Water Quality Lab -- TOPS



Part 1


How can adding nutrients or heat to water change it as a habitat?


In this lab you will compare the productivity of a yeast culture at varying nutrient concentrations.

If partially treated sewage and other wastes are placed into water, they serve as food for microorganisms. The population of decomposers increases as the quantity of nutrients increases. The increase in microorganisms will change water in ways you will discover by doing this lab. When microorganisms consume organic matter in a water solution, they also use oxygen in the water, just as people use oxygen to metabolize their nutrition. The amount of oxygen needed for all of the living things in an ecosystem is called the Biological Oxygen Demand, or BOD. As the number of organisms in a system increases so does the BOD.

When we circulate water from a lake, river, or the ocean, through an industrial plant to help cool equipment, the water is placed back into its natural source at a higher temperature than when it left. Increasing water temperature also changes the water as a dwelling place for organisms. You will look at some environmental factors (abiotic) that are altered as the water temperature rises.

As the temperature of a liquid rises, its ability to hold dissolved gas decreases. This is what happens to a soft drink. A warm "Coke" goes flat faster than an ice cold "Coke." An "Alka-seltzer" will "bubble" faster in a glass of hot water than in a glass of cold water. Similarly, cold water can hold more of the gas produced by the "Alka-seltzer" than warm water. Oxygen is a gas and more of this gas can be dissolved in cold water than warm water. Refer to graph #1.

Optional: When a gas dissolves in a liquid heat is normally given off. This is an exothermic process. The kinetic energy of the rapidly moving gas particles is lost to the environment as the gas enters the more structured world of the liquid. As heat is added to this solution the dissolved gas particles can absorb the energy by moving faster and so be more likely to evaporate from the surface &emdash; an endothermic process. This corresponds to Le Chatelier's principle &emdash; When a system in a state of dynamic equilibrium is acted on by some outside stress, the system will, if possible, shift to a new position of equilibrium to reduce the effect of the stress.

Your test solution represents polluted water that is being "cleaned" by the action of a microorganism, yeast. In order to remove the pollutant (milk), the yeast consumes some of the dissolved oxygen (D.O.) from the solution. The amount of D.O. consumed depends on the pollutant concentration. The difference between the initial D.O. concentration and the D.O. minimum is a measure of the amount of D.O. needed to degrade the pollutant, and this value is referred to as the biological oxygen demand or BOD. The more organic matter in the solution, the more oxygen is demanded by microorganisms to remove it, and the greater the BOD.

Methylene blue is an indirect indicator for oxygen. During respiration, as the yeast consumes the available food, oxygen is also consumed and carbon dioxide is produced. The blue color disappears as the oxygen concentration drops and the carbon dioxide level rises. Carbon dioxide combines with water to form carbonic acid which changes the indicator color.