Membrane treatment of side-stream cooling tower water for reduction of water usage Susan J. Altman a, ⁎, Richard P. Jensen b , Malynda A. Cappelle c , Andres L. Sanchez d , Randy L. Everett a , Howard L. Anderson Jr. a , Lucas K. McGrath e a Geochemistry Department, Sandia National Laboratories, P.O. Box 5800, MS-0754, Albuquerque, NM 87185-0754, USA b National Security Applications Department, Sandia National Laboratories, P.O. Box 5800, MS-0751, Albuquerque, NM 87185-0751, USA c Center for Inland Desalination System, The University of Texas at El Paso, 500 West University Ave, Burges Hall 216, El Paso, TX 79968, USA d Fire and Aerosol Sciences Department, Sandia National Laboratories, P.O. Box 5800, MS-1135, Albuquerque, NM 87185-01135, USA e LMATA Government Services LLC, P.O. Box 5800, MS-0754, Albuquerque, NM 87185-0754, USA abstract article info Article history: Received 11 July 2011 Received in revised form 26 September 2011 Accepted 27 September 2011 Available online 13 November 2011 Keywords: Cooling tower Nanofiltration Reverse osmosis Water treatment Water use reduction Silica scaling A pilot study was conducted to determine whether membrane treatment on a side stream of recirculating cooling-tower water could reduce overall water usage and discharge. The treated permeate was returned to the cooling tower while the concentrate was discharged to the sanitary sewer. Flow rates, pressures and water chemistry were monitored. The pilot demonstrated potential substantial water savings. Maximum make-up water and discharge reduction were 16% and 49%, respectively. As high as possible permeate recov- ery is needed to maximize water conservation. Silica scaling on the membranes limited water savings in this pilot. Development of membranes with a solute-rejection capacity less than the 92% average of the mem- branes used in the pilot would assist in optimizing water savings. Decreased water outlays compensated for the additional energy used by membrane treatment. Scaling control is critical for economic operation. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Large volumes of water are needed to support thermoelectric power generation. The majority of this water is used for cooling. Thermoelectric generation was responsible for 49% of freshwater withdrawals in the United States in 2005 [1] and 3% of freshwater consumption [2], where water consumption represents the amount of water withdrawal that is not returned to the source. This translates to approximately 10 million m 3 per day (3 billion gallons per day) freshwater consumption by ther- moelectric power generation [3]. As freshwater supplies become scarcer there will be increased competition and cost for freshwater usage. In addition, there can be costs associated with water disposal. It is to the power generation industries' benefit to develop methods to minimize water consumption and maximize water usage efficiency. Maximizing water usage efficiency may allow power plants to expand operations while minimizing new water supply and water disposal costs. Thermoelectric power plants will benefit from optimization of water usage by cooling water towers. Recirculating (closed loop) wet cooling towers are used in 41.9% of existing thermoelectric power plants [3]. The recirculating cooling tower first withdraws water from a fresh- water source and recirculates this water through the cooling tower. Salt concentrations increase as water is recirculated and the cooling water evaporates until dissolved salt concentrations reach a threshold value (the set-point). To keep the concentrations of dissolved salt at or below the threshold value, the concentrated water is discharged as blowdown and water from the fresh-water source (make-up water) is added to the tower. Schematically, the fluxes of water and salt in the system are represented by Fig. 1a. Blowdown and make-up water are required to control the higher salt loads that can lead to scale formation of, for example, calcite and silica. Side-stream treatment can be applied to mitigate scale formation and reduce water usage. Examples of in-use side-stream treatment in- clude lime-soda softening [4] and brine concentrators [5]. Lime-soda softening precipitates calcium, magnesium (as calcium carbonate and magnesium hydroxide floc), and silica (by adsorption onto the magne- sium hydroxide floc) by adding lime (CaO and/or Ca(OH) 2 ) and soda ash (Na 2 CO 3 ) to the blowdown. The treated water is clarified, filtered, pH adjusted, and added as make-up. Sodium, potassium, sulfate and chloride are not removed in this process. Matson and Harris [4] give two examples of industrial plants achieving zero liquid discharge by lime-soda softening. They report the major problems facing side-stream softening are drift losses and sludge disposal. DiFilippo [5] describes the San Juan Generating Station where blowdown is treated by brine con- centrators. The brine concentrator is reported to recover over 98% of the inflow. Thermal evaporative systems have the advantage of creating Desalination 285 (2012) 177–183 ⁎ Corresponding author. Tel.: + 1 505 844 2397; fax: + 1 505 844 7354. E-mail address: sjaltma@sandia.gov (S.J. Altman). 0011-9164/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2011.09.052 Contents lists available at SciVerse ScienceDirect Desalination journal homepage: www.elsevier.com/locate/desal