Optimization of algal turf scrubbers for large-scale super-intensive aquaculture
Mitchell, Jacqueline M.
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Since their invention in the 1980s, algal turf scrubbers (ATS) have been successfully used to remove nitrogen and phosphorus from eutrophic waterways. Along with the nutrient mitigation properties of ATS, the biomass generated is also of interest as a potential source of biofuels. Much of the research has focused on nutrient removal by ATS systems rather than increasing biomass production. Therefore, two experiments were conducted sequentially to test the effect of substrate material and harvest interval on biomass productivity. For the substrate experiment, three substrate types (2-D, 3-D, and advanced 3-D) were tested against a control (no mesh substrate). Mean (n=3, ±s.d.) productivity of the advanced 3-D substrate (6.1±2.5 g AFDW/m2/day) was significantly higher than both the control (3.5±2.4, P < 0.001) and the 2-D substrate (4.5±1.9, P= 0.033), but not the 3-D substrate (4.9±1.8, P > 0.05). The advanced 3-D material was then used to test the effect of harvest interval (4, 7, 10, and 14 days) on biomass production. The 4 day harvest schedule produced significantly (P < 0.004) more biomass (21.9 ± 7.0 g AFDW/m2/day) than the other treatments (7, 10, and 14 day harvest intervals were 17.7 ± 7.1, 13.0 ± 5.0, and 10.3 ± 4.6, respectively). Compositional analysis was performed for the ATS biomass in each experiment to determine its potential use as a biofuel. For both experiments there was no difference for protein (% of dry biomass), fat (%), and fiber (%) content among treatments (P > 0.05). Protein, fat, and fiber averaged 8.1% (± 2.1), 0.09% (± 0.1), and 1.66% (±0.5), respectively, for the substrate experiment and 7.03% (± 0.6), 0.06% (±0.1), and 1.34% (±0.4), respectively, for the harvest interval experiment. During each of the experiments water samples were taken from the head tank and at the end of each ATS lane to determine the nitrogen removal rate. The low ammonia-nitrogen levels in the incoming water combined with the short residence time on the flow ways and measurement variation between samples prevented the nitrogen removal rate from being determined for either experiment. With these parameters somewhat optimized, future experiments should determine the nitrogen removal rate for the ATS system so that an appropriately sized ATS for super intensive shrimp aquaculture can be modeled.