Modelling the distribution of micro-nutrient metals in the anaerobic digestion for fischer-tropsch reaction water.

Abstract

Sasol, one of South Africa’s largest petrochemical producers; derives water from the Fischer Tropsch process and is referred to as Fischer-Tropsch Reaction Water (FTRW). FTRW has high hydrocarbon content and a very low pH, hence, it must be treated prior reuse in the system. In the purification process, the organic content in the FTRW is mainly broken down to CO2 and CH4 by microorganisms which use the organic content as nourishment, bringing about its decomposition. Macro and micronutrients are necessary for survival of microorganisms, and this study is focused on the optimisation of micronutrients (micro-metals) to supplement the treatment of FTRW. This study was aimed at developing a model to predict the distribution of micronutrients in the various phases present in anaerobic digester sludge: soluble ions, precipitates and an adsorbed phase. The aim was to introduce an adsorbed phase to the precipitates-soluble phase model to reduce the deviation between model and experimental sludge concentrations. The model would thereafter be used for optimization of micronutrients dosing techniques employed by Sasol to reduce operating costs of anaerobic treatment of FTRW. The model was sufficiently extended to include the adsorbed phase by ionic representation of the biomass, however, the accuracy to which the model represents reality could not be tested by model validation due to these crucial limitations: incomplete set of experimental soluble phase concentrations, imprecise experimental data for metals entrapped in precipitates only and in the adsorbed phase only, to allow for regression of governing equations, as well lack of experimental representation of the relationship between sulphide and sulphate by concentrations. The integrated Ionic speciation model was used to point out the importance of the sulphatesulphide system, as the phase control varies between the precipitates and the adsorbed phase, depending on the behaviour of the anions, specifically the sulphides. A series of further experimental work needs to be completed to ensure a robust model outcome, such that the model best represents the speciation of metals in an anaerobic digester. The hypothesis therefore could not be proven to be true based on the data at hand. The next steps will be to carry out detailed experimental work; showing initial conditions, sulphate and sulphide concentration as well as experimental partitioning of metals in the various phases. In the meantime, Sasol can look into incorporation of a donor cation with a greater affinity to sulphide ions and adsorption on biomass while not impacting the decomposition reaction, such that the metal cations essential for anaerobic digestion are kept in solution to enhance the microbial activity for the treatment of FTRW.