An article in Journal of Hazardous Materials [“Toxicity Assessment from Electro-Coagulation Treated-Textile Dye Waste Waters by Bioassays,” 2009, Vol. 172(1), pp. 330–337] discussed a study of pollutant removal from textile dyeing waste water with an electro-coagulation technique. Chemical oxygen demand (COD) (a common measure of water pollution) was used as the response, and three different values for electrolysis time were considered. The following data were extracted from a larger study. Suppose that a randomized complete block experiment was conducted with three blocks based on initial pH values.

(a) Is there an effect of electrolysis time at α = 0.05? Calculate the P-value.

(b) Analyze the residuals from the experiment.

(c) Calculate a 95% confidence interval on mean COD removal when the electrolysis time is 15 minutes.

(d) Perform an ANOVA assuming that all data are collected at a single pH value. Comment on differences from part (a).

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Initial pH Electrolysis time (min) 3 7 11 15 77.1 75.2 42.2 30 80.1 76.8 45.0 45 82.8 75.2 46.8