Removing nitrogen from toxic wastewater. Highly toxic wastewater is produced during the manufacturing of dry-spun acrylic fiber. One way to lessen toxicity is to remove the nitrogen from the wastewater. A group of environmental engineers investigated a promising method—called anaerobic ammonium oxidation—for nitrogen removal and reported the results in the Chemical Engineering Journal (April 2013). A sample of 120 specimens of toxic wastewater was collected and each specimen was treated with the nitrogen removal method. The amount of nitrogen removed (measured in milligrams per liter) was determined, as well as the amount of ammonium (milligrams per liter) used in the removal process. These data (simulated from information provided in the journal article) are saved in the file. The data for the first 5 specimens are shown below. Consider a simple linear regression analysis, where y = amount of nitrogen removed and x = amount of ammonium used.

a. Assess, statistically, the adequacy of the fit of the linear model. Do you recommend using the model for predicting nitrogen amount?

b. Find a 95% prediction interval for nitrogen amount when amount of ammonium used is 100 milligrams per liter. Practically interpret the result.

c. Will a 95% confidence interval for the mean nitrogen amount when amount of ammonium used is 100 milligrams per liter be wider or narrower than the interval, part b? Verify your answer by finding the 95% confidence interval for the mean.

d. Will a 90% confidence interval for the mean nitrogen amount when amount of ammonium used is 100 milligrams per liter be wider or narrower than the interval, part c? Verify your answer by finding the 90% confidence interval for the mean. (first 5 observations of 120 shown) Nitrogen Ammonium 18.87 67.40 17.01 12.49 23.88 61.96 10.45 15.63 36.03 83.66