Effect of Scale Deposition on Surface Tension of Seawater and Membrane Distillation

High liquid entry pressure (LEP) is one of the desired characteristics of a membrane distillation (MD) membrane. Maintaining LEP above operating feed pressure conditions is critical to prevent permeate quality worsening by saline feed liquid passage through the membrane. Air gap membrane distillation experiments under controlled salinity and temperature operating conditions indicate that in addition to flux decline, inorganic scaling can also lead to feed breakthrough and permeate salinity increase in some cases. The relationship between calcium sulfate and carbonate scaling and breakthrough was further illustrated by comparing the permeate quality obtained using clean and fouled membrane with seawater brine of S = 120 g/kg as feed at t = 70 °C. While contact angle changes and pore size variation have been considered to explain salt passage and LEP decrease, the effect of surface tension of the feed liquid under MD operating conditions has not been investigated. In a previous work, the authors measured reference data and created correlations for surface tension of seawater for 1 ≤ t ≤ 92 °C and salinities 0 ≤ S ≤ 131 g/kg. In the present study, new measurements investigating the effect of scaling on the surface tension of seawater are presented. Bulk precipitation was observed in seawater solutions for t > 50 °C. Membrane distillation operating conditions were simulated in the surface tension measurements where the test-beaker was reused between experiments after rinsing, which left behind a thin layer of scalant on the glass surface. Repeatable results under these conditions show marked decrease in surface tension (up to 30% at S = 120 g/kg) from standard correlations. This reduction in surface tension can cause LEP to reduce and increase salt passage in MD systems at t > 50 °C, as has been reported in the literature. The role of surface tension changes on MD separation effectiveness is recognized.