There is one plug flow and one completely mixed reactor operating in parallel to treat wastewater. What will be the removal performance of the treatment plant if reactors operate in series? The wastewater flow rate is $872$ m$^2/$d with a COD load of $210$ mg$/$L$.$ The reactor dimensions are $4$ m for the cubic tank and $2\mathrm{.}9$ m in height, $2$ m in width, and $11$ m in length for the prism tank.