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Solid waste degradation on a screen in a cold, oxygenated saline environment : applications for salmon farming Buryniuk, Mardell

Abstract

High accumulations of solid wastes generated by coastal fish farms may affect both the caged fish and the surrounding environment, and it is this concern, universal to fish farmers and environmentalists alike, that was the impetus for this study. A dynamic, deterministic simulation model was developed using a combination of empirical and mechanistic approaches to simulate the accumulation of solid waste on a screen-like device located beneath a fish farm. Solid waste was freshly collected and size-fractioned on fish farms, and transported back to the laboratory for experiments to determine the order and rate of the degradation equation. The solid waste was fragile and the larger particles easily broke down into smaller pieces. The mesh opening size required for 50% retention of the particles on a screen was approximately 8 mm. This result is required for further engineering of the screen-like device. The order of the degradation equation for the solid waste was 1.19, and the-associated rate constant was 0.0262. The major assumptions used to develop the model were that all solids produced by the fish farm would be trapped and would not include wasted feed, no erosion would occur, bacterial growth would not affect the mass on the screen, and the water temperature was a constant 8°C (the lowest water temperature at the site where the solid wastes used in the experiments were collected). Additionally it was assumed that the waste would fall in layers and mixing would not occur. These assumptions were built into the model in order to create a "worst case" scenario. It was assumed that biological degradation removed mass from the screens in the parameter estimation experiments and because the experiments were all done in oxygenated environments, it was assumed that the water surrounding the screen-like device below a fish cage would contain enough oxygen such that oxygen would not become a limiting factor in degradation. Accumulation reached 27 kg m⁻² (dry weight basis) after a 500-day production cycle under typical British Columbian coastal fish farm conditions. Assuming a harvest of all the fish on day 500, the waste would be 75 % (by mass) degraded in 78 days, 90 % degraded in 135 days, and the waste would be completely degraded within 275 days. The model was validated using waste outputs that represented approximately one week of solid waste output from near the end of the production cycle. Future work might be concentrated on the fate of the tiny particles passing through the screen-like device.

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