Single-Cell vs Membrane HOCl Generators

Every hypochlorous acid generator is built around an electrolytic cell, and the design of that cell is the most consequential specification difference between systems. It decides what pH comes out, whether there is a second stream to deal with, how much of what you produce is usable, what needs replacing and how the solution behaves in storage. This page explains the two main designs — single-compartment and membrane — fairly, including where each makes sense.

What happens in any cell

Brine flows between a coated titanium anode and a cathode while a low-voltage direct current is applied. At the anode, chloride is oxidised to chlorine, which hydrolyses immediately to hypochlorous acid. At the cathode, water is reduced to hydrogen gas and hydroxide. The anode side therefore tends towards acidic and the cathode side towards alkaline. The design question is simple: do you keep those two sides apart, or let them combine?

Membrane (two- and three-compartment) cells

A membrane or diaphragm cell places an ion-permeable barrier between the anode and cathode. Three-compartment designs add a central brine chamber between two membranes. The result is two separate output streams:

• Anolyte from the anode side: acidic, typically around pH 2–3, with hypochlorous acid and, at the lowest pH, some dissolved chlorine.
• Catholyte from the cathode side: alkaline, typically around pH 11–12, with little or no available chlorine.

Membrane cells give tight control over each stream and can produce a very low-pH anolyte, which is why they dominate much of the older acidic electrolyzed water research. The trade-offs are a second stream that must be used or discharged, membranes that can foul and need replacement, and an acidic product that loses chlorine more readily and is harder on some metals. Many membrane systems also adjust the anolyte pH upwards, by blending with catholyte or by other means, to reach a near-neutral product.

Single-compartment cells

A single-compartment cell has no barrier. The products of both electrodes mix as they are formed, and the chemistry settles into one stream. With the right control of current, flow and brine, that stream is near-neutral — the pH range where free chlorine is mostly hypochlorous acid. There is no membrane to foul and no second stream. Slightly acidic electrolyzed water in the research literature is often produced this way.

EcoloxTech generators use a patented single-pass, single-compartment design that produces one near-neutral stream at pH 5.0–6.5, with the whole flow usable.

How output pH is controlled

In a single-compartment cell, output pH is not a fixed property of the design; it is the result of how the cell is run. Current density, flow rate, brine concentration and residence time all shift the balance between the acid formed at the anode and the hydroxide formed at the cathode. A well-engineered system holds those variables steady so the output stays in its target window, which is why the output pH range is worth asking about specifically. Membrane systems control pH differently: each stream’s pH follows from the separation, and any near-neutral product is made by adjusting or blending the anolyte.

Side by side

Usable yield and cost per litre

The price of a generator is only half the comparison. What matters is the cost per litre of solution you actually use. If a membrane system splits its flow into two streams and only one is needed, the cost of the salt, water, power and cell life behind the unused stream is carried by the usable one. A single-compartment cell, where the whole flow is usable product, avoids that. When you compare quotations, ask each supplier for the usable output at your required concentration and pH, not the total flow through the cell.

Maintenance and feed water

Both designs share the same enemy: hardness. Calcium and magnesium precipitate as scale on the cathode, reducing efficiency and distorting output. Softening and filtration ahead of the cell are standard on commercial installations. Membrane cells add a second wear component: membranes can foul with scale, iron or organic matter, and are a recurring replacement cost. In both cases, salt quality matters — food-grade, non-iodised salt without anti-caking agents, or a ready-mixed brine — because additives foul cells.

Electrode coatings

In both designs, the anode is usually titanium with a mixed-metal oxide coating. The coating does the electrochemical work and takes the wear, and its formulation and application largely decide cell life. Operating outside the design conditions — hard feed water, the wrong salt, excessive current or running the cell dry — shortens it. Keeping feed water softened and filtered, using clean salt or ready-mixed brine, and following the service schedule are what protect the investment.

Materials and corrosion

Any oxidising solution can affect some metals over time, and lower pH generally makes that worse. Near-neutral solution is gentler on stainless steel and aluminium than low-pH anolyte or high-strength chlorine. The research hub includes work on corrosion behaviour of electrolyzed water. Whatever the design, check material compatibility for the equipment and surfaces the solution will touch, and wipe or rinse where your procedure requires.

Hydrogen and installation

Every electrolytic cell, whatever its design, produces a small amount of hydrogen at the cathode as part of normal operation. Systems are designed to vent it, and installation guidance calls for the generator to be placed in a ventilated area, away from ignition sources, with the manufacturer’s clearances respected. It is a routine installation point rather than a hazard in daily use, but it belongs in the site survey.

What the research examines

Studies in the hub compare solutions produced by different methods, including slightly acidic electrolyzed water produced by different methods, and examine membraneless systems in practice, such as free chlorine loss during spraying of membraneless electrolyzed water and membrane-less electrolyzed water fogging in a hen house. Stability comparisons, such as this study of electrolyzed oxidizing water stability, show why storage conditions and pH matter. These are published studies, not claims for any product.

Questions to ask any supplier

1. Is the cell single-compartment or membrane? If membrane, what happens to the second stream?
2. What is the usable output, in litres per hour, at the concentration and pH I need?
3. What is the output pH range, and how is it controlled?
4. What are the wear parts, their expected life and their replacement cost?
5. What feed-water treatment is required, and is it included?
6. What documentation comes with the system: SDS for the solution and inputs, manuals, establishment registration?
7. What claims do you make for the solution — and are they on an EPA-accepted label?

Choosing

For general facility sanitation — food plants, restaurants, schools, hotels, ships, farms — a near-neutral single-compartment system is usually the simpler and more economical choice: one stream, no membranes, whole flow usable. Membrane systems remain a reasonable choice where a very low-pH anolyte or a separate alkaline cleaning stream is specifically wanted. The buyer’s guide covers sizing, and the free sanitation audit sizes a system for your site.

Frequently asked questions