Ashley Bird, Environmental Engineer, Division of Drinking and Groundwater, Ohio Environmental Protection Agency
Karen Mancl, Professor Food, Agricultural and Biological Engineering, The Ohio State University
Microorganisms are a part of the natural environment. Most have little or no effect on human health. Some microorganisms are beneficial and even essential to human health. Unfortunately, a few microorganisms cause disease when they are present in drinking water. Waterborne diseases have caused serious illness and even epidemics. Listed below are common waterborne diseases along with their symptoms.
Waterborne Diseases
| Waterborne Disease | Causitive Organism | Source of Organism in Water | Symptom |
|---|---|---|---|
| Gastroenteritis | Rotavirus Salmonella (bacterium) Enteropathogenic E. coli |
Human feces Human or animal feces Human feces |
Acute diarrhea or vomiting Acute diarrhea or vomiting Acute diarrhea or vomiting |
| Typhoid | Salmonella typhosa (bacterium) | Human feces | Inflamed intestine, enlarged spleen, high temperature; sometimes fatal |
| Dysentery | Shigella (bacterium) | Human feces | Diarrhea; rarely fatal |
| Cholera | Vibrio comma (bacterium) | Human feces | Vomiting, severe diarrhea, rapid dehydration, mineral loss; high mortality |
| Infectious hepatitis | Hepatitis A (virus) | Human feces, shellfish grown in polluted waters | Yellowed skin, enlarged liver, abdominal pain; low mortality; lasts up to 4 months |
| Amoebic dysentery | Entamoeba histolytica (protozoan) | Human feces | Mild diarrhea, chronic dysentery |
| Giardiasis | Giardia lamblia (protozoan) | Human and animal feces | Diarrhea, cramps, nausea, and general weakness; not fatal; lasts 1 to 30 weeks |
| Cryptosporidiosis | Cryptoporidium (protozoan) | Human and animal feces | Diarrhea, stomach pain; lasts an average of 5 days |
One of the critical goals of water treatment is to remove disease-causing microbes from water. The removal techniques vary based on the size of the microbe being removed. Parasites are the largest disease-causing organisms, followed by bacteria. Viruses are the smallest pathogens.
Removing microbes from drinking water is a multi-step process. A variety of water treatment techniques are used to effectively and reliably filter out pathogens. For groundwater supplies filtration is a natural process, while artificial systems are used for surface water supplies.
The natural properties of unsaturated soil attract and trap pathogenic microbes. Once trapped, disease-causing organisms either die-off or become food for soil microbes. This wonderful, simple system protects groundwater from pathogens.
Surface water sources have not been filtered through unsaturated soil. Also, some groundwater supplies are directly connected to surface water sources and can be contaminated with disease-causing microorganisms. To treat surface water, artificial systems substitute for the properties of natural soil. Four types of systems filter disease-causing organisms from surface water.
1. Slow-sand filters
Slow-sand filter systems have been used for over a century to treat drinking water. Water is ponded on the surface of more than a 1-foot deep layer of fine sand. The sand is colonized by natural soil microbes forming a layer of organic matter on the sand surface. Pathogens become trapped in the layer of organic matter, called a schmutzdecke, and are either preyed upon by the natural soil microbes or are periodically removed by scraping off a portion of the accumulated organic matter.
2. Membrane filters
First introduced for drinking water treatment in the 1980s, membrane filtration has been used in the food processing and pharmaceutical industries for decades. Simply put, membrane filters with very small pores strain out the parasites, bacteria, and in some cases, even viruses from water.
3. Bag and cartridge filters
Another approach to filtering out larger pathogens like parasites and some bacteria is to use filters constructed of wound fibers or bags filled with fibers. While initially less expensive than membranes, their reliability must be established. Bags and cartridges must be frequently replaced.
4. Rapid sand filters
Rapid sand filters are currently the most established and widely used method of removing pathogens from drinking water. Treatment occurs through a multiple-step process in a series of tanks as shown in Figure 1.
Figure 1. Flow chart of rapid sand filtration process.
Surface water contains suspended particles, sometimes called turbidity, and pathogens that naturally repel each other. Chemicals, called coagulants, are first added to the water that can break down the natural repulsion and allow the suspended particles and the pathogens to stick together.
The next step is to gently stir the water to cause the particles to collide and form bigger particles called floc. Once the floc particles are large and dense enough, they are separated from the water in basins called settling basins or clarifiers.
The last step is to filter out any remaining turbidity in a tank containing specially selected sand several feet deep.
Overall the final filtration of drinking water is the most critical step in pathogen removal. Proper water filtration through either natural, unsaturated soil, or artificial filter systems protects water system customers from disease.
To protect the public health, water filtration systems require careful: