| The detection of water toxicity using luminescent bacteria has been listed as one of the standard detection methods because it has the advantage of easy operation and quick response. Therefore, it is necessary to have the online continuous detection of water quality with the early warning function to ensure the water quality. The fact that major watersheds in China have their own monitoring stations makes it high demand for the equipments for online and in-situ detection of water toxicity. However, the imported equipments is bulky, expensive and complicated in operation, and the domestic equipment is less automated, which makes both of them difficult to meet the requirement of water quality online and in-situ detection. On the other hand, the decrease of luminosity of luminescent bacteria at ambient temperature limits its application in water toxicity detection. Thus, we prepared and compared the luminescence and stability of the continuous culture, lyophilized powder and immobilized bacteria of Photobacterium phosphoreum in order to identify the optimum forms for the in-situ, online and continuous detection of water quality. The results are as follows:(1) Continuous cultivation:the continuous cultivation was conducted in the 5 L fermentation tank for multiple batches. The longest cultivation time lasted for 12 days and the shortest one lasted for eight days. The specific luminosity during the successful continuous cultivation was above 700 mV/OD which met the requirement of continuous toxicity detection. However, this method may be not a good option for water quality online detection because the failure rate was as high as 50% due to the complicated device, operation difficulty and high chance of being contaminated.(2) Lyophilized powder:lyophilized powder was prepared by freeze-drying method. The prepared lyophilized powder remained active for two weeks under the temperature range of 4-35℃, which met the requirement of outdoor online continuous toxicity detection. The luminosity of bacteria was kept within the range of 1200-2000 mV and its variation among different batches was not obvious. Using lyophilized powder as the detection media, the correlation between luminescence intensity and ZnSO4 concentration was good. Combined with light detector, this method can be applied in the online and in-situ detection system of water quality.(3) Immobilization of bacteria:luminescent bacteria was immobilized into beads and films. The bacteria beads had better stability and uniformity with luminosity between 170 and 500 mV, and its discrete coefficient of dispersion ranged from 2.2 to 6.6%. On the other hand, the luminosity of bacteria films fluctuated within a wider range of 300-1400 mV with the dispersion coefficient ranged from 4.8 to 20.9%. Stored at -4℃, the luminosity of both of them was high enough for detection for one week. The bacteria beads showed good performance in toxicity detection using ZnSO4 as the test material. The short maintenance time of luminosity makes it not applicable to online and insitu detection of water toxicity.(4) Based on the above results, lyophilized powder was used for the detection of the performance of self-designed water toxicity detection. Photomultiplier tube of model H5784 was chosen as the photo sensor detector; and multi-meter of Victor 97 was chosen to convert A/D and acquire data. Voltage of 0.75 V was determined to be the working voltage. The detection of light signal indicated that the system has good stability and repeatability. Compared with the commercial DXY-3 biological toxicity detector, the self-made detector had the similar results. When it was used for detecting toxicity of ZnSO4, good correlation between luminosity and the test material concentration was observed.The results of this study provide theoretic and practical basis for the development of online, insitu and continuous detection system for water toxicity. |
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