ASSESSMENT OF DIFFERENT SOLID-LIQUID SEPARATION TECHNIQUES FOR LIVESTOCK SLURRY

Abstract

Livestock manure is a valuable resource for fertilizing and amendment, due to the high concentrations of plant nutrients. However, the increasing intensification of livestock farms combined to an improper manure management led to nutrient losses to air, water and soil. Among the manure treatment technologies that allow to reduce the environmental impact of livestock activities, solid-liquid separation is a widely applied technique that leads to a more appropriate nutrient application to fields and, thus, minimize nutrient losses and their related environmental problems. The evaluation of the different separating system and the optimization of this process is crucial in order to improve the performances of the treatment systems and the minimization of the environmental impact related to manure management. The present study aims to: (i) evaluate the solid-liquid separation treatment in a treatment plant under different operating conditions; (ii) investigate the optimization of the separation process through natural and synthetic chemicals. The performances of different solid liquid-separation techniques were evaluated under different operative conditions. In particular, the separation efficiency of two screw press separators were compared to the separation efficiency of a decanting centrifuge, in order to evaluate the separation system that allow a better biological removal nitrogen. Both the screw press and the decanting centrifuge treated digested slurry; moreover, the decanting centrifuge was run with the liquid fraction produced by the screw press and with the addition of flocculants. In all the trials, the decanting centrifuge presented a higher separation efficiency, producing a liquid fraction with a lower dry matter concentration and enabling a correct biological nitrogen removal. The effect of manure pre-treatments on solid liquid separation performances was also evaluated. In particular, the effect of manure acidification on three different separation techniques was studied. Pig slurry (pH = 7) and acidified pig slurry (pH = 5.5) were treated through (i) screw press, (ii) decanting centrifuge, (iii) flocculation combined to a belt thickener drainage. Acidified manure was easier to drain, leading to a lower retention of solid particles in the liquid fraction. This resulted in a decrease of the separation efficiency for all the analyzed parameters. Furthermore, The acidification caused the concentration in the solid fraction of particle related species to decrease, of dissolved monovalent species not to be affected, and of divalent cations to be increased. Chemical pre-treatments were investigated in order to improve the separation efficiency of mechanical separators for nutrient, especially nitrogen and phosphorus. In particular, the utilization of natural flocculants (such as chitosan) was compared to a synthetic flocculation treatment. A laboratory test was run treating the manure produced by pig livestock and anaerobic digestion plants were treated. In particular, 20 different manure samples were used:…