RSM-D-optimal modeling approach for COD removal from low strength wastewater by microalgae, sludge, and activated carbon- case study Mashhad

•Microalgae-based processes for advanced wastewater treatment provide numerous advantages including simultaneous removal COD, N, and P.•At optimum conditions, 91% COD removal was obtained.•In low-strength municipal wastewater, activated carbon was an effective parameter for removing COD.•Observed and predicted values were in close agreement with each other. [Display omitted] This work removes Chemical Oxygen Demand (COD) from the effluent of a domestic wastewater treatment plant in Mashhad, Iran, low strength wastewater (COD 70 mg/L). Using two processes: activated carbon-activated sludge (AC-AS) and activated sludge- microalgae (AS- Microalgae) was studied. As independent variables, the concentrations of AC and AS for the AC-AS system and the concentrations of AS and microalgae for the AS-algal system were selected and optimized using response surface methodology (RSM) involving a four-level and a five-level optimal design (D-Optimal), respectively. The optimum experimental conditions for the AC-AS process (i.e. 0.05 g/L AC and 7 ml/L AS) led to a COD removal of 95% in 8 h. As for the AS-Microalgae process, a COD removal of 85% in 7 h was attained under optimal experimental conditions (i.e. 1 ml/L AS and 7 ml/L microalgae). The predicted and experimental removal efficiencies obtained for the AC-AS and the AS-Microalgae system were (91%, 91%) and (82%, 86%), respectively. An excellent correlation between the experimental and predicted values was also observed with determination coefficients of 0.99 (AC-AS system) and 0.83 (AS- Microalgae system). According to WHO discharge standards, the effluent treated by AS- Microalgae can be applied for agricultural lands, golf course irrigation, and industrial usages, reaching a typical value of COD lower than 30 mg/L. In addition, the effluent treated by AC-AS can be used for edible crops irrigation, parks and schoolyards irrigation, dishwashers and toilet flash tanks with a COD value lower than 12 mg/L.