by Edward W. Moore Harvard University
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W O R K B O O K
WASTES F E A T U R E
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Thermal "Pollution" of Streams Thermal loading of streams creates problems for industry and conservation agencies alike. Here are some points to be considered
ORGANIC
pollution of streams and
the biochemical oxygen demand (B.O.D.) created by these wastes are important industrial wastes problems. Another important problem, not too well known, is that of waste heat discharged into streams. Indeed, B.t.u.'s may soon be as much a problem as B.O.D.'s. Where does the heat come from? Steam power plants and certain industries are major contributors to thermal pollution, but all industrial activity contributes in some degree. Power plant contributions are measurable directly in the temperature rise (usually about 10° F.) of the very large volumes of cooling water drawn from the stream. Steel plants and petroleum refineries are big contributors. Simonsen (4) states that a typical 50,000 barrels-per-day refinery burns about a billion B.t.u. per hour, half of which must be carried away by water. T h e importance of accurate figures on heat load
Raccoon Creek
from various industries is obvious. Before any regulatory body can do anything, it must first know the facts. Present estimates of existing and future thermal loads on streams leave much to be desired ; in fact, estimates by competent engineers differ by factors greater than 20. W h a t A r e t h e Effects of T h e r m a l Load?
When the temperature of a stream goes up, the stream becomes less satisfactory for use as either municipal or industrial water. T h e natural range of stream temperature in the northern United States is about 32° to 85° F. Extreme thermal loads have caused temperatures of 120° F. or more in a few streams. The direct effects of thermal load are shown in the accompanying figures, replottings of curves sent the author by Edward J. Cleary, Executive Director and Chief Engineer,
Mahoning River
Ohio River Valley Water Sanitation Commission; they represent a portion of the comprehensive data being collected by O R S A N C O for a study on the reaction of the Ohio and its tributaries to thermal loading. Raccoon Creek, receiving no thermal load, shows close correspondence of water and air temperature except during the summer months, when water temperature is measurably lower. At the other extreme is the Mahoning River, a classic example of a stream with a heavy thermal load. T h e Ohio River, which receives a large heat load (estimated to be as much as 1012 B.t.u. daily), appears to be coping with this load in a satisfactory manner; the only noticeable effects are a summer temperature equal to or somewhat above air temperature in recent years, and a winter temperature more obviously above air temperature. This would seem to indicate that streams are capable of re-
Ohio River
Figures show h o w t h e r m a l loads a f f e c t w a t e r t e m p e r a t u r e . Raccoon Creek receives no t h e r m a l l o a d a n d its w a t e r t e m p e r a t u r e corresponds v e r y closely t o the a i r t e m p e r a t u r e . But the w a t e r in M a h o n i n g River, which gets a h e a v y t h e r m a l l o a d , has a n a v e r a g e t e m p e r a t u r e w a y a b o v e t h a t o f the surrounding air. The O h i o River, which has a p r e t t y high t h e r m a l l o a d , a p p e a r s to b e coping with it in a satisfactory manner
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INDUSTRIAL WASTES
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covering from very considerable heat loads without damage. Evalu ation of this "self-purification" capac ity will permit it to be intelligently used for the benefit of all. Physical a n d C h e m i c a l Effects
When stream temperature goes up, the density and viscosity of the water go down. This may have far-reach ing effects on the mode of flow of the stream. One is the production of dead areas of thermal stratification in pools and reservoirs, where the warm water slides over pockets of cool water with very little mixing. This means that any added pollu tant gets downstream much faster; this may be good or bad in any partic ular instance, but it means that the self-purification capacity of the stream is not being effectively uti lized. The most familiar effect of in creased stream temperature, the one most frequently cited as an objection to thermal "pollution," is the dimi nution of the solubility of oxygen in the stream water. Using the newer values of oxygen solubility suggested by Truesdale, Downing, and Lowden (6) it appears that equilibrium oxygen concentration for water in contact with air is about halved be tween 0° G. (14.16 mg. per liter) and 35° C. (7.04 mg. per liter); or, choosing another range of tempera tures, the reduction is better than 2 5 % between 68° F. (20° C , 8.84 mg. per liter) and 104° F. (40° C , 6.59 ,mg. per liter). This means there is less oxygen available in the stream for coping with organic pol lution, a serious problem for the stream biota, which is faced with in creased metabolic rates, and less oxygen available for metabolism. The solubility of other gases is likewise decreased, and other chemi cal equilibria, such as those govern ing pH, alkalinity, and hardness, are altered ; rate processes such as the coagulation of colloids going on in the stream are accelerated, but these effects are secondary. T e m p e r a t u r e a n d the O x y g e n
Sag
The rate at which oxygen is ab sorbed from the air by an oxygendeficient stream is accelerated by increasing the water temperature. 88 A
A W o r k b o o k Feature
But so also is the rate of B.O.D. ex erted by organic pollution. The classical theory of the oxygen sag indicates a greater increase in the latter, per degree increase in temper ature, and therefore predicts a con siderably greater drop in dissolved oxygen level (oxygen deficit) at a point nearer the source of pollution, when stream temperatures are raised. The effect can be calculated quickly by using the equations and diagrams developed by Fair (7). More recent studies by LeBosquet and Tsivoglou (3) and Thomas (5) suggest that the classical theory is in error, in that both dcoxygenation and reaeration rates appear to be affected about equally by temperature. Con sequently, the maximum oxygen deficit should be substantially un changed over a fairly wide tempera ture range, although the point at which it occurs is altered. This would present one of the bad effects of thermal loading as less drastic than it was formerly supposed to be, although the oxygen remaining in the stream at the low point of the sag would still be less at the higher tem peratures, by virtue of the decrease in the equilibrium solubility of oxy gen with temperature. The matter is of sufficient importance to demand further investigation. It seems doubtful that temperature effects can be dismissed at the high levels pre vailing in streams receiving heat loads. Recovery f r o m T h e r m a l
Loading
A stream has several ways of "unloading" its heat pollution; chief of these is natural cooling by con vection, radiation, and evaporation. Dilution, by admixture of unheatcd tributaries, and by percolation of ground water during seasons of low flow, is an important way of losing heat. These are analogous to the ways in which the stream spreads out and eliminates a load of organic pollution, but we know much less about the "heat sag" than about the oxygen sag. Le Bosquet attempted to formulate cooling rates in streams (2), using what is essentially a firstorder formula, now believed to un derestimate the rate of stream recov ery because it neglected heat loss by evaporation. Recent studies on evaporation rates, notably those of
INDUSTRIAL AND ENGINEERING CHEMISTRY
the U. S. Geological Survey, may help in formulating stream recovery rates. Control of T h e r m a l Pollution
The measures available for the control of thermal pollution are es sentially the same as those used for controlling other kinds of pollution— dilution or elimination by waste treatment. Included under dilution is the provision of other sources of cooling water—for example, ground waters—which are then added to the stream along with the heat load. The construction of reservoirs to pro vide increased flow during the criti cal summer months, as was done on the Mahoning River, is also essen tially a dilution process. What industry commonly thinks of as water-conservation methods— use of cooling towers, spray ponds, and the like in recirculating systems —can be equally well considered as treatment devices for the elimination to the air of potential heat pollution. As with other forms of pollution control, either method costs industry money—returns are indirect and in tangible. The most desirable solu tion is full use of the recovery powers of the stream, with control costs held to the necessary minimum. This solution requires more knowledge of how streams handle a thermal load. Literature Cited
(1) Fair, G. M., Gcycr, J. C , "Water Sup ply and Waste Water Disposal," pp. 524-5, 841-8, Wiley, New York, 1954. (2) EeBosquet, M., J. New England Water Works Assoc. 60, 111 (1946). (3) LeBosquet, M., Tsivoglou, E. G , Sewage and Ind. Wastes 22, 1054 (1950). (4) Simonsen, R. N., Ibid., 24, 1372 (1952). (5) Thomas, Η. Α., Jr., Seminar on Waste Water Treatment and Dis posal, Boston Society of Civil Engi neers, 1957. (6) Truesdale, G. Α., Downing, A. L., Lowden, G. F., J. Appl. Chem. 5, 5362 (1955).
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