SO, Control Processes for Stack Gases Reach Commercial Status The latest development in the fastmoving field of control of sulfur oxides from power plant flue gases is Monsanto’s decision to offer commercially the catalytic oxidation process it has had under development since 1961. The process underwent a year’s testing at a prototype plant at Metropolitan Edison’s Portland, Pa., station. In making the announcement, Monsanto vice president John Eck says the company has now “accumulated enough in-depth data to design full-scale units.” The Monsanto move is one of a series of announcements over the past few months that may herald an advance in SO, control technology on a broad front. Among these moves: Within a few days of the Monsanto announcement, Combustion Engineering, Inc., put on stream a sulfur oxides control unit at Union Electric’s Meramec plant in South St. Louis County, Mo.; later this year, Kansas Power and Light will start a similar unit at Lawrence, Kan. The Tennessee Valley Authority has just completed a design study, sponsored by the National Air Pollution Control Administration, of the dry limestone injection process. TVA will use the results of the study in planning full-scale tests by 1969 of the dry limestone process at its Shawnee and Paradise plants in Kentucky. Last June, Wellman-Lord, Inc., announced plans for a 25 megawatt Clean Air Demonstration Plant in Baltimore County, Md., for evaluating its SO, control process. Cooperating with Wellman-Lord in the project are Baltimore Gas & Electric Co., Potomac Electric Power Co., and W. R.Grace & Co. Stone & Webster Engineering Corp. and Ionics, Inc., have disclosed plans for a jointly developed process for SO, control that was field tested last year at Tampa Electric’s Gannon station. In addition to these processes, several other approaches have been undergoing evaluation-for example, the alkalized alumina process for sulfur recovery which the Bureau of Mines is
994 Environmental Science and Technology
studying at a pilot plant in Bruceton, Pa. Recently, North American Rockwell’s Atomics International Division and Princeton Chemical Research both disclosed plans for pilot scale evaluation of SO, control processes. North American’s process uses molten carbonate as the SO, absorbant; Princeton Chemical’s process features catalytic reaction of SO, with hydrogen sulfide. Both these processes recover elemental sulfur as byproduct. Major problem
The background for this flurry of activity in SO2 removal processes is clear. NAPCA, whose largest single R&D effort is in sulfur oxides control, some time ago released figures that outline the problem: The combustion of fuel for power generation accounts for 46% of the sulfur dioxide emitted to the atmosphere, and 58% of the total results from the combustion of coal. Air quality criteria for sulfur dioxide, the prelude to federal control standards, are due from the Department of Health, Education, and Welfare by the
end of the year. And some metropolitan regions, most notably New York and Chicago, have taken action to limit sulfur content of fuels used for power generation. The coal industry, while acknowledging the scope of the SOy emission problem, takes a dim view of sulfur limits on fuel and is pushing for stack emission limits instead. The reasoning behind this stand is cogent: The amount of naturally occurring low-sulfur coal is limited, and the technology available for sulfur dioxide removal from stack gases is more advanced than coal desulfurization techniques and likely to be less costly. Determining the costs
The cost question is of vital interest to the coal industry. Coal has been all but eliminated from such traditional markets as railroads and domestic heating. Now the industry faces a formidable challenge to its major remaining market, generation of electricity, from nuclear power. To play up the fact that projected
economies of nuclear power are based on adequate supplies of low-cost fuel, either from high-grade uranium ore or breeder reactors, the coal industry makes the claim that it can satisfy within a year “a 20% increase in demand at a guaranteed price.” Coal industry spokesmen are aware that this guaranteed price will now have to include costs for SO, control. And, indeed, figures from the National Coal Association indicate that coal prices can absorb pollution control costs (thermal and air) of $1 per ton and still remain competitive with nuclear power. Thus, the power industry will follow with keen interest the several attempts to bring SO, control processes to fullscale commercial status. Sharper fipures on operating costs and removal efficiencies will help resolve the dilemma facing the utility operators in selecting SO, control-whether to choose a simpler. low-capital-cost process such as limestone injection and absorb the lower operating costs with higher power changes, or to use a more complex chemical absorption process that relies on sulfur recovery to recoup capital and operating costs. Limestone injection
Combustion Engineering says that one assumption behind the company‘s decision on alkaline earth injection process development is that “power companies don’t want to get into the
chemicals business.” Combustion Engineering says that recovery of sulfur from plants using its process may eventually prove feasible, and the company is conducting some development work along these lines. For the time being, however, the company is selling its process on the basis of relatively low capital cost, adaptability to existing power plants, and capability for better than 83% removal of sulfur dioxide. In the Combustion Engineering process, the additive is introduced into the boiler where it calcines and reacts with SO, and some of the SO,. Wet scrubbing combines the remaining SO, with calcined additive and water to form sulfates and sulfites. The major equipment required for installation in an existing plant are the wet flue gas scrubber, associated mills and feed equipment for injecting the additive, and settling tanks for handling the fly ash and slurry. Basing its comments on test work at Detroit Edison’s St. Clair plant, the company says the process has demonstrated 9 0 5 or more removal of SO, under a variety of conditions, and 98% removal under optimum conditions. Added benefits
The introduction of additive into the boiler furnace has other beneficial effects, according to Combustion Engineering. For example, high tempera-
OUTLOOK ture corrosion of boiler tubes, caused by the reaction of complex alkali-iron trisulfates with tube metal, is lower. The probable protecting mechanism is the reaction of calcium and magnesium sulfates which combine with the sodium and potassium sulfates to form double salts, instead of the reactive compounds. Corrosion within the boiler air heaters by SO, is also reduced, since most of the SO, formed in the boiler is removed by the additive. Such savings in operating costs due to corrosion reduction are an important factor in credits to the total operating costs for the system, according to the company. Combustion Engineering says that capital costs for a 500 megawatt unit burning coal with 3% sulfur and 10% ash are less than that for an electrostatic precipitator and a taller stack. Gross operating costs are 63.1 cents per ton of coal. Against this projected operating cost, the company applies credits to the system of 27.0 cents per ton of coal, which represents savings due to such things as reduced corrosion. elimination of precipitator operating costs, and improved thermal efficiency. The net operating cost for the system, 36.1 cents per ton of coal burned, does not include any return for the sulfur content of the fly ash. Dry limestone process
The dry limestone process, that is being investigated for the two TVA plants, also uses injection of pulverized limestone or dolomite. But, in this system, in the absence of a wet scrubber, the calcined limestone reacts with the sulfur oxides in the gas phase. The calcium or magnesium sulfates are removed with the fly ash in conventional dry scrubbers, either electrostatic precipitators or cyclone separators.
Limestone. Combustion Engineering’s wet limestone injection process being installed at Kansas Power & Light (flow sheet) is similar to unit just started u p (photo) at Union Electric Meramec plant
Volume 2, Number 11, November 1968 995
Generation of SO2 in t h e atmosphere-1966 By type of industry:
Combustion of fossil fuel for power generation
46 %
Combustion of fossil fuel for other purposes
32
Smelting of ores
12
Petroleum refinery operations
5
Miscellaneous sources (coke processing plants, sulfuric acid plants,
5
paper mills, coal refuse banks, incinerators. etc)
By type of fuel: Coa I
58 %
Petroleum products (mostly residual fuel oil)
20
Other sources
22
Source: Department of Health. Education. & Welfare
TVA admits the process is relatively inefficient-only 50% or less of the sulfur oxides are removed with reasonable amounts of excess limestone. Nevertheless, even this removal could lead to an improvement in air quality. Total annual operating costs for the system are very sensitive to plant size, according to the TVA design study. But in the smallest plant unit described-a 200 megawatt unit burning 2 % sulfur coal-operating costs amount to about 50 cents per ton of coal burned. Most of the other major SO, removal methods being studied are dependent on the recovery and sale of sulfur in some commercially useful form. Capital cost requirements are relatively high. Economic evaluations of these processes are also fairly involved because they must take into account such things as the form in which sulfur is recovered, the local markets for sulfur products, and a price structure which varies throughout the country. Catalytic oxidation
The process which Monsanto is now offering commercially for sulfur oxides
996 Environmental Science and Technology
control is based on catalytic oxidation of the SO, in the stack gas to SO, for conversion to dilute sulfuric acid. The first stage of the system is a hot electrostatic precipitator which removes more than 99% of the fly ash. Following this stage, the gases pass through a converter where they are oxidized by a vanadium pentoxide catalyst. As much as 90% conversion of the SO, is obtained over a wide range of conditions, according to Monsanto. Waste heat from the stack gases is recovered at this point in an economizer and air preheater, and the flue gas is then fed to a packed absorption tower. The absorbent is a stream of cool sulfuric acid which condenses the vapor; the heated acid drains to the bottom of the tower and is cooled. The final stage before venting the SO,-free gases to the stack is removal of entrained acid in a mist eliminator. The acid from the eliminator is added to the excess acid from the tower, which is sent to storage. Monsanto says the capital costs for the process will vary, but that the range of $20-$30 per kilowatt of installed power capacity will cover most cases.
Using a typical installed capital cost of $25 per kilowatt, the company points out tnat recovery of $13.50 per ton for t t e product acid will cover the operating, maintenance, and capital costs for a plant burning 3% sulfur coal. The same plant using 5% coal would reach the breakeven point if only $8 per ton of acid is realized. Wellman-Lord’s approach to SO, recovery is also based on chemical absorption, followed by regeneration of the adsorption liquid and stripping of the SO2 from the process liquid. The product is recovered as SO, vapor, and the economic evaluations can be based on the sale of liquefied SO,, or conversion in integrated plants to elemental sulfur or sulfuric acid as the local market conditions warrant. The company has not divulged much of the engineering details of the process, because of the pending patents and trade secrets involved. But based on a few patent disclosures issued to date, the company has roughly outlined the basic chemistry involved: following removal of 99% of the fly ash and SO, by wet scrubbing, SO, is removed by reaction with an aqueous solution of potassium sulfite to form potassium bisulfite. The reaction temperature, although elevated, is maintained low enough to prevent conversion of the bisulfite to potassium sulfite. Subsequently, the potassium bisulfite is cooled until a portion of the bisulfite is crystallized as potassium pyrosulfite. The crystals obtained are sufficiently elevated to convert the pyrosulfite into potassium sulfite and SO,. The sulfiteSO, stream is fed to a steam stripping column for recovery of SO,. Wellman-Lord makes its estimates of capital costs for its process on the basis of value of the SO, recovered, and typical costs range from about $4.5 million for a 500 megawatt plant to about $9.5 million for a 1500 megawatt unit. Actual capital costs again vary somewhat with the sulfur content of the coal. For a typical central Florida power plant location which realized a return of $15.60 per short ton of SO?, operation of the 500 megawatt unit would closely approximate a break-even situation. The 1500 megawatt plant using 3% sulfur coal would realize a net return on capital costs equivalent to 0.08 mills per kilowatt hour and 0.17 mills per kilowatt hour with 4% sulfur.
Other techniques
In addition to these processes which have reached commercial status, a number of other routes to recovery of stack gas sulfur are being investigated, The demonstration plant being jointly designed by Stone & Webster Engineering Corp. and Ionics, Inc., will use caustic soda to scrub the SO2 from the flue gas, and convert it to sodium bisulfite. The bisulfite is fed to a stripping column for recovery of the SO,. Key to this process is an electrolytic technique for regeneration of the caustic soda absorbant from the sodium sulfate from the bottom of the stripping column. Other byproducts of the electrolytic regeneration are dilute sulfuric acid, oxygen, and hydrogen. The dilute sulfuric acid and the oxygen can be used for an integrated plant that converts the SO., to sulfuric acid; the hydrogen can be bottled and sold in local markets. Alkalized alumina process
The Bureau of Mines has selected the alkalized alumina process, which it has had under study since 1957, as a candidate for large-scale development, because it is one of the few sulfur recovery processes that is readily adaptable for the control of SO, from large, new, fossil-fueled power stations. The Bureau has had a pilot plant in operation at its Bruceton, Pa., facility for some time. NAPCA is negotiating with Britain’s Central Electricity Generating Board for large-scale testing of the process, and basic data for construction of a U. S. version of the process are being generated at a number of laboratories in this country. In the Bureau of Mines process, stack gas is contacted with the alkalized alumina, a coprecipitate of sodium and aluminum oxides, and converted to sodium sulfate. Regeneration is accomplished by high temperature reduction of the sodium sulfate to sodium oxide and hydrogen sulfide. The regenerated alumina is recycled, and the hydrogen sulfide converted to elemental sulfur by the Claus reaction. Variations on the alkalized alumina process are also being studied. For example, a process, financed by Slick Industrial Corp. and developed at Southwest Research Institute, substitutes sodium aluminate for alkalized alumina. According to recent results
published by workers at SRI, the sodium aluminate is more efficient, and permits the use of smaller equipment and reduces handling costs. The change is significant, in that one of the drawbacks of the alkalized alumina process is the difficulty of circulating large tonnages of solid absorbent at high temperatures. A major point in favor of alkalized alumina process is that it recovers elemental sulfur instead of sulfuric acid. More variations
Another process that converts SO, to elemental sulfur is one developed by Princeton Chemical Research. In this process, sulfur dioxide-containing stack gas reacts with hydrogen sulfide in the presence of a catalyst to produce sulfur and water. Some of the sulfur is reacted with methane and water to produce carbon dioxide and more HzS for reaction with the stack gas. Princeton Chemical Research is planning a pilot plant to confirm its early studies on process feasibility. North American Rockwell’s Atomics International Division also has brought its molten carbonate absorption process past the bench-scale phase and is now planning a pilot operation. Studies of the process chemistry and testing of the process components have shown the molten carbonate to be a satisfactory absorbent for SO,, the company says, and preliminary estimates of operating costs are within the range of what could be recovered by sale of the recovered sulfur. The carbonate mixture in the North American Rockwell process is a mixture of lithium, sodium, and potassium carbonates. They react with SO2 to produce mixed sulfites and sulfates, which remain dissolved in the unreacted carbonate. The salt mixture is then circulated to a regeneration system where the sulfites and sulfates are reacted with CO and hydrogen and converted to hydrogen sulfite, and the carbonates regenerated. Preliminary estimates by North American Rockwell indicate capital requirements for a full-scale process at close to $10.25 per kw.; operating costs are estimated at $1.30 per ton of coal before credits for recovered sulfur. The company states that the operating costs would be completely offset by the sale of sulfur at $48 per ton. A sulfur price of $22 per ton
could reduce the actual operating costs to about those estimated for the dolomite process. Sulfur’s contribution
j,
The recent surge of development work in SO, control appears to give the utility industry a variety of technical approaches to choose from. What is still far from clear, however, is to what ext7nt sulfur recovery can be expected to become part of control technology. Stack gases, of course, offer considerable potential as a sulfur source. For example, a typical 1000 megawatt plant emits the equivalent of 300 tons of sulfur per day as sulfur dioxide. TVA has estimated that the recovery of even one third of this sulfur now emitted from the nation’s power plants would satisfy about two thirds of the current consumption of sulfur by the fertilizer industry. The possibility of tapping this potential profit on the way to coming to grips with a serious pollution problem has been a strong factor in bringing some of the processes to their current status. The economic feasibility claimed by developers of the various recovery processes usually contains strong qualifications regarding local sulfur markets. Nevertheless, sulfur is currently in short supply, is likely to remain so for some time, and prices are on an upward trend. There are other factors, however, which will have a direct bearin2 on the future of sulfur recovery in the SO2 control field. The argument that power companies would not prefer the burden of selling a byproduct to recover the cost of operating pollution control equipment seems valid. Although the utility industry has recently expressed some interest in the sale of byproduct fly ash, the problems of sulfur recovery and marketing are much greater. In addition, much of the current need for SOz control is for existing plants, and fitting a complex sulfur recovery plant into these may not be practical. Many of these plants are not large enough to make sulfur recovery a profitable operation. Finally, power station operations are quite variable, with units periodically shut down or started up in response to variations in demand for power. Such cyclic operation would drastically affect the efficiency of any down stream recovery process.
Volume 2, Number 11, November 1968 997
Definitive Water Quality Criteria Needs Demand More Research Interior’s National Technical Advisory Committee lists the unknown, problem, and investigate areas in need of scientific answers for improvement of the quality of the nation’s
Research Needs, the companion report to an earlier report entitled Water Quality Criteria, details the research programs that are both needed and recommended for obtaining scientifically based data for various water uses. Prepared by the National Technical Advisory Committee (NTAC), Research Needs, the second document, covers the same five major water uses as the earlier one (ES&T, September, page 662), and makes research recommendations for the following water uses: Recreation and aesthetics. Public water supplies. Aquatic life and wildlife. Agriculture. Industry. The research needs specified in this report do not necessarily coincide with research goals, priorities, or policies of various agencies within the Department of the Interior. The committee recommends research leading to new or improved methods for the following: Determining the limits and desirable levels of dissolved oxygen and temperature necessary to protect aquatic life. Defining acceptable bacteriological limits in waters used for recreation. Defining limits on minerals and taste- and odor-causing materials in water used for agriculture and public water supplies. Determining the chronic effects which minute concentrations of pollutants might have on life systems. Recreation and aesthetics
From the viewpoints of health aspects, and availability and economics, the committee agrees that research needs in the area of recreation and aesthetics should center on monitoring and methods for indicating the potential presence of pathogenic viruses. But the committee cautions that the
998 Environmental Science and Technology
greatest values to be realized work on pathogenic organisms w in applied research rather than in applications of monitoring. Additionally, the committee mends research in the following Treatment and control of wastes affecting recreational uses of waters. Disinfection of domestic sewage, other wastes, and storm water runoff. Effective control-or disinfection -to maintain desirable sanitary levels at bathing beaches. Investigation of the influence of aesthetic factors on human attitudes toward acceptance of water conditions. Public water supplies
Tastes and odors in public water supplies remain the most vexing problems of water treatment plant operation, according to NTAC. Methods for judging tastes and odors by chemical or other specific analytical techniques are needed. The lack of suitable analytical techniques for the measurement and control of dissolved organic materials also indicates the need for research attention. The microbiological problem requiring attention is methodology for the enumeration and isolation of bacteria and viruses. Additionally, the committee lists the following topics for research: Development of data on eutrophication and enrichment phenomena. Collection of more and better data for waste treatment operations. Determination of the physiological effects of constituents such as boron, herbicides, and others in water. Evaluation of the economic effects of water quality and effects of storm water and combined storm water overflow on receiving water quality. Aquatic life and wildlife
The development of microanalytical and other toxicological methods for
ing toxic materials in waters and heads the list of research needs area of aquatic life and wildlife. rime importance, these methods be designed to measure these mas in their toxic forms. For example, methods must be developed to detect and measure undissociated hydrocyanic acid in mixtures of complex cyanides. The development of such a method is essential for the meaningful toxicological evaluation of cyanide wastes. according to NTAC. Similarly, methods must be developed for the H,S molecule, unionized NH, molecule, and the ”,OH molecule. In the past, too many analyses of toxicants have been confined to those materials or chemical species for which analytical methods had already been developed. The problem has been that they do not measure the toxic species or forms of the material being studied. The committee also notes that field studies must be conducted, in addition to laboratory research studies. For example, ecological studies are needed to verify laboratory findings for the favorable ranges of environmental factors such as temperature, dissolved oxygen, carbon dioxide, dissolved solids, SUSpended solids, settleable solids, turbidity, color, light requirements, currents, salinity, and pH. The committee finds that the research needs for wildlife are met largely if the requirements for aquatic life are satisfied. Nevertheless, the committee points out the following areas for additional research studies: Studies for the distribution of toxicants in the wildlife food chain and in tissues of various species such as migratory birds. Studies of the amount of toxicant in the environment and comparison with the amount available for absorption by plants and animals.
OUTLOOK Studies of plants and animals that concentrate toxicants and are thereby capable of intoxicating other organisms in the food chain. Studies of effects of environmental nutrient elements present either in excessive amounts or in deleterious combinations. Agricultural water supplies
Problem oriented research is specified for problems relating to agricultural water supplies, according to NTAC. Since pollution abatement is so closely associated with use requirements, coordinated research efforts should be encouraged to supply the information necessary for effective water quality control. But, the committee notes, such research needs related to agricultural water supplies are not limited to water use requirements. The research needs include other related subjects such as pollution control and water management. The committee observes that agriculture’s contribution to the water pollution problem is difficult to evaluate because of the lack of sound research data. For example. the quality of water resources may be affected by agricultural practices-directly, as a result of irrigation, or indirectly, through the influence of agricultural chemicals and animal wastes on seepage and runoff from agricultural watersheds. So, the committee points out, research effort is needed along the following lines: Rapid procedures for evaluating water quality for agricultural uses. Improved monitoring methods for agricultural water uses. *Improved data bank on current quality of our water resources. Development of more significant indicators of water quality. Studies are needed, for example, to find more effective ways of determining the number of coliforms, fecal coli, and fecal streptococci with the objective of using these organisms as indicators of the number of salmonella. shigella. and other animal and human pathogens in water. The committee also says that a particularly difficult problem is posed by chemicals adsorbed by the soil and sub-
Declining bird population may be due to pollution The decline in the osprey population may be caused by environmental pollutants, according to the Bureau of Sport Fisheries and Wildlife. Take the case of the osprey colony at the mouth of the polluted Connecticut River-the colony declined from 150 t o 10 nesting pairs. According to research workers of the Patuxent Wildlife Research Center (Laurel, Md.), perhaps some pollutant obtained by the adult osprey from its environment was passed on to the egg, resulting in death of the embryo. The osprey, also known as the fish hawk, eats a large diet of fish, mainly obtained by diving for them. It is possible that pesticides accumulate in the tissue of larger fish, for example, as smaller fish are eaten by larger ones. Thus, the osprey, at t h e t o p of the food chain, may be eating fish that contain high concentration of pesticides which have built up over a long time.
sequently washed into the waterways as pollutants. To solve this problem, the committee recommends the following research studies: Evaluation of improved management practices in lakes, reservoirs, and other water sources which constitute a major source of water for agricultural and other uses. Development of techniques for predicting effects of proposed reservoirs on downstream water quality. Establishment of economic surveys of existing conditions, pollution abatement costs, water treatment costs for various uses, and the resulting potential economic effects of various water uses. Industrial water supplies
The overall objective of water pollution abatement is the prevention of further degradation to surface waters and reduction of existing pollution by optimum management of our water resources. Accordingly, two separate R&D approaches should be practiced, NTAC observes. Maximum utilization of existing facilities, manpower, and knowledge. Construction of needed facilities, the training of more and better qualified manpower, and the development of better information. An intensive effort should be made to assure that the capabilities of existing plants are fully utilized, observes NTAC. And the committee calls attention to the definite need for filling the gap between research and practice. Top priority should be given to more widespread dissemination of available
concepts and practices, NTAC says. The committee notes that an assessment of the potential benefit versus cost for each research study should be made as a prerequisite in planning the overall research program. The committee specified the following four research areas which require additional effort: Pollutant effects. Flow measurement, sampling, and analyses. Control, treatment, and dispersal. Social, political, and economic considerations. Research on flow measurement devices should have low priority, according to the committee. What is needed, however, is a compilation of present installations and utilization of flow measuring devices. Perhaps, the Federal Water Pollution Control Administration could prepare this compilation, NTAC observers. Although the committee says that there is no urgent need for research on control measures, the committee does point out that a real need exists for disseminating information on water and waste water control practices already in use at various geographical areas and in some industrial plants. The engineering profession must recognize the necessity for the routine inclusion of water and waste water control measures as an integral part of process and plant design, according to the committee. At the same time, local management must assume the responsibility for continuing pollution control through plant operating practices.
Volume 2, Number 11, November 1968
999
Electron Spectrometer Detects
OUTLOOK
Low Carbon Monoxide Levels
Scientists at the National Bureau of Standards have found that the electron impact spectrometer, an instrument originally developed for atomic and particle physics research, may be a valuable tool for air pollution studies. An NBS group has already shown that the high-resolution, high-sensitivity instrument can detect, at the part-per-million level, for example, the hard-to-distinguish gaseous contaminant, carbon monoxide. The full capabilities of the instrument in gas analysis are being studied in a joint program between NBS and the National Aeronautics and Space Administration’s Langley Research Center. NBS’s John A. Simpson says the present instrument, originally developed for atomic studies, will have to be redesigned for the NASA study. NASA is interested in the instrument for studying spacecraft cabin movements, and hopes to use it for mon-
Adjusting. NBS’s Stanley Mielczarek adjusts electron spectrometer. High resolution and sensitivity make the instrument useful for trace gas analyses
itoring a number of toxic compounds. With the buildup in experience and instrument capability, Simpson hopes to move on to study more complex gaseous compounds and make a fuller assessment of the capabilities for trace gas determinations. Simpson, who is chief of the electron physics section of NBS’s Institute for Basic Standards, points out that the principle of electron impact spectrometry has been known since about 1920. In the past nearly 50 years, numerous instrument designs have been developed for use in atomic physics research. Interest in the instrument was heightened c few years ago when, at the peak of a resurgence in particle physics studies, high resolution monochromaters became available. Unexpected effects
Simpson and his coworkers, Chris E. Kuyatt and Stanley R. Mielczarek, developed the electron impact spectrometer in 1963. In the course of experimental work with it, they noticed that many more well-defined peaks appeared than were expected for the gas samples under study. Apparently, other gases, present in trace quantities, showed up in the spectra. To test this hypothesis and to get a feel for the instrument’s possibilities in trace chemical analysis, Simpson “asked around for the toughest problem in air pollution studies,” and was told of the difficulties with low level carbon monoxide determination by mass spectrometry. In subsequent studies, the NBS workers were able to detect carbon monoxide levels as low as 15 parts per million, even in the presence of what had previously been interfering compounds. Trace air elements such as argon and helium were detected at 5 p.p.m. I m p a c t energies
As the name of the instrument implies, operation of the spectrometer depends on the collision of electrons 1000 Environmental Science and Technology
with gaseous atoms or simple molecules. On collision, the electrons transfer their energy to the electrons bound by the atom. The electrons, in turn, are raised from the stable state to an excited energy level. The spectrometer disperses the collision electrons according to their energies after impact, and the resultant energy spectrum corresponds to the optical absorption spectrum of the gas under study. The NBS apparatus itself includes an electron gun, monochromater, analyzer, and collector. This assembly is bent into an S shape for compactness and is enclosed in a one-foot long housing. The electron beam generated by the source passes through the monochromater and makes a 180’ turn. The electrons then collide with the atoms in the gas sample, after which they are dispersed in the analyzer, execute another 180’ turn, and are then collected. The result is a complete absorption spectrum of the sample. Simpson points out that the electron impact technique is “as close as one can come to measuring the intrinsic property of a gas: the energy state of its constituent atoms or molecules.” The instrument is about as complex as a mass spectrometer, the instrument it most resembles. However, the electron impact spectrometer is smaller and lighter, and for most atoms and simple molecules, much less prone to be affected by interferences between constituents of mixtures. For example, in the case of carbon monoxide and nitrogen, which have an atomic weight difference in the third decimal place, difficulties present in the analytical mass spectrometer are absent in the impact spectrometer. Moreover, the problem of identifying compounds by their fragments does not arise. Compared with an optical absorption spectrometer, the electron impact spectrometer has a wider wavelength range (x-ray to visible region). In addition, response to concentration is linear instead of exponential.
Reusing Storm Runoff This aproach to the problem of combined sewers involves never letting the two flows combine. Rather, storm water is collected and stored locally, then treated and reused
In a few years, some residents of one of America’s “new towns” should be tapping a new source of water and licking an old source of pollution, As much as half of their water requirements may be filled by a system that collects and stores storm runoff in small, local reservoirs, then treats it to make it potable. The net cost of such a system is estimated at less than 65% of that required for conventional treatment of the runoff in a central facility to control pollution. With storm flows reduced, erosion and sedimentation should also be reduced. The new town is Columbia, Md.. a fledgling community rising along the Washington-Baltimore corridor. Designer of the combined sewer system is Hittman Associates, Inc., a research and development contractor who moved to Columbia about a year ago from Baltimore. The project was supported by a oneyear, $200,000 contract with the Federal Water Pollution Control Administration. Negotiations are now underway for financing a demonstration project to construct and evaluate part of the system. The cost and technical information gathered will determine if other communities may one day apply the same concept. “This idea is really not new-the
Romans, for example, used cisterns,” C. W. Mallory, head of Hittman’s environmental engineering group points out. “But this is, we believe, the first time a community without a water shortage has taken a serious and systematic look at reusing rain water. And, of the combined sewer projects FWPCA supports, ours is the only one to deal exclusively with separating the storm water at the source.” Defining reuse criteria
When Hittman started out on the project, the company felt the simplest approach was to find uses requiring large volumes of subpotable water. “But to find the uses,” according to Mallory, “we first had to create classifications relating quality and the proposed use-very little had been done in this area.” Hittman came up with this scheme: AA-Potable, required for human consumption and general kitchen use. A-Safe for humans, but generally undesirable because of taste and odor; suitable for bathing, laundering, swimming pools, and general interior cleaning. B-Safe for humans, but generally undesirable; suitable for irrigation of lawns and parks, so mustn’t contain salt or organics harmful to vegetation.
Sedimentation. Runoff-due to construction-threatens both the beauty and recreational facilities of Wilde Lake, a 21-acre artificial lake in Columbia, M d .
1002 Environmental Science and Technology
C E o l i d s removed, free of bacterial pathogens; suitable for toilet flushing, street cleaning, fire fighting, heat transfer, and air conditioners. This classification was later upgraded to make it suitable for water contact recreation. “As we considered the lower categories, we quickly realized we had a problem,” Mallory declares. “Industries frequently are big water users, but the area of Columbia we are concerned with will have no industry. Things like washing cars and sprinkling lawns don’t really need a lot of water-and the demand tends to be seasonal. Toilet flushing represents a large constant volume use,” he points out, “ but we wanted to avoid taking the water into the house because of the safety factor and also because of cost. So we had to revise our thinking to include the possibility of bringing the water up to potable quality.” Hittman considered many combinations of reuse, and decided to study intensively the following: A uses only. A uses plus B uses. A uses plus C uses. A uses plus all lower uses. B uses plus all lower uses. C uses only. No reuse.
period. With this one-year storm criterion, we should be able to capture 94% of the runoff.” With the reservoirs sized and located, Hittman next turned to the treatment problems. “Again we found little in the literature to help us. The pollutants in rain are complicated, and, of course, get even more so when the rain hits the ground,” Mallory adds. Hittman elected to treat the storm water in two stages, both using advanced methods. (Conventional methods can’t do the job in these small reservoirs.) Pretreatment will bring the water to quality C. Thus, if no reuse is planned, the receiving stream will still be suitable for water contact recreation. The kind of reuse planned dictates final treatment. A 10-pond system
Picking a watershed
The area Hittman chose to study is the 1 100-acre area draining into Wilde Lake, a 21-acre artificial lake. The Village of Wilde Lake-the first of the 9 villages to be built around the downtown core of Columbia-has a population of about 1500, with 12,000 planned for 1980. The Wilde Lake watershed is divided into 22 subwatersheds, each with a single spot where all the natural drainage can be intercepted. Thus, the entire system can be tied together; each subwatershed can be looked at independently or in combinations. About 10,000 combinations are possible, considering numbers of collection systems, watershed runoffs, and locations; this number increases by an order of magnitude for each additional factor considered-for example, type of reservoir and treatment facility. By use of a computer program, Hittman considered more than 20,000 possible system configurations. The technique involved the development of a generalized system diagram in which the design and performance characteristics and cost information have been displayed as the critical parameters. Even with the computer making all the calculations, searching for the best answers would
have been a monumental task. So, the system analysis computer program included the necessary logic to search the answers, select the best approaches, and display the results. The results were displayed on single output sheets which could be scanned easily to select the best system considering the actual constraints of the site. Predicting runoffs for the 22 subwatersheds proved a difficult job. Very little rainfall data existed-the nearest Weather Bureau gages were more than 5 miles away; no stream flow data at all existed for the Columbia area. “We ended up collecting the information available, then creating the methods we needed,” Mallory declares. Hittman located a watershed similar to the Wilde Lake watershed in size, soil, vegetation, and degree of urbanization, and one for which five years of data on rainfall and runoff were available. “From these data, we developed a mathematical model that enables us to predict the probable volume of runoff for a given amount of rain.” Mallory says. A related problem was to predict peak rates of flow, to prevent flooding. “Here, the technology existed,” according to Mallory, “we just had to adapt it to our specific situation. We chose as our criterion the biggest storm likely to occur over a one-year
Using these techniques, Hittman has designed a system for the Wilde Lake watershed that collects and stores storm water in 10 ponds of 1-2 acres each. Most will be in open space or the flood plains. (To get the required zoning from Howard County, the Rouse Co., developers of Columbia, had to dedicate 20% of the area to open spaces.) The ponds are sized to collect the runoff, then drain at a rate at which the water can be properly treated. The ponds have a minimum level for aesthetic reasons and to preclude weeds from growing. Although there is a chance of the ponds not being emptied sufficiently to receive a second storm, the probability of this occurring is quite low; even then, the amount that would overflow without treatment is small. Alongside each pond will be a pretreatment plant. First, the storm water passes through a nest of inclined tubes, which will give a much higher rate of sedimentation for a given size reservoir. Treatment with conventiona? chemicals (alum, for example) and with polyelectrolytes, followed by chlorination, brings the water to quality C. Exploring reuse alternatives
To design the last part of the system, Hittman examined three reuse alternatives in detail, then compared them with a traditional system. Volume 2, Number 11, November 1968
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Modeling an Ungaged Watershed When Hittman Associates came to establishing the volumes of water the local storage system for Wilde Lake watershed should be prepared t o handle, i t faced some pretty knotty problems: i t had very little rainfall data on the watershed-and no runoff data at all. On top of that, the watershed was largely undeveloped: t h e storage sys. tem, however, would have io kiandle runoff f r o m a n u r banized community abounding i n impervious surfaces producing large, but unknown, quantities of runoff. The only t h i n g Hittman knew for sure was what four Weather Bureau rain gages had to say over the past 20 years: that Wilde Lake's rainfall was most probably similar to that at FrkndshiD Airport a few miles away. Friendstill no runoff data. ship had a lot of rain data-but "We then decided t o look for a watershed of similar size. soil. vegetation, a n d eventual degree of urbaniza. l i o n as Wilde Lake." says John J . Boland. According t o Eoland, a systems analysts on the project, "We found it i,sst north of Baltimore in t h e West Branch of Herring Run." Herring Run was blessed with five years of Weather Bureau data on rainfall and five years of Geological Sur. vey data on runoff. The task then became one of develop. ing a mathematical model that would predict runoff for a given rainfall. "We analyzed the data i n many ways-five springs, five July's, five years, to cite just a few examples. From these analyses." Boland says, "we developed three m o d els based on seasonal, monthly, and yearly data." Herring Run data were then fed back into the eqtiations. to see how well these general, statistical relationships predicted runoff on individual days. The seasonal model proved to give the best correlations. It accounted for sea son a I va ria t i on s i n ra i i i fa I I. r u noff re lat io n s h i ps without producing ail excessively complex model.
Potable reuse. Capital costs on a potable reuse system will be moderate, but maintenance and operating costs will be high. However, the system could supply half of the Wilde Lake area's water needs. At 45d/1000 gallons, what Columbia now pays for water, the net cost of this system drops to $10.38 per dwelling unit per year. Because the primary purpose of the system is pollution control, the water is used only as a supplemental supply source. Subpotable reuse. Because of the cost of distribution piping, the capital costs will be very high on a system for subpotable reuse. Operating and maintenance costs will be moderately high. Even selling the water will not make this an attractive alternative in most communities. Cost per dwelling unit per year will be $25.76. However, a community fortunate enough t o have a big subpotable customer could avoid
1004 Environmental Science and Technology
"However, we had broken our watershed u p into 22 subwatersheds to help locate t h e storage ponds," Bo. land points out. "While Herring Run watershed overall was similar to Wilde Lake watershed overall, the simi. larity d i d not extend t o each subwatershed. So, we had to derive coe3icients that permitted the question t o take into account the different amounts of construction-or impervious areas-in each of the 22 subwatersheds," according t o Boland. The final equation is
Q
A
-:
-+ BR, .i. CR,' 4- DR,:]
in which
Q,
R,
:runoff
i n cubic feet per second days/square mile
rainfall i n inches/day
With the aid of a computer, five typical years were then prepared for all 22 subwatersheds. "From t h i s point. we were able t o assign values to the expected daily r u n offs from each subwatershed. We finally had what we needed-the daily hydrograph, the day-by-day record of the storm runoffs we should plan for," he says. On the basis of the daily hydrograph. Hittman decided t o plan for a runoff of about 250 million gallons per year. But the problem of flooding remained. ,,To calculate peak flows." Boland says. we used "a linear storage reservoir model t o find storm hydrographs for about 50 different tnypothetical watersheds. From the results. we used a multiple regression technique t o derive a formula which calculates peak flows for any watershed. given area. i m oerviousness. lag time. and storm intensity." Armed "itli these data. Hittman was ready to size and locate the reservoirs and t o design the remainder of the system
an extensive distribution system and might end up making a profit on its system. Also, the system might be applicable in an area with a water shortage. No reuse. With no reuse, the storm water will be brought to quality C and sent to local streams. Capital, operating, and maintenance costs will be low. Thus, for water pollution control alone each dwelling unit will pay $15.35 per year. Conventional treatment facility. For comparative purposes, Hittman contracted with Whitman, Requardt and Associates of Baltimore to design a central facility for conventional treatment of the storm water. The system consisted of interceptor sewers, two pumping stations, a 2-acre holding pond, and a 10-acre treatment pond. Operating and maintenance costs are moderate, but the capital costs are very high. The resulting cost is the highest
of any of the systems excluding subpotable reuse-$24.97 per dwelling unit per year. "The dollar comparisons don't tell the whole story of what we feel a system of local reservoirs can do," Mallory maintains. "Consider for a minute what happens to the pretty little stream that graces many a new development. Runoff will be two to three times greater after development than before. As storm after storm rushes through the stream, the banks begin to erode. Trees fall in. The stream begins to look messy. Pretty soon the local government cements in the sides and bottom, or even puts in a big storm sewer and covers the old stream bed over entirely. "Local reservoir storage will dampen the raging storm flows and preserve a stream as it is. At the same time, the amount of sediment carried by streams will be reduced. In our test area, sedi-
Alternative Systems for Handling Runoff of Wilde Lake Watershed 0 pe r a t Ing a n d r n a i n t e n - Value o f Annual net ance costs water cost/ day -eused/day dwelling unit
i y b t 1:nis
C a p i t a l costs
Lncdl collection, storage, and
$1,403,000
$197
$291
$10 38
2,606,000
126
235
25 76
treatment-potable
reuse
-subpotable reuse ( v ~t hi d i st ri b ut ion PIPlnR) -no
reuse
Co n v e iit io n a I t re it ni):I 111
t
ccntral facil ty
859,000
35
15.35
1.315.000
68
24 97
mentation in Wilde Lake is becoming a problem because of construction. A large amount of sediment has been dredged on two occasions. It is questionable whether the lake will be suitable for swimming and other recreation even after lawns are planted because of the much greater runoff occurring,” Mallory concludes. Compared to centrally collecting and treating the runoff, the local system offers another advantage: It avoids a lot of massive concrete structures. Normal drainage channels should be able to collect the runoff. The ponds will need a minimum of concreteprobably no more than a strip down the middle for maintenance purposes. The ponds proved easy to locate in the Wilde Lake watershed, most of them ending up on the flood plain where no homes are located. The pretreatment plants alongside each pond will measure about 10 feet by 12 feet. They will be buried, with only a foot or so being above ground. The treatment plant will also be constructed underground; it will contain about 500 square feet of floor space. “The local reservoirs use only a small percentage of the land to protect the beauty of the area. The reservoirs themselves are small and with quite simple landscaping can also be made very attractive-even centers of attraction within the flood plains, which are normally not used because of possible storm damage. With the type of planning that has gone into Columbia. you can be sure that the ponds won’t be allowed to become eyesores.” Mallory notes. Demonstrating t h e concept
But all of this is still largely a paper concept-very little physical data were
gathered. The next phase of the Hittman plan calls for a demonstration project that will answer some basic questions about the concept: Can a system of small, local reservoirs adequately collect and treat storm water and thereby control pollution? Can the reservoirs dampen storm water flows and decrease erosion and sedimentation? Can the reservoirs be made aesthetically acceptable in urban areas? Can potable water be reliably produced by small treatment plants with part-time operators? Will the public accept reused water? Aside from these bigger questions some of Hittman’s design and analysis methods must be verified in practice. These include such questions as: How accurately can system analysis techniques predict the rather unpredictable acts of nature? What are the overall effects of converting a cow pasture into an urban community? Can mechanical devices perform adequately to protect a natural environment? What price must be paid to control storm water pollution? To answer these questions, and as part of its work for FWPCA, Hittman is working out the details of a proposal that calls for demonstration of the key components of the storm water reuse concept. The demonstration program will be directed toward gathering the information necessary to apply the concept to an entire watershed for abatement of storm water pollution. The program will include construction of one reservoir in the Wilde Lake watershed. A pretreatment and treatment facility will also be installed with
appropriate instrumentation. The goal of the program will be to demonstrate the ability to produce potable water reliably from storm water using a small, partially attended facility. Efforts will be made to produce potable water from the system but the water will not be distributed or reused during the demonstration phase. The ability to produce potable water reliably will have to be demonstrated for many months before public health officials would permit the water in the existing distribution system. The planned demonstration program will answer most of the questions that pertain to the technical, economic, aesthetic, and public health aspects of the storm water reuse system. The next step would be application of the system to a complete watershed to effectively combat pollution from storm water, or combined sanitary and storm water flows. Other applications
Hittman feels the concept will be particularly valuable in areas just beginning to develop. “In many respects we are too late for Wilde Lake, even in a well-planned area like Columbia. Had such a system been included in the original planning, a lower cost and great benefit could be attained. More importantly, it would be particularly valuable in the building phase, since erosion is so high during this period,” Mallory points out. The concept might also be applied in communities with a combined sewer problem. Several studies are now underway using process control computers to program combined flows so that the most polluted flow goes to the treatment plant and the less polluted flow goes directly to the river. In such situations, the local reservoir concept could be used to dampen storm flows. Or the concept could be used if a combined sewer has to be split. In such cases, the existing sewer usually becomes the storm sewer, and a new sanitary sewer is built, since it is usually smaller in volume. With local reservoir concept, the storm flow could be diverted, converting the combined sewer to only a sanitary sewer. Natural ponds-by far the cheapest way to store water-might not be possible. However, constructed underground tanks could be used in urban redevelopment situations.
Volume 2, Number 11, November 1968
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