Generating Disinfectants From Water: Could In Situ Electrolytic Production Be The Future?

Chemical disinfection using strong oxidants involves the transportation, storage, and handling of unstable and/or aggressive and hazardous chemicals. The production of oxidants at the location where they are to be used has therefore been a preferred option, especially in small-scale or remote installations. On-site electrolysis of NaCl for the local production of chlorine or hypochlorite, or the production of ozone-enriched oxygen by silent discharge has been standard for years.  However, it is possible to take the idea of producing chemical disinfectants locally a step further and look at in situ techniques, i.e. at devices, which produce oxidants in and from the water to be treated.

In situ techniques offer a number of advantages as they can be introduced into the water systems where needed. They do not require extra equipment for contacting and the dosage can be controlled very easily by the electric input into the device. An example of such a promising new technology is a miniaturized low-cost electrochemical ozone generator that can be integrated very easily into distribution piping (Fig. 1).

Fig. 1: Miniaturized electrochemical cell (compared to a one Euro cent coin) producing ozone from tap water

The Process

Water can be the source of a number of more-or-less short-lived oxidants ranging from the extremely unstable and reactive hydroxyl (HO^Ÿ) and hydroperoxyl (HOO^Ÿ) radicals to more common disinfectants such as ozone or hydrogen peroxide, and eventually molecular oxygen. One important property of water-derived disinfectants is that they decompose to harmless species, i.e. water or molecular oxygen. This means the formation of potentially hazardous secondary products can be minimized.

The production of oxidants from water requires that the energy for splitting water molecules be overcome. For this to work, either electricity or vacuum UV radiation (VUV, wavelength <185 nm) can be used. Electric discharge technologies that apply high voltages directly to the water have been studied in the laboratory; however it is not clear whether they can be made efficient, reliable and safe enough for widespread application. VUV can split water to produce hydroxyl (HO^Ÿ) and hydroperoxyl (HOO^Ÿ) radicals. This technology has been used to oxidize trace TOC, with the main application in TOC monitors for ultrapure water systems. The radicals produced by VUV-photolysis of water are very short-lived and have no practical importance for disinfection purposes. In comparison, electrochemical processes operating at low voltages of a few volts dc seem more appropriate for decentralized applications and have been shown to be tunable to produce the full range of oxidants from water.

An electrochemical cell for the electrolysis of water consists of two electrodes (anode and cathode) and a strong aqueous electrolyte that does not decompose in the process. Feeding a dc current to such a cell yields the products of water splitting, i.e. hydrogen, at the reducing cathode, and oxygen at the oxidizing anode in stoichiometric quantities proportional to the electric charge that passes through the electrodes. The oxidation process at the anode that ultimately leads to the evolution of oxygen gas proceeds via a number of reactive intermediate oxygen species. Which intermediates are formed with what yield depends on the surface properties of the anode material. It has been long known that, by using specific electrolytes, electro-catalytic anode materials, and appropriate process conditions, fractions of up to 50 percent of the oxygen can be produced in the form of highly reactive ozone. As the conductivity of water as an electrolyte is a limiting factor in the operation of in situ electrolysis cells, practical applications have used a chemically inert polymer electrolyte membrane (PEM), which exhibits a high ionic conductivity and allows the operation of cells with high current densities and hence high production rates (Fig. 2).

Fig. 2: Water electrolysis process in a PEM cell.

The use of ozone-generating anode materials in an electrochemical cell using PEM as an electrolyte was first described more than 30 years ago and has proved to be the most interesting option for producing ozone in situ from water.

Anodes for the production of ozone consist of corrosion resistant metal structures with coatings of β-PbO₂, doped SnO₂ or, more recently, thin films of semiconducting diamond. While β-PbO₂ anodes only produce mixtures of ozone and oxygen, the primary product of doped diamond are OH^Ÿ radicals, which can then either react directly with substrates or recombine to form H₂O₂ and/or ozone.

Application of ozone generating PEM electrolysis cells

Ozone-generating PEM cells were originally developed for the efficient disinfection of industrial ultrapure water systems.  Since they can be operated to maintain a very low (<10 ppb) background concentration of ozone in the system, they are very suitable for keeping water systems sterile. Typically an ultrapure water loop of some 20m³/h capacity can be kept clean with an electrolysis device with 250 W power input. In order to guarantee the absence of ozone in the water in the pharmaceutical production process, UV lamps are situated at the point of use to destroy residual ozone. In situ ozonation has been the state-of-the-art world-wide for more than 20 years now, replacing the much more energy intensive hot storage technology still in use in many pharmaceutical plants, especially in the US. The set-up of an ozone generating electrolysis cell in a typical application to keep an ultra-pure water loop in a pharmaceutical production plant sterile is shown in Fig. 3.

Fig. 3: Ultra-pure water sterilizing unit, complete with electrochemical ozone generator, power supply unit, pumps, uv lamp for ozone removal and tubing.
(Photo courtesy BWT Group).