OMZ - Ozone Microcell
The system should not be used in water containing high amounts of chloride (seawater, brackish water) because side products as chlorine respectively hypochlorite will be formed. Those side products also act as disinfectants but need much more time to degrade. Prolonged electrolysis in stagnant water can cause the formation of chlorate. However, because of the high current density causing a limitation of substance transport towards the active electrode surface the generation of chlorine in drinking water is negligible.
It is a fact that ozone oxidizes bromide to bromate. Bromates are cancerogenic. Therefore this chemical reaction is undesirable. The drinking water ordinance contains limits for those substances. Bromate fomation can be decreased by lowering the pH value of the water.
The ozone generating anodes of the cellhearts are coated with PbO2 (approx. 1 mg Pb per anode). This coating is chemically stable for several 1000 h under enormal electrolysis conditions.
The easiest indication for a damaged cell is the voltage: If a single cell needs >15V for sustaining a current of 200 mA then it should be replaced. A cell voltage below 3V (at 200 mA) is an indicator for a short-circuit within the cell, which leads to zero production.
The most important indicator is the ozone production rate. This can be verified with a check measurement. It is possible that the cell voltage and current are within normal parameter range but the cell does not produce the expected ozone quantity (e.g. damaged cellheart). If the ozone rate is just too low the lifetime of the cell can be extended for a short period by increasing the current to 300 mA.
The production rate is proportional to the number of cells and the current. The adjustment of the production rate depends on the specific demand of an application. Please note: doubling the production rate by doubling the number of cells does not necessarily lead to doubled ozone concentration. The additionally produced ozone might lead to increased degedation rate and/or losses by increased amounts of ozone transferred to the gas phase.
The microcell is optimized for an ambient temperature of 10 to 30 °C. It is possible to run the cell within 0 °C and 60 °C without damaging it. However please note that operation at higher temperature decreases the ozone production and its solubility in water. Furthermore the ozone loss through decomposition reactions is increased.
The electrolytic ozone generation is independent of pressure to a large extent. Microcells are designed to be operated in pressurized lines. The operation under pressure has the advantage of higher solubility and the dispersal of ozone is increased. The solubility of ozone is proportional to the system pressure.
The active components of the Microcell are very robust versus degradation. In average quality drinking water, a service life of one year in continuous operation has been verified. At the beginning of a continouos operation the cells need a running-in period of approx. 10 days. During that time the cell voltage increases slightly and the ozone production rate decreases slightly. After that a steady-state phase follows usually. In very hard water calcification on the cathode might lead to cell voltage over 15V. The lifetime of the cellhearts can go down to several 100 hours. This effect has been minimized – however not eliminated completely – through our new cellheart design.
The ozone production rate of electrolytic ozone generators with proton conducting membranes and PbO2 anodes has been determined for demineralized water. At ambient temperature and a current density of 1 A/cm² it is 18%. Experience shows that the presence of dissolved salts or organic impurities reduce the current efficiency. A realistic value is 8%. One reason is the reduced catalytic reaction at the electrode. Another reason is that ozone quickly reacts with organic substances thus distorting the current efficiency result. For an exact determination of the performance the current efficiency for a given water quality needs to be determined experimentally by measuring the concentrations of ozone in water and gas.
The maximum achievable ozone concentration in a fixed volume depends on various factors:
- The ozone production rate
- The solubility coefficient of ozone in water. According to Henry’s law the solubility of gasious ozone in water at a given temperature is proportional to the partial pressure of ozone. You can find values for the solubility coefficient in relevant literature . You may also calculate them using the empiric formula: logS = -0.25 – 0.013T (T: temperature in °C, S: concentration in water divided by concentration in gas in mol/liter)
- The efficiency of ozone transfer to water. It depends on the contact surface gas/water and the contact time. It is determined mainly by the bubble size and the height of the bubble column. Pressure and turbulence are positive factors for the transfer of ozone to water.
- The decay rate of ozone in water. As a rule of thumb, the rate of ozone decay and/or its reaction with dissolved reducing agents is proportional to its concentration in water. In order to keep a specific concentration level of ozone the production rate needs to be adjusted.
If Microcells are installed in pipes the ozone concentration additionally depends on factors like flow rate and speed as well as the residence time of the gas bubbles in the pipe.
 J. Carell Morris, The aqueous solubility of ozone – a review (Ozone News, Nr.1, 1988)
Please refer to related literature as this topic is extensive. You find some basic information about concentration and residence time in our Microcell presentation under Downloads.
Using DC voltage between 5 and 15 V the Microcell splits the water into its element according to the following equation:
H2O -> H2 + n/3 O3 + (1-n/2) O2
Oxygen and ozone are generated on the anode and hydrogen on the cathode. The percentage of ozone formed depends on the current efficiency. The useless byproducts H2 and O2 dissolve in the water or are released as gas. As the generation rate of those gases is very low there is no risk of explosive gas accumulation.
Electrolysis causes a pH-gradient within the cell: the anode area gets acidic and the cathode area gets basic. In total the water will not change its pH-value. However the change of pH-value near the cathode causes precipitation of carbonates (esp. Calcium) which may lead to incrustation and, in the long term, failure of the cell. The microcell technology has been upgraded to cope with water hardness.