Sewage treatment process refinement and intensification using multi-criteria decision making approach: A case study

In assessing sewage treatment systems, decision makers now have to consider sustainability, affordability, and reliability beyond functionality. To facilitate this complex assessment, multi-criteria decision making (MCDM) tools have evolved recently. This case study illustrates an application of the Preference Ranking Organization METHod for Enrichment of Evaluations (PROMETHEE) method in the MCDM process in evaluating five sewage treatment systems: activated sludge treatment followed by anaerobic digestion (P1); photoautotrophic algal treatment followed by hydrothermal liquefaction (P2) or by anaerobic digestion (P3); and, mixotrophic algal treatment followed by hydrothermal liquefaction (P4) or by anaerobic digestion (P5). This evaluation is based on 15 criteria aligned with the United Nation's sustainable development goals. Based on energy recovery, the five processes ranked as follows: P2; P3; P4; P5; and P1. But, when all the 15 criteria are considered, P4 ranked as the most preferred option followed by P5 and P2. Utility of the MCDM approach in highlighting drawbacks of the options and identifying areas for improvement is illustrated. This study indicated that recovering phosphates and energy from the digested sludge in P1 offer greater potential to improve its ranking than by minimizing its energy demand for aeration. Increasing biomass density in the mixotrophic system can further improve its ranking, minimizing footprint and maximizing recoveries. Since the energy-intensity of hydrothermal liquefaction in P2 and P4 (4.57 and 23.23 kWh/kg BOD, respectively) is higher than that of anaerobic digestion in P1 (0.01 kWh/kg BOD), nutrient recovery from its byproducts has to be maximized to justify its selection.