A symbiotic system exits whereby all aquatic organisms depend on themselves for survival. Most of the dissolved oxygen (DO) in water comes from photosynthesis and atmosphere, about 8 10mg/l is needed to maintain 100% saturation in water. However, the level of dissolved oxygen in water varies, the amount or concentration of oxygen (O2) rises from morning through o evening, this is as a result of serious activity by plants in water. At this time photosynthesis is taking place and so oxygen (O2) is given off, whereas at night, the level of oxygen drops because photosynthesis has stopped.
Nonetheless, plants and animals continue to consume oxygen. The level of oxygen drops, this drop may bring down the level of to about 4mg/l and this is the minimum amount that is required to sustain the living organisms in water. Man and nature contribute significantly to the level of dissolved oxygen (DO) in water both in a positive and negative way. Nature The levels of dissolved oxygen in water can greatly be affected by weather conditions, these are; temperature, pressure, erosion, sedimentation and ice cover.
These factors affect the solubility of oxygen (O2) in water. An increase in temperature reduces the amount of dissolved oxygen. As the temperature increases the saturation concentration decreases (Gray N. F 1999). Table (1) shows the relationship between dissolved oxygen in water and temperature at 1 atmosphere. Temperature (oC) Dissolved Oxygen (mg/l) 0
Table (1) source; Gray N. F 1999 pg 67 From table (1) above it can be observed that as water gets warmer, there is a reduction in dissolved oxygen (DO), this is because the oxygen molecule becomes energised and diffuses to the water surface thereby leaving fewer dissolved oxygen in water. Dissolved minerals in water could be as a result erosion, sedimentation and weathering. Dissolved salt in water reduces the concentration of dissolved oxygen in water; water being a universal solvent dissolves salt.
Oxygen is used up to form other compounds as shown below; SiO2(s) + 2H2O Si(OH)4 (1) Fe2O3(s) + 3H2O Fe 2O3. 3H2O(s) (2) CaO(s) +H2O Ca(OH)(aq) (3) CaCO3(s) + CO2 + H2O Ca2+(aq) + 2HCO3- (4) (Equations from Harrison R. M et al 1996) When these solid minerals dissolve in water, oxygen is used up to another compound. Man The quest by man for a better living has in a way impinged on the quality of water. To improve agricultural yield, fertilizer is added to plant root for growth, however, plants take up few amounts and the rest is washed into river and or lake.
Nitrogen and phosphorus are the major constituents of fertilizer, if in excess, both elements in a compound form causes eutrophication thereby reducing the amount of dissolved oxygen in water. In developed countries treated sewage is disposed off into river, this waste contains microorganisms, detergents, and other waste product. If untreated or regulated these causes severe reduction of dissolved oxygen in a water body. Sewage contribute large amount of nutrients into the river, plants and microorganisms use up these nutrients, rivers or lake that contains essential mineral nutrient may support heavy growth of algae (Manaham S.
E 1993). Biochemical Oxygen Demand (BOD) In water dissolved oxygen (DO) place an important role in maintaining a balance as enumerated above, for instance, when oil or for that matter any substance that is a stranger is introduced into a water body, some biodegradation will take place to break down the hydrocarbon molecules, usually oxygen is the fuel that is used up or consumed to carry out this breakdown of the hydrocarbon molecules. Biodegradation can be chemical reaction or biological that is caused by living organisms like; bacteria, fungi, sulphate reducing bacteria etc.
The amount of oxygen required to completely breakdown the hydrocarbon molecules by chemical reaction is called the Chemical Oxygen Demand (COD), this is a measure of the amount of oxygen required to breakdown the molecules. Also the amount of oxygen required to breakdown completely the hydrocarbon molecule by biological activity is called the Biochemical Oxygen demand (BOD). However, in these report Biochemical Oxygen Demand (BOD) will be focused on rather than Chemical Oxygen Demand (COD).
Biochemical Oxygen Demand (BOD) can simply be defined as the amount of oxygen used up by microorganism (e.g. aerobic bacteria) in water. The rate at which oxygen is used up is perhaps more important than the determination of dissolved oxygen (Pierce J. J et al¦ 1997). How much clean a water is can well be determined by the amount of BOD, this is because the amount of oxygen present is determined and also the amount available for both plants and organisms to use up and maintain a balance ecosystem. Increase of biochemical oxygen demand in a water body can be caused by; * If there is high level of organic pollutant High level of nutrients.
Increase in BOD causes species or microroganisms that are sensitive to lower dissolved oxygen to be replaced by organisms that are more tolerant to low dissolved oxygen, these results in a shift in the ecosystem. Organic and nitrogenous compounds are responsible for high levels of BOD, basically biochemical oxygen demand is divide into carbonaceous and nitrogenous oxygen demand. Aerobic organisms utilize organic and nitrogenous nutrients and these processes require high amount of dissolved oxygen. This is represented thus; Organic C6H2O6 + 6O2(aq) + bacteria 6CO2 (aq) + H2O (5) Nitrogenous.
COHNS + O2 + bacteria CO2 + NH3 + energy (6) From equations 5 and 6, it is obvious that oxygen is consumed to the detriment of the water body. Factors that causes high Biochemical Oxygen Demand Organic matters are the source of high biochemical oxygen demand, the sources of these contaminant are, industrial effluents, leaves, dead plants, animal manure, sewage treatment works, urban runoff and agricultural runoff. However, in Nigeria the most significant contribution of organic compounds in water are the petroleum and petrochemical industries, oil exploration activities are being carried out.