## 6 The reverse osmosis (RO) process

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Take a look at the reverse osmosis process, as explained in Video 6, then answer the questions below.

Video 6 The reverse osmosis processVideo player: 6a%20RO.mp4

Transcript: https://www.open.edu/openlearncreate/mod/page/view.php?id=161929

Look at the specific energy consumption (SEC) for reverse osmosis (RO) in the equation sheet (314.8 KB)  ('Ctrl click' to open in a new window). The numerators of the two main terms in brackets both refer to various contributors to the pressure, which are defined in the equations list or in the symbols list on the following page. The denominator includes the concentration factor, which is directly related to the recovery.

In this spreadsheet, you can adjust the values of the concentration factor (CF) and the salt concentration to determine the impact on the RO SEC. Note that the CF must, by definition, be >1. Try incrementally increasing the value and recording what happens to the SEC value.

Question 16:

Seawater typically has a concentration of around 3.5%. What is the optimum CF at this concentration, and what is the resultant SEC?

Question 17:

The wastewater has a dissolved solids concentration of 1000 mg/L. What is this in wt%, and how does the SEC change with CF at this concentration? What does this imply about the energy recovery turbine?

Question 18:

Pre-treatment is by membrane filtration, similar to the membrane filtration component of the MBR where the energy consumption is from permeate pumping. What is the approximate SEC for this step?

Question 19:

What is the optimal total SEC for the RO desalination route?

Question 20:

What is the optimal total SEC for the MBR wastewater recycle route?

Question 21:

What do you conclude from your analysis? Which of the two options − municipal water reuse or seawater desalination − is the most cost effective in terms of energy?

You've now finished all the calculations for both of the two options for water supply. It's time to finish off.

You can now move to Section 7 Conclusion