Pineapple peel waste enhances manure protein degradation: Statistical optimization

Animal farms generate large amounts of manure suitable as feedstock for producing biogas by anaerobic digestion (AD). However, AD encounters difficulties when manure contains excessive protein levels. This study investigates using pineapple peel waste (PPW)-derived protease enzyme (bromelain) to enhance manure’s protein degradation and improve biogas production. It aims to improve the degradation of manure protein and mitigate the inhibitory ammonia accumulation problem. The study applied a Box–Behnken design and analyzed the data using the Response Surface Methodology (RSM) to optimize protein reduction and diminish ammonia levels. It examined the single and two-way impacts of parameters such as manure dosage, PPW dosage, and degradation duration. The statistically derived optimal degradation conditions of 9 g VS_manure L⁻¹, 4 g VS_PPW L⁻¹, and 48 h degradation time achieved 36 ± 0.25 % protein reduction. However, the highest reduction of ammonia nitrogen (NH₃-N) of 72 ± 0.48 % was achieved under the optimal combinations of 6.5 g VS_manure L⁻¹, and 7 g VS_PPW L⁻¹ , and 48 h. Fourier Transform Infrared (FTIR) spectroscopy and Scanning Electron Microscopy (SEM) analyses revealed changes, particularly weakening and cleavage of hydrogen and amide I, II, and III bonds, confirming hydrolyzed manure's protein structural and morphological alterations. The hydrolyzed substrate characterization, paired with the rigorously developed statistical data, strongly supports using PPW as an effective agent to address the ammonia accumulation challenges. The PPW significantly and effectively enhances protein breakdown within manure, potentially increasing hydrogen and methane generation during AD.