Vertical Loop Reactor (VLR) is designed based on loop reactors in series that allow DO stratification. VLR is installed in a rectangular tank and the entire tank is divided in upper & lower compartments through a horizontal baffle running across the tank length almost at mid depth. The VLR combines ORBAL and Diffused Aeration to achieve its proven high efficiency level. The VLR tank can be much deeper and thus reduced footprint. The Air Diffusers are installed below the baffle plate and thus travel the full length of the tank before being released after a minimum retention time and thus higher mass transfer efficiency. The mixing conditions in the VLR tank are achieved through the ORBAL disc for which more details are available in the relevant ORBAL section. The VLR units can be provided for nutrient removal as well with simultaneous nitrification / de-nitrification and biological phosphorous removal. Existing rectangular tank can be retrofitted to VLR design if they are meeting in minimum sizing requirements.