Examination of Silica Removal with Solids Recycle for Reverse Osmosis Pretreatment
Water treatment methods serve to remove harmful constituents from ground and surface waters prior to municipal distribution by exploiting the physical and chemical properties of those constituents. Properties can include size, charge, or solubility. Silica (SiO2) is difficult to remove during water treatment because it can exist in ground and surface waters as various dissolved and particulate species, all of which are defined by different properties. Silica speciation is dependent on variables like pH, temperature, concentration, and ionic composition. Consumption of silica as it exists in drinking water is not dangerous to humans or animals, but it can form damaging scales on the surfaces of industrial equipment and reverse osmosis membranes. For this reason, the subject of silica removal has been the focus of many studies; yet, there remains much to be learned. This thesis aimed to study the complex issue of silica removal as a pretreatment step to RO. A review of the literature indicated that silica is most commonly removed during a lime softening process but that removal is tied mostly to the presence of magnesium. Furthermore, studies have shown that removal is best when the chemical softening process is operated at a pH of 10 or above. Magnesium hydroxide precipitates in this higher pH range thereby removing silica via a co-precipitation or adsorption mechanism. This project explored the relationship between magnesium concentration, pH, and silica removal in two phases. Phase 1 included 21 jar tests that studied silica removal within a pH range of 7 to 12 and with different concentrations of dissolved magnesium chloride (MgCl2) and freshly precipitated magnesium hydroxide (Mg(OH)2); freshly precipitated ferric hydroxide (Fe(OH)3) was also studied in one test. Phase 2 flow-through experimentation consisted of 5 tests that were conducted in a system operating around a pH of 9.5 or 10 with solids recirculation. Mg(OH)2 and Fe(OH)2 were tested in Phase 2. The concentrations of dissolved MgCl2 and freshly precipitated (also referred to as preformed) Mg(OH)2 used in Phase 1 were 100, 200, 600, 1,000, 1,200, and 10,000 mg/L as Mg2+. The concentration of freshly precipitated (also referred to as preformed) Fe(OH)3 used in Phase 1 was 2,300 mg/L as Fe3+. In Phase 2, preformed Mg(OH)2 concentrations were 0.5, 1, and 3 mg/L as Mg2+ and preformed Fe(OH)3 concentrations were 1.15 and 2.3 mg/L as Fe3+. Phase 1 demonstrated that in tests were no calcium was present, preformed Mg(OH)2 solids removed more silica than Mg(OH)2 solids that precipitated during tests with dissolved MgCl2. This result suggested that adsorption was a more dominant removal mechanism than co-precipitation. The first 11 tests with MgCl2 and Mg(OH)2 were compared on the basis of initial pH, but it was determined that final pH would offer a more accurate comparison. The jar testing completed during Phase 1 served two purposes, the first of which was to explore the question of which mechanism — adsorption or co-precipitation — is more dominant in silica removal. The second goal was to establish some ideal operating parameters (i.e., magnesium concentration, pH, hydraulic residence time) to be used during phase two of the project. The flow-through experimentation phase looked at silica removal over an extended period of time within a system that recirculated solids. Results showed that freshly precipitated magnesium hydroxide solids achieve significant silica removal in the pH range of 9.5 to 10, despite the common assertion that good removal can only be achieved at a pH greater than 10. Furthermore, findings from Phase 1 suggested that adsorption was the more dominant removal mechanism during the softening process. Phase 2 results supported findings of Phase 1, and demonstrated that freshly precipitated Mg(OH)2 solids were good silica adsorbents but that silica removal was limited by adsorption capacity of the solids present in the system. Phase 2 results also supported the hypothesis that increased solids recycle in a system will enhance silica removal.
silica removal, reverse osmosis pretreatment, silica scale, membrane fouling, solids recycle, adsorption, co-precipitation, magnesium hydroxide, flow-through experimentation, solids recirculation
Networked Digital Library of Theses and Dissertations