Electrochemical activation: from salt to disinfectant.

The concept of electrochemical activation took shape in the 1970s through the work of V. M. Bakhir and his team in Tashkent; the term “electrochemical activation” came into use in 1977. The first applications were industrial, such as treating drilling fluids by electrolysis instead of chemical additives.

In the 1980s and 1990s the method spread to medical disinfection, food processing and water treatment. Today, electrolysed water is studied in detail in the peer-reviewed literature as a disinfection method for the food industry whose only chemical input is salt and which can be produced at the point of use [8].

Electrolysis means passing direct current through a solution containing ions to drive reactions that would not occur spontaneously.

A cell has two electrodes. The anode, connected to the positive terminal of the power supply, draws electrons; this is where oxidation takes place. The cathode, connected to the negative terminal, supplies electrons; this is where reduction takes place.

In salt water the current is carried by ions: positively charged sodium ions (Na⁺) migrate towards the cathode, and negatively charged chloride ions (Cl⁻) towards the anode. At the electrodes, these ions and water molecules gain or lose electrons and are converted into new substances.

How much is produced is set by Faraday’s law: output is directly proportional to the electric charge passed through the cell. In real cells part of the current goes into side reactions such as oxygen evolution; the share that goes to the desired product is called the current efficiency.

At the heart of the Aquamirus E.C.A. system is a diaphragm electrochemical reactor that converts salt water into two separate products. The anode and cathode compartments are separated by a ceramic diaphragm; the anolyte, dominated by hypochlorous acid, and the catholyte, containing sodium hydroxide, are drawn off as separate streams without mixing.

The reactor works on a flow-through basis: salt water passes continuously through the reactor, and the electrochemical conversion takes place during this passage. ECA reactors are generally coaxial and cylindrical, with the anode at the centre, the cathode on the outside and a porous ceramic diaphragm between them. The connections at the two ends of the reactor carry the inlet and outlet lines of each compartment.

The diaphragm does more than separate the products; it is also key to keeping the pH of the anolyte in the range where hypochlorous acid dominates [1]. The reactors are modular: as a facility’s needs grow, capacity is increased by operating several reactors together.

According to manufacturer data, more than one million reactors have been produced with ECA technology to date; the latest-generation reactors used in Aquamirus E.C.A. systems have been in production since 2011.

• Flow-throughSalt water passes continuously through the reactor; production starts and stops as needed.
• DiaphragmA ceramic diaphragm separates the anode and cathode compartments.
• Two separate outletsAnolyte (HOCl) and catholyte (NaOH) are drawn off through separate lines.
• ModularCapacity is scaled up by operating reactors together.

In the anode compartment, chloride ions give up electrons and become chlorine; the chlorine reacts immediately with water to form hypochlorous acid.

Both reactions release H⁺ ions, so the pH of the anolyte drops. This drop is intended: hypochlorous acid is the dominant form at low and near-neutral pH [1].

If the pH falls too far (below about 3), however, the equilibrium shifts towards dissolved chlorine (Cl₂), and chlorine gas starts to leave the solution. In a well-designed cell the aim is to keep the anolyte in the range where HOCl dominates. Salt concentration, current and flow rate together determine this.

In the cathode compartment, water molecules take up electrons, forming hydrogen gas and hydroxide ions. Together with the sodium ions that pass through the diaphragm, this gives sodium hydroxide (caustic).

The catholyte is a high-pH, alkaline solution. It is collected in a separate tank and can be used on site for alkaline cleaning (CIP steps, removal of grease and organic soil) and for pH correction in suitable processes.

The hydrogen released at the cathode is a safety parameter: its lower flammability limit in air is about 4% by volume. The gas at the cell outlet is therefore separated from the liquid, diluted with ventilation air and vented in a controlled way.

Chlorine added to water does not remain a single substance; depending on pH, it is distributed between hypochlorous acid (HOCl) and the hypochlorite ion (OCl⁻).

The pKa of this equilibrium is about 7.5 [1]. At a pH equal to the pKa, half of the chlorine is in the HOCl form and half in the OCl⁻ form. At pH 6 about 97% of the free chlorine is HOCl; at pH 8 this falls to about 26%, and at pH 9 to about 3%.

The practical consequence: the same free chlorine reading means very different disinfecting power at different pH values. That is why pH must always be measured together with chlorine.

Hypochlorous acid is a small, uncharged molecule that passes easily through the cell wall and membrane of a microorganism. The hypochlorite ion, by contrast, is negatively charged and is repelled by the similarly charged microbial surface.

Once inside the cell, HOCl oxidises sulfur-containing groups in enzymes, as well as proteins and nucleic acids, and stops the cell from functioning. Neutrophils in the human immune system produce the same oxidant against microbes, so it is not surprising that it does not irritate tissue at working concentrations [7].

The standard way to measure disinfection strength is the CT value: the product of disinfectant concentration (C) and contact time (T). The official tables of the US Environmental Protection Agency show how the required CT rises with pH at the same temperature and the same free chlorine [2].

ORP (oxidation-reduction potential) expresses a solution’s tendency to take up electrons in millivolts (mV) and is measured against a reference electrode. The higher the value, the stronger the solution is as an oxidant.

For electrolysed oxidising water, ORP has been shown to be the decisive variable for efficacy, more so than pH or residual chlorine [4]. On-site generation systems therefore monitor ORP together with free chlorine and pH to manage dosing. Because ORP is affected by pH, it is interpreted together with these measurements, not on its own.

A biofilm is a community of microorganisms attached to pipe and tank surfaces and protected by a matrix they produce themselves. Surface-attached bacteria have been found to be 150 to 3,000 times more resistant to hypochlorous acid than free-floating cells. The same study showed that the limiting factor is transport of the disinfectant into the biofilm, and that raising the dose did not improve efficacy [3].

In mains water, the survival of bacteria despite chlorination involves surface attachment, protection inside particles and adaptation from previous exposure; a measured residual alone does not guarantee safety [10]. For Legionella, too, risk is managed not by one-off measurements but through a water safety plan that includes a continuous disinfectant residual [9].

In stored sodium hypochlorite solutions, oxyhalogen species such as chlorate, bromate and perchlorate form after production. Their rate of formation depends on temperature, ionic strength, hypochlorite concentration and transition metals; in other words, a ready-made chemical weakens over its shelf life while its by-product load grows [5].

The formation of disinfection by-products (for example trihalomethanes), on the other hand, is linked to the dose of disinfectant and its contact time with organic matter in the water [6]. Delivering the right dose at the right point is therefore a matter not only of efficiency but of water quality.

The cell alone is not a system. Below are the components of an on-site generation setup, from feed-water preparation to the dosing point. Hover over or tap a component to see its role.

Electrochemical activation (ECA)

Electrolysis of a dilute salt solution in a diaphragm cell, converting it into two separate solutions called anolyte and catholyte.

Electrolysis

Driving a chemical reaction by passing direct current through a solution containing ions.

Anode (+)

The positive electrode, where oxidation takes place. In ECA, HOCl forms here.

Cathode (−)

The negative electrode, where reduction takes place. In ECA, NaOH and hydrogen form here.

Diaphragm

A porous separator between the anode and cathode compartments that allows ions to pass.

Anolyte

The oxidant solution, mainly HOCl, that leaves the anode compartment.

Catholyte

The alkaline solution containing sodium hydroxide that leaves the cathode compartment.

Free chlorine

The sum of HOCl and OCl⁻ in water. Combined chlorine (chloramines) is not included in this total.

Combined chlorine (chloramine)

Compounds formed by chlorine with ammonia and nitrogen-containing organic matter; the main cause of the “chlorine smell” in pools.

pKa

The pH at which half of an acid is dissociated. About 7.5 for HOCl.

CT value

The product of disinfectant concentration (mg/L) and contact time (min); the disinfection “dose” required for a given microbial reduction.

Log reduction

Reduction in the number of microorganisms in powers of ten: 3-log = 99.9%, 4-log = 99.99%.

ORP (redox potential)

A solution’s tendency to oxidise or reduce; measured in mV.

Biofilm

A community of microorganisms attached to a surface and protected by a matrix it produces itself.

Current efficiency

The share of the electric charge passed through the cell that is converted into the desired product.

Meta-stable

A state that stays active for a time but gradually converts into more stable (and less active) forms; the reason the anolyte is used fresh.

NTU

Unit of turbidity (nephelometric turbidity unit).