| The discharge standard of municipal wastewater has been improved in China recently in order to control serious eutrophication. To guarantee the concentration of nitrogen(N) and phosphorus(P) in effluent meets with the First A Standard, it is necessary to explore new technologies in the removal of N and P. Algal-bacterial symbiosis system can utilize N and P to constitute the cellular structure. And the nutrients in sewage were removed when algae, aquatic plants that easily uptake N and P, were co-cultured with bacteria. Thus, the establishment of algal-bacterial symbiosis system with suitable species of algae and bacteria can improve the nutrient removal efficiency as well as recycle bioresource with low energy consumption. This new technology has been considered as a promising method in wastewater treatment.This paper establised a high efficient algal-bacterial symbiosis system with Oedogonium brevicingulatum, a dominant alga identified in a secondary setting tank in a municipal wastewater treatment plant, and activated sludge from the same plant. And then, the removal efficiency of N and P by the system was evaluated. Meanwhile, the process of N and P removal was explored in detail. Based on the above, the effects of water quality conditions, e.g., ratio of carbon to nitrogen(C/N), water temperature, p H value and toxic pollutants, on the removal efficiency were investigated. Furthermore, the treatment efficiencies of wastewater samples from a certain municipal wasterwater plant by the algal-bacterial symbiosis system were obtained. The following conclusions were obtained:(1) The removal efficiencies of N and P by the algal-bacterial symbiosis systems were higher than those by single algal or single bacteria(activated sludge) system. A n optimal symbiosis system with the ratio of algae and activated sludge(RAS) of 1:1 was obtained with the best removal efficiencies of ammonium(NH4+-N, 99.0%) and phosphorus(PO43--P, 97.0%). In the nutrient removal process by the optimal symbiosis system, it was observed that the bacteria grew fast in the initial 3 days(with the biomass of activated sludge increasing from initial 0.09 g L-1 to 0.23 g L-1). After that, the number of bacteria decreased(with the biomass of activated sludge decreasing to 0.19 g L-1), however the algae started to grow fast(with the biomass of algae increasing from initial 0.09 g L-1 to 0.20 g L-1). This phenomenan led to the high nutrients removal efficiencies during the whole cultivation process. Uptake of N and P by the biomass was confirmed to be the main mechanism of nutrient removal in the optimal system, which separately accounted 80.7% and 57.0% of NH4+-N and PO43--P removal at the 6th day. Besides, there were good positive linear correlations between dissolved oxygen(DO) level and the removal efficiency of NH4+-N and PO43--P, the generation of NO3--N and the RAS value, with R value of 0.88, 0.82, 0.97 and 0.88 respectively in Pearson correlation analysis, which suggest that the DO level plays an important role in nutrient removal process by the symbiosis system. Furthermore, the settlability of the algal-bacterial system was superior to single algae or single bacteria(activated sludge) system, with the stable settle ratio of 22% achieved after settling 6 min and the turbidity of suspention liquid of 4 NTU achieved after settling 30 min.(2) Under laboratory conditions, the nutrients removal efficiency were affected significantly by C/N and water temperature, but little influenced by p H value. As to toxic pollutants in wastewater, Cetyltrimethylammonium C hloride(CTAC) had higher inhibition effect on the nutrient removal than heavy metal Cd2+ did. After 6 d, the removal efficiencies of NH4+-N and PO43--P increased from 27.5% to 99.5% and from 19.9% to 98.7% respectively when C/N increased from 2.5 to 25, and increased from 31.6% to 99.0% and from 57.9% and 97.0% respectively when water temperature ranged from 5oC to 25 oC, while remained 95.7%~99.3% and 96.9%~99.6% respectively with p H value ranging in 6~8. When the concentration of Cd2+ in wastewater increased from 0 to 3 mg L-1, the removal efficiencies of NH4+-N and PO43--P by the system varied little from 99.0% to 98.0% and from 97.0% to 97.6% respectively, however decreased from 99.0% to 65.0% and from 97.0% and 61.9% respectively when the concentration of CTAC in wastewater increased from 0 to 3 mg L-1.(3) When ultilizing the optimal algal-bacterial symbiosis system to treat samples from the influent, the effluent of oxidation ditch, the effluent of secondary settlementation tank of a certain municipal wastewater treatment plant, the removal efficiencies of NH4+-N, TN, TP and COD of the influent were 98.2%, 83.1%, 98.1% and 92.5% after 6 d, respectively, those of the effluent of oxidation ditch were 92. 8%, 74.1%, 99.3% and 76.4%, respectively, and those of the effluent of secondary settlementation tank were 66.8%, 34.3%, 98.8% and 47.4%, respectively. Meanwhile, the p H value in all samples remained at 6.15~7.04 during the treatment process. All the above indexes reached the first grade A standard of Discharge Standard of Pollutants for municipal wastewater treatment plant(GB18918-2002). |
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