Page 145 - ro membanes

128 6. CONDITIONING OF SALINE WATER
some types of antiscalants. Scale inhibitor feed systems typically include positive- displacement metering pumps (or centrifugal pumps for large systems) drawing from a day tank or other storage devices (for small plants) such as 55-gallon drums or larger- capacity totes.
6.5 ADDITION OF SODIUM HYDROXIDE
Sodium hydroxide systems are typically used to adjust the pH of the feed seawater to the first or second pass of RO systems designed for enhanced boron removal. Addition of sodium hydroxide allows the conversion of boron, which naturally occurs in seawater as boric acid, into borate, which has larger molecule of stronger charge, and is, therefore, easier to reject by the RO membranes than boric acid.
In SWRO plants with two-pass membrane systems, sodium hydroxide (NaOH) is usually introduced into the first-pass permeate because this water does not contain a significant amount of scaling compounds and the antiscalant does not need to be added for its treatment through the second RO pass. Sodium hydroxide dosage usually varies between 2 and 20 mg/L, and based on practical experience, most SWRO plants apply 12e15 mg/L of sodium hydroxide for enhanced boron removal.
In single-pass SWRO systems, sodium hydroxide is added to the feed of the RO systemd typically downstream of the point of addition of antiscalant. In such systems, feed of antisca- lant is needed because source water pH increase above 6.8 even at plant recovery of 40%e45% would trigger scale formation on the last two SWRO elements within several days to a week. SWRO operation at recoveries higher than 45% may result in scaling at even lower pH. Practical experience shows that if sodium hydroxide is added to feed of the first pass of the RO system, achieving the same boron rejection effect requires approxi- mately 15%e20% higher NaOH application dosage.
Ultimately, NaOH could be added to both the first- and the second-pass RO system feed waters. Practical experience shows that in this case, if the pH of the first-pass feed is increased to only 8e8.6, addition of antiscalant could be avoided for waters of high-salinity (i.e., TDS concentration >40 ppt). The NaOH dosage needed to achieve this pH is usually only 5e8 mg/L. Under this two-pass NaOH feed arrangement, the total boron removal could be increased with 10%e15% compared to the conventional practice of NaOH addition to the second-pass feed only.
In most cases, overdosing of sodium hydroxide would not have a negative impact on plant RO system performance except for increased scaling potential if this chemical is added to the feed of the first-pass RO system. As indicated previously, such scaling could be controlled successfully by the addition of antiscalant.
If sodium hydroxide is underdosed, the RO permeate will have elevated boron level. To determine the optimum dosage of sodium hydroxide, it is recommended to incrementally in- crease its concentration in the range of 10e20 mg/L in 2 mg/L increments every 2 h. For each test, NaOH dosage and boron levels in the RO feed and permeate should be measured and boron removal rate determined. The optimum dosage of sodium hydroxide is determined as the minimum dosage at which boron levels in the RO permeate reach steady state and further increase in NaOH does not yield significant additional boron removal.