A Phase II USDA SBIR grant to develop cost effective technologies for the indoor recirculating production of the marine shrimp (Litopenaeus vannamei) was awarded to AST. An obstacle to obtaining commercial success was identified as low tank production densities. Phase I studies demonstrated commercially attainable production densities of 5 kg/m3 (25 count) across both biofloc and fixed film formats. In the first Phase II study, a fixed film water treatment format was evaluated at a small 250-L scale in triplicate resulting in a mean production density of 10 kg/m3 falling short of the 15 kg/m3 required for cost effective tank production in small-scale operations. Loss of larger molting shrimp during periods of elevated nitrite levels (>0.6 mg-N/L) appearing to limit higher densities. The findings were consistent with the working hypothesis that that RAS densities are limited by water quality although the nitrite peaks are suspected of causing the issues here was much lower than expected. The experiments are now being repeated with more stringent nitrite control mechanisms.
Moving forward the AST RAS design will focus on the use of a fixed film PolyGeyser® floating bead filter supported by an aerated sludge basin (ASB) that will provide addition nitrification support external to the tank. Phase II studies that employed bead filter with an in tank biofloc nitrification were thwarted by the complexity of maintaining target tank suspended solids in the tank while a bead bioclarifier was constantly removing it. Balance was achievable in well trained hands, but, deemed unfeasible for small scale commercial facilities. Moving the biofloc to an external sludge basin significantly simplified the integration of the biofloc and fixed film technologies. The emerging designs will employ pneumatic sludge and water transfers between the PolyGeyser® and the ASB while water movement with the tank will be implemented by airlift. It is projected that these design improvements will lower capital cost while the density studies improve production improving economic feasibility.