Disposal of animal by-products by wet air oxidation: Performance optimization and kinetics

This paper describes the application of subcritical wet oxidation to the disposal of sheep animal by-products originating from slaughterhouse. Animal by-products (ABPs) from categories 1 and 3 (gall, head, tail, spinal cord, offal, ileum and blood) were oxidized at high pressure and moderate temperature (P = 12.5–20 MPa, T = 200–320 °C). The oxidation experiments were performed on individual samples or on a reconstituted mixture representing the ABPs of a slaughtered sheep. The oxidation kinetics of a representative sample was studied and the apparent activation energy was found to be 42.9 kJ mol−1. The chemical by-products were also identified and quantified in the final oxidized solution: acetic acid and ammonia were identified in all samples as the major by-products representing around 31% and 69%, respectively, of residual TOC and initial nitrogen after the oxidation of a representative sample of ABPs containing initially 5 g L−1 of total organic carbon (TOC). The contribution of the experimental factors temperature, reaction time and concentration of the feed solution to remove the organic matter was assessed and optimized using an experimental design based on the response surface methodology. Fitting of the experimental data showed that the 2nd order polynomial model represented the data best. A multicriteria optimization, using the desirability function, allowed the determination of the best region of the experimental domain to optimise the TOC removal and the energy consumption.