The BioDenipho Process can best be understood by following the nitrogen and phosphorous removal stages through one complete 4-hour cycle of operation. One cycle consists of six separate phases (A,B,C,D,E and F). Phases D, E, and F mirror phases A, B, and C.

In all of the phases, the influent wastewater is mixed with the return activated sludge (RAS) in the anaerobic selector prior to being directed to either ditch, or train.

In the first phase, the mixed liquor is directed to bacteria operating in an anoxic state. Bacteria begin feeding off the organic matter in the influent wastewater, utilizing nitrate rather than oxygen and release nitrogen gas harmlessly into the atmosphere, thus the nitrate concentration decreases within the reactor while the ammonia concentration rises. It is during this phase that submersible mixers without aerators are utilized to maintain the flow velocity of the mixed liquor. The second reactor operates in the oxic stage.

Phase A is initiated by changing the flow direction from the distribution chamber. The automated flap gate-type flow distributor in the distribution chamber switches positions and directs the influent to the inlet pipe for ditch two instead of ditch one. In phase A, ditch one is isolated from the influent. Ditch one is oxic and will remain oxic throughout phase A.

Ditch two is in the denitrification mode of operation. The rotors in ditch two are turned off and will remain off throughout phase B to produce anoxic conditions. The activated sludge in ditch two is kept in suspension by submerged mixers. The ammonia concentration in ditch one will increase due to the oxidation of ammonia and organic material in the influent. The concentration of nitrate, which was produced during an oxic phase in ditch two in the previous cycle will decrease as a result of denitrification.

Denitrification requires an organic carbon source to fuel the process. This carbon source is present in the influent wastewater. Because the influent wastewater is directed to the anoxic ditch, the organic substrate is readily available for use during denitrification. If the influent had been initially sent to the oxic ditch, the organic substrate would have been degraded aerobically and an external carbon source, such as methanol, would have to be added to promote denitrification.

The rotors operate to aerate the entire basin. This increase in oxygen allows ammonia to be converted into nitrate and BOD to be removed. At the same time, bacteria feed off the phosphorus found in wastewater which is incorporated into the sludge that later settles into the secondary clarifiers.

During the second phase of operation both reactors are in an oxic state, aerating both basins to ensure a minimum amount of ammonia and phosphorus in the influent. In this phase, the effluent weir in ditch two is raised and the effluent weir in ditch one is lowered.

The hydraulic gradient is now shifted so that the flow direction is from ditch two to ditch one, with ditch one discharging effluent. Ditch one receives influent from ditch two. The rotors in ditch one are in operation and the ditch, which was oxic in phase A, will remain oxic through phase B. Influent ammonia that was not oxidized in ditch two will be oxidized in ditch one. Ditch two, which was anoxic in phase A, will remain off throughout phase B. This will maintain the anoxic conditions in ditch two, and enable denitrification to continue. The activated sludge in ditch two is kept in suspension by submerged mixers.

The third phase of the process is simply a mirror-image of phase one, with the two reactors changing operating conditions. Both ditches are oxic in phase C, and nitrification will be carried out in each.

The fourth phase of the BioDenipho process is similar to phase two with both reactors operating in an oxic state. This phase is initiated by the distributor redirecting the influent to ditch one. The rotors are turned off to promote anoxic conditions so that the nitrate produced during the previous four oxic phases for ditch 1 can be denitrified.

In phase E, the effluent weir in ditch one is raised, and the effluent weir in ditch two is lowered. The hydraulic gradient produced causes the direction of flow to be from ditch one to ditch two, and on to the clarifiers.

Ditch one is in the nitrification mode of operation. The rotors in ditch two are turned on and will remain on throughout phase F to produce oxic conditions unless the dissolved oxygen concentration (DO) reaches the upper limit. If the DO concentration reaches the upper limit, a rotor will be turned off to control the DO. The ammonia concentration in ditch one will decrease due to the oxidation of ammonia (nitrification). The concentration of nitrate will increase as a result of the nitrification.

Ditch two is receiving influent from ditch one. The ammonia concentration in ditch two will decrease further, while the nitrate concentration increases because the oxic environment promotes nitrification. The motorized effluent weir is lowered in ditch two to allow the activated sludge to exit the ditch and proceed to the clarifier. As the influent enters ditch one, a hydraulic gradient is produced that forces the mixed liquor from ditch one to ditch two, over the effluent weir and on to further treatment.

Based on the Hydraulic Retention Time (HRT) of 10.5 hours, one complete four-hour cycle accounts for only 38 percent of the HRT. Therefore, the average influent wastewater particle will experience approximately two and a half cycles before exiting the oxidation ditches.

When these processes are repeated, the effluent results are achievable. General design criteria permit the total nitrogen to be between 4-6 milligrams per liter and total phosphorous below one milligram per liter from the secondary clarifier. However, more stringent effluent criteria can be met if needed.

This treated and clarified effluent receives further treatment, such as filtration and disinfection. The underflow from the secondary clarifier is then discharged to a wet well by means of a telescoping valve. The sludge is then either wasted or directed back to the anaerobic selector.