Element Six Pioneers Diamond-Based Waste Water Treatment Process
Heralded for its unique properties for advanced semiconductor manufacturing and many industrial processes, synthetic diamond from Element Six has found a new home treating some of industry's most toxic waste water.
Globalized heavy industry has largely contributed positively to modern economies by creating employment and providing low cost finished goods. However, many industrial processes also create low volumes of concentrated hazardous waste water, some so harmful that it is difficult to handle, let alone dispose of safely. Sometimes they are simply diluted down and allowed to enter conventional water treatment processes.
The solution to this growing tidal wave of toxicity? Synthetic diamond.
Toxic waste water isn't just a problem in long-industrialized countries. Neither is it limited to emerging economies who welcome some types of manufacturing without fully understanding the harmful impact of waste waters that they can generate, or the consequences of safely handling these effluents. A solution now exists to treat in situ those wastes associated with toxic dissolved organic compounds that issue from a wide number of industrial processes and chemical manufacturing.
Figure 1: (right) The Element Six "˜bolt-in' waste treatment unit comprising a Diamox cell with solid BDD electrodes, housing and all electrical and pipework connections is delivered ready to install into Electrochemical Advanced Oxidation Process (EAOP) systems.
Technical editor for Silicon Semiconductor, Mark Andrews, spoke with Tim Mollart (PhD), Principal Applications Engineer at Element Six, about the hardware and materials technology underlying the treatment cells his company calls "˜Diamox.' Mollart leads the research and application of his company's synthetic boron doped diamond electrodes, including their use in the Diamox system.
Originally introduced in 2016, Diamox is Element Six's second-generation technology, a cost-effective and efficient wastewater treatment electrochemical cell, designed using freestanding, boron-doped diamond electrodes. Diamox is effective in removing the dissolved organic content of contaminated industrial wastewater that cannot be treated using biological methods. Element Six describes its packaged reactor as simple to implement in on-site advanced oxidation technology, providing an environmentally cleaner and versatile solution that can be used across various types of effluents. There are no hazardous chemical additions and therefore no solid residue or sludge.
Mollart said that Diamox has been successfully implemented in pilot projects with an industry-leading wastewater treatment company, delivering electrochemical advanced oxidation capacity that can be scaled to meet industrial requirements. The company is now in the process of widely marketing the technology, since its amassed data establishes Diamox's effectiveness in multiple, real-world situations.
Mollart further explained that while biological treatment processes are adequate when handling simple organic and inorganic compounds, the situation changes completely when recalcitrant compounds such as phenols are introduced into waste streams. Further, that biological processes (such as those found in municipal water treatment systems and across industry,) are easily upset or rendered ineffective when ammonia, nitrates, and phosphates"”all common to multiple industries including semiconductor manufacturing"”are present within waste streams.
Advanced oxidization processes (AOPs) have been utilized by industry since the 1990s, to deal with some of the most difficult compounds found in waste streams. Often these work by combining UV or electrochemical activation processes with strong oxidizing chemicals such as hydrogen peroxide or ozone to generate the hydroxyl radical, which has the oxidative power to mineralize these recalcitrant species. What Mollart called an "˜attractively simple route' to avoiding the addition of more chemicals to an already complex mix has been enabled through the use of diamond electrodes. The electrodes directly oxidize hydroxyl ions - occurring naturally in water - to create the hydroxyl radicals on the surface of the electrode. All other types of metal-based electrodes are less effective since they are rapidly self-consumed by any hydroxyl radicals that they generate. Diamond passes the tests that other materials fail thanks to its unique properties that make it such a highly prized industrial workhorse. Its extraordinary chemical inertness enables the hydroxyl radical to exist for a few critical nanoseconds on the surface of water undergoing treatment and be available to oxidize dissolved pollutants in wastewater streams.
Figure 2: (Left) Diamox contains a stack of bipolar BDD electrodes to maximize the effluent's exposure to oxidizing species generated at the electrode surface.
So why isn't the world already using synthetic diamond reaction elements to treat its most hazardous waste? Early generations of thin-film diamond electrodes, created for use in electrochemical advanced oxidization processes, were uncompetitive in a price-sensitive industry. This was due to their oxidation capacity and production costs when compared to alternative advanced oxidation water treatment techniques. Beyond these issues, a key failure mechanism also existed in earlier types of diamond electrodes relative to applications in wastewater treatment. Earlier processes utilized substrates that would dissolve in common treatment applications. These factors largely delayed development until companies like Element Six pioneered more effective means for cost-effectively manufacturing diamond electrodes.
The microwave plasma-enhanced CVD reactor process that Element Six developed can be used to create high-purity, freestanding boron-doped diamond.
The high deposition rates that can be achieved by Element Six's synthesis technique enables self-supporting, freestanding, high-purity diamond electrode fabrication. With no substrate to be dissolved by the hydroxyl radical, the primary failure mechanism that plagued older thin-film diamond electrodes was eliminated. A further benefit of switching synthesis techniques is that solid boron-doped diamond (BDD) electrodes have a wider solvent window, enhancing electrochemical processing efficiency. While high-rate deposition techniques are limited by the area, this is mitigated by the ability to operate the electrodes at 10 to 20 times the current density when compared to conventional electrodes. The work done is proportional to the total charge flowing through the cell. A significant advantage of solid diamond electrodes is their ability to operate at <10,000 Am-2 without the loss of effectiveness; this dramatically lowers the consumable costs when expressed per kilogram of COD removed.
Now that the stage was set for synthetic diamond electrodes to play a role in cleaning-up industry's more toxic waste products, Element Six had to demonstrate cost-effectiveness and viability compared to other treatment scenarios. When selecting an advanced oxidation technique, physical plant operators typically consider consumable costs as a critical factor. In the case of electrochemical advanced oxidation, the dominant cost is the electrical power used to generate the hydroxyl radical and the consumable cost of the diamond electrodes. The amount of energy used to mineralize dissolved organic pollutants, expressed in kilograms of oxygen demand, into carbon dioxide (CO2) is in the range of 20 to 60 kWh, depending on the conductivity and concentration of the dissolved contaminate of the effluent.
Figure 3: Diamox uses BDD for the anode and the cathode. The designed allows for switching the polarity of electrodes to prevent fouling, enabling operation in highly contaminated environments.
To maximize the utilization of the oxidation capacity of solid diamond electrodes, ultra-compact electrochemical cells are needed. Bipolar cells (see Figure 3) that can be operated in either polarity are preferred since other electrodes cannot survive in these conditions; fouling during contact with industrial effluents can occur. Cell materials that are capable of operating at pH extremes are also preferred due to the nature of industrial effluent they will experience. Solid diamond electrodes are preferred since they can be operated in extremely oxidative or reductive effluents. Supporting robust but brittle electrodes while enabling high recirculation flow rates, up to 25 m3 h-1, requires a significant engineering development process.
Figure 4: The COD reduction of dissolved recalcitrant is demonstrated over time.
Figure 5: EAOPs are compact and simple water treatment systems. The effluent is held in a process tank and then pumped through the Diamox cell.
In determining the final element of cost-effectiveness - mass transport characteristics - Mollart said that one needs to review the measured rate of removal of dissolved organics (expressed as the coefficient of oxygen demand [COD]) removal, and map these characteristics (see Figure 4). The process maintains high efficiency throughout higher levels of concentration and is at its most efficient. Like any system, this decreases somewhat over time until the dissolved species are fully depleted.
Electrochemical advanced oxidation with solid diamond electrodes is an environmentally friendly method for treating waste streams. In particular, streams contaminated with toxic and persistent chemicals such as herbicides, pesticides, chloro- and nitrophenols, polychlorinated biphenyls, pharmaceuticals, and other hard-to-treat compounds. Diamox has demonstrated its effectiveness in the treatment of a wide range of complex compounds, including high COD and ammonia-containing streams (such as landfill leachates), as well as for treating process waters used in many manufacturing environments. These include waste waters from oil and natural gas exploration and production. The Element Six Diamox solution is well suited for low volumes of waste streams containing high concentrations or recalcitrant COD that otherwise overwhelm treatment programs utilizing hydrogen peroxide and/or ozone.