An optimum strategy for substrate mixture and pretreatment in biogas plants: Potential application for high-pH waste...

Optimization of anaerobic digestion poses many practical constraints. Therefore, a new approach was developed by integrating mathematical modeling and desirability analysis to determine optimum amounts of input factors. For this purpose, different proportions of co-digestion (grapefruit waste:cow manure) and pretreatments (NaOH and H₂O₂) were tested on the basis of a combined D-optimal experimental design. Different models were developed for certain slurry properties and CH₄ production (responses) depending on the input factors. To improve the models, Box-Cox transformation was used to transform the models into more accurate formats. The improved models were then used in desirability analysis, and optimum ranges were determined instead of optimum absolute values in three cases of given constraints, (i) constraints of the slurry properties, (ii) increase in CH₄ production, and (iii) constraints on all the responses (more important for plant efficiency). The optimum ranges were extracted for the desirability levels with values greater than 0.9 × (maximum desirability value). For the constraints given in all the responses, the optimum range of grapefruit waste:cow manure proportion was 2.5%:97.5% to 25%:75%, the optimum range of NaOH pretreatment was 0.3-2.64%, and the optimum range of H₂O₂ was range experimented except for 1.32-1.68%. Similar trends were determined for the other cases of constraints. To clarify the method of desirability analysis, the overlaying method was used to determine regions of interest according to some predefined constraints. Simultaneous consideration of range optimization and region of interest showed that (i) an amount greater than 25% grapefruit waste in the digestion decreased the CH₄ production significantly, and (ii) plant efficiency was improved with all amounts of H₂O₂ except 1.32-1.68%, and with amounts of 0.3-1.2% and 1.8-2.64% of NaOH. As shown the optimum ranges are more common for practical use in plants.