MANAGEMENT OF BLOWDOWN FROM CLOSED LOOP COOLING SYSTEMS USING IMPAIRED WATERS

摘要

Management of cooling tower blowdown is one of the key components in cooling tower operation and usually requires treatment to meet local, state or federal discharge regulations. Typical options for power plants blowdown management include discharge to surface waters or wastewater treatment plants (WWTPs) and zero liquid discharge (ZLD). Compared to other options, ZLD is preferred as it helps to decrease environmental impact of power generation, especially for power plants using reclaimed water for their recirculating cooling systems. ZLD typically includes one or more of the following advanced treatment technologies: (1) Membrane filtration; (2) Electrodialysis (ED); and (3) Evaporation.The objectives of cooling tower blowdown treatment is to recover water with the quality equal to or better than the makeup water so that it can be returned to the cooling loop. In this study, membrane filtration was evaluated as a possible treatment option for blowdown from cooling towers operated using secondary treated municipal wastewater ("MWW blowdown") and acid mine drainage ("AMD blowdown") as make up water.Treatability studies with microfiltration (MF), ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO) membranes, were carried out for the MWW and AMD blowdown in a bench-scale membrane filtration unit. It was observed that the nanofiltration with BW30 membrane was effective in reducing the concentration of dissolved species in MWW blowdown: TDS and sulfate decreased from 3,060 and 326 mg/L to 379 and 31 mg/L, respectively. Sequential filtration by NF90 and BW30 membranes was required to reduce the TDS and sulfate in AMD blowdown from 5,810 and 3,079 mg/L to 192 and 107 mg/L, respectively. Preliminary studies also indicated that a trans-membrane pressure of 135 psi is sufficient to achieve acceptable permeate flux. Lowering pH of the feed water from 7.4 to 6.0 resulted in higher permeate flux, while increasing it to 9.0 had an adverse impact on permeate flux.Laboratory tests performed in this study indicate that nanofiltration could be a viable alternative for the treatment of blowdown from cooling tower systems using impaired waters (i.e., secondary treated municipal wastewater or passively treated acid mine drainage).