Statistics for Environmental Engineers

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Case Study: Ban on Phosphate Detergents

Wisconsin passed a law in 1978 that resulted in two interventions that were intended to change the amount of phosphorus entering into the environment. The law required that after July 1, 1979, household laundry detergents could contain no more than 0.5% phosphorus (P) by weight. Before this virtual ban on phosphate detergents went into effect, detergents contained approximately 5% phosphorus. In 1982, the phosphate ban lapsed and in July 1982 detergents containing phosphates started to reappear, although product reformulation and marketing changes had reduced the average detergent phosphorus content from pre-ban levels. A few years later, a new ban was imposed.

From the mid-1970s until the mid-1980s, there was controversy about how much the ban actually reduced the phosphate loading to wastewater treatment plants. Of course, the detergent manufacturers knew how much less phosphate had been sold in detergents, but confirmation of their estimates from treatment plant data was desired. Making this estimate was difficult because the potential reduction was relatively small compared to the natural random fluctuations of the relevant environmental series.

The largest treatment plants in Wisconsin are the Jones Island and South Shore plants in Milwaukee. Both plants have expert management and reliable measurement processes. The combined mass load of influent phosphorus to these two plants is plotted in Figure 54.1. The Wisconsin phosphate detergent ban stretched from July 1979 to June 1982, and indeed the lowest phosphorus concentrations are recorded during this period. The record shows a downward trend starting long before the imposition of the ban, and it is not obvious from the plot how to distinguish the effect of the ban from this general trend. If an average value for the ban period is to be compared to a pre-ban average, the difference would depend heavily on how far the pre-ban average is extended into the past.

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