A Pathogen Primer

Guidelines for reducing health risks related to exposure to disease pathogens at wastewater treatment plants

Wastewater operators are exposed daily to numerous health risks. These risks include exposure to gases, chemicals, endotoxins, exotoxins, and pathogens. Asphyxiating, irritating, and toxic gases produced through the anaerobic degradation of carbonaceous wastes include ammonia (NH3), carbon dioxide (CO2), carbon monoxide (CO), hydrogen sulfide (H2S), and methane (CH4). In addition to these gases, chemicals such as vaporized, volatile organic compounds (VOCs) from wastewater also represent a health risk. Dead and living bacterial cells release endotoxins and exotoxins, respectively. These toxins attack cells and tissues in the human body and cause gastrointestinal, respiratory tract, and nervous system diseases. Examples of several diseases caused by exotoxins include anthrax, food poisoning, and tetanus. Of all health risks associated with wastewater treatment facilities, perhaps disease transmission is of most concern to wastewater operators.

Profiling Pathogens
Pathogens include viruses, bacteria, fungi, protozoa, and helminthes (or worms). Exposure to pathogens and the potential for disease transmission through contact with pathogen contaminated wastewater, aerosols, compost, foam, sludge, and work surfaces are considered to be risks for wastewater personnel. 

Pathogens enter wastewater treatment facilities from the bodily wastes of infected individuals, which may be human, domestic animals, or wild animals. Fecal waste and urine from cats and dogs enter wastewater treatment facilities through inflow and infiltration (I/I). Slaughterhouse waste from poultry, pork, and beef industries also contains many pathogens that are capable of infecting humans. Fecal waste and urine from rodents in the sanitary and combined sewers represent sources of pathogens.

The Culprits: Pathogens of Concern
Viruses are ultramicroscopic agents and are inert. They are not capable of independent growth or reproduction and are not considered to be living organisms. Viruses increase in number through replication. For replication to occur, a virus must enter a living cell (host) and cause disease. Although numerous viral groups are present in wastewater, the principle viruses of concern include the enteroviruses and the hepatitis viral group. Enteroviruses attack the gastrointestinal tract, while hepatitis viruses attack the liver.

Bacteria are simple, unicellular organisms. Most bacteria range in size from 0.1 micrometers (µm) to 15 µm, and the shape of most bacteria is rod (bacillus), spherical (coccus), or spiral (sprillium). Bacteria are ubiquitous in nature, and most are harmless. They reproduce asexually, usually by splitting in half, and may be found as individual cells, clusters of cells, or chains of cells (filaments).

There are two types of pathogenic bacteria. "True" pathogens such as Leptospira interrogans are aggressive and cause disease. "Opportunistic" pathogens such as Escherichia coli are typically found on or in the human body and do not cause disease unless the body's immune system is weakened by injury, a "true" pathogen, or physiological disease.

Although numerous pathogenic bacteria are found in wastewater, the principle pathogenic bacteria of concern are Camplyobacter jejuni and Leptospira interrogans. Camplyobacter jejuni attacks the gastrointestinal tract, while Leptospiria interrogans attacks the kidneys.

Fungi are a diverse group of organisms. Some fungi such as yeast are unicellular, while other fungi such as molds and mushrooms are multicellular. Fungi are saprophytes and obtain their nourishment from dead organic matter or living organisms. Although there are few pathogenic fungi, these fungi are not obligate parasites. The principle fungus of concern is Aspergillus fumigatus. This organism attacks the respiratory tract and often is associated with thermophillic composting operations.

Protozoa are single-celled organisms that are animal-like, fungus-like, or plant-like. Protozoa often are grouped or identified by their ability or lack of ability for locomotion. Some protozoa move by the beating action of hair-like structures or cilia (ciliates) or whip-like structures or flagella (flagellates). Some protozoa (amoebae) move by a pseudopodia action, i.e., a streaming of the cytoplasm or intracellular content against the cell membrane.

Most protozoa are free-living, but several parasitic ciliated, flagellated, and amoeboid protozoa are found in wastewater. Of these parasitic protozoa, two protozoa are of concern to wastewater personnel. These protozoa are Crytosporidium parvum and Giardia lamblia. These organisms infest the intestinal tract and cause profuse and watery diarrhea.

There are many helminthes of concern. These worms include the roundworms (nematodes) and flatworm (tapeworms and flukes). The principle worms of concern are the roundworm Ascaris lumbricoides and the dwarf tapeworm Hymenolepis nana. Not only humans, but also dogs and cats carry Ascaris lumbricoides. The principle carriers of Hymenolepis nana are Norway rats (Rattus norwegicus) and roof rats (Rattus rattus). These rats also carry other pathogens as well as fleas, lice, mites, and ticks. Therefore, the control of rats in sanitary and combined sewers and rodent-infested neighborhoods often may be required to control disease transmission in communities.

Pathogens that enter wastewater treatment facilities come from a variety of sources including infected community members, domestic animals, and wild animals. Infected community members may display acute or chronic symptoms of disease or may display no disease symptoms (asymptomatic). Regardless of disease manifestation (symptoms), infected individuals release pathogens in their bodily wastes. Travelers and military personnel, as well as migrant workers, represent an additional risk of the introduction of a new pathogen to the wastewater treatment facility.

Pathogens Meet Their Fate
At a wastewater treatment, facility pathogens have four fates. Pathogens may be removed, inactivated, destroyed, or pass-through. Pathogens are removed from wastewater through their adsorption to solids such as sediment in the sanitary sewer, rags in the bar screen, or floc particles in the activated sludge process. Therefore, these solids should be considered contaminated and handled with appropriate safety precautions. Some pathogens may be inactivated within the wastewater treatment facility. Examples of inactivation include pathogen exposure to ultraviolet light in the final effluent or increase in pH of sludge. Inactivated pathogens may become reactivated some time later after exposure to ultraviolet light or when the pH of the sludge decreases.

Pathogens may be destroyed by biological or chemical processes in the wastewater treatment facility. For example, pathogens may be destroyed by septicity, the lack of oxygen, elevated temperature, and entrapment by bacteria in the anaerobic digester. Pathogens may be destroyed by free chlorine in the chlorine contact tank.

Although no conclusive statement can be offered with respect to what pathogen is destroyed in any treatment tank or condition, the exposure of any pathogen to as many different treatment conditions as possible increases its probability of destruction. The assessment of pathogen destruction is complicated by the presence of protective structures of some pathogens and the complex life cycles of many protozoa and helminthes.

If pathogens are not removed from the wastewater, inactivated, or destroyed, they leave the wastewater treatment facility through aerosols, final effluent, or sludge. Aerosols are a source of airborne pathogens. Although the infective stages of protozoa (cysts and oocysts) and the eggs of helminthes are too heavy to be carried on aerosols, many viruses, bacteria, and fungi are carried on aerosols. The aerosols represent a health risk not only to wastewater personnel, but also community members living downwind of a wastewater treatment facility. Exposure to pathogens in aerosols as well as sludge represents a health risk to community members living adjacent to sludge disposal sites.

Keeping Wastewater Workers Safe
In order for a wastewater worker to become infected with a pathogen, three steps in disease transmission must be satisfied. First, the pathogen must leave an infected individual within the community. Second, the pathogen must come in contact with a worker; and third, the pathogens must enter the worker.

There are three common routes of entrance or portals of entry for pathogens. These routes are ingestion (fecal-oral), inhalation, and invasion. The most common route of entrance is ingestion. For an infection to occur in a new individual, an adequate number of viable pathogens must enter the individual and overcome the individual's bodily defenses. Regardless of the risk assigned for disease transmission from any pathogen, that risk can be significantly decreased or eliminated through the use of proper hygiene measures, protective equipment, and common sense.

The use of proper hygiene measures, protective equipment, and common sense prevent contact with pathogens or block their portals of entry. These measures prevent infection. Measures available to wastewater personnel to prevent infection include the use of antimicrobial agents, automation, cleanliness and consumption precautions and restrictions, first aid, proper sampling practices, protective clothing, records, training, and ventilation. In addition to these measures, the use of immunobiologicals (vaccines and immunizations) also helps to prevent pathogen infection.

Antimicrobial agents destroy pathogens by damaging cellular components. Antimicrobial agents can be used to disinfect the hands or hard surfaces such as lunch tables and laboratory counters. Automation, for example, automatic collection of wastewater samples and automatic cleaning of bar screens, reduces personnel contact with wastewater and pathogens. Cleanliness and consumption precautions and restrictions make use of good common sense, appropriate hygiene measures, and personal protective equipment where appropriate.

Immediate first aid should be given to any cut or abrasion that occurs at a wastewater treatment facility. A physician should treat more serious injuries. Proper sampling techniques should prevent breakage and spillage. Sample bottles should have a wide mouth opening and, whenever possible, should be plastic. If glass containers are required, the glass should be coated with plastic. Lids for bottles should be tight fitting. Bottles and lids should be cleaned after each use with squirt bottles and paper towels. Carriers for sample bottles should be compartmentalized to prevent breakage, and sampling stations should be hosed down to wash away pathogens that may be present due to spillage or leaking bottles.

Protective clothing consists of uniforms, shoes or boots, masks, gloves, and goggles. Protective clothing remains at the wastewater treatment facility and prevents wastewater personnel from bringing pathogens home. Protective clothing should be washed, dried, and stored at the wastewater treatment plant or cleaned professionally. Separate lockers should be provided for work clothes and street clothes.

Safety records should be maintained for all wastewater personnel. The records should include information addressing accidents, immunobiologicals, major and minor illnesses, and training. Training should provide information regarding the hazards of pathogens found in wastewater, areas of significant exposure to pathogens, and the use of appropriate hygiene measures and protective equipment. Training should also review significant pathogens that are present in wastewater, their transmission and portals of entry, clinical symptoms of gastrointestinal and respiratory tract infection, and available immunobiologicals.

Proper ventilation helps to reduce the risk of infection from pathogenic agents including allergens and toxins by reducing their numbers. Pathogenic agents are present in higher concentration in poorly ventilated areas as compared with outside areas and properly ventilated areas. Areas of poor ventilation usually are bar screens, grit chambers, lift stations, sludge dewatering facilities, and wet wells.

Several immunobiologicals are commonly recommended for wastewater personnel and adults. Immunobiologicals made available to wastewater personnel are based upon diseases that are endemic to the area where the wastewater treatment facility is located and risks associated with the immunobiologicals. In lieu of immunobiologicals, appropriate hygiene measures and protective equipment are recommended.

Due to the frequent, often daily, exposure to large numbers and large diversity of pathogens, wastewater personnel have occupational concerns related to disease transmission from wastewater pathogens. Operator concerns also exist over the emergence of new diseases such as bird flu virus, monkeypox, and West Nile virus. Although most health hazards or risks related to date with wastewater pathogens are minimal or nil, unique and potential hazards do exist, and much research work needs to be performed to better evaluate existing and potential hazards. Again, regardless of the risk assigned to the transmission of any wastewater pathogen, this risk can be reduced or eliminated through the use of proper hygiene measures and appropriate protective equipment.

Table 1

Group Pathogens
Viruses Enterovirus, Hepatitis viral group
Bacteria Campylobacter jejuni, Leptospira interrogans
Fungi Aspergillus fumigatus
Protozoa Giardia lamblia
Helminthes Ascaris lumbricoides, Hymenolepis nanas

Table 2

Chemical Agent Action
Alcohols Denature proteins
Alkalis (in soaps) Denature proteins
Detergents and soaps Lower surface tension of pathogens, making them susceptible to other chemical agents
Halogens Oxidize cellular components
Heavy metals (in disinfectants) Denature proteins
Oxidizing agents Denature proteins
Phenol and phenolic compounds Damage cell membrane and denature proteins

Table 3

Cleanliness and Consumption Precautions and Restrictions
Avoid touching the ears, eyes, mouth, and nose with your hands,
unless you have just washed.
Confine eating, drinking, smoking, and the use of smokeless tobacco products to designated areas.
Keep your fingernails short; use a stiff soapy brush to clean under your nails.
Wash your hands frequently and properly after contacting wastewater and before
eating, drinking, or smoking, use of smokeless tobacco products, and at the end of work.
Wear appropriate gloves where necessary, especially when hands are chapped, cut, or burned.

Table 4 -- Protective Clothing

Clothing Action or Item
Gloves Wear appropriate gloves at each work site; elbow-length gloves may be necessary; never submerge top of glove; wash or dispose of gloves after use; wash hands immediately after work when gloves cannot be used.
Goggles Protect eyes from pathogens in aerosols and dust; wash goggles after use.
Masks Prevents inhalation of pathogens in aerosols and dust; ensure proper fit of masks; wash or dispose of masks as directed after use.
Uniforms Leave uniforms at work; use separate lockers for work and street clothing; wash work uniforms at work or use professional service; use bleach on heavily soiled uniforms

Table 5

Immunizations Commonly Recommended for Wastewater Personnel and Adults
Hepatitis A
Hepatitis B

Influenza (annually for adults 65 years or older)

Measles (for adults born in 1957 or later)
Mumps (if not previously infected)
Pneumococcal (for adults 65 years or older)
Rubella (if not previously exposed)
Tetanus and Diphtheria (TD) every 10 years after initial dose;
after wounds, unless less than 5 years since last dose

This article originally appeared in the 07/01/2005 issue of Environmental Protection.

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