Denitrification is an anaerobic biological process employed to convert the nitrate-nitrogen in the effluent from the activated sludge-nitrification process into nitrogen gas. Denitrification takes place in the deep bed, mono-media denitrification filters. A supplemented carbon source (methanol) is added to the denitrification filter influent to provide a food source for the denitrifying culture in the filters.
There are 32 denitrification filters arranged for control in two groups of ten and two groups of six. There are three principle filter cycles, as follows:
- Normal Filter Cycle
- Nitrogen Release Cycle
- Full Backwash Cycle
During the normal filter cycle, the nitrified effluent with the supplemental carbon source (methanol) enters the filters and passes through the filter media where it comes in contact with the anaerobic denitrifying bacteria. It is here that the bacteria utilize the oxygen on the nitrate molecule, leaving nitrogen gas in the filter media.
The nitrogen gas forms small gas bubbles within the filter. The media and down flow of the liquid prevent the nitrogen gas bubbles from rising to the surface and escaping into the atmosphere. The nitrogen release cycle is a short backwash cycle to release the nitrogen gas which becomes trapped in the filter media. Generally, the nitrogen release cycle backwash is done at 4 to 6-hour intervals. If the nitrogen release backwash cycle is not performed at the required interval, the nitrogen gas will continue to accumulate and the head loss through the filter will increase just as in a dirty filter. The flow rate through the affected filter will be reduced. Automatic controls are arranged to provide an adjustable duration water backwash, in sequence, to each group.
Greater than 90 percent of the nitrogen in the influent of this process will be converted to nitrogen gas, and thereby removed from the wastewater.
As discussed above, denitrification is an anaerobic process. Therefore, the effluent from denitrification will have little or no dissolved oxygen (DO). Re-aeration of the denitrified effluent is provided in the post-aeration-chlorination tanks by diffused air. The DO in the effluent is raised to at least 5.0 mg/l.