The Orbal® system incorporates a trademarked concentric loop design that creates dedicated zones for specific treatment processes. These treatment zones operate in series which is essential to the simultaneous nitrification-denitrification (SND) process. The volume split of the concentric loop design naturally applies the correct volume-to-oxygen input ratio required in the design of an SND plant.
By design, the aeration discs placed in the outer channel supply approximately 50% of the systems's total oxygen requirement. The design ensures a constant oxygen deficit condition throughout this channel with the DO set-point below zero.
The oxygen deficit environment of the outer channel delivers an overall denitrification performance rate of 80% without internal recycle. The majority of the system’s nitrification takes place in the outer channel where the anoxic condition also channels where the anoxic condition also drives denitrification. This is the definition of a TrueSND™ biological treatment system.
The mixed liquor then flows hydraulically to the middle channel where DO conditions swing depending on daily load variation. Finally, the mixed liquor flows to the inner channel where the DO is designed to operation at 2.0 mg/l. Final nitrification is completed in the inner channel (or oxygenation zone) in surplus oxygen conditions.