The diagram below shows how salt is removed from sea water to make it drinkable.

The flow chart elucidates the process of saltwater treatment by deliverance of salt from it. There are basic stages of processing before water is fit for drinking.

First of all, seawater flows via a pipe to the first stage of a pre-treatment filter, where being under explosure of processing, is disjoined to debris, sending them back to the sea as backwash, and to the quite treated water, directed to the next filter. Saltwater is subsequently forced through a membrance at high pressure so as to detach salt and other undrinkable dashes from water molecules by returning them back to the sea as seawater concentrate. Afterwards, freshwater is post-treated by adding various elements such as lime, chlorine and flouride, in order to kill any harmful bacteria, minimizing its risk to people's health. Thereafter, refined water is stored in warehouses before it is purveyed for minor trade points.

Overall, after such multistage process, through physical and chemical purification, water is ready for safe utilization.

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As long as I learn about writing task 1, this is an appropriate approach:
1. Introduction paragraph; you have to restate the prompt
2. Overall; here you must mention the main trend. I read from an IELTS book which informs that we should write an overview in the second paragraph because of writing task 1 need overview instead of conclusion paragraph.

3. Body paragraph 1; you can write detail information of diagram above.
4. Body paragraph 2; explain detail information in depth.

I think you should summarize how many stage for this process before you describe it