Chapter 9: Chemical Analysis in Water Treatment 

Introduction to Chemical Analysis in Water Treatment 

Source water sampling is vital to the design phase of industrial plants. Samples are ideally collected over a period of several months or longer to capture seasonal chemistry variations. This is particularly important when surface water is the makeup source. The data provides critical information that plant personnel and the water treatment chemical supplier use to develop the most effective treatment programs for the various plant systems. Additionally, this data is crucial for selecting makeup treatment equipment and determining the appropriate materials for heat exchangers and other water system components. Design engineers often select materials without considering water chemistry, only to learn later that some of the selected metals are ill-suited. For existing facilities, periodic makeup, cooling, and wastewater analyses are necessary to evaluate process conditions and discover if treatment programs are effective or in need of modifications.

This chapter highlights laboratory capabilities and provides examples of how data is utilized to solve customer issues at ChemTreat’s state-of-the-art Technology and Innovation Center; however, water is not the only constituent of importance. Addressing scaling or microbiological fouling issues requires performing deposit analyses, while metallurgical analyses are often needed for corrosion investigations, as will be explored in this chapter. When troubleshooting water system issues, it is also critical to consider operational parameters, equipment manuals, and manufacturer’s guidelines.

Figure 9.1 ChemTreat Technology and Innovation Center

 Interpreting a Water Analysis

The document below demonstrates the analyses ChemTreat lab personnel perform on a standard sample, and it features water chemistry data from an industrial facility. Additional analyses are available upon client request or as necessitated by specific circumstances.

Figure 9.2. An analysis of makeup and cooling water chemistry at an industrial plant.

The first item to note is that this plant has two possible makeup sources, groundwater and municipal wastewater treatment plant effluent (designated in this report as WTP). WTP is the primary source at this facility. The WTP sample contains a high concentration of dissolved solids, which can affect makeup pre-treatment equipment selection and influence the potential for corrosion, scaling, and microbiological fouling in the cooling systems. One may ask, “Why would the plant accept WTP effluent as raw makeup instead of well water?” There are a few potential possibilities.

• The groundwater capacity may be limited and unable to supply the full needs of the plant.
• The WTP effluent may be much cheaper.
• In some places, like California, regulations require plants to take wastewater plant effluent for makeup. In other cases, plant management makes that choice as support for sustainable technology.

Now we will examine other constituents in the WTP stream that have significant impact on plant operations or on treatment programs and materials selection. It is important to note that Figure 9.2 is only a snapshot analysis. Conditions can fluctuate significantly within short periods of time or seasonally.

• Higher calcium, magnesium, alkalinity, and phosphate concentrations increase the potential for scale formation in cooling systems, particularly in heat exchangers where temperatures are higher. At this facility, sulfuric acid is injected to the cooling makeup to remove most alkalinity, minimizing the potential for calcium carbonate scale formation. This treatment method is reflected by the high sulfate concentration in the cooling water sample. As previously outlined, monitoring of calcium sulfate scaling potential becomes important in systems that utilize acid feed to reduce alkalinity.
• The relatively high WTP dissolved solids concentration appears to limit the cooling tower to roughly three cycles of concentration (COC). The blowdown rate would be considerably lower if the COC could be adjusted upwards, potentially to five, six, or seven.
  • The data clearly illustrates why on-line specific conductivity measurement is common for cooling tower COC and blowdown control. The difference in conductivity between the WTP effluent and cooling water is nearly threefold, reflecting the presence of hard-to-measure constituents like chloride and magnesium 
• The total organic carbon (TOC) concentration in the sample is 6.1 mg/L. This value is relatively high, and likely fluctuates significantly. TOC can impact operation in several ways.
  • Organics provide food for microbes that foul water systems.
  • Large organic compounds can foul the reverse osmosis membranes and ion exchange resins of high-purity makeup systems.
  • TOC reduction may require installation of tertiary treatment equipment at the industrial plant. Membrane bioreactors (MBR) and moving-bed bioreactors (MBBR) are two examples of emerging technologies. Installation of either adds cost and complexity to plant operations.
  • TOC can consume oxidizing biocides in cooling water systems, requiring elevated feed rates.
  • TOC can lead to trihalomethane compounds when oxidized using sodium hypochlorite or chlorine gas.
• The ammonia, nitrate, and phosphate concentrations are all much higher than typically seen in freshwater or groundwater supplies. These species are primary nutrients for microorganisms and can promote rapid microbiological growth in cooling water systems.
• The relatively high chloride concentration in the WTP effluent cycles up in the cooling tower. Chloride is a primary pitting agent of stainless steel, particularly the 300 series. Yet, on numerous occasions, ChemTreat personnel have seen new project specifications that call for 304 or 316 stainless steel steam surface condenser tubes without any consideration given to water quality, including chloride content. As outlined in Chapter 7, Introduction to Corrosion, Scale, and Biofouling Control in Cooling Systems, 304 SS can suffer pitting at chloride concentrations as low as 200 ppm, particularly if suspended solids or microbes have deposited on the tubes. The recommended limit for 316 SS is 400 ppm. Thus, these materials would be inadequate for this application.
• The elevation in sulfate concentration per acid feed could require a higher grade of concrete for the cooling tower basin. Standard concrete is susceptible to sulfate degradation, which is an additional issue that the design engineers may not typically consider.

We will examine other constituents of a water sample that are significant in many plant operations, but it is first necessary to discuss the fundamental concept of balancing the major cations and anions. 

Other Applications

The preceding sections highlighted important analyses for cooling water and makeup water systems. The following section will address additional applications.

Condensate Return

At many industrial facilities, a percentage (often large) of the steam utilized for process heating is recovered as condensate and returned to the boilers. Depending on the plant unit operations, or those facilities to which steam might be exported, condensate return may contain a wide variety of impurities ranging from corrosion products to chemicals leaking in at heat exchangers. Space limitations prevent a review of the many contaminants that could be present, but at refineries, petrochemical plants, and similar facilities, TOC is a critical measurement. Reliable on-line TOC analyzers are now available and can provide immediate notification of upset conditions.

Iron is another critical parameter to monitor in condensate samples, primarily because most of the condensate system is fabricated with carbon steel components. It is essential to measure total iron, because under the operating conditions of a well-functioning condensate system, most iron in the sample will be insoluble and require acid digestion for measurement.

Wastewater Trace Metal Analyses

Trace metal analyses can be particularly important for monitoring plant discharge streams, including those into and out of the facility’s wastewater treatment plant. The following table outlines harmful metals and metalloids that may appear in industrial wastewater for which regular analyses may be required.

Deposit Analysis

Analyses of solids deposits and density are valuable in determining the factors behind scale formation or other deposition. The figure below shows a typical qualitative analysis of a boiler tube deposit.

Figure 9.3. Deposit analysis from the mud drum of an industrial steam generator.

This report illustrates a common problem in steam generators, buildup of iron oxides on boiler tubes and internals. The iron oxides are often corrosion products generated in condensate return or feedwater systems. When the particulates reach the boiler, the high temperatures induce deposition. Heavy deposits restrict heat transfer and can cause tube overheating. More critical is the potential for under-deposit corrosion by either acidic or alkaline species. The data shown in Figure 9.3 suggests that investigation is needed into sources of carbon steel corrosion within the steam generating network. Water sample iron analyses would assist in the investigation, potentially through grab-sampling methods and on-line techniques like nephelometry. 

Presence of other constituents in a deposit analysis suggest additional issues. For example:

• Noticeable concentrations of calcium, magnesium, and silica indicate cooling water ingress from leaking steam surface condensers or heat exchanger tubes. Such ingress also allows additional contaminants to enter, most notably chloride and sulfate. In high-pressure boilers, these compounds can cause significant under-deposit corrosion, leading to premature tube failure.
• The presence of hardness deposits may also indicate makeup water system upsets. Many industrial plants have low- to medium-pressure steam generators, where the primary makeup treatment method is sodium zeolite softening. Frequently, when ChemTreat representatives visit a plant for the first time, the plant staff mentions boiler tube failures. A review of plant records often shows softening system upsets, which may allow hardness-laden makeup to enter the boilers.
• Copper in the deposit typically suggests corrosion of copper-alloy heat exchangers. Copper deposition will considerably increase the difficulty of chemically cleaning the equipment.
• Loss on Ignition (LOI). Similar to TOC for water samples, LOI in solids samples typically suggests the infiltration of organic compounds into the system. As noted, organic ingress to condensate return is common at refineries and organic chemical plants. The organic compounds may bake onto boiler tubes and reduce heat transfer. Organics can also cause foaming in steam drums, which in turn induces solids carryover into steam. Rapid superheater tube failures are a well-known outcome of this phenomenon.  

Analytical techniques for deposit chemistry include:

• Loss on Ignition (LOI)
• X-ray Diffraction (XRD)
• X-ray Fluorescence (XRF)
• Scanning Electron Microscopy (SEM)
• Thermogravimetric Analysis (TGA)

Figure 9.4. A scanning electron microscope in use at ChemTreat’s analytical lab.

Conclusion

Earlier chapters outlined the importance of on-line instrumentation for continuous monitoring of makeup, process, and cooling water chemistry. However, laboratory analyses are still vital for evaluating system conditions and revealing pathways for the selection or enhancement of chemical treatment programs. For new projects, pre-design raw water analyses are important for materials selection and preparation of chemical treatment programs. Always remember to consult with a team of experts when developing a treatment program.

References

1. Manahan, S.E., Fundamentals of Environmental Chemistry, CRC Press LLC,
   Boca Raton, FL, 1993.
2. Buecker, B., Kuruc, K., and F. Murphy, “Iron Monitoring in Industrial Steam Generating Systems”; Water Technology, Jan/Feb 2021.

Acknowledgements

The ChemTreat Water Essentials Handbook would not have been possible without the contributions of many people. See the full list of contributors.