Treatment critical to curbing waterborne disease.

The combination of wastewater treatment and drinking-water treatment has dramatically reduced cholera, typhoid, and dysentery in the United States and other developed countries. This is not the case in the developing countries, where wastewater treatment and drinking-water disinfection are inadequate or nonexistent.

"Almost half the world's population suffers from diseases associated with insufficient or contaminated water and is at risk from waterborne and foodborne diseases, of which diarrheal diseases are the most deadly," according to a World Health Organization (WHO) report. These diseases "caused over three million deaths in 1995, 80 percent of them among children under age five."

This is not to say that the United States is free of waterborne illness. Individual cases and occasional outbreaks continue to be reported, most notably the 1993 cryptosporidiosis outbreak in Milwaukee that was linked to 104 deaths and illnesses among an estimated 403,000 people. Cryptosporidiosis is caused by the Cryptosporidium parasite, which is found in water contaminated by human or animal feces.

Experts warn that with the prevalence of air travel, a disease outbreak in another part of the world can quickly pose a threat to U.S. residents. The challenge for the United States is to ensure that we continue efforts to properly treat wastewater and drinking water, while not becoming complacent about our advances to date.

Death Rate Declining in the United States

In addition to the diseases mentioned above, waterborne illnesses include giardiasis, hepatitis, and viral gastroenteritis, among others. These diseases are the result of a variety of bacteria, viruses, and protozoa that can find their way into the drinking-water supply or contaminate the foods we eat. Some general symptoms of waterborne disease include fever, abdominal discomfort, diarrhea, and vomiting.

During the past century, the number of reported outbreaks and deaths in the United States linked to these diseases has dropped considerably. Between 1920 and 1991, there were 1,087 deaths associated with waterborne diseases, according to the U.S. Environmental Protection Agency (U.S. EPA). Of those deaths, 83 percent occurred before 1936 and less than one percent occurred after 1970.

Mentioning specific numbers when talking about waterborne diseases can be tricky, however. Symptoms can be mild, and may not occur until many days after the exposure. Most cases are never reported to health officials and therefore are not linked to waterborne sources. Even when illnesses are reported, the pathogen responsible is identified only about half of the time. In fact, some experts believe that only 10 to 20 percent of cases are ever reported. Thus the exact number of cases cannot be determined.

Why We Treat Wastewater

The role wastewater treatment plays in disease reduction is not always apparent. Inadequate treatment can reduce the effectiveness of drinking-water treatment systems downstream and can contaminate the groundwater and nearby wells. Poorly treated effluent from wastewater systems discharged into rivers and lakes can also be a direct health hazard to swimmers and water skiers and can threaten commercial fishing. It can cause the degradation or even death of bodies of water that become overloaded with nutrients. For these reasons, it is essential to properly treat and dispose of wastewater.

Centralized Treatment

In densely populated areas, central collection and treatment of wastewater with direct discharge to a body of water is usually the most effective and economical method of dealing with wastewater.

The treatment plant normally uses a number of treatment steps designed to reduce the oxygen demand of the influent sewage and, most importantly from a preventive standpoint, remove as many solids as possible. Removing solids not only helps to eliminate some pathogens directly from the wastewater stream, but also ensures that the effluent is prepared for the final treatment step - disinfection.

Disinfection, the process of reducing or totally destroying pathogenic organisms, is most effective when the concentration of suspended solids in the wastewater is at a minimum.

Chlorine is the most common disinfectant presently in use. It is a chemical disinfectant, as are ozone and bromine chloride. Ultraviolet light is coming into increasing use as a disinfectant because it has the [TABULAR DATA OMITTED] advantage of not requiring the handling of potentially dangerous chemicals.

On-Site Systems

In less populated areas, where lot sizes are large and homes are spread apart, on-site wastewater treatment with subsurface discharge is generally the most practical option for wastewater disposal. This generally - but not always - involves the use of a septic tank and drainfield.

A septic tank is normally designed to retain wastewater for at least 24 hours. This retention period promotes the settling of solids and the floating of grease and other lighter materials.

The rest of the on-site treatment system (i.e., the distribution field) ensures the removal of the remaining pathogens as the septic tank effluent moves through the drainfield and the soil. Insects, bacteria, and various organisms already present in the soil digest the organisms and other materials present in the effluent.

This degradation process, along with the filtration of the effluent as it passes through soil, completes the treatment of the septic system effluent.

System Failures Can Spell Trouble

Both treatment plants and on-site systems can have problems ranging from poor design or installation, to inadequate maintenance or process control, to insufficient capacity or aging equipment.

For community treatment works, if the design and construction are adequate, poor maintenance or process control may result in inadequate treatment. Wastewater treatment plants are usually designed not to completely remove all pathogens, but to reduce the concentration of indicator organisms significantly - from millions to hundreds. Passage through a drinking-water plant can then easily deal with the remaining pathogens.

Problems can arise if the required reduction in organisms does not take place at the wastewater treatment plant, which puts an extra burden on the other steps in the treatment chain. In on-site systems, problems can occur because of poor site evaluation, design, installation, or maintenance. Usually these factors contribute to drainfield failure. This can take the form of discharge to groundwater without adequate treatment (e.g., through limestone or fractured bedrock) or surfacing caused by tight soils or solids buildup, which can result in direct runoff or ponding.

Such conditions can pose a threat to wells or, in the case of runoff or ponding, a health threat through potential direct contact with human beings. Ponding can also provide a breeding ground for disease-carrying insects.

The key to the prevention of waterborne disease is vigilance, including the maintenance of all of our protective mechanisms. More often than not, the prevention of waterborne disease starts with the proper treatment of wastewater.

Environmental Health-'Net

* National Association of Physicians for the Environment http://www.intr.net/napenet

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* Uncle Sam Wants Comments on Healthy People 2010 http://web.health.gov/healthypeople

On September 15, 1997, a new web site opened to accept public comments on Healthy People 2010, the U.S. health goals for the first decade of the next century. The new URL address was announced in the Federal Register on September 5. This notice calls for public comments on the proposed structure of Healthy People 2010 - its goals, focus areas, and focus area arrangement. Commenters also have the opportunity to suggest modifications and deletions to existing Healthy People 2000 objectives, as well as to propose new objectives.