CHAPTER 37COMMERCIAL, INSTITUTIONAL

AND RESIDENTIAL WATERTREATMENT

This chapter covers the special requirements in treating water for nonindustrialfacilities such as office buildings, hospitals, apartments, schools, universities,department store chains, hotels/motels, shopping centers, and commercial enter-prises such as laundries. The systems in such establishments that require watertreatment are heating, air conditioning, and domestic water.

In some respects the water problems encountered in buildings and institutionsare similar to those in industrial plants. But there are several important differ-ences. First, the consequences of improper water treatment may be even morecritical to a hospital or living complex than to a large industrial plant, sincehuman health, comfort, and even survival may be at stake. Second, the utilityservices are often required intermittently on an instantaneous demand basis, sothe system may be severely taxed physically. This is particularly true duringspring and fall heating seasons when heat may be required at night but not duringthe day; a steam heating system under these conditions is subject to oxygen cor-rosion during the idle daylight hours when condensate receivers draw in air, oftenaggravated by CO2 corrosion when heat is again called for and cold condensatereturns to the steam plant.

Designers of modern commercial, institutional, and residential facilities arewell aware of the importance of water treatment. They provide sophisticatedchemical feeding and control systems for these installations (Figure 37.1), whichare designed to use the same kinds of chemicals as industrial systems. The oper-ating personnel in charge are skilled in the operation of mechanical systems, andare well trained in proper chemical control testing.

Older, smaller facilities may lack good feed systems and skilled personnel. Thisrequires some modification of the chemical program: simple feeders are devisedto apply multipurpose chemical formulas, containing a variety of ingredients forcontrolling any of the problems that might be encountered (Figure 37.2).

HEATINGSYSTEMS

Heating systems using water are of two basic designs: (1) steam boilers and (2)hot water "boilers." Steam boilers are usually package-type firetube or watertube

FIG. 37.1 In most modern buildings, chemical treatment of waterfor heating or cooling is automated for simple, reliable control.

FIG. 37.2 Where there are multiple units tobe treated, a simple feeder charged with bri-quetted chemical is often more practical thanindividual mechanical feeders.

Drum LevelGauge

Chemical Pump

Wall BracKet.Pump Mtg.

55 GaI DrumNALPAC

WaterMake-up

115V,1o, 60Hz

Water Meterand Contactor

F. W. Tank

Process

SteamDrum

NaI QuM

BoilerFeed Pump

FIG. 37.3 A typical packaged steam generator with complete fireside andwaterside controls. (Courtesy of Cleaver Brooks, division of Aqua-Chem,Inc.)

design operating at less than 200 lb/in2 (13.8 bars) (Figure 37.3). However, insome of the more sophisticated installations, such as an energy supply complexfor a consortium of medical centers or a large university campus, there is a trendtoward cogeneration and high-pressure (900 to 1200 lb/in2) boilers. These arebeing installed as topping plants to produce electrical energy from turbines thatthen discharge steam at perhaps 100 lb/in2 to the campus central heating system.

Most smaller institutional boilers are usually gas- or oil-fired, but occasionallyan electrically heated boiler is found. Fuel oils can range from No. 2 to No. 6,including mixtures of several grades. Many boilers contain burners designed toburn either gas or fuel oil, a useful feature in areas where shortages of either fuelcan occur. Coal-fired boilers in commercial buildings and institutions are rare.

In heating applications the steam generated in these boilers is used in one oftwo ways: (1) directly; i.e., circulated throughout the building, where the heat isextracted through radiators and the condensate is returned to the boiler to be usedas feedwater; (2) indirectly; i.e., passed through a heat exchanger located relativelyclose to the boiler, with the condensate returning to the boiler. The other side ofthe heat exchanger is part of a closed heating water loop which extends throughoutthe building. The water heated by the steam is pumped around the loop, givingup its heat through radiators, mixing boxes, and other types of heat exchangers.

The steam generated in these boilers can have other uses, such as humidifica-tion, dishwashing, food preparation, and sterilization in autoclaves (as in hospi-tals and research laboratories). A careful study must be made of the uses to whichthe steam is put so that acceptable, safe treatment chemicals can be used. If anypart of the steam contacts food, for example, FDA-approved chemicals are

required. If the steam is used for humidification, OSHA guidelines for condensatecorrosion inhibitors must be followed and care must be taken that condensatecorrosion inhibitors be safe and odor-free. In some university, medical, orresearch centers, pure steam is needed in small quantities for these and other uses.Often the quantity needed at a research site or a hospital work station may beonly 100 to 200 Ib/h, a very tiny fraction of the 200,000 to 500,000 Ib/h generatedby the central boiler house. It is essential that the multimillion dollar steam-con-densate system be thoroughly protected against corrosion, yet the corrosion inhib-itors in the steam may be objectionable for these small critical uses. In that case,the institution must install a "pure steam generator," where needed, to convertplant steam to research-quality steam free of the inhibitors.

Water treatment in these steam plants is similar to that used in most industrialboilers. The chemical products used should control scale, corrosion, and foamingin the boiler, and corrosion in the condensate system. The types of products usedare similar to those used in industrial systems; however, they are often blendedand packaged so as to be convenient and easy to handle, feed, and control.

The condensate return system of a university complex is subject to unusualcorrosion problems because of the stress of seasonal changes. Because of this, inaddition to having a central chemical treatment station in the boiler house, sat-ellite systems are sometimes installed to supply booster shots of steam treatmentto correct for seasonal upsets. The quality of the condensate must be under con-stant surveillance, because often the steam heating coils in the individual build-ing's hot water supply system may fail, and the city water is then diverted backthrough the condensate return lines to the boiler plant. It is common practice toinstall ion-exchange-type condensate polishers in the boiler house to handle suchfailures and prevent having to dump large amounts of valuable hot condensatebefore the source of coil failure is found. Monitoring of conductivity at significantcondensate return junction points helps to locate and isolate the failure promptly.

Hot water "boilers" are really misnamed since they do not boil water and pro-duce no steam. They can heat water to 50O0F (26O0C), but typically operate attemperatures between 180° to 25O0F in a vessel almost identical to a steam boiler.The heated water is circulated through the building to various heat exchangersand radiators and back to the boiler.

In addition to hot water boilers, which use gas or fuel oil for heat, there arealso electric boilers. These use either enclosed, clad-type immersion heaters, orresistance heaters, which operate by a flow of current between electrodes in theboiler water, which is conductive because of its mineral content.

These systems are designed to be closed, so theoretically there should be nomakeup water required, but normally a small amount is required to replace thatlost by leakage of liquid or vapor. In these systems, the primary concern is pre-vention of corrosion. Scale formation, although not usually a major factor, canalso occur in hot water boilers. This may be caused by a combination of very hardwater, used to fill the system initially and to provide makeup, and the high tem-perature of the boiler tube surfaces. The gradual accumulation of calcium andalkalinity precipitates CaCO3.

In addition to corrosion and scaling, foaming can also occur. This can lead tosevere cavitation-erosion in recirculating pump impellers, caused by the impinge-ment of foam bubbles on the metal at high velocity. The preferred chemical treat-ment in hot water boilers is a borate/nitrite-based product. Such products containa mixture of corrosion inhibitors to protect steel, aluminum, copper, and admi-ralty; a scale control agent; and high- and low-temperature antifoams. This treat-ment is initially charged at high dosage levels for maximum protection. Since hot

water boiler systems have little makeup and the chemicals are relatively stable,only small additions of treatment are required thereafter to maintain protection.'

Occasionally, hot water systems require high makeup, caused by excessiveleaks of water or vapor from the system. In some cases these systems are veryextensive, such as in a university complex with a major portion of the systemburied underground. This makes repair of leaks costly compared to the cost ofmakeup water, so the system has to be treated accordingly. Maintaining high leveldosages of a borate/nitrite-based inhibitor becomes prohibitive economically.Under these conditions less expensive treatments are used. These still provideacceptable protection, but need more attention for control of chemicals. Sodiumsilicate treatments, combination hydrazine/amine programs and glassy phosphatetreatments fall into this category.

AIR-CONDITIONING SYSTEMS

Most large buildings, shopping centers, and similar installations use water-cooledair-conditioning units. These fall into three categories:

1. Reciprocating compressor units, usually electrically driven and using Freon*as the refrigerant (Figure 37.4).

FIG. 37.4 Packaged water chiller, using a reciprocating-type compres-sor. (Courtesy of The Trane Company.)

2. Centrifugal compressor units, driven by electric motor, steam turbine, or gasengine, also use Freon as refrigerant (Figure 37.5).

3. Absorption refrigeration units, in which the refrigerant (water) is absorbed inconcentrated lithium bromide solution and then evaporated by steam (Figure37.6).

In the reciprocating and centrifugal systems, the hot compressed refrigerantvapor first passes through a water-cooled condenser (usually a shell-and-tube-

* Registered trademark of the E. I. du Pont de Nemours & Co.

FIG. 37.6 An absorption-type refrigeration system. (Courtesy of TheTrane Company.)

FIG. 37.5 A centrifugal-type compressor is used in this*package refrigeration unit. (Courtesy of The TraneCompany.)

FIG. 37.7 Typical air-conditioner schematic.

The water used to cool the refrigerant in the condenser is itself cooled in atypical cooling tower, and then recycled back to the condenser. Typically, waterenters the condenser at 80 to 9O0F (28 to 320C), and leaves at 90 to 10O0F (32 to380C). This is known as the open recirculating portion of the cooling water sys-tem, since the cooling tower is open to the atmosphere.

The evaporator, or chiller, also a shell-and-tube-type heat exchanger, provideschilled water for air-conditioning systems. When the condensed refrigerant leavesthe condenser, it has been cooled, but is still at a high pressure. In the tube sideof the chiller, the Freon vaporizes by extracting heat from the relatively warm[550F (130C)] chilled water entering the shell side. This cools the chilled water toabout 450F (80C) for circulation throughout the building to provide air condition-ing. The air being cooled warms this water back to 550F (130C) for return to thechiller. The chilled water system is not open to the atmosphere and is known asa closed recirculating system (Figure 37.8).

In simple terms, the heat within a building is absorbed by the chilled water,which then gives up this heat to the refrigerant in the chiller. The refrigerant isthen compresed and passed into the condenser, giving up the heat to the openrecirculating cooling water. The cooling water then gives up this heat to the atmo-sphere as it evaporates while passing through the cooling tower. An air-condition-ing system is nothing more than a mechanism designed to move heat from insidethe building to the outside.

Steam absorption units are more complicated in their operation. They usewater as a refrigerant, a lithium bromide solution as refrigerant absorber, and

type heat exchanger) where it is cooled and condensed into a liquid. From thereit passes into an evaporator (commonly called a chiller) where it boils underreduced pressure into a cool vapor. The boiling liquid is used to extract heat andthe vapor is then compressed, repeating the cycle (Figure 37.7).

To coolingtower

•

Cold water

Hot water

Condenser

Hot gas (Freon)at high pressure

CondensedFreon athigh pressure

Chiller (evaporator)

Low-pressure gas(suction side)

Chilled water

Expansion valve

Compressor

FIG. 37.8 Schematic of all components of a complete air-conditioning system. Atcertain times of the year, the cooled water in the cooling tower basic can bypass therefrigerant system and go directly through the chilled water loop ("free cooling") ata significant cost savings.

steam as a heat source to evaporate the water (Figure 37.9). Absorption units usecondensers, cooled by water from a cooling tower, and chillers to provide chilledwater for air conditioning. From the water treatment point of view, the operatingprinciples of the open recirculating and chilled water systems are the same in allof the above systems, except that much higher temperatures and heat fluxes areinvolved.

In order for water-cooled air-conditioning systems to operate at design effi-ciency, careful attention must be paid to the condition of both the open recircu-lating water and the chilled water. The chemical makeup of water varies widely,depending on the source. Even water from the same metropolitan area can havewide variation. For example, Chicago's municipal water, obtained from LakeMichigan, has approximately 120 mg/L hardness, but many suburban commu-nities outside Chicago obtain their water from wells, typically with hardness inthe 300 to 500 mg/L range. In some areas of the country, such as the eastern

Pump

CHILLED WATER LOOP

Fan coil units

Chiller

Vapor tocompressor

Compressor Sealedmotor

80° F in

Pump

Condenser

Makeup water

Meter

Cooling tower

OPEN RECIRCULATING WATER LOOP

90°Fout

45°Fout

Pump andexpansion valve

Bleedoff

FIG. 37.9 Flow diagram of absorption refrigeration system.

seaboard and the Pacific Northwest, waters of exceptionally low hardness arecommon.

Problems that can occur on the water side of air-conditioning systems are:

1. Formation of scale, especially on heat transfer tubes2. Corrosion of the metals used in the system3. Microbial fouling4. Fouling from dirt and silt accumulation

As with industrial heat exchangers, scale and fouling deposits cause poor heattransfer through the exchanger tubes, reducing efficiency and increasing energycosts. The problem is worse in air-conditioning systems than in most applicationsbecause the temperature driving force is much lower than in most other heattransfer applications (see Table 37.1). Corrosion causes failure of the system parts,resulting in expensive repair or replacement.

Water treatment procedures for these systems are the same as for industrialsystems. Scale and corrosion control in open recirculating systems are maintainedeither by: (1) keeping the water in a nonscaling condition by using acid to lowerthe pH, and adding chromate to prevent corrosion or (2) keeping the water in analkaline condition, and preventing scale by the addition of organophosphate scalemodifying agents. To provide the required copper corrosion protection, organicnitrogen compounds are added.

In systems where concentration results in a pH of 8.5 or above and sufficientalkalinity is present, the addition of corrosion inhibitors may not be necessary.Corrosion coupons at critical spots in the system can be inserted to provide thedata to determine whether inhibitors are needed.

Solutionpump

Dilutesolution

Absorbingsolution

Coolingwater

Absorber

Heatexchanger

Refrigerant vapor

Concentratedsolution

Steam

Concentrator

Refrigerant vapor

Condenser

Condensercooling water

Refrigerant

Evaporator

4O0F chilled water toair conditioners

550F return

Refr igerant

Refrigerantpump

* Assumptions: (1) unit operating 8 h/day, 240 days/yr, (2) one ton of A/C consumes 0.7 kWh, (3)electricity costs 7.5C per kWh.

Some cooling towers are located in areas where appreciable quantities of air-borne dirt and silt are drawn into the recirculating water. These impuritiesbecome suspended in the water, and tend to settle out in areas of low velocity,such as in heat exchangers. In addition to impeding heat transfer, settled sludgecauses corrosion and encourages microbial activity underneath these deposits. Adispersant may be added to keep the particulates in suspension, until they areeventually removed from the system with the blowdown water. Dispersants areavailable as separate products, or they can be included in the formulation of mul-tipurpose products, which also contain scale and corrosion inhibitors.

Problems can be caused by aerobic slime-forming microorganisms, anaerobicbacteria, molds, yeasts, and algae. Slime-forming bacteria cause loss of heat trans-fer efficiency in condensers; in severe cases they can completely block the flow ofwater through the tubes.

Anaerobic bacteria generally grow under deposits in areas where no oxygen isavailable. These produce corrosive by-products such as hydrogen sulfide, whichcan eat holes in piping quite rapidly. Algae growth on tower decks and other areasexposed to sunlight can become so severe as to plug the drain holes on the towerdeck. The application of microbial control chemicals is essential to correct theseproblems.

Closed chilled water recirculating systems, like closed hot water systems, aregenerally not susceptible to scale formation, since little or no evaporation occursin a properly operating system. However, leaks and evaporation can result in scaleformation. The primary goals in treating these systems are to prevent metal cor-rosion, to control anaerobic microorganisms, and to prevent foaming and ironfouling.

The preferred chemical treatment for corrosion, scale, and foaming is the sameas for hot water boilers—a borate/nitrite-based formulation containing scale con-trol agents and antifoams.

Life cannot exist in hot water systems, because of the high temperaturesinvolved; but microbes can and do grow in chilled water systems. Typical exam-ples are nitrate-reducing and sulfate-reducing bacteria. The end result of the activ-ity of these microbes is corrosion of the system metal, which can become severe.Proper control is maintained by periodic analysis of the water and by the appli-cation of biocides when necessary.

In colder climates, the addition of ethylene glycol antifreeze solutions to bothhot and chilled water systems is common. The chemical treatment selected mustbe compatible with the antifreeze. For example, borate/nitrate-based inhibitorsare usually compatible, while chromate-based inhibitors are not.

A/Ctonnage

500100025005000

Light0.001 fouling factor

(X84 in)

$ 5,10010,20025,50051,000

Scale thickness

Moderate0.002 fouling factor

(& in)

$ 11,10022,20055,500

111,000

Heavy0.003 fouling factor

(/32 in)

$ 15,60031,20078,000

156,000

TABLE 37.1 The Cost of Energy Loss Caused by Scale in an Air-Conditioner Condenser*

FIG. 37.11 An automated feed and controlsystem for an air-conditioner cooling tower.

An example of an institutional air-conditioning system is shown in Figure37.10, representing a typical hospital installation. The completely automatedchemical feed system for the open cooling tower is shown in Figure 37.11.

ENERGYSTORAGE

The public utility practice of offering a special discount on electrical costs to cus-tomers able to use off-peak energy has brought a new factor into the economicsof air-conditioning operations called "load-leveling." One way to take advantageof off-peak electrical rates for comfort cooling is to manufacture ice during thecheaper off-peak hours and to melt it during peak periods. Another method is toprovide storage of refrigerated water in large, insulated reservoirs which may holda 12-h supply of chilled water. Taking advantage of this method of electrical cost

FIG. 37.10 These two large centrifugal chilling machines provide for the air conditioning of anentire hospital. (Courtesy of Carrier Corporation.)

saving, however, requires special consideration of the chemical treatment pro-gram to prevent microbial attack and concentration cell corrosion beneath thesediment that invariably forms in large, relatively quiescent reservoirs.

DOMESTIC WATER

Domestic water is that used for drinking, cooking, bathing, washing of dishes andclothes, toilet flushing, and lawn or garden sprinkling. It is usually municipalwater but may be from private wells. Because of the large size of modern build-ings, hot water is often recirculated throughout the building from large holdingtanks.

Domestic cold water problems are limited to corrosion of the water piping andholding tanks. Domestic hot water problems include both scale and corrosion.Tastes and odors may be the cause of occasional complaints, but these are notmajor problems because they do not involve equipment damage and they applyto a very small percentage of the total water used.

The corrosion in both hot and cold systems is caused by (1) water that is nat-urally corrosive as supplied to the building, and (2) the use of a zeolite softener,which replaces Mg and Ca ions with Na ion, creating a more corrosive soft water.

Scale in hot water systems occurs in the hot water heater tubes, and is causedby the precipitation of CaCO3, which becomes less soluble as the temperaturerises. The EPA has responsibility for granting approval of those chemical products

FIG. 37.12 Typical packaged sewage treatment plan used by institutions and commercialcenters not served by a municipal sewage plant. (Courtesy of Smith & Loveless Division, Eco-dyne Corporation.)

Second stage aerationchamber

Comminutor,

Rawsewagemlet

Airdiffusers

First stage aerationchamber for raw sewage

Aerated wasteo f f t a k e to f ina lc la r i f ie r

Treatedeffluent

Shell off ina l clari f ier

Aerobicsludgedigestionchamber

.Airdiffusers

Return sludgel i f t controls Air hf t

to decantsupernatant tof i rs t stage

that are safe to use in potable systems. In potable water systems within buildings,the accepted practice is to use only those chemicals that are approved for use inmunicipal water supplies. These chemicals are usually polyphosphates, sodiumsilicates, or combinations thereof. These products provide both scale and corro-sion protection. These chemicals are applied through the simple feeder shown inFigure 37.2.

WASTE TREATMENT

In the building and institutional field, most individual installations dischargetheir wastes into the municipal sewer system. In some water-short areas, such asLos Angeles, noncontaminated wastes may be segregated from contaminatedwastes, such as domestic sewage, for recovery and return to the potable watersource.

There are certain commercial operations, such as laundries, where strongwastes are given special treatment before discharge. In the case of laundries, thecost of special treatment is partly offset by heat recovery and use of rinse water inthe washing cycle. The recovered water contains both valuable heat and unspentdetergent. Some institutions, such as hospitals and hotels/motels, operate theirown laundries, but dilution with other wastewater is usually adequate to producea combined effluent meeting local discharge standards, which usually apply tooils, pH, BOD, and suspended solids.

Some shopping centers, resort hotels, hospitals, and similar establishmentsmay not have access to municipal sewers. These are required to install waste treat-ment facilities to meet the same standards applied to municipal sewage planteffluents. An illustration of a complete packaged sewage treatment facility servingsuch an installation is shown in Figure 37.12.