Effect of calcium and iron(III) on membrane fouling under conditions typical of submerged membrane bioreactor treatment of wastewaters

Abstract

While the use of submerged membrane bioreactor (MBR) technology has risen dramatically during last decade, the most significant challenge still remaining is the reduction in the severity of membrane fouling. In addition, this filtration technology performs poorly in removal of dissolved contaminants such as phosphorus with addition of adsorbing chemicals such as iron necessary to ensure satisfactory effluent quality. The addition of chemicals such as iron salts, which readily hydrolyse and precipitate as iron oxyhydroxides on addition to the wastewater stream, may however exacerbate the fouling problem, particularly in submerged MBRs where a sedimentation step is not used. In such situations, optimizing the dosage of iron salts is critical in ensuring the most cost effective performance of the MBR however, the relationship between iron dosage and membrane fouling is not completely understood. In particular, the presence of other wastewater constituents such as monovalent and divalent ions (such as sodium and calcium) may significantly influence the interaction of iron with soluble microbial products (SMP) present in the MBR supernatant. In this thesis, the model polysaccharide alginate is used to investigate the interplay between SMP and iron and calcium under conditions typical of a submerged membrane bioreactor. The concentration of calcium present is shown to be a critical determinant of the severity of membrane fouling with low concentrations inducing alginate gelation and resultant severe membrane fouling while higher calcium concentrations result in gel breakage and alginate aggregation resulting in formation of porous cakes which facilitate rapid filtration. Our results also demonstrate that the presence of sodium may lead to a worsening of fouling as these ions block binding sites and limit the ability of calcium to induce aggregation. Comparison of the properties of the alginate assemblages formed in the presence of iron indicate that the Fe-alginate deposits induce even more severe fouling than Ca-alginate gels with lower concentrations of iron than calcium required to induce gelation. Increasing the concentration of iron leads eventually to a reduction in fouling propensity, most likely as a result of the adsorption of alginate to oxyhydroxide surfaces rather than alginate bridging as was the case for calcium. Importantly, the presence of calcium in a system to which iron salts are dosed is shown to lead to a significant reduction in fouling propensity. Investigations with SMP from an actual wastewater plant reveal similar interplay with iron and calcium as observed in the alginate system and highlight the possibility of fouling control through careful manipulation of iron and calcium concentrations in the supernatant.