# Fooling in Membrane Distillation, Osmotic Distillation and Osmotic Membrane Distillation
## 5 Methodologies for Membrane Fouling Prevention and Reduction
### Use of Antiscalants  
Antiscalants or scale inhibitors are surface active materials that inhibit inorganic scaling not only at low temperature but also at high temperature processes. These agents can prevent scaling not only in MD but also in OD or OMD [46] specially in pilot-scale desalination or RO brine treatment. The antiscalants are used to minimize the potential of scale formation on membrane surface. Antiscalants interfere with precipitation reactions in three primary ways: threshold inhibition, crystal formation and dispersion [146]. The first one refers to the capacity of an antiscalar to maintain supersaturated solutions of sparingly soluble salts (e.g., CaCO\({}_{3}\), CaSO\({}_{4}\), MgCO\({}_{3}\), Fe(OH)\({}_{3}\), CaF2, Fe(OH)\({}_{3}\), Al(OH)\({}_{3}\), etc.). The second one concerns the process of an antiscalants to deform crystal shapes, resulting in soft non adherent scales. The third one means the ability of some antiscalants to adsorb on crystals or colloidal particles and impart a highly negative charge to the crystal thereby keeping them separated and preventing propagation. Early antiscalants used sodium hexametaphosphate (SHMP) as a threshold agent to inhibit the growth of calcium carbonate and sulfate-based scales. Care must be taken to avoid hydrolysis of SHMP in the dosing tank that may decrease the scale inhibition efficiency. The following reaction can describe the steps of polyphosphate hydrolysis to ortho-phosphate (PO\({}_{3}^{-}\) + H\({}_{2}\)O \(\rightarrow\) H\({}_{2}\)PO\({}_{4}^{-}\)\(\rightarrow\) HO\({}_{4}^{3-}\)) [146; 147]. In addition to the hydrolysis problem there is a calcium phosphate scaling risk that can be described as (3 Ca\({}^{2+}\) + 2 PO\({}_{4}^{3-}\)\(\rightarrow\) Ca\({}_{3}\)(PO\({}_{4}\))\({}_{2}\)). The most used antiscalants in desalination are derived from three chemical groups [148]. Those including condensed polyphosphates, organophosphonates and polyelectrolytes. Effective polyelectrolyte inhibitors are mostly polycarboxylic acids (e.g., polyacrylic acid, polymethacrylic acid and polymaleic acid) [46]. In general, the most effective antiscalar contains a blend of polyacrylic acid (PAA) and phosphoric acid or polyacrylate and a hydroxyethylidene diphosphate (HEDP) [46]. [PERSON] [46] investigated the effect of polyphosphates antiscalar on the formation of CaCO\({}_{3}\) using a PP capillary module under DCMD configuration. When the feed without antiscalant was used the results of XRD analysis of the deposit formed on the PP membrane surface showed the calcite polymorphic form of CaCO\({}_{3}\) scale. However, when polyphosphates were applied as antiscalant under various compositions (2-20 ppm), practically no CaCO\({}_{3}\) crystal was detected on the membrane surface. According to [PERSON], et al. [149] five different commercial antiscalants, namely K797 (water acrylic terpolymer), K752 (polyacrylamide acid and sodium polyacrylate based compound), GHR (solution of a nitrogen containing organo-phosphorus compound), GLF (organo-phosphorus antiscalant) and GSI (based on neutralized carboxylic and phosphonic acids), were tested in DCMD process with different concentrations in the range 0.6 to 70 mg/L using a PP hollow fiber membrane coated with fluorosilcone on its external surface. The antiscalant K752 was found to be more effective in inhibiting CaCO\({}_{4}\) scaling compared with the other tested antiscalants due to its excellent thermal stability compared with polyphosphates and other copolymers antiscalants. In addition, GHR reduced calcite scaling, whereas not much difference was observed between the performances of the other antiscalants. It is necessary to point out that no wetting problem was detected because the surface tension of antiscalants solutions (71.5 mN/m) was near to that of tap water (71.8 mN/m).