Technical and Economic Assessment of the Fourth Stage of Plovdiv Wastewater Treatment – Calculations, Scenarios and Cost-Effectiveness

This paper assesses the technical and economic feasibility of implementing quaternary (fourth-stage) treatment for micropollutant removal at the Plovdiv wastewater treatment plant (WWTP) located in Bulgaria (378 454 PE design, Qavg 129 047 m³/d). The cost structure is prepared based on a technical design calculation that takes into account specific dosages of ozone, powdered and granular activated carbon, and on-site PSA generation of oxygen for the ozonation step, tailored to the specific conditions of the plant. Seven scenarios are evaluated with three ozone doses (5, 7, 10 mg O₃/L) followed by a biologically active filter (O₃+BAF), a combined O₃+GAC scheme, GAC stand-alone, and two PAC dosing options (15 mg/L after secondary treatment and 25 mg/L in the bioreactor). For O₃+BAF, CAPEX is estimated at 16.48 €/PE (6.24 M€). The minimum effective ozone dose (5 mg/L) yields the lowest total annual cost of 2.89 €/PE.y, the typical operating dose (7 mg/L) yields 3.25 €/PE·y. The PAC options are substantially more expensive, reaching 5.62 €/PE·y for dosing after secondary treatment and 7.62 €/PE·y for dosing in the bioreactor. Electricity demand is dominated by ozone production (7.9 GWh/y) and PAC options shift costs towards consumables and sludge handling. The O₃+GAC configuration provides the highest removal performance at intermediate costs (3.25 €/PE.y). A tornado sensitivity analysis around the 3.25 €/PE·y baseline shows that the recommended O₃+BAF configuration remains below 4.16 €/PE·y under the tested one-at-a-time variations. The analysis confirms that electricity price and ozone dose are the dominant cost drivers. Therefore, the final technology recommendation is interpreted together with the multi-criteria comparison rather than as a purely cost-based ranking. A new six-criterion multi-criteria comparison covering cost, indicative carbon footprint, sludge implications, transformation-product risk, operational risk and sorbent regenerability complements the cost ranking. The results confirm significant economies of scale at large wastewater treatment plants. The recommended O₃+BAF configuration results in total annual costs of €2.89 - 3.25/PE·year for Plovdiv and a unit cost of €0.026/m³, which places it below the overall average of the European reference set (€7.3/PE·year). Novelty. To the best of the author’s knowledge, this is the first systematic techno-economic comparison of quaternary-treatment scenarios for a Bulgarian wastewater treatment plant prepared for the peer-reviewed literature. It additionally contributes a sensitivity-informed identification of the cost drivers that govern the choice between O₃, PAC and GAC for a large-scale Balkan agglomeration, a setting under-represented in the existing micropollutant-removal literature, in which most studies originate from other countries.