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EVALUATION OF WATER QUALITY MODELLING PARAMETERS-TOWARDS THE EVOLVEMENT OF REAERATION COEFFICIENT FOR RIVERS IN NIGERIAN ENVIRONMENT
1.1 Background of Study
When a lake or river becomes stratified, many changes in the physical, chemical and biological characteristics of the water can occur.
These changes are often undesirable for many reasons, in the hypolimnion (lower layer) of the stratified lake, oxygen levels are lower, nutrient concentration higher and temperature less than in the epilimnion (upper layer). Water withdrawn from the hypolimnion for domestic and industrial usage needs more treatment to remove nutrients and add oxygen. Treatment costs are thus higher. Reducing conditions present in the hypolimnion change the chemistry: oxidation products are no longer released and instead large volumes of hydrogen sulphide and methane may be produced; anaerobic conditions may prevail such that aerobic bacteria and respiring organisms are replaced by anaerobic bacteria; the rate of release of phosphates from the sediments is increased by a large percentage.
Oxygen transfer at the surface of lakes and streams is an effective process for the environmental quality of the aquatic ecosystem; in fact, oxygen transferred from the atmosphere by means of natural reaeration replaces the amount consumed due to oxidation of organic mater discharged into the water body. Otherwise, if the dissolved oxygen (DO) levels drops below acceptable values, aquatic ecosystem health could be seriously impaired and desirable uses of water resources could be precluded. Therefore, water quality standards and criteria for DO are provided by environmental regulation of many countries, such as USA, UK, Germany, Italy and Japan; in Italy,D.L.152/1999 has recently established for streams and lakes a classification system which considered
Dissolved Oxygen (DO) as key water quality parameters (Ciaravino and Gualtieri; 1999). General speaking, DO levels are the result of an interaction among processes, i.e., sources and sinks which affect DO concentration; the first one, such as atmospheric reaeration and photosynthetic production, tends to increase oxygen levels, while the second ones, such as oxidation of carbonaceous and nitrogenous wastes material, respiration of aquatic plants and Sediments Oxygen Demand (SOD) produce a decrease of DO concentration; each one of these phenomena can be expressed through a different kinetic expressions in order to quantify its effect. Reaeration, that is, the physical absorption of oxygen from the atmosphere by water, is the most relevant natural means by which a water body may recover DO concentration; thus, reaeration amount should be carefully estimated. Several empirical equations have been proposed in literature and some of these ones are widely applied in water quality studies, but recent investigations have demonstrated that they generally result in a poor fit with field data (Melching and Flores, 1999; Gualtieri and Ciaravino,1999). In fact, most of the equations are derived from relatively small set of laboratory or field data for a relatively localized group of streams; so that, if they are applied to other field data different from which they were developed, they provide a poor estimation; moreover, most of them were developed using field data obtained by the dissolved oxygen-balance, which is generally affected by high errors. Thus, none of available equations appears to be applicable to all steam hydrodynamic conditions, but, on the contrary, they remain stream-specific, probably since some parameters involved in this process have been neglected in their formulation.
Furthermore, oxygen dissolves by diffusion from the surrounding air; aeration of water that has tumbled over falls and rapids; and as a waste product of photosynthesis. A simplified formula is given below:
Photosynthesis (in the presence of light and chlorophyll)
Carbon dioxide + Water Oxygen + Carbon-Rich foods
CO2 H2O O2 C6H12O6
Fish and aquatic animals cannot split oxygen from water (H2O) or other oxygencontaining compounds. Only green plant and some bacteria can do that through photosynthesis and similar processes. Virtually all the oxygen we breathe is manufactured by green plants. A total of three-fourth of the earth‟s oxygen supply is produced by photosynthesis in the ocean.
If water is too warm, there may not be enough oxygen in it. When there are too many bacteria or aquatic animals in the area, they may overpopulate, using DO in great amount. Oxygen levels also can be reduced through over fertilization of water plants by runoff from farm fields containing phosphates and nitrates (the ingredients of fertilizer). Under these conditions, the numbers and sizes of water plants increase. Then, if the weather becomes cloudy for several days, respiring plants will use much of the available DO. When these plants die, they become foods for bacteria, which in turn multiply and use large amount of oxygen and thus depleting the oxygen. How much DO an aquatic organism needs depends upon its species, its physical state, water temperature, pollutants present, and so on.
Consequently, it is impossible to accurately predict minimum DO levels for specific fish and aquatic animals. For example, at 50C (41oF), trout uses about 50-60 milligram (mg) of oxygen per hour, at 25oC (77oF), they may need five times that amount. Fish are coldblooded animals. They use more oxygen at higher temperatures because their metabolic rates increase.
Numerous scientific studies suggest that 4-5 parts per million (ppm) of DO is the minimum amount that will support a large, diverse fish population. The DO levels in good fishing waters generally averages about 9.0 parts per million (ppm).
The environmental impact of dissolved gas is as explained below. Total dissolved gas concentration in water should not exceed 110 percent. Concentration above this level can be harmful to aquatic life. Fish in waters containing excessive dissolved gases may suffer from “gas bubble disease”; however this is a very rare occurrence. The bubbles or emboli block the flow of blood through blood vessels causing death of the aquatic organisms. External bubbles (emphysema) can also occur and be seen on the fins or skin and on other tissues. Aquatic invertebrates are also affected by gas bubbles disease but at level higher than those lethal to fish.
Adequate dissolved oxygen is necessary for good water quality. Oxygen is a necessary element to all forms of life. Natural streams purification processes require adequate oxygen levels in order to provide for aerobic life forms. As dissolved oxygen levels in water drops below 5.0mg/l, aquatic life is put under stress; the lower the concentration, the greater the stress. Oxygen levels that remain below 1-2mg/l for a few hours can result in a large fish kill.
Biologically speaking, however, the level of oxygen is much more important measure of water quality than faecal coliform. Dissolved oxygen is absolutely essential for the survival of all aquatic life.
Moreover, oxygen affects a vast number of other water indicators, not only biochemical but esthetic ones like odour, clarity and taste. Consequently, oxygen is perhaps the most well established indicator of water quality.
Dissolved oxygen affects water quality in that a high DO in community water supply is good because it makes drinking water tastes better. However, high DO levels speeds up corrosion in water pipes, for this reason, industries use water with the least possible amount of dissolved oxygen.
Several empirical equations have been proposed in literature and some of these ones are widely applied in water quality studies, but recent investigations have demonstrated that they generally result in a poor fit with field data. Most of these equations were derived from a relatively small set of laboratory or field data for relatively localized groups of streams; so that, if they are applied to other field data different from which they were originally developed, they provide a poor estimation. Moreover, most of them were developed using field data obtained by the dissolved oxygen-balance which is generally affected by high errors. Thus none of the available equations appears to be applicable to all stream hydrodynamic conditions. (Melching and Flores, 1999; Ciaravino and Gualtieri, 1999).
The objectives of the research are as follows:
Estimation of the reaeration coefficient of a river is essential for
The scope of the research is limited to investigation and data collection with regard to
Adada river only within the boundary between Aku in Igbo-Etiti and Nkpologu in UzoUwani Local Government Area, both in Enugu State using only two hydraulic
parameters: the mean flow rate, U (m/s) and hydraulic radius or depth. This study will not consider the effect of photosynthesis, over fertilization of water plant by run-off from the farm fields containing phosphate and nitrates, weather and climatic condition, oxidation of carbonaceous and nitrogenous waste materials and respiration of aquatic plant on the reaeration coefficient (K2)
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