Lyophilization and Reconstitution of Reverse-Osmosis Concentrated Natural Organic Matter from a Drinking Water Source

Drinking water treatment research can be complicated by difficulties in shipping large water quantities and changing natural organic matter (NOM) characteristics over time. To overcome these issues, it is advantageous to have a reliable method for concentrating NOM with minimal loss and maximum shelf-life. NOM concentration and preservation by lyophilization (freeze-drying) has been practiced for many years; however, little information for lyophilizing and reconstituting NOM exists in the literature. The purpose of this research was to both evaluate and improve the lyophilization process of a concentrated NOM solution and to determine the appropriate conditions (pH, mixing time, and concentration factor) under which to reconstitute lyophilized NOM for maximum total organic carbon (TOC) and ultraviolet absorbance at 254-nm $(\mathrm{U}{\mathrm{V}}_{254})$ recovery. To achieve this, a new lyophilization method was developed, in which NOM concentrate is supercooled below its freezing point and heat transfer is carefully regulated to avoid ice melt-back. The lyophilized NOM was then reconstituted in 36 aliquots, at every unique combination of three pH values (6, 8, and 10), three mixing times (1, 4, and 24 h), and four concentration factors ($1\times$, $10\times$, $100\times$, $1000\times$). The overall mean TOC recovery was 101% ($\pm 3.3\%$). The improved method recovered 11% more TOC than a conventional procedure. No statistically significant ($p<0.05$) differences in TOC concentration were observed with respect to pH, mixing time, or concentration factor. The mean recovery of $\mathrm{U}{\mathrm{V}}_{254}$ was 91% with significantly better recovery at pH 10 than at pH 8 or 6. Mixing time did not significantly affect $\mathrm{U}{\mathrm{V}}_{254}$ recovery. No statistically significant differences in $\mathrm{U}{\mathrm{V}}_{254}$ were observed between reconstitutions at $1\times$, $10\times$, and $100\times$. A small but significant drop (4.8%, $p=0.029$) in $\mathrm{U}{\mathrm{V}}_{254}$ recovery was observed between $100\times$ and $1000\times$ samples. Finally, it was found that lyophilization and reconstitution reduces the dissolved silicon concentration in the final sample