5.2 What are the infectious organisms in wastewater?
In many countries, excreta-related infections are common, and excreta and wastewater contain correspondingly high concentrations of infectious organisms.
Without an adequate wastewater treatment process, these pathogens may survive for a long time in the environment. Humid environments, clay soils and soils with high organic content, and low temperatures are just a few factors that favour pathogen survival.
On the other hand, natural factors that facilitate pathogen inactivation are:
- Dry environments,
- High temperatures or freezing temperatures,
- Alkaline soils for viruses’ die-off (but may favour bacterial survival),
- Direct sunlight,
- Competition with native flora and fauna (antagonistic effects from bacteria or algae; bacteria may be preyed upon by protozoa).
If those natural conditions cannot be guaranteed and effluent is intended for specific purposes, disinfection becomes a must. There are different types of infectious organisms (viruses, bacteria, protozoa and helminths) that differ in their properties and in their disinfection requirements, which is why they are presented in more detail in the following section.
Viruses
Viruses are the smallest parasitic organisms (0.01 – 0.3 μm), formed by the association of genetic material (DNA or RNA) and a protein structure. Viruses are not a normal part of the faecal flora. They occur in infected persons and appear in septic tank effluent intermittently, in varying numbers. Once an infection has occurred, faeces are estimated to contain 106 to 1010 viral particles per gram. There are more than 150 types of enteric viruses capable of producing infections that multiply in the intestine and are expelled in faeces. Some of the most common types of viruses and the diseases they can cause are listed in the following table.
| Types of Viruses | Diseases Caused |
|---|---|
| Enteroviruses (72 types, e.g. polio, echo) | Gastroenteritis, heart anomalies, meningitis |
| Hepatitis A virus | Infectious hepatitis – fever, malaise, anorexia, nausea, abdominal discomfort, jaundice |
| Norwak agent | Gastroenteritis |
| Rotavirus | Gastroenteritis |
Viruses are not or only partially removed by filtration or sedimentation and require disinfection by chlorination or UV radiation. Nonetheless, they are more resistant than bacteria to inactivation by disinfection. Without disinfection measures, survival times, exemplified by enteroviruses in the table below, can extend to several weeks.
| Organisms | Survival Times (days) | ||
| Fresh Water and Sewage | Crops | Soil | |
| Enteroviruses | <120, usually <50 | <60, usually <15 | <100, usually <20 |
Bacteria
Bacteria are unicellular organisms that are present in various forms and sizes (0.2 – 10 μm). Bacteria are the main organisms responsible for the stabilisation of organic matter, while some are pathogenic with the potential to cause intestinal diseases.
Bacteria constitute the most diverse group of human pathogens in wastewater. Some major types of bacterial pathogens and the diseases they cause are listed in the following table.
| Types of Bacteria | Disease Caused |
|---|---|
| Escherichia coli (enterotoxigenic) (type of thermotolerant coliforms) | Gastroenteritis |
| Salmonella typhi | Typhoid fever/paratyphoid fever – headache, fever, malaise, anorexia, cough, bradycardia, splenomegaly |
| Salmonella (2,100 serotypes) | Salmonellosis – diarrhoea, fever, abdominal cramps |
| Shigella (4 spp.) | Shigellosis – dysentery (bloody diarrhoea), vomiting, cramps, fever |
| Vibrio cholerae | Cholera – watery diarrhoea, lethal if severe and untreated |
While filtration processes are shown relatively efficient in the removal of viruses, bacteria reach better removal efficiencies through biological treatment and sedimentation processes. Chlorination and UV radiation are both suitable for disinfection of the wastewater. Without proper treatment, bacteria have similar or partially shorter survival times compared to viruses. The following table presents survival times for various types of bacteria.
| Type of Bacteria | Survival Times (days) | ||
| Fresh Water and Sewage | Crops | Soil | |
| Thermotolerant coliforms | <60, usually <30 | <30, usually <15 | <70, usually <20 |
| Salmonella spp. | <60, usually <30 | <30, usually <15 | <70, usually <20 |
| Shigella spp. | <30, usually <10 | <10, usually <5 | No data |
| V. cholerae | No data | <5, usually y2 | <20, usually <10 |
The increased production and use of antibiotics in many countries has led to the awareness of the presence of antibiotics and antibiotic-resistant bacteria in wastewater. There are studies that chlorine disinfection may enrich antibiotic-resistance genes in bacteria. A study on this topic can be found in the Materials tab of this chapter.
Protozoa
Protozoa are usually unicellular organisms without a cell wall (between 2 and 60 μm in size). They can be up to 10 times larger than bacteria.
The three main groups of these microorganisms are Ciliates, Flagellates and Amoebae. Ciliates are often the largest and most complex protozoa found in wastewater. They are primarily predatory, feeding on bacteria, algae, and other microorganisms in the wastewater. Similar to the other two main groups, their presence can contribute to the reduction of organic matter and the removal of pathogens. In this sense, protozoa are essential in biological treatment.
| Type of Protozoa | Disease Caused |
|---|---|
| Cryptosporidium parvum | Cryptosporidiosis – watery diarrhoea, abdominal cramps and pain |
| Entamoeba histolytica | Amebiasis (amoebic dysentery) – abdominal discomfort, fever, chills |
| Giardia lamblia | Giardiasis – diarrhoea, abdominal cramps, malaise, weight loss |
Due to larger dimensions, protozoan cysts are not inactivated by disinfection measures but are separated by sedimentation or filtration. Without treatment, some types of protozoa have long survival times, especially in water/sewage and soil, as the following table shows.
| Type of Protozoa | Survival Times (days) | ||
| Fresh Water and Sewage | Crops | Soil | |
| Cryptosporidium parvum | <180, usually <70 | <3, usually <2 | <150, usually <75 |
| Entamoeba histolytica | <30, usually <15 | <10, usually <2 | <20, usually <10 |
As protozoans remain in the sludge, their elimination is part of the sludge treatment and is covered in the “Biosolids Management” module.
Helminths
Helminth eggs are the infective agents for the worm diseases known globally as helminthiases. Although helminths are pluricellular animals, their eggs are microscopic (around 20 – 80 μm for those relevant in the sanitary field). The eggs of different helminth species vary in shape, size, and resistance.
Due to variable infarction rates in different countries, the type of helminth eggs and their content in wastewater also varies. The eggs are not always infectious. To be infectious, they need to be viable, and larval development needs to occur. This occurs after about 10 days of incubation at the required temperature and moisture levels. The diseases that can be caused by selected types of helminths are listed in the following table.
| Type of Helminths | Disease Caused |
|---|---|
| Ascaris lumbricoides (roundworm) | Ascariasis – generally no or few symptoms, wheezing, coughing, fever, enteritis, pulmonary eosinophilia |
| T. solidum (tapeworm) | Taeniasis |
| Trichuris trichiurid (whipworm) | Trichuriasis – unapparent through vague digestive tract distress to emaciation with dry skin and diarrhoea |
Helminth eggs remain viable for a long time. Due to a highly resistant biological structure, they are not inactivated by chlorine, UV light, or ozone. The resistance derives from the eggshell that consists of a variable number of layers (3 to 4 layers depending on the species), each providing mechanical resistance or protection from toxic compounds. Without treatment, they can have the longest survival times up to years, especially in water/sewage and soil.
| Type of Helminths | Survival Times (days) | ||
| Fresh Water and Sewage | Crops | Soil | |
| Ascaris eggs | Years | <60, usually <30 | Years |
| Tapeworm eggs | Many months | <60, usually <30 | Many months |
To inactivate helminth eggs, it is recommended to raise temperature above 40°C and/or to reduce moisture below 5% and maintain these conditions for an extended period. These conditions are better assured in biosolid treatment. That is why processes that remove particles, such as sedimentation, filtration, or coagulation-flocculation, are recommended to be employed. The elimination of helminths is part of the sludge treatment and is covered in the module “Biosolids Management”.
According to WHO guidelines, if the effluent is intended for the irrigation of crops that will be eaten raw, it should not contain any helminth eggs. Similarly, for aquaculture, it is proposed that no eggs should remain at all, as certain types of eggs (trematode eggs, mostly Schistosoma spp.) can multiply in an intermediary host (snails) before infecting fish and humans.
For further information, please click on the Materials tab at the top of the page.
