Wastewater’s effect on air quality
Pinpointing the cause of poor air quality can be difficult at times – leading to unnecessarily prolonged exposure to hazardous pollutants for workers, neighbours and the environment. In this expert series, we take a closer look at specific industries and some of the more common contaminants that could affect the air in your workplace or home, as well as treatment strategies to deal with it.
Wastewater is inevitable
There are more than 7.5 billion people on Earth and all of them produce waste. In the process of treating this waste, many compounds that can linger in the air and cause distress or harm are formed.
Through the various treatment phases, different leftover chemical compounds are produced. These must be dealt with in order to reduce the environment hazard level.
Sulphur compounds can be one of the more challenging components to handle in the wastewater treatment process. They are often toxic and can produce a nauseating odour even at very low concentrations.
When wastewater is treated aerobically (using oxygen), the sulphur has access to oxygen molecules and can form odourless sulphates. But when it is treated anaerobically – or is kept away from oxygen for prolonged periods of time – more problematic compounds are formed.
The challenge often comes from cost effectively neutralizing small concentrations of these dangerous anaerobic products (Lebrecht & Hannay, 2015).
The most commonly discussed odorous compound associated with wastewater is hydrogen sulphide (H2S), often referred to as `rotten egg’ gas. H2S has an odour detection threshold in air (i.e. the lowest concentration in which humans can detect it) of about 0.5 parts per billion (ppb). Concentrations just five times this odour detection level can cause headaches, and eye and lung irritation. Elevated H2S levels can even cause unconsciousness in a few breaths and, in extreme concentrations, it is lethal (Government of Western Australia, Department of Health, 2019).
Mercaptans (or thiols) are also created in anaerobic processes (oxygen free) where sulphur is present. Mercaptans have a strong and repulsive odour resembling that of garlic. Methyl mercaptan, for instance, has an odour detection threshold about half that of H2S (0.25 ppb). Many mercaptans have been shown to be toxic. They can produce serious effects such as nervous system damage, muscle weakness and even death (Lv, et al., 2016). Often difficult to electronically detect at such low concentrations, care must be taken to remove or neutralise these compounds before the water leaves the plant. This is because our sense of smell can often detect these sulphur-based compounds at concentrations below the limits of electronic sensors.
Oxygen control directly regulates these compounds. Supplying oxygen can convert them into more breathable gases. However, techniques for reducing one of these specific compounds will often introduce or neglect many other sulphides and sulphates.
It is important to be aware of the leftover gases present after the primary target of the treatment has been controlled. Depending on the initial technique to control or transform the sulphur in wastewater, further treatment procedures are often required. This could be to balance levels of safe sulphur compounds or to ensure they do not reform into dangerous ones later in the process (Lebrecht & Hannay, 2015).
Human urea and industrial by-products can introduce ammonia (NH3) and other nitrogen-based compounds such as amines (derivatives of ammonia) to wastewater. If not treated correctly this can pose a severe threat to aquatic life as well as human health via the consumption of seafood (Karri, Sahu, & Chimmiri, 2018).
Typically, the ammonia undergoes two processes which convert it into a harmless, odourless gas and both use a different kind of bacteria (Metrohm Australia, 2019).
First, one species of bacteria consumes the NH3 and ammonium (NH4+) molecules as food. They excrete waste products in a process called nitrification that combine with oxygen to form nitrate. This nitrate is then further processed in the absence of oxygen with a different specialised heterotrophic bacterium. This process is called denitrification and separates the oxygen from the nitrate to create the waste product of pure nitrogen gas (i.e. N2). N2 comprises about 78 per cent of the atmosphere and is safe to release into the environment.
Various changes in pH, temperature and relative oxygen levels can disturb the balance between these two processes. It is important to monitor the nitrite, nitrate and ammonia levels at all stages of the water treatment process so that toxic gases are not distributed in the surrounding area.
Volatile organic compounds
Another large family of chemicals that are major contributors to air pollution are carbon-based chemicals that easily evaporate at room temperatures. These volatile organic compounds (VOCs) are known as hazardous air pollutants that have serious health effects if inhaled, ranging from throat irritation and headaches, to liver, kidney and central nervous system damage (Yang, et al., 2014) Many are considered carcinogenic, while others still do not have well known health effects.
Common VOCs present in wastewater treatment cycles include toulene, benzene, ethylene glycol, tetrachloroethene, xylenes and chloroform – but the list goes on (Yang, et al., 2014; Quigley & Corsi, 2012). Such a large family of compounds means that VOC treatment methods are often overestimated.
In wastewater treatment, VOCs are released through several pathways. The most important one is via evaporation into the air during treatment phases that require aeration (such as the nitrification mentioned previously).
If the surface of the wastewater or sludge is exposed to the open air and sunlight, then VOCs will evaporate into the ambient atmosphere. This is due to both the temperature difference between the water and the air, and air flow passing along the surface of the wastewater for the aeration process.
This means that weather conditions and temperature can influence odour and VOC emissions extensively. Hot temperatures and strong winds exacerbate the VOC evaporation rate and can blow any pollutants far into the surrounding environment. Summer months are an important time to be monitoring any potential causes of all the above-mentioned compounds.
Wastewater treatment processes have many sources of chemical compounds that must be regularly monitored in order to fully control emissions. Oxygen control is one of the most important. It is a vital part of the treatment processes, but also a possible cause for unforeseen emissions.
A careful balance of waste treatment and emission control of several chemical families must always be present such that the plant can run safely and efficiently.
References and Further Reading
Government of Western Australia, Department of Health. (2019). Hydrogen sulfide and public health.
Karri, R. R., Sahu, J. N., & Chimmiri, V. (2018). Critical review of abatement of ammonia from wastewater. Journal of Molecular Liquids, 21-31.
Lebrecht, T., & Hannay, N. (2015, February). Treating Sulfur in Wastewater. Hydrocarbon Engineering, pp. 59-67.
Lv, Z., Sun, Z., Song, C., Lu, S., Chen, G., & You, J. (2016). Sensitive and background-free determination of thols from wastewater samples by MOF-5 extraction coupled with high-performance liquid chromatography with fluorescence detection using a novel fluorescnce probe of carbazole-9-ethyl-2-maleimie. Talanta, 228-237.
Metrohm Australia. (2019). Wastewater Treatment Plants: Nitrogn Removal – Simultaneous analysis of Ammonia, Nitrate and Nitrite.
Quigley, C. J., & Corsi, R. L. (2012). Emissions of VOCs from a Municipal Sewer. Journal of the Air & Waste Management Association, 395-403.
Yang, J., Wang, K., Zhao, Q., Huang, L., Yuan, C.-S., Chen, W.-H., & Yang, W.-B. (2014). Underestimated public health risks caused by overestimated VOC removal in wastewater treatment processes. Environmental Science; Processes & Impacts, 271-279.