Rather than pay for methanol as a carbon source for denitrification, it is quite possible to design a nitrification-denitrification system that uses the carbon present in raw sewage (the carbon associated with endogenous respiration of the microbial sludge is also significant). In these systems nitrification and denitrification occur in a singe vessel with alternating aerobic and anaerobic zones. Raw sewage is delivered not at a single point, but wherever denitrification occurs. Sufficient recycle is required to prevent the effluent from containing excessive ammonia concentrations. Several systems have been developed along with these design elements; and, of these, the two most successful have been the Bardenpho process and the denitrifying oxidation ditch.
A nitrification-denitrification four stage Bardenpho flow diagram (figure is adapted from Metcalf and Eddy, 1979) would be:
The extent of denitrification is determined mainly by the fraction of mix liquor suspended solids recycle. This recycle line should make it clear that there is a tradeoff between detention time and performance in combined nitrification-denitrification systems. There will always be a small percentage of nitrates left in the effluent(if you understand what's going on, you should be able to figure out why). Nevertheless, Bardenpho systems generally run at around 90% nitrogen removal efficiency.
Oxidation ditches are racetrack shaped reactor in which rotating brushes both aerate and move the mixed liquor at several points in the loop (the author would like to apologize: a photographic image is currently under construction. In the mean time the reader is referred to Metcalf and Eddy, 11-7 for a schematic). The amount of oxygen supplied is determined by the level of the brush rotor in the mixed liquor, and the velocity of the flow remains constant.
The region just downstream of the brush aerators is aerobic so nitrification occurs. As the liquor travels downstream and the oxygen is consumed, an anaerobic zone is formed. By routing a small portion of the raw sewage influent (as a carbon source) to this zone , denitrification occurs. The mixed liquor then contacts another brush aerator so that the organic nitrogen produced by the denitrifying bacteria is oxidized. The number of anaerobic zones and aerators required is a design parameter that depends on the capacity and loading of the plant.
Denitrifying oxidation ditches are capable of extremely high efficiencies. The 3MGD oxidation ditch in Stonybrook, New York regularly maintains 97% nitrogen removal efficiency. Oxidation ditches are also one of the easiest wastewater processes to operate and maintain. There is of course a drawback: they require a large land mass. Land is often simply too expensive in high density population regions that could benefit from the high efficiency rates associated with a modern oxidation ditch.
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