chapter 22 control of microbial activity

CHAPTER 22CONTROL OF MICROBIAL

ACTIVITY

All water treatment processes are affected by the presence of microbes. Many oxi-dation-reduction reactions are biologically mediated. In most cases, microbialeffects are detrimental to the water-using process or system. However, certainindustrial operations put microbes to work in a useful way: the activated sludgeprocess uses microbes for digestion of organic wastes; microbes are used for thefermentation of beverages; microbial enzymes are useful for leather processing;and bacteria are used in the recovery of metal values—especially copper—fromtailings, the residue of mineral beneficiation processes.

Disinfection of municipal drinking water and sterilization of food processingand hospital equipment are examples of applications of biocides (chemicals toxicto microbes) where the goal is to kill all microbes. However, in the treatment ofnonpotable water, a complete kill is often costly and not always necessary. Cool-ing water in utilities, steel mills, refineries, and other industrial plants is treatedto control microbe populations at levels that experience has proven to be tolerableto the system without complete sterilization. Papermaking systems, unlike cool-ing water systems, are designed to operate with large amounts of suspended solids,so the tolerable levels of microbe populations are considerably higher than forcooling water. The tolerable microbe count in a paper mill varies with the type ofpaper being made and machine operating conditions, such as pH andtemperature.

Planning an effective microbial control program for a specific water treatmentprocess requires an examination of:

1. The types of organisms present in the water system and the associated prob-lems they can cause.

2. The population of each type of organism that may be tolerated before causinga significant problem.

Typical microbes encountered in water treatment and the problems they causeare summarized by Table 22.1.

Bacteria, the largest group of troublesome organisms, cause the most variedproblems. They are usually classified in water treatment by the types of problemsthey cause: slime-forming bacteria, iron-depositors, sulfate-reducers, and nitrify-ing bacteria. Each group has its preferred environment and thrives in specificareas of a water system. Aerobic bacteria, for example, require oxygen, so theyare found in aerated waters such as in a cooling tower basin or white water in a

paper machine wire pit. Anaerobic bacteria, on the other hand, don't use oxygenand obtain their energy from reactions other than the oxidation of organic sub-stances. The reduction of sulfur in sulfate to the sulfide ion is an example. Sinceanaerobes don't need oxygen, they are found in oxygen-deficient areas, such asunder deposits, in crevices, and in sludges.

Iron-depositors occur in water high in ferrous iron, which they convert toinsoluble ferric hydroxide and which becomes part of the mucilaginous sheatharound the cell. These deposit and accelerate corrosion rates, which producesadditional soluble iron, further increasing the population of iron-depositors in thesystem. The cycle accelerates until the whole system is plugged with iron deposits(Figure 22.1).

Nitrifying bacteria oxidize ammonia to nitrate. This nitrification reactionsometimes occurs in iron removal filters, accompanied by a reduction of oxygenand pH. These bacteria are often found in ammonia plants where leakage of

TABLE 22.1 Typical Microorganisms and Their Associated Problems

Type of organism

A. Bacteria1 . Slime-forming bacteria

2. Spore-forming bacteria

3. Iron-depositingbacteria

4. Nitrifying bacteria

5. Sulfate-reducingbacteria

6. Anaerobic corrosivebacteria

B. FungiYeasts and molds

C. Algae

D. Protozoa

E. Higher life forms

Type of problem

Form dense, sticky slime with subsequent fouling.Water flows can be impeded and promotion ofother organism growth occurs.

Become inert when their environment becomeshostile to them. However, growth recurswhenever the environment becomes suitableagain. Difficult to control if complete kill isrequired. However, most processes are notaffected by spore formers when the organism isin the spore form.

Cause the oxidation and subsequent deposition ofinsoluble iron from soluble iron.

Generate nitric acid from ammonia contamination.Can cause severe corrosion.

Generate sulfides from sulfates and can causeserious localized corrosion.

Create corrosive localized environments bysecreting corrosive wastes. They are alwaysfound underneath other deposits in oxygendeficient locations.

Cause the degradation of wood in contact with thewater system. Cause spots on paper products.

Grow in sunlit areas in dense fibrous mats. Cancause plugging of distribution holes on coolingtower decks or dense growths on reservoirs andevaporation ponds.

Grow in almost any water which is contaminatedwith bacteria; indicate poor disinfection.

Clams and other shell fish plug inlet screens.

ammonia into cooling water encourages their growth. A pH drop caused by theconversion of ammonia to nitrate is often the clue to their presence.

Sulfate-reducing bacteria are found in many systems subject to deposit prob-lems. The sulfides produced are corrosive to most metals used in water systems,including mild steel, stainless steel, and aluminum. Evidence of the sulfate-reduc-ers is the unique pit etched on the metal surface, sometimes in the form of con-centric rings (Figure 22.2).

Many bacteria secrete a mucilaginous substance that encapsulates the cell (Fig-ure 22.3), shielding it from direct contact with water, so that the cell is protectedfrom simple toxic biocides. Control of encapsulated bacteria usually requires bothoxidation and dispersion of the protective sheath so that the biocide can reachthe cell.

FIG. 22.3 A biofilm in development. Biofilm is the mucus-like coating produced by slime-forming bacteria. Organisms are Pseudomonas aeruginosa at 7000 X.

FIG. 22.1 Iron-depositing bacteria initiatedthe tuberculation attack on this steel distribu-tion pipe.

FIG. 22.2 The development of pits filled withvoluminous products of corrosion from theattack of sulfate-reducing bacteria on steelpipe.

Yeasts and molds can live on dead or inert organic matter. Fungi are oftenfound on wooden structures, such as cooling tower fill and supporting members,and sometimes under bacterial or algal masses. Fungal attack of wood usuallymeans permanent loss of strength of the wood structure, so protection of the woodrequires control of fungi from the time the structure is put into service. Periodictesting of the wood to determine its resistance to fungal attack is an importantmaintenance step. Very thin sections of wood specimens taken from susceptiblelocations are examined to determine the extent of attack if there has been any(Figure 22.4).

With few exceptions, algae need sunlight to grow, so they are found on open,exposed areas, such as cooling tower decks or on the surface of reservoirs, ponds,

FIG. 22.4 Microtome of wood section from a cooling tower,showing fungal attack.

and lakes. Most algae grow in dense, fibrous mats that not only plug distributionpiping and flumes, but also provide areas for subsequent growth of anaerobic bac-teria under the algae deposits.

A century ago, a Danish biologist, Christian Gram, developed a method ofstaining bacterial cultures as a means of separating them into two broad categoriesas an aid to identification: those that retain a blue color produced by an iodinetreatment are called Gram-positive; those not retaining the blue color and accept-ing a red dye following iodine treatment are Gram-negative. Most aquaticmicrobes are Gram-negative. They all are negatively charged colloids (have a neg-ative zeta potential), a property not related to the Gram staining technique.Because they are negatively charged colloids, they are affected by cationic poly-mers and biocides.

PHYSICAL FACTORS AFFECTING MICROBE GROWTH

Many species of microbes indigenous to soil, water, and vertebrate organismsthrive in a rather broad temperature range of 10 to 450C. Nature has producedselect organisms that can live at temperatures as low as O0C and as high as 10O0C.Higher temperatures kill all common microbes, but scientists report finding lifein hot springs and adjacent to ocean vents on the sea floor at temperatures of over20O0C.

Denaturation of proteins, which causes coagulation within the cell, occurs attemperatures below 7O0C. Commercial pasteurization is a denaturation process.Milk is usually pasteurized at 630C by holding that temperature for 30 min; if thetemperature is raised to 720C, pasteurization is completed in only 15 s. This pro-cess kills all disease-producing (pathogenic) organisms but does not produce ster-ile milk; some microbes remain to cause the milk to spoil in time. Most activelygrowing microbes of interest in water treatment technology are killed at 7O0C inless than 5 min. Although pasteurization has a long history of success in foodprocessing, it has never been reported in use for water disinfection, except in occa-sional emergencies where a community water supply may be contaminated andthe public is warned to boil all drinking water until the crisis has passed.

Maintaining low temperatures is not an effective means of killing microbes. AtO to 50C, organisms become dormant. Freezing kills many cells, but those thatsurvive are capable of complete recovery from the shock. One procedure used topreserve microbes involves freezing cells rapidly at -7O0C and then removingthe ice crystals as vapor (sublimation). This process is lyophilization.

Dry heat results in dehydration of all cellular matter and oxidation of intra-cellular constituents. Sterilization of laboratory media is usually carried out in anautoclave at 1210C. Sterilization of glassware is done with dry heat at 16O0C fortwo hours.

Moisture is required for microorganisms to grow actively. Many species ofpathogenic organisms are killed quickly by drying. However, organisms in thespore or cyst state can survive low moisture environments; and, if transported bywind or animals to a location where moisture levels suit them, they revive andform new colonies. To prevent attack by microbes, lumber and other vulnerablematerials are dried to less than 20% moisture content.

Organisms containing chlorophyll are able to use the radiant energy of the sunor artificial lighting to convert CO2 to carbohydrates, which they need for cellsynthesis. However, not all radiant energy is useful to the cell and certain fre-quencies of radiation are harmful. Radiation is therefore one method of microbecontrol.

Short-wavelength forms of energy, such as gamma rays (0.01 to 1 A) and x-rays (1 to 100 A) are particularly useful. These create free hydrogen and hydroxylradicals and some peroxides when they pass through the cell, causing cell damageor death. These forms of radiation are hazardous. Energy in the ultraviolet region(1000-4000 A) is also useful for killing microbes. In this case, the energy isabsorbed by the nucleic acids, creating chemical reactions that are lethal to thecell. This form of energy, however, has poor penetrating ability, so the use ofultraviolet light for disinfection requires a treatment unit of special design so thatthe energy does not have to penetrate deeply into the water. Ultraviolet steriliza-tion uses about 0.2 kWh of electric energy per thousand gallons of water treated(0.05 kWh/m3), so it is economically attractive in situations where the microor-

ganisms are not shielded by large agglomerated masses or by suspended solids.This method of disinfection is widely used in ultrapure water systems.

Osmosis is the diffusion of water through a semipermeable membrane sepa-rating two solutions of different solute concentrations. The water flows in a direc-tion to equalize the concentrations. When microbes are placed in 10 to 15% saltsolutions or 50 to 70% sugar solutions, the water inside the cells is extracted bythe surrounding medium. This dehydrates the cells so they are unable to grow orare killed. This technique is used commercially to preseve food. Bees use thisprinciple to preserve honey, concentrating it by fanning the comb with theirwings.

The interfaces between a liquid and a gas (such as surrounding a bubble of airin water), between two liquids (oil droplets in water), and between a solid and aliquid (sand grains in water) are characterized by unbalanced forces of attractionbetween the molecules of water at the surface and those in the fluid body. Theseforces are closely associated with the metabolic processes of the microbe. The cellmust be able to accumulate nutrients at its surface for assimilation, and wasteproducts must be eliminated from the cell and carried away. Therefore, thegrowth and well-being of a cell are influenced by surface forces in the surroundingaquatic environment. Substances having surface tension depressing effects (sur-factants) tend to have a detrimental effect on microbes if the concentration is highenough. These materials can alter cell division, growth, and survival. Surfactantsare often used to increase the effectiveness of biocides by dispersing cell coloniesand protective sheaths to allow the toxicant to contact the cells. Some toxicantsare themselves surface active (such as phenols and quaternaries) and tend to accu-mulate on cell surfaces by adsorption. This prevents entrance and utilization offood substances by the cell. Quaternaries will sometimes cause leakage of cellularmaterial out of the cell wall by changes in surface tension at the membranesurface.

CHEMICAL FACTORS AFFECTING MICROBIALGROWTH

Microbes have been found to exist in the broad pH range of 1 to 13. However,the most common microbes associated with water—algae and bacteria—usuallymaintain their internal pH at 7, so they prefer a neutral aquatic environment.

Generally, yeasts and molds favor depressed pH, in the range of 3 to 4. Dilutealum solution is sometimes contaminated by fungi, causing the plugging of alumfeed lines and rotameters while at the same time the solution is free of bacterialgrowth.

Bacteria and fungi can both contribute to industrial problems over a pH rangeof 5 to 10. Other chemical factors, discussed in earlier chapters, include the pres-ence of organic matter to serve as food for the microbes, and a supply of the com-mon nutrients such as nitrogen and phosphorus required for cell metabolism.

One of the surprising facts of microbe life is that there is such a profusion andvariety of forms that some can almost always be found that will resist damage by,or even thrive on, chemicals that are toxicfto animal and plant life. For example,phenol was one of the early chemical biocides used for sterilization in medicalpractice, yet at low concentrations—up to 100 mg/L, which is sometimes foundin coke plant wastes—it is readily digested in activated sludge waste treatment

plants. Similarly, some bacteria thrive in wastewaters that contain herbicides, pes-ticides, cyanide, arsenic compounds, and a variety of other chemicals normallyconsidered toxic.

METHODS FOR CONTROLLING MICROBIAL ACTIVITY

For practical reasons, in most industrial water systems, only limited use can bemade of the physical conditions that inhibit or destroy microbial life. For exam-ple, heating water may control microbial activity, but if the water is used for cool-ing purposes, this is not useful. Radiation is sometimes used, but its adoption ona widespread basis would require the development of more efficient energysources and better designs of equipment to expose the water to the radiant energy.Among chemical conditions that might be used for microbe control, pH is theonly likely candidate for practical results. Even this is limited unless the systemwater can be kept at a pH over 10. However, pH does have important effects onthe performance of biocides, as shown later when chlorine reactions are discussed.

Contact time, min

FIG. 22.5 Relationship of time versus concentration of HOCl required for 99%destruction or inactivation of a common bacteria (Escherichia coli) and a virus, at 0°to 6° C. (From Water Pollution Control Federation Publication Deeds & Data, Jan-uary 1980.)

Ava

ilabl

e ch

lori

ne

as H

OC

I,m

g/L

Since neither physical nor aquatic chemical conditions can be changed in apractical way to control microbial growth, toxic chemicals must be applied as bio-cides. The two commonly used types are oxidizing and nonoxidizing. Regardlessof which type is used, there is a relationship for all chemical biocides thatexpresses effectiveness, measured as percent kill or inactivation, concentration ofbiocide applied to the water, and time of contact of the biocide with the organismor virus. This was first discovered by H. Chick in 1908 and further developed byH. E. Watson in the same year, and is now designated as the Chick-Watson law:

N/N0 = exp(-^rt)

where TV0 represents the bacterial population at the time zeroTV is the reduced population at time t, after biocide application

exp = base of natural logarithm system, 2.718k is a rate constantc is the concentration of the biocide, mg/Ln is an empirical value/ is the time of contact, min

As an illustration of this law, Figure 22.5 shows the concentration versus timerelationship for application of chlorine to water for destruction of Escherichiacoli, the coliform bacteria used as an indicator organism in evaluating disinfectionof municipal water supplies, and for inactivation of polio virus 1. This graph istypical of all biocidal charts, demonstrating the Chick-Watson law. In each case,the graph is specific for both the organism and the water supply, as there may beside reactions of the biocide with other constituents of the water.

OXIDIZING BIOCIDES (SEE ALSO CHAPTER 19)

Chlorine gas, the chemical biocide most commonly used in the United States,hydrolyzes rapidly when dissolved in water according to the following equation:

Cl2 + H2O - H + H - Cl- + HOCl (1)

Hydrolysis occurs in less than a second at 650F (180C). Hypochlorous acid (HOCl)is the active ingredient formed by this reaction. This weak acid tends to undergopartial dissociation as follows:

HOCl - H+ + OCl- (2)

This reaction produces a hypochlorite ion and a hydrogen ion. Depending onpH and concentration, chlorine in water exists as free chlorine gas, hypochlorousacid, or hypochlorite ion. Figure 22.6 illustrates the distribution of these compo-nents at varying pH values. Above pH 7.5, hypochlorite ions predominate, andthey are the exclusive form when the pH exceeds 9. The sum of the hypochlorousacid and hypochlorite ions is defined as free available chlorine. Hypochlorite saltssuch as calcium hypochlorite ionize in water to yield these two species, dependingon pH.

Ca(OCl)2 - Ca2+ + 2OCr (3)

2OCr H- H2O — HOCl H- OCT H- OH~ (4)

FIG. 22.6 Distribution of hypochlorite andhypobromite in water as affected by pH.

Chemicals other than chlorine gas that liberate hypochlorite ions are comparedto one another in oxidizing power on the basis of "available chlorine." The avail-able chlorine values of a number of disinfectants are shown in Table 22.2.

Chlorine is a strong oxidizing agent capable of reacting with many impuritiesin water including ammonia, amino acids, proteins, carbonaceous material, Fe2+,Mn2+, S2-, and CIST.

The amount of chlorine needed to react with these substances is called thechlorine demand. Chlorine reacts with ammonia to form three differentchloramines:

HOCl + NH3 — NH2Cl (monochloramine) + H2O (5)

NH2Cl + HOCl — NHCl2 (dichloramine) + H2O (6)

NHCl2 + HOCl — NCl3 (trichloramine) + H2O (7)

These chloramine compounds also have biocidal properties; they are referred toas the combined residual chlorine. In general, the chloramines are slower actingthan free residual chlorine, but have the advantage of being more effective at pHvalues above 10. Chloramines may also be more persistent in a water system.

Breakpoint chlorination is the addition of sufficient chlorine to satisfy the chlo-rine demand and produce free residual chlorine. When breakpoint chlorination isused, the ammonia nitrogen content is destroyed and the residual chlorineremaining will be almost entirely free available chlorine (Figure 22.8).

Thus, the same equilibria are established whether elemental chlorine or hypo-chlorite is used for chlorination.

Liquefied chlorine is available in bulk or in cylinders. A typical installation forfeeding chlorine is shown in Figure 22.1a and b.

Per

cent

u

n-i

on

ize

d fo

rm

(HO

CI

or

HO

Br)

Pe

rce

nt

ion

ize

d fo

rm

(OC

I or

OB

r )

FIG. 22.7 (a) Typical chlorination system. In this plant, eight chlorinators eachmeter up to 8000 Ib Cl2/day to municipal sewage plant effluent. Each is suppliedby a chlorine evaporator (in background, right). (Courtesy of Wallace & Tier-nan.) (b) Ton cylinders of chlorine are mounted on a scale to monitor consump-tion by weight loss, and the discharge is converted to Cl2 gas by the evaporatoron the right. The chlorine supply room is isolated from operators in the controlroom. (Courtesy of Wallace & Tiernan.)

C h l o r i n e appl ied, mq/l(b reakpo in t can be es t imated at 10 X NH3, mg/1)

FIG. 22.8 Breakpoint chlorination curves showing reaction of Cl2 with N-compounds.

the cell. The superiority of HOCl over OCl may be due to the small molecularsize and electrical neutrality of HOCl, which allow it to pass through the cellmembrane.

On-site production of hypochlorite from seawater or brine is becoming popu-lar as it limits the exposure of operating personnel to chlorine gas or hypochloritecompounds. An installation of such a system is shown in Figure 22.9.

Chlorine reacts with a variety of organic materials, and attention is beingdirected to the presence of chlorinated compounds in water thought to be pro-

Material

Chlorine gas (Cl2)Chlorine dioxide (ClO2)Hypochlorites (OCl)

Calcium, HTH, Ca(OCl)2

Sodium, NaOClIndustrial gradeDomestic grade

Lithium, LiOCl, laundry gradeChlorinated isocyanuric acid (CONCl)3

Percentavailable Cl2

100263

70

12-153-5

3585

Chlorine also reacts with organic nitrogen in water. This is found in compo-nents of living cells, protein, polysaccharides, and amino acids. The toxicity ofchlorine is thought to be derived not from the chlorine itself or its release of nas-cent oxygen, but rather from the reaction of the HOCl with the enzyme system of

TABLE 22.2 Available Chlorine of Chlorination Chemicals

Breakpoint : Left of breakpoint Cl2 is combined;to the r igh t , it is free

This curve is typical of contaminated water supply

Theoret ical

This curve is typical ofc lean water supply

Chlo

rine

resi

dual

,mg/

1(a

fter

fixed

per

iod,

15m

m mi

nimum

)

FIG. 22.9 An electrolytic cell designed to produce chlorine from brine. Thisunit is generating Cl2 from seawater for chlorination of sewage planteffluent. (Courtesy of Electrolytic Systems Division, Diamond Shamrock.)

For this reason, the electric utility industry has studied chlorine minimizationtechniques and alternatives to chlorination to meet the strict discharge limitswithout jeopardizing condenser performance. One such alternative is an activatedbromide program: a chlorine-bromide mixture produces oxidant species that pen-etrate the biofilm; chlorine activates a bromide compound, according to thereaction

HOCl H- Br" -* HOBr H- Cl" (8)

The Cl: Br ratio can be varied to produce a Br2 residual, a Cl2 residual, or a mix-ture, as conditions may vary and require a ratio change. With high ammonia lev-els, the bromamines that form degrade more rapidly than do chloramines, so theyare less persistent in the environment. The combination reduces the total residualchlorine and aids in compliance.

Like chlorine, bromine residuals exist either as unionized or ionized species inwater, as seen in Figure 22.6. At pH 7.5, 50% of the available chlorine is presentas HOCl, with the balance as OCl". With hypobromous acid, at pH 7.5, over 90%of the oxidant is present as HOBr, the more active form, just as HOCl is moreactive than OCl", as mentioned earlier. These curves demonstrate why bromineresiduals provide better biocidal performance than chlorine in systems operatingat higher pH values. In an 8-month field study, a utility system treated with theactivated bromide-chlorine blend required only about 10% of the oxidant used bya similar unit treated with chlorine alone, and in this period, the 0.2 mg/L chlo-

duced by chlorination. Chloroform is one of these materials. Because of concernfor the potentially adverse physiologic effects of these chlorinated compunds, reg-ulatory agencies are severely restricting chlorine applications to large effluentflows. For example, treatment of utility station condenser water with chlorinemay be limited to a total period of only 2 h/day at residuals averaging not over0.2 mg/L Cl2 (EPA limit mandated July 1, 1984).

rine residual specified by the EPA was never exceeded. (Note: The conventionaltotal chlorine test reacts with both chlorine and bromine.)

Chlorine dioxide, ClO2, is used to a limited extent in water treatment for thecontrol of taste and odor problems and for the degradation of phenol. It is usedextensively in the pulp and paper industry for bleaching. This compound mustbe generated from the reaction of chlorine with sodium chlorite as shown:

2NaClO2 + Cl2 - 2ClO2 + 2NaCl (9)

Generally an excess of chlorine is used to drive the reaction to completion. Forsafety and to preserve its stability, the material is generated on-site. Since chlorinedioxide does not react with ammonia, it is useful in systems containing ammonia.

The next most common oxidizing biocide is ozone, O3. This is in common usecommercially throughout Europe in preference to chlorine and is finding growingacceptance in certain municipalities in the United States for disinfection of pota-ble water. It is also used in certain waste treatment applications to avoid the resid-ual chloramines that result from the usual chlorination of wastewater effluent.Ozone is produced on-site by an electric corona discharge through air or oxygen.A typical ozone generator is shown in Figure 22.10.

Oxidizing biocides such as chlorine, hypochlorites, and organochlorine mate-rials will kill all organisms in the system quickly, if the free chlorine comes intodirect contact with the organisms long enough and at a strong enough dosage

FIG. 22.10 A battery of three 110 Ib/day (49 kg/day) ozonators installed in a municipal plant.These produce O3 from air. (Courtesy oflnfilco Degremont, Inc.)

level. They also retain their effectiveness because organisms cannot adapt to orbecome resistant to chlorine.

However, oxidizing biocides also react with contaminants like H2S, NH3, pulplignins, wood sugars, and other organics. This increases the amount of chlorinerequired for biocidal effects. They are not persistent, and they decay quickly afterthe chemical feed stops. They do not penetrate slime masses, so they do not reachsubdeposit microbes.

Thus oxidizing biocides require complementary treatments to improve theireffectiveness. These include biodispersants to remove existing slime masses andto prevent organisms from settling on heat transfer surfaces; penetrants to per-meate organic masses and to expose and kill subsurface organisms; and biocidesfor control of organisms in systems contaminated with H2S, NH3, and otherreducing agents.

NONOXIDIZING BIOCIDES

Nonoxidizing biocides offer a way to control microbial activity in systems incom-patible with chlorine, such as water systems high in organic matter or ammonia.With few exceptions (e.g. copper sulfate), they are organic chemicals. They pro-vide the following features:

1. Activity independent of pH2. Persistency3. Control of organisms such as fungi, bacteria, and algae

Since all of these benefits are usually not available from a single penetratingbiocide, individual ingredients are formulated into proprietary products designedto increase overall performance in very specific applications, e.g., paper machinesystems, open cooling water systems, and process water in food plants. Slug feedis the preferred method of application.

Just as ammonia, sulfides, and reducing agents can interfere with the perfor-mance of oxidizing biocides, certain substances can reduce the effectiveness ofnonoxidizing biocides; e.g., cationic biocides may react with anionic dispersantsto cancel the effectiveness of both.

Organic Compounds

Methylene-bis-thiocyanate (MBT), (SCN)-CH2-(SCN), is a well-known organo-sulfur biocide. It is usually recommended for applications in paper mills and cool-ing systems where effluent limitations are strict, and where control of slime-form-ing bacteria is the main problem. Holding time and pH affect the half-life of MBT,which hydrolyzes in water to form less toxic substances.

Figure 22.11 illustrates the pH dependency of the half-life of MBT. At a pH of11, it is destroyed in seconds.

A continuous feed of 1 mg/L to the makeup of a cooling system will controlorganisms entering with the water. If an alkaline cooling water treatment programis being used, the relatively higher pH will cause the MBT to hydrolyze faster. Inmost cooling systems, the molecule will eventually be destroyed if holding timeis sufficiently long. Most of the degradation products are volatile and are strippedin the tower.

Time, hours

FIG. 22.11 Degradation of methylene-bis-thiocyanate at pH 6, 7, 8, and 9.

Dibromonitrilopropionamide (DBNPA) is also excellent where antibacterialactivity and environmental acceptability are needed. DBPNA kills quickly, thendecomposes to nontoxic compounds.

Chlorinated phenols are highly effective against most common organisms,especially fungi and algae. Tower fill is sometimes sprayed with chlorinated phen-ols to increase the lumber's resistance to fungal attack. These compounds areeffective biocides when fed directly to cooling water. However, because of toxicityand danger to the environment, chlorinated phenols are banned in the UnitedStates and many other countries.

Organotriazine compounds, such as isothiazolones, are broad spectrum non-oxidizing biocides, particularly effective against bacteria, and active over a widepH range. Isothiazolones are not inhibited by most organic and inorganic contam-inants found in cooling waters, and are compatible with ionic and nonionic dis-persants. They can be used with any of the oxidizing biocides.

A typical example of the organometallic group of biocides is the compoundbis-tributyl tin oxide (TBTO), (H9C4)3=Sn-O-Sn=5(C4H9)3. This compound isused to control fungi and algae. It also has a tendency to adsorb on surfaces inthe system, especially on wood, and therefore analyses usually do not show tinlevels as high as would be expected from dosage calculations. This adsorptionprovides for residual algae and fungi control after treatment is discontinued.Although its toxicity is greatest on fungi and algae, it also provides good controlof anaerobic corrosive bacteria.

CA TIONIC BIOCIDES

If penetration of slime and algae masses is desired along with persistency, thenamines and quaternaries (quats) are usually applied. The use of amines or quatswith chlorination usually permits the dosage of chlorine to be reduced.

A study to assess the effect of MBT on biological sewage treatment programsindicated that 0.5 to 2.0 mg/L had no measurable effect on BOD and suspendedsolids removal. So it is feasible to treat a system with methylene-bis-thiocyanateto control microbes and still have an effluent amenable to biological treatment.

Pe

rce

nt

MT

re

ma

inin

g

Many of these biocides are surface-active and therefore can disperse slimemasses. This allows chlorine and other toxicants to contact organisms which,under normal circumstances, they would not harm. The wettability of organicslime masses is increased so that the toxicants can reach under the mass to get atanaerobic corrosive bacteria.

In a typical amine program in a cooling tower system, a once-per-month slugof an amine biocide at 75 mg/L for 48 h is used to supplement continuous chlo-rination at 0.5 to 1.0 mg/L free residual for 6 to 8 h/day.

METALLIC COMPOUNDS

The detection of mercury in aquatic organisms in the late 1960s—and especiallyin tuna and other edible fish—as a result of mercury discharges from chloralkaliplants alerted the public to the potential threat of toxic metals in the environment.As a result of subsequent studies of the toxicity of heavy metals, some that hadbeen used as biocides (mercuric salts) and others that were not used as poisonsbut had simply found their way into the environment indirectly (such as leadaklyls in gasoline), these compounds came under scrutiny and were withdrawnfrom the market or had their use greatly curtailed.

One of the most common of the metallics used as a biocide is copper in ionicform. It has been applied as copper sulfate to ponds and reservoirs for many yearsfor control of algae at a concentration usually below 1 mg/L. Its solubility falls offrapidly as pH increases, so a chelating agent, such as citric acid, is often appliedto improve the effectiveness of treatment. Since algae have their individual sea-sons for blooming, the copper sulfate is usually applied only seasonally, and itseffective dosage is below the concentration permitted in potable water. The mech-anism of action against algae is not clear, but copper ions form complexes withamines; the effectiveness of copper ions is probably a result of their reaction withthe essential amino acids. Toxic metals can be transported across the cell mem-brane more readily in nonionic form, so a variety of neutral metallo-organic com-pounds have been developed to increase the toxicity of certain heavy metals asbiocides, as discussed earlier (TBTO).

MICROBIAL MONITORING PROCEDURES

In systems where microbe populations may be maintained within a certainacceptable control range and complete sterilization is not required, frequentmicrobe analyses should be run for monitoring the program. Sometimes onlytotal counts are run to indicate overall microbe population levels. However, insystems such as industrial recirculating cooling water loops, analyses of more spe-cific organisms are required. Changes in total count do not always indicatechanges in fungi, anaerobic bacteria, sulfate-reducing bacteria, or algae. Sincethese organisms can be troublesome, they should be monitored specifically.

In industrial waters, a microbial analysis usually expresses counts per 1-mLsample. Since a billion bacteria weigh on the order of 1 mg, a high count of10,000,000/mL of sample represents only about 10 mg/L of suspended solids.

Since there are interfering substances that can reduce the effectiveness of cer-tain biocides, a change in microbial population may be due to causes other thanthe inherent toxicity of the biocide. So, a review of plant conditions (temperature,

chemical environment, sources of inoculation) should always accompany anysampling for microbe analysis.

COOLING WATER SYSTEMS

A typical report of those organisms most commonly found in industrial watersystems is shown in Figure 22.12. This analysis is typical of the weekly analyses

From:Analysis No. B 41616Date Sampled 12/ 1/83Date Received 12/14/83

Sample Marked: Date Printed 12/21/83Cooling Water - Ammonia Plant

>» MICROBIOLOGICAL EVALUATION <«

PHYSICAL APPEARANCE : Yellow Liquid

TOTAL AEROBIC BACTERIA 500,000Aerobacter 20,000Pigmented 50,000Mucoids <1,000Pseudomonas 70,000Others 360,000

TOTAL ANAEROBIC BACTERIA <10Sulfate Reducers <10

IRON-DEPOSITINGGallionella NoneSphaerotilus None

TOTAL FUNGI <10Molds <10Yeast <10

ALGAEFilamentous NoneNon-Filamentous None

OTHER ORGANISMS : None

Lab Comments:(All Counts Express Colony Forming Organisms per ML of Sample.)

FIG. 22.12 Analysis—a cooling system under control.


Sample Marked: Date Printed 12/21/83Ammonia Plant


PHYSICAL APPEARANCE : Yellow Liquid with Floe

TOTAL AEROBIC BACTERIA 14,000,000Aerobacter 70,000Pigmented 200,000Mucoids <1,000Pseudomonas 400,000Others 13,330,000

TOTAL ANAEROBIC BACTERIA 20Sulfate Reducers 20


TOTAL FUNGI <10Molds <10Yeast <10


OTHER ORGANISMS : Few Protozoa


FIG. 22.13 Analysis—the system in Fig 22.12 out of control because of an exchanger leak.

for microbe counts taken on a cooling system water in an ammonia plant, sam-pled when the system is under control. The notation "NEG IN 1/1000" meansthat no organisms were observed in a 1/1000 dilution of the sampled water. Thetotal count is usually higher than the sum of individual aerobic slime-formingbacteria shown above it. This is because there are many more types of aerobicbacteria in cooling water than those specifically reported as troublesome.

Figure 22.13 demonstrates changes in microbe populations produced by an


Sample Marked: Date Printed 12/21/83Well Water


PHYSICAL APPEARANCE : Liquid with Floe

TOTAL AEROBIC BACTERIA NoneAerobacter <10Pigmented NoneMucoids NonePseudomonas NoneOthers None

TOTAL ANAEROBIC BACTERIA 15Sulfate Reducers 10Clostridia 5

IRON-DEPOSITINGGallionella NoneSphaerotilus Few

TOTAL FUNGI NoneMolds NoneYeast None


OTHER ORGANISMS : None


FIG. 22.14 A well water with low counts can still produce troublesome iron growths.

ammonia leak into the cooling water system described above. This sudden influxof nutrient caused aerobic slime-formers to multiply. The resulting deposition ofthese aerobes increased the shelter for anaerobic corrosive bacteria underneaththe deposits.

Using analyses such as these, an industrial plant can optimize biocide usagesto minimize water treatment costs while preventing unnecessary shutdownscaused by rampant microbial contamination.

A common problem in once-through cooling water systems is iron fouling. Inmany instances the iron fouling is actually a result of contamination with iron-depositing bacteria such as Sphaerotilus or Gallionella. By routinely monitoringthe makeup water with microbiological analyses, potential problems with irondepositors can be anticipated and treatment adjusted before the problem gets outof hand.

Figure 22.14 shows a typical makeup water analysis with iron-depositors pres-ent. A total count analysis would have indicated this system to be under goodcontrol and would have missed the potential for iron fouling from the iron-depos-iting bacteria.

PAPER MILL WATER SYSTEMS

In the manufacture of unbleached grades of paper or board, total counts of 30 to40 million bacteria per milliliter can often be tolerated in the system without caus-ing slime problems, because the solids are highly dispersed by residual ligninmaterials remaining in the system. By contrast, in the production of bleachedgrades, slimes and other microbial problems would be uncontrollable at such hightotal counts because of the absence of the dispersive lignins.

The high temperatures of linerboard production systems often allow themachines to run virtually slime-free even though total counts may approach the30 million to 40 million range. Cylinder machines can tolerate extremely highlevels of bacteria because the machines run slowly and are relatively immune toproduction interruptions from slime. They usually produce heavyweight, multi-ply board which can incorporate slime spots without damage to its properties.

Molds and yeasts are more responsible for slime problems in papermaking sys-tems than are bacteria. Molds create more problems than yeasts because theirthreadlike branched form can trap fiber, filler material, and debris, and bind theminto a tenacious deposit. Yeasts cause more problems than bacteria because theyare considerably larger and have a tacky coating around the cell membrane.

Figure 22.15 shows an analysis of a deposit taken from a paper machine frame.The important features of this deposit are the high levels of yeasts, the presenceof anaerobic corrosive bacteria, and the presence of coliform bacteria (E. coli).Deposits caused by aerobic bacteria require total counts many times higher thanthese, so this is not primarily a bacterial problem. The major culprit is yeast. Cor-rosion may be a problem in this system because of the high level of anaerobiccorrosives associated with the deposits. Protozoa in the deposit are indicators thatthere are problems in treating the fresh water brought into the mill. Improperchlorination or filtration of the fresh water may be the cause.

STORING, HANDLING, AND FEEDING PRECAUTIONS

Because the purpose of biocides is to kill living organisms, the need for care inplanning an effective but safe program to handle these toxic chemicals is obvious.Proper protective clothing, gloves, masks, and respirators must be worn by oper-


Sample Marked: Date Printed 12/21/83Tray Deposit (No. 1 Machine)


PHYSICAL APPEARANCE : Green Deposit

TOTAL AEROBIC BACTERIA 56,000,000Aerobacter 50,000Pigmented 1,000,000Mucoids <10,000Pseudomonas 5,000,000Sporeformers <100Others 49,950,000

TOTAL ANAEROBIC BACTERIA 10,200Sulfate Reducers 10,000Clostridia 200


TOTAL FUNGI 20,100Molds 100Yeast 20,000


OTHER ORGANISMS : Many Protozoa

Lab Comments:(All Counts Express Colony Forming Organisms per GRAM of Sample.)

Microscopic Examination - Many Bacteria, Moderate Fibers and Fines.

FIG. 22.15 Bioanalysis—paper machine frame.

ators responsible for charging tanks, adjusting feeders, and controlling the pro-gram by testing. Showers and eye baths must be readily accessible.

Bulk supply has advantages in minimizing exposure of personnel to biocidesand eliminating the problem of package disposal. If products are obtained indrums, provision must be made for proper drum handling, piping hookup, anddrum disposal. (Figure 22.15). Not only is the safety of plant personnel at risk,but also the safety of the environment. It is important to know the fate of the

biocide selected when it reaches the waste treatment plant and eventually a receiv-ing stream. Bench testing or pilot plant studies may be needed to assure the plantmanager that the biocide program most effective in the plant will be compatiblenot only with discharge permit requirements, but also with the health of thereceiving stream.

chapter 22 control of microbial activity

Documents