Optimised effluent treatment by stabilised trout faeces

The efficiency of mechanical effluent treatment depends on the ratio of particle bound waste load to total waste load and on the size distribution of the suspended solids. We tested an innovative approach to increase mechanical treatment efficiency by adding small amounts of indigestible binders to trout feed. The hypothesis was that the addition of binders would enhance faecal stability and improve shear resistance. This should slow the breakdown of faeces by water turbulences resulting in larger particles. These are easier to remove by filtration and should be more resistant to leaching than small particles, so more soluble waste remains particle-bound and removable. In two separate trials, portion size trout were fed diets containing indigestible binders, while control groups received the same basal diet without binder. The fish were reared in 500 L circular tanks. The effect of each binder on the stability (viscosity and elastic resistance) of dissected faeces was measured by a rheometer. Both stability parameters increased significantly by the addition of the binders. Addition of Guar gum increased the viscosity of the faeces best by 183% and 140% in Trial 1 and Trial 2 respectively, while Alginate increased elastic resistance best by 173% and 125%. The digestibility of the diets was not affected. The dissected faeces were exposed to controlled turbulence in the laboratory. The particle size distributions of the suspensions were measured using a laser-based technique applying the 'time-of-transition' theory. The Guar gum and Alginate treatments led to significantly larger particles compared to the control. Based on these data the beneficial effect of the Guar gum binder on post-filtration suspended solid load (100 micrometer gauze) was calculated to be 40.2% and 18.2% for Trials 1 and 2 respectively. In a leaching experiment, faecal particles were suspended in distilled water and allowed to leach for 1 h. The solids were fractionated by sieving and their relative phosphorus and nitrogen content was determined. The relative phosphorus and nitrogen content of the particles increased significantly with increasing size class. In a second experiment, larger particles of the Guar gum treatment yielded significantly higher total amounts of phosphorus and more dry matter remained solid-bound compared to the control. The present study demonstrates promising potential for binders as tools in significantly reducing the effluent load of fish farms without affecting feed efficiency.