Desalination 184 (2005) 185–195 The effect of feed ionic strength on salt passage through reverse osmosis membranes Craig Bartels1, Rich Franks1, Stefan Rybar2, Manfred Schierach3, Mark Wilf1* 1Hydranautics Inc., 401 Jones Dr, Oceanside, CA 92054, USA Tel. þ1 760 901 2548; Fax þ1 760 901 2664/2578; email: [email protected] 2Hydranautics, Inc., Europe and Middle East Section, Turnburry Wynd 45, UK 3Hydranautics GmbH, Zeppelinstrasse 4, 85399 Munich, Germany Received 23 March 2005; accepted 12 April 2005 Abstract Several parameters are known to influence the passage of salts through a reverse osmosis membrane. These parameters include characteristics of both the membrane and the feed water. Of these parameters, the least understood is the effect feed water concentration has on salt passage. At high and very low feed salinities, salt passage can increase by a factor of two or more. As an increasing number of RO systems are designed to treat water at these salinity extremes, a better understanding of this salinity effect is necessary to accurately predict the permeate quality of these systems. This study seeks to demonstrate and characterize the salinity effect on different RO elements treating different feed waters. The magnitude of the salinity effect at any given feed salinity is shown to be influenced by membrane charge and feed water composition. The results of the study on individual elements are used to accurately predict the salt passage in an existing full scale RO system. Keywords: RO membranes, Feed water, Salt passage 1. Introduction feed water characteristics. Specifically, mem- It is well known in the membrane industry brane age, chemistry, thickness, pore size, that salt passage through a reverse osmosis and charge density as well as feed water tem- membrane is affected by both membrane and perature and composition all contribute in varying degrees to the passage of ions *Correspondence. through the membrane. But less widely Presented at the Conference on Desalination and the Environment, Santa Margherita, Italy, 22–26 May 2005. European Desalination Society. Presented at the AWWA Membrane Technology Conference, Phoenix, AZ, March, 6–9, 2005. 0011-9164/05/$– See front matter Ó 2005 Elsevier B.V. All rights reserved 186 C. Bartels et al. / Desalination 184 (2005) 185–195 recognized is the role feed salinity has on salt In a single spiral wound element, the con- passage. Though this feed salinity effect has centration on the feed side increases as the appeared in experimental and theoretical stream flows from the feed end to the brine studies, only in the past few years has it been end of the element and volume is reduced due acknowledged by the industry when projecting to the removal of permeate. Salt passage membrane performance. As demand for RO through an element is measured using the treated water increases, an increasing number average of the feed concentration and brine of brackish systems are being designed to treat concentration (Cfb) so that Eq. 2 becomes: higher salinity waters at higher recoveries. For this reason, a practical understanding of the SP% ¼ðCp=CfbÞ100 ð3Þ effect of feed salinity on salt passage is essential for predicting and optimizing the design and Though the actual mechanism of mem- operation of these RO systems. brane salt passage is not well understood, The experiments presented in the paper will theories have been developed to predict the investigate the effect feed salinity has on salt concentration of salt on the permeate side passage. Six membranes will be tested over a given the characteristics of the membrane range of salinities on two different feed waters and the feed water. Basic calculations for consisting of sodium chloride and concen- predicting salt passage treat the membrane trated city water. A consistent trend will be as a black box and require little understand- shown in which salt passage increases at very ing of the transport mechanism within the low and high feed salinities. The increase in membrane [1]. These theories consider the salt passage can be as high as four times the following parameters when predicting salt salt passage at standard test conditions. passage: Temperature. An increase in feed water temperature will lead to an increase in 2. Theoretical Background membrane salt passage. A temperature A typical RO membrane allows only a small correction factor is used to compensate percentage of the feed ions to pass. Given the for this increase in salt passage when nor- concentration of salt in the permeate (Cp) and malizing data of an RO system. in the feed (Cf), the salt flux is given as: Membrane type. Brackish membranes, for example, have higher passage rates than Js ¼ B ðCf À CpÞð1Þ seawater membranes. Even among brack- ish membranes, salt passage varies with where B, referred to as the salt permeabil- the specific membrane chemistry. ity coefficient or simply the B-value, is a Membrane age. Wearing of the membrane function of the membrane thickness and the from continuous use and repeated clean- membrane’s diffusivity. Salt permeability is ings will increase salt passage over time. specific to different membrane types and is Feed water composition. Certain ionic arrived at by analytical methods. species in the feed water, such as mono- The passage of salt through a membrane is valent ions, pass more readily than other expressed as a percentage using the following ions such as the divalent ions. equation Given the nature of the membrane and an understanding of the degree that the mem- SP% ¼ðCp=CfÞ100 ð2Þ brane allows different ions to pass, basic C. Bartels et al. / Desalination 184 (2005) 185–195 187 theories assume salt passage is unaffected by membrane with a strong negative charge will feed concentration. have better rejection than a membrane with a A more in depth understanding seeks to weak negative charge. explain the passage of salt through the mem- The benefit of the Donnan potential in the brane in terms of several interacting mechan- form of increased rejection is greatest at low to isms including convection, diffusion, and mid salinities (1000 mg/L < TDS < 3000 mg/ charge repulsion. Specifically, the combined L). At very low salinity (TDS < 300) the con- influence of membrane charge and feed ionic centration of anions and cations is so low that strength is known to play a significant role in the Donnan potential is negligible. The Don- rejecting salts. As shown in Fig. 1, when a nan potential is also affected at high feed sali- typical feed solution containing both posi- nity. Increasing the feed salinity beyond tively charged ions (cations) and negatively 3000 mg/L weakens the Donnan potential charge ions (anions) comes in contact with and leads to a decrease in membrane rejection. the negatively charged membrane, the con- The theory behind this phenomenon suggests centration of the cations in the membrane is that along with increasing feed concentration, greater than their concentration in the bulk comes an increase in the cations at the mem- solution. At the same time, the concentration brane surface and thus a shielding of the Don- of the anions in the membrane becomes less nan potential (Fig. 2). As salinity continues to than that of the bulk solution. This ion shift increase, the rejection advantage created by creates an electrical potential known as the the Donnan potential is gradually diminished Donnan potential at the boundary between to a point where it is no longer effective and the membrane and the solution. The Donnan the increase in salt passage with feed salinity potential attracts cations to the membrane levels off [3]. while repelling anions away, thus increasing Donnan potential is also known to be anion rejection. According to this theory, the heavily influenced by the valance of ions pre- overall salt rejection is heavily dependent on sent in the feed. Specifically, the Donnan the rejection of anions. Therefore, a higher potential is weakest in solutions with a higher Donnan potential leads to an increase in concentration of divalent cations [4]. This is overall salt rejection of the membrane [2,3]. because the divalent cations at the membrane It can also be inferred from this theory that a surface shield the repulsive force of the Strong Negatively Charged Membrane Weak Negatively Charged Membrane 3 Fig. 1. Donnan potential created by the repulsion of anions and attraction of cations by a negatively charged membrane. The membrane with a strong negative charge will produce a greater repulsive force than a membrane with a weak negative charge. 188 C. Bartels et al. / Desalination 184 (2005) 185–195 Fig. 2. An increase in feed salinity leads to an Fig. 3. The divalent cations at the membrane surface increase in cation concentration at the membrane shield the repulsive force of the membrane’s negative surface which shields the repulsive force of the mem- charge on the anions in the bulk solution. brane’s negative charge on the anions in the bulk solution. concentrated city water feed was created by membrane’s negative charge on the anions running municipal city water through an RO (Fig. 3). while returning concentrate to the feed tank and sending permeate to drain. This was done until the desired concentration was achieved 3. Laboratory studies (up to 5000 ppm) and then spiked with 3.1. Single element test sodium chloride when a higher salinity was needed. After running an element for Testing for this study was done on 30 min, measurements were taken for pH, composite polyamide brackish RO elements temperature, and conductivity (feed, brine, of various permeabilities and rejections. permeate). Table 1 compares each of the elements used in this study at a single standard test condition. 3.2. Cell test Element testing was done on a closed loop Cell testing was also performed on several RO system at 15 gfd, 13% rec, and 25C.