A chemical fertilizer plant in Kedah, Malaysia produced high Ammonia-nitrogen content in the discharge outlet, exceeding the allowable standard limit (Standard B). The standard is based on Environment Quality Act (EQA) 1974, to ensure the integrity of water is maintained. At the plant, current wastewater treatment used is reed bed system, which is incapable to treat the excessive amount of Ammonia-nitrogen. Feasibility of extended aeration activated sludge process (ASP) was then discovered in this study. Objective of this study is to determine the impact of treating wastewater generated from the chemical fertilizer plant using bench scale extended aeration activated sludge system at 20 of contamination, diluted with domestic wastewater of an average of 500 mg/L chemical oxygen demand concentration. Bench scale ASP was set up using 5000 mg/L initial biomass, a 20 L� influent tank with a heavy-duty mixer, connected to a pump via 10 mm tube at 0.1 rpm which is equivalent to 5 L of water infused into the aeration tank for 24 h consistently, and finally discharged into an effluent tank. Influent and effluent samples were monitored for Total Suspended Solids (TSS), Chemical Oxygen Demand (COD), Nitrate (NO3�1 ), Ammonia (NH3), and Total Phosphorus (PO43�) while the mixed liquor sample was monitored for Mixed Liquor Suspended Solids (MLSS) and Mixed Liquor Volatile Suspended Solids (MLVSS). © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2021.