Treatment Of Undissolved Effluent From Aquaculture By NFT-cultured Botanical Filter

Aquaculture is the fastest growing food production industry in the world. With the development of scientific technology, it will probably be the second largest food industry. Recirculating aquacultural system has become the development trend for its high density, high resource utilization rate and sound benefit. Because recirculating aquacultural system is intensive, there is no doubt if the accumulated waste were discharged at random, it would destroy the environment. Research on the wastewater from aquaculture was conducted in a wide range and some technologies were applied in practical use, which achieved ideal effect. However, traditionally, effluent containing settleable solids was used to recycle in the way of land application and composing. Though these methods are convenient and easily operated, it will still have negative effect on environment. We need adopt more ecological, more beneficial management to treat this undissolved effluent. Undissolved effluent mainly consisted of organic matter which contained plenty of N, P. Nitrogen and Phosphorus is the necessity for plant growing, meanwhile, its interlaced root zone could play the role of mechanical filter. Upon above mechanism, if we could use plant as treatment core by some engineering measure, it will be a kind of engineering technology with economical, ecological and social benefit.In this project, "treatment of undissolved effluent from aquaculture by NFT-cultured botanical filter", the characteristics of undissolved effluent was analyzed and its inner variation was investigated by two treatments, covered and exposure. The concept of integrated filtration layer was brought forward on the purpose of the removal of undissolved effluent. In order to achieve better filtration efficiency, we considered number of layers and filtration time as optimization parameter. On the basis of above two experiments, the NFT-cultured botanical filter was used to treat undissolved effluent after 1 d. treatment and its purification effect was evaluated after filtering and circulating irrigation. Details were listed as followed:(1)Undissolved effluent from sedimentation tank contained high TS with concentration of 700mg/l and NO3-N was extremely high, reaching 8.6mg/L. On the contrary, the concentration of TAN was only 1.2mg/L. Ryegrass could tolerate relatively high TAN by absorbing, but high content of NO3-N is not beneficial for grass absorption and purification. It is necessary to find some way to minimize the NO3-N.(2)Two different treatment of covered and exposure had significant influence on undissolved effluent during the placement. The results indicate that, under the covered treatment, ammonification and denitrification occurred in the inner of effluent for it provided a relatively anoxic environment. TAN and UIA increased linearly with daily increment of 0.67 mg/L On the contrary,NO₂^-1-N, NO₃^-1-N, TN.COD decresed.especially, NO₃^-1-N decreased from 7.65 mg/L to 0.7 mg/L and its conversion rate is as high as 91%. Under exposure state, nitrifying bacteria were inhibited by unionized ammonia, so its inner occurred denitrification and mineralization initially with the increase of TAN,UIA and the decrease of NO₂^-1-N, NO₃^-1-N, TN . During the last days of experiment, nitrifying bacteria were accustomed to the environment, nitrification took place. Because in the exposure state, sunshine entered into the tank, a great quantity of algae appeared which increased the content of DO and COD.Two treatments had no significant effect on the conversion and stabilization of Phosphorus. The study showed that both of two treatments could reduce NO3''-N, TN and attained prospective requirement. While, the covered treatment weighed out of exposure treatment,(3)The integrated filtration layer was the treatment core including the stem and leaves of grass, layers of unwoven fabric, plate bottom and root. We optimized integrated filtration layer by different layers of unwoven fabric and filtration times and investigated the removal efficiency of TS, TVS, and TSS. 1-layer fabric, 3-layers fabric, 5-layers fabric,7-layers fabric were four treatment. The results suggested that different layers of unwoven fabric had different effect on removal of total solid (TS), total volatile solid (TVS).After undissolved effluent was filtered by integrated filtration layer, TS removal rate increased with layers. 3-layer fabric removed 22.2% TS more than 1-layer fabric, 5-layer fabric removed 5.5% TS more than 3-layer fabric, and 7-layer fabric removed 0.8% TS more than 7-layer fabric. Similar results were obtained about TVS. By contrary, the removal of TSS is so low that showed no difference in layers. Taken the effect and economical factor into consideration, 3 layers is the most suitable choice. Basing on the previous experiment, we studied on the relationship between different filtration times and removal effect through 3-layers. As a result, l-time filtration reduced TS, TVS, TSS by 50.3%, 51.2%, 2.0% respectively.3-time filtration and 5-time filtration removed 3.2%,2.0%% TS, 3.2%,1.3TVS, and 1.7%, 1.5% TSS more than l-time filtration. The experiment indicated that increasing filtration time in short period didn't help to remove solid and it should need a long time.(4) After 1-day treatment using NFT-cultured botanical filter N, P in the effluent were purified greatly with average removal rate of 63.7% and 78.8% respectively. Especially, the removal of P showed that it's a good candidate for treatment. The average removal rate of TS, TVS, and TSS were also high, reaching66%, 70.9% and 87.6% respectively. This experiment suggested the mechanical filtration is not the main mechanism for TSS removal and circulating regulation create a favorable condition for the growth and propagation of microorganism around rhizosphere. Suspended solid were flocculated, decomposed and absorbed by microorganism. The experiment were carried out for 9 times and the results explained that in the range of study, the relationship between TN removal load and its pollution load existed linearly, so did TSS. It also pointed out the relationship between TP removal amount and TSS removal amount was exponential. With the increase of pollution load, the removal amount of TN, TP, and TSS will enhance accordingly, so it requires pre-treatment to enhance the concentration of undissolved effluent in order to obtain a high removal rate.