|Other Abstract||Biofilms play an important role in wastewater treatment by constructed wetlands. The structure, time and spatial distributions, dehydrogenase activity and polysaccharide, N and P purification capacity of substrate biofilms in the integrated vertical-flow constructed wetland (IVCW) as well as pH changes of suspended solution of substrate biofilms in the case of different nutrients were studied. The main results were summarized as follows:
1. The thickness of biofilms increased with the culture time in the lakes investigated. The dry weight of biofilms in the depth of 0.3 m was higher than that in the depth of 1.5 m. Only some algae, especially diatom species, were observed in the substrate biofilms of the IVCW system, but no zooplankton were observed.
2. The dehydrogenase activity and polysaccharide content of substrate biofilms were significantly correlated with the volatile weight of substrate biofilms in the up-flow and down-flow chambers, respectively (P<0.001). For the same chamber, the dehydrogenase activity, polysaccharide content and volatile weight of biofilms were significantly different among different depths. For the surface layer, the values of all the parameters measured in the down-flow chamber were significantly higher than that in the up-flow chamber. The activity and polysaccharide content of biofilms in summer were respectively higher than the activity and polysaccharide content in autumn, and the volatile weight of biofilms in summer was almost the same as that in autumn.
3. The dehydrogenase activity of biofilms increased with the extended sampling time, and the activity data at different concentrations of carbon sources were fit with a quadratic curve.
The dehydrogenase activities of biofilms showed a quadratic curve relationship to the concentration of nitrogen source. The nitrogen sources inhibited the activity of biofilms within 72 hours. When KNO3, NaNO2 or NH4HCO3 was add in the artificial wastewater, the polysaccharide content of biofilms increased with the increasing concentration of nitrogen source. When the concentration of P was in excess of 3.2 mg/L, the dehydrogenase activity of biofilms was improved and the polysaccharide was accumulated advantageously.
Results of the orthogonal array showed that C6H12O6 and KNO3 were the main factors for the dehydrogenase activity and polysaccharide content of biofilms, respectively. The optimal combination for dehydrogenase activity was obtained by locating the concentrations of C6H12O6, KNO3 and NaH2PO4 at 0.2, 0.8 and 0.05 g/L, and the optimal combination for polysaccharide content was obtained by locating the concentrations of C6H12O6, KNO3 and NaH2PO4 at 0.2, 0.4 and 0.2 g/L. The corresponding maximum activity and polysaccharide content were 5.40 µg TF/g /12h and 3454.6 µg/g, respectively.
Of the dehydrogenase activities of biofilms at 15 °C, 25 °C and 37 °C, the activity at 37 °C was the highest before 72 h, and the activity at 25 °C was the highest after 72 h.
The dehydrogenase activity of biofilms was the highest when the solution was neutral (pH=7). At the same time, the more the acidity or basicity of the solution increased, the more the activity of biofilms declined.
The dehydrogenase activity of biofilms which were cultured in the dark was higher than the activity of biofilms which were cultured with light before 48 h, subsequently the light condition took advantage over the dark condition.
Within 72 hours, PCP was disadvantageous to the dehydrogenase activity and polysaccharide of biofims. Moreover, both vitamin C and humic acid could stimulate the dehydrogenase activity and polysaccharide of biofilms inhibited by PCP.
2 mg/L Zn2+ additons benefited the dehydrogenase activity and polysaccharide of biofilms within 72 hours. Co2+ and Mn2+ inhibited the dehydrogenase activity of biofilms, and the inhibitory effects became increasingly obvious with the increasing concentrations of these two metal ions and extended experimental time. Besides, Co2+ and Mn2+ presented no obvious effects on the polysaccharide content of biofilms.
Dehydrogenase activities decreased significantly with the increasing concentrations of Cd2+ and Pb2+ at 6 h and 24 h in the case of single and combined treatments. A synergistic effect of Cd2+ and Pb2+ was observed. For polysaccharide content of biofilms, there were no significant statistical differences within the range of concentration and time studied, whether singly or in combination.
4. NO3--N could be always removed by the substrate biofilms in the IVCW system in this study. The optimal time for N removal was 48 hours. N removal rate was about 15% at 15 °C, and the rate decreased with the increasing temperature. The alkaline condition (pH=8.5) benefited N removal. Exogenous humid acid additions benefited N removal. C6H12O6 at higher concentrations improved N removal rate, whereas KNO3 at higher concentrations deteriorated the rate. It indicated a weak linear relationship between NO3--N removal rate and the dehydrogenase activity of substrate biofilms (R2 =0.415).
Although the negative removal rate of P occurred in this study, the effects of time, temperature and humic acid on P removal rate were similar to that of time, temperature and humic acid on N removal rate. The acidic condition (pH=5.5) benefited P removal. In opposition to the effect of C6H12O6 on N removal rate, C6H12O6 at higher concentrations was disadvantageous to P removal, and the combinations of C6H12O6 and KNO3 at higher concentrations caused the negative removal rate. The polysaccharide contents of substrate biofilms were cubicly correlated with the P removal rates (R2 =0.518).
5. Of the six treatments studied, pH changes in treatment 1 (CO(NH2)2 and CH3COONa were nitrogen and carbon sources, respectively), treatment 4 (NH4HCO3 was nitrogen source as well as carbon source) and treatment 5 ((NH4)2SO4 and NaHCO3 were nitrogen and carbon sources, respectively) were similar, namely pH decreased first, then increased. However, the ranges of their respective changes in these three treatments were different. On the other hand, pH changes in treatment 2 (NaNO2 and CH3COONa were nitrogen and carbon sources, respectively), treatment 3 (KNO3 and CH3COONa were nitrogen and carbon sources, respectively) and treatment 6 (KH2PO4 and NaHCO3 were nitrogen and carbon sources, respectively) were also similar, namely pH values ranged from 7.3 to 7.4 before carbon source was added, and increased to about 9.0 after carbon source was added.|