Sewage Sludge as a Renewable Resource for Citric Acid Production: Optimization of Supplementary Nutrients

The optimization of supplementary nutrients in the sewage sludge as a basal medium was carried out to determine the media components on the citric acid production. Two statistical designs were employed in the citric acid productions which are Plackett-Burman (PB) and Central Composite Design (CCD). For the media screening, the two-level PB design was applied. Under the experimental conditions, it was observed that total suspended solids of sewage sludge (TSS), sugar, cassava, methanol, potassium di-hydrogen phosphate and urea to be the major factors in maximizing citric acid production. A study of single factor was conducted to find the optimum ranges of parameters for citric acid production followed by the Central Composite Design (CCD) under the responses surface methodology. A polynomial model was created to correlate the relationship between two factors (sewage sludge TSS and sugar) and citric acid production. The optimum yield of citric acid, 29.12 g L-1 was produced with 0.8% TSS sewage sludge and 7% sugar in four days of fermentation. The determination of coefficient (R2) from the analysis was 0.9527, indicating that the model of this experiment is significant. This study shows an effective solution to the sludge management for further development through the production of citric acid.