Sodium hypochlorite is used to activate and condition the catalytic surfaces of DMI-65 filter medium. The method is described below followed by a step by step guide. The chemical process of curing of the DMI-65 catalytic surfaces – is done by soaking the media for at least 3 hours prior to its use. Ideally the longer the soaking and conditioning period is, the stronger and harder the catalytic surfaces become.

Clean water is added to fill the filtration unit, fill until 50% of the free board is submerged. Typically a solution of 12.5% sodium hypochlorite is then introduced into the unit in a ratio of 10 liters of 12.5% solution sodium hypochlorite to 1m3 of DMI-65. DMI-65 is poured into the filter unit, with the allowance of sufficient support media such as filter quartz, so that the bottom of the DMI-65 bed sits above the lowest backwash laterals. The mixture is then lightly agitated and the wash valve opened to drain off the water until the water is at the level of the surface of the filtration medium. The wash valve is then closed. The DMI-65 is then soaked for at least 3 hours. The longer the soaking, the better.

The wash valve is then opened and all the water is drained from the system. The sodium hypochlorite of the soak water will be concentrated to several hundred ppm and discharge must be considered. The DMI-65 is then continuously backwashed while sodium hypochlorite is continually injected, maintaining a chlorine residual level of at least 0.1 – 0.3 mg/L (ppm) in the backwash water. This process should continue until the residual manganese level in the backwash water reduces to a value that is “three times the times the maximum contaminant limit” of manganese in your region. Typically the manganese level should be reduced to about 0.15mg/L (or ppm).  In this process, the excess manganese dioxides left over from the proprietary infusion manufacturing process, that have not cured to the DMI-65 catalytic layers need to be backwashed washed into waste. The time taken for this to occur is usually between 20 and 40 minutes and up to a number of hours for larger applications, depending on backwash velocity and filter volume.

Once the manganese concentration has fallen to 0.15 ppm or less, the injection of hypochlorite or chlorine is adjusted to bring the level of residual chlorine to between 0.1 and 0.3 ppm in the filtrate. The use of a reducing agent such as sodium thiosulphate may be necessary to neutralise high levels of residual chlorine when discharge of the backwash water is not allowable.

Hypochlorite and chlorine are in a chemical equilibrium in water; the position of the equilibrium is pH dependent and low pH (acidic) favors chlorine. Therefore the prescribed ratio of: 12.5% solution of chlorine, at a ratio 10 litres per 1m3 of DMI-65, which is used to condition the DMI-65 will naturally be at a low pH.

Addition of chlorine to water gives both hydrochloric acid (HCl) and hypochlorous acid and inevitably will lower pH. Because the pH scale is logarithmic, pH 6 is 10 times more acidic than neutral pH 7. And pH 5 is 100 times more acidic than neutral. This means during the soaking and conditioning procedure when 10

liters of chlorine (12.5%) per 1m3 of DMI-65 is lowering pH to less than pH 3, we can estimate the solution is more than 10,000 times more acidic than neutral.

During service filter mode, strong chlorine dosing causes highly acidic conditions that are less than pH 5.8. This should never be allowed to occur with a programmed chemical dosing pumps fitted for regulated dosing to maintain operation in a neutral to basic pH range. It’s definitely wise to have monitoring system and alarm in place to maintain free chlorine residual level 0.1 – 0.3 ppm and a neutral to basic pH.

This will encourage high oxidation rates of target heavy metals and a long whole of life