design of Basket filter

With a designed flow rate of 9000000 liter per hour which is 9000 kilo liters per hour. I do think it would easily hand 1200 kilo liter per hour.

I think there is something wrong in your question.

You have stated the design flow as 9000000 LPH i.e. 9000 kiloliter per hour (kl/hr) and enquired whether the flow can be increased to 1200 kl/hr. If the design flow is 9000 kl/hr then with a reduced flow of 1200 kl/hr, there should not be any problem.

Increasing or decreasing the flow rate is not a problem, anyway you have to take into consideration the recommended velocity of that fluid to avoid maximizing of pressure drop through that filter.

Note. I think you mean 12 000 m³/hr not 1 200 m³/hr.