Kiwi peel waste enhances manure protein degradation: Statistical optimization using Box-Behnken design and response surface methodology

Anaerobic digestion (AD) of protein-rich waste is challenged by ammonia accumulation. This study explores the potential of Kiwi peel waste (KPW)-derived proteases (actinidin) to enhance manure protein degradation. It used Box–Behnken response surface design to optimize the protein quantity and ammonia reduction in batch experiments. It studied the individual and interactive effects of the manure degradation parameters (manure dosage, KPW dosage, and time). The optimal manure protein quantity reduction (39 ± 0.54 %) was obtained at manure dosages of 4 g VS L⁻¹, KPW dosage of 7.5 g VS L⁻¹, and 48 h. However, the optimum conditions for reducing ammonia by 64 ± 0.65 % are manure dosage of 9 g VS L⁻¹, KPW dosage of 7.5 g VS L⁻¹, and 48 h. A highly predictive second-order polynomial model predicted reduction consistent with those observed experimentally (R² = 0.99). Change and decrease in FTIR peak intensity from 3200 to 3400 cm⁻¹ confirmed the disturbance of hydrogen bonds and the breaking of amide or N-H bonds within side chains in the hydrolyzed manure sample. The tests characterizing the hydrolyzed substrate and the statistical model data affirm that employing KPW for manure degradation is a feasible strategy to tackle ammonia buildup. This approach can potentially enhance protein degradation in manure and increase methane yield in AD. Future studies may explore the effects of using different types of manure or other slaughterhouse waste to understand the model’s viability on various protein-rich wastes. The stability of actinidin needs to be investigated under different environmental conditions, such as temperature, pH, nutrients, etc.