Gas Hazards in Waste Water Treatment

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Gas Hazards in Waste Water Treatment

Water is vital to our daily lives, both for personal and domestic use and industrial/commercial applications. It is everywhere, promoting some chemical reactions and inhibiting others. Being used to clean surfaces, carry chemicals to where they are used and to carry unwanted chemicals away. There are so many permutations of things that can come together and react, dissolved gases that can come out of solution, dissolved liquids and solids that can react to generate gases. Additionally, you must determine what gases you generate when you collect, clean, store, transport or use water. Gas detectors must be chosen to suit the specific environment in which they operate, in this case highly humid, often dirty, but rarely outside the temperature range from 4 to 30 °C. All the risks are present in these complex environments, with multiple toxic and flammable gas hazards and often the additional risk of oxygen depletion.

Gas Hazards

Apart from common gas hazards known in the industry; methane, hydrogen sulphide, and oxygen, there are bi-product gas hazards and cleaning material gas hazards that occur from purifying chemicals such as ammonia, chlorine, chlorine dioxide or ozone that are used in the decontamination of the waste and effluent water, or to remove microbes from clean water. There is great potential for many toxic or explosive gases to exist as a result of the chemicals used in the water industry. And added to these are chemicals that may be spilled or dumped into the waste system from industry, farming or building work.

Chlorine (Cl2) gas appears yellow green in colour, used to sterilise drinking water. However, most chlorine is used in the chemical industry with typical applications including water treatment as well as within the plastics and cleaning agents. Chlorine gas can be recognised by its pungent, irritating odour, which is like the odour of bleach. The strong smell may provide adequate warning to people that they are exposed. Chlorine itself is not flammable, but it can react explosively or form flammable compounds with other chemicals such as turpentine and ammonia.

Ammonia (NH3) is a compound of nitrogen and hydrogen and is a colourless and pungent gas, also known to be highly soluble when in contact with water. This means that ammonia dissolves quickly into the water supply. Found at very low levels in humans and in nature. It is also often used in some household cleaning solutions. Although ammonia has many benefits, it can be corrosive and dangerous in certain circumstances. Ammonia can enter wastewater from several different sources, including urine, manure, cleaning chemicals, process chemicals and amino acid products. If ammonia enters a copper piping system, it can cause extensive corrosion. If ammonia enters water, its toxicity varies depending on the exact pH of the water. It is possible for ammonia to break down into ammonium ions, which can react with other compounds present.

Chlorine dioxide (ClO2) is an oxidising gas commonly used to disinfect drinking water. When used in very small quantities, it is safe and does not lead to significant health risks. But chlorine dioxide is a strong disinfectant that kills bacteria, viruses, and fungi, and when used in high doses, it can be dangerous to people since it can damage red blood cells and the lining of the gastrointestinal tract.

Ozone (O3) is a gas with an antiseptic smell and no colour that, mostly, forms naturally in the environment. When inhaled, ozone can have a range of harmful effects on the body. As it is colourless gas it is difficult to trace without an effective detection system in place. Even when relatively small amounts are inhaled, the gas can have a damaging impact on the respiratory tract, causing inflammation and chest pain, alongside coughing, shortness of breath and throat irritation. It can also act as a trigger causing diseases such as asthma to worsen.

Confined Space Entry

The pipelines used to transport water require regular cleaning and safety checks; during these operations, portable multi-gas monitors are used to protect the workforce. Pre-entry checks must be completed prior to entering any confined space and commonly O2, CO, H2S and CH4 are monitored. Confined spaces are small, so portable monitors must be compact and unobtrusive for the user, yet able to withstand the wet and dirty environments in which they must perform. Clear and prompt indication of any increase in gas monitored (or any decrease for oxygen) is of paramount importance – loud and bright alarms are effective in raising the alarm to the user.


European Commission Directive 2017/164 established an increased list of indicative occupational exposure limit values (IOELVs). IOELV are health-based, non-binding values, derived from the most recent scientific data available and considering the availability of reliable measurement techniques. Non-binding but best practice. The list includes carbon monoxide, nitrogen monoxide, nitrogen dioxide, sulphur dioxide, hydrogen cyanide, manganese, diacetyl and many other chemicals. The list is based on Council Directive 98/24/EC that considers the protection of the health and safety of workers from the risks related to chemical agents in the workplace. For any chemical agent for which an IOELV has been set at Union level, Member States are required to establish a national occupational exposure limit value. They also are required to take into account the Union limit value, determining the nature of the national limit value in accordance with national legislation and practice. Member States will be able to benefit from a transitional period ending at the latest on 21 August 2023.

The Health and Safety Executive (HSE) state that each year several workers will suffer from at least one episode of work-related illness. Although, most illnesses are relatively mild cases of gastroenteritis, there is also a risk for potentially fatal diseases, such as leptospirosis (Weil’s disease) and hepatitis.

Under domestic law of the Health and Safety at Work etc Act 1974, employers are responsible for ensuring the safety of their employees and others. This responsibility is reinforced by regulations.

The Confined Spaces Regulations 1997 applies where the assessment identifies risks of serious injury from work in confined spaces. These regulations contain the following key duties:

  • Avoid entry to confined spaces, e.g., by doing the work from the outside.
  • If entry to a confined space is unavoidable, follow a safe system of work.
  • Put in place adequate emergency arrangements before the work start.

The Management of Health and Safety at Work Regulations 1999 requires employers and self-employed people to carry out a suitable and sufficient assessment of the risks for all work activities for the purpose of deciding what measures are necessary for safety. For work in confined spaces this means identifying the hazards present, assessing the risks and determining what precautions to take.



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