Recovery of cooling tower blowdown water for reuse: The investigation of different types of pretreatment prior nanofiltration and reverse osmosis

The suitability of two different pretreatment methods, i.e., coagulation-filtration and ultrafiltration (UF), and two final membrane treatment technologies, namely nanofiltration (NF) and reverse osmosis (RO), for desalination of a cooling tower blowdown (CTBD) was investigated. Particular attention was paid to ensuring that the best pretreatment method could enhance the permeate flux and lifespan of the NF and RO membranes and decrease the membranes' fouling characteristics. Furthermore, the difference of NF and RO performances in CTBD treatment was investigated. In order to find the most appropriate type of coagulant, coagulant dosage, pH and co-coagulant dosage, 21 jar tests were performed. The results showed that 50 mg/L of Polyaluminium chloride (PACl) in the presence of 0.5 ppm co-coagulant in pH of 6.5-7.5 has the best treatment performance. Silt density index (SDI), chemical oxygen demand (COD), turbidity, electrical conductivity, and membrane permeate flux tests were performed for both pretreatment and treatment stages. Both pretreatment methods produced appropriate feed for NF and RO in terms of SDI and turbidity. Using the coagulation-filtration pretreated water instead of raw water as a feed for NF and RO membranes showed about a 25 and 33 percent improvement in permeate flux after 100 min in 10 and 15 bar applied pressure for NF and RO, respectively.