Many medium-sized towns in Morocco and other semi-arid countries are served by natural lagoon wastewater treatment plants (WWTPs) that now operate well beyond their design capacity, and utilities must decide how to upgrade them. This study documents the in-situ conversion of the 30-year-old Mrirt WWTP (Morocco) from natural lagooning to a two-stage aerated lagoon system, using twelve months of operator monitoring data before (2015) and after (2024) conversion. Mean BOD5 removal increased from 73.9% to 89.2% and COD removal from 67.7% to 78.9% (Welch's t-test, p ≤ 0.001), and effluent BOD5 and COD complied with the national discharge limits in 12 of 12 months, compared with 2 and 1 months before conversion. Effluent TSS did not change (144 vs 147 mg/L, p = 0.86) and exceeded the 150 mg/L limit in 5 months; the lower TSS removal efficiency (51.0% vs 68.1%) reflects a lower influent TSS concentration. The improvement came at a large energy cost: specific consumption rose from 0.071 to 0.85 kWh/m³ (2.15 kWh/kg BOD5 removed), i.e. about 8 kWh per additional kilogram of BOD5 removed. Electricity use was nearly constant regardless of flow or load, and effluent dissolved oxygen remained at 7.0–9.4 mg/L, indicating scope for savings through dissolved-oxygen-based aeration control. These single-site results support considering in-situ conversion among upgrading options, provided that energy use, TSS polishing and salinity are addressed.