Characterization of Spent Bleaching Earth and Its Utilization for Improving Manure-based Biogas Production

Abstract

Aim of the Study: To utilize spent bleaching earth (SBE) to improve biogas production efficiency through co-digesting with manure and provide a practical way of disposing the SBE. Design of the Study: The SBE was collected from two edible oil refineries in Dar-es salaam, Tanzania. Composition was analysed in triplicate to determine its suitability for biogas production. Thereafter, the SBE was mixed with manure at (%) 0, 10, 20, 30, 40, 50 and 100 for biogas production. Methodology: The residual oil from the SBE was recovered via solvent extraction system (soxtherm). Total solids, volatile solids and pH were determined using standard methods.  Physicochemical parameters of the SBE were determined using AOCS Official methods. Total carbon and total nitrogen were analysed using elemental analyser whereas the fatty acid profile of the residue oil was established using gas-chromatography techniques. Methane potential of mixtures SBE and manure at different proportions was investigated using a multi-channel analyser, methane potential test system in 500 mL glass bottles with reaction volume of 300 mL. Place and Duration of Study: The study was completed in 2 years from 2015-2016. Samples of SBE were collected from two edible oil refineries in Dar-es salaam, Tanzania from March to July 2015 and all experiements were conducted between January and April 2016 at Tanzania Industrial Research and Development Organization. Results: Two types of waste; soapstock ca.3150 metric tons and SBE ca.2800 metric tons are generated per year. SBE contains high percentage (%) of total solids ca.98, volatiles solids 35, oil content 30 and C: N ratio 258. Macronutrients expressed in percentage (%) were MgO 4.6, CaO 4.2, K2O 1.0, P2O 55.2, whereas as micronutrients were (%) F2O3 8.0 and MnO2 0.1. Mixing of cow dung manure with SBE at the rate of 50% on volatile solids bases improved methane yield by 9-fold (508ml/gVS compared to 55ml/gVs for only manure). The methane yield for only SBE was 16-fold (867 mL/gVS) compared to only manure (55ml/gVs). The SBE is therefore a cheap, abundantly available, excellent substrate for biogas production.