Treatability of wastewater from the cold-rolled coilsteel industry using membrane bioreactors

The steel industry plays a strategic role in the development of modern economies by supplying essential raw materials to many sectors, such as construction, automotive, energy, white goods, and defense industries. Turkey is one of the leading steel producers in Europe, and has become an important player both regionally and globally due to its production capacity, export potential, and geographical advantages. In this sector, which is the fourth largest industrial branch in Turkey, Borçelik A.Ş. operates a plant in Gemlik, Bursa, where it produces hotdip galvanized, cold-rolled, and hot-rolled steel. In line with the company's annual production capacity of 1.5 million tons, approximately 480-500 m³ of wastewater is generated daily, with the majority of this wastewater being detergent and oilbased, resulting from the washing of iron during the production process. This thesis investigates the potential outcomes of replacing the biological pond in the existing wastewater treatment plant at Borçelik with a membrane bioreactor, through laboratory-scale experiments. The study was conducted using real wastewater, which had been subjected to pretreatment and chemical treatment processes at Borçelik. The characteristic properties of the wastewater from Borçelik include a COD concentration of 1000 mg/L, Nitrogen content of 3-4 mg/L, Phosphorus content of 2-3 mg/L, a pH of 10, and a conductivity value ranging between 4000-5500 μS/cm. This wastewater was treated in a laboratory-scale membrane bioreactor for 94 days. The total volume of the laboratory-scale membrane bioreactor was 4.5 L, with an active volume of approximately 2.5 L. A 10 cm x 10 cm membrane module was used, with a membrane material of Polyethersulfone (PES) and a pore size of 0.04 μm. In the study, the hydraulic retention time (HRT) was initially set to 24 hours and was gradually reduced to 6 hours in order to assess the effects of the retention time on the quality of the effluent. In the study, after reducing the HRT to 6 hours, the inlet COD concentration was measured as 716.80±68.0 mg/L, while the outlet COD concentration was 52.80±10.06 mg/L, resulting in a COD removal efficiency of 92.62%. The COD value of the effluent sample taken from the reactor was 134.00±25.61 mg/L, which was higher than the outlet COD, suggesting that membrane filtration and biofilm effects played a significant role in the removal process.