Denitrification/Filtration Technology Helps Wastewater Treatment Plant Meet China's Stringent Class IA Standards

For more than 20 years, the Denite technology has been used throughout the world for denitrification in secondary, tertiary and advanced wastewater treatment applications. Using deep bed filtration as the bioreactor to achieve denitrification, microorganisms are encouraged to grow on the surface of gravel or sand media, providing the biological reaction. This biofiltration process treats secondary wastewater to produce a final effluent capable of meeting China's new effluent discharge regulations. However, such a post-denitrification filtration technology had not been previously used at any WWTP in China. So, a pilot test of the Denite system was undertaken at the Wuxi Lucun WWTP site to determine if the system was able to remove NO_x-N and SS to the mandated levels for Phase 4 of the site.

The Denite system requires one-tenth of the space used by suspended growth systems, greatly facilitating expansion and retrofitting requirements. With Denite, the denitrification process and the filtration process are combined in a single system. NO₃-N is converted to nitrogen gas and captured within the media bed along with suspended solids and biomass formed from the denitrification. The Denite gravity filter system operated in a downflow mode to maintain excellent suspended solids removal, thus avoiding the necessity for clarifiers or additional effluent polishing filters.

During the denitrification process, wastewater is forced to flow around nitrogen gas bubbles that accumulate in media voids in the filtration vessel, improving biomass contact and filtration efficiency. Effective removal of NO₃-N is accomplished by introducing sodium acetate, an external carbon source, using an automatic dosing controls scheme. A "bump" operation is employed to remove or purge accumulated nitrogen gas that can potentially build up in the fi lter media bed. If desired, this "bumping" can be accomplished without removing the reactor from service using the patented SpeedBump®, which applies backwash water to the bottom of the filter, releasing the entrapped gas into the atmosphere and reducing head loss.