Aquaculture Waste Sediment Stability: Implications for Waste Migration

Abstract

Experiments were conducted in an annular flume using waste bed sediment from a discontinued aquaculture operation to assess its stability against erosion. Critical shear stress for erosion was measured under different flow conditions and after three different consolidation periods (2, 7 and 14 days). The influence of biostabilization was also assessed as a mechanism for controlling the bed sediment stability. Results suggest a moderate increase in bed sediment strength with time as evidenced by the increasing critical bed shear stress for erosion with increasing consolidation times. Critical bed shear stress for erosion ranged from 0.06 to 0.1 Pa. Eroded floc size and settling velocities were in the range that would allow for significant horizontal transport of sediments provided a flow was present (i.e. transport outside of the aquaculture pens). The increase in sediment strength with time is believed to be more strongly influenced by biofilm integration on and within the surface sediment layer than to consolidation and dewatering effects. Extensive biofilm growth was visibly evident and microscopy confirmed the presence of extensive filamentous organisms (likely of a fungal origin) and bacteria. The point of failure of this biostabilized sediment was significantly lower than that reported for other natural freshwater and salt water sediments. Regardless of the sediment type, however, biostabilization is a consistent and important mechanism which controls the stability of sediments. Factors such as the microbial community and sediment floc structure will need to be considered in order to improve our understanding of the mechanisms of bed sediment stability and erosion for the environmentally sustainable operations of aquaculture facilities.