Treatments of stainless steel wastewater containing a high concentration of nitrate using reverse osmosis and nanomembranes

A nanomembrane (NTR 729HF) and three different types of reverse osmosis (RO) membranes (CPA2, LFC1, and ESPA1) were tested for the treatment of high concentration of nitrate wastewater from stainless steel industry. All the tested RO membranes showed higher rejection rates (90-99% at 1000-60 mg/L of NO3-N) than the NF membrane. The rejection rate and flux of RO membranes were not highly affected by pH variation and Ca2+ as co-existing ion. However, the rejection rate of NF, which was 67% at 60 mg/L of feed concentration, was decreased as pH decreased and Ca2+ concentration increased indicating that charge repulsion is one of the major rejection mechanisms. As nitrate concentration increased from 20 to 1000 mg/L in feed water, the removal rate decreased from 67 to 20% in NF membrane. The flux of RO was relatively high and ESPAI (a low-pressure RO type) showed more than two times higher flux compared to the NF. ESPA1 was successfully tested for a long-term experiment with real stainless steel wastewater for 30 days of experimental period. Current study implicates that RO membranes could be an alternative for the treatment of stainless steel wastewater.