| In recent years, due to water pollution, the traditional way of eel culture by replacing large quantity of clean fresh water faces great challenges. This makes the development of adopting circulating water system for eel culture to be of great significance. Two sets of different aquaculture sewage treatments (A set and B set) in a circulating water system have been designed and practiced in eel culture. The areas of A set and B set were 302.86 m2 and 196.17 m2 respectively. The sewage from eel farming pond in A set was treated in order of descending polyamide yarn pool, uplink polyamide yarn pool, descending and uplink oyster shell pools. The sewage in B set was treated only by descending and uplink oyster shell pools. After treatment, water was reused for eel culture. The present research was mainly involved in the efficency of the two sets of sewage treatment. A study was carried out in A set on different daily treating sewage quantities to evaluate its efficiency. The treatment effect was also compared between A set and B set, and between polyamide yarn in A set and oyster shells in B set. Aerobic effect was evaluated by using different equipment in eel ponds. The culture effect was compared between the traditional way and circulating water aquiculture model. The results are as follows.1.Sewage treatment effect of A set. When daily treating sewage of 672m3 in A set, the removal efficiencies of TAN(tatol ammonia nitrogen),NO2--N and turbidity were 86.53±2.03%,95.32±1.54% and 90.5±2.4% respectively; the pH value was 7.39 in influent, and increased to 8.56 in effluent. When daily treating sewage of 846 m3, the removal efficiencies of TAN,NO2--N and turbidity were 82.3±3.83 %,94.2±1.34 % and 91.5±2.6%; the pH value increased from 7.39 in influent to 8.26 in effluent. When treating sewage of 1560 m3, the removal efficiencies of TAN,NO2--N and turbidity were 58.25±8.88 %,86.92±1.51 % and 88.9±3.5%; the pH value increased from 7.53 in influent to 8.05 in effluent. The results indicated that the removal efficency decreased with the increasing of the treating quantity,but the effluent water complied with the water quality standard of fishery.2.Comparative study on the sewage treatment effect between A set and B set. When daily treating sewage of 432 m3, the pH values in A set and B set rised from 7.36±0.07 in influent to 8.65 and 8.87 in effluent respectively. The DO(dissolved oxygen) in both sets was 8.89±0.93mg/L in influent, then were 9.34±0.69mg/L and 8.28±0.75 mg/L in effluent respectively. The average turbidity removal efficiencies in A set and B set were 94.54±2.43% and 91.73±3.94% respectively. The TAN removal quantities per unit area in A set and B set were 1325.13±171.49 mg/(m2·d) and 1838.81±269.16 mg/(m2·d), the former significantly lower than the latter (P < 0.05). The NO2--N removal quantity per unit area was 687.53±91.66 mg/(m2·d) and 678.27±122.74 mg/(m2·d) respectively, both had no significant difference (P > 0.05).3.Comparison of the treatment effects between the polyamide elastic packing in A set and the oyster shells packing in B set. When daily treating sewage of 432 m3, the removal quantities per unit area of TAN by polyamide elastic and oyster shells were 2238.63±168.83 mg/m2·d and 1625.94±181.28 mg/m2·d respectively; the nitrite nitrogen were 1142.64±317.29 mg/m2·d and 862.52±236.12 mg/m2·d; the activity phosphorus were 1347.84±535.17 mg/m2·d and 1548.86±491.76 mg/m2·d. The CODcr were 91597.36±8262.62 mg/m2·d and 68157.21±7415.47 mg/m2·d. When daily treating swage of 672 m3, the removal quantity per unit area of ammonia nitrogen by polyamide elastic and oyster shells were 2217.64±878.19 mg/m2·d or 1486.47±664.19 mg/m2·d; the nitrite nitrogen were 937.43±147.19 mg/m2·d and 611.10±95.78 mg/m2·d; the removal quantity of activity phosphorus were 923.44±233.00 mg/m2·d and 1607.59±300.34 mg/m2·d; the CODcr were 73734.93±3934.23 mg/m2·d and 55935.16±2050.11 mg/m2·d.4.The comparison of culture effects between circulating water aquaculture mode in A set and traditional way. The results showed that the growth rate of Auguilla rostrata in the circulating water model was 56.9%, feed conversion rate was 60.6%, and the survival rate was 99.9%, while the growth rate of Anguilla rostrata in the traditional way was 25.0%, the feed conversion rate was 56.5%, and the survival rate was 97.6%, In the circulating water model, the TAN concentration was 0.61±0.31 mg/L, NO2--N was 0.15±0.10 mg/L, the DO was 5.46±0.56 mg/L, pH was 7.36±0.26. while in traditional way, the TAN concentration was 0.84±0.15 mg/L, NO2--N was 0.11±0.06 mg/L, DO was 6.08±0.60mg/L, pH was 7.53±0.06. In energy conservation and emission reduction, the circulating water model could save more than 10.2% energy than traditional way and reuse 83.7% of discharged water.5.The comparison on the increasing oxygen by application of micropore aerators and waterwheel aerators in eel ponds. The results showed that there was significant difference in the aeration efficiency between the ways of aeration under unloading fish(P < 0.01). The aeration capacity of the micropore aerator was 15.85% higher than that of the waterwheel aerator, and the aeration efficiency of the former was 2.36 time as that of the latter. But under the case of loading fish, the average dissolved oxygen of surface water by the former was 5.81±0.14 mg/L, and that of the latter was 6.39±0.25mg/L; the dissolved oxygen of bottom water were 5.74±0.14 mg/L by the former and 5.75±0.26 mg/L by the latter. Using micropore aerator can save 57.6% electricity and was much safer than using waterwheel aerator. The eel grew much faster by the former than the latter.The effluent water from circulating water system was stability, and conform to the requirements for quality of the eel growth. The pH of effluent water rised obviously in both A set and B set. The removal effects of turbidity and nitrite nitrogen in B set were close to those of A set, and because of simple structure and low cost, B set had better application prospect. The treatment effect of polyamide elastic packing in A set and oyster shells packing in B had significant difference. The polyamide had better effect in the removal of ammonia nitrogen, nitrite nitrogen and CODcr because of its large surface area than the packing of oyster shells, but the removal effects of active phosphorus and turbidity of the polyamide were lower than those of oyster shells. It must be point out that the B set was new constructed, and there was less little biomass on the packing which might effect the treatment effect and need further study. In circulating water model in A set, the growth rate, feed conversion rate, survival rate and the cultivation effect were higher than those in traditional way. In comparison with waterwheel aerator, the way of microporous aeration achieved better cultural effect, and was more safety and energy saving.
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