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Difficulty: HardDirect Fact Retrieval

During field operations, tactical combat casualty care units deploy reverse osmosis water purification units (ROWPU) to treat brackish groundwater. Before contaminated intake water reaches the high-pressure pump, a dual-stage multimedia filter removes suspended particulate matter exceeding 25 microns. The fluid is then injected with sodium hexametaphosphate at a concentration of 4 parts per million to prevent mineral scale accumulation along the spiral-wound polyamide membranes. Operating at a sustained manifold pressure of 800 pounds per square inch, the system produces 600 gallons of potable water per hour while discharging concentrated brine. To prevent biological fouling during downtime, operators flush the assembly with a 2 percent citric acid solution.

According to the passage, what specific chemical and concentration are added to inhibit mineral scale buildup on the membranes?

  1. A
    A 2 percent citric acid solution
  2. Sodium hexametaphosphate at a concentration of 4 parts per millionAnswer
  3. C
    Sodium hexametaphosphate at a concentration of 25 parts per million
  4. D
    Citric acid at a concentration of 4 parts per million

Answer

Sodium hexametaphosphate at a concentration of 4 parts per million
The passage explicitly states that fluid is injected with sodium hexametaphosphate at a concentration of 4 parts per million specifically to prevent mineral scale accumulation along the spiral-wound polyamide membranes.

Step-by-Step Solution

1
Scan the text for the specific key terms: 'mineral scale' and 'chemical/concentration'.
Located the target sentence: 'The fluid is then injected with sodium hexametaphosphate at a concentration of 4 parts per million to prevent mineral scale accumulation along the spiral-wound polyamide membranes.'
Direct fact retrieval requires locating the exact sentence addressing the stated condition.
2
Extract the exact chemical identity and corresponding concentration value from the located sentence.
Identified 'sodium hexametaphosphate' and '4 parts per million'.
Ensure that numbers and chemical names from other sentences (such as 25 microns or 2 percent citric acid) are not erroneously conflated.
3
Match the extracted fact directly to the corresponding answer choice.
The option specifying 'Sodium hexametaphosphate at a concentration of 4 parts per million' matches the explicit text perfectly.
Confirms factual precision without relying on assumptions or extraneous data.

Key Concept

Direct Fact Retrieval
Estimated Time:1m 30s
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Topics

Direct Fact Retrieval

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