Optimal ratios of oyster to Gracilaria improving removal efficiency of nitrogen nutrients in aquaculture wastewater

Abstract: Currently, intensive marine aquaculture has been widely adopted to obtain higher yields of aquatic food production. However, the effluent unfiltered wastewater, including organic and inorganic nitrogen and phosphorous can damage the physiology of farmed species and cause hyper-trophication of adjacent ecosystems. Using bivalve mollusc and macroalgae to purify wastewater before discharged is gaining extensive attention due to their cost effect and efficient features. The present study was conducted to investigate the effectiveness of an oyster-Gracilaria system. The study focused on the effects of Gracilaria lemaneiformis densities and Crassostrea gigas to Gracilaria lemaneiformis ratios. This experiment contained two parts. In the first part, four densities of Gracilaria (0.8, 1.6, 2.4 and 3.2 g/L) were set up. The results showed that all the four treatments had a low total ammonia nitrogen removal rate (less than 40%) before 48 h. However, treatments of 2.4 g/L and 3.2 g/L had a relative high TAN removal rate in 60 h (54.34% and 67.70%, respectively), 72 h (39.77% and 48.27%, respectively). Gracilaria of 1.6 g/L and 2.4 g/L removed relative more NO2-N in 6 h (56.64% and 55.26%), and in 12 h. In comparison, 2.4 g/L had a significant higher (P<0.05) NO2-N removal rate (57.33%) than other groups. Significant difference (P<0.05) of NO3-N removal rate occurred in 6, 18 and 72 h, with the highest removal rate in sets of 3.2 g/L (11.72%), 2.4 g/L (15.65%) and 2.4 g/L (7.61%), respectively. In the second part, a two-factor experiment was designed. Three gradient densities of oyster (1.6, 2.4, 3.2 g/L) and Gracilaria were set up and combined to each other. Results showed that 2.4 g/L oyster and 2.4 g/L Gracilaria lemaneiformis combination had higher NO2-N removal rate (40.29%) than other treatments. There were significant (P<0.05?) interaction on TAN removal efficiency between the densities of oyster and Gracilaria lemaneiformis and Gracilaria lemaneiformis played a leading role (Gracilaria of 2.4 g/L removed 74.44% of TAN). The combination of 2.4g/L oyster and 1.6g/L Gracilaria lemaneiformis had better NO3-N removal rates, while there was no significant (P>0.05) difference with Gracilaria lemaneiformis of 2.4 g/L. Therefore, both oyster and Gracilaria lemaneiformis had the best efficiency in treating wastewater at the density of 2.4 g/L (Oyster: Gracilaria=1:1).