By Brad Buecker
Issues with declining fresh water availability coupled with continued efforts towards sustainability are requiring personnel at new and existing power and industrial plants to look for alternative sources of plant makeup water.
One increasingly common supply is secondary treated municipal wastewater (gray water), but other supplies such as deep groundwater and seawater may be options as well. These supplies often contain some impurities in much higher concentration than in fresh water. For gray water, ammonia, phosphorus, and organic compounds are notable examples. Deep aquifer water can have an exceptionally high dissolved solids concentration, with chloride, sulfate, alkalinity, sodium, calcium, and other ions contributing to the total.
This article provides an overview of some emerging technologies for dealing with these difficult makeup supplies.
The Times They Are A Changin’
During the author’s many years of direct experience in the power industry (and for a bit in the chemical industry), the primary source of water at two of the facilities was lake water and in the third shallow well water. The table below outlines the typical chemistry in one of the lake water supplies.
As can be seen, the concentrations of the major dissolved solids are in a moderate parts-per-million (ppm) range. Such waters generally allow for straightforward makeup water treatment.
But now in many areas of the country, either by choice or mandate, plants must use some form of reclaim water or other off-spec water for makeup. Consider the common case of secondary-treated municipal wastewater effluent. Additional impurities outside of those listed above typically include:
- Suspended solids
- Ammonia (perhaps up to 35 to 40 mg/L in concentration)
- Organics (often expressed as biochemical oxygen demand [BOD] and chemical oxygen demand [COD])
- Phosphorus (as phosphate) of perhaps up to 10 mg/L in concentration.
If this water were to be sent directly to plant processes, say for cooling tower makeup, severe problems would result. Even with fresh water, cooling towers and cooling systems serve as the ideal environment for microbiological growth by providing warm and wet conditions.
Biocides such as chlorine and bromine, which are effective in treating fresh water supplies, may be quickly overwhelmed by the impurities in municipal wastewater effluent. Chlorine reacts irreversibly with ammonia and is also, along with bromine, consumed by organics to form halogenated organic compounds that may run afoul of regulatory requirements.
Becoming increasingly popular are technologies to remove organic compounds and microbiological nutrients from makeup streams. Two are membrane bioreactors (MBR) and moving bed bioreactors (MBBR). The flow diagram of one of a variety of MBR configurations is shown below.
|Microbiologically fouled heat exchanger tubes and cooling tower film fill. These deposits severely restrict heat transfer, fluid flow, and may lead to severe under-deposit corrosion.|
The core of a basic MBR system is a combination of an activated sludge bioreactor (suspended growth) and low-pressure microfiltration. In this process, secondary treated wastewater is first filtered and then treated in an aerated bioreactor, with perhaps some chemical addition to enhance performance. This is followed by low-pressure membrane filtration (microfiltration) for solid and liquid separation. Typical mixed liquor suspended solids concentrations in MBR systems range from 10,000 mg/L to 12,000 mg/L. Two membrane configurations are common for MBR, hollow fiber and flat sheet. Microfiltration produces the required effluent for feed to the service water / plant water system, and from there, to the cooling water and high-purity makeup systems. MBR treatment potentially can reduce ammonia concentrations to less than 1 mg/L, BOD to less than 5 mg/L, and COD to less than 50 mg/L. Microfiltration typically will reduce the effluent turbidity well below 1 nephelometric turbidity units (NTU), and often below 0.1 NTU.
The table below lists several advantages, and also disadvantages, of MBR. The ability of MBR to handle influent of varying quality plus the excellent effluent water quality from these systems are the major reasons for many installations of these units over the past several decades.
The reader may have already observed the anoxic reaction vessel of the MBR shown in Figure 2. Basic MBR, with just activated sludge treatment, will remove ammonia but only by converting it to nitrite (NO2) and nitrate (NO3). Thus, the nitrogen is just transformed but not removed. More complex MBR units with anoxic and perhaps even anaerobic zones allow for conversion of ammonia and nitrate intermediates to elemental nitrogen (N2), which, of course, escapes to the atmosphere. System design can also be modified to enhance organics and phosphorus removal. The proper design depends upon raw water chemistry and the needs of the plant.
In a moving bed bioreactor, the activated sludge vessel or vessels contain mobile media, which provides a substrate for microorganisms to attach and grow.
Flow through the vessel and air injection to establish the activated-sludge process keep the media circulating and in suspension, and which allows the attached bio-organisms to fully contact the process stream. MBBR does not include internal microfiltration, as does MBR, so typically external filtration is required. MBBR and MBR may even be used in combination to achieve the desired reduction of organics and nutrients from the influent. The author, who has direct experience with conventional activated sludge and has also observed MBR in action, can attest to the claims that the modern technologies require a much smaller footprint. This aspect can be a decided advantage when selecting a treatment technology for physically-constrained sites.
On occasion, the only source of water may be a deep aquifer with very high total dissolved solids (TDS) concentration. The next section outlines how reverse osmosis (RO) has become quite popular as part of high-purity makeup configurations, but some of these deep aquifer supplies may have dissolved solids concentrations well in excess of what RO can handle. A technology that is emerging is forward osmosis (FO), where a concentrated solution (commonly termed a “draw” solution) containing a recoverable material such as ammonium carbonate flows along one side of the RO membrane, with the raw water on the other. Even though the raw water has a high TDS concentration, the higher concentration on the process side extracts dissolved solids by basic osmosis. The purified water is then released when the draw solution is regenerated.
While energy is required to regenerate the draw solution, an advantage is that natural osmosis is the primary process, unlike reverse osmosis which requires high pump pressure to produce purified water.
For makeup water supplies that are high in hardness and/or alkalinity and silica, lime/soda ash softening clarification is a quite viable technique. But, when clarifiers are mentioned many in the industry still think of the large, circular clarifiers that were common technology for years. Now, high-rate clarification has become a mature technology whereby advanced sludge recirculation techniques or use and recycle of ballasted sand (or other materials), much higher process rates are possible.
High-Purity Makeup Water and Cooling Water – Influences on Plant Discharge
During the heyday of coal-fired power plants, many lessons were learned regarding the importance of steam generation chemistry and that even seemingly trace levels of impurities could cause serious and sometimes even catastrophic failures in boilers and turbines. These lessons have only partially carried over to the combined-cycle industry, in part due to the fact that many plants are only minimally staffed and have few or no personnel who truly understand water/steam chemistry. With regard to high-purity makeup treatment, the following guidelines from the Electric Power Research Institute (EPRI) for heat recovery steam generators (HRSGs) are quite important.
ppb = part-per-billion
But for the main purpose of this article, an examination of issues related to high-purity makeup water treatment is important. The common method that has evolved for production of this water is:
- Suspended solids filtration. Micro- and ultrafiltration have become very popular for protecting reverse osmosis (RO) membranes from suspended solids fouling, but clarification may be needed in some scenarios.
- Reverse osmosis for primary demineralization
- Final polishing by either mixed-bed ion exchange or electrodeionization (EDI), or sometimes both
While this arrangement is still quite popular, issues are arising with regard to waste stream discharge, in large measure related to water conservation and minimized waste volumes. Consider makeup water RO treatment. Modern RO systems will remove 99-plus percent of dissolved ions. However, even with fresh water as the makeup supply, a typical RO can only convert about 75% of the stream to product (permeate). The remaining 25 percent (reject) must be discharged as waste, with a dissolved solids concentration four times greater than the inlet water. (Advanced technologies are changing this recovery ratio, as we will explore later.) A potentially much larger discharge volume exists if the plant has a cooling tower or towers, in which the tower blowdown may be several hundred gallons-per-minute (gpm).
Examples of MBBR Mobile Media
So, now we have streams with potentially significant dissolved solids concentrations, but in an environmental climate where an increasing number of plants are facing tighter and tighter restrictions on discharge quality and sometimes quantity. In more than one case, the author has seen discharge limitations that are more stringent than those of the raw water entering the plant. Thus, if nothing were done to the raw water at all during passage through the plant, treatment would still be required before the water could be discharged. Apart from quality issues, discharge quantity may be restricted, sometimes to the point of zero liquid discharge (ZLD). What are emerging or established technologies to deal with these issues?
One approach being selected at some new plants is air-cooled condensers (ACC) as opposed to wet cooling towers. Obviously, two advantages of ACCs over cooling towers are greatly reduced makeup water requirements and the corresponding water discharge from blowdown. However, there are important tradeoffs. ACCs are massive structures that can only condense steam to an approach to dry bulb temperature. Thus, in warm weather they are considerably less efficient than cooling towers, which approach the wet bulb temperature. ACCs with their many finned tubes through which air is blown can be troublesome to keep clean. The many tubes in an ACC are also very susceptible to two-phase flow-accelerated corrosion (FAC), which typically requires condensate filtration to prevent excessive transport of iron oxide particulates to the steam generator.
At other plants, cooling towers are the choice, and the tower basin often serves as a collection point for other streams such as RO reject and boiler blowdown. Thus, cooling tower blowdown constitutes the major discharge. Treatment of the blowdown via a combination of techniques, potentially including clarification, microfiltration, water softening, and reverse osmosis can recover up to 90 percent of the stream for recycle to plant. At this point, mention of a cutting-edge RO technology is appropriate. This is exemplified by the Closed Circuit Desalination technology as offered by Desalitech. In this process, the RO is operated in semi-batch mode where during set periods only permeate is produced with the increasingly concentrated raw water being constantly recirculated. At a pre-determined set point, production ceases and the concentrated water is drained from the system, and a new batch of raw water is introduced. The company claims that up to 98 percent recovery is possible with this technology, which offers excellent potential for water recycling and wastewater volume reduction. The latter can be particularly important if the plant must be a ZLD facility. Potential ZLD techniques include thermal-mechanical evaporation, disposal in evaporation ponds, deep-well injection, or perhaps even trucking the material off-site to a licensed disposal facility. Rarely are any of these methods simple or inexpensive, and upfront volume reduction can be very beneficial from cost, energy, and maintenance perspectives.
This article has touched upon several well-developed and emerging technologies for preparing plant makeup water and for treating process water before discharge. Others, such as specialty ion exchange resins exist for removing particular impurities such as arsenic, heavy metals and metalloids, and others from waste streams. Clarification processes can be modified to adjust pH and/or feed compounds such as organic sulfides for metals removal. In today’s increasingly stringent regulatory climate, plant personnel must often think beyond traditional ideas when designing the plant water treatment systems.
Brad Buecker is a process specialist with Kiewit Power Engineers in Lenexa, Kan., and a contributing editor for Power Engineering.