Evaluating the feasibility of green scaling inhibitors in fertilizer-driven forward osmosis process: Application of methylglycinediacetic acid (MGDA) and glutamic acid diacetate (GLDA) to replace commercial scaling inhibitors

Abstract

Fertilizer-drawn forward osmosis (FDFO) is a strategy used to establish forward osmosis (FO) as a single water treatment method. In FDFO, phosphate from the draw solution is transported to the feed side and induces membrane scaling by interacting with divalent ions on the membrane surface. Ethylenediaminetetraacetic acid (EDTA) is generally applied to prevent membrane scaling in FDFO, but it can lead to the discharge of toxic metal ions into the water system. Therefore, in this study, methylglycinediacetic acid (MGDA; dicarboxymethyl alaninate) and glutamic acid diacetate (GLDA) were evaluated as potential “green” scaling inhibitors of FDFO desalination, based on feed conditions—scaling inhibitor concentration, feed pH, and temperature—and biodegradation extent. Among these, EDTA and MGDA were more effective in mitigating scale formation than GLDA because of their high chelation effect on divalent ions in batch experiments. With FDFO, the flux decline ratio was improved under the following three cases: (1) increasing the scaling inhibitor concentration to over 10 ppm by enhancing the chelation effect and hindering phosphate transport, (2) decreasing the feed pH by maintaining protonated phosphate, and (3) decreasing the feed temperature by reducing the reaction energy. Moreover, MGDA showed a 100 % biodegradation rate within 21 days, whereas EDTA was not degraded by microorganisms, even after 28 days. This study confirmed that MGDA has the potential to replace EDTA in FDFO and water treatment plants and suggested an outstanding eco-friendly operational procedure for its application.