Ozone demand

Disinfection. Ozone is a more effective broad-spectmm disinfectant than chlorine-based compounds (105). Ozone is very effective against bacteria because even concentrations as low as 0.01 ppm are toxic to bacteria. Whereas disinfection of bacteria by chlorine involves the diffusion of HOGl through the ceU membrane, disinfection by ozone occurs with the lysing (ie, mpture) of the ceU wall. The disinfection rate depends on the type of organism and is affected by ozone concentration, temperature (106), pH, turbidity, clumping of organisms, oxidizable substances, and the type of contactor employed (107). The presence of oxidizable substances in ordinary water can retard disinfection until the initial ozone demand is satisfied, at which point rapid disinfection is observed. 

Ozone has proven to be effeetive against viruses. Franee has adopted a standard for the use of ozone to inaetivate viruses. When an ozone residual of 0.4 mg/I ean be measured 4 minutes after the initial ozone demand has been met, viral inaetivation is satisfied. This property plus ozone s freedom from residual formation are important eonsiderations in the publie health aspects of ozonation. When ozonation is eombined with aetivated earbon filtration, a high degree of organie removal ean be aehieved. Coneerning the toxieity of oxidation produets of ozone and the removal of speeifie eompounds via ozonation, available evidenee does not indieate any major health hazards assoeiated with the use of ozone in wastewater treatment. 

Ozonolysis can be applied for assessing the pollution level of potable and wastewaters, by determining the ozone demand in an apparatus including an ozone generator and UVD for measuring the amount of O3 supplied. A correlation can be established between the ozone demand and the chemical oxygen demand, which is a standard method for pollution evaluation. The advantage of the ozonolysis route is its speed (a few minutes for sample... 

Another consideration connected with these multiple effects is the optimum placement of the ozonation stages within a whole treatment scheme. The efficiency of every ozonation unit and the ozone demand depend on the water and waste water quality produced by the preceding process units (e. g. particle removal or biodegradation). Ozonation will also have pronounced effects downstream in the treatment sequence, e. g. improved biodegradation of dissolved organics. 

The first three tasks are much more relevant and applicable to full-scale plants, compared to the last topic. The reason is the high ozone demand for direct chemical mineralization, with typically more than 3 g 03 g l DOC initially present needed to achieve a removal efficiency of 20 % or more. 

Typical ozone dosage is 1.0 to 5.3 kg/1000 m of treated water at a power consumption of 10 to 20 kW/kg of ozone. It has been observed that complete destruction of poliovirus in distilled water is accomplished with a residual of 0.3 mg/L of ozone in 3 min. As in the case of chlorination where a chlorine demand is first exerted before the actual disinfection process can take place, an ozone demand in ozonation is also... 

The immediate ozone demand parallels that of the immediate chlorine demand. Recall that this demand is due to ferrous, manganous, nitrites, and hydrogen sulhde. The immediate demand reactions with ozone are as follows ... 

This is one of the advantages in the use of ozone the effluent is saturated with dissolved oxygen. Of course, the previous reactions are simply for the immediate ozone demand and have nothing to do with disinfection. As mentioned, these reactions must be satisfied first before the actual act of disinfecting commences. 

Then 0.01 mole of potassium cyanate was dissolved in a small amount of water and added to a saturated solution of ozone in 3100 ml. of neutral distilled water, which was agitated thoroughly. The redox potential fell to a minimum point, rose slightly, and then fell off in a normal decay curve. The raising of the pH which would follow upon addition of cyanate could catalyze the decomposition of ozone, but would not account for the minimum in the curve. Cyanate solutions will, therefore, exercise an ozone demand, whether through hydrolysis or oxidation, and ozone will be consumed until the demand is satisfied. 

Figure 8 shows the effect of ozone on the redox potential of ferrocyanide solutions. Successive additions of ozone show a decreased ozone demand in the solution. 

The lethal dose of ozone for E, coli (0.4 to 0.5 mg. per liter) under these experimental conditions is higher than the values obtained by other workers 3, S), who determined the ozone residual as total oxidizable constituents remaining after bubbling the ozone through a test solution. This slightly higher ozone value may be the result of temperature dependency or of use of the dose required rather than residual accumulated. Each test solution can be assumed to have a certain ozone demand. 

The actual ozone dosage at Saint-Maur based on 3.5 years operation, is of the order of 1 p.p.m. The minimum is 0.6 p.p.m. The amount actually used or lost in the water is less, however, as these figures include the excess ozone at the column outlets. Because the ozonizers cannot be operated below certain limits, this causes excess dosages, when the ozone demand of the water is less than 0.6 p.p.m., even at maximum column output of 50,000 cubic meters (twice rated flow). 

Once or twice a year, for several hours or days, there is an exceptionally heavy ozone demand, presumably caused by the dumping of industrial wastes into the river. 

The water saturated with ozone leaves compartment A at the bottom where the ozone is dissolved, and moves slowly through compartment B. In compartment B, because of the absence of turbulence, the ozone tends to remain in solution at its maximum ratio, and decomposes slowly unless the ozone demand of the water is so high that oxidizable substances still are present. 

The preozonation of the water by means of excess ozone from the ozonation chamber has been introduced chiefly to utilize all the ozone injected into the ozonation chamber water. In eliminating ozone losses, the first concern was to reduce the operating and installation costs appreciably. But it was also desired to see if, by satisfying part of the ozone demand of the water before the actual disinfection operation by means of the recovered ozone, it would be possible to obtain a subsequently more powerful bactericidal action with smaller ozone dosages. The object has been attained. 

C. J. Johnson and P. C. Singer, Impact of a magnetic ion exchange resin on ozone demand and bromate formation dnring drinking water treattnent, Water Research 38, 3738-3750 (2004). 

Ozone requirements for partial destraction of phenols range from one to five parts per part of phenol. The actual ozone demand will be a function of phenol concentration, pH, and retention time. 

In various studies concerning ozonation and ozone-based AOP the formation of bromate is observed [31,62,66-68]. The extent of bromate formation is dependent on the ozone exposure. Higher ozone doses usually result in higher bromate concentrations. However, if the ozone demand of the water itself is high, the ozone exposure for bromide is smaller and less bromate is formed [31,62,66]. During experiments with the combined process... 

The rate of ozone decomposition reaction and instantaneous ozone demand also depend on the content of dissolved organic matter. An example of different treated waters with varying content of total organic carbon (TOC) is shown in Figure 6. 

The actual ozone dose utihzed at a water treatment plant is determined by the quaUty of the raw water (i.e., how much ozone demand will be exerted) and what is the ultimate objective for using ozone. Plants using ozone for iron and manganese oxidation will need to apply a different dose than plants using ozone as a disinfectant for inactivation of protozoans such as Cryptosporidium. 

Herron, T Huie, R. E. J. Phys. Chem. 1969, 73 3327 Hureiki, L. Crou J. R Le-gube, B., Dore, M. Ozonation of amino acids Ozone demand and aldehyde formation. Ozone Science and Engineering 1998, 20 381-402. 

Determining the optimum ozone dosage depends on several factors, including CT requirements, ozone residual target, transfer efficiency, ozone demand, oxidation objective, and ozone decay characteristics. Dosage calculations are included in chapter 4. 

The amounts of ozone needed to perform each of these functions depend upon a number of factors, but primarily upon the ozone demand of the constituents of the water/wastewater to be ozonized. 

A concentration of residual ozone can be achieved and monitored during disinfection and/or viral inactivation of waters that have little extraneous ozone demand (drinking water, swimming pool water, cooling water, etc.). For other drinking water or industrial water treatment applications, and for most wastewater applications, control of ozonation processes must be monitored by a surrogate analytical technique. Such process controls are not based upon the monitoring of dissolved residual ozone. 

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