Clean freshwater is a critical component of life. Decreasing supplies coupled with growing populations have stressed conventional sources of drinking water such as surface water and groundwater. Pollution of existing groundwater has further reduced sources of drinking water and increased requirements for expensive cleanup. In addition, recent headlines have reported on school children who have been exposed to arsenic and lead through school drinking fountains.
Desalination is becoming a worldwide solution to these water woes. Technology has improved and costs have decreased to the point that, when compared to the rising cost of clean water obtained from other sources, desalination has become an attractive option.
Large-scale desalination technology has been in use for decades; in fact, Aruba and Curacao have celebrated more than 70 years of continuous desalination operation. More than 10,000 facilities operate worldwide, with a significant number located in the U.S. Although seawater is the primary feedstock, brackish water and treated wastewater are potential starting materials for inland communities.
Supplying quality drinking water remains the primary goal of desalination. Source water must be analyzed for contaminants before the desalination process. The final treated water must also be analyzed to help optimize the treatment process, and to ensure regulatory requirements are met. Analytical challenges in the analysis of various types of water for desalination arise because of the matrix, which may vary and often has a high salt content.
Water Analysis Requirements
Desalinated water output is generally used for drinking water and must meet regulatory requirements for microbiological, organic and inorganic contaminants. Table 1 shows a few of the 69 regulated primary organic and inorganic contaminants, and the maximum amounts allowed in drinking water supplies, as prescribed in 40CFR, part 141. The types of technology specified to measure these compounds are shown in the last column. Most of the techniques used for analysis have been improved over the years, but ICP-MS (inductively coupled plasma mass spectrometry) has also been used in a new way. Advances in ICP-MS analysis used in the desalination process will be described later.
Speciated Inorganic Compounds
Disinfection byproducts may add to the list of measured compounds. One compound that may be observed when ozone is used for disinfection is bromate (BrO3-), regulated at a maximum concentration of 10 ng/mL. Measurement of this compound may be accomplished by measuring the total amount of bromine and assuming it is all present as the bromate. This is not viable since bromide is normally present in waters at much higher concentrations. If the limit is not exceeded, then the total measurement is enough.
But, what if the limit is exceeded? Rather than assume the bromate is not in compliance, it makes sense to measure the exact compound of interest. This measurement can be performed by using two techniques together (EPA Method 321.8). One is used to separate the compounds of interest, and the other is used to detect them. High-performance liquid chromatography is used for the separation of the bromine compounds coupled with ICP-MS for the detection. Figure 1 shows the results obtained with this technique.
Source Water & Brine
Source water for desalination provides a more challenging matrix for analysis. Varying salt content may cause interferences in the measurement of the analytes of interest. Brine, a byproduct of desalination, must also be evaluated before discharge because it has approximately twice the salinity. In the U.S., metals determination must be done by comparing it with the specifications of the National Pollutant Discharge Elimination System (NPDES) permit.
One example of this is the measurement of trace elements, such as arsenic (As), at low levels in the presence of high concentrations of chloride in seawater. Several techniques are available to measure As, such as atomic absorption, inductively coupled plasma optical emission and ICP-MS. The technique with the lowest detection limit is ICP-MS; however, it is subject to interference from argon chloride (40Ar35Cl+) at the mass usually measured for As, mass 75. The sample can be diluted, or an adsorbent can be used to separate the analyte from the matrix, but the detection limit may be degraded or losses may occur.
Newer cell technology can help remove the interference before the analyte of interest gets to the mass spectrometer where it is measured. This cell allows a gas to react with the sample, and under controlled conditions, the interference is chemically separated from the analyte. Figure 2 shows how the dynamic reaction cell operates. Detection limits are improved compared to other approaches, and the precision in varying matrices is improved. An instrument detection limit for arsenic measured in 1,000 mg/L NaCl was shown to be 2.3 ng/L, comparing favorably to detection limits of 0.6 to 1.8 ng/L, measured in 1% nitric acid solution, using this technique.
As desalination use for the production of drinking water increases, analytical challenges will also increase, particularly in source water analysis. Routine analyses for source and final drinking water can be enhanced with the use of new technology. Inorganic speciation analysis and the development of technology to more effectively remove measurement interferences improve analyses for desalination applications.