How to Remove Phosphate From Waste Water

Phosphate that contaminates waste water is in part responsible for a process known as eutrophication. Eutrophication causes excess plant growth, which slowly depletes the oxygen available in water. An example of this process is the growth of algae in ponds polluted by fertilizer run-off.Calcium hydroxide is used to separate phosphorus from solution, forming a precipitant. This process is known as precipitation. The precipitant is then it is filtered from the waste water. This procedure makes for a great classroom or home experiment. Separating phosphate from waste water is a common water treatment procedure. Using chemicals for the process is convenient since phosphate can be immediately eliminated from the water.

Things You'll Need

• Calcium hydroxide
• Waste water
• Filter paper
• Filter
• Water sample bottle
• Labels and pen
• Glass stirring pen
• 250-ml flasks
• Gram scale
• Watch glass
• Hot plate

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Instructions

• 1

  Collect water samples from a pond, lake, or waste water treatment center. Call the waste water treatment center ahead of time and ask for permission to take water samples. Place the water in sample bottles and label them according to the date, time and place at which the samples were taken.

• 2

  Filter solids from the waste water using a paper lab filter. The phosphorus remains dissolved within the water during this filtration. Collect the filtered water for the next portion of the experiment.

• 3

  Pour 200 ml of each water sample into a 250-ml flask, and label accordingly.

• 4

  Add calcium hydroxide to the water in 0.2-gram increments, while stirring. Keep adding calcium hydroxide in 0.2-gram increments, while stirring, until precipitate no longer forms. Precipitate is a solid that appears when it becomes insoluble in solution. When the calcium hydroxide reacts with the phosphorus, it produces water and phosphorus pentoxide. Calcium hydroxide is available at fish supply stores, and is commonly found in high school or college laboratories. It can also be purchased online from a scientific supply company. The chemical typically costs about $10 per pound.

• 5

  Filter the phosphorus pentoxide precipitate from the water using a paper lab filter. Collect the filtered water. Extract any remaining precipitate by adding more calcium hydroxide to the filtered water. If precipitation occurs, this means some phosphorus still remains dissolved in the sample of water. Repeat step 4 if this occurs, and re-filter to obtain the remaining precipitant.

• 6

  Dry the recovered phosphorus pentoxide from the waste water by placing the filter paper on top of a watch glass. A watch glass is a circular piece of glass commonly used in chemistry for holding solids while weighing, evaporating liquids, and as a beaker cover. Place a 250-ml flask filled with water onto a hot plate, and adjust the heat to boiling temperature. Place the watch glass over the 250-ml flask and carefully lay the filter paper with the wet phosphorus pentoxide over the watch glass. Allow the steam to fully dry the the phosphorus pentoxide. It should be completely dry five minutes after it has a dry appearance.

• 7

  Weigh the recovered phosphorus pentoxide. Compare the amount recovered between the different water samples to determine how much phosphorus was present in each water sample.