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تاريخ التسجيل : 15/09/2009
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|موضوع: 4-كتالوج اجهزة معمل تحاليل المياه والهندسة الصحية النظرى(الاكسجين المذاب) الأحد يناير 23, 2011 3:56 pm|| |
[b]chemical parameters of water
Carbon dioxide + Water --------------> Oxygen + Carbon-rich foods
CO2 H2O O2 C6H12O6
Dissolved Oxygen (DO) is the amount of oxygen dissolved in the water.
DO is a very important indicator of a water body's ability to support aquatic life. Fish "breathe" by absorbing dissolved oxygen through their gills.
Oxygen enters the water by absorption directly from the atmosphere or by aquatic plant and algae photosynthesis.
Oxygen is removed from the water by respiration and decomposition of organic matter.
The amount of DO in water depends on several factors, including temperature (the colder the water, the more oxygen can be dissolved); the volume and velocity of water flowing in the water body; and the amount of organisms using oxygen for respiration.
The amount of oxygen dissolved in water is expressed as a concentration, in milligrams per liter (mg/l) of water.
Human activities that affect DO levels include the removal of riparian vegetation,
runoff from roads, and sewage discharge.
• Water Quality Standard - Values below the standard (5 mg/l for freshwater) indicate poor water quality
Dissolved oxygen is measured in mg/L.
0-2 mg/L: not enough oxygen to support life
2-4 mg/L: only a few fish and aquatic insects can survive.
4-7 mg/L: good for many aquatic animals, low for cold water fish
7-11 mg/L: very good for most stream fish
DISSOLVED OXYGEN (DO)
Dissolved Oxygen (DO) is found in microscopic bubbles of oxygen that are mixed in the water and occur between water molecules. DO is a very important indicator of a water body's ability to support aquatic life. Fish "breathe" by absorbing dissolved oxygen through their gills. Oxygen enters the water by absorption directly from the atmosphere or by aquatic plant and algae photosynthesis. Oxygen is removed from the water by respiration and decomposition of organic matter.
Measurement of DO
Dissolved Oxygen can be measured with an electrode and meter or with field test kits.
The electronic meter does not measure oxygen directly; rather, it uses electrodes to measure the partial pressure of oxygen in the water, which is converted to oxygen mass weight concentration. The field test kits (such as a drop bottle, a microburet, or a digital titrator) involve adding a solution of known strength to a treated sample of water from the stream. The amount of solution required to change the color of the sample reflects the concentration of DO in the sample. The amount of oxygen dissolved in water is expressed as a concentration, in milligrams per liter (mg/l) of water.
Dissolved oxygen levels are also often reported in percent saturation. Temperature affects DO concentrations, and calculating the percent saturation will factor out the effect of temperature. The "saturation level" is the maximum concentration of dissolved oxygen that would be present in water at a specific temperature, in the absence of other factors. Scientists have determined the saturation DO level for various temperatures. Saturation levels also vary with elevation. Percent saturation is calculated by dividing the measured dissolved oxygen concentration by the saturation level and multiplying by 100.
This equation is shown as:
% Saturation = (DO / Saturation Level) x 100
The Importance of Dissolved Oxygen
• Dissolved oxygen is defined as the amount of oxygen that has dissolved into a body of water. The presence of oxygen in water is an important factor for sustaining life within a pond, lake, river or ocean. Without proper saturation of dissolved oxygen, fish and other life forms can't survive. There are three mechanisms that contribute to how much oxygen is dissolved within a specific body of water: diffusion, aeration and waste products from plants.
• Diffusion refers to the natural inclination of chemical substances to move from an area of high concentration to an area of low concentration. When gaseous oxygen in the atmosphere is of a higher concentration than dissolved oxygen in the water, diffusion occurs. This results in the displacement of oxygen into the water. Factors that increase atmospheric and barometric pressure of oxygen in the atmosphere--such as cold weather--result in increased levels of diffusion.
• The surface area of a body of water plays a key factor in dissolving oxygen. Simply put, additional aeration occurs as the surface area of a body of water increases. For example, a swift-moving river creates additional turbulence, splashes and ripples that combine to maximize the amount of water molecules that make contact with oxygen molecules in the atmosphere. This aeration process facilitates diffusion and results in additional dissolving of oxygen.
Factors Affecting DO
Volume and velocity of water flowing in the water body
In fast-moving streams, rushing water is aerated by bubbles as it churns over rocks and falls down hundreds of tiny waterfalls. These streams, if unpolluted, are usually saturated with oxygen. In slow, stagnant waters, oxygen only enters the top layer of water, and deeper water is often low in DO concentration due to decomposition of organic matter by bacteria that live on or near the bottom of the reservoir.
Dams slow water down, and therefore can affect the DO concentration of water downstream. If water is released from the top of the reservoir, it can be warmer because the dam has slowed the water, giving it more time to warm up and lose oxygen. If dams release water from the bottom of a reservoir, this water will be cooler, but may be low in DO due to decomposition of organic matter by bacteria.
The colder the water, the more oxygen can be dissolved in the water. Therefore, DO concentrations at one location are usually higher in the winter than in the summer.
During dry seasons, water levels decrease and the flow rate of a river slows down. As the water moves slower, it mixes less with the air, and the DO concentration decreases. During rainy seasons, oxygen concentrations tend to be higher because the rain interacts with oxygen in the air as it falls.
More sunlight and warmer temperatures also bring increased activity levels in plant and animal life; depending on what organisms are present, this may increase or decrease the DO concentration.
The type and number of organisms in the water body
During photosynthesis, plants release oxygen into the water. During respiration, plants remove oxygen from the water. Bacteria and fungi use oxygen as they decompose dead organic matter in the stream. The type of organisms present (plant, bacteria, fungi) affect the DO concentration in a water body. If many plants are present, the water can be supersaturated with DO during the day, as photosynthesis occurs. Concentrations of oxygen can decrease significantly during the night, due to respiration. DO concentrations are usually highest in the late afternoon, because photosynthesis has been occurring all day. For an example of how DO can vary from day to night, select here .
Oxygen is more easily dissolved into water at low altitudes than at high altitudes, because of higher atmospheric pressure.
Dissolved or suspended solids
Oxygen is more easily dissolved into water with low levels of dissolved or suspended solids. Waters with high amounts of salt, such as the ocean (which contains about 35 grams of salt for each 1000 grams of water) have low concentrations of DO. Freshwater lakes, streams, and tap water generally contain much less salt, so DO concentrations are higher. As the amount of salt in any body of water increases, the amount of dissolved oxygen decreases. An increase in salt concentration due to evaporation of water from an ecosystem tends to reduce the dissolved oxygen available to the ecosystem’s inhabitants.
Runoff from roads and other paved surfaces can bring salts and sediments into stream water, increasing the dissolved and suspended solids in the water.
Amount of nutrients in the water
Nutrients are food for algae, and water with high amounts of nutrients can produce algae in large quantities. When these algae die, bacteria decompose them, and use up oxygen. This process is called eutrophication. DO concentrations can drop too low for fish to breathe, leading to fish kills. However, nutrients can also lead to increased plant growth. This can lead to high DO concentrations during the day as photosynthesis occurs, and low DO concentrations during the night when photosynthesis stops and plants and animals use the oxygen during respiration. For an example of how DO can vary from day to night, select here.
Nitrate and phosphate are nutrients. Nitrate is found in sewage discharge, fertilizer runoff, and leakage from septic systems. Phosphate is found in fertilizer and some detergents.
Organic wastes are the remains of any living or once-living organism. Organic wastes that can enter a body of water include leaves, grass clippings, dead plants or animals, animal droppings, and sewage. Organic waste is decomposed by bacteria; these bacteria remove dissolved oxygen from the water when they breathe. If more food (organic waste) is available for the bacteria, more bacteria will grow and use oxygen, and the DO concentration will drop.
Directly downstream from where sewage effluent is discharged to a river, DO content often decreases, because of the increase in growth rate of bacteria that consume the organic matter contained in the effluent. The degree and extent of the DO "sag" depends on the Biological Oxygen Demand (BOD) of the effluent (how much oxygen the effluent can consume) (Giller and Malmqvist, 1998).
Shading tends to lower average summer temperature and reduce the daily duration of higher temperature. Removing trees reduces shade on the creek, allowing the sun to warm the water. This can affect DO concentrations in different ways. As mentioned above, in general, as water temperature increases, DO drops. Also, the bare soil exposed from removing the tree can erode, increasing the amount of dissolved and suspended solids in the water. This also leads to a decrease in DO concentrations. However, direct sunlight, along with increased nutrients can increase the growth rate of aquatic plants. These plants release oxygen to the water during the day, but then remove oxygen from the water at night. This can cause DO concentrations to become very high during the day, then very low during the night. For an example of how DO can vary from day to night, select here .
The amount of groundwater entering a river or stream can influence oxygen levels. Groundwater usually has low concentrations of DO, but it is also often colder than stream water. Therefore, groundwater may at first lower the DO concentration, but as groundwater cools the stream or river, the ability of the water to hold oxygen improves.