|Other Abstract||With the development of aquaculture industry in China, the traditional culture mode has made serious impacts on the sustainable development of the aquaculture industry, including the deterioration of water quality, the occurrence of disease, the decline of aquaculture product quality, and so on. Aiming at these problems, the present research was carried out to apply the Integrated Vertical-Flow Constructed Wetland to the purification and management of the pond water. The water quality in the recirculating aquaculture system had been monitored for three years to find out whether these problems encountered in aquaculture industry could be solved by the constructed wetland. Researches were carried out from four aspects as follows: 1) the effect of the operation time on the purification performance of the constructed wetland; 2) the effect of the hydraulic loading rate on the purification performance of the constructed wetland; 3) the function of the constructed wetland in the nitrogen budget of the constructed wetland-pond recirculating aquaculture system; 4) what caused the off-flavor problems and the performance of the constructed wetland on the control of the off-flavor problems. The main research contents and the main results are summarized as follows:
1) Significant differences of several physicochemical parameters (TSS, CODcr, BOD5, TN, NH4+-N and NO3--N) were found between the influent and effluent of the constructed wetland through three years’ (2005-2007) monitoring. There was high treatment efficiency of the pond water by the constructed wetland, especially referring to the physicochemical parameters. The average removal rates of TSS, CODcr and BOD5 were 57.3%, 40.1% and 52.0%, respectively; those of TP, TN, NH4+-N and NO3--N were 21.3%, 52.6%、44.5% and 34.0%, respectively. The removal rates of Chl.a, CODcr, BOD5, TN, NH4+-N and NO3--N were stable all through three years. However, there was a decrease tendency of the removal rates of the TSS and TP from 2005 to 2007. All parameters except dissolved oxygen in the effluent of the constructed wetland could meet the standard of national fishery water quality. After all, dissolved oxygen could be recovered by the oxygen supplying equipment.
2) The effect of the hydraulic loading rate to the purification performance of the constructed wetland was studied on a small-scale constructed wetland with an area of 2 m2. There were high purification performances of CODcr, BOD5, Chla, TSS, TP, IP, TN and NO3--N at the hydraulic loading rates from 200mm/d to 800mm/d. At a lower hydraulic loading rate, the removal process was mainly found in the upper layer of the down-flow chamber. As the hydraulic loading rates increased from 200mm/d to 1000mm/d, the treatment efficiency of the wetland decreased accordingly. Negative removal rates of NO3--N, TP, Chla and TSS were found at 1000mm/d.
3) The nitrogen budget in the constructed wetland-pond recirculating aquaculture system showed that feed was the main source of the nitrogen gains, which accounted for 84.9% of the nitrogen gains, while catfish harvest was a main source of nitrogen losses, which accounted for 34.1% of the nitrogen losses. Constructed wetland was helpful for the control of nitrogen accumulation in the system. The nitrogen removed by the constructed wetland accounted for 49.8% of the total nitrogen gains; it was concluded that denitrification in wetland accounted for 41.7%, while substrate accumulation and plant assimilation accounted for 4.0% and 4.1%, respectively.
4) Relevant study on the management of off-flavor problems from 2006 to 2007 showed that the concentrations of physicochemical parameters such as TN, TP, CODcr and TSS in the static pond were significantly higher than the recirculating pond and the concentrations of TP, BOD5 and CODcr were significantly higher in the recirculating pond with a higher fish density, the increase of which accomplied with increases of the concentrations of TP, BOD5 and CODcr. Water recirculation by the constructed wetland was helpful for the control of dominance of cyanobacteria, and the species and density of cyanobacteria in recirculating pond were lower than those in the static pond. In 2007, MIB, geosmin and β-cyclocitral were detected in the static pond, with average concentrations of 63.3ng/L, 152.6ng/L and 254.8ng/L, respectively. However, only lower concentration of geosmin was detected in the recirculating pond. In 2006, these odorous compounds were also detected in the recirculating pond with higher fish density. Among them, the concentration of geosmin was higher than that in the static pond, while the concentration of β-cyclocitral was lower. It was found that the occurrence of Oscillatoria sp., Microcystis aeruginosa and O. kawamurae would be related with the three odorous compounds, and the trophic level was the main reason of the off-flavor problems.
5) As the concentration of geosmin in the influent varied from 5.6-255.3ng/L in 2006, the removal rate of the constructed wetland was from 67% to 81%. In 2007, the average removal rate was 88% when geosmin concentration varied from 3.5-88.0ng/L. In the small-scale constructed wetland, a high removal rate of geosmin was found at hydraulic loading rates from 200mm/d to 400mm/d, while there was a negative removal rate as the hydraulic loading rate increased to 1000mm/d. The average removal efficiency of β-cyclocitral was 94% as the hydraulic loading rate varied from 200mm/d to 800mm/d. However, a negative removal rate was also found as the hydraulic loading increased to 1000mm/d.
6) The concentrations of two odorous compounds (geosmin and MIB) detected in fish samples from the static pond were significantly higher than those from the recirculating pond in 2007. Geosmin concentration in fish samples from the static pond was above OTC from July to October and MIB concentration was above OTC on October, while the concentrations of both gesomin and MIB were below OTC in the recirculating pond. In sediment samples, both geosmin and MIB concentrations in the static pond were significantly higher compared with those in the recirculating pond. According to the correlation analysis of the odorous compounds in water, fish and sediment samples, significant correlation relationships were found between each two of them. In addition, significant correlation relationships of actinomycete numbers in sediment samples and geosmin concentration in fish and sediment samples were also observed.|