ORBAL® multichannel oxidation ditch for biological nutrient removal is designed for conventional activated sludge, advanced secondary sludge treatment, simultaneous nitrification-denitrification, enhanced nutrient removal and storm water treatment. It is a complete mix, looped reactor system.

A typical Orbal® system uses the 0-1-2 punch for enhanced nutrient removal. The Orbal® basin has three concentric channels, with the outer channel having approximately 50% of the total volume. After screening and grit removal (optional), influent enters the outer channel which is operated under an oxygen deficit condition to promote simultaneous nitrification-denitrification. The actual oxygen demand of the first channel can be as high as 75% of the total but the aeration discs allotted to the channel only supply 30 to 60% of the system's overall oxygen requirements to ensure a constant oxygen deficit condition and a dissolved oxygen (DO) operating concentration of zero throughout the channel.

The simultaneous nitrificationdenitrification environment of the first channel results in an overall denitrification
performance rate of 80%, without internal recycle. The first channel, where the majority of the process "work" takes place, is classified as an aerated anoxic reactor. Despite the zero DO conditions, the majority of the system's nitrification takes place in this channel.

The DO of the second aeration channel operates in a "swing" mode. Although designed for 1 mg/l DO, the actual operating DO varies with the daily load conditions, being reduced to near zero during the peak loads of the day, and rising to 2 mg/I during low load conditions.

To maximize enhanced nutrient removal, the oxygen delivery in the first channel of the Orbal® basin is reduced to levels lower than normal, allowing the oxygen deficit condition to become much stronger. This strong deficit condition in the first channel, coupled with an elevated sludge yield rate, will allow most
plants to achieve effluent phosphorus levels of 1 mg/l or lower without chemical addition.

When both TN and TP removal are required an anaerobic selector can be added. Typical Orbal® systems with an anaerobic selector generate an activated sludge with 5 to 6% by weight phosphorus and can achieve effluent TP values of 0.3 to 0.5mg/l without chemical addition.

As with any enhanced nutrient removal system, chemical polishing and backup is recommended for reliability.