Oxygen: Water Pollutant Or Life-Giver?

Water pollution is a pressing issue that affects aquatic ecosystems, human safety, and drinking water supplies. One of the key indicators of water quality is dissolved oxygen (DO) content, which is the amount of oxygen available to aquatic organisms for respiration. High DO levels generally indicate clean water and support the growth of clean-water fish, while low DO levels can be fatal for fish and other organisms. Oxygen-demanding waste, such as sewage and agricultural runoff, can lead to reduced DO levels in water, creating an oxygen-deficient environment that negatively impacts aquatic life. This is often associated with cultural eutrophication, where high levels of nitrogen and phosphorus from human activities cause rapid growth of algae, leading to oxygen depletion during their decomposition. Therefore, understanding the role of oxygen in water pollution is crucial for maintaining water quality, protecting ecosystems, and ensuring safe drinking water.

Oxygen-demanding waste

The BOD is a commonly used measure in water quality management. It is measured as milligrams per litre of oxygen consumed over five days at 20°C. A high BOD indicates a high level of decaying organic matter in the water. The major sources of oxygen-demanding waste are biodegradable organic substances found in municipal wastewater, which is mainly human waste and food residue.

Other sources include industrial wastewater from the tanning, textile, paper and pulp, and food processing industries. Minor sources include crop residue, decaying plants and leaves, animal droppings, and the oxidation of certain inorganic compounds. The BOD is affected by temperature, nutrient concentrations, and the enzymes available for indigenous microbial populations.

Biochemical oxygen demand (BOD)

BOD is calculated as the number of milligrams of oxygen consumed per litre of water over five days at 20°C. A high BOD indicates a high level of decaying organic matter in the water. Dead organic matter in streams and rivers comes from natural sources (e.g. dead leaves from a forest), as well as from agriculture and urban sewage. Sewage is the major source of dead organic matter in most natural waters.

The presence of sufficient concentrations of dissolved oxygen is critical to maintaining the aquatic life and aesthetic quality of streams and lakes. Streams or rivers with rapidly moving water usually contain more dissolved oxygen than stagnant water due to aeration increases. As organic matter decays, bacteria present in the water reduce the amount of dissolved oxygen. Therefore, water that contains high levels of organic matter can quickly become oxygen-deficient, affecting the water quality and causing aquatic organisms to die.

Dissolved oxygen (DO) plays a crucial role in groundwater quality. It supports bacteria that break down pollutants and minimises harmful substances like iron and manganese. DO is also monitored in a wide range of industries and applications to maintain water quality. DO concentrations above 6.5-8 mg/L and between 80-120% are considered good water quality.

Eutrophication

Natural eutrophication occurs through the gradual accumulation of sediment and nutrients over time. However, human activities have significantly accelerated eutrophication through the discharge of limiting nutrients, such as nitrogen and phosphorus, into aquatic ecosystems. This is known as cultural or anthropogenic eutrophication.

Cultural eutrophication is caused by human activities such as the release of sewage, industrial wastewater, fertilizer runoff, and other nutrient sources into the environment. High concentrations of nitrogen and phosphorus from these sources can cause eutrophication, leading to rapid growth of aquatic plants, particularly algae, in a process known as an algal bloom. These blooms form thick mats that cover the surface of the water, blocking sunlight from reaching plants below, causing them to die.

The decomposition of the prolific algal layer consumes oxygen, creating very low oxygen levels in the water, known as hypoxia or dead zones. These zones can lead to the death of organisms that are unable to leave the environment, including fish, mammals, birds, and even humans. Approximately 50% of lakes in North America, Europe, and Asia are negatively impacted by cultural eutrophication, and the size and number of marine hypoxic zones have increased significantly in recent decades.

Drinking water quality

The absence of uniform international standards for drinking water quality allows for a risk-benefit approach to be taken when establishing national standards and regulations. This approach considers local or national environmental, social, economic, and cultural conditions. For example, the WHO issues guidance for developing national and regional drinking water standards, which countries can then adapt according to their specific needs and resources.

Drinking water contamination is a widespread issue, even in developed countries with established drinking water regulations, such as the US. Numerous chemical contaminants from anthropogenic and natural sources continue to pose chronic health concerns. Some of the contaminants of concern in drinking water include arsenic, disinfection by-products, fracking-related substances, lead, nitrate, per- and polyfluorinated alkyl substances (PFAS), and uranium.

One of the most critical indicators of drinking water quality is dissolved oxygen (DO) concentration. High DO levels improve the taste of drinking water but can also cause corrosion in water pipes. DO plays a crucial role in groundwater quality by supporting bacteria that break down pollutants and minimizing harmful substances like iron and manganese. DO is also essential for fish and other aquatic organisms, as low DO levels can lead to their death.

The presence of excessive nutrients, particularly nitrogen (N) and phosphorus (P), in water can lead to cultural eutrophication, characterized by rapid plant growth, especially algae. These algal blooms can create water pollution, damage aquatic ecosystems, and reduce oxygen levels in the water as they decompose, further endangering aquatic life. High levels of nitrogen in drinking water can also cause health issues in humans, including skin discolouration, increased heart rate, weakness, fatigue, and dizziness. Therefore, maintaining adequate DO levels in drinking water supplies is essential for both ecological and human health.

Aquatic life

Oxygen is not a water pollutant. On the contrary, dissolved oxygen (DO) is a vital indicator of water quality and plays a crucial role in groundwater quality. Most aquatic animals and plants require oxygen to survive. For example, fish cannot survive for long in water with dissolved oxygen less than 5 mg/L. A low level of dissolved oxygen in water is a sign of contamination and is an important factor in determining water quality, pollution control, and treatment processes.

The amount of oxygen used by aerobic bacterial decomposition of organic matter is called biochemical oxygen demand (BOD). A high BOD indicates a high level of decaying organic matter in the water. Dead organic matter in streams and rivers comes from natural sources, such as dead leaves from a forest, as well as from agriculture and urban sewage. Sewage is the major source of dead organic matter in most natural waters.

In the context of aquatic life, oxygen-demanding waste is an extremely important pollutant to ecosystems. As organic matter decays, bacteria present in the water reduce the amount of dissolved oxygen. Therefore, water that contains high levels of organic matter can quickly become oxygen-deficient, affecting the water quality and causing aquatic organisms to die. Streams or rivers with rapidly moving water usually contain more dissolved oxygen than stagnant water due to aeration increases.

Exponential growths of algae due to high concentrations of nitrogen and phosphorus from human sources (mostly agricultural and urban runoff) can cause cultural eutrophication, which involves the rapid growth of aquatic plants, particularly algae, called an algal bloom. When the prolific algal layer dies, it becomes oxygen-demanding waste, which can create very low O2 water (<2 ppm O2), called hypoxia or a dead zone, as it causes the death of organisms that are unable to leave that environment.

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