Ozone disinfection in water treatment process
In this article we are going to discuss about Ozone disinfection in water treatment. Ozone gas, 03, is a powerful oxidising agent widely used for disinfection and oxidation. The bactericidal effect of ozone is rapid, the usual contact time being between 4 and 10 minutes with dosages of the order of 2-3 mg/1. It is also known to be more effective than chlorine in killing viruses, cysts and oocysts.
The WHO Guidelines 3 state: ‘Ozone has been shown to be an effective viral disinfectant, preferably for clean water, if residuals of 0.2-0.4 mg/litre are maintained for 4 minutes’. Therefore the criterion adopted by European designers for disinfection is to maintain a free ozone residual of 0.4 mg/1 for 4 minutes (i.e. Ct = 1.6 mg.min/1); this residual being the required level after the initial ozone demand is satisfied. The US practice is to design for the Ct values based on the inactivation of cysts and viruses given in the SWTR
How ozone can be produced for disinfection of drinking water?
Ozone is produced on a commercial scale by passing air or oxygen through a silent electrical discharge. A high voltage AC current is applied between two electrodes separated by a dielectric and a narrow gap through which the air or oxygen to be ozonised is passed. The gap width is a function of the feed gas, the dialectric material, and the power supply’s characteristics. Typical gap widths are between 0.5 and 3 mm. Electrical power options commonly available for electrolysis are low frequency (50-60 Hz at 14-19 kV); medium frequency (60-1000 Hz at 9-14 kV); and high frequency (greater than 1000 Hz typically 5000-7000 Hz at 10 kV).
Medium frequency generators are used primarily for oxygen feed systems, they have a smaller discharge gap and allow higher ozone production per tube. Low frequency systems have the simplest power supply. High frequency systems have the advantage of low voltage and therefore few dielectric failures concentric tube designs, the outer electrode is a stainless steel tube. The dielectric is a glass or ceramic material, which can be plated onto the outside tube or the inner electrode.
Historically a glass tube with an inner metallised coating forms the inner electrode. A typical ozone generator consists of several hundred of such tubes assembled in a large,vessel (see Fig. 9.4 and Plate 25). The high voltage is applied to the inner tube; the low voltage is connected to the stainless steel outer tube. Approximately 90-95% of the energy input appears as heat and must be removed by applying cooling water. Manufacturers recommend that cooling water has a chloride content <30 mg/1 to minimise corrosion of stainless steel.
Depending on the temperature and the chloride content the cooling water could be once-through type using filtered plant water or closed-circuit type which uses water-cooled heat exchanger or refrigerated water chillers. The system capacity should be adequate to limit the increase in gas stream temperature to less than 5~ The feed (air or oxygen) to the ozoniser must be completely oil-free to prevent detonation (hydrocarbon content < 10 ml/m3), dust free to a level better than 99% at 0.5 micron to reduce electrode fouling and very dry; the normal requirement is that the feed should have a dew point below -80~ to prevent sparking and formation of nitric acid. In the air feed to achieve this dryness it is usual to both refrigerate the feed and pass it through a desiccant.
The feed should be cool with a temperature below 25~ and compressors or blowers used for the feed must be of the oil-free type. With air as the feed, typical production is up to 4% w/w ozone in air (52.3 g O3/m 3 of air at 0~ 1.013 bar). Air feed systems can be either low pressure (<2 bar g) or high pressure (> 4 bar g). In a high pressure system most of the moisture can be removed in the aftercooler and eliminates the need for a refrigerant dryer. High pressure systems are commonly used in packaged ozone generators.mWith oxygen as the feed typical production is about 10% w/w ozone in oxygen (149 g O3/m 3 of oxygen at 0~ 1.013 bar).
Methods of supplying oxygen include use of liquid oxygen (LOX), delivered in bulk to site by tanker or pipeline, on-site production of LOX,mor on-site generation of gaseous oxygen (GOX). LOX delivered in bulk is almost 100% pure oxygen with a dew point lower than -80~ and therefore does not normally require further treatment. It is stored on site in a vacuum insulated storage tank. Vaporisers which use ambient air, warm water, steam or electrical energy convert LOX to gas for use in the ozone generators.
Another Ozone disinfection in water treatment can be done using LOX system. With oxygen as the feed typical production is about 10% w/w ozone in oxygen (149 g O3/m 3 of oxygen at 0~ 1.013 bar). Methods of supplying oxygen include use of liquid oxygen (LOX), delivered in bulk to site by tanker or pipeline, on-site production of LOX, or on-site generation of gaseous oxygen (GOX).
LOX delivered in bulk is almost 100% pure oxygen with a dew point lower than -80~ and therefore does not normally require further treatment. It is stored on site in a vacuum insulated storage tank. Vaporisers which use ambient air, warm water, steam or electrical energy convert LOX to gas for use in the ozone generators.
LOX is produced on-site in pre-engineered packaged plants with capacities > 20 t/day by cryogenic air separation where liquefaction of air is followed by fractional distillation to separate oxygen and nitrogen. This method is commonly used in commercial gas production plants where both oxygen and nitrogen are required as products. Oxygen concentration in the product gas is usually greater than 95%.This is Ozone disinfection in water treatment process.