Research On Treatment Of Organic Wastewater By Psychrotrophs

Biotreatment techniques are widely used in wastewater treatment. The predominant bacteria used in wastewater treatment are mesophilic bacteria, which are highly effective and active in normal temperature. But in cold winter, especially in north area of China, the sewage temperature always below 10℃, the activity of mesophilic bacteria existing in activated sludge were strongly weaken and even deactivated, which cause low efficiency and great difficulties meeting standards. The traditional process employed nowadays is to reduce the sludge loading rates and to prolong the hydraulic retention time, but it means a larger factory area and higher cost of operation. An essential way to improve the efficiency of low- temperature wastewater treatment is to enhance the effect of cold-adapted microorganisms.This paper made an attempt to study this issue from the view of microbiology. Nine psychrotrophs with high activity in low temperature(5~15℃) were isolated and selected from samples of northern China. These psychrotrophs were identified by morphological, physiological and growing characteristics identification. Two strains, with top high-rate growth, had been selected to research. The identification of 16S rDNA showed these two isolates belong to Pseudomonas sp. and Shewanella sp., respectively. They were designated as Pseudomonas sp. N and Shewanella sp. J5. According to the result of growth characteristics, Pseudomonas sp. N and Shewanella sp. J5 can both keep high growth rate in low temperature; their most suitable growth temperature is about 20℃; their suitable initial pH ranges are 5.5~8.5,6.0~8.5, respectively; their suitable salinity range are respectively 0~3%,1~5%. Besides, they can use many kinds of carbon and nitrogen sources, which mean they may have potential in degrading different types of organic wastewater.In order to explore the degradation characteristics of psychrotrophs from the enzymatic mechanism, two types of key enzyme in metabolism were studied. Results showed the two advantage psychrotrophs could keep high dehydrogenase activity in low temperature; Shewanella sp. J5 can both produce low-temperature protease and low-temperature amylase. Characterization of protease showed this enzyme had 20% catalytic activity at 0℃, the optimal pH was 8.0 and it could obviously stimulated by Mn2+,Fe2+. According to HPSEC results, it showed this enzyme has ability in degrading macromolecular peptides. The characterization of amylase showed this enzyme could keep 40% catalytic activity at 0℃, the optimal pH range was 7.0~10.0 and it depended little on metal ions. All these results indicated the two isolates had good degradation characteristics of organic pollutants under psychrophilic conditions.From a practical application point of view, the degradation capability of the two psychrotrophs was studied under aerobic and low-temperature conditions. Based on that, the removal capability of mixed strain and its application in activated sludge were simultaneously studied. Results showed the two strains were suitable for treatment of moderate or high strength wastewater; the best degradation conditions were as follows: pH7.0~8.0, DO 3.2~4.8 mg/L, aeration time 8 h. Under the best degradation conditions, the removal rates of COD,NH3-N by Pseudomonas sp. N could all reach 90%, while the removal rates by Shewanella sp. J5 could reach 77% and 70%. After mixed culture, the two strains showed better COD removal efficency, about 10~20% higher than single strain. Compared with activated sludge, below 10℃,the dehydrogenase activity of mixed strain is manifestly higher than activated sludge. It also found that the removal efficiency of low-temperature sewage can significantly improved by adding mixed psychrotrophs in activated sludge, especially when their VSS ratio is 1:1, the removal rates of COD,NH3-N could be increased by 40% and 70%. Additionally, it's confirmed that the mixed strain had certain capacity in degradation of different types of organic wastewater.In conclusion, the psychrotrophilic bacteria have great and bright potential in domestic wastewater treatment in cold area.