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SIMULTANEOUS WATER CONSERVATION/RECYCLING/REUSE AND WASTE REDUCTION IN SEMICONDUCTOR MANUFACTURING
Impact/Purpose:
Water is increasingly limited in some areas of the Gulf Coast and is likely to be a major factor limiting growth. Industries that use large quantities of water are striving to conserve by using less in their manufacturing, by recycling (using the water again in the same process), and by reuse (using the water again in a different process). One example is the semiconductor industry. In current research partially funded by the Center, the Principal Investigator has worked with Motorola to identify wastestreams that could be recycled to the ultrapure water production facility. That work has progressed well, as Motorola is now instituting many recommendations, and an article that will extend the benefit to other industries is in preparation.
In the system developed in the current research, wastestreams were categorized into three classes. These groups included streams that were considered likely to be recyclable (Group I) based on sufficiently high quantity and quality, those unlikely to be recyclable (Group III), and an intermediate Group II that were too difficult to place definitively in the other groups without additional study. In the application at Motorola, over 200 wastestreams were analyzed, and approximately 40% of the flow was considered to fall into Group I. In most cases, these streams could be recycled without additional treatment. As a side benefit, several wastestreams with unnecessarily high flows were identified, and these are being reduced by Motorola. However, several wastestreams that are now considered non-recyclable still have potential to be recycled.
The primary goal of this proposed research is to extend our work with the microchip manufacturing plants to investigate both treatment options and waste reduction options for currently non-recyclable wastestreams to make them recyclable. We believe that several types of wastes fit this descript
Water is increasingly limited in some areas of the Gulf Coast and is likely to be a major factor limiting growth. Industries that use large quantities of water are striving to conserve by using less in their manufacturing, by recycling (using the water again in the same process), and by reuse (using the water again in a different process). One example is the semiconductor industry. In current research partially funded by the Center, the Principal Investigator has worked with Motorola to identify wastestreams that could be recycled to the ultrapure water production facility. That work has progressed well, as Motorola is now instituting many recommendations, and an article that will extend the benefit to other industries is in preparation.
In the system developed in the current research, wastestreams were categorized into three classes. These groups included streams that were considered likely to be recyclable (Group I) based on sufficiently high quantity and quality, those unlikely to be recyclable (Group III), and an intermediate Group II that were too difficult to place definitively in the other groups without additional study. In the application at Motorola, over 200 wastestreams were analyzed, and approximately 40% of the flow was considered to fall into Group I. In most cases, these streams could be recycled without additional treatment. As a side benefit, several wastestreams with unnecessarily high flows were identified, and these are being reduced by Motorola. However, several wastestreams that are now considered non-recyclable still have potential to be recycled.
The primary goal of this proposed research is to extend our work with the microchip manufacturing plants to investigate both treatment options and waste reduction options for currently non-recyclable wastestreams to make them recyclable. We believe that several types of wastes fit this descrip
Description:
The project was devoted to two separate arms of research. The overall goals of this research was to reduce the water use in the semi-conductor industry through a comprehensive program to reduce water usage in manufacturing processes, to investigate opportunities to recycle or reuse water, and consider treatment of wastestreams to allow recycling or reuse. The work was done with the cooperation of personnel from Motorola, Inc., which operates two semi-conductor manufacturing plants in Austin.
In earlier work, we had identified numerous opportunities for water saving, recycling, and reuse. Motorola personnel were able to implement many of our recommendations with a net substantial savings of water. At this point, few opportunities remained within their plant for water savings for individual manufacturing processes, and we were encouraged to consider broader water conservation and water reuse ideas during this project period. Throughout this project, we worked on two ideas.
The first idea was to consider biological treatment of all of the remaining wastewaters not otherwise recycled. This water is that which is currently discharged to the local publicly owned treatment works (POTW) for treatment there and consequent disposal into receiving waters.
The second idea was also primarily directed toward water conservation, and one that would have broad implications beyond the semiconductor industry. This one was to consider reducing the water loss in cooling towers, a substantial overall use of water not only in semiconductor manufacturing sites but in many industries.
The two semiconductor fabrication facilities operated by Motorola, Inc. in Austin, TX are committed to reducing water consumption and wastewater discharge. Motorola engineers, in part due to our previous work with them, have made great progress reducing water use through a variety of strategies, including by separation and reclamation of the relatively clean wastestreams. However, after the clean wastewater streams have been segregated, the remaining wastewater is of a lower quality, and requires extensive treatment before reclamation can be considered. The wastewater stream that is generated at the Motorola’s MOS-13 plant consists primarily of spent ultrapure water (UPW) from wafer cleaning processes and water from scrubber operations.
Treatment of semiconductor wastewater typically focuses on segregation of well-characterized streams; however, further segregation is no longer feasible at MOS-13; therefore, the entire wastewater stream must be treated. This industrial wastewater stream is the largest at MOS-13, accounting for approximately 700 gallons per minute, and organic contaminants are the primary concern in considering reclaim. A treatment system that efficiently eliminates the organic contaminants would result in reclamation of substantially more wastewater.
The objective of this part of the research was to determine the feasibility of biological degradation of the organic contaminants in the industrial wastewater. Biological reactors were operated in the laboratory and inoculated with activated sludge from a municipal wastewater treatment plant. Municipal wastewater was the initial organic feed to the bioreactors until a functioning system was developed. Neutralized Motorola industrial wastewater (IW) was gradually increased as a proportion of the feed stream to acclimate the microbial population to the organic compounds in the industrial wastewater. Eventually, the entire system was maintained on the neutralized IW alone. Various analyses were performed on the bioreactors and the effluent to monitor the effectiveness of the treatment.
The feasibility of biodegradation of the organic constituents in this wastewater is marginal. On the positive side, the industrial wastewater contains the organic constituents and nutrients (nitrogen and phosphorus) necessary to sustain biological activity; therefore, no additional nutrients are required. Also, isopropanol, acetone, and ethylene glycol were effectively degraded in the reactors. Ethylene glycol accounted for approximately one-half of the TOC of the industrial wastewater; therefore, the degradation of glycol represents a major fraction of the overall removal observed. However, the wastewater evidently contains some organic compounds that are recalcitrant to treatment. Biodegradation appears to be able to achieve no better effluent quality than approximately 10 mg/L TOC with the semi-continuous, non-recycle system used in this research. The remaining organic constituents are non-oxidizable by the microorganisms present in our reactors.
Nevertheless, several positive aspects of treating this wastewater with biological oxidation were found. The air diffuser system did not cause stripping, i.e., the removal of the organic constituents into the air stream. Therefore, aeration can be used for wastewater treatment, and the treated water could be reused in cooling towers or scrubbers; despite the presence of organic carbon in the water, the lack of degradability suggests that biofilms would not grow on the surfaces of the cooling towers, for example.
The successful biodegradation and removal of light organics means that recycle of the treated water back into the ultrapure water production system can be considered. Low molecular weight organics are not effectively removed by reverse osmosis (RO) or adsorption on activated carbon, and therefore the presence of such compounds is a major worry in considering recycling at semiconductor plants. However, RO effectively removes the heavier or larger organics and is used routinely in UPW production to accomplish the removal of the natural organic matter present in most municipal water supplies.
Some organic nitrogen compounds appear to be recalcitrant to biodegradation, and remain in the effluent of the bioreactor. The most likely constituent among these compounds is tetramethylammonium hydroxide (TMAH). Identification of other non-degradable constituents was not performed in this research.
Fluctuations in the organic loading of the industrial wastewater represent a major problem for consistent treatment. The TOC of this wastewater ranged from 30-150 mg/L. This variation caused changes in the effluent from the reactors. A full-scale system would probably require an equalization tank to reduce this fluctuation in the influent to the bio-reactors. Economic analysis indicated that a full aerobic treatment system, utilizing sequencing batch reactors, would have a return on investment period of 3.4 years. This period is usually considered too long for funding in most industrial plants. A more cost-effective solution is segregation of the less contaminated organic wastewater for treatment through a biofilter system, but this segregation can only reasonably be considered in the design of new semiconductor fabrication plants.
In summary, the total organic carbon (TOC) concentration of this wastewater was quite low, but we successfully operated laboratory scale activated sludge reactors and achieved sufficient treatment that the resulting water could be used for lawn watering, for example. The nitrogen levels within the wastewater could be considered an advantage for that use, essentially replacing the need for fertilizer. However, the technical success was not matched with economic success; the cost of installing and operating a treatment process for this purpose was greater than the combined current costs for disposal and purchase of new water for lawn watering.
The second study evaluated the feasibility of conserving water in the cooling towers at Motorola’s Ed Bluestein plant. A significant amount of water (87 million gallons per year) is used in the Motorola-Ed Bluestein cooling towers. Of this water usage, 90% (78 million gallons per year) is lost to evaporation, while the remaining 9 million gallons per year is wasted as blowdown. The feasibility of implementing several water conservation techniques in the Motorola-Ed Bluestein cooling towers was estimated. The techniques include converting the existing evaporative cooling towers to dry or wet/dry cooling towers, changing the chemistry of the cooling water, capturing water droplets in the cooling tower outlet air using a cyclone, capturing moisture from the cooling tower outlet air using a desiccant wheel, and condensing the water vapor leaving the cooling tower using parallel condensing, Peltier chip technology, a water-cooled coil, or a water-cooled drift eliminator. A preliminary design was completed for most of the water conservation methods. Each design was evaluated based on the expected reduction in water losses, cost, return on investment, power requirements, ease of operation, extent of technology development, and byproducts.
For each technology considered, the total capital investment (TCI) and total annual cost (TAC) were estimated and converted into a cost per 1000 gallons of water saved. For several of the technologies, these costs were more than ten times the current cost of water—so far from being economically viable that they have little chance of ever becoming viable for this application. Technologies that fell into this category included:
- Cyclones to capture water droplets that leave the cooling towers in the air stream
- A dessicant wheel to dehumidify the air stream (which requires much lower temperatures than those in Texas)
- Parallel condensing systems, consisting of an air-cooled condenser and a surface condenser in parallel
- Peltier chip technology for condensation; these systems consist of an electrical circuit made of dissimilar conducting materials so that heat is absorbed at one junction and released at another, and the cool side can be used for vapor condensation
While none of the technologies proved to be economically viable, the following ideas had some merit, inasmuch as their cost were low enough that they could become economically viable in the future if conditions changed sufficiently.
The conversion of the existing evaporative cooling towers to dry cooling towers eliminates evaporation and reduces water losses by nearly 100%, compared to current losses. However, dry cooling towers are much less efficient than conventional evaporative cooling towers. A large heat exchanger is needed for the dry cooling tower, and the capital and operational costs of the system do not balance the water savings. Under current operating conditions, the total capital investment (TCI) and total annual cost (TAC) for the dry cooling tower are $700,000 and $721,000, respectively. The cost of this system per 1000 gallons of water saved is $19/1000 gallons, considerably greater than the cost of water. This technology is not a feasible option for Motorola’s cooling towers at this time.
The conversion of the existing evaporative cooling towers to wet/dry cooling towers, where a portion of the hot water flow passes through a dry cooling tower section, can significantly reduce evaporative losses. The wet/dry cooling tower was designed to reduce the current water losses by 25%. Like the dry cooling tower, the capital and operational costs of the wet/dry cooling tower do not balance the water savings. Under current operating conditions, the TCI and TAC for the wet/dry cooling tower are $175,000 and $216,000, respectively. The cost of this system per 1000 gallons of water saved is $23/1000 gallons. This technology does not appear to be a feasible option for Motorola’s cooling towers.
Condensation of the water vapor leaving the cooling tower can be achieved through the use of a water-cooled coil. The water-cooled coil system was designed for a 25% reduction in the current water losses. Under current operating conditions, the TCI and TAC for the air-cooled condenser are $68,000 and $243,000, respectively. The cost of this system per 1000 gallons of water saved is $26/1000 gallons. This technology does not appear to be a feasible option for Motorola’s cooling towers.
Condensation of the water vapor leaving the cooling tower could also be achieved through the use of a water-cooled drift eliminator. The performance of a water-cooled drift eliminator is expected to be similar to that of a water-cooled coil. This technology does not appear to be a feasible option for Motorola’s cooling towers.
The water requirements for a cooling tower are affected by the chemistry of the circulating water. A maximum reduction of 10% of the current water losses can be achieved by changing the water chemistry; this reduction would occur if the recirculation were increased to the point that blowdown was completely eliminated. Recirculation is typically limited by scaling. Scaling can be reduced by several methods, including chemical addition, pH control, and lime softening. The calcium concentration was found to control the scaling that occurred in the normal pH operating range (7-9).
Chemicals can be added to the circulating water to reduce scale and corrosion. The addition of 1-hydroxy-ethylidine-1,1-diphosphonic acid (HEDP) was predicted through mathematical modeling to be highly effective in the reduction of calcite formation. The addition of scale- and corrosion-inhibiting chemicals appears to be the most cost-effective way to reduce cooling tower water usage. None of the other water treatment methods studied were as cost-effective as the current operation, which includes the addition of scale- and corrosion-inhibiting chemicals.
It is not feasible to limit scaling by pH control, because the pH at which scale begins to form is low for nearly all cycles of concentration (number of times on average that water passes through the cooling towers). The chemical requirements to reduce the pH to a range where precipitation does not occur are high, which results in high chemical costs. In addition, pH control is only applicable to low cycles of concentration; the make-up water requirements at low cycles of concentration are much higher than for the current operation.
Lime softening is not a feasible method for controlling scaling due to its high chemical requirements and the extremely low concentrations required of the softened water stream. Operation at higher cycles of concentration for water conservation, using lime softening for scale control, does not provide a cost benefit over current operation, because the water savings are very low. Operation at lower cycles of concentration with lime softening for scale control does not provide a chemical cost benefit over current operation, because the cost of make-up water increases dramatically at lower cycles of concentration.
The results show that evaporative cooling is a very efficient process. The use of heat exchanger coils to conserve water in cooling towers results in high operational costs that greatly exceed the water savings obtained. Alternative water treatment methods, including pH control and lime softening, were not cost-effective compared to the current practice of adding scale- and corrosion-inhibiting chemicals. The current use of reclaim water means that water savings are minimal with all of the technologies, compared to current operation, since the amount of water that is currently purchased is small compared to the total water usage. The current operation of Motorola’s cooling towers appears to minimize water usage and is more cost-effective than any technology analyzed in this study. No changes to the operation of the cooling towers are recommended at this time.