Your filters need to be washed every day. What would your energy, water and waste treatment saving be if they only required washing every four or five days? You need to increase the output of the works. Are the filters the existing throttle point? Can you achieve your revised target by reducing wash water rather than building more filters while simultaneously reducing operating costs? Can you save the expense of building new clean and dirty wash water tanks by reducing backwash requirements?
The velocity of your existing backwash system is not sufficient to produce your required filter bed expansion. What is the cost of replacing your backwash system including pumps, pipes and wash water tanks? Will installation of a lighter density filter media solve the problem while also increasing filter run times with associated cost reductions?