Chemical pre-treatments effect for reducing lignin on cocoa pulp waste for biogas production

There are many cocoa pulp wastes in Indonesia; unfortunately, still a few of them has been utilized. Cocoa pulp contains high enough of cellulose and hemicellulose that able to be converted to reducing sugar with hydrolysis process. However, it also has an immense lignin content that inhibits the enzymatic hydrolysis process. How to reduce lignin contents will be studied in this research by comparing pretreatments using NaOH solution and using NaOH-H2O2 mixture solution. Lignin, hemicellulose, and pectin contents were analyzed by using UV-Vis spectrophotometer and gravimetric method was used to analyze cellulose contents. Morphology, and crystallinity index (CrI) were analyzed by using Scanning Electron Microscope (SEM), and X-ray Diffraction Pattern (XRD), respectively. The experimental results showed that pretreatment with NaOH-H2O2 was the best method with a decrease in lignin content up to 85.67%. This result is supported by SEM results showing that with the pretreatment, the previously smooth surface of cocoa pulp becomes rough surface and many void/cracks. XRD results show that the CrI of cocoa pulp waste decrease after being chemically pretreatments, especially in NaOH-H2O2 pretreatments. COD removal with NaOH-H2O2 is the highest than others. Total Solid (TS) and Volatile Solid (VS) decrease with the increasing of fermentation time. Contribution of acetic acid in Volatile Fatty Acid (VFA) total is higher than others and achieved maximum value after 15 days and decrease rapidly after 20 days and being constant after 25 days of fermentation. This result agrees with biogas production and resulting methane with concentration of 66.07% and yield of 0.734 m3 CH4/kg VS.