Environmental Pollution Control Microbiology - Chapter 12 pdf

The use of staged filters can minimize the impact of suspended solids by removing the large suspended particles ahead of the microorganisms.. A few specialized filters will remove small

Chapter 12

AIR MICROBIOLOGY

Air microbiology has long fascinated a few environmental microbiologists Primary concern has been with the spread of pathogenic microbes through the air Originally, people believed that diseases were transmitted through the air Once it was demonstrated that most bacterial pathogens could not survive very long, much less increase in the air, many microbiologist lost interest in air microbiology There is no doubt that air microbiology is unique, requiring more imagination and creativity than water or soil microbiology Recently, interest in air microbiology has been stimulated with the construction of relatively air-tight houses and buildings Heating and air conditioning systems have created environments that allow various microorganisms to survive and adversely affect the health of people Emphasis on reuse of wastewaters in spray irrigation has raised concern over the potential spread of pathogenic organisms through the air

in fine mists Even wastewater treatment plants are being examined as potential sources of airborne pathogens Recently, the concern has been raised that terrorists could disperse extreme pathogens through the air As populations increase and greater demands are made on the environment, there will be additional concerns about air microbiology For these reasons it is important that environmental microbiologists understand air microbiology, as well as, water microbiology and soil microbiology

THE AIR ENVIRONMENT

Air is composed primarily of nitrogen and oxygen, 78% by volume nitrogen and 21% by volume oxygen Air also contains water vapor in varying amounts

Water vapor can help the microbes survive in the unfavorable air environment There are also many tiny suspended particles in the air Microorganisms are part

of those suspended particles dispersed in the air Gravity is the major force affecting the removal of suspended particles from air Large particles quickly settle onto solid surfaces; while small particles settle slowly, and very small particles settle very slowly Wind currents determine the ability of suspended particles to remain in suspension Natural wind currents can move suspended particles for hundreds of miles Not only can wind currents carry suspended particles long distance, but wind currents can also lift tiny suspended particles from the land surfaces Movement of cars, trucks, and people in urban areas help generate micro-currents that combine with natural wind currents to keep particles

in motion Industrial operations often result in the production of gaseous wastes that contain particulates The velocity of the discharge gases can carry the particulates high into the air, allowing wind currents to carry tiny particulates great distances

People sneezing and coughing discharge microorganisms into the adjacent air environment In close quarters people can breathe in some of the suspended microorganisms before the discharged particulates settle to the ground Splashing water also discharges microbes into the air environment in tiny droplets As the water evaporates, the active microorganisms lose their protective environment and quickly die Spore forming microorganisms have the ability to survive as spores in the air environment, since the spore coating protects the nuclear material from drying out The microbes not only suffer from

a loss of moisture, but also from ultra-violet light in natural sunlight Ultra-violet light can kill vegetative cells Only the dense spore cells are partially protected from ultra-violet light Long-term exposure to sunlight can even kill microbial spores and cysts The oxygen in air adversely affects anaerobic bacteria that are discharged into the air environment The hostile air environment has greatly limited the spread of diseases except for closely confined populations Schools, hospitals, factories, and offices provide environments that allow some pathogenic microorganisms to be spread from person to person through the air The decomposition of crop materials left on agricultural fields by actinomycetes and fungi results in the production of large quantities of aerial spores Natural wind currents can carry the fungi spores for long distances Most of the spores settle out onto surfaces that are not suitable for their germination Still, a sufficient number of spores find a suitable environment for growth and continued survival There is no doubt that the survival of spores in the air has helped transmit specific microorganisms to different parts of the world

SAMPLING TECHNIQUES

Initially, the exposure of solid biological media to air provided the air microorganisms for study Sterile nutrient media plates were exposed to the air

by lifting the glass cover and exposing the media surface to air for a given period

of time Spores and a few vegetative cells, which settled out on the media surface, found the media suitable for rapid growth, allowing them to be examined in detail While this technique is reasonable for a qualitative measure

of the microbes in the immediate vicinity, it was not suitable for quantitative evaluation of microbes in the air In 1931 W F Wells at Harvard University developed an air sampler that allowed quantitative measurement of the microorganisms in air The Wells air sampler was a combination centrifuge that sucked in air at a measured flow rate and forced the suspended particles onto the surface of biological media that coated the surface of clear glass centrifuge tubes As the centrifugal action of the sampler pushed the microbial particles against the media surface, the microbes found an environment for rapid growth Based on initial studies on the survival of bacteria in the air, Wells and Stone found that bacteria could survive for a sufficient period to allow pathogenic bacteria to be transmitted from person to person through the air

As interest increased in collecting data on all types of air contaminants, the impingement type bubbler sampler was developed The impingement sampler used a small air compressor to pull air through a series of bubbler tubes to collect the air contaminants in water tubes Different size orifices in the air tubing controlled the airflow rate through the sampler The glass tube inside the sampler was drawn to form a nozzle, allowing the entering air to create maximum mixing when air bubbles were formed in the water Rapid mixing insured that the contaminants in the air were quickly transferred to the water in the sampler The use of three bubbler tubes in series provided good collection of most of contaminants In 1970 David Armstrong reported on an impingement method for collecting samples from incinerator stacks Instead of pulling the air through the liquid, he placed the inlet tube just above a phosphate buffer collection fluid The velocity of airflow through the inlet tube caused the microbial particles to impinge on the phosphate buffer solution and to be captured for later transfer to normal microbiological media The key was in the mass of the bacteria and the velocity of airflow The surface of the phosphate buffer solution was placed about 1.27 cm below the exit from the sampling tube Currently, the AGI-30 impingement sampler is considered the standard air sampler for liquid collection of microbial samples

After World War II, concern over the use of pathogenic organisms for biological

warfare led to the development of the membrane filter for collecting air samples for microbial growth Membrane filters were cellulose acetate filters of specific pore sizes to allow retention of different size particles Measured volumes of air were drawn through the membrane filters to capture all particles of a certain size

By using a series of different filters a spectrum of particle sizes could be measured After passing the desired amount of air through the membrane filter, the membrane filter was placed on top of a porous pad containing specific microbiological media to stimulate the growth of the desired microbes Eventually, the membrane filters were made available for general use in air and water microbiology The primary concern with membrane filters is damage to viable cells during collection High concentrations of suspended particulates in the air sample can suppress microbial growth by completely covering the microbes and not allowing them to obtain nutrients for growth The use of staged filters can minimize the impact of suspended solids by removing the large suspended particles ahead of the microorganisms Polycarbonate membrane filters eventually became more popular than cellulose acetate membrane filters in

ah1 microbiology since the polycarbonate membrane filters had a more uniform pore size than the cellulose acetate membrane filters

The Anderson cascade impactor has become one of the more popular microbiological air samplers The Anderson sampler utilizes a series of stacked agar plates in which the air impacts on the surface of agar media plates similar to the Wells air sampler Anderson samplers have either two stages or six stages A series of plates with specific size holes separate the different stages in the Anderson sampler The size of the holes decreases as the ah- moves through the sampler In effect, different particle size organisms are removed at the different levels The major differences between the Wells sampler and the Anderson sampler are the lack of centrifuging by the Anderson sampler and its multi-staging Recently, increased interest in air microbiology has stimulated the development of new air samplers and evaluation of the old samplers In 1994

Juozaitis et al reported on their research using several commercially available air samplers, as well as, two new samplers for the capture of Pseudomonas fluorescens from air in a bioaerosol chamber They found that a new, agar slide

impinger was the most efficient sampler, capturing 26% of the viable cells expected in the air sample The Anderson two-stage impactor recovered 1.25%

of the viable cells The Anderson six-stage compactor captured 16.8% of the Ps fluorescens in the air sample These data indicate the limitations of the current

air sampling equipment The lack of interest in air microbiology has prevented

the development of more efficient sampling equipment Air sampling equipment

is highly specialized with a limited market As better air sampling equipment is developed, there should be an increase in air microbiology research

CENTRAL HEATING AND AIR CONDITIONING

The development of central heating and air conditioning systems has created problems that are stimulating new interest in air microbiology Houses and office buildings are being constructed tighter to prevent significant energy losses Heated air in the winter and cool air in the summer are recirculated on a semi-continuous to semi-continuous basis for maximum efficiency Various types of filters are placed ahead of the circulation fans to remove particulates from the recycled air Most of the simple filters trap large particles A few specialized filters will remove small particles, such as bacteria In large buildings electrostatic precipitators are used to remove microbial particles from the air Electrostatic

Negatively Charged

Bacteria Electric Charge on Plates

DIRTY ^^W + | \ CLEAN AIR M^T I 1/ AIR

CHARGED PLATES

Figure 12-1 SCHEMATIC DIAGRAM OF AN ELECTROSTATIC PRECIPITATOR

precipitators use high voltage, about 50,000 volts, across a series of flat metal plates to pull the tiny charged particles onto the plate surfaces Figure 12-1 illustrates the removal of bacteria by an electrostatic precipitator The velocity of airflow, the spacing between plates, the area of the plates, and the number of plates are important design parameters Periodically, the metal plates are cleaned

to remove the attached particles Since bacteria and colloidal particles in the air tend to be negatively charged, all of the tiny particles are removed together onto the positively charged plates Electrostatic precipitators can be very efficient in removing microorganisms from the air

The use of humidifiers to adjust the moisture content of internal air can stimulate microbial growth that can be dispersed through central ventilation ducts to all

parts of the building Most humidifiers pick up moisture by passing the air through a mesh screen coated with water The air removes some of the water from the screen; but the water picks up various contaminants from the air Over time, microorganisms are able to grow on the wetted mesh screen Any splashing allows the microbes to be transferred from the water to the air and circulated through the ventilation ducts Dust and suspended particles also tend to accumulate in the ventilation ducts, especially around bends and at junctions of several ducts The moist air and the nutrients in the settled particles can allow fungi to grow and produce aerial mycelia with discharge of large numbers of spores into the flowing air It is normal to see fungi and some bacteria circulated through household air ducts, especially during the warm weather when air conditioning is widely used People bring microbes into the house as they come and go The central ventilation system moves the indoor air around the house The settling of dust on furniture is indicative of the increase in particulates in the home environment The normal household vacuum cleaner collects the large dirt particles and suspends the tiny microbial particles in the adjacent air Recently, some vacuum cleaner manufacturers have added fine filters to minimize the return of small particles to the adjacent environment The use of air filters in the return air ducts removes the larger suspended particles with limited removal of microorganisms By removing the larger particles of dust and dirt, the microorganisms have less opportunity to grow in the air ducts The overall effect

is reduced spore production and distribution through the central ventilation system even though the air filters do not remove many microorganisms, if any Some air filters use finer materials to remove small particles These filters increase resistance to airflow, providing less air circulation The finer material filters also clog more quickly, requiring greater maintenance

The discovery of the transmission of Legionella in 1976 through the central

ventilation system in a Philadelphia hotel focused attention on the importance of

internal air contamination in the transmission of disease Legionella was

considered to be a non-pathogenic bacteria that normally lived in water It became pathogenic when introduced into the lungs of certain individuals through

contaminated aerosols Legionella appears to be a secondary pathogen, affecting people with reduced immune systems While Legionella has been found in

normal water systems and in hot tubs, it is easily controlled by maintaining a chlorine residual in the water system As primary pathogens are reduced in the environment, allowing more people to live longer, secondary pathogens that have not been significant, will move to the level of primary pathogens Non-pathogenic bacteria suddenly become Non-pathogenic, creating undue concern among the public The problem is simply one of bacteria moving into an available niche when opportunities exist for increased growth New pathogens will arise at regular intervals to replace the old pathogens brought under control by modern

science and technology This is part of our natural environment and it should be accepted as such without fear or panic It simply means that science will never fully eliminate microbial pathogens

Spores from bacteria, fungi, and actinomycetes, as well as, protozoa cysts, are the primary microbial forms circulated through central ventilation systems Spores have the ability to remain dormant until they fall onto sites where they can grow into vegetative cells Since moisture is a major factor affecting microbial growth, it is not surprising that most microbial spores do not germinate Fungi and actinomycetes tend to grow on paper, cloth or leather objects in damp basements where the moisture level is sufficient The spores produced by the fungi and actinomycetes in basements are easily picked up by ventilation systems and circulated through the house While the spores may not cause disease, they adversely affect people who are allergic to microbial proteins Allergies are more of a problem with central ventilation systems than pathogens Microbial spores can also prove to be contaminants in industrial products unless care is taken to eliminate them Many industrial plants have clean rooms with very fine air filters and ultra-violet lights to kill any vegetative cells Multiple doors and negative pressure vents are used to remove contaminants before people enter the clean process rooms With reasonable care, indoor microbial contamination can be controlled

SEWAGE IRRIGATION AND SLUDGE APPLICATION

The environmental movement favored simple concepts for wastewater disposal over modern technology They suggested that municipal wastewaters could be used for irrigation This would allow reuse of the nutrients contained in the wastewater and reduce the demand on the use of clean water for agriculture This positive approach to solving the sewage disposal problem had its corresponding negative side Concerns were raised about the potential spread of pathogenic microorganisms, as well as, the accumulation of toxic materials in agricultural soils It is not surprising that wastewater irrigation research studies were approached from two different points of view One group wanted to show that wastewater irrigation represented a definite health hazard and should be stopped The other group wanted to demonstrate that wastewater irrigation was a sound agricultural practice that did not create a health hazard, even to the workers in the immediate vicinity of the irrigation projects Much of the initial research focused on spray irrigation systems A study in Israel in 1978 showed that coliform bacteria and enteric viruses were found downwind from a spray irrigation system using treated wastewater This research was followed in 1980

by a more detailed study that showed that low levels of Salmonella, coliform

bacteria, and enteroviruses could be detected in mists sampled from 40m (131 ft)

to 100m (328 ft) downwind from the spray nozzles discharging treated wastewater The data showed that coliform bacteria were contained in aerosols generated by spray irrigation Fortunately, the aerosols did not travel very far from the spray nozzles None of the data showed that pathogenic microorganisms survived in sufficient numbers to create measurable cases of enteric diseases Since the research was carried out in areas where few pathogens existed in the wastewaters, the results were not surprising and could be misleading for developing areas of the world where wastewaters contain large numbers of pathogenic organisms The 1978 study carried out in Israel indicated that survival of coliform bacteria in aerosols was related to the relative humidity

of the air Here again, the moisture in the air helped the coliform bacteria survive Survival was also greater at night than during the daylight hours Part of the increased survival was related to the increased relative humidity and part was related to the lack of sunlight with its lethal uv radiation and its dehydration effect Ultraviolet radiation definitely reduced the survival of coliforms as did increased temperatures There is no doubt that spray irrigation of treated wastewaters poses a risk, based on the potential pathogenic microorganisms in the wastewater being used With the reduction in pathogens in wastewaters before and after treatment, the risk of spreading contagious diseases is quite low Unfortunately the public perception of the risk of spreading diseases remains quite high and will continue to remain high for some time in the future

In many locations wastewater sludge is applied to the land as a source of nutrients and for ultimate disposal back onto the land environment When the liquid sludge is applied to land from trucks, aerosols may be formed Application of dewatered sludge does not normally produce aerosols As the dewatered sludge dries out, it breaks down into smaller particles Wind action can pick up and carry small particles great distances, causing concern that pathogens might be transported within the tiny particles Studies have shown that wastewater sludge applied to land quickly lose their fecal coliform bacteria Air sampling did not indicate that fecal coliform bacteria were present in the air particles One of the problems with agricultural sampling lies with normal bacteria from the soil and from animals in the vicinity Again, it should be emphasized that wastewater sludge in the United States does not contain as many pathogens as sludge in some of the developing countries where enteric diseases are still endemic The probability of American wastewater and wastewater sludge having significant numbers of pathogens is quite low even though many environmentalists and microbiologists do not want to recognize that simple fact

It has required a large number of federally funded research projects to confirm that the application of sewage sludge to agricultural lands does not pose a health

hazard from pathogens in the air One of the major problems with the current university education is that knowledge is only accepted when the individual raising the questions carries out the experimental research Existing knowledge has no real value Common sense and logic are not considered of importance in evaluating research problems As a net result, the same research is carried out over and over again with varying conclusions, depending upon the individual researcher's objectives

AERATION TANKS

Aerosol generation is common in modern wastewater treatment plants Aerosols are formed where wastewaters undergo turbulent mixing by normal hydraulic currents or by mechanical agitation The prime sources of aerosols are the aeration tanks used for grit removal and for organic stabilization in activated sludge plants Trickling filters also produce aerosols as the wastewaters splash

on the rock media from the distribution arms Primary and secondary sedimentation tanks produce aerosols when the treated effluent is collected around the periphery of the sedimentation tanks and drops into the pipe inlet box Most of the aerosol research to date in municipal wastewater treatment plants has been carried out on aerosols created in activated sludge aeration tanks The basic problem with aeration tanks lies in their potential for creating of large quantities of aerosols Moisture around the aeration tanks and brown colored particulates coating nearby surfaces indicate where the majority of the aerosols are deposited Only the tiny droplets move any significant distance from the aeration tanks One of the problems in collecting aerosol data from aeration tanks is the large number of heterotrophic bacteria growing in activated sludge The heterotrophic bacteria easily overgrow the fecal coliform bacteria, making data collection and evaluation difficult

The Chicago Metropolitan Water Reclamation District has carried out studies on the hazards of aerosols from activated sludge aeration Their research demonstrated that the air in the residential environment before their WWTP was constructed, contained enteric microorganisms of unknown origin As expected, the microbial populations were highest at night when the relative humidity was the highest and there was no ultra-violet radiation After the WWTP was constructed and placed into operation, the microbial populations in aerosol samples increased on the downwind side of the WWTP, as would be expected Enteric viruses were isolated from the downwind samples It appeared that there was a potential for pathogenic microbes to be produced in the aeration tank aerosols Unfortunately, the lack of microbial discharge data from the aeration

tank made it difficult to evaluate the degree of reduction in the number of bacteria as the distance from the aeration tank increased In 1993 researchers at Chicago reported on their study to measure the bacteria discharged into the air above an aeration tank They used a six stage, Anderson, aerosol sampler in a smokestack tower to eliminate extraneous bacteria from other parts of the aeration tank The sampler was located approximately 15 cm above the surface

of the aeration tank Their data indicated a standard plate count between 0.66 and 2.65 bacteria/m2/sec with between 0.02 and 0.40 coliform bacteria/m2/sec Since the Anderson sampler had an air flow rate of 0.47 1/s and the aeration tank air flow rate up the tower was 0.33 1/s, it was assumed that most of the air from the aeration tank passed through the sampler, giving reasonable bacteria capture from the aeration tank

Activated sludge plants have been in operation in the United States for about 80 years Many of the WWTP are located in large cities with people living immediately adjacent to the plants The people living adjacent to the activated sludge plants and the plant operators form the base population of individuals who could have been potentially affected by the aerosols from the aeration tanks

No data have been produced to date to show that either the WWTP operators or significant numbers of the people living adjacent to the WWTP have suffered from enteric diseases or other diseases that might have been transmitted by pathogenic microorganisms carried in wastewater aerosols The number of potential pathogens in American municipal wastewater is simply not great enough to cause serious concern for the spread of enteric diseases by aeration tank aerosols Yet, this lack of serious danger does not mean that operators do not have to be concerned about basic sanitation and personal hygiene As already indicated, common bacteria can become pathogenic under the right circumstances The important aspect of the studies to date is that there have been

no serious health hazards from activated sludge tank aerosols The public living next to a wastewater treatment plant should not have to worry about potential health problems from the treatment plant, provided the plant is properly designed and operated

The 1965 design of an aeration only, activated sludge system below the floor in a confined hog building led to the production of aerosols around the aerator Additional designs in larger, finishing buildings resulted in several aerators located directly adjacent to the hog pens While some experts predicted that the aeration equipment would quickly produce aerosols that would spread disease throughout the confined hog population within the buildings, there was never any disease transmission related to the aeration aerosols The potential for serious contamination in a confined animal building is far greater than in open aeration tanks in wastewater treatment plants Not only were the animals healthy in the