Treatment of dairy wastewater by a two-stage membrane process with ultrafiltration (UF) and nanofiltration (NF) was investigated. The results showed that the flux of UF was higher at pH = 4.6 than that at pH = 8 because the resistance of the fouling membrane was lower at the isoelectric point of protein (pH = 4.6) in UF operation. Protein rejection exceeded 99% by UF + NF operation. Lactose rejections were 98.5 and 54% for UF + NF90 and UF + NF270 respectively. Experiments on membrane cleaning showed that the fouling layer of UF and NF was mainly protein and casein which could be removed by aqueous NaOH with pH = 10. The result of long-term experiments showed that the chemical oxygen demand (COD) of NF90 permeates was below 70 mg/L consistently and the wastewater could be concentrated to 24% by a two-stage membrane process.
Keywords: dairy wastewater, lactose, nanofiltration, protein, ultrafiltration