The scarcity of water is an imminent problem in many Brazilian regions, configuring the reuse of water as a great ally in the business sustainability of many companies. Reverse Osmosis (RO) systems, also known as Reverse Osmosis, are capable of purifying water by removing dissolved salts from semi-permeable membranes, with a production of low salinity water around 75% of the input water. Several tests can be applied to evaluate the performance of the system, one of them is the SDI (Silt Density Index).
The challenge in operating RO systems lies in the ability to produce low-salinity water with as little intervention as possible with periodic chemical cleaning stops when their flow rate decreases, the pressure differential increases, and/or the salt rejection decreases. Typically, interventions occur due to the characteristic of the RO system’s own feed water, and may be associated with a need for pretreatment or inefficient pretreatment. In the field, some analyses must be performed to monitor the equipment, among them the SDI. SDI is a recommended test to be performed directly in the field on feed water, as a function of the fouling variation of a microfiltration membrane at a constant pressure for the time required to filter the volume defined by the standardized analytical protocol. Below is the schematic of figure 1:
After stabilizing the pressure at 2.1 bar (30 psi), the test is started by collecting volumes of 500 mL at different times, initial and final, and applying the formula below the SDI, the result is obtained and interpreted according to the table below. To calculate the SDI the following formula is used: SDI =100*(1-Ti/Tf)/T In which:
- Ti = Time needed to reach 500 ml.
- Tf = Time needed to reach 500 ml after filtration of 15 minutes.
- T = Total test time.
The result obtained is interpreted in this table 1, favoring immediate action, taking into account the need for continuous improvement of the process with the goal of high productivity.
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