The Role Of Cooling Towers In Achieving Zero Discharge

By Ernest Q. Petrey, Jr., Petrolite Corp., Tretolite Div. How can cooling towers be utilized to contribute to pollution control and to approach zero discharge? There are two basic functions that cooling towers with proper treatment and control can perform. The first i s water conservation and the second is the reduction of undesirable compounds in the water stream by direct oxidation from atmospheric oxygen and the aerobic reduction o f degrad- able matter by micro-organisms present in cooling water systems.

The normal mode o f operation of cooling towers is to use makeup water that has been pre-treated as effectively as possible by clarification, filtration, softening and chlorina- tion, and the discharge of the cooling tower blowdown to waste. We are now fast approaching an era when this luxury will no longer be available. What can be done toovercome this situation? One apparent answer to achieve the reduc- tion o f water usage in a plant, is the education of operating personnel to the need to conserve water and to solicit their ideas on this matter. Several studies of before and after results o f programs o f this nature show that a reduction of from about 10% to about 30% reduction has been derived from this method. After education, what next? The next step is the reuse o f all possible water in an operation. While this requires the cooperation of many individuals in each facility, the ultimate outcome is radically reduced water consumption.

First, stream surveys What procedures are followed in order to reuse these wastewaters? First, a survey of the streams o f both the volume and the composition should be made. Second, the data accumulated should then be analyzed to show which streams can be reused without further treatment and which may have to undergo either chemical and/or physical treatment.

Consider the utilization o f wastewater streams in a refinery. Sour water condensates previously sent to the sewer are now being “stripped” to remove hydrogen sulfide and ammonia. This “stripped” sour water i s essentially

distilled water, and therefore i s an excellent source for water for use in the desalters. Desalter water that was previously sent to the sewer goes to the A.P.I. separator together with other refinery water waste streams, including boiler blowdown. These combined effluents are treated with or without chemicals in the A.P.I. separator and can be processed through an air flotation unit to remove a greater percentage of the oil and suspended solids. The air flotation effluent can then be used as cooling tower makeup. The cooling tower blowdown i s then sent to waste. Alternately the cooling tower blowdown can be reprocessed through the air flotation unit to remove suspended solids that contribute to BOD. and COD. and a portion of the air flotation effluent can be used as blowdown. A water conservation program such as this can result in the reduction from 50% to 80% of the water usage in a plant. In one such operation the water consumption has been reduced to approximately 15 gallons per barrel of crude run.

When water is reused, usually the corrosiveness o f the water decreases and fouling tendencies of the water increase. Biological problems With the reuse of waste waters, more nutrients are added to the cooling water system and as a result, the biological population will show a marked increase, This must be anticipated and steps taken to counter this increase. Increased chlorination and increased biocide usage will occur. This i s expecially true in the case of algae growth. The inclusion of e f fec t i ve wetting and dispersing agents will aid in the prevention of problems related to the increased b io I og i ca I growth .

The reuse of water also usually results in a large increase in the population of sulfate reducing bacteria relative to the increase in the total bacterial population. The sulfate reducing bacteria, being anerobic, thrive in those low flow areas that have been oxygen depleated by the accumulation of general debris. Table 1 shows the results o f this increased growth rate.

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In the foregoing example chlorination was maintained at 4 hours per day and proprietary biocide addition at 100 ppm on days 5 , 8, 11, and 13 was effected. The total bacterial population was determined with T.G.E. Agar using the plate count technique, and the sulfate reducing bacteria population was determined with A.P.I. Both using the serial dilution technique.

Fouling problems Fouling in water reuse i s a problem that can be overcome with proper chemical treatment. The normal foulants, calcium carbonate and calcium sulfate may be controlled in the usual manner; i.e. cycles of concentration and/or acid feed; or by chemical addition.

There are two principle foulants that require additional concentration when water reuse i s undertaken. These are calcium phosphate and organic and inorganic debris. In so far as calcium phosphate is concerned, the limit of concentration as shown in Figure 1 would result in the cycles of concentration in most systems being maintained at 1.5 - 2.0 which i s far too low for desirable operation. Therefore, chemical treatment must be instituted to pre- vent the deposition of calcium phosphate. The phosphate content o f such combined waste water streams that may be utilized for cooling tower makeup range from 2 to 20 ppm as orthophosphate.

The problem of fouling by organic and inorganic debris i s greater when reusing water due to the contaminants contained therein. Suitable wetting and dispersing agents should be used to control this problem. In addition to the control o f fouling, these agents will promote the biological reduction of contaminants in the system.

Biological reduction of contaminants The role of the cooling tower and related biological results are shown on a reduction of phenols in the operation in two different cooling towers. In Table 2 it is shown that the phenolic content o f wastewater can be reduced by sending the water to the cooling tower.

It i s important to maintain the necessary balance of chlorination and biocide addition in order not to reduce the microbiological population to such a level so as to prevent the removal o f the phenolic contaminants.

A wastewater stream from an air flotation unit was used as makeup water to a developmental cooling tower that had a circulation rate o f 200 gpm and a temperature differential of 20F. The cycles o f concentration were maintained at 4.0 to 4.5, and the total biological population was maintained a t 400,000 to 750,000 colonies/ml.

The waste water stream was of the following average composition: phenols, 88 mg/l; oil, 18 mg/l; BOD, 102 mg/l; COD, 413 mgll. After four weeks of operation, the blowdown was o f the following average composition: phenols, 34 rng/l; oil, 32 mg/l; BOD, 328 mg/l; COD, 482 mg/l. The total reduction o f contaminants i s shown in Table 3.

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Currently evaluations are under way to ascertain the effectiveness o f chemical additives to a water reuse system for the improved reduction of contaminants.

Reduction o f hydraulic flow The resultant added contamination from various process streams in actuality may be beneficial to the resultant wastewater discharge, In Table 4 the total hardness of the makeup water was reduced by 8S%, which is the current limitation of cycles in the cooling tower. With chlorination (which is currently being applied to the makeup water) and pH adjustment, this water stream is suitable for makeup to the cooling towers.

To illustrate the reduction o f effluent flows, the following example is shown: An induced draft, two-cell tower with a circulation rate o f 20,000 gpm and controlled at a concentration o f 3.5 cycles a t a temperature differ- ential of lOF, requires a makeup water flow of 280 gpm and a blowdown rate of 60 gpm. If we substitute the water as shown in the Table 4, a concentration of f ive cycles can be maintained. The blowdown is reduced to 30 gpm. This is a total reduction of 90% of the previous discharge rate.

Operation Without Water Reuse ~~

Circulation Rate 20,000 gpm AT 10 F Dri f t 0.1% Cycles 3.5

Makeup Evaporation Drift Blowdown

Operation With Water Reuse

Circulation Rate AT Dri f t Cycles

Makeup Evaporation Drift Blowdown

Waste Water Discharge Cooling Tower Blowdown

Water Discharge Without Reuse

Water Discharge With Reuse

20,000 gpm 10 F 0.1% 5.0

250 gpm 200 gpm

30 gpm 30 gpm

250 gpm 60 gpm

31 0 gpm

30 gpm

Percent Reduction 87%

Disposal o f cooling tower blowdown Reuse of waters in an operation and education of operating personnel can reduce the total effluent by approximately 60 to 90%. How can this be utilized to approach zero discharge? There are several developmental processes in operation and under consideration.

=Solar Ponds. Solar ponds that also serve as fire water ponds to receive the discharge from the plant. These ponds are constructed in a series so that the first pond is used also as a settling basin for suspended matter and the flow from this pond to subsequent ponds gives the desired volume of water for fire protection purposes.

Discharge to Domestic Sewerage Systems. The greatly reduced effluent can now in some instances be combined with domestic sewage in a municipal/industrial waste treating plant. Further, the reuse o f water will remove a considerable amount of the components that are unaccept- able to a municipal treatment plant.

=Evaporation. The use of the evaporation can result in a concentration of up to 98% solids discharge with an equivalent 98% distilled water recovery. While the cost of this process is rated a t approximately $2.00 per 1,000 gallons, this recovered distilled water can be used for boiler makeup water and the resulting total credits from this method will lower the water cost to approximately 50d per 1,000 gallons.

By using this source of distilled water a reduction in the discharge of pretreatment effluent such as line softening residues and clarification residues also i s possible.

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NEW PRODUCTS Low-temperature waste crystallization Bulletin describes the use o f scrape crystallizers in waste control processes.. . E.g. Inorganic salts: Very often plants which run sulfonation reactions or other processes involving sulfuric acid, will have small streams rich in glaubers salt (sodium sulfate decahydrate). When sewer disposal is no longer a possibility, scraped crystallizers are well suited to stripping the glaubers salt out. Small-scale and low-temperature operation favor the cooling crystallizer over vacuum crys- tallizers which are inherently large volume, high-temperature devices. . . Often electrochemical plants produce waste streams which contain troublesome sodium sulfate, or carbonates, which can be easily separated at low tempera- tures. . , Organic chemicals: There are many purge or other streams in organic chemical plants which are rich in off-grade product, which are currently sewered. Many of these will yield to cooling crystallization. In most cases, the product recovered can be recycled into the process and represent a yield increase.. . Again, because o f the small quantities involved, scraped crystallizers are favored for the following reasons: To reach the low temperatures involved i t is often difficult to use a vacuum crystallizer, due to the very low vapor pressure, and high boiling point rise, of the mixtures. The equipment investment i s normally much lower for a small scraped surface crystallizer than for a vacuum crystallizer with i t s associated condensers, instrumentation, pumps, and high stream and cooling water consumption. The crystals produced by a small continuous scraped crystallizer will normally be better than those produced by a single-stage vacuum crystallizer in this type o f service.. Richard M. Armstrong Co.

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Simple newsprint as filter media Brochure describes a patented rotary vacuum drum filter that employs a con- tinuous sheet o f paper, the most common of which is ordinary newsprint, to jewater slurries previously proven either to be unmanageable or to require the Ase o f diatamaceous earth on conventional drum filters. . . the ‘Trommel’ filter :onsists of a perforated drum suspended in an open tank which contains the ilurry to be filtered. The drum is covered with newsprint (or other type paper filter) and rotates slowly through the slurry. The wetted paper seals against the :nds o f the drum to prevent solids leakage into the filtrate. Vacuum, which i s ipplied to the drum internally, causes the liquid portion (filtrate) of the slurry .O pass through the paper. Solids contained in the slurry are deposited on the laper. The paper containing the filter cake i s discharged from the drum by a ;arrier belt. The carrier belt is returned to the drum and fresh paper fed con- :inuously to it. A seal plate i s used in the area between the paper take-off and .he slurry level to prevent air leakage and vacuum loss through the exposed urface of the drum. The filtrate is carried through the drum drainline spokes, 1ollow center shaft and rotary joint-valve to the filtrate receiver. Technics/

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MAUE4JP EVAPORATION

POLVHETRICS REVERSE

Electroplating rinsewater treatment Bulletin 856 describes eleclroplatinl rinsewater treatment by reverse osmo sis.. . . Plating rinsewater is pumpec under pressure through the RO sys tem. The major portion of the wate permeates the membrane and is puri fied, while the remainder, togethei with the concentrated plating salts, i! utilized to flush the membrane and i! returned to the plating bath. * Po&. we trics

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Self dean i ng clams ‘CLAM’ (Cleansimatic Liquid Analysis Meters) is a family o f self-cleaning electronic instruments for measuring turbidity, suspended solids, and sludge levels. . . They consist of a sensor and a control unit and can measure con- centrations up to 10,000 ppm. . . Self- cleaning and detection principles: a mo to r driven reciprocating piston within a glass tube draws and expels a sample every 15 seconds while wiping the optical surface of the sampling chamber on each cycle. This method of continuous cleaning prevents inter- ference from dirt and slime accumula- tions on the sampling chamber and eliminates clogging o f the lines. , . The instrument operates on the principle of light scatter and/or adsorption to measure the suspended solids concen- tration of the sample, photocell out- put i s displayed as a linear response on the meter scale in ppm. Biospherics

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