The key idea is that dairy wastewater BOD is variable but centers around a fairly high value. It’s typically about 1,000 mg/L, with potential spikes up to around 10,000 mg/L depending on factors like process activity and cleaning cycles. So the statement that best fits is that dairy wastewater BOD is usually near 1,000 mg/L but can reach as high as 10,000 mg/L. The other descriptions don’t match this variability: 100 mg/L is far too low for dairy waste, 50,000 mg/L exceeds the stated maximum, and saying it’s always exactly 1,000 mg/L ignores natural fluctuations.

Understanding how many gallons fit in a cubic foot comes from using the standard US definitions for volume. A cubic foot is 12 inches on each side, so it equals 12 × 12 × 12 = 1728 cubic inches. A US gallon is defined as 231 cubic inches. So the number of gallons in a cubic foot is 1728 ÷ 231, which is about 7.484—roughly 7.48 gallons. The other options aren’t close because they would correspond to using different gallon sizes or rough rounding that doesn’t match the standard US definitions. If you’re comparing to imperial gallons, you’d get a different value (about 6.23), which isn’t what’s asked here.

When trying to stop reverse flow in a discharge line after the pump stops, you want a device that automatically blocks flow in the opposite direction. A check valve does exactly that. It allows fluid to move forward with the pump but closes automatically if the flow tries to go backward, so the discharge line isn’t drawn back toward the pump. Check valves come in types like swing checks and lift checks. A swing check uses a disc on a hinge that swings closed when reverse pressure appears. A lift check has a guided disc that lifts with forward flow and drops back to seal when flow reverses. Proper orientation and installation are important to ensure reliable closure. Other valves—the globe, ball, or butterfly—are primarily for isolation or throttling. They don’t automatically close to prevent backflow when the pump stops, so they don’t reliably protect against reverse flow on shutdown.

Flow rate through a pipe is the product of cross-sectional area and velocity (Q = vA). With the same flow rate in both pipes, the velocity is inversely proportional to the area. A circle’s area is A = πD^2/4, so the 6-in pipe has A6 = 9π in^2 and the 12-in pipe has A12 = 36π in^2. The velocity in each is v = Q/A, so the ratio v6/v12 = A12/A6 = (36π)/(9π) = 4. This means the 6-in pipe’s velocity is four times that of the 12-in pipe, not twice.