Semi-closed circulation integrated multi-trophic aquaculture treatment system using aquatic organisms as biofilters to improve shrimp effluent quality

Abstract

Rapid development of aquaculture industry has contributed to the degradation of the coastal environment, and the waterways receiving effluent discharged. The wastewater contains high amount of excess nutrients and total suspended solids (TSS). Shrimp wastewater is rich in excretory waste products because the cultured shrimps could assimilate only 23-31% nitrogen and 10-13% phosphorus of the total inputs. This significantly causes water quality deterioration, outbreaks of shrimp diseases and affect shrimp production. This study focused on biological treatment option using macroalgae to balance the negative impact to the ecosystem. Thereby, Gracilaria edulis and Ulva lactuca were selected and potential as biofilters to improve shrimp water quality has been tested and verified in several laboratory and outdoor tank-scale experiments. Biofiltration potential of macroalgae in outdoor tank shrimp wastewater recirculation system had demonstrated considerably high nutrient removal efficiencies for ammonium, nitrate and phosphate concentrations such as 86%, 53% and 78% for G. edulis and 70%, 42% and 90% for U. lactuca, respectively. Furthermore, semi-closed circulation integrated multitrophic aquaculture treatment system fabricated at outdoor of laboratory with integration of tiger shrimp cultivation and treatment units such as sedimentation tank, green lipped mussel in a spray tank cultivation system and macroalgae tank. Sedimentation tank, reduced TSS, chlorophyll-a, and turbidity by 40%, 22% and 43%, respectively. Mussel system depicted reduction of 65%, 67% and 54.0%, respectively. Whereas, macroalgae had presented remarkable removal efficiencies for ammonium, nitrate and phosphate concentrations by 98%, 79% and 89% for G. edulis and 85%, 63% and 96% for U. lactuca. Besides that, G. edulis had exhibited removal for total nitrogen (TN) by 40%. On the other hand, U. lactuca had shown greater removal for total phosphorus (TP) with 80% removal efficiency, followed by G. edulis by 62.0%. Tissue analysis had demonstrated that the final nitrogen content almost doubled than the initial value in both macroalgae. The final phosphorus content of the tissue in G. edulis has doubled and U. lactuca has shown an increase of about 1.5 times. In addition, the mean growth rate for G. edulis and U. lactuca were about 4.4 % d-1 and 3.7% d-1 respectively. The mean growth rate of shrimp in treatment tank was 1.31 ± 0.76 % d-1, doubled compared to control tank with 100% survival rate. Furthermore, maximum sustainable yield approach revealed the optimum macroalgae biomass for harvest to improve the performance of biofiltration capacity. Thus, G. edulis and U. lactuca are suitable as biofilters and potential applications of these findings include improvement of shrimp water quality to an acceptable level that ultimately enhance shrimp and macroalgae productivity besides produces an ecologically sustainable treatment and integrated system.