Overall, this is good. I just have a few improvements in grammatical points of view
The water which is collected in the dam then is sent to water treatment plant using a pipe.
This sentence describes a method or means. Therefore, I would be nice if you use participle clauses with adverbial meaning.
By collecting in the dam, the rain water is sent to the water treatment plant using a pipe.Noted: I use 'the' so as to refer to the picture given.
Be consistent for the sequencers
Firstly
The second
Secondly
Finally
Lastly
WhenAs the waste-water is being purified, the excess treated water is sent to the river.
The reason why I use
As instead of
When is to say that one thing changes while another thing changes in the same time
when the rain falls into the roof and water tank, the water is stored in the tank. The water which is collected in the tank then is sent to the garden.
On falling into the roof and water tank, the rain water stored is sent to the garden.
Hope this helps :D