China turns to bio-treatment
One of the impacts of China's exponential industrial growth is the pollution of its waterways. But the country's government is making inroads into the issue, with biological treatment a favourite, writes Dr Jeffrey J Chen.
The recent pilot testing of an innovative deep-bed biofiltration system is good news for the 1.3 billion people requiring wastewater services.
China's emergence as a world player in the global economy has come at a steep environmental cost. Air quality was a stark sign of the country's growth to TV viewers of the Olympic Games. But the country's rapid industrialisation has also put pollution stresses on its water system.
Pollution has depleted oxygen resources in waterways, inducing eutrophication in surface waters and exerting toxicity to water resources. Organics and nitrogen in their various forms can cause damage throughout the water system. Particularly hard hit is China's northern region, where rainfall is scarce.
To create sustainable economic development, the China Central Government has put in place measures to manage natural-resource consumption and environmental degradation.
Among those enacted 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:
- COD less than 50mg/l
- BOD less than 10mg/l
- Ammonium-nitrogen (NH4-N) to less than 5mg/l at more than 12˚C water temperature and less than 8mg/l at less than 12˚C
- Total suspended solids (TSS) to less than 10mg/l
- Total-nitrogen to less than 15mg/l
- Total-phosphorous to less than 0.5mg/l
- pH to 6-9
Financial resources are limited and, despite the country's enormity, 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 for many new treatment facilities. An added benefit of biological treatment is that it is a natural method for waste disposal.
A family of biological treatment technology that has been used successfully throughout the world for organic oxidation, nitrification and denitrification is the TETRA fixed-film biological reactor from Severn Trent Services (STS). Its fixed-film biological system, which received a US patent in April 2006, has also been used for pre-treatment, secondary treatment and tertiary polishing.
In the US, Florida's wastewater treatment plants and water reclamation facilities so commonly use the TETRA system that 12 of the last 15 winners of the Florida Water Environment Association's top advanced wastewater treatment award count on the system for biological nutrient removal. STS is currently commissioning the largest biological tertiary wastewater filter in the UK at Strongford wastewater works.
Using TETRA T-Blocks - a common design of filter floor 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. Three stages of biofiltration treat the 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 submerged denitrification filter (SDF), which provides pre-denitrification in an anoxic environment using BOD from the incoming wastewater as the carbon source for denitrifying the oxidised nitrogen (NOx-N) in the recycled flow from the submerged aerated filter (SAF).
The second stage is the SAF bioreactor, also in an up-flow mode, which oxidises the remaining BOD and NH4-N from the SDF effluent in an aerobic condition using oxygen supplied by compressed air. The media used in both SDF and SAF bioreactors is 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 a Denite denitrification sand filter, which removes TSS and NOx-N from the final effluent.
The filter is operated in a down-flow mode. And, if additional 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 performed to dislodge the retained TSS and excess biomass. During the denitrification operation, the Denite filter uses a bumping procedure - a very short duration of water backwash - that removes the nitrogen gas accumulated in the filter bed.
The TETRA three-stage, fixed-film biological treatment system was pilot-tested at the Jiashan Yao-chuang wastewater treatment plant in south-east China's Zhejiang Province. It consisted of three-stage bioreactors.
The first stage was the SDF pilot reactor, with a dimension of 0.406m in diameter and 2m gravel media or 0.259m3 of media volume. At a designed flow of 145l/h to the pilot plant, the hydraulic retention time (HRT) was 107 minutes or 36.7 minutes with a two times influent flow (2Q) recycle.
The second stage was the SAF pilot reactor with a size of 0.406m in diameter and 3.5m gravel media or 0.455m3 of media volume.
At the same flow rate of 145l/h, the HRT for the SAF was 188 minutes or 63 minutes with a 2Q recycle. The last stage was the DeepBed filter with a diameter of 0.406m and 1.8m sand media for a filtration rate of 18.34l/min/m2 at the same flow rate.
The two-month pilot study at the Jiashan Yao-chuang wastewater treatment plant demonstrated the ability of the TETRA Denite 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 200mg/l to 2mg/l, NH4-N from 82mg/l to 3mg/l, T-N from 88mg/l to 11mg/l and TSS from 165 to non-detect.
Based on the results from the pilot study, Jiashan Dadi Sewage Treatment Engineering began construction of a full-size treatment plant with an average wastewater flow of 20,000m3/d using the three-stage treatment process.
The full-size treatment plant is currently under construction in Jiashan, China, and start-up is expected in soon.
Dr Jeffrey J Chen, DEE, EMBA, Severn Trent Services.
T: 00 1 650 424 1876, firstname.lastname@example.org