Which of the following would be a reason to clean the RO vessels?

Cleaning the reverse osmosis (RO) vessels is crucial for maintaining the efficiency and lifespan of the system. When the total dissolved solids (TDS) in the product water exceed a threshold, in this case, 1000 psid, it indicates that the quality of the water produced is deteriorating. This high level of TDS suggests that the RO membranes may be fouled or scaling, which can severely impact the system's performance by decreasing its permeate flow rate and increasing the pressure required to achieve the same output.

Maintaining product water quality is vital for the end-use application, as high TDS can lead to adverse effects on processes that depend on purified water. Therefore, cleaning the RO vessels becomes necessary when the TDS level surpasses acceptable limits, ensuring that the membranes function correctly, and the water quality meets the required standards. The other scenarios presented do not directly indicate the need for cleaning based on the critical aspect of water purity; thus, they do not signify the same urgency for vessel maintenance.