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Sludge dewatering technology is reaching the market and ready to wring out high-disposal costs.
By David Engle
Peering into a misty vat in his Moscow laboratory, in 1809, naturalist F.F. Reuss discovered a curious phenomenon: a little electricity can move water. DC current, continuously applied to it in a porous medium, will make the charged cations steer water silently towards a cathode—leaving behind whatever material the water was in.
At the molecular level, the voltage gradient produces electro-osmosis; water separates from its element. Even with minimal voltage, this works, as Reuss and others have found repeatedly.
These days, innovators and entrepreneurs on several continents are succeeding, in putting Reuss’s discovery to use by integrating conventional sludge treatment with electro-dewatering. Conventional presses have probably reached their limit, but, the addition of this new technology will enable them to slash the tonnage of sludge, now piling up as a result of wastewater treatment.
Much of the recent developmental research to apply electrokinetic geosynthetics (using the broader term) is being done in the UK at the University of Newcastle; they, in turn, have licensed patents to a local firm, Electrokinetic Ltd. In 2003, Electrokinetics began partnering, to pioneer electro-dewatering, with US equipment-maker, Ashbrook Simon-Hartley, of Houston.
At present, four actual installations of electro-dewatering appear to be in operation: two pilots programs from Elode, and the two commercial Cinetiks in Canada. Along with Ashbrook, the three contenders are making impressive strides, and the respective hardware being rolled out will compete over technical advantages and tradeoffs.
Here, then, is a roundup of market-leading efforts to put DC dewatering to work in sludge reduction.
SELO, HELO, BELO: Multistage Dehydratin
The Korean-based Elode (“ELectrO-DEwatering”) began development of its line of electro-dewatering systems, in May 2002. In Elode’s resulting design concept—a technology hybrid, which is rather more similar to that of Ashbrook—a combination of conventional mechanical presses and electrolysis are integrated. The critical cathode element is built into a large roller, and, facing it, an electrified sludge conveyor belt, wraps around the roller, electrified as the anode.
Elode’s Singapore-based representative, C. K. Chua of Ace Korea, who has been working with Elode since mid-2006, explains the process (which is also well-presented on the firm’s Web site) as follows: A sludge cake is first generated by a conventional mechanical dewatering process, such as a belt or filter press.
Next, a conveyor delivers the sludge into the above-mentioned anode drum for compression, by the cathode-charged belt, during which both electrophoresis (i.e., electrokinetic movement of sludge particles) and electro-osmosis occur.
As noted, Elode’s approach is hybrid, represented by three variations of this model. A standalone, “single Elode” (“SELO”), may be installed at the end of an existing dewatering system, such as a press, decanter, or gravity dehydrator, and this produces a moisture level below 95%. The two other models offer tighter electro-dewatering integration: a “gravity built-in” system (“GELO”), and a belt-press built-in machine (“BELO”).
Each of the three comes in six sizes—representing belt width in mm, respectively—at increments of 500, 1,000, 1,500, 2,000, 2,500, and 3,000. Treatment capacities in tons per hour range from 0.3 to 1.8, and power usage ranges, in increments from 25 to 170 kWh. Equipment life is rated at five years (20,000 hours, 12-hour days), although the first installed machine is still only two years old. Elode warranties their drum qualities for twelve months, the belt filter for six months, and the larger hardware system for five years. Due to the introduction of heat and electrolysis to achieve the drying results, vapors, and potentially heavy odors become a problem. Typically, this will necessitate a hood and venting system.
Elode’s design—applying mechanical, electro-dewatering, and dehydration with heat—consumes relatively more power than the other designs do, but produces a drier, lighter cake. Resulting dryness attained will vary, depending on the cake’s condition after mechanical treatment; but, for say, a municipal wastewater plant, the volume/weight, starting with a moisture content of between 70%–90%, can be cut to 40%–60% water content—meaning, notes Chua, that the cake weight drops by “at least 50%” of its former weight.
Elode’s first sale came in 2005, and pilot projects at several Asian chemical and industrial plants have followed; including a digestive wastewater plant installed for a Hyundai Motors, where two installations are still in operation, according to a source in Korea.
A performance report on the Hyundai plant, posted at Elode.com, provides data on two years’ operation. Prior to installation, the conventional sludge process yielded 3% suspended-solids, with annual waste output for disposal at 130 MT. Installation of a belt press reduced the water content to 86%; installation of a SELO unit reduced water to 53%. Following the installation of these, annual disposal weight was reduced to 40.3 MT, at a total net annual savings of $3.6 million.
Abu-Orf notes that Elode’s drum machine design is similar to the one that Elcotech had formerly developed, but, later abandoned in favor of the Cinetik. In the drum technology, he notes, the thickness between the cathode and anode remains constant, thus making it, he says, “less efficient” than the Elcotech concept. However, he adds, Elode’s strategy looks promising, because the company offers both integrated electro-dewatering on conventional machines or a stand-alone option. This, he thinks, may open more doors than a single-solution approach.
Abu-Orf also notes that the machines of both of these contenders—unlike Ashbrook’s— “require pre-dewatering or thickening to at least 10% cake solids in order for the technology to work … [and also] need to remove the free or easily drained water from the sludge, in order to reduce the energy consumption.”
What this may mean, then, is that, even after the electro-dewatering market ripens in the years ahead, there will still remain a niche, “for old, dewatering equipment, or good, thickening equipment needed ahead of their dewatering devices.”
Cleaner, Lighter, More Useful, and Cheaper
Decker sees the potential impact of electro-dewatering on sludge disposal as “huge,” noting that, currently, treatment systems struggle to improve sludge weight by a percentage or two. In comparison, “at Long Ridge,” he says, “we had an increase of fourteen percentage point in dry solids … a quantum leap in dryness.”
At this magnitude, annual savings on disposal costs would easily tally to six or seven figures.
Other benefits of putting voltage through sludge include reduction in the polymer dosages needed for dewatering, ammonia venting, improved biodegradability, and cell lysis. Potentially better still, the weak DC voltage has also reportedly disinfected some sludge by killing E. coli, salmonella, enteric viruses, and parasites.
Pasteurization of this kind was confirmed recently, by another enterprise that is in pursuit of electro-dewatering, a firm called Elcotech, of Sherbrooke, QC. Founded in 2001, Elcotech’s wastewater equipment has advanced through R&D, and is already at the product marketing stage. In 2006 and 2007, came the commissioning of the first two Cinetik systems at wastewater plants near Montreal.
Elcotech market development manager Scott McKay says that the pasteurization effect appears significant enough to allow electrolytically processed sludge to qualify as class-A biosolids. If this indeed comes to pass, he says, it will confer “a higher value on the sludge for agricultural use, or even for incineration” as a biofuel. “When you get this cake up to the 35% solid range and eliminate pathogens,” he adds, “then your options suddenly open up.”
By being rendered so dry, cakes will no longer leak obnoxiously during transport; they will be readily stackable, easily storable, and odorless. In addition, an electro-dewatered cake “has no hydrogen sulfide,” compared to regular cakes that release large amounts of this gas. A recent 31-day test measurement of electro-dewatered cakes found they resulted in “zero hydrogen sulfide” emissions. If this result holds up, it will mark an enormous improvement, “that will make sludge much more convenient to farmers,” he adds.
The boost in value and versatility, though, is just a bonus over and above the sharply reduced weight. Electrolyzing provides tons of water removal at a cost of one-fifth to one-tenth the expense of conventional drying, by the application of heat. As McKay says of his equipment’s process: “We don’t evaporate water,” as in a dehydrator, “we strip it off, using electricity.”
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Photo: Elcotech |
| Elcotech's Cinetik electro-dewatering system adds electrolysis to a conventionally dewatered sludge cake. |
Ashbrook and Elcotech’s efforts to turn electro-osmosis into some kind of a working technology have been ongoing for a couple of decades already, as Abu-Orf has found in his own market tracking. In his analysis, the machinery being developed to do this will pursue any of several basic design strategies: either integrating electrolysis into a conventional machine, as Ashbrook is doing with its Klampress and other machines; creating a separate stand-alone system that works as an additional piece added to the treatment stream; or offering a hybrid.
A Modular, Standalone Cinetik
Of the three aforementioned technology strategies, Elcotech’s entry would represent the second, as a stand-alone machine. McKay explains: “Instead of trying to introduce electro-osmosis inside a piece of traditional electro-mechanical dewatering, we developed it as an add-on, that can fit and be integrated after any type of dewatering device,” as a final processing step. The Cinetik is also designed to be modular, meaning that one or more units can be applied to the process stream, being either stacked or positioned end-to-end within a metal frame, according to space needs or preference. Modules come in sizes 600 and 840, the numbers representing anode surface.
In this system’s dewatering process, sludge cakes first emerge from the conventional mechanical system (from, say, an existing belt filter press) on a conveyor. As cakes move through the frame, voltage is applied and electro-dewatering occurs. Power levels can be varied, and adjustment occurs automatically to match sludge characteristics.
In 2006, a wastewater treatment plant in Victoriaville, QC, became the first Cinetik application; it was followed by a second in Valleyfield, QC, in early 2007. Both, McKay notes, were confronted with a tough dewatering challenge posed by mostly straight waste-activated biological sludge without primary solids. Cakes that were formerly processed on simple belt-filter presses attaining up to 15% solids, are now getting an electro-osmotic cycle, by which the result is being raised, he says, “to around 35% total solids range.” Sludge volumes at both sites in Quebec are thus being reduced by around 60%.
As for practical benefits, at Valleyfield this reduction now enables the plant to dispose of its sludge far more inexpensively than before, by means of auto combustion using plasma oxidation (i.e., without the need and expense of natural gas for incineration). Currently, says McKay, Valleyfield is even exploring the novel idea of marketability of its cakes as a cheap fuel source, for a regional cement kiln and paper mill.
At Victoriaville, results are also quite positive, he says. Following the first year of operation, the Cinetik-equipped plant estimates its annual savings in hauling and disposal (net after the additional electricity expenditure) will total over $500,000. (Note: at this writing, Canadian and US dollars are near parity). Victoriaville spent about $1.6 million on four Cinetiks, the modules costing about $400,000 each, thus, payback should come in just three to four years.
Payback will vary with each project, of course, but McKay suggests that in the prospective applications, that are likeliest to happen early—such as those with tough secondary sludge or high disposal costs—the value impact should be significant enough to bring payback in five, or, as little as, two to three years.
The following model case quantifies benefits, assuming these conditions: 26,000 wet MT of sludge annually; mechanical dewatering with a screw press; 10 kg/dry ton of polymer added to achieve dryness of 15%; disposal costs at (a high) $72.50 per WT. Given these assumptions, then, the total yearly cost of disposal without electro-dewatering comes to about $1,885,000.
Adding Cinetik electro-dewatering modules to achieve volume reduction of 62.4%, through increasing cake dryness from 15% to 40%, will presumably gain class-A waste classification; using low voltage will yield these benefits:
- Disposal cost per wet ton: $42.50
- Annual disposal cost: $414,375
- Gross savings: $1,470,625
- Cost of additional energy: $219,375
- Net saving: $1,251,250
Elcotech embarked on its quest five years ago with the guiding concept in mind, says McKay, that product-developers should, “first, master the principles of electro-dewatering.” An earlier very different prototype design was tried, based on a rotating-drum anode. However, as McKay concedes, “after a few years of trying to improve this, Elcotech concluded that [problems with] fundamental fouling limited its capabilities.” Thus, Elcotech switched to a very different design, now finalized as the Cinetik, which, he says, “we are now comfortable to market worldwide.”
Abu-Orf observes Elcotech’s design change as, going “better with their theory… [That], as water is squeezed-out by the combined effects of DC current and the mechanical process, the volume and thickness of the sludge cake moving through the machine is reduced.” Therefore, too, as the cake moves through the belt, the gap between anode and cathode should narrow. “This allows material to be kept with no voids,” he explains, enabling it to achieve, “more effective conductivity with the DC application.”
Lastly, Abu-Orf adds the suggestion from McKay regarding elimination of cake odor and fecal coliform regrowth during transport and disposal, if accurate, would be hugely important. “These two drawbacks are major obstacles to beneficial-reuse programs in the US,” he says, and so, overcoming them would truly be a breakthrough.
Ashbrook Simon-Hartley’s Dewatering Belts
After partnering in 2003, Electrokinetics Ashbrook Simon-Hartley spent the next few years applying Reuss’s voltage gradient along Ashbrook’s 2-meter (12 rollers) Klampress, to see just how much water this electrolysis could extract. Ashbrook’s Vice President of Biosolids, Bill Decker, describes the experiments that were done to dewater an aerobically digested biosolids of combined primary and secondary waste-activated sludge, at the Long Ridge Sewage Works near London.
First, to establish a baseline, a conventional mechanical Klampress was applied; it yielded dry solids of “about 19%,” he says. Next, came the moment of truth. An electrically charged second Klampress produced, he says, “fantastic results on the first trial.”
Colleague John Lamont-Black, operations director at Electrokinetic, quantified further: by electrolyzing sludge at 16–17 DC V he recalls, “We raised that to around 31% dry solids, with the additional power requirement of just 7 or 8 kilowatt-hours.” Results were easily repeated. Comparatively, the concepts developed by Elode and Elcotech both assume a preliminary conventional mechanical dewatering that occurs with, say, a belt press, centrifuge, screw press, etc. But, in Ashbrook’s ambitious and more integral design, the low 15–30 V DC electrolysis is incorporated into the filter-press belts themselves. “Conductors are woven into the geosynthetic belt material at a specific spacing,” he says. Voltage in one belt makes it the cathode; the other becomes the anode. Sludge captured and pressed between them is electro-dewatered.
The ingenious idea here is to do electro-dewatering within the belts themselves. This strategy allows a longer total exposure-time at low voltage, resulting in drier sludge at lower cost. And, notes Decker, Ashbrook belts will eventually be usable on any manufacturer’s belt-filter-press machines. With this design, the process can occur with very modest additional power requirements—calculated in recent trials in the US and Britain “down to 55 cents a ton,” Decker notes. Thus, the key to Ashbrook’s competitive strategy is the anticipation of offering relatively lower
operational costs.
As for the dewatering mechanism itself: In a belt-filter press, gravity enables material to be strained of some water, through an underlying conveyor; the sludge then continues on this, to a point where two facing belts turn a right angle and converge. Here, squeezing and dewatering occurs as the sludge is compressed between them. Belts also engage a series of six to a dozen pressure rollers (depending on the equipment make and model), “and follow a kind of serpentine path,” says Decker.
Ashbrook’s belt-based electro-dewatering occurs here: “We’re attempting to apply electrokinetic geosynthetics in the pressure section on our machines, around those pressure rollers,” he says.
End-to-end, the combination of compression, dilation, shearing, and electrolysis result in “a higher level of … dewatering,” at a low power-level, he says. Combining it all into a one-process stream also makes for greater efficiency, in that the system achieves higher output. Once the belt electrode technology is perfected, machines will be able “to take single-step sludge processing from liquid sludge into finished cake,” Decker says. Belts will be affordably replaceable.
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Photo: Elcotech |
| Depending on initial sludge condition, dry solids can be improved to as high as 40% and weight reduced over 60%. |
As for the economics: Since the belts and machines aren’t yet being made, it’s premature to project this, but, based on the London-area trials, Lamont-Black observes, “dewatering at 31% resulted in a 40% reduction in volume.” Thus, “taking a fairly conservative estimate for pounds [£] per meter cubed to be removed, that worked out to a savings about £120,000 to £130,000 per year per machine. For dollars, double that” (i.e., $240–$160,000 per annum).
Abu-Orf believes that “the technology that eventually will ‘make it big in the market’ … will be either Ashbrook’s or a similar one, which combines dewatering via filtration, and electro-dewatering at a later stage to produce [greater than] 45% cake solids.” He notes that Australia’s Commonwealth Scientific and Research Organization undertook a similarly promising approach some years ago, but, to date, it has not yielded any products, at least none on the market. If all goes as planned, Ashbrook’s first electro-dewatering Winklepress will roll out sometime in 2008.
The Next Big Thing
Abu-Orf cautions that occasional claims from equipment-makers, suggesting that destruction of sludge cell membranes will enhance dewatering, are, “scientifically not accurate. Breaking the cell membrane does not yield more dewaterable sludge, but [in fact makes it] harder to dewater.”
Also, regarding the claims that electrolysis destroys enteric viruses and parasites, Abu-Org suggests that a prospective customer might wish to await verification of this by independent testing. Regarding the claimed reduction of hydrogen sulfide, he adds that, while this may be accurate of the end product, it is noteworthy that, during the electro-dewatering process itself, hydrogen sulfide gases will be present, “thus, requiring onsite air collection and treatment.”
All in all, the stakes in achieving drier cakes are clearly enormous; this technology’s success or failure will likely shape what happens next in wastewater treatment—i.e., determining whether facilities that treat wastewater onsite will soon be pleasantly finding cost-avoidance opportunities, or struggling with even higher disposal bills. Over the years, Abu-Orf notes, milestones have been reached—like the application of polymers in solids conditioning. “Electro-dewatering,” he suggests, will be the industry’s next big leap, “narrowing the gap between dewatering and drying equipment performance.”
La Mesa, CA-based writer David Engle specializes in construction-related topics. Special thanks go to Mohammad Abu-Orf, Ph.D., National Biosolids Leader at Metcalf & Eddy, for technical review of this article.
OW - March/April 2008 |
