- In wastewater treatment plants, ozone is used to disinfect and remove/reduce pollutants, odor, turbidity, color, and BOD.
- Ozone is a strong oxidant and reactive element and can corrode metals, be explosive at high concentrations, and cause severe health problems.
- The plant design and work safety protocols should eliminate the risk of operators’ exposure to ozone leaks from weak connections and in ozone production and destructor areas.
- Constant monitoring is necessary to prevent ozone accumulation from small leaks persisting for a long time in the air.
Ozone Use in Waste Treatment Plants
Ozone is a colorless to blue, pungent-smelling gas replacing traditional agents like chlorine in wastewater treatment.
Ozone is used in medium to large wastewater treatment plants in the final stages of the process.
Ozone (O3) is a highly reactive molecule and a potent oxidizing agent. It destroys the cell walls of microbes and organic matter through oxidation and breaks carbon-nitrogen bonds. It also damages the genetic matter. As a result of these actions, ozonation is helpful for disinfection, pollutant removal, odor control, and Biological oxygen demand (BOD) and chemical oxygen demand (COD) reduction.
- Disinfection: Ozone efficiently kills helminths, bacteria, protozoa, and viruses, making water safe for discharge into the environment.
- Pollutant Removal: Ozone oxidizes and breaks down organic and inorganic pollutants in wastewater to reduce sludge.
- Odor, Color, and Turbidity Control: Ozone helps control unpleasant odors by breaking down iron and manganese odor-causing compounds. The color and turbidity of organic material and flocculants can be reduced significantly by ozone as it breaks down unwanted compounds.
- Reduction of BOD/COD: Ozone acts as an algae control to reduce BOD; it degrades organic matter to limit COD.
Ozone helps improve treated water’s overall quality to meet safe discharge standards.
Ozone has several advantages: it acts fast, within 10-30 minutes, and is a more effective disinfectant than chlorine or UV treatment, killing more microbes. Moreover, because it is unstable, it breaks into oxygen, produces no residual toxic byproduct or residue, and is therefore environmentally friendly.
Because of its advantages, ozonation is widely used for municipal, industrial, hospital, and electroplating wastewater treatments. The applications are increasing and are also used for treating landfill leachates, rubber additive wastewaters, etc.
Figure 1.: Ozonation schematic diagram, EPA. (Credits: https://www3.epa.gov/npdes/pubs/ozon.pdf)
How Ozone is Used In Wastewater Treatment Plants
Ozone is unstable, has a relatively short lifespan of only 20 to 90 minutes, and decomposes to oxygen quickly after production. Therefore, ozone generation is done onsite at wastewater treatment plants.
Ozone is generated using ultraviolet lights and oxygen feed, then transferred into a contact tank for mixing with wastewater.
The contact tank plays a critical role in the disinfection process. Its primary purpose is to facilitate ozone transfer and allow sufficient contact time with the wastewater to disinfect it effectively. Given ozone’s relatively short lifespan, uniform, efficient contact is crucial for maximum effectiveness and is achieved by using near-plug flow contactors.
Despite its various advantages, ozone use has disadvantages because of its properties.
Ozone Occupational Hazards
Ozone’s strong oxidative and reactivity, which make it an excellent agent for waste treatment plants, can also lead to safety issues.
Reaction Risk
Ozone is reactive and corrosive and causes safety issues in pipes and containers not made of stainless steel.
The whole plant must be designed so that operators do not come in contact with any ozone that escapes wastewater.
Explosion Risk
Ozone gas is explosive at concentrations of 240 g/m3. Therefore, gaseous ozone concentrations should be below this level (generally within 50 to 200 g/m3) to prevent hazards. Ozone in gaseous form also remains hazardous for a long time, so exercising extreme caution when handling ozone gas systems is necessary.
Heath Risk
Ozone is present in the Earth’s upper atmosphere and ground level. While ozone in the upper atmosphere protects us from the sun’s harmful ultraviolet rays, ground-level ozone can pose health risks, particularly in high concentrations.
Acute health risks occur immediately or shortly after ozone exposure.
- Skin and Eye Irritation: Liquefied ozone can cause irritation and severe burns upon contact with the skin or eyes.
- Respiratory Irritation: Breathing ozone irritates the nose and throat, and exposure to higher gas levels can lead to headaches, chest pain or tightness, upset stomach, and vomiting. Ozone can also irritate the lungs, resulting in coughing and shortness of breath. Higher exposures might cause pulmonary edema—a fluid buildup in the lungs—leading to severe shortness of breath, constituting a medical emergency.
Repeated and chronic exposure can produce symptoms after a while and last for months to years.
- Mutations: Ozone causes mutations (changes in genetic matter).
- Reproductive Effects: Ozone exposure may potentially harm the developing fetus.
- Lung Damage: Repeated exposure causes lung damage.
- Cancer Hazard: Limited evidence shows ozone causes lung cancer in animals.
It’s important to note that the severity of these health effects largely depends on the concentration and duration of exposure to ozone. People with pre-existing respiratory conditions such as asthma may be particularly susceptible to the effects of ozone exposure.
Preventing Ozone Exposure Risks
Since ozone poses serious health hazards and can be explosive, off-gases from the contactor tank must be sent for ozone destruction or recycled to prevent worker exposure, see Figure 1. This is where the importance of ozone detection in wastewater treatment comes into play.
Guidelines emphasize the importance of regular monitoring, maintenance, and strict adherence to safety measures and established protocols during ozone generation and wastewater treatment while minimizing risks to personnel.
Gas Monitoring: Safety managers at the plant must ensure no leakage at connections or near the ozone generator.
- Operators have to monitor subunits to check for overheating regularly.
- Routine checks for leaks are necessary, as even a tiny leak can lead to unacceptable air ozone levels.
- To ensure accuracy and reliability, the gas measuring devices must undergo regular bump testing and calibration tests as per equipment manufacturer instructions.
Ozone Contact and Reactivity: Safety managers must ensure the effective diffusion of ozone into wastewater within the contactor due to ozone’s limited solubility and rapid decomposition in water. Maintaining proper covering of the contactor will optimize ozone diffusion.
Purging Systems: Before opening any ozone system or subsystem, operators should ensure the purging of inlet piping for the ozone generator, distribution, contracting, off-gas, and destructor to prevent the release of trapped ozone gas or oxygen deficiencies. Operators must be cautious when entering the ozone contactor due to potential trapped ozone gas or oxygen deficiencies despite purging efforts.
Emergency Operating Procedures: Operators must be aware of all operating procedures in case of a problem. All necessary safety equipment must be readily available for operators in emergencies. Check detailed information on emergency measures here.
Regulatory Compliance: Maintain ambient ozone levels within the limits specified by safety regulations to ensure compliance and safety for personnel and the environment.
Safe exposure limits
Safety managers should ensure that the ozone levels in their plants are well below the permissible levels prescribed in their country of operation.
The safe limits prescribed in the USA by various agencies are as follows:
- Occupational Safety and Health Administration (OSHA) has set the legal airborne permissible exposure limit (PEL) averaged over an 8-hour work shift at 0.1 ppm (parts per million). See Table 1.
- National Institute for Occupational Safety and Health (NIOSH)’s recommended airborne exposure limit, which should not be exceeded at any given time, is 0.1 ppm.
- American Conference of Governmental Industrial Hygienists (ACGIH) recommends airborne exposure limits averaged over an 8-hour work shift based on the nature of work:
- for heavy work, it is 0.05 ppm
- for moderate work, the limit is 0.08 ppm
- for light work, the limit is 0.1 ppm
- limits for work less than 2 hours is 0.20 ppm
Table 1: “Safe exposure limits,” OSHA. (Credits: https://www.osha.gov/chemicaldata/9)
How to Measure Ozone
Portable instruments like the GasD 8000 Series Gas Analyzers can be used in addition to fixed sensors to measure ozone levels rapidly, within a second, for routine work and in case of leakages. Some places where operators should take a portable device are as follows:
- before entering the ozone contactor, where escaped ozone gas could be trapped
- near ozone generators and destructors to detect trace amounts of ozone that may have leaked through loose connections.
The GasD 8000 Series Gas Analyzers have two options: one measures ozone in the range of 0-2000 ppb (parts per billion) with a resolution of 1 ppb, and the second measures 0-20 ppm at a resolution of 0.01 ppm.
Reducing exposure to ozone by following air quality regulations is crucial in mitigating health risks and damage to wastewater treatment plants’ property.
Sources
EPA. (1999 September). Wastewater Technology Fact Sheet Ozone Disinfection. Retrieved from https://www3.epa.gov/npdes/pubs/ozon.pdf
Ozone hazard summary ozone – the official web site for the state of New Jersey. (n.d.). https://nj.gov/health/eoh/rtkweb/documents/fs/1451.pdf
Ozone. Occupational Safety and Health Administration. (n.d.). https://www.osha.gov/chemicaldata/9
Rip G. Rice (1996) Applications of ozone for industrial wastewater treatment — A review, Ozone: Science & Engineering, 18:6, 477-515, DOI: 10.1080/01919512.1997.10382859
Sallanko, J., & Okkonen, J. (2009). Effect of ozonation on treated municipal wastewater. Journal of Environmental Science and Health, Part A, 44(1), 57–61. https://doi.org/10.1080/10934520802515350