Optimization of electrocoagulation process and combination of anaerobic digestion for the treatment of pistachio processing wastewater

It is very challenging to treat raw pistachio processing wastewater (PPW) by one step process due to its high chemical oxygen demand (COD) and total phenols (TP). In this study, electrochemical (EC) process and anaerobic digestion are combined to improve treatment efficiency. Central composite design (CCD) was used in EC experimental design with three factors at three levels of the independent variables. Impact of current density (50-100 A/m(2)), electrode distance (2-6 cm), and reaction time (60-300 min) on the removal of COD and TP were systematically investigated. A maximum removal efficiency of 43.7 and 56.4% of COD and TP, respectively, was obtained with the current density of 100 A/m(2), electrode distance of 6 cm, and reaction time of 293 min. Specific energy consumption (SEC) was measured as 7.80 kWh/kgCOD for the same experimental conditions. In order to achieve further treatment, a batch anaerobic reactor was combined with EC process to produce biogas. COD and TP removal efficiency were measured in the anaerobic reactors at appropriate time intervals (0, 10, 25, 30, 40, and 50th d) and biochemical methane potential (BMP) tests lasted up to 50 d. BMP enhancement of raw PPW was found as 35.7 and 16.5% for electrocoagulated PPW under maximum COD and TP removal efficiency condition (100 A/m(2) current density, 6 cm electrode distance, and 293 min reaction time) and under cost driven approach condition (53 A/m(2) current density, 2 cm electrode distance, and 182 min reaction time) compared to raw PPW. The results showed that pre-treated PPW with EC process had the highest COD and TP removal efficiencies and enhanced the biochemical methane production, by decreasing the concentration of total phenols in the PPW. Energy balance revealed that EC under cost driven approach condition combined with anaerobic digestion can be used as an energy positive process for the treatment of PPW. (C) 2018 Published by Elsevier Ltd.