Design Example 3 – Low Recovery

At low recovery, there are fewer cycles needed to reach the volumetric recovery set point. This corresponds to a low CC duration and may create a challenge to meet the guideline CC duration of greater than 5 minutes. In such cases, it’s useful to recall that the feed spacer adds volume to the system to increase the duration of CC Mode.

The below example demonstrates the necessary adjustments when operating at low recovery or trying to avoid CC Duration warnings (see Figure 1):

1. Increase the ‘Pressure Vessel Size’ (from 6 to 7 in this example). This change allows for a second spacer per PV and is the primary method to increase CC Duration.
2. Increasing the ‘Feed Ratio’ will also slightly increase the CC Duration (this method works by increasing the number of cycles).
3. Note increased CC Duration (increasing from 4.75 (3A) to 6.17 (3B) minutes in this example).

Figure 1: Ex. 3: Flow Calculator - Low Recovery

Notes:

• The alternative to adding spacers is decreasing the flux rate (or using a combined approach).
• At very low recoveries, (approaching 60%), the designer will be required to decrease the flux rate as well as increase the number of spacers per PV.
• WAVE CCRO forces the user to have more than 2 cycles, one PF and one complete CC cycle. Otherwise it is essentially traditional RO. The designer is unable to exit the flow calculator page until the number of cycles is increased. Increasing the ‘Conc. Recycle’ flow rate will increase the CC cycles (by decreasing the cycle recovery).