Here are the answers to the Oct. 8, 2020 edition of TPO's Exam Study Guide:

1. What type of trickling filter is best used for BOD reduction, but not for reporting purposes?

The answer to this question is D. High-rate trickling filters are typically loaded at max organic loading capability. This rate may reach levels of 100 pounds/BOD/1,000 cubic feet. Due to high amounts of flow through this filter, some short circuiting and bleed-through is common, as these units are rarely used for strict discharge standards. High-rate trickling filters commonly use plastic media to account for the high hydraulic loading rates. Trickling filters are a major form of secondary treatment and knowledge of their characteristics is key for certification standards.

2. Using the data in the table, what is the 90th percentile concentration for lead?

The correct answer is A. Determining the 90th percentile concentration for lead is based off the number of samples collected. For this example, place all of the concentrations in order from smallest to largest and then multiply 0.9 by the number of samples collected. Using the number yielded from the multiplication, identify which sample concentration represents the 90th percentile. If the number yielded is not whole (6 samples X 0.9 = 5.4), additional calculations must be performed to determine the 90th percentile concentration level between sample 5 and sample 6.

6 samples X 0.9 = 5.4 (not a whole number)

15.4 ug/L (sample 6) – 11.1 ug/L (sample 5) = 4.3 ug/L

4.3 ug/L X 0.4 = 1.72 ug/L

1.72 ug/L + 11.1 ug/L = 12.82 ug/L

90th percentile lead concentration = 12.82 ug/L

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