Your company has recently acquired an old pulp mill and needs to install an new secondary treatment reactor to remove BOD from it's wastewater. There isn't a lot of space available for the construction. You are asked to compare three designs: CSTR, 4-CSTR in series, and plug flow, to determine which would require the smallest volume. You know that the facility plans to produce 1000 o.d. metric tons/day secondary fiber with no deinking. Which design do you recommend?