Answer to problem 4

4.The town you live in is concerned that the local lake is eutrifying. As a result, The town council is considering upgrading the local sewage plant from secondary some form of tertiary treatment. Which piece of information would be most helpful to you right now (assume that the existing plant works fine):
  1. Phosphorus, not nitrogen is the limiting nutrient in the lake ecosystem.
  2. The kind of aquatic life that naturally inhabit the lake
  3. The strength of the influent to the sewage plant.
  4. The pH of the lake.

A is the correct answer. Although B through D are interesting parameters that may need to be addressed at some point, the limiting nutrient will determine what kind of tertiary treatment is appropriate. Recall that phosphorus removal is also a kind of tertiary treatment. If phosphorus were the limiting nutrient for the lake, denitrification might not solve the problem.

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