Understanding Pollutants: Impact On Dissolved Oxygen Levels

Dissolved oxygen (DO) is a vital indicator of water quality and plays a critical role in the survival of aquatic life. DO refers to the concentration of oxygen gas dissolved in water, which is essential for the respiration of aquatic organisms such as fish, invertebrates, and plants. The presence of pollutants in water bodies can significantly impact DO levels, affecting water quality and the health of aquatic ecosystems. Human activities, such as agricultural runoff, industrial waste discharge, and urban development, are major contributors to the introduction of pollutants that affect DO. These pollutants can lead to oxygen-deficient conditions, known as eutrophic or hypoxic zones, which can result in the decline or death of aquatic life. Understanding and managing these pollutants are crucial for maintaining healthy aquatic ecosystems and ensuring clean water supplies for human use.

High temperatures

DO refers to the concentration of oxygen gas incorporated in water, which is essential for the growth and reproduction of aquatic life. It is a critical indicator of water quality, and high temperatures can negatively affect this. When water temperatures rise, the gas and water molecules gain more energy, breaking the weak molecular interactions between them and causing oxygen to escape. This results in decreased solubility of oxygen in water, which increases water pollution and harms aquatic habitats and organisms.

The consequences of high temperatures and low DO levels can be severe. Aquatic plants and animals cannot tolerate DO levels below 3 mg/L, and levels below 1 mg/L are considered hypoxic, leading to the death of fish and disruption of aquatic ecosystems. Low DO levels also affect the life stages of fish embryos, including the number of eggs that hatch and larval development.

Additionally, high temperatures and low DO levels impact water treatment systems. In wastewater treatments, low DO concentrations allow filamentous growth to take over, killing beneficial microbes. Furthermore, drinking water systems are affected as low DO levels allow minerals to dissolve into the water, impacting taste and quality.

To address low DO levels due to high temperatures, aeration methods can be employed, such as using aerators, diffusers, or pumps to increase water movement and enhance gas exchange. Adding rooted aquatic plants can also promote photosynthesis and increase oxygen levels. However, it is crucial to monitor DO levels closely to avoid supersaturation, as extremely high oxygen levels can also be detrimental to aquatic life.

Nutrient run-off

The effects of nutrient run-off on dissolved oxygen levels can vary with depth. In deep water, dissolved oxygen levels are often depleted as photosynthesis from surface algae is limited. Additionally, when algae cover the water's surface, they block light from penetrating deeper, causing the algae to die and sink to the bottom, further reducing oxygen levels as they decompose.

To address the issue of nutrient run-off, it is crucial to implement effective wastewater treatment practices and promote sustainable farming and landscaping methods that minimise the use of fertilisers and reduce organic waste. These measures will help maintain healthy dissolved oxygen levels in water bodies, ensuring the survival of aquatic ecosystems and the safety of drinking water for human consumption.

Sediment build-up

• Decreased Light Penetration: Sediment particles in the water reduce the amount of light that penetrates the water. This affects the photosynthetic activity of aquatic plants, leading to lower oxygen production.
• Increased Water Temperature: Sediment particles absorb warmth from the sun, raising the water temperature. Higher water temperatures decrease the solubility of oxygen, resulting in lower dissolved oxygen levels.
• Oxygen Consumption during Decomposition: Excess organic matter in eroded sediments contributes to oxygen depletion as it decomposes. This process reduces the amount of oxygen available for aquatic organisms.
• Interference with Fish Reproduction: Settling sediments can bury and suffocate fish eggs, impacting fish populations. Additionally, suspended sediments can dislodge plants and invertebrates, affecting the food sources of fish.
• Toxic Compound Transport: Sediment can carry toxic agricultural and industrial compounds, such as heavy metals and pesticides, into water bodies. These compounds can be released into the habitat, causing abnormalities or death in fish populations.
• Increased Erosion and Instability: Sediment loads in waterways can lead to further erosion and instability of stream banks, resulting in wider and shallower channels. This, in turn, contributes to warmer water temperatures, which affect oxygen solubility.
• Reduction in Turbulence: Sediment build-up can reduce turbulence in water, particularly when vegetation along the banks of surface waters is removed. This decrease in turbulence leads to reduced aeration and lower dissolved oxygen levels.

It is important to address sediment build-up to maintain healthy aquatic ecosystems and ensure sufficient dissolved oxygen levels for aquatic life. Implementing sediment control plans, minimizing land disturbance, and utilizing techniques like silt fences and sediment traps can help mitigate the negative impacts of sediment build-up on dissolved oxygen levels.

Ammonia

The presence of ammonia in water exerts a biochemical oxygen demand as bacteria and other microbes oxidize it into nitrite and nitrate. This process consumes DO, leading to reduced species diversity and, in some cases, fish kills. Ammonia can also contribute to heavy plant growth, or eutrophication, due to its nutrient properties.

In water treatment processes, ammonia is a critical parameter to monitor. High levels of ammonia in drinking water can lead to health issues such as bladder cancer and reproductive problems. Therefore, reducing total organic carbon and nitrogen levels in untreated drinking water is a priority for water treatment facilities.

To ensure effective nitrification in wastewater treatment, maintaining a minimum DO level of 1.5 mg/L is essential. Complete nitrification of ammonium ions to nitrate requires approximately 4.6 kg of oxygen for every kg of ammonium ions. Additionally, nitrifiers, the bacteria responsible for nitrification, are obligate aerobes, meaning they require free molecular oxygen to survive.

In summary, ammonia is a significant pollutant that affects dissolved oxygen levels in waterbodies. Its presence has detrimental effects on aquatic life and ecosystems, and its management is crucial for maintaining water quality and protecting human health.

Organic waste

The presence of organic waste in water suggests low DO levels, as indicated by certain characteristics. The water may exhibit a grayish hue with visible sludge deposits. Additionally, there may be a bad odor, resembling rotten eggs or sour cabbage, which is indicative of low oxygen conditions. The color of the water may also change, ranging from light green to pea-soup green, brown, gray, or black. These color changes are due to the presence of dark sediments caused by metal sulfides, which indicate anoxic conditions.

The decomposition of organic waste by microorganisms has a significant impact on DO levels. As organic matter breaks down, it consumes oxygen, leading to a decline in DO. This is particularly notable in eutrophic lakes, where excessive nutrients from fertilizers and agricultural runoff cause excessive plant growth. When these plants die, they are decomposed by bacteria and fungi, resulting in increased oxygen demand.

The addition of organic waste from sewage and manure, as well as organic fibers from textile and paper processing, contributes to the BOD. The microorganisms responsible for decomposing this waste utilize oxygen, further decreasing DO levels.

To maintain healthy aquatic ecosystems and ensure water quality, it is crucial to manage and reduce organic waste. This can be achieved through proper waste disposal, wastewater treatment, and by minimizing the use of fertilizers and other sources of excess nutrients.

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