# Exam Study Guide

WASTEWATER

Effluent quality improves in lagoon systems that are operated in which configuration?

A. Parallel arrangement

B. Series arrangement

C. Anaerobic only

D. Aerobic only

Answer: B. Lagoons operated in series tend to produce higher-quality effluent as the waste stream passes from lagoon to lagoon. Series operation means the lagoon system piping is configured so that the influent of the second lagoon is the treated effluent of the first lagoon; the influent of the third lagoon is the treated effluent of the second lagoon, and so on. Lagoons operated in parallel essentially run side by side, and the influent wastewater is split equally to two or three lagoons; the outlets of all three are combined into one effluent. Series configuration allows for improved treatment because the influent of each downstream lagoon has considerably less BOD and TSS loading. Lagoons can be operated in combinations of anaerobic, aerobic, or facultative modes, but they will still produce higher-quality effluent if operated in series.

DRINKING WATER

The Langelier Index (LI), sometimes referred to as the Langelier Saturation Index (LSI), is a valuable tool to determine the corrosivity of potable water in a distribution system. The formula for Langelier Index is LI = pH – pHs.

What factors are used to determine the pHs?

A. Temperature, alkalinity, dissolved oxygen, free chlorine residual

B. Temperature, total dissolved solids, calcium hardness, alkalinity

C. Chlorine demand, total dissolved solids, dissolved oxygen, total hardness

D. Temperature, alkalinity, velocity, total hardness

Answer: B. The calculation of the pHs portion of Langelier Index is based off T.E. Larson’s method of determining calcium and bicarbonate concentrations. A table of values used in determining the pHs was developed. They include the water temperature, the total dissolved solids (TDS) in mg/L, the logarithmic value (Log10) of the calcium hardness, and alkalinity, both of these last two as mg/L CaCO3. These factors play a part in how saturated the water is with respect to its ability to form a protective scale on the water distribution mains.

Once the pHs is determined, the calculation is performed to produce a number that is either negative, positive or neutral. A positive Langelier Index, for example +0.5, indicates that the water is slightly scale forming (noncorrosive). A negative Langelier Index, such as -0.5, indicates that the water is corrosive, or aggressive. A neutral result (neither positive nor negative) indicates a stable water that will neither form a scale inside the waterlines nor be corrosive. The pH portion of the Langelier Index calculation is just as it seems — the water’s pH value.