F R A N K
E. C L A R K E
Who is re-using? What are the ppes o f systems? How can quantip be increased and qualip improved?
INDUSTRIAL RE-USE it grows to sustain our expanding population, Management is aware that much of its life blood is flowing to waste through wide-open valves, and numerous water re-use systems already are used to stretch water supplies. By increased use of such systems, industries can survive in present locations and even flourish in arid climates where industrialization once seemed impossible. s
A industry will not die of thirst.
Requirements
The terms water intake, water use, and water consumption are not synonymous. Industry may use large quantities of water but draw relatively little from the original source. Some users, like carbon black plants, consume virtually all the water they withdraw, but average consumption for all industries, including steamelectric power generation, is only about 2%. Water requirements of industries vary widely. Major users such as the chemical and metals industries withdraw 25 to 30 times as much as minor users such as the rubber industry. Within a given industry, one plant may use 10 to 30 times as much water per unit output as another making identical products. Numerous factors contribute to intra-industry variation : Factories in arid climates have high water-use efficiencies; large plants tend to use less water per unit than small plants; and, because of economic pressures and legislation, modern plants are generally more water-conscious than older plants. Every industry and most individual plants can reduce their primary water needs by process improvement, water conservation, and water re-use. Where cooling is
F. E. Clarke is a Chemical Engineer with the Water Resources Division, U. S. Geological Survey. He has authored many articles on industrial water. AUTHOR
18
I N D U S T R I A L A N D ENGINEERING C H E M I S T R Y
the principal use, substitution of recycle for oncethrough operation can reduce intake by as much as 90%. Re-use alone might counteract the twofold increase in industrial water requirement predicted for 1980. Doubling the present 100% re-use figure would make this possible by providing more than three gallons of plant water for each gallon withdrawn. Tripling re-use to 3o070 would allow doubling the current production rate without increasing water intake. The number and variety of return-flow and on-site re-use systems are increasing. Pertinent information from the latest census of manufacturers (1959) was just made available as I&EC went to press. These data, tabulated on page 20, show that over-all re-use in industries other than steam-electric generation increased from 82y0 to 119% between 1954 and 1959, and that re-use among the re-users increased from 106% to 136y0 during the same period. The 1959 census did not include all manufacturers withdrawing 20 million gallons or more, as in 1954, but it did include the major users who account for more than 90% of the total water withdrawn (steam-electric generation excluded). Return Flow Systems
Surface Wuters. Industry as a whole consumes only 270 of the 140 million gallons per day of water withdrawn and eventually returns the remainder for downstream users. About 80% of this return is cooling water contaminated only with heat. The remainder carries a great variety of industrial wastes. Where industries are scattered and stream flow is abundant, chemical, physical, and biological forces of nature recondition the water adequately between successive users. If streams are sluggish or the ratio of damand to supply is great, processes are less effrctive and each successive user receives water of poorer quality. Withdrawal from the Verdegris River (Kansas) is as much as 17 times its total flow in certain locations during periods of drought. Concentration of industrial plants can create the same
OF WATER
VOL 54
NO. 2
FEBRUARY 1962
19
Industry May Use Large Quantities of Water, but Withdraw Relatively little from the Orginal Source" ( 1 million gallons per year; slight discrepancies i n total result from independent rounding
Users ~
Industry Primary metal industries (steel, etc.)
Total intake
Reusers,b
70
Total
5f raw data) Re- Users Actual Intake (I) Use ( u ) consumjDischarge ( D ) to Fresh Brackish including tion Surface Ground Other water water re-use ( I - D ) waters waters users
c;'o re-use
(7)
3702
72
3284
2998
285
5255
142
3100
20
22
60
Chemicals, allied products
3240
74
2633
1768
864
4617
163
2345
17
109
75
Paper, allied products
1937
88
1884
1724
159
5989
110
1765
9
1
218
Petroleum, coal products
1319
88
1232
655
577
5692
114
1111
5
2
362
Food, allied products
624
70
518
458
60
1192
48
433
20
17
130
Transportation equipment
260
72
235
180
55
497
31
201
3
1
112
Stone, china, glass products
295
61
223
207
16
587
22
200
3
c c
163
Machinery, less electrical
171
70
141
117
23
222
5
134
1
Lumber, wood products
140
74
110
88
23
154
10
95
1
3
40
Rubber, plastics products
127
78
97
93
4
306
8
87
2
1
216
Textile mill products
135
58
91
90
137
12
78
C
1
51
77
Electrical machinery
93
70
82
72
11
149
5
Fabricated metal products
44
59
30
29
1
56
2
Instruments, related products
23
74
20
20
C
58
1
Ordnance, m i x . manufacturing
26
78
18
17
E
49
Apparel, related products
10
68
9
9
E
10
Leather, leather products
12
50
7
7
0
Printing, publishing
13
66
5
5
3
87
3
3
79
2
Tobacco products Furniture, fixtures Total Corresponding a b C
3 12,177
70
10,624
- 2083 2 8542
72
E
82
1
C
c
c
87
19
E
c
190
2
15
1
C
172
0
8
27
11
0
1
29
9
0
7
39
1
5
C
680
44
1
2
C
1367
3 25,065
0 677
2 9711
0
84
-
50
157
136
1957 Census of Manufacturing Establishments. Steam-electric generation excluded, Includes all major users among firms withdrawing 20 million gallons or more per year. Incomplete coverage among firms withdrawing 20 million gallons or more may affect this column slightly. Less than 500 million gallons.
situation on otherwise healthy streams by increasing the ratio of waste water to dilution water and reducing the distance between users. A 1959 survey revealed 254 installations of 10 different types on the Delaware River. Return flow re-use in such an area is complicated by the mixed return and the relatively low concentrations of wastes which must be removed from large volumes of influent water before it can be used in critical processes. With the expected increase in industrial activity during the next 20 years, it is clear that more attention must be paid to the effects of industrial discharges on surface waters. Industries must either withdraw only makeup for the water consumed and depend on re-use, or devise waste treatments capable of keeping pace with the increasingly restrictive effluent quality standards. Organizations such as the Ohio River Valley Water Sanitation Commission and some industries have gained a head start in reducing return flow problems. Ground Water. Ground water recharge is increasing in importance as a special type of return flow re-use. Although industry draws about 10 times as much water from surface sources as from aquifers, ground water 20
5
57
INDUSTRIAL AND ENGINEERING C H E M I S T R Y
may supply 75y0 or more of the demand in areas like the Dakotas. Withdrawal at rates exceeding local infiltration has resulted in serious water table decline, particularly in the southwestern states. Danger of saline water intrusion and the economic burden of going farther or deeper for water has prompted legislators and industries alike to think more seriously about ground water recharge. Artificial recharge with industrial wastes amounted to 47 million gallons per day in 1955. In 1958, the American Water Works Association reported 13 projects which involved recharging with industrial effluents. New York State has a requirement for returning cooling waters to Long Island aquifers, and other states are considering similar legislation. The 1959 census showed that some ground water recharge is practiced by most industries. If the estimated 4oyO increase in ground water use actually occurs by 1975, more recharge can be expected. Water is returned to aquifers through direct well injection or by surface spreading in ditches, ridge and furrow systems, and basins. Well injection is three to seven times as expensive as surface spreading and involves the risk that waste waters may clog the aquifer
Sewage. Tandem re-use is a modified form of return flow in which one user passes his effluent directly to another instead of returning it to the source. The most common and promising combination is industrial re-use of domestic sewage effluent. The volume of municipal sewage effluent is roughly 20% of the total industrial water intake; however, only about 1% of the available supply is used by industry. Population centers generally coincide with industrial complexes, making sewage readily available. Competition of industrial and domestic users for the same primary water supply increases the incentive for sewage re-use. Bethlehem Steel Co.’s re-use of Baltimore City sewage is the classic example of salvaging domestic waste water. At one time Bethlehem used well water for critical cooling
and process needs, where saline harbor water would not suffice. When the declining water table necessitated another source, sewage effluent was used. The quantity of sewage consumed has increased from 25 million gallons per day in 1942 to 125 today. Supplementary settling and chlorination are the only special treatments applied to the combined trickle filteractivated sludge effluent before piping it five miles to Bethlehem’s plant at Sparrow’s Point, Md. Bethlehem paid for the supplementary treatment equipment and continues to pay the cost of its operation. The waste water is purchased on a sliding scale based on the quantity withdrawn. Texaco’s Amarillo refinery uses city sewage for a somewhat different reason. Both the city and the refinery originally drew from the same aquifer and both are expanding. Rather than compete for the declining ground water supply, Texaco shared the cost of a new activated sludge type municipal sewage plant and now uses sewage effluent for 98% of its needs. The water cost, although high because of equipment amortization, is expected to decline as other users appear. I t is still less than the cost of city water. A number of industries use sewage effluent from their own housing developments, particularly where water is in short supply. The Santa Rita Copper plant in New Mexico uses sewage waste in the precipitation process and then re-uses it in leaching operations. Industrial Waste. Tandem re-use of industrial waste water also is practiced, but not to the maximum practicable extent, considering the heavy concentrations of industry in certain areas. An interesting example is the cooperative re-use system employed by Allied Mills and Hopper Paper Co. in Taylorville, Ill. These companies once competed for water from the same source. Now Allied Mills uses the water first for cooling and then passes it on to Hopper.
Plants built adjacent to riuers can use that source for cooling water. In steam operated plants, where the steam must be con-
densed, the amount of water is Z1/2 to 3 times that needed for cooling compressed air
or contaminate the supply. At present, well injection is best suited to cooling water return, where heat is the only pollutant involved. Even here, differences in pH, gas content, and oxidation-reduction potential can create serious problems. I n surface spreading, percolation through the soil removes a variety of industrial contaminants. I n Wisconsin, for example, ridge and furrow irrigation with cheese processing wastes and spray disposal of paper mill wastes are carried on extensively with little trouble from ground water pollution. I n a New Jersey community, spray disposal of food processing wastes raised the water table 22 feet in a single year with no ill effects. Germany has made spectacular use of surface recharge in returning industrially contaminated water to aquifers via a series of large basins in the Ruhr Valley. The next decade will see much more research directed toward conditioning water for safe, effective recharge, both for perpetuating ground water supplies and for safe disposal of waste. Tandem Systems
VOL. 5 4
NO. 2 F E B R U A R Y 1 9 6 2
21
An interesting mxampk of tandem re-use where water remums art shored by two plants of the Chiw Mitlls Division of Kennecott Copper C o ~ p which . arc located at Sonh Rita-and Hurley, N. M. Both plantr murt re-use to the maximum cxtenf, including sewage and dminopc. Whm Smta Rita's supfib exceeds demand, excess water is diverted to the Hurler plant ma White Water Creek which is cssentiully an open pipelint
l o w Cost Encourages Re-Use of Sewage cort, ilIo00 Gal. Alternnas cflW"t Jourccc
Szmge
City Baltimore, Md. Big Spring, Tex. Amarillo, Tex. Grand Canyon, Ariz. n
Indurtr-y
Steel plant Oil refinery Oil refinery Power plant
1.75" 4.9 13.Ob 37
13.5 1 1
200
Coat is 3.5 if fixed charges are includd.
b Indudu
ammfiution of new sewage plant cquiprncnt.
In Many Areas, Treated Sewage Effluent Is of Better Quality than the Natural Water Supply Component or AmmBoltiLos Enid, La Ropety ill0 mom Almnos Okln. Vegm
7.6 6 . 7 t o 7 . 0
PH
-
8.4
7.1
10
B.O.D.
8
20
70-90
10-30
30 50-75
18-33
700-
856-
P (
Sulfate, Phosph PO, ~. Nitrate, 22 Nos Ammonia, 15 NH, Silica,-SiOz 79 Suspended 10 Solid Total solid 623 Equiv./million Hardness Total Alkalinity
6 6
66 3.6 13-21 18
60 25
2405x0 ...
1.8-
m n n
900 ~
0.8
L.L
1.63 3.2
1026
~~
3-5
8.8-9.5
8-10
-
Better Waste Treatments Make Inter-Industry Re-Use More Feasible Component
PH Temperature, F. P.p.m. Dissolved oxygen Nitrate, NO3 Ammonia. NH, Phenol C.O.D. B.O.D. Sumended solid Tocal solid ~~~~
D
7.8-8.2 74-90 6.3-8.6 0.43-21.6 0.3-2.8 00.003 74.5-207 0 040 1,16-224 ~~
6.6-8.2 86
4-5 27 3.9-5.6 0,003 127-154 0-8.2 15-32 1522-1606
Trafalga. Rdncry of Cilics Scrvicc Go., Canada.
Were it not /or esthetic conridnationr, many trcotcd wastes would octuolly be suitable fw domestic we. 22
Large plants situated in water-rich areas may find it uneconomical to return a relatively high quality effluent to the point of need. Critical processes and cooling requirements may not tolerate an increase in dissolved solid, or effluent may represent such a small fraction of total use, as in the Trafalgar example, that its reclamation is not justified. However, where geographic location is favorable, it is better to pass effluent to another less critical user than to discharge it to a surface supply from which it must be reclaimed at greater cost. Some advantage is being taken of this situation. I n the 1959 census about half of the industries listed indicated some tandem re-use and the chemical industry was shown to use 109 billion gallons in this manner.
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
On-Site Re-Use Systems
On-site re-use obviously is relatively common, otherwise industry as a whole could not use more than twice as much water as it withdraws. These systems may be divided into two general types: multiple recycle and cascade, but combinations of the two normally are used. Multiple Recycle. A multiple recycle system consists of a number of parallel circuits carrying various types of cooling waters and process waters. This arrangement segregates wastes for simplified treatment and provides for variation in temperature, pressure, and makeup water quality. The individual circuits may have integral clean-up systems for maintaining the desired water qualities. More often they operate with continuous blowdown draining to limit circuit retention time and deliver the resulting wastes to less critical systems or waste treatment facilities. If blowdowu waste passes to another process or cooling system, the plant is a compound multicycle-cascade type. The elaborate multiple recycle of The Dow Chemical Co. at Midland, Mich., handles high pressure service and cooling water, domestic water low pressure cooling water, high grade cooling and process water, and demineralized boiler-feed water in five separate supply systems involving 75 miles of distribution lines. The
~
over-all re-use is estimated to be fourfold. Eventually the various circuits intermingle to form a semicascade system and the effluent waste waters are integrated and treated in a central location. Many organic contaminants are reclaimed from their respective process waters and returned to process before final effluent integration. Particularly objectionable waste concentrates are withheld from the normal discharge and injected in waste disposal wells. Cascade Systems. Cascade re-use systems depend on integration rather than differentiation of process and cooling waters. Water first flows to the equipment or process which requires the coldest or cleanest supply and continues to successive operations where progressively higher temperatures and poorer watm nualities can be tolerated. The effluent of one operr Y'
A Multiple Recycle System in Its Purest Form Gd./Min. Total
Cooling
Areas Shviwd
Make-
UP Crude unit, catalytic 950-1000 . cracking, thermal cracking, delayed cokmg unit Light ends tower 20CL250 (gasoline stabilization, gaseous constituents) Catalytic reformer 150 Alkylation unit 30-40 Power plant surface 60-150 condensers
systm
Main Tower Tower
Tower Tower Spray pond
c*nJotion
41,000
16,000
8,000 4,500 11,000
The Ohia Oil Co., Rabimon, nl., wilhdrows only 58 gollom of wolrr per 6aWel of d e oil, 6ut we$ 1240 gollonr. T h , 2o-fold m-we is mhewd. Blowdownr from the Jim rtcydcx we inugrntcd for through tremlmmt and the eflumt is of sxcellant quality.
becomes the influent of the next until intolerable temperature or contamination develops. Either single passes or recycle circuits may form the various steps of a cascade, but most systems of this type involve considerable recycle. Some element of cascade re-use can be found in most industrial water systems. The cascade system does not reduce water losses in cooling operations or decrease the waste which eventually must be treated. However, it generally reduces the number of water-cooling and waste-treatment facilities required and increases the over-all percentage of reuse. Designing heat exchangers and process equipment to tolerate relatively poor quality water and problems of reclaiming specific wastes from integrated streams are recognized disadvantages. Kaiser's Fontana, Calif., steel plant is an excellent example of this type of re-use system. The influent mixture of well water and local company water is used step-wise for cooling motors and heating furnaces, mill cooling and scale-flushing, and cooling open hearths, blast furnaces, and the coke plant. Then it is split in three parallel sd'eams for further cascading. These parallel streams handle less critical operations, l i e gas washing, precipitator flushing,and blast furnace cooling before fmally being used for slag quenching. Some water is diverted from the parallel circuits for use in the tin mill and some is reclaimed as slag-pit runoff. Much of the last-stage water evaporates in the slag-quenching operation, thereby depositing its load of waste in a form which is essentially harmless, considering the arid climate. Process Integration Systems. Occasionally, recycle and cascade systems are plagued by production of particularly noxious wastes which cannot be released without high dilution and from which the troublesome agents cannot be removed economically. The black liquors produced by sulfite and neutral sulfite-semichemical (NSSC) paper mills are examples. There is no profitable process for reclaiming raw materials from such liquors, and the relatively high concentration of oxygenconsuming organic wastes will cause serious stream
MAKEUP AND STEAM FOR ALTERNATE OPEN HEATING SYSTEM ~~~
I I I I
MANUFACTURING PROCESS
I I I
rl
3 I ~
HOT OR COLD WATER TUB
STEAM OR REFRIGERANT
The American Cyanamid Co.'s chemicd plant at Bound prim$lc types of w-we ciraits which we 50 per &y .f rooling and procm water but withdraws o gallons. Much of this luotcr is usedfor heating m chilling pocmes. has four
'
VOL 5 4
NO. 2 F E B R U A R Y 1 9 6 2
29'
pollution. Until recently, roadbinding use and spray disposal on wastelands were the most practicable solutions to the black liquor pFOblem. Now Sonoco Products Co. of Hartsville, S. C., has an interesting plan for reclaiming formic and acetic acids from the liquor and combining the residue with Kraft mill wastes for regeneration of raw materials useful in the Kraft process. It is possible to reclaim 98% of the NSSC black liquor water in evaporators. A flow sketch of this integrated re-use system was shown in October 1961 issue of I&EC, p. 775. Other opportunities for improving reuse through process integrations have been and will be exploited as necessity continues to encourage invention.
I
I
Maintainin& Quality
The practicablekxtent of re-use is determined to a considerable degree by the effectiveness of quality control treatments in the re-use systems. Roughly 80% of all industrial water is used for cooling and special attention has been given to heat dissipation. The cooling methods applicable to recycle are spray ponds, cooling towers. evaporative condensers, and aircooled heat exchangers. All but the last depend on the cooling effect of evaporation. Approximately 1% water is evaporated for every IOo F. temperature rrduction, and loss is about the same regardless of the system. Even when the windage factor is included, cooling tower water loss generally is no more than twice that of a once-through return-flow cooling system. Wet-bulb temperature is the lower limit of evaporative cooling. This may range from a degree or two below atmospheric temperature at high humidity to 30 degrees or more below atmospheric temperatures when the air is very dry. A spray pond is less efficient than a cooling tower, and the forced-draft cooling tower, with its bottom fan and countercurrent air flow, is now favored over the induced draft (top fan) and natural draft types. The evaporative condenser is best suited to relatively small cooling loads. I t combines cooling tower and process cooler functions in a single operation. For example, cooling water return may be sprayed on condenser tubes of a refrigerating system to condense the refrigerant and release heat to a circulating air stream. At General Electric’s Electronics Park site, 68 evaporative condensers save approximately 41 5 millions of gallons of water per year. Air-cooled heat exchangers waste no water but they can cool only to a few degrees above atmospheric temperature, and thus are limited to relatively high temperature applications. Compound systems sometimes cool with air as far as possible and then continue temperature reduction with a cooling tower or other evaporative system. Open cooling circuits commonly are treated with chlorine or algacides to minimize biological growths, although many British towers depend on light tightness for this control. Antirot treatments are employed on wooden towers to a considerably smaller degree. 24
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
=
7
K&?s F m r t o ~ , Calif., g s t m of rnulti-rtcycling cnrcadc is so @&e that the plant withdraws only 1400 gallons of water per ton of rtcel produced, whereas as much (IS 65,000gallo? ore required in rorncplants. It con be said that the wotn is wrung dry in this 3000% re-use system
Corrosion inhibitors such as soluble oils, chromates, and dianodic phosphate mixtures may be required. Organic and mineral acids sometimes are fed to maintain proper pH and minimize deposit formation. It is feasible to control p H by diverting a part of the cooling water through an acid-type cation exchange resin instead of by feeding acid. The British “starvation process” is a particularly interesting variety of this treatment. Here a carboxylic acid-type exchange resin is used which reacts only with carbonate alkalinity, thus making it impossible to overacidify the circuit or to generate free mineral acid. Unlike strongly acid exchangers, regeneration of this type requires no more acid than would be required by direct acid feed to the cooling circuit. The reaction is:
+ NO+HCOa + RCOONa + Con + H 2 0 or + Ca++ 2HCOs- RCOO> Ca + 2COn +
RCOOH 2RCOOH
-
RCOO
2Hn0
Some of the recycled cooling water must be continuously diverted to waste (blowdown) to prevent excessive concentration of solids by evaporation and process contamination. Equations based on water analysis are useful in calculating proper blowdown. Denman’s equations for control of calcium sulfate are particularly pertinent because of the persistence of
Significant economic benefit may result from byproduct recovery. The Sonoco organic acid-recovery system in the integrated NSSC-Kraft paper process
literally trades 1 cent per pound sulfuric acid for acetic and formic acids worth 10 and 15 cents per pound, respectively. The Emscher River Association in Germany reclaims about 5000 tons of phenol per year at a profit of $60 per ton. Armco Steel at Middletown, Ohio, salvages u p to 200,000 pounds of iron oxide per day and this is 70% iron. Plant size is a factor in determining if by-product recovery is worth while. The Sonoco process is not profitable for plants producing Less than 300 tons of paper per day. Phenol concentration generally must be above 1500 p.p.m. to make its recoverypay the cost. Re-use systems also can reduce water costs. Industrial waters generally cost from 2 to 25 cents per 1000 gallons as compared with 1 to 5 cents for recycled cooling water and 2 to 13 cents per thousand for treated sewage effluent. This difference undoubtedly will become greater in the future. Other advantages of re-use are: reduction in chemical, thermal, and biological stream pollution; increased flexibility for plant expansion without providing larger water mains and sewers in congested areas; relative simplicity of treating and otherwise handling the concentrated waste waters which result from recycle; and maximum freedom from water pollution by upstream users. Reduced cost of corrosion control in a closed system should not be overlooked. Conserving water resources for future expansion and facilities for establishing new plants in water-starved environments also is important. It is foolish to think that re-use involves no problems. Biological growths, scale formations, and corrosion may tax the resourcefulness of the water technologist despite the variety of conventional treatments at his dispowl
Sometimes ingenious combinationr o j processes me used. The Sun Oil Go. at Toledo, Ohio, user cooling towns as tricklc JiItcrs for secondary oxidation o j phenol. Phenol-conrurning bacteria dcmlop readily on thc tower packing und no problems from other bnctnd
growth hove been exptrienced. The waters are subjected 10 @-gas rhipping for s@de removal, parsed through conventional API separatorsfor oil removal, and impounded in a lnrgc basin for equolizalion undprimq biologid ozidation before sh,b,bing ofphenol
deposits that result from excessive retention and concentration of this solute (I&EC, October 1961, p. 817). Waste disposal and by-product recovery are important aspects of quality control in re-use systems. A variety of organic and some inorganic wastes can be separated by standard processes like coagulation, sedimentation, demulsification, skimming, flue-gas stripping, filtration, centrifugation, and biological or chemical oxidation. Removal of certain soluble inorganics requires more costly treatment such as evaporation, precipitation, and ion exchange. The variety of by-product recovery processes is limited only by the resourcefulness of research staffs and economic considerations. Paper mills filter large amounts of re-usable fiber from wash water, refineries reclaim a variety of oils from their integrated wastes, platers salvage copper from cyanide baths, steel mills recover sizable quantities of iron as oxide scale, and brewers recover vitamins and antibiotics from waste waters. Numerous other examples could be cited. A particularly interesting and relatively new recovery process involves use of ion exchange resins to remove continuously metal ions from circulating rinse waters. When used in a closed countercurrent rinse cycle of a plating operation, an exchange resin ensures continuous supply of high quality rinse water and, on regeneration, yields valuable metals for recharging the plating bath.
-
*
Profits and Problems
VOL 54
NO. 2
FEBRUARY 1 9 6 2
25
Foaming agents and phosphate compounds in sewage effluent may require hot pr-ssing and abnormal chemical oxidation. Special alloys, carefully designed heat exchangers, and high circulation rates might be required to handle waters concentrated tenfold or more through re-use. Eventual rot of wooden cooling towers can be expected regardless of the care they receive. I t costs money to cope with these problems. Initial investment for a cascade system, such as that of Kaiser Fontana Plant, might be several times that of a oncethrough counterpart. There is also the very real problem of disposing of the concentrated wastes resulting from re-use. Release of such wastes to surface streams in slug concentrations is far more objectionable than untreated once-through return flow and generally is illegal. Metered discharge, although better than slug discharge, still returns essentially the same amount of pollution as in a oncethrough operation. If conventional multi-process treatments do not provide a practicable solution to this problem, more ingenious solutions must be sought, such as Kaiser’s adsorption of wastes on quenched slag, the sulfite paper industry’s disposal of black liquor through road-building operations, and The Dow Chemical Co.’s deep-well injection oftroublesome organics. A Gallon Saved Can Be Re-Used
Good management of water distribution systems is as important in re-use as it is in once-through systems. The potential savings of the best re-use plan can be squandered by water-handling equipment with excessive pressures, leaky valves, uninsulated hot water lines, dirty heat exchangers, and uncontrolled cooling-water flow. Allowing drinking-water fountains to flow continuously cost one firm 30 million gallons of water per year. Even a ‘/,-inch diameter leak from a single faucet operating at normal public supply pressure can waste a million gallons annually. Water use in a large plant is likely to vary as the square of the pressure. Low pressure service with booster pumps for high pressure needs is one defense against this situation. Processes with unjustifiably large water appetites also can defeat water conservation programs. Too short a drain pause above a plating bath increases unnecessarily the quantity of drag-out rinse that must be applied. Concurrent drag-out rinse uses more water than equally effective countercurrent, fog, and spray rinses. Improved detergents and proper pH control can reduce wash water requirements in many operations. Substitution of dry drags for scale flushing has saved considerable water in some steel mills.
auggesrions for Water Consorvation
-Meter the water and issue water “bills” to each department -Use automatic valves whenpracticable -Control pressures and temjeratures at optimum conditions, using fail-safe systems -Designprocesses to use water eficiently -Use separate system fur variousgrades andpressures of water -Keep heat exchanger clean -Use saline ur brackish water instrad of fresh water if the salt effects are tolerable -Use reservoirs and surge tanks to saveperiodic surplus -Insulate water lines to avoid wastefuljushing -Establish central management of water use
Advantages of good housekeeping in the plant can be lost at the last minute by using the hoarded water to dilute concentrated wastes for legal discharge. Special emphasis must be placed on devising other answers to the waste disposal problem. Many industries have learned these lessons the hard way and excellent water conservation policies are becoming more prevalent. The Dow Chemical Co. developed its present conservation policy when a special committee found that it cost more to treat and pump effluent water than to obtain makeup water. In addition to instituting good housekeeping practices, a thorough water-metering program was established and each department now receives regular statements of its water use. This attention to water consumption saves the company about $125,000 per year in direct feed-water costs. It eliminated the immediate need for installing additional costly water mains and sewers. Similar attention to housekeeping practices reduced water requirements at a General Motors plant from 2.3 million to 800,000 gallons per day during a 10-year period.
-
The Safe Approacn to Kcuse
A Thorough Survey Should Cover:
Plan, not panic, is the keyword in solving the water problems of industry. Whether a new plant is being designed or an existing one is being revised, the entire operation should be studied with a view to improving efficiency of process and reducing over-all cost of operation. Reduction in water requirements is an automatic dividend of this approach.
-Cost and availability of waterfrom alternate sources -Quantities of uarzous waters withdrawn, used, and Lonsumed with special attention topreoentton of waste -Water-use eficiemy of uariousprocesses -Adaptability of plant to water re-use systems -Cost andpracticability of alternate waste disposal procedures
The Picture in 2000 A. D.?
Industry lives continuously on the threshold of a new era. Like Janus, it looks simultaneously forward and backward, profiting by experience in planning for the future. For this reason it will not run out of water or even suffer a permanent critical water shortage. Certainly it will not evaporate the rivers to com-
It is generally more effective to have the survey made by an unbiased specialist in the industrial water field; however, excellent surveys and revised water-use programs have been made with no outside help. Whichever plan is used, it is important to recognize that water management is a dynamic affair and that water use must be restudied periodically to ensure that the latest most effective treatments are being used.
Whcn cart of h c a h t for re-u.rc mrd & x h g e m'm'des w'th cost of complete demimmliration and cleanup, maximum 18-use, pnhaps of thc type shown hcrc, will be used. T h ,thc problem of adqlrntc supply will be solucd, at least to the point whm makeup done epunls thc proportion of total mmff availablefor industrial usms
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