Design and calculations pertaining to a bio-fuel endeavor.

I am proposing an idea to a national petroleum company to change human waste into bio-fuel. I know the bio-digesters can be built using brick. However, bricks are heavy and because of this the building time is long, which enables bricks to be stolen. I am wondering if iron/ore can be melted down and poured into forms that an engineer designs. Once the iron/ore is cooled the pieces could be assembled offsite in to modular pieces and shipped to the building locations. Once at the building location the modular pieces could be attached, and when done the building would be complete. This would reduce theft and the build time, once the pieces were created, would be less than if bricks were used. If iron-ore cannot be used is there another substance that could be used that would be cost effective and meet the parameters of the project. Also I need to know how big the digester’s tank needs to accommodate 600 people if every person per-day discharges 300-1000 grams of waste and it takes 30 days for the human waste to change into bio-fuel.