Bulk density of microbial biomass is dependent on organisms present and growing conditions.  You need to provide more information on the growing conditions you are using and predominant type(s) of microflora you are concerned about in your work.  You mention microbial count (cfu/mg), biofilm thickness and surface area of diffusion column.  A count as cfu/mg and the other terms you use suggest you may be using a membrane filtration method for cell enumeration and may be working with waste water or other water/aquatic media.  I would have reservations about using membrane filtration as a method for enumeration of a mixed microflora if you are after bulk properties -- membrane filtration typically is too selective to provide reliable general estimate of cell types/counts.  You need to be more specific about what you are trying to do.

CFU counting typically concerns something between 1 and 10% of the real natural microbiota, as only "culturable" strains can grow on culture media under lab conditions. Therefore it is very difficult to estimate the actual biomass from that technique.

Likewise for biofilm thickness: biofilms are not composed only of microbial cells and polymeric matrix thickness depends closely on environmental conditions as well as on the metabolic state of the community, both varying a lot in the field and under lab conditions.  

Determining biomass in "real" populations is always a challenge and no technique is 100% reliable. It seems to me that using two or more different approaches helps obtaining a more accurate evaluation from the ensemble of results.

The microbial count by serial dilutions gives cfu/ml

The bulk density is weight of microbial mass per unit volume

cfu / mg can be deduced from how much sample is taken for serial dilutions in g or mg

The same thing can also be equated to bio film if we know the thickness

1 mg can occupy how much area

Thank you so much sir.