How to Determine the Ozone Dosing Location and Dosage in Wastewater Treatment

However, ozone oxidation has strong selectivity, and its applicability varies greatly among different industries' industrial wastewater. Even for the same industry's industrial wastewater, differences in production raw materials and processes can cause differences in wastewater concentration and composition, changing the applicability of ozone oxidation.

Ozone oxidation technology does not have a removal effect on all refractory pollutants and is not suitable for all difficult-to-treat industrial wastewater. If ozone oxidation cannot meet the standards, other physicochemical processes such as Fenton's method, ultrasound, electrolysis, reverse osmosis, ion exchange, electrodialysis, or other advanced oxidation processes need to be considered.

The positions of ozone oxidation in the process flow are generally the following three or a combination of the three:
(1) The first position is pre-ozone oxidation.
Placing ozone oxidation before biological treatment as pre-ozone oxidation aims to improve the biodegradability of wastewater. However, this method is not applicable to all wastewater. It should be noted that ozone will preferentially oxidize easily degradable pollutants. Easily degradable pollutants consume ozone first, while refractory pollutants require more ozone dosage and longer reaction time to degrade into biodegradable substances, thereby improving the biodegradability of wastewater. Therefore, pre-ozone oxidation may result in a large ozone consumption.

(2) The second position is deep treatment ozone oxidation.
Placing ozone oxidation downstream of biological treatment as deep treatment to treat unbiodegraded pollutants with strong oxidation action and make the final effluent meet the standards. Therefore, it is suitable for wastewater treatment projects with practical engineering experience or test data support. For some refractory wastewater, ozone oxidation needs to be combined with photocatalysis or activated carbon to achieve the desired results.

(3) The third position is deep treatment to improve the biodegradability of wastewater.
When the biodegradability of wastewater is low after biological treatment, if all the remaining refractory biodegradable pollutants are treated by ozone oxidation to meet the standards, it may result in excessive ozone consumption, high investment and operating costs, and economic inefficiency. In this case, ozone oxidation can be considered as a deep treatment facility to improve the biodegradability of wastewater. After ozone oxidation, a second biological treatment (usually using aerated biological filters, biological activated carbon, etc.) can be performed to make the effluent meet the standards, which significantly reduces the ozone consumption and cost.

The dosing amount and contact reaction time of ozone are determined based on engineering experience and vary with the function of ozone oxidation and the characteristics of wastewater. If there is no experience to refer to, pilot tests or bench tests should be conducted to determine the appropriate values. The specific situations are as follows:

(1) For drinking water disinfection and sterilization, the ozone dosage is 1-3mg/L. If the water source quality is poor, it needs to be increased to 3-5mg/L, with a contact time of 12-15min, and the residual ozone in water should be less than or equal to 0.3mg/L, with a removal rate of 99%. Under the same dosage and contact time conditions, the removal rates for fouling, deodorization, and color removal of contaminated water sources are 80%-90%.

(2) The ozone dosage for swimming pool recirculating water treatment is 2mg/L.

(3) The dosage of ozone used for municipal decolorization is mostly 5mg/L, with a reaction time of 15min.

(4) The order of ozone oxidation for organic compounds is: alkanes > amines > phenols > polycyclic aromatic hydrocarbons > alcohols > aldehydes > alkanes.Oxidizing 1mg of cyanide consumes 1.87mg of ozone, while oxidizing 1mg of CN- requires 2-2.5mg of ozone. When used for removing impurities such as CN-, phenols, and ABS from water, a contact time of 5-10min can achieve a removal rate of up to 90%. Currently, the dosage of advanced oxidation in municipal sewage treatment is between 20-30mg/L, with a reaction time of 40-60min.

(5) For refractory pollutants without practical data, the estimated ozone dosing amount is 1-3mg (ozone)/mg organic matter.

The output of ozone generators should meet the maximum ozone dosing requirement and consider standby capacity. When selecting an ozone generator system, it is necessary to understand whether the ozone treatment process is controlled by mass transfer for disinfection or by reaction rate control for ozone oxidation, what the available water head is, what the overall gas pressure of the ozone generation system is, how much ozone is required for use, and what the liquid ozone uptake rate is.