Can the water be used in the garden & veggie garden ? is it safe not harmful to plants ??

Yes Amed all your backwash water should go to your garden if you can. Just be careful when you use salt it will damage your soil and certain plants.
Your pond water normally will contain some Nitrates which is a type of fertilizer.

I use my pond's water for plants most of the time. No problem at all but as Neville said, be careful when you use salt

tanx guys , Unfortunately i use salt and that explains why some plant have just dissapeared lol