Establishing treatment processes for reliable high-purity makeup in power and co-generation boilers (Part 1)

By Brad Buecker, Buecker & Associates, LLC

By Katie Perryman, ChemTreat, Inc.

It has been over a century since steam was first utilized to drive turbines/generators for electrical production. As boiler technology advanced from early designs, power plant owners, operators, and technical personnel began to realize that the increasing pressures and temperatures of steam generators required high-purity makeup water to minimize corrosion and scale formation. This led to the advancement of ion exchange (IX) technology to produce boiler makeup with low part-per-billion (ppb) concentrations of impurities.

In the last several decades, membrane methods, notably reverse osmosis (RO), have become popular for primary demineralization, with ion exchange now serving to “polish” the RO product for steam generator makeup. In this series, we will examine various aspects of current technologies and the capabilities of modern systems. Part 1 offers a discussion of pretreatment methods, which are very important for reducing fouling, scaling, and other chemistry upsets within RO membranes and IX resins.

“Fresh” water still has many impurities

Although freshwater supplies are in decline (subject to regional fluctuations), many industrial facilities still use makeup from lakes, reservoirs, or rivers. Water moves around the globe in a process known as the hydrologic cycle.

*Figure 1. Basic schematic of the hydrologic cycle. (1)*

Water vapor may be transported many miles before atmospheric conditions cause condensation and precipitation. Along the way, water vapor can absorb gases from the atmosphere, including pollutants, which alter its chemistry. Water chemistry is also influenced by the soil, mineral deposits, and vegetation over which water flows (or filters through to become groundwater).

Table 1 provides a snapshot analysis from several years ago of the major constituents in a Midwestern lake.

For utility heat recovery steam generators (HRSGs) and conventional fossil-fired boilers, common makeup water treatment effluent guidelines are:

Sodium: ≤2 ppb
Silica: ≤10 ppb
Specific conductivity: ≤0.1 μS/cm

When comparing Table 1 to these guidelines, it becomes apparent that even systems with freshwater as the raw makeup source may need to reduce impurity concentrations dramatically before sending the water to high-pressure boilers. Most modern power systems, such as combined cycle units with HRSGs, rely primarily on RO and IX polishing to produce high-purity water.

*Figure 2. Typical core process for high-purity makeup water production. (1)*

It is common for contractors to change out exhausted IX “bottles” with vessels containing freshly regenerated resin, eliminating the need for on-site regeneration with acid and caustic.

Exploring pretreatment options

For the configuration shown in Figure 2, pretreatment largely focuses on reducing fouling and organic growth on RO membranes.

In this article, we spotlight pretreatment options for surface water issues, including:

The spiral-wound configuration of RO membranes, which makes them susceptible to particulate fouling.
The importance of raw water biocide treatment for reducing microbiological growth (keeping in mind that oxidizing biocides, particularly chlorine, can severely damage RO membranes)
The accumulation of large organic molecules from decaying vegetation found in many freshwater supplies, which can coat RO membranes, inhibiting flow, reducing capacity, and raising the transmembrane pressure

In the 20th century, clarification with multi-media filtration was the common method for removing particulates from clarifier effluent. A well-designed and operated clarifier/filter can produce water with less than 1 NTU turbidity. However, micro- and ultrafiltration membrane technologies have become a popular replacement for clarification, unless lime softening is necessary to lower hardness and alkalinity concentrations, which may be elevated in some groundwater supplies. Figure 3 below shows a 300 gallon-per-minute (gpm) microfiltration (MF) unit chosen as a replacement for an aging power plant clarifier.

Figure 3. Microfilter skid including the 24 modules required to produce 300 gpm of filtered RO feedwater. The inlet raw water holding tank, with forwarding and backwash pumps, is on the left. Photo by Brad Buecker.

The unit reduced RO makeup turbidity from a typical range of 0.5–1.0 NTU to less than 0.05 NTU. (2) This led to a dramatic reduction in RO cartridge filter and membrane cleaning frequency. Regularly adjusting clarifier coagulant and flocculant dosages to match changing flow rates was no longer required. This particular MF unit proved to be extremely reliable, provided it was given a thorough off-line cleaning every two to three months. For this application (and also for auxiliary heat exchanger cleanings throughout the facility), plant mechanics fabricated a portable vessel with mixer, heater, hoses, and a circulating pump to warm cleaning solutions to near 100^oF.

*Figure 4. Cleaning cart showing tank and heater. Photo by Brad Buecker.*

Normally, a two-step cleaning starts with circulating a relatively dilute but powerful caustic and bleach solution to remove organics and microbes. Following a rinse, dilute citric acid circulation removes iron oxide particles.

The normal process for MF and UF operation involves producing filtered water for a set period, e.g., 20 minutes, followed by a one- to two-minute backflush/air scour process to remove particulates that have collected on the membranes. The solids exit in a small wastewater stream. Modern units also include a periodic chemically-enhanced backwash (CEB) step, in which caustic or a chelant (often citric acid) is added to the backwash water to help clean the membranes. The chemical choice depends on the typical solids that collect on the membranes.

Membrane design options

Three designs exist for MF/UF membranes:

Hollow fiber
Tubular
Spiral wound

The hollow fiber design is most common, with pressurized and vacuum systems available for different application needs.

*Figure 5. Cutaway view of the spaghetti-like hollow fiber membranes in the MF pressure vessels shown in Figure 3. Photo courtesy of Pall Corporation.*

Typical membrane materials include polyethersulfone (PES), polyvinylidene fluoride (PVDF), polypropylene (PP), and polysulfone (PS), with PES and PVDF being the most common. Both are hydrophilic, meaning the lumen surface becomes completely wetted to help resist organic fouling. PES has a slightly better permeability than PVDF. These materials easily tolerate a continuous oxidizing feed, a common method to minimize microbiological fouling.

PES has a higher caustic tolerance for organics removal during off-line cleanings, whereas PVDF has a higher chlorine tolerance and membrane durability. These are important factors when deciding which material is better for particular water sources or operational factors.

Pressurized or submerged membrane designs are also available. Pressurized systems can have either an inside-out or outside-in flow path, whereas submersible designs, with the membranes suspended in a tank containing the feedwater, are outside-in, with mild vacuum pulling the water into the central core of the membranes.

Suspended solids excursions and the importance of historical water quality data

The potential for intense suspended solids excursions is an important consideration when designing membrane systems. Such excursions are most common in river waters following heavy precipitation. Some form of particulate pre-screening or settling may be necessary upstream of MF or UF, although submerged membranes can handle much higher solids concentrations than pressurized systems.

Historical water quality data can be very valuable for process and equipment selection in these instances. For example, the turbidity in some rivers can increase from single digits to hundreds or even thousands of NTU during heavy rain. Without seasonal analyses to confirm such fluctuations, extreme conditions may cause pretreatment system failure.

The impact of oxidizing compounds on RO membranes

Oxidizing biocide (often bleach) feed is a typical treatment option for inhibiting microbiological fouling in water networks and treatment equipment in most raw water makeup systems. Unfortunately, the primary material of most RO membranes (not the spacer or support material) has a polyamide chemistry that contains nitrogen. Chlorine bonds with the nitrogen molecules and irreversibly damages the membranes. A common rule-of-thumb for membrane longevity is 1,000-ppm-hours, meaning membranes remain functional for approximately 1,000 hours at a 1 ppm chlorine concentration (or one hour at a 1,000-ppm chlorine concentration). However, the presence of heavy metals like iron can decrease this tolerance to as low as 200-ppm hours. Given that a normal membrane life expectancy typically ranges from 3 to 7 years, chlorine removal ahead of the RO membranes is an important step for improving membrane longevity. Of course, so is control of suspended solids fouling and scale formation.

The two primary methods for oxidizing biocide removal from RO/demineralizer feed are activated carbon (AC) filtration and reducing agent injection. However, oxidizing biocides react within the first few inches of an AC bed, leaving the remainder of the bed as a breeding ground for organisms that survive treatment. This is exacerbated by the AC bed’s ability to remove organics, which then become food for the organisms. Accordingly, many modern systems are designed with reducing agent injection to remove residual oxidizers. While a number of reducing agents are available, the two most common are:

Sodium bisulfite (NaHSO₃): The most popular and inexpensive reducing agent, usually supplied as a 30% liquid solution.
Sodium metabisulfite (Na₂S₂O₅): The granular form of sodium bisulfite.

The reactions of these two compounds with chlorine are shown below.

2HOCl + 2NaHSO₃ → 2H₂SO₄ + 2NaCl Eq. 1

2HOCl + Na₂S₂O₅ + H₂O → 2H₂SO₄ + 2NaCl Eq. 2

Continuous monitoring is very important downstream of the reducing agent injection point. The primary measurement is chlorine residual with oxidation-reduction potential (ORP) as a potential supplement. The priority is to provide an alarm in the event of reducing agent feed malfunction to protect RO membranes. However, modern systems can also be designed to adjust reducing agent feed with analyzer signals to minimize overfeed while reducing the impact of chlorine on membrane life.

Figure 6. Hach ULR CL-17sc continuous chlorine analyzer. Photo courtesy of Hach.

If possible, the reducing agent injection point should be placed after the RO cartridge filters. If that is not an option, the injection point should be as close to the cartridge filer housing as possible. Some organisms go into hibernation when contacted by an oxidizing biocide, re-emerging once the biocide residual disappears. The surviving microbes can establish large colonies in RO pre-filters and membranes.

Dealing with organic foulants

Many surface water sources contain significant concentrations of large organic compounds, e.g., tannins, lignin, and humic acids that can foul membrane surfaces. These compounds are normally measured as total organic carbon (TOC). TOC of less than 3 ppm in an RO feed is typically recommended. MF and UF are used primarily for particulate filtration, although some large organic removal may be possible. Supplemental AC filtration may be needed to remove other organics. If so, concerns about downstream microbiological fouling potential should be addressed before AC filtration is implemented.

Conclusion

This first installment of the series explores several important aspects of high-purity makeup system pretreatment. Deficient RO pretreatment is a leading cause for RO membrane failure/premature membrane change-out.

Please remember that each system is different and has unique treatment needs. As with all other technologies, due diligence is necessary to determine the feasibility for utilizing the methods discussed in this article. Difficulties have arisen at some sites where the inlet water had impurities that reacted with pretreatment or backwash chemicals to produce foulants or scale-forming deposits. Always consult your equipment manuals and guides and contact a water treatment professional before making changes to system operation.

References

B. Buecker, Tech. Ed., “Water Essentials”; ChemTreat, Inc., 2023. (This is the industrial water handbook that is currently being released digitally on a chapter-by-chapter basis.) Information is available at www.chemtreat.com.
B. Buecker, “Microfiltration: An Up and Coming Approach to Pre-Treatment for the Power Industry”; presented at the 26^th Annual Electric Utility Chemistry Workshop, May 9-11, 2006, Champaign, Illinois.

Brad Buecker is president of Buecker & Associates, LLC, consulting and technical writing/marketing. Most recently he served as Senior Technical Publicist with ChemTreat, Inc. He has over four decades of experience in or supporting the power and industrial water treatment industries, much of it in steam generation chemistry, water treatment, air quality control, and results engineering positions with City Water, Light & Power (Springfield, Illinois) and Kansas City Power & Light Company’s (now Evergy) La Cygne, Kansas station. Buecker has a B.S. in chemistry from Iowa State University with additional course work in fluid mechanics, energy and materials balances, and advanced inorganic chemistry. He has authored or co-authored over 250 articles for various technical trade magazines and has written three books on power plant chemistry and air pollution control. He may be reached at [email protected].

Katie Perryman is Manager of the Pretreatment Technical Team at ChemTreat. She has nine years in the water treatment industry with a focus on pretreatment applications that include filtration, membrane separation and ion exchange systems. She has spent her time at ChemTreat supporting a wide variety of customers in the power, chemical, food & beverage, and transportation industries, among others. Perryman has acted as a corporate trainer and presenter both internally and externally for several years at conferences such as the Southwest Chemistry Workshop and ChemTreat’s Power conference. Perryman has a B.S. in Chemistry from Virginia Tech. She may be reached at [email protected].