As a result of China's rapid economic development in recent years, the country has implemented more stringent environmental standards. Local environmental protection departments now require most urban wastewater treatment plants (WWTP) to apply strict enforcement measures to meet Class IA effluent discharge standards according to the Discharge Standard of Pollutants for Municipal Wastewater Treatment Plant (GB18918-2002).
So, how did the earliest and the largest multifunctional WWTP in Jiangsu Province meet the stringent Class IA effluent discharge standards?
The Wuxi Lucun WWTP is an important facility in the Wuxi Taihu Lake Project. The facility serves more than 700,000 people over an 89 sq km area. The Wuxi Lucun facility Phase 1 to 3 projects have an average treatment capacity of 200,000 cu meters per day. In order to control cyanobacteria, the Wuxi Lucun WWTP needed to upgrade its denitrificiation and filtration process by adding a Phase 4 of the project with a treatment capacity of 100,000 cu meters per day. The upgrade to the treatment process was designed to remove total nitrogen (TN) and total phosphorus (TP), the primary causes of eutrophication.
De Nora Water Technologies' TETRA DeepBed Denite system was selected as the treatment method for the Phase 4 project following a successful pilot test demonstrating the system's ability to meet Class IA standards. Because of Denite's ability to perform full denitrification to levels far below those required for Class IA compliance, it was possible to design a facility to treat only half of the Phase 4 flow of 100,000 cu meters per day, and then blend treated and untreated flow to meet Class 1A standards. The DeepBed Denite system is a fixed-film biological denitrification process that also serves as a deep bed filtration system capable of removing suspended solids. Installed after the water outflow from the secondary sedimentation tank, the system integrates well with other treatment processes to provide simultaneous removal of TN, TP and total suspended solids (TSS) to meet Class IA effluent standards.
Pilot Testing the Denite Technology
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 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 TN and TSS to the mandated levels for Phase 4 of the site.
With Denite, the denitrification process and the filtration process are combined in a single system. Nitrate (NO3-N) is converted to nitrogen gas and captured within the media bed along with suspended solids and biomass. The Denite gravity filter system is operated in a downflow mode to maintain excellent suspended-solids removal, 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. Removal of NO3-N is accomplished by introducing sodium acetate, an external carbon source, using the TETRAPace automatic dosing control. A "bump" operation is employed to remove or purge accumulated nitrogen gas that can potentially build up in the filter 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.
During the pilot test, the Denite filter demonstrated excellent removal of NO3-N and TSS. The effluent NO3-N measured was between 1 and 2 mg/L. When TN in water ingress was between 14 and 24 mg/L, the effluent TN was consistently below 5 mg/L.
After the successful pilot test, a full-scale Denite system was installed at the Wuxi Lucun WWTP Phase 4 project. Full-scale system testing began on May 31, 2010. On the morning of June 2, the results of water samples were found to be in compliance with the Class IA standards. Backwashing took place on June 4. Results showed that when the Denite system is in the appropriate amount of sodium acetate dosing case, it can be stabilized to ensure effluent TN, TSS and TP are less than 5 mg/L, 5 mg/L and 0.5 mg/L, respectively.
Upon completion of the Phase 4 project, the upgrade is enabling the Wuxi WWTP to continue operating the plant effectively and efficiently. More importantly, the plant's water outflow meets the stringent Class IA effluent discharge standards.