Seawater Treatment For Biological Resource In Land-based Intensive Aquaculture

With the development of aquaculture, land-based intensive aquaculture produces pollution effect to coastal waters, because of the discharge of a large quantity of entropic water. This is one key factor that restricts healthy and sustainable development of aquaculture industry of China. There are a lot of N,P nutrients and suspended solids (rich in organic matter) in industrialized mariculture water and waste water. When physical-chemistry and biofilm water treatment technology are chose to deal with this kind of water, much energy will be used and many resources will be wasted. In order to acquire biological resources from aquaculture waste water, and to provide a reference for aquaculture plant, in this study field experiments have been done using two kinds of filter-feeding bivalve (Crassostrea gigas, Mytilus galloprovincialis)to remove suspended solids, and two kinds of macroalgaes (Gracilaria lemaneiformis,Laminaria japonica) to absorb and utilize nutrient in the water. The following are the main results:(1) Shellfish- macroalgae biofilter treated the water in the water reserve pool :high deposition efficiency for suspended solids,removal of many nutrient by Gracilaria lemaneiformis and Laminaria japonica。At suitable temperature(17～25oC),in the water reserve pool,bio-depositionrates of the two kinds of filter-feeding bivalves (BDR) were: 1.08¹.32 g?ind^-1?d^-1 (Crassostrea gigas) and 0.65⁰.85 g?ind^-1?d^-1(Mytilus galloprovincialis), respectively; but when the temperature was lower or higher, the BDR was lower, and the bivalve would die at high temperature. Though cultivation of a large number of bivalves in the pool, this study found that the DR (TSS) with bivalves cultivation was higher than no bivalves.The Gracilaria lemaneiformis had higher photosynthetic rate and N uptake rate in the pool. in the cultivation period of 40 days; and its weights increased 3.73 times; Also, Laminaria japonica increased 4.8 times in weight in 68 days. Results indicated that the two kinds of algae could grow well in the pool; and through analysis of the C,N,P contents in algae, we found that the two kinds of macroalgaes could remove some nutrients from the water in the pool.Therefore, in the water reserve pool, bivalve and algae could remove many pollutants, which could not only reduce the pollutant load of discharged water, but also realize resource utilization as the algae grew, and then obtained certain economic products.(2) Filter-feeding bivalve biofilter had good filtering effect on SS in Cynoglossus semilaevis Günther aquaculture wastewater.In the experimental conditions, water temperature 20.1～24.6 oC; DO 6.04～7.83 mg?L^-1; pH 7.6～7.8; Salinity21±0.5‰,Q=40¹00L/h,the BDR was 11.94⁷7.84 mg?ind^-1?d^-1(Crassostrea gigas),0.81⁶. 37mg?ind^-1?d^-1(Mytilus galloprovincialis). When Q=80L/h, the BDR was maximum: 77.84±7.77 mg?ind^-1?d^-1 (Crassostrea gigas); 6.37±0.27 mg?ind^-1?d^-1(Mytilus galloprovincialis).Thus oyster's BDR was higher than mussel's.Through determination of POM,POC,TN,TP,IP,OP in sediments, this study showed that their contents for the two kinds of bivalve system were all lower than those for the control system. So the bivalves could absorb and use part of the organic matter in SS in aquaculture wastewater. The analysis of significance of difference in organic matter composition showed both the two kinds of bivalves'sediment were much significantly different to the control system, when the water-flow rate was 60 L/h.Therefore, we concluded that the two species of Crassostrea gigas and Mytilus galloprovincialis had a considerable potential to filter and removed suspended particles from aquaculture wastewater,and simultaneously yielded the biological resource use.(3) Gracilaria lemaneiformis absorbed and used the nutrient in Cynoglossus semilaevis aquaculture wastewater.This study showed that the removal rate of NH4-N, NO3-N, NO2-N and PO4-P of aquaculture wastewater in 24h hour were 57.45%,6.50%,0.31%,8.17%, respectively; and in 72h were 87.02%, 21.21%, 7.85%, 21.68%, respectively. The Gracilaria lemaneiformis first absorbed the NH4-N, then NO3-N and NO2-N. The daily growth rate of Gracilaria lemaneiformis was 5.22±0.057% in flowing Cynoglossus semilaevis aquaculture wastewater. So the Gracilaria lemaneiformis could grow in this wastewater and absorb nutrient.A conclusion is drawn in this study: filter-feeding-bivalve biofilter had good filtering effect on SS in both the water–reserve-pool water and Cynoglossus semilaevis Günther aquaculture wastewater, playing a role in both pretreatment and treatment of the culture water. Particularly the average levels of organic matter composition in sediments of the two kinds of bivalve system were all lower than those in the control system, with achievement of resource. Also, the two kinds of macroalgaes could grow in the water-reserve pool with abundant nutrient. Particularly, the higher growth rate and absorbance rate (N, P) in Cynoglossus semilaevis aquaculture wastewater, indicated that microalgae can complete two task of removing pollutants and achieving resources. Therefore, we can construct one bivalves-algae filtration system to efficiently remove and use SS and nutrient in recirculation aquaculture farm.