Increasingly publicized in recent years, China’s emergence as a notable player in the world economy has come at a significant environmental cost. The country’s rapid industrialization has resulted in increased pollution, particularly to its water system. Man-made pollution has contributed to the depletion of oxygen resources in waterways, inducing eutrofication in surface waters and exerting toxicity to water resources. Organics and nitrogen in their various forms can cause significant damage throughout the water system.
In order to create sustainable economic development, the China Central Government has enacted measures to manage natural resource consumption and environment degradation. Among measures promulgated by the government in 2003 was a new wastewater discharge standard. The Class IA effluent discharge standards, among the more stringent effluent parameters required in the new regulation, call for an 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 efficient and 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. From the standpoint of cost and technical efficiency, biological treatment has been selected as the treatment of choice in most cases. An added benefit of biological treatment is that it is a natural method for waste disposal.
How the Three-Step Process Works
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. A patented three-stage biofiltration process using a fixed-film biological nutrient removal system can be used to treat raw wastewater to produce a final effluent capable of meeting China’s new effluent discharge regulations or as required for wastewater reuse.
The first stage is an up-flow SDFTM - submerged denitrification filter - which provides pre-denitrification in an anoxic environment using BOD from the incoming wastewater as the carbon source for denitrifying the oxidized nitrogen (NOx-N) in the recycled flow from the submerged aerated filter. The second stage is the submerged aerated filter bio-reactor, also in an up-flow mode that oxidizes the remaining BOD and NH4-N from the submerged denitrification filter effluent in an aerobic condition using oxygen supplied by compressed air. The media used in both submerged denitrification filter and submerged aerated filter bioreactors are coarse gravel, enabling excess biomass to be regularly sloughed off naturally without any intentional backwash requirements. The final treatment stage is a polishing step provided by the sand filter, which removes total suspended solids (TSS) and NOx-N from the final effluent.
The filter is operated in a down-flow mode and if denitrification is required, a supplemental carbon source, such as methanol, is added to the filter influent to facilitate the bio-denitrification reaction. Periodic air and water backwash are made to dislodge the retained TSS and excess biomass. During denitrification operation, the filter utilizes a bumping procedure – a very short duration of water backwash – which removes the nitrogen gas accumulated in the filter bed.
Pilot Plant in Zhejiang Province
One particular denitrification system, TETRA® Denite®, was pilot tested in Jiashan City, located in southeast China’s Zhejiang Province. The plant consisted of three-stage, fixed-film biological nutrient removal systems. The first stage was the submerged denitrification filter pilot reactor, with a dimension of 0.406 m in diameter and 2 m gravel media or 0.259 m3 of media volume. At a designed flow of 145 l/hr to the pilot plant, the hydraulic retention time (HRT) was 107 minutes or 36.7 minutes with a 2Q recycle. The second stage was the submerged aerated filter pilot reactor with a size of 0.406 m in diameter and 3.5 m gravel media or 0.455 m3 of media volume. At the same flow rate of 145 l/hr, the HRT for the submerged aerated filter was 188 minutes or 63 minutes with 2Q recycle. The last stage was a DeepBed filter with a diameter of 0.406 m and 1.8 m sand media for a filtration rate of 0.45 gpm/ft2 at the same flow rate.
The two-month pilot study at the Jiashan Yao-chuang wastewater treatment plant demonstrated the ability of the fixed-film bio-treatment process to deliver a final effluent meeting the new Class IA effluent discharge standards in China. During the pilot study, average BOD was reduced from 200 to 2 mg/L, NH4-N from 82 to 3 mg/L, T-N from 88 to 11 mg/L and TSS from 165 to non-detect.
Based on the results from the pilot study, a full-size treatment plant with an average wastewater flow of 20,000 m3/day was designed utilizing the three-stage treatment process. The plant is currently under construction in Jiashan, and is scheduled to be in operation in early 2008.
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