A performance evaluation of a multi-soil-layering for the treatment of an olive mill wastewater

Mitigating the volumes of Olive Mill Wastewaters(OMWs) created each year presents a substantial environmental challenge for Mediterranean countries.Even though multi-soil-layering (MSL) technologies have demonstrated effectiveness in treating different types of wastewater via affordable and energy-efficient filtration systems, existing soil mixed block (SMB) mixture compositions use traditional carbon components and metal adsorbents that have only been tested independently.Consequently, little is known about the utilization of local agro-industrial residues together with iron-based adsorbents for OMW treatment.This study addresses this gap by evaluating the performance of an MSL system (consisting of an alternation of permeable layers(made of a pozzolan) and a locally sourced, low-cost SMBs (made of 70% soil, 10% charcoal, 10% Phoenix Dactylifera and 10% iron debris))-a previously untested combination for the treatment of a 50% diluted OMW.The influence of hydraulic load rate(HLR)(200,400 and 500 L day^-1m^-2) on physico-chemical parameters, such as pH, turbidity, conductivity, concentrations of ammonium (NH4⁺), orthophosphates (PO4^3-), phenolic compounds and Chemical Oxygen Demand (COD) was studied.The 200 L day^-1 m^-2 load rate allowed the MSL to treat effectively the OMW, as the highest removal rates for NH4⁺,PO4^3-, phenolic compounds and COD reached 93.42%,99.67%,96.8% and 97.41% respectively- exceeding removal rates reported for MSL systems using conventional carbon and iron amendments.An inferential statistical approach (ANOVA,Tukey HSD post-hoc testing,effect-size calculation)-not previously applied to validate MSL-OMW treatment performance-confirmed the robustness of these removal rates and identified COD as the only parameter significantly sensitive to HLR, providing quantitative design guidance for scale-up.The data revealed how SMB composition influences the geochemical retention of nutrients and phenolic compounds in the MSL, validating the SMB as an efficient barrier for controlling the human health and ecotoxicological risks associated with the disposal of untreated OMW in Mediterranean olive-growing areas.