Importance of Combined Electrochemical Process Sequence and Electrode Arrangements: A Lab-scale Trial of Real Reverse Osmosis Landfill Leachate Concentrate.

• Combined electrochemical process (CEP) reduced the COD by 96% in 3.5 hours • Electrocoagulation-electrooxidation enhanced the BOD 5 /COD from 0.015 to 0.530 • Cathodic electrocoagulation was dominant in electrooxidation-electrocoagulation • Chloride can be a surrogate to determine electrocoagulation efficiency in CEPs • Changes in CEP sequence can deter Pb dissolution from Ti/ß-PbO 2 anodes Reverse osmosis (RO) is a widely applied technique for wastewater effluent reuse and landfill leachate treatment. The latter generates a refractory RO leachate concentrate (ROLC), for which cost-effective treatment is required. This study focuses on a two-step electrochemical method consisting of aluminum-based electrocoagulation (EC), and simultaneous electrooxidation-electrocoagulation with a titanium-based lead dioxide (Ti/ß-PbO 2) anode and aluminum cathode (EO EC) assembly. The sequence and electrode arrangements of the combined electrochemical process were investigated to determine the organic transformation, Ti/ß-PbO 2 anode viability, and energy consumption. Series-based EC-EO EC decreased the total chemical oxygen demand (COD) from 8750 mg L−1 to 380 mg L−1, a 96% removal efficiency, in 3.5 hours at 141 A m−2. Under a low energy consumption of 28.7 kWh kg COD −1, the ROLC biodegradability (BOD 5 /COD) significantly increased from 0.015 to 0.530, which was ascribed to aromatic removal (e.g., -C=C) and an increase in -COOH functional groups. Furthermore, the rapid removal of natural organic matter and increase in pH elevation from EC suppressed the dissolution of Pb from the Ti/ß-PbO 2 anode during the subsequent EO EC , thereby leaving 0.061 mg L−1 in the ROLC after treatment. The treatment cost was 3.86 USD kg COD −1, which was approximately 34% lower than that of previously reported electrochemical processes for ROLC treatment. These findings obtained with a real RO concentrate provide a foundation for scaling up this new electrochemical treatment approach. Image, graphical abstract [ABSTRACT FROM AUTHOR]