Impact of treatment and co-treatment of different types of discharges on the improvement of receiving surface water quality

• Effect of urban, landfill, slaughterhouse, olive mill wastewater of ZAT river is studied. • Wastewater treatment/co-treatment scenarios have been proposed to reduce this impact. • PEGASE model predicts river water quality improvement for the best treatment scenario. • Co-treatment (with 1.1 % OMW) will meet good water quality status of ZAT river. • Co-treatment (with 0.5 % OMW) will meet discharge authorization and good water quality status of ZAT river. Protecting and maintaining aquatic environments while meeting strict regulations regarding pollutant discharge into surface waters presents a significant challenge for stakeholders. This paper assesses treatment and co-treatment scenarios for different types of discharges, including urban wastewater, slaughterhouse wastewater, landfill leachate and olive mill wastewater, aimed at improving the water quality of the ZAT River in Morocco. Through a combination of field measurements and mathematical simulations, different wastewater treatment scenarios were developed and evaluated using the PEGASE model. Results from the simulations indicate that treating each effluent individually failed to meet the required COD concentration limits for discharge (<250 mgO 2 /L) of the Moroccan standards. Furthermore, this approach fell to achieve the desired good water quality status with a COD concentration target below 35 mgO 2 /L in the river water quality grid. Conversely, co-treatment involving a mixt of all effluents and 1.1 % (v/v) olive mill wastewater, successfully reached the COD concentration target in the ZAT River water column. In addition, co-treatment with a 0.5 % (v/v) olive mill wastewater met both standard COD concentrations in the outlet and complied with the COD concentration limit below 35 mgO 2 /L in the receiving river regulations. The study shows that co-treatment is more effective than individual treatments. It also shows that maintaining discharge concentration limits in line with Moroccan standards enables the river to achieve good water quality status. This highlights the need for stricter regulations and better wastewater management to protect aquatic ecosystems. This suggests that co-treatment is a viable option in contexts where resources are limited. These results advance sustainable wastewater practices, particularly in arid regions. Promote the application of mathematical models such as PEGASE for compliance assessment and decision-making.