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What is Biological oxygen demand and how does it affect water quality?

Chemicals…oxygen…demand…what do all these words mean and what does it have to do with your water? For a quick look into the world of biochemical oxygen demand just have a look below. Biochemical oxygen demand is a measure of the quantity of oxygen used by microorganisms (e.g., aerobic bacteria) in the oxidation of organic matter. Natural sources of organic matter include plant decay and leaf fall. However, plant growth and decay may be unnaturally accelerated when nutrients and sunlight are overly abundant due to human influence. Urban runoff carries pet wastes from streets and sidewalks; nutrients from lawn fertilizers; leaves, grass clippings, and paper from residential areas, which increase oxygen demand. Oxygen consumed in the decomposition process robs other aquatic organisms of the oxygen they need to live. Organisms that are more tolerant of lower dissolved oxygen levels may replace a diversity of natural water systems contain bacteria, which need oxygen (aerobic) to survive. Most of them feed on dead algae and other dead organisms and are part of the decomposition cycle. Algae and other producers in the water take up inorganic nutrients and use them in the process of building up their organic tissues.
Consumers like fish and other aquatic animals eat some of the producers, and the nutrients move up the food chain. When these organisms die, bacteria decompose the organic compounds and release into the water inorganic nutrients such as nitrate, phosphate, calcium, and others. Some of these nutrients end up down stream or in sediments, but most of them recycle again and again. Most of the bacteria in the aquatic water column are aerobic. That means that they use oxygen to perform their metabolic activities of decomposition. Remember that we learned in other related exercises that under normal conditions, dissolved oxygen exists in very low concentrations. Natural levels of oxygen in aquatic systems are always somewhat depleted by normal levels of aerobic bacterial activity. In most cases, if dissolved oxygen concentrations drop below 5 parts per million (ppm), fish will be unable to live for very long. All clean water species such as trout or salmon will die well above this level and even low oxygen fish such as catfish and carp will be at risk below 5 ppm.
When abnormally high levels of aerobic bacterial activity takes place, however, the level of dissolved oxygen can drop dramatically. Under what circumstances does this happen? Generally, this occurs when there is some sort of abnormal "pollution" introduced into the system. This can occur in the form of organic pollution for sources such as domestic sewage, septic tank leakage, and fertilizer runoff, or could be in the form of inorganics from domestic or industrial sources. Natural sources of organic compounds can also come into aquatic systems by means of floods, landslides, and erosion.
One of the most important nutrients, which affected BOD in aquatic systems in the recent past is phosphate pollution from American households. It was discovered decades ago that the addition of phosphorous to soaps and detergents made them clean better. By the 1960's, millions of households and businesses were dumping tons and tons of phosphate down the drain. Eventually, much of this important nutrient made its way to the watercourses of America. Because phosphorous is one of the most important limiting factors (necessary nutrients) in aquatic systems, there began numerous and widespread algal blooms. Algal blooms are dramatic population outbursts of growth in which often one or two species of algae suddenly find the conditions right for rapid growth.
Because most unicellular algae reproduce asexually by rapid cell division, it doesn't take long for a species of algae to suddenly and literally turn the water green with billions and billions of new cells. Because the conditions necessary to these algal blooms are sometimes temporary or because the algae exceed the threshold level of some other limiting factor, the blooms are only temporary. They often last only a few days. What happens when the bloom is over? The algal cells don't have enough nutrients and most of them die. At this point, the aerobic bacteria become important and start to decompose the algae. Because there is so much food for them, they also experience a sort of bloom, and they literally suck the oxygen out of the water. When the oxygen is gone, the bacteria and most other aerobic creatures in the aquatic system start to die.
During the 1960's and the 1970's, this phenomenon was widespread with dramatic fish kills and large segments of slow-moving rivers and lakes becoming almost abiotic (lifeless) because of high BOD caused by pollution. The procedures followed in this exercise involve the collection of water and the measurement of dissolved oxygen and pH at the time of the collection. The samples are placed in bottles full to the brim and sealed off by a lid. The sample bottles are covered completely with aluminum foil and placed in a dark place. This limits the photosynthesis, which could happen with captured algae. After five days, the bottles are uncorked and the dissolved oxygen is probed. The difference between the first and the last of the samples is called the BOD. A low number generally means little pollution and/or little aerobic activity. A high BOD means the opposite. 

source: http://www.freedrinkingwater.com/water_quality/quality1/1-bod-effects-on-water-quality.htm

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