and geosciences.
Initially, the research was looking into the development of geosynthetic materials for ground engineering applications.
Following a three-year £1.49M project, part-funded by the DTI under the Technology Programme, the concept is now close to commercial application. For the municipal wastewater treatment industry, it has the potential to dramatically reduce sludge cake disposal costs and provide a much-needed solution for the draining and solidifying of sludge lagoons.
In 1939, the pioneering geotechnical engineer Arthur Casagrande demonstrated the potential of electrokinetics by deploying it to stabilise railway cuttings. But since then its widespread application has been prevented by the limitations of available electrodes. These problems have been eliminated by Electrokinetic's development of novel materials known as electrokinetic geosynthetics (EKG) to create geosynthetic electrodes.
To date, the EKG concept has been successfully applied to civil-engineering challenges such as construction of reinforced soil banks and stabilisation of slopes and embankments. They are also developing it to improve sludge conditioning and composting, and to enhance the performance and growth of sports turf under the low lighting conditions of large stadia.
Five years ago, Electrokinetic directors John Lamont-Black and Colin Jones spotted the opportunity to apply EKG to sludge dewatering. Since then, the concept has been developed in partnership with water and wastewater treatment specialists Ashbrook Simon-Hartley to make it compatible with their belt filter press technology.
Previous attempts to exploit electrokinetic phenomena in belt filter press applications have been severely limited due to problems of current discharge, electrode corrosion and the complexity of belt design required to overcome them. Dr Lamont-Black says EKG has overcome these hurdles and allows electrodes to be incorporated into belts very similar in form to those currently used in the Klampress. As a result, the existing Klampress has required minimal modification. EKG-conductive belts have replaced the conventional belts. A DC power panel with variable voltage control has been added and transfer brushes located to deliver power to the belts as they pass through the compression zone.
These brushes are designed to resist wear, minimise voltage drop and maintain uninterrupted contact with the conductor strips in the belts. In Electrokinetic's patented design, the top belt operates as an anode and the bottom belt as a cathode, so the sludge captured between the two is exposed to an electric field to induce electro-osmotic flow. Water is attracted to the cathode and sludge particles to the anode resulting in far greater removal of water than is possible by compression alone.
The key parameters determining dewatering performance are the applied voltage, sludge residence time in the shear section and the belts' conducting elements. In laboratory tests, a clear relationship between voltage and dry solids output was established showing optimum performance at 17V across a wide range of sludge types.
Full-scale field trials of the EKG belt system have now been undertaken with very encouraging results. At Thames Water's Long Reach WwTW, it was trialled with mixed SAS and primary sludge pre-thickened to 2.5% dry solids and fed at 14m3/h. Compared with a conventional Klampress, that would typically produce a cake of 18-20% the Klampress EKG consistently exceeded 30% dry solids.
Throughout the trial, the increase in cake dry solids reduced its volume by almost 40% from 2.84m3/h to 1.74m3/h. Interpreting these results Ashbrook Simon-Hartley's process manager, Mark Thomason says this would reduce the annual volume of cake produced by 8,800m3 assuming typical operation of 8,000 hours a year. He also predicts that additional benefits would accrue from better mechanical handling and reduction in cake storage space requirement.
Surely this performance must come at a cost? Yes, but one that will be dwarfed by the dramatic reduction in disposal costs according to Thomason. In the trial quoted, the EKG's power consumption was 15kW compared with a conventional Klampress 5kW requirement.
"In the light of the potential cost savings this is marginal," he says. "Assuming an all-in disposal cost of £15 per tonne, the operator's disposal bill would be slashed by £132,000 a year." He also points out that the EKG system's power requirement will still be less than half that of a centrifuge.
Over the coming months, further trials will take place on both sides of the Atlantic to refine the power to cake dry solids relationship and test belt material longevity. The EKG's power panel will be also be linked into the Klampress main control system for unmanned operation. Open days are being planned to take place at a UK site for a week this June to give the industry a pre-launch opportunity to experience this revolutionary new technology first hand.
With the research and development nearing completion, the Klampress EKG is moving into commercial development with the intention of bringing it to market early in 2008.
Initially, it will be offered as a retrofit kit for existing installations followed by launch of a new Klampress model with EKG factory fitted. At this point, Thomason says pricing cannot be revealed but is already confident that it will allow a payback in six to 12 months at most installations derived from savings on cake transport and disposal costs.
Leveraging their know-how in slope stabilisation, another EKG possibility close to commercialisation is for sludge lagoon drainage. Historically sewage sludge cake has been deposited in purpose made holding lagoons. In arid parts of the globe, the material in such lagoons slowly dries out. But in more temperate latitudes, rainfall prevents the drying out of the material. In many cases the material can remain in a lagoon for decades with little or no reduction in moisture content or increase in solids content.
These lagoons are often sited next to rivers and on the edges of urban areas. This means that lagoons can at the same time pose an environmental threat but also present a development opportunity. Therefore there are economic and environmental drivers to clean these areas up. But the peculiar nature of sludge cake makes this a significant technical challenge.
The alternatives all have drawbacks. Disposal of the material to landfill is subject to environmental and economic constraints. Encapsulation with a strong membrane reduces the leaching in time but prevents the ground from being developed. Mixing a stabilising material into the sludge to strengthen is a slow, expensive and hazardous process.
The EKG approach is to dewater the sludge in situ either as a precursor to disposal or a first stage of ground improvement for development. Laboratory scale electro-osmotic consolidation carried out on sewage sludge samples has shown an overall volume reduction of 57%, an increase in dry solids from 19% to a maximum of 42% and an increase in shear strength from approximately 2kPa to a maximum of 29kPa.
Subsequent tests on sewage sludges indicate that, although these materials vary appreciably in their composition, they invariably support the application of electro-osmosis. While hydraulic permeability is controlled by the grain size of the material, electro-osmotic permeability is relatively constant among materials that support electro-osmosis. Because most municipal sludges are made up of colloidal sized particles, electro-osmosis is an ideal solution for sludge dewatering either in process or in situ.
Although the development of EKG is currently most advanced in its application to sludge dewatering it potentially offers far broader benefits to the waste management sector as a whole.
Encouraging results have already come from laboratory scale tests exploring its potential for accelerating composting. Electrokinetic's research in South Africa is also looking into dewatering of construction waste slurries and mine tailings.
Dr Lamont-Black expects that EKG will play an important role in minimising man's disturbance of the environment and use of natural resources in the future. "The environmental applications will help industry accommodate the ever changing environment it finds itself in, and make previously prohibitive waste treatment economically attractive. There is also a very strong economic incentive with the potential annual value of this technology to the UK economy estimated at £555M."