Treatment and valorization of olive mill wastewater by hydroxyapatite co-precipitation using experimental design

In this study, the reduction of phenolic compounds content (PCC) and chemical oxygen demand (COD) from olive mill wastewater (OMW) by hydroxyapatite (HA) co-precipitation was investigated using experimental design. Six parameters namely reaction time (t), stirring rate (V), pH, HA/OMW ratio (S/L), reaction temperature (T) and CaCl2 amount (m) were studied using Hadamard matrix in screening to identify the factors which have a significant influence on PCC and COD reduction. The selected factors in screening (t, pH, and S/L) were investigated in a quantitative study using full factorial design. Results obtained show that the main effects of t, pH, and S/L were significant for both responses. Only the first-order interaction effect pH-S/L was significant for the PCC. For the COD, the first-order interactions between t-pH, t-S/L, pH-S/L, and the second-order interaction t-pH-S/L were significant. Under the conditions minimizing the responses (t = 30 min, pH = 12 and S/L = 10 g/L), there was a substantial reduction of PCC and COD contents (87.30% and 76.30%, respectively). The regenerated HA after PCC extraction was reused for the OMW treatment using the selected optimal conditions. In this case, PCC and COD reductions obtained (72.40% and 71.90%, respectively) show that the proposed process could be a good alternative for OMW treatment.