Zero discharge programsrequire careful planning
By Brad Buecker,Contributing Editor
Several water discharge programs are availableto address the complex needs of each unique plant site
Environmental regulations, increasing scarcity of fresh water supplies and rising costs of clean water production have contributed to greater water conservation throughout the country. Reduced or zero discharge programs have become much more commonplace at electric utilities, chemical process industries and manufacturing facilities. Water conservation can be a very complicated issue, with methods varying considerably from site to site. The issues that influence water reduction programs include:
¥ quality, quantity and cost of fresh water available to the plant;
¥ quality of water needed for various plant processes;
¥ ability to recycle wastewater streams to other plant processes;
¥ techniques for water treatment;
¥ capital, operations, maintenance and labor costs, and floor space and construction requirements for water treatment equipment,
¥ pilot testing of proposed treatment methods and
¥ environmental restrictions on the quantity and quality of any wastewater that may bedischarged.
Quality, quantity andcost of fresh water
Makeup water may come from a lake, river, ocean, wells or other supply source. Even sewage treatment plant effluent serves as a source at some facilities. Surface water from a lake or river can vary quite widely in quality and is greatly influenced by seasonal climate changes. Surface waters often contain fewer dissolved solids than well waters but contains more suspended solids. The supply may become very turbid during periods of heavy rainfall or snowmelt, which introduces particulates, loose vegetation and sometimes collodial silica to plant systems. Surface waters frequently contain several ppm or more of complex organics that are a product of decayed vegetation. Ocean water is, of course, quite saline and presents special problems, including those related to macrofouling.
Well waters generally contain few organics but are more heavily concentrated with calcium, magnesium, iron, alkalines and sometimes silica. Ion loading may result as the water percolates through soil and dissolves various constituents. The type of soil and underlying rock formations naturally have an effect on the groundwater quality. Clay or sandy soil will introduce silica to well water, while water percolating through limestone beds picks up hardness and alkalinity. Even potable waters can cause problems in water treatment systems. They may have hardness due to the source of supply, or they may contain chemicals added during the water treatment process that affect other downstream chemical treatments.
The composition of the water supply will greatly influence any choice of treatment equipment. It is highly important that project designers perform comprehensive chemical analyses of the makeup source before developing any treatment schemes. Ideally, historical chemistry data will be available, but often this is not the case. If not, the project planners should have samples analyzed periodically as far in advance as possible of the equipment selection date. Analytical data covering a several-month or, better yet, yearly time span will reveal variations in chemistry caused by seasonal changes.
The availability of the supply also affects any water conservation plans. Zero discharge may be the only alternative for semiarid and arid climates where surface water supplies dwindle in summer or well water usage is closely regulated. At the other extreme, plants situated by the ocean or a very large body of fresh water may have an unlimited source of supply.
Costs for water are variable, too. Purchase costs are negligible for a plant with its own source of supply. Water from potable water treatment facilities may cost as little as $0.50 to as much as $2.50 or greater per 1,000 gallons. Even sewage treatment facilities may charge a fee for use of plant effluent. Purchase costs are typically not the primary economic factor that drives a zero or reduced discharge project, but the costs should still be considered in the overall evaluation.
Water quality forvarious plant processes
Water serves many different processes in industrial facilities, and the acceptable water quality for each process can differ widely. For instance, at an electric utility, high-purity water is essential for boiler makeup. The high-purity treatment system may consist of carbon filters, cation demineralizers, anion demineralizers and mixed-bed polishers. This arrangement will produce water that contains parts-per-billion (ppb) levels of contaminants. At the opposite extreme, water used to wet ash in preparation for hauling the ash away may need no treatment at all. Even clarifier/softener blowdown, with its entrained sludge, is often suitable for this purpose. Reject water from reverse osmosis units or other treatment equipment can sometimes be recycled to the cooling tower, where it is absorbed in a large volume.
Project planners should carefully evaluate the water quality needed for various processes, because it may be possible to send lower quality waters to compatible processes. This can reduce the number of treatment systems and help project planners from overdesigning systems.
Techniques for water treatment
A reduced or zero discharge process must treat significant quantities of wastewater, such that the waste stream volume is greatly reduced or eliminated and the bulk becomes reusable. The principal constituents that treatment systems may have to remove include: dissolved inorganic ions (calcium, magnesium, alkalinity, chloride, etc.); dissolved organic ions; suspended solids and turbidity-producing compounds; dissolved gases; silica, both dissolved and collodial; and large debris such as logs, leaves and trash.
It is often possible through conservation techniques to reduce discharge volumes before adding treatment equipment. Cooling tower blowdown is frequently the largest discharge at a facility. New treatment chemicals are available that better inhibit corrosion and scaling, thus allowing an increase in the cooling tower cycles of concentration. Higher cycles result in a smaller discharge. As an example, consider a cooling tower operating with a circulating flow rate of 100,000 gallons per minute (gpm), a temperature range of 20 F, and an average evaporation factor of 0.75. Blowdown is 365gpm at five cycles of concentration, 157 gpm at 10 cycles and 97 gpm at 15 cycles. Increasing the cycles of concentration has a significant effect on the size of any blowdown treatment system.
The following list outlines some of the primary treatment methods and how they affect reduced/zero discharge programs.
Clarifier/softening–This treatment will remove hardness-forming compounds, silica, suspended solids and organics from plant influent or cooling tower blowdown. Cooling tower cycles of concentration can often be increased with softened water as makeup. Side-stream or full-stream clarifier/softening of the cooling tower blowdown is an alternative technique.
Filtration–Filtration is an important part of most water treatment systems. Multimedia filters are usually placed downstream of clarifier/softeners to remove floc that may have carried over in the effluent. Side-stream filtration of a cooling tower prevents accumulation of suspended solids. Cartridge filtration ahead of a reverse osmosis unit is standard and helps protect the system membranes from fouling. Also common is activated carbon filtration upstream of a demineralizer. Activated carbon removes organics and oxidizing biocides that would otherwise foul or degrade the resin beds.
Oxidation of organics–Organic compounds play havoc with demineralizers and can cause chemistry upsets in boilers. Oxidizing chemicals such as chlorine dioxide and ozone have proven effective in destroying organics, and may be used in lieu of activated carbon filters. (Some processes, such as silicon chip production, require ultrapure water. An activated carbon filter is unsuitable for these applications because the filter removes all of the oxidizer in the top portion of the bed, leaving the bottom of the bed as a good breeding ground for microbes.) Chlorine will destroy organics, but chlorine also reacts with organics to form halogenated compounds. Because many halogenated organics are suspected carcinogens, chlorine is not recommended for organic destruction. However, it is still an effective biocide, although the carcinogen issue has caused some to discontinue its use in favor of other biocides. The combination of ultraviolet light and ozone is becoming more popular for organic destruction, especially in low-volume waste streams.
Ion exchange–For processes that do not require complete ion removal, hardness reduction by sodium-cycle ion exchange may be suitable. This process is one of the simplest forms of demineralization, yet can reduce calcium and magnesium concentrations to low-ppm levels. Softeners are regenerated with inexpensive common salt.
Complete ion exchange reduces cations and anions in a water stream to low-ppb concentration. The high cost of regeneration for these systems prohibits their use for bulk water treatment. Complete demineralizers serve primarily as polishers for production of high-purity water for boilers, microchip fabrication, etc.
Reverse osmosis–Reverse osmosis (RO) units use pressure to force water through semipermeable membranes. These systems are cost effective for treating high total-dissolved-solids waters, and they will remove colloidal. and dissolved silica RO is often very suitable for purifying boiler makeup (alone for low-pressure units and as pretreatment to a demineralizer for high-pressure units) and for recovering a portion of cooling tower blowdown. RO membranes are subject to fouling and usually must be protected by upstream filtration. An RO system produces a concentrated waste stream that is often 25 percent or more of the influent flow volume.
Electrodialysis and electrodialysis reversal–Electrodialysis (ED) and electrodialysis reversal (EDR) are also membrane processes, but instead of pressure forcing water through the membranes, an electrostatic field pulls ions out of the water and past the membranes. Because of this, ED and EDR membranes have a lesser tendency to foul than RO membranes. EDR differs from ED in that the polarity of the electrostatic fields alternates every 15 to 30 minutes. This helps to loosen any large, suspended solids that may have attached to the membranes. An EDR system can remove 85 percent of the salt content and recover 75 percent or more of the influent. It will not remove silica.
Evaporator/crystallizer–Evaporators and crystallizers are commonly used to treat reject streams from RO systems, electrodialysis units, and other concentrated sources to produce a solid material for disposal and return the remaining small quantity of water to the plant. Thus, they are often the final step in a zero discharge system. Evaporators and crystallizers can, in some instances, serve in a stand-alone capacity, but the high capital cost of these systems usually makes them better as a final treatment device after the waste stream volume has been reduced by other methods.
Solar ponds–In areas where evaporation rates are high, solar ponds may be an option. The principal advantage of a solar pond is that maintenance costs are quite low. However, capital costs are high due to land purchases, excavation and installation of a pond liner. Solar ponds sometimes have a place in conjunction with cooling tower blowdown reduction programs, wherein the reduced cooling tower blowdown is discharged to a relatively small pond.
Effect of modificationson plant processes
Careful consideration must be given to the effect any modifications may have on plant processes. For example, increasing the cycles of concentration in a cooling tower increases the potential for scale formation if the chemical feed system malfunctions. Softening of cooling tower makeup reduces scaling potential but increases corrosion potential. Addition of carbon filters to a process could increase microbiological release to the makeup water. An evaporator/crystallizer eliminates a liquid waste stream but produces solids that must be disposed. These factors and more must be taken into account.
Equipment, O&M and labor costs
Costs for equipment, O&M and labor can be quite different for the various water treatment methods. A ballpark estimate for capital and installation costs of a standard 100 gpm RO system is about $200,000. A similarly sized EDR system may cost 30 to 80 percent more, while the price to install a 100-gpm solar pond may be several million dollars. If the correct system has been installed for the application, O&M and labor costs may be very low. For example, a membrane system manufacturer indicated that its equipment typically required an average operator attention time of only one hour per day. Assuming a labor rate of $25 per hour, the cost over the course of a year would be $9,125. However, if for some reason the membranes fouled, maintenance personnel would have to clean them, and O&M costs would increase dramatically. The labor cost of a four-person crew working for one week at the $25 per hour rate would amount to $5,600.
Some equipment, such as evaporator/crystallizers, are much more labor intensive and may require round-the-clock operation. Solar ponds, on the other hand, require almost no supervision. The choice of the reduced/zero discharge scheme may partially depend on the plant`s management philosophy regarding capital costs vs. O&M costs. Energy costs are also important. They may be relatively low (in the thousands or low tens of thousands of dollars) for RO and EDR but much higher for energy-intensive equipment such as evaporator/crystallizers.
Equipment size and construction requirements must also be taken into account. Large clarifiers may have to be placed in-ground, but for applications of several hundred gpm or less, package or field-erected clarifiers may cost considerably less. With regard to construction and fabrication, RO units and other sophisticated equipment should be located in an enclosed structure. Some plants do not have room in existing buildings for new equipment that might take up hundreds of square feet of floor space, so new buildings must be erected. Control system design will also significantly affect the cost of the overall project. Some plant personnel prefer that the water treatment equipment contain local controls and monitors and be operated from a stand-alone workstation. Others prefer that the controls be integrated into a plant distributed control system or programmable logic control system.
Pilot testing of proposedtreatment methods
Many existing reduced/zero discharge plants have been modified from their original treatment process because some piece of equipment proved incompatible. It is very difficult to determine the most appropriate system without prior laboratory and pilot testing. Jar tests of the raw makeup water are important if a clarifier/softener is proposed as part of the treatment scheme. Pilot testing of an EDR or RO can be extremely useful in determining if these systems will be subject to fouling or scaling. A stepwise approach might be the best method for determining system design. For example, if the program calls for increased cooling tower cycles of concentration, it may be impossible to determine the effects on an EDR or RO treatment system without first cycling up the towers and then pilot testing the equipment.
A plant`s environmental permit may call for zero discharge or place a maximum limit on the amount of water that can be released. Conversely, some plants in arid locations may be required to return a certain quantity of water to the source in order not to affect downstream users. Certainly, any discharges must meet the plant`s National Pollutant Discharge Elimination System (NPDES) permit. Criteria now include pH; oil and grease; total suspended solids; boron; and some transition or heavy metals including iron, copper and silver. Other constituents may be added to this list, and it is imperative to select a treatment system that produces discharge water of NPDES quality. z
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