Biostability characterization in a full‐scale hybrid NF/RO treatment system

The objective of this study was to quantify the assimilable organic carbon (AOC) in processing water at several stages of a full‐scale nanofiltration (NF) water treatment plant. The NF membrane plant investigated was a 45,400‐m 3 /d (12‐mgd) water‐softening facility in Plantation, Fla. The average AOC concentration of raw feedwater was estimated at 158 μg/L acetate‐C. After pretreatment (acid and antiscalant addition), AOC levels increased by 12.7% ( p ‐value = 0.055), suggesting that pretreatment chemicals used to control scaling may enhance bacterial growth potential on the membrane surface. The results also demonstrated that the membrane system was capable of effectively removing 63.4% of AOC and showed a decrease in membrane productivity over time (declined linearly at a rate of approximately 1.2 times 10 ‐4 L/m 2 /h ÷ kPa [4.9 times 10 ‐4 gfd ÷ psi] per d). This decrease could be attributed to biofouling, as observed by a steady increase in differential pressure during operation, and to natural organic matter fouling, as shown by the composition of the organic matter in the feedwater (54.9% hydrophobic and 18.1% hydrophilic neutral compounds).