Here are the answers to the Nov. 30, 2021 edition of TPO's Exam Study Guide:

A digester is being fed with sludge that is 60% volatile solids. The sludge withdrawn is 40% volatile solids. What is the percent reduction in volatile solids?
The answer to the question is C, 55.6%. The formula for reduction of volatile solids percentage is: Volatile Solids In - Volatile Solids Out / Volatile Solids In - (Volatile Solids In x Volatile Solids Out) x 100%.

The key to successfully solving this math problem is to remember to change the percentages to decimal form before doing any operations. This is a math problem very typical of digesters and is found on many certification examinations. More information may be found in the Association of Boards of Certification Formulas, which is found in many textbooks and guides.

How many repeat bacteriological (total coliform) samples should an operator collect, if a routine bacteriological sample is total coliform positive?
The correct answer is C, three. Routine and repeat bacteriological samples are collected from approved sample site locations throughout the distribution system and analyzed for total coliform. Typically, systems monitor total coliform every month by collecting the appropriate number of routine samples throughout the sampling period. If a routine sample tests positive for total coliform, the operator must collect three repeat samples within 24 hours of being notified. Repeat samples must be collected from the original site within five service connections downstream, and within five service connections upstream, from the original site. An alternative sample location can be proposed to the governing agency if it is likely to better represent a pathway for contamination into the distribution system.

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