Biological Wastewater Treatment Process Becoming Technology
of Choice in China

Perhaps the world’s most challenging wastewater issues are being faced in China, a country in which continuing population growth and rapid industrialization have outpaced environmental regulatory measures.

For many years, the construction of urban drainage, sewage and wastewater treatment has lagged far behind the development and environmental requirements of the country’s cities. Vast quantities of untreated domestic and industrial wastewater flow into China’s rivers, lakes and streams. And changes in the country’s agricultural practices, including the increased application of chemical fertilizers, have overloaded its waterways with organics and nitrogen in their various forms.

As the country’s dynamic growth continues, so does the need for its precious water resources. From 1995 to 2030, residential demand for water is projected to increase from 31 billion tons to 134 billion tons. The demand for water by industry is projected to grow even faster, from 52 billion tons to 269 billion tons. Water shortages are estimated to affect nearly half of the country’s cities, and in areas of severe shortage farmers have been using untreated wastewater for agricultural purposes.

In recent years, the China Central Government has enacted a number of regulatory measures to create sustainable economic development through the management of natural resource consumption and environmental degradation. One of the more significant wastewater-related measures is the Class IA discharge standard of pollutants for municipal wastewater treatment plants (GB18918-2002), effective as of July 1, 2003. Among the standard’s more stringent effluent parameters are for effluent BOD to be reduced to <10 mg/L, ammonium-nitrogen (NH₄-N) to <5 mg/L and total-nitrogen <15 mg/L.

Finding the most cost-effective treatment system capable of meeting the new effluent criteria is a particular challenge in many Chinese municipalities. Financial resources are limited and land available for development is scarce. While many physical, chemical and biological treatment methods are capable of removing organics and nitrogen from water or wastewater, system costs and land availability are critical determining factors. One biological treatment process that is meeting with increased acceptance in China is a technology that has been used successfully worldwide: TETRA® biological wastewater systems from Severn Trent Services.

Biological treatment used worldwide now being used in China
The TETRA® DeepBed™ Denite® System is a fixed-film biological denitrification process that also serves as a deep bed filtration system capable of removing suspended solids. The system integrates with other treatment processes to provide effective total nitrogen (TN) and phosphorous removal.

The TETRA system has been used throughout the world for organic oxidation, nitrification and denitrification in secondary, tertiary and advanced wastewater treatment applications. Using deep bed filtration as the bioreactor to achieve bio-oxidation and denitrification, microorganisms are encouraged to grow on the surface of gravel or sand media, providing the biological reaction. This biofiltration process treats raw wastewater to produce a final effluent capable of meeting China’s new effluent discharge regulations or as required for wastewater reuse.

In 2008, a full-size Denite treatment plant with an average wastewater flow of 20,000 m³/day was designed and installed at the Jiashan Yaozhuang wastewater treatment plant in Jiashan City, located in southeast China’s Zhejiang Province. The Denite system has also been selected for use at other facilities in China, including for a 25,000 m³/day plant in the Huishan district of Wuxi City. The project is scheduled for completion in November 2009.

Benefits of the Denite process
Biological denitrification processes can be of the fixed-film or suspended growth type. The TETRA Denite system requires one-tenth of the space used with suspended growth systems, greatly facilitating expansion or 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 reaction. The Denite gravity filter system operates 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 methanol 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.

TETRAPace uses the filter influent flow rate and the influent and effluent NO₃-N concentrations to attain an operator-inputted setpoint value for effluent NO₃-N concentration. This is done by continuous, automatic adjustments of the methanol dosage rate. An alternative to this system is one incorporating either a flow-paced, a feedforward or feedback system. But TETRAPace is far more efficient.

The advantages of tighter methanol control can be significant if a plant has a stringent BOD limit in combination with a low TN limit. Under these conditions, the tighter control and reduced risk can be critical components in ensuring the plant meets limits reliably. The accuracy of the proprietary algorithm used to feed methanol during the denitrification process enables TETRAPace to yield significant savings of up to 30 percent in methanol consumption costs while guaranteeing effluent quality with no net total organic carbon pickup across the filter system.

According to Marwan Nesicolaci, vice president of international sales for Severn Trent Services, the TETRA biological treatment system is being selected for many projects worldwide for a variety of reasons. “From the standpoint of cost and technical efficiency, biological treatment has become a preferred treatment technology for many new treatment facilities in China. In addition, biological treatment is a natural method for waste disposal, which has increasing appeal no matter what country the plant is located in. And Severn Trent Services has been designing and placing our award-winning TETRA systems in wastewater plants worldwide for more then 20 years. Wastewater plant managers have come to see these biological treatment systems as cost-effective solutions.”

For more information, e-mail info@severntrentservices.com.