When is citric acid fed to the RO system relating to product water?

Citric acid is commonly fed to a reverse osmosis (RO) system primarily to manage the pH levels of the water being processed. This is important because maintaining the appropriate pH can help prevent scaling on the membranes, thereby enhancing their performance and longevity.

In the context of the question, the correct condition for adding citric acid occurs when the pH of the RO system rises above 8. A higher pH may indicate alkaline conditions that can lead to precipitation of calcium and magnesium salts, resulting in scale formation. By introducing citric acid, the system's pH can be lowered, helping to keep the membrane surfaces clear and operational.

Additionally, the reference to "20 hours of production time" suggests a maintenance aspect where citric acid dosing should also be considered after a certain operational period to ensure continued effectiveness and quality of the product water. This combined parameter ensures that both pH balance and operational time are monitored to prevent any detrimental effects on the RO system.

Therefore, the specific combination of pH above 8 and the threshold of production time directly connects to optimal RO system maintenance and product water quality.