Is the flux 32m/hr (velocity) or 32 m³/hr? What length of time is the backwash running? What is the overhead volume? What is the length of the filter run between backwashes? What is the size of the filter media? What is the filter bed expansion during backwash? Personally, I would check these items directly during a backwash.

grrun

There seems to be some confusion here.  

This is a filter system with "sea water" flowing through the filters, where the seawater has a TSS concentration of 5 mg/liter (or, 5 parts per million)?  What is the flow rate of this sea water through the filters?

You are expressing this "flux rate" in terms of "meters per hour," which is velocity of flow (I assume).  But, this is the back wash rate?

You have expressed the "waste per backwash" in terms of "less than 150 square meters."  Again, that is velocity, or what?  

Since you have given the filter area of 29 m2 each, times 4 filters online, that would be (4)(29 m2)(32 m/hour) = 3712 m3/hour if that is in fact the flow of seawater through the filters.

If that is indeed the "dirty" water flow through the filters, and NOT the backwash, then:

Assuming 100% capture of the TSS in the filters, and complete cleaning of the filters during a back wash, the TSS captured and backwashed would be    (3712 m3/hour)(1000 liters/m3)(5 mg/liter)(1,000,000 mg/kg) = 18.56 kg/hour of solids removed from the seawater

But, that would be the total TSS removed per hour.  Since we don't know the frequency of the backwash cycles, we don't know how much TSS is removed per backwash.  Or, at least that's my take on the problem.