B. Buecker, Buecker & Associates LLC, Lawrence, Kansas; and P. KALAKODIMI, ChemTreat Inc., Ashland, Virginia
As fresh water sources diminish in some areas of the U.S. and around the world, water conservation is becoming increasingly important. Wastewater reclaim is a vital tool in these conservation efforts, where at least two main themes have emerged: effluent use from publicly owned treatment works [(POTW), i.e., municipal wastewater] as industrial plant makeup, and the recovery/reuse of wastewater generated within the industrial plant. This article examines potential difficulties with these processes and considers methods for avoiding pitfalls.
Growing reclaim water use. It is well-known that drought conditions in the southwestern U.S. continue to increase in severity over the long term. Accordingly, conserving water by using alternatives to fresh water has become vital. California has mandated that industrial plants take POTW effluent for makeup. To various degrees, this philosophy is being adopted at other locations within the country. However, the variability and complexity of POTW effluent chemistry and quality makes reuse a challenging task. Many wastewater plants employ primary and secondary treatment, which remove suspended solids and pathogens before discharge; however, tertiary treatment is often missing, allowing elevated concentrations of many dissolved solids to remain. TABLE 1 offers a comparison between four fresh water supplies and four reclaim waters (listed by another common name, “grey water”).
Compare the surface water supplies, highlighted in blue, with the grey water supplies, highlighted in orange. The grey water exhibits much higher concentrations of the major cations (calcium, magnesium, potassium, sodium), anions (chloride, nitrate, phosphate, sulfate) and, of course, conductivity. Ammonia in surface waters is usually low but often appears in the double digits as mg/l for reclaim water, as in the Sonora and Pittsburg samples in TABLE 1, unless the POTW employs a nitrification step to convert the ammonia to nitrate. Not shown is total organic carbon (TOC), which is also typically much higher in reclaim water than fresh water. Municipal wastewater effluent may have significant concentrations of suspended solids. The nitrogen species, phosphate and organics serve as nutrients and food for microorganisms.
Dealing with the “nasties” in reclaim water. Anyone who has dealt with cooling water treatment knows that microbiological fouling control can be a severe challenge. Consider systems with cooling towers. Hordes of microbes enter from both the air flow and makeup water to the tower. The tower and associated cooling system provide warm, wet conditions for the development and growth of microbiological colonies. Once colonies become established, some organisms secrete a protective polysaccharide (slime) layer that protects the microbes (FIG. 1). The slime, plus any silt and other debris that it captures, can quickly and severely foul heat exchangers throughout the system (FIG. 2).
A well-designed, maintained and operated chemical feed program is necessary for fouling control. An oxidizing compound (chlorine, chlorine-activated bromine, chlorine dioxide, etc.) serves as the primary biocide, with perhaps periodic feed of a non-oxidizing biocide as a supplement.2 However, throughout modern industrial history, severe fouling has repeatedly occurred from upsets in treatment system design or operation. Now, imagine what could happen with a makeup stream that has the chemistry of the grey water samples shown in TABLE 1. The elevated levels of nitrogen and phosphorus, combined with plenty of organic food, can lead to explosive microbiological growth. Furthermore, fouling is not limited to cooling networks and can affect other systems, including pretreatment for high-purity water production, service water lines, etc. For example, large organic molecules can severely foul RO membranes, apart from microbiological issues.
This is a primary challenge for POTW effluent reuse as industrial plant makeup. In cases where the stream contains a constant elevated suspended solids concentration, clarification might be in order. Clarifier design has significantly evolved from the large, circular types that were ubiquitous during the last century. A common rise rate (effluent flow divided by the surface area at the top of the clarifier) of 1 gpm/ft2 or slightly less is a common standard for these older designs, as the gentle flow allows particles to settle. However, in systems such as the ballast type (ballast material is typically micro-sand or magnetite), after coagulant injection, the influent encounters the ballasted recirculating flow in the flocculation zone (FIG. 3).
The coagulated/flocculated solids attach to the ballast material. The microsand or magnetite is much heavier than the sludge, therefore the settling efficiency increases dramatically. Rise rates of 25 gpm/ft2 or higher have been reported for some of these technologies. The ballast material is recovered in hydrocyclones (via the underflow stream) for reuse, while the sludge exits with the hydrocyclone overflow for disposal.
If the makeup water is high in hardness, lime softening clarification may be a consideration, although this process generates large quantities of sludge that must be disposed. Lime softening, and the resultant elevated pH, will precipitate phosphate and hardness that could otherwise cause downstream difficulties. However, the added complexity of lime softening and sludge disposal diminishes its viability for many applications.
Clarification does nothing for nitrogen-based impurities, particularly ammonia and nitrite/nitrate. Also, many organics may be carried through a clarifier. Therefore, even with clarification for suspended solids and possibly phosphate removal, many nutrients and food can still enter plant water systems. Accordingly, for POTW effluent from a facility with only secondary treatment, the industrial plant may need to strongly consider independent tertiary treatment. Proven methods have been in place for decades. The activated sludge process—in which, following large solids and grit removal, the waste stream flows into a large basin or basins filled with beneficial microorganisms that consume the organics and nitrogen/phosphorus nutrients—is common. The term activated comes from the fact that air is injected into the basin, often at numerous locations, to provide an aerobic environment for the microbes. A common variation on this technology is the trickling filter, in which beneficial organisms attach to fixed media that wastewater flows over. This arrangement gives the microbes a stable base to carry out waste removal.
A difficulty with both technologies is that the treatment processes are slow and require large pond volumes or media surface area. Among several advanced systems, two have emerged as leading candidates: membrane bioreactors (MBR) and moving-bed bioreactors (MBBR). A basic MBR schematic is shown in FIG. 4.
The fundamental process is similar to conventional activated sludge, wherein the beneficial organisms consume food and nutrients that enter the main vessel from the mixing zone. A recycle stream helps to bring active, well-established microbes to the inlet of the mixing zone. However, modern units have a much smaller footprint than older, traditional activated sludge systems. Another major difference of MBR from conventional activated sludge is the use of microfilter membranes rather than a clarifier to separate solids from the effluent. The microfiltration process produces a very clear stream, essentially free of suspended solids. Conscientious care of the filters is necessary to maintain reliability. This includes periodic membrane cleaning.
One deficiency of this most basic MBR process is that while ammonia in the stream is converted to nitrite/nitrate, the nitrogen remains. These species can still serve as a nutrient in industrial plant water systems. The problem can be solved, if necessary, by expanding the MBR unit to include anoxic or anaerobic reaction chambers, where other beneficial microorganisms convert nitrite/nitrate to elemental nitrogen. However, this step adds complexity to the process.
The prime feature of MBBR is a mechanically stirred main reaction vessel that contains mobile plastic media. The media (shown in FIG. 5) serves as sites for the beneficial microorganisms to attach and then consume the nutrients and food from the influent.
In some respects, MBBR can be thought of as an advanced version of the trickling bed wastewater treatment process, but with mobile rather than fixed reaction sites. Due to the presence of circulating solid media in the reaction vessel, microfiltration membranes cannot be placed in this compartment. Rather, the microfilters must be external to the reactor.
A common concern when evaluating system installation is staffing requirements to operate the unit. Two potential solutions stand out: if the plant is located close to the POTW, it may be possible to place the MBR or MBBR at the POTW and have that staff operate it; or many reputable manufacturers offer “build-own-operate-maintain (BOOM)” programs and will, for an annual fee, manage all equipment and operating details.
Consider another issue with the reclaim water chemistry outlined in TABLE 1: the rather high chloride concentrations. Common materials for combined-cycle and co-generation steam surface condenser tubing are either 304 or 316 stainless steel. Chloride can induce serious pitting in these metals, which may severely shorten tube life. The recommended chloride limits for clean tubes have been gradually reduced as more data has become available about corrosion potential. One top industry expert now suggests chloride limits of 200 ppm for 304 SS and 400 ppm for 316 SS.3 Standard pretreatment processes do not remove chloride, and it is evident from TABLE 1 that if any of those reclaim waters were selected as makeup, the concentration increase induced by cooling tower cycling would quite likely cause a chloride exceedance. Note the emphasis above on clean tubes. Deposits would decidedly increase the corrosion potential.
Internal water conservation. Multiple waste streams may be generated at large industrial facilities. Cooling tower blowdown is one example that has received increased attention for water conservation. FIG. 6 outlines a treatment process at a combined-cycle power plant in a semi-arid region of the U.S.
The system recovers 90% of the blowdown as high-purity reverse osmosis (RO) permeate, with the remainder discharged to an evaporation pond. The configuration allows the plant to operate as a zero-liquid-discharge (ZLD) facility.
Keys to the process are:
Lessons learned and modifications made during the commissioning phase of this system include:
As many readers may have observed, this particular ZLD application is somewhat simplified because the final concentrated wastewater stream can be disposed of by natural evaporation. That option is not viable at many locations, and more intensive methods such as evaporation/crystallization may be needed to reach ZLD.4 Per the example above, pre-concentration reduces the wastewater volume ahead of final treatment. Of the remaining waste stream, perhaps 90% can be recovered in a brine concentrator and recycled to the plant. The crystallizer converts the remaining liquid stream to a solid product. These final steps can be energy intensive and often require precise chemical feed to reduce the scaling potential of the concentrated waste stream. Equipment costs may be high for exotic materials to resist corrosion. Regular equipment cleaning is usually necessary to remove scale and restore proper heat transfer.
Condensate return. At times, condensate return chemistry affects water conservation, but also often has a strong influence on equipment performance and reliability. Consider the basic co-generation schematic in FIG. 7.
The diagram includes automatic condensate return dump valves that are activated by conductivity excursions—a feature common for many systems. The size and frequency of upsets may vary greatly from plant to plant, or even between the condensate streams within a plant. Dumping can result in considerable water loss. Furthermore, this water would already have gone through makeup treatment and energy input from the boiler(s), making dumping costly from that standpoint. If condensate polishing is considered, the types of impurities in the condensate will dictate polisher design. Conventional ion exchange resins are effective for mineral salt and hardness removal, subject to resin temperature limitations.
Granular activated carbon (GAC) is a possibility for some organic compounds, particularly for larger organic molecules. Ion exchange resins, without the active sites, may also be effective for organic removal, as the millions of beads with their many minute passageways offer an enormous surface area for organic adsorption. Direct media filtration may work well on particulates such as iron oxide corrosion products; however, a better approach is addressing the root cause(s) of corrosion with proper condensate chemical treatment, whenever possible.
As a final thought, consider a reverse scenario to the discussion above. Years ago, author Buecker and a colleague investigated a fouling issue at an organic chemicals plant that produced a primary product plus four closely-related derivatives. Steam generation came from several 550-psig drum-type boilers in which solids deposition in the superheaters and the associated tube overheating required tube bundle replacement every 1.5 yr–2 yr. An extracted superheater bundle had internal deposits up to approximately ¼ in. deep. It was also discovered that foam was being issued from every boiler-saturated steam sample line. The plant had minimal steam generation analytical chemistry equipment—with tests performed by operators, not chemists—so internal data was sparse. However, condensate return chemistry data provided by the plant’s water treatment vendor showed condensate total organic carbon (TOC) concentrations as high as 200 ppm. Compare this to the recommended upper limit of 0.5 ppm in the ASME industrial steam generator guidelines.5 The excessive organic contamination in the boilers generated the foam that carried over to the superheaters. Condensate dumping would have produced a large and potentially hazardous waste stream and required a substantial upgrade to the makeup treatment unit to provide the lost volume. However, no condensate polishers were in place to remove the organic compounds. So, without some modifications, the superheater fouling was destined to continue.
Takeaways. Water conservation is becoming increasingly important, as many parts of the world face drying conditions. Various methods are available to recover and recycle both external and internal wastewater streams for industrial plant use. However, pretreatment to varying degrees is necessary to minimize the fouling, scaling and corrosion potential of makeup supplies and internally recycled streams. Costs of unit shutdowns can be much greater than the expenditures for design, installation and operation of reliable reclaim water treatment systems.
Please remember that each system is different and has unique treatment needs. Due diligence is necessary to determine the feasibility for utilizing the methods discussed in this article. Always consult your equipment manuals and guides and contact a water treatment professional before making changes to your systems and treatment processes. HP
REFERENCES
BRAD BUECKER is President of Buecker & Associates LLC, specializing in consulting and technical writing/marketing. Most recently, he served as Senior Technical Publicist with ChemTreat Inc. Buecker has more than four decades of experience in or supporting the power and water treatment industries, much of it in steam generation chemistry, cooling 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. His work has also included 11 yr with two engineering firms, Burns & McDonnell and Kiewit, and 2 yr as Acting Water/Wastewater Supervisor at a chemical plant. Buecker earned a BS degree 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 more than 250 articles for various technical trade magazines, and has written three books on power plant chemistry and air pollution control. He is a member of the ACS, AIChE, AIST, ASME, AWT, NACE (now AMPP), the Electric Utility Chemistry Workshop planning committee, the Power-Gen International planning committee, and he is active with the International Water Conference. He may be reached at beakertoo@aol.com.
RAJENDRA P. KALAKODIMI is the Director of Cooling Water for ChemTreat Inc. in Ashland, Virginia. He received an MS degree in physical chemistry at Andhra University and a PhD in electrochemistry at the Indian Institute of Science in Bangalore in 2003. While in India, Dr. Kalakodimi served as the Engineering Technical Leader at the GE India Technology Centre in Bangalore and as Product Manager for Chemicals and Monitoring Solutions for GE Water. He has more than 20 patent filings, 20 international publications and various conference presentations.