The term biologic hazards usually refers to diseases caused by pathogens such as viruses, bacteria, prions, fungi, and parasites. It should be noted that the product of an infection can cause disease as well as the live pathogen. For example, in Ghana and other countries where foodstuffs may be stored in a damp state, fungal infections of tubers or maize (corn) produce aflatoxins, proteins that are potent carcinogens that especially affect the liver.
An astounding 94% of the disability-adjusted life years (DALYs) of disease burden due to diarrhea, principally caused by viruses and bacteria, is environmental in origin.1 Approximately 1.5 million deaths a year, mostly in young children, are caused by poor sanitation, contaminated water, and lack of hygiene (a complex behavioral and socioeconomic component). When feces and urine are not disposed of carefully, and when hygiene (the ability to wash hands with soap) is absent, human pathogens contaminate food, surface and groundwater, and hands. Lifespan in the United States increased in the period from 1900 to 2000 by more than 3 decades, and a good two thirds of this increase has been attributed to clean water, clean food, and sanitation.6 One reason that some countries with limited budgets for health, such as Cuba and Costa Rica, have recently achieved major increases in lifespan and major decreases in childhood mortality and adult morbidity is that they have focused on water and sanitation risks.7 If you take a single point away from this chapter, it is the crucial importance of keeping human and animal feces out of water and food.
Pathogens found in human feces are exquisitely adapted to causing human disease, and it should be obvious that breaking the cycle of transmission through basic sanitation and the provision of clean water should be an extraordinarily high societal priority.8 The major pathogens causing illness and death through these transmission pathways include rotavirus, enteroviruses (a group that includes polio), Salmonella, Shigella, Escherichia coli, Cryptosporidium, Campylobacter, and hepatitis A and E viruses. Some of these are shared with domestic or peridomestic animals of economic importance, such as Salmonella, Campylobacter, E. coli, and Cryptosporidium. Typhoid (Salmonella typhi), in contrast to most of these pathogens, is a disease only of humans, and one way to judge the adequacy of water treatment and sanitation is to look at decreases in typhoid incidence.
The issues of water supply and sanitation in the developing world are of great importance. As the saying goes, “An ounce of prevention is worth a pound of cure.” More to the point is the fact that public health engineers have saved many more lives than doctors over the course of human history. Safe, clean drinking water and adequate sanitation are critical needs for the developing world. However, projects to improve these conditions must include appropriate technology, cultural sensitivity, and long-term management procedures or they will quickly fail.
Water supply starts at the source, either surface water or groundwater. Surface water (i.e., streams, lakes, rivers, or ponds) is easily found and used. Large rivers and lakes provide year-round sources of water, whereas small streams and ponds may fail in dry seasons. Impoundments (dams) may be used to store stream flow from the wet season to supply community needs during the dry season. However, all surface water sources are unprotected. That is, they are very susceptible to pollution and should not be used without treatment.
Groundwater falls into two sources, shallow and deep. Shallow groundwater comes from water infiltrating the soil and trickling down until it is caught on top of the bedrock. This upper aquifer, or water table, fluctuates in depth depending on the season, dropping in dry seasons and rising in wet weather. Although the soil can filter out many pollutants, shallow groundwater is susceptible to pollution and should be used with care.
Shallow groundwater is tapped through wells or springs. Shallow wells are typically hand dug down to the water table and use a hand pump or bucket to bring water up. Springs are natural points where the water table meets the ground surface and water seeps out. Typically these occur at the toes of slopes or on hillsides.
Shallow wells and springs are very common water sources for developing countries. Although they are not pristine sources of water, the method of getting water from them can add considerable pollution to the water, and the solutions are usually easy and inexpensive to use. For wells, a small wall around the top of the well made of stone, brick, or concrete serves to keep animals and small children from falling in and diverts rainwater runoff from entering the well. A cover also serves to protect the well from trash and pollution; providing a bucket and rope, with a windlass to gather the rope when out of use, solely for the well helps keep these items clean and keeps dirt out of the well (Figure 6-1).
Protected well. (Photo by Edward H. Winant.)
Hand pumps provide the best protection for a shallow well because the well remains covered while the water is withdrawn. Further, using a bucket and rope will introduce some contamination because buckets are frequently set on the ground, and ropes pass through the unwashed hands of the users. Many types of hand pumps are available in developing countries, and their use is limited only by cost. They typically cost more than submersible electric pumps in the United States but have the advantage of working without power (Figure 6-2).
Hand pump. (Photo by Edward H. Winant.)
If an event occurs to contaminate a well, perhaps an animal drowning in the bottom, it is possible to “shock” the well to cleanse it. This will remove the existing pollution but will not guard against recontamination. The procedure is to add chlorine, typically in the form of bleach, to the well and then draw out all of the chlorinated water and dispose of it. This prevents people from drinking overchlorinated, and dangerous water, and removes the source of contamination. The bleach should be added to a bucket of water, mixed well, and then lowered into the well to mix with all of the water at the bottom. It is necessary to get an idea of how much water is in the bottom of the well so that the proper amount can be withdrawn. Without too much math, the volume is the area of the well times the water depth. A typical circular well, 1 meter in diameter (3.28 feet) with a water depth of 2 meters (6.56 feet), holds
2 × (3.14 × 12/4) = 1.6 cubic meters or 1,600 liters (410 gallons)
For a spring, a spring box or house helps gather the water from the ground and stores it in a protected place until needed. Typically, a pipe drains the box and allows users to fill their buckets under the pipe, keeping buckets and dirt out of the spring. Also, washing and bathing activities then take place downstream of the spring and do not affect the water quality at the source.
Deep wells, or boreholes, tap a water source (aquifer) that is much deeper and more protected than shallow wells. These deeper aquifers are contained in water-bearing rock layers under layers of impermeable rock. Thus their waters are safe from most forms of pollution but are also more of a finite resource because they are so hard to recharge.
Reaching these deep aquifers can be quite a challenge. They need to be drilled or bored into the rock with specialized machinery. Further, the hole has to be cased through the upper soil levels to keep potentially dirty waters out of the well. Finally, an electric submersible pump is extended into the hole to access the deep waters, thus requiring modern machinery and electricity for use.
A final source of drinking water is collecting rainwater. This is most commonly done on the roofs of houses, with gutters to collect and carry the rain to a storage basin, the prototypical American rain barrel. Gutters are fairly easy to install, but sizing the storage basin can be a problem. Ideally, it would be large enough to store all the water needed by the inhabitants of the building from one rainfall until the next. The difficulty arises because so many locations on earth have wet seasons and dry seasons, and the time between rainfalls may last weeks or even months. Storing enough rainfall from a rainy season to last through a dry season requires large and expensive storage tanks (Figure 6-3).
Rain water. (Photo by Edward H. Winant.)
The other aspect of rainwater collection is that roofs are typically quite dirty, with leaves, sticks, and bird droppings. There are two solutions to the problem of a dirty roof: foul flush tanks and filters. The foul flush tank diverts and stores the initial rainfall, which is assumed to rinse the roof clean. After a short time, the rest of the rain is collected in the main storage tank. Filters are usually sand columns placed on top of the storage tank to remove contamination. Filtration is discussed in more detail later in the chapter.
Once the water source has been identified and developed, some thought must be given to getting it to the users. The most common and low-tech method is to have users come to the water source and carry their daily supply home in buckets or jars. This requires a lot of human effort and also serves to reduce daily consumption. People are not inclined to take long baths or to waste water when they have to tote it a long way. In these cases, water use is usually restricted to cooking, cleaning, drinking, and occasionally bathing. Washing clothes and bathing may take place nearer to the source. This reduces the need for transporting water but usually leads to further contamination of the source water unless protective steps are taken as outlined previously.
Another method of water delivery is the commercial water cart. Here, larger supplies of water are brought to homes by cart, and the water is sold to the homeowner. Carts can vary from small pushcarts carrying 50 gallons or so to animal-drawn carts with a few hundred gallons, or to tanker trucks capable of delivering thousands of gallons.
The “modern,” or preferred, method of water delivery is through pipes. Laying pipes in the ground is an expensive investment in community infrastructure, which is the main drawback to its universal adoption. When using pipes, it is also necessary to provide pressure to force the water through them. This is typically done by pumps, which require a power source. Water towers are usually included in the system because the demand for water varies through the day and can exceed the pumping rate. Thus towers store water at night, when demand is low, and assist the pumps in the morning and evening, when demand is highest. In some places pressure may be provided by gravity, if the water source is sufficiently elevated above the users. When relying on gravity, storage tanks are sometimes required to maintain a sufficient supply of water.
A commonly adopted system is to pipe water into a community center from a remote source and then require users to carry their daily supply from the tap to their homes. This reduces individual treks to find water from miles of walking to more reasonable distances, but it also saves money on laying pipes throughout the community to every home.
What options exist for poor rural people in the developing world? It must be recognized that in many places, an adequate quantity of water is more important than quality. Water may have to be carried, sometimes for miles, which consumes a huge amount of time, principally for women and children. In developed countries, the basic assumption is that each person uses 50 to 70 gallons per day (195 to 275 liters per day). Of course, this covers various uses such as watering the lawn, washing cars, laundry, automatic dishwashers, and teenagers taking long showers. The WHO suggests that the minimum amount in the developing world, where people must carry their own water, is 2.5 gallons per day (10 liters per day). An adult in the setting of drought needs at least 5 liters of water per day for basic food and hydration needs, without even considering the needs for basic hygiene (washing hands, etc.).9 Actual use will fall somewhere in this range and will tend to increase if water is piped directly into each house. Thus efforts to decrease the environmental risks of unclean water must often address both quantity and quality.
- Feces should be kept out of water supplies with the use of basic or improved pit latrines.
- Water can be boiled if there is sufficient fuel in the area.
- Simple filtration (e.g., through cloth, such as a sari) will remove some pathogens, as has been amply demonstrated in Bangladesh and India.10
- It is being increasingly recognized that simply letting water settle after collection will carry many pathogens down with the sediment.
- Relatively simple methods for treatment at the household point of use—such as chlorinating water with the use of household bleach, or storing water in translucent or transparent vessels that allow ultraviolet (sunlight) sterilization to occur—are being tested.
- Communities can organize themselves to build simple water distribution systems, using PVC or similar pipes, where the source water is upstream of the community and therefore unlikely to be fecally contaminated.
With water provided to homes, the next thought is treating it to improve the water quality and reduce incidents of sickness. Treatment may occur on many levels, from small doses for the individual, to a household system for all occupants, to communitywide treatment systems. However, the basic steps and methods of treatment are similar at all levels.
Water treatment consists of three basic steps, although not every method includes all the steps.
Primary, or physical, treatment consists of settling out particles in the water.
Secondary, or biologic, treatment involves filtering the water through a benign biologic layer to reduce organic contamination. The biologic layer is typically fixed or suspended in some type of filter medium, such as sand. Modern plants in the developed world sometimes use plastic shapes or grids for the same purpose.
Tertiary, or chemical, treatment (also known as disinfection) is aimed at killing and removing harmful pathogens in the water. The most common chemical for disinfection is chlorine.
Personal water treatment, mostly used in travel situations, consists of either portable filters (backpacking water filters) or chemical tablets. Backpacking filters use a hand pump to force water through extremely small pores in a filter medium and remove particles, organic materials, and possibly pathogens, depending on the pore size. Tablets, either chlorine or iodine, disinfect the water, killing pathogens but not removing any silt, particles, or organic material. These tablets will not remove color or existing bad taste from the water.
Household treatment accounts for daily water use for several people. The simplest method is to store water in large covered barrels. This form of primary treatment will settle out particles in the water that lead to bad taste and color. Secondary treatment, or filtration, can be achieved with a range of commercially bought units that use porcelain candles or fabric bags to strain out contaminants. In general, the pore sizes on these filters are not small enough to remove pathogens, so a disinfection step is also required. Forcing water through a pore size small enough to remove pathogens requires pressure, and this complication would make most home-sized filters too complex (Figure 6-4).
Home water. (Photo from “Water in Africa” U.S. Peace Corps Photograph Archive. http://www.peacecorps.gov/wws/educators/enrichment/africa/index.html.)
The most accepted method of disinfection for a household is boiling. Water temperatures higher than 140°F (60°C) will kill pathogens. Of course, without a thermometer it is hard to judge 140°F, so bringing the water to boiling temperature is a nice visual indication of the proper amount of heat. Some authorities recommend boiling water for 30 minutes to ensure complete disinfection. This can be quite wasteful of fuel, however, and simply bringing the water to a rolling boil at sea level is sufficient. At higher elevations, boiling water for 5 minutes or less will typically give good results. The water should be boiled in a covered pot for protection and be allowed to cool. When sufficiently cool, it may be poured into the filter or other storage container.
Household filters can also be constructed using local materials. Typically the container is an oil drum or other large barrel. Gravel is placed at the bottom around the outlet pipe, which needs to be punched through the barrel wall. The gravel should be small enough, such as pea gravel, so that the sand does not settle into the pore spaces. Over the gravel, at least 24 inches (0.60 m) of sand should be placed. This should leave enough room at the top of the barrel for water to stay while it filters through the sand. The outlet pipe should also be equipped with a tap, so that water may be withdrawn without problem. Of course, this means the filter needs to be raised enough to get a container under the tap.
Another good household disinfection method is using clay filters treated with colloidal silver, such as those made by Potters for Peace.11 These filters, which can be made locally in almost any village, are inexpensive and do a fair job of destroying pathogens. The silver impregnation lasts for about a year of normal use before replacement is needed.
Of all these household treatment methods, the single most important is boiling because this does an effective job of removing pathogens, and every household has a way of heating water. Thus teaching villagers to boil water is the single most effective way of getting them to improve their water quality. It can be difficult to convince them of the need, however, because the fuel cost of boiling all drinking water can be excessive. However, this simple step can reduce the incidence of sickness dramatically, especially for infants, young children, and the aged.
Community water treatment is generally an extension of the procedures just mentioned. Settling basins are used to remove solid particles suspended in the water. Filters are then used to further purify the water, and then it is disinfected and stored.
Historically, the first community filters were slow sand filters. These were large beds of sand through which water slowly percolated. The slow rate of application kept the sand from getting clogged too often. When the sand had trapped enough contamination to clog the filter and reduce the percolation, the filter was cleaned by manually raking the sand and removing the top layer. As demand for water in cities grew, these slow filters soon became too large, and rapid sand filters were introduced. As the name implies, the water is applied much more quickly to a rapid sand filter, and the filter tends to clog much sooner. The cleaning method is to apply a backwash periodically to the filter. Backwashing means forcing water through the filter in the reverse direction, which expands the sand, cleans out the clogging material, and readies the filter for continued operation. In developing countries with available land, especially for small communities, slow sand filters are preferred for their low cost and easily understood maintenance. Where land is not available, rapid sand filters should be considered.
For disinfection, the most commonly used chemical is chlorine. It comes in three forms: gas, liquid, and solid tablet. The gas form can be somewhat tricky, so for small systems, a liquid drip is the preferred method. This drip is introduced by a small feed pump into the water line so that the concentration of chlorine in the water is roughly constant. Chlorine is a dangerous chemical, both for the operator and for the end user if the concentration is too high. However, it is well understood, relatively inexpensive, and leaves a residual in the water line that continues to protect the quality of the water throughout transmission.
People living in the developed world as well as the developing world have the need to maintain rigorous water treatment and sanitation practices. The methods used for water treatment—halogenation, usually with chlorine or chloramines, and then filtration—were devised over a century ago, and although effective when optimally implemented, they suffer the deficiencies of old technology. Chlorination is highly effective against bacterial and viral infections, and when first instituted it uniformly leads to major decreases in the burden of disease due to these infections. However, it is ineffective against a number of emerging pathogens that are chlorine resistant. Many of these resistant pathogens are most active where especially susceptible populations exist, such as people with acquired immunodeficiency syndrome (AIDS) or pregnant women.
An epidemic of waterborne toxoplasmosis was detected in Vancouver, Canada, stemming from the use of water from a reservoir that was chlorinated but not filtered. Astute clinicians noted an increase in the number of cases of in utero (congenital) Toxoplasma infections, as well as retinal disease in the general population. An epidemiologic investigation revealed that cougar feces in the watershed contained Toxoplasma oocysts. Presumably, the infectious oocysts were washed by rainfall into the reservoir and (unaffected by the chlorination) then directly entered the drinking water supply.12 To globalize this incident, one only needs to reflect on the absence of filtration in many countries where basic chlorination is provided. Estimates from Central America and Africa suggest that most cases of toxoplasmosis are the result of infection with the oocyst form of the parasite, which is only excreted by felines. In the United States and Europe, most toxoplasmosis is the result of eating undercooked meat that contains Toxoplasma cysts.13 The addition of filtration to water treatment, even simple sand filtration, is believed to decrease the risk of infection from pathogens such as Giardia, Cryptosporidium (and, one supposes, Toxoplasma) by about 100-fold.14
Filtration is not a perfect defense, even though it may remove the vast majority of pathogens (99.00% to 99.99% of pathogens is typical for modern conventional treatment plants).14 Unfortunately, the infectious dose needed to infect 50% of people for Cryptosporidium is under 10 oocysts for some strains,15 suggesting that even the rare organism that slips through the filtration system can cause illness. The largest outbreak of waterborne disease in the history of the United States occurred in Milwaukee in 1993 when more than 400,000 people became clinically ill with cryptosporidiosis when one of the two filtration plants in Milwaukee failed.16 Of note, infection rates in households with tap water filters were approximately 80% lower than in households without them. In 1994, an epidemic of cryptosporidiosis in Las Vegas in people with AIDS was epidemiologically linked to the municipal water supply, even though it met all relevant chlorination and filtration standards.17 It must be emphasized that the two major causes of persistent diarrhea in people with HIV/AIDS in the developing world are cryptosporidiosis and microsporidiosis.18 These pathogens cause chronic diarrhea and malabsorption with wasting. Neither of these diseases has reliably effective drug treatment, and thus prevention (through paired drinking water treatment and sanitation) is the only real option against these scourges.
The use of halogens in treating water introduces variable levels of these elements into water. Halogens have been linked (at higher levels) to bladder cancer, fetal congenital defects, and miscarriages.19 The balance between a halogen level sufficient to kill pathogens and low enough to minimize other risks is a delicate but necessary one.20 Failures of chlorination have led to outbreaks of dysentery and diarrhea in Canada21 and typhoid in Central Asia,22 proving the point that water treatment systems cannot be allowed to fail, no matter the location. One of the ironies of water treatment practices is that source water protection (e.g., not letting fecal material enter source water for drinking purposes) has been ignored in many communities, under the assumption that water treatment will invariably render the water completely safe.
A safer, although somewhat more expensive, method of disinfection is ultraviolet (UV) light. This involves passing the water past UV light bulbs, where the radiation kills off the pathogens. UV disinfection requires relatively “clean” water, meaning that most of the suspended solids have been removed.
Of course, the application of water treatment depends heavily on the source water available. Surface waters, being unprotected, are usually suspected of being highly contaminated with organic material and pathogens. Further, many rivers and streams carry a high silt load, so settling basins (primary treatment) are almost always required when treating surface waters. Springs and shallow wells may be contaminated, depending on what is “upstream” of them in a groundwater sense. Mountaintop springs, which basically are fed by pure rainwater, can be of very high quality. Springs situated below farms or houses are likely to be quite contaminated. Still, groundwater does not carry the silt loads that surface waters do, so in many cases disinfection is all that is required of spring or well water. Deep wells, if properly constructed and drawing from a quality source, may not require any treatment to be safe for drinking.
Sanitation deals with treating the waste products of human society and making them safe for the environment and for public health. This section discusses human waste (feces and urine) and solid waste (garbage).
As with water treatment, wastewater treatment falls into the same three levels: primary (physical), secondary (biologic), and tertiary (disinfection and polishing). Further, these apply to all wastewater treatment, from individual house systems up to the largest municipal plants.
Sanitation provides benefits beyond those of decreasing diarrheal disease. For example, intestinal nematode infections from Ascaris, Trichuris, and hookworm are all transmitted after fecal contamination of soil. The first two nematodes are ingested (either in soil or in contaminated uncooked food), and hookworm larvae penetrate the skin of people without shoes. All three of these cause diseases that contribute to malnutrition and to anemia but can be completely prevented by the implementation of adequate disposal of feces. In the United States, rural poor farmers residing in the southern areas of the country were once regarded as lazy, before it was understood that most of them were severely anemic from hookworm infections. After World War I, the Rockefeller Foundation devoted enormous resources to convincing people to spend scarce resources on shoes and sanitation facilities.23 Schistosomiasis, a trematode infection, affects hundreds of millions of people, and the infectious eggs are all excreted in urine or feces. Again, simple sanitation would abolish this disease over time in affected regions.
The most common form of sanitation in developing countries is the latrine or outhouse. Latrines may be provided for individual houses or combined into a community facility. The latrine is a simple pit, covered with a durable slab, where users go to relieve themselves. Concrete slabs, at least 6 inches (15 cm) thick and reinforced with iron bars, make the best latrine floors. They are easy to clean, last a long time, and are very sturdy. A less expensive floor may make use of wooden planks or even logs.
Because no water is used to transport the waste, there is no need for wastewater treatment. The pit holds the solids, allowing for some biologic degradation, but in general serves only for storage. Eventually the pit will reach capacity, leading to removal of the solids or digging a new pit. Ash or lime is sometimes added to the pit to help control odors.
The pit is usually left unlined if it is dug in a stable soil such as clay. Where sandy soils predominate, some reinforcement of the pit may be required. Although there is no effluent to treat, the urine will seep out the pit floor and walls into the surrounding soil. This is normal, even beneficial, but some care must be taken to separate the pit from the surrounding groundwater.
Two feet (0.61 m) of soil separation is sufficient to protect the groundwater. Thus the pit should not be dug deeper than 2 feet above the water table. The water table may be determined roughly by the water depth of nearby shallow wells. Remember, however, that the water table will fluctuate according to wet and dry seasons; the latrine should be sited using the wet season water levels. If the groundwater level reaches the bottom of the pit, it will become contaminated and threaten nearby shallow wells and springs.
Where there are high groundwater tables, it may be necessary to construct vault latrines. This variation includes a lining for the pit of concrete, brick, or stonework to prevent groundwater contamination. Obviously, this method is more expensive, and so it is used only when absolutely required to protect the groundwater (Figure 6-5).
Vault latrine. (Photo by Edward H. Winant.)
An implementation that many groups prefer is the ventilated, improved pit latrine (VIP latrine). This involves building solid walls that do not allow light through, a tight roof covering, and a screened ventilation pipe running to the top of the roof. The doorway must be set away from the prevailing winds, so that the wind draws air out of the latrine rather than forcing it in. Further, with no light in the latrine, the only light available to the pit comes from the pipe, which draws the flies up to the screen. Unable to escape, they die (Figure 6-6).
VIP latrine. (Photo by Edward H. Winant.)
Culturally, VIPs are unacceptable to some people. Because they are so used to latrines, they consider it improper to relieve oneself inside. Adding a roof to a latrine makes it a building, and thus not suitable. Many latrines are thus simple affairs with walls of plaited leaves or sheet metal for privacy, or situated so they are screened naturally by trees.
Another improvement on the common latrine is the composting latrine. This involves improvements to the pit so that air and heat are available to promote the composting process. Additionally, access is needed because the pile needs to be turned. This is usually done manually with a pitchfork to stir the accumulated waste. A carbon source is also needed and can be provided by sprinkling sawdust or throwing paper waste down the latrine hole after every use.
Once water is provided to individual homes, flush toilets can be installed, at which point wastewater becomes a much larger problem. The first step toward treatment is usually to pipe wastewater to the latrine pit, thus making it into a cesspool. However, this pit now has to deal with a great quantity of polluted water in addition to the solids it was storing. The water will leach out the sides of the pit; given the amounts of water used and the organic contamination, this will eventually clog the soil around the pit and back up into the house.
Septic systems, in which the solids settle into a tank and the effluent passes to a field of perforated pipes to soak into the ground, are much more effective in the long term. They require additional investment and a larger area for application, however. In areas of low housing density, they are undoubtedly the best method for wastewater treatment.
For areas of greater housing density, sewers are the preferred method. These large pipes, laid so gravity will convey the sewage, collect wastewater and convey it to a central treatment location. This treatment plant usually consists of settling tanks (primary treatment); biologic treatment, such as filters or aeration tanks (secondary treatment); perhaps some polishing or additional filtration, and then disinfection (tertiary treatment). The treated effluent is then discharged to a nearby surface water body; the removed solids, now called sludge, will be deposited in a land fill, incinerated, or used as a soil amendment or crop fertilizer.
Biologic treatment for community wastewater may be in several forms. The least technological is lagoons, or sewage ponds, where the sewage is contained for long periods of time to allow proper treatment. Lagoons do occupy large areas of land, but they do not require much maintenance. Sand filters are also frequently used to treat wastewater, as are aerobic tanks and wetlands. Aerobic tanks introduce air into the sewage to promote the growth of aerobic bacteria, which are very efficient at consuming organic waste. Filters work much the same as for water treatment, supporting a layer of bacteria that consume the organic waste, as well as physically straining the water to remove solids.
Constructed wetlands combine both methods of treatment. Here, the effluent flows through a gravel bed, which performs the tasks of a filter. Water-loving plants, such as reeds or cattails, grow in the gravel, and their roots provide oxygen for treatment, take up some of the effluent for their water needs, and also remove nitrogen and phosphorus from the waste as plant nutrients.
Proper sanitation also includes solid waste, or garbage. Sadly, this is commonly overlooked, and many villages have no way of dealing with their accumulating garbage. It is frequently piled in heaps or scattered about carelessly. Both conditions are unsightly, smelly, and can support rodents, insects, and other disease vectors.
When garbage is collected in central locations around the community, it is common to periodically set fire to the collected waste and burn off the combustibles. Although incineration is certainly an accepted method of reducing the volume of solid waste, it is helpful to attempt it in controlled conditions. Certain materials, notably plastics and tires, give off noxious fumes when burned. In general, the smoke from trash fires can be hazardous and is certainly annoying to nearby residents. Finally, some materials will not burn and will remain after the attempted incineration.
Another method of solid waste treatment is land filling. This requires a suitable area of land set aside to receive the solid waste, preferably away from most residents. It also requires soil to be placed over layers of garbage to contain the odors and disease vectors. The soil cover, with accompanying ditches and landscaping to control runoff, is important to keep water contaminated by the waste (termed leachate) away from other sources of water that serve the community. If possible, the floor of the landfill should be a heavy clay soil, compacted by machinery to further contain the leachate.
Given that landfills and dumps should not be too close to communities, a garbage collection and transport method should be established. Although it is certainly possible for each resident to make a trip to the town dump, it is more convenient to have local community collection points throughout the town and have the garbage picked up and taken to the landfill using community resources.
Recreational Water Exposure
Recreational water exposure has also been recognized as a potent source of fecal-oral contamination. Indeed, the U.S. Centers for Disease Control and Prevention defined swimming in pools and other recreational waters as “communal bathing” and has published studies on the average mass of feces carried by swimmers into pools. In the United States, epidemics of disease caused by Giardia, Shigella, and Cryptosporidium occur every year because of fecal contamination of recreational swimming sites such as pools, lakes, rivers, and beaches.24 Recreational waters in some countries are contaminated not only by diarrheal fecal pathogens but also by parasitic pathogens such as schistosomes and by viruses such as polio, other enteroviruses, and hepatitis A. Historically, polio was frequently waterborne in the now-developed world, and swimming in rivers, ponds, or lakes (which are obviously not chlorinated) was a recognized risk for the disease.
Management of Water and Sanitation Systems
Use of community resources brings up an important point: management. In many cases, it is not the technology or even the resources that are a barrier to project implementation; it is the continued management of the project that causes failure. Any village in the world can dig latrines and shallow wells and provide for trash collection. What is lacking is the community management needed to marshal the community resources to accomplish these tasks and see to their continued operation and maintenance.
For instance, tales of broken and unrepaired pumps, caved-in wells, or dilapidated spring boxes are common throughout the developing world. Many of these projects are installed by well-meaning and dedicated volunteers and nonprofit organizations. The projects work well and are much appreciated for several years. But something eventually breaks, and there is no money to fix it. The impressive and helpful project goes to waste, and the residents return to their previous ways of getting water or eliminating waste.
It is important to involve local residents in project planning and implementation. This means more than just asking the opinions of the village elders. In many cases the elders, usually men, want a project that will bring prestige or honor to their village. However, the women and children who must make use of the new infrastructure have other ideas. For instance, when it is the children’s job to get water, it is no good installing a hand pump that requires great strength to use. It is also sometimes the case that women who walk a good way to gather water at a remote spring cherish the communal time they have together and resent the piping of this spring into the village to relieve them of some of their hard work.
Thus project planning should involve representatives of all groups in a community. It should also deal with requirements for upkeep and use of the installed equipment. The minimum level of management should be the creation of a community committee charged with overseeing and maintaining the project. This committee should have a maintenance budget and a way to collect money from the users. For example, a hand pump costing $1,500 and expected to last 20 years should have $75 collected each year in a replacement fund ($6.25 each month). If this pump serves 25 homes, then each family would be expected to contribute $0.25 each month.
What happens, in many cases, is that some people do not pay, even when they have the best of intentions. Perhaps the harvest failed, or a child was sick and they needed the money for medicine. If they fail to pay and yet can still use the community resource (pump or spring box or latrine), they have less incentive to pay in the future. Neighbors, seeing this, are also less inclined to pay. Sadly, it is very common for no one to pay into community repair funds. Then, 15 or 20 years down the road, when the pump breaks or the latrine is full, there is no money to fix it.
It is helpful, therefore, to have some enforcement capacity for the committee. At the very least, peer pressure can be exerted on noncontributing families to encourage their participation. The most effective management, of course, is the utility model, where users pay for the amount of water or sanitation services that they use and can be cut off for nonpayment. This provides an enforcement action to ensure continued participation.
Disturbance of the Natural Environment and Risks for Infectious Diseases
Some ecosystems support Anopheles, Culex, or Aedes mosquitoes, which transmit diseases such as malaria, filariasis, dengue, and yellow fever. Brackish water, as found in coastal mangrove swamps, is a reservoir for Vibrio cholerae, the agent of cholera. Perhaps by convention or out of reverence for the natural environment, we do not usually consider a pristine swamp an environmental hazard. However, it is clear that our forebears did, for they industriously drained swamps to provide more arable land and to decrease the risks of diseases such as malaria. One of the greatest accomplishments of the fascist Italian dictator Mussolini was the drainage of the swamps near Rome and the eradication of malaria from the region. When the Tennessee Valley Authority in the United States built dams in the 1930s and 1940s to provide electricity to Appalachian areas, studious care was taken to alter the water levels periodically in dams and rivers to disrupt the hatching of mosquito eggs. This had, at times, devastating effects on the aquatic habitat of the affected rivers, but the incidence of malaria was dampened by these tactics.
A counterexample demonstrating the importance of intact ecosystems to human health is that of the Naivasha Lake region in the Rift Valley of Kenya. The town of Naivasha does not treat its sewage, which flows into a lake that is used for both drinking water purposes and fishing. Fortunately, the Kenya Wildlife Service maintains a game preserve where the contaminated water from the town flows into wetlands. The wetlands detoxify and decontaminate the wastes before they enter the lake. Indeed, the use of artificial wetlands in tropical countries is being promoted globally as a way to treat wastewater without the capital expense of a modern treatment plant.25
There is substantial evidence that the destruction of ecosystems increases the hazards of infectious diseases. Malaria epidemics often follow the construction of roads and houses in forested areas because new water pools (breeding sites for mosquitoes) are unintentionally left without adequate drainage near the construction. Indeed, deforestation of tropical forests, and the concomitant construction of crude logging roads to remove the trees, results in predictable increases in mosquito-borne infections. As an example, yellow fever is maintained in a high forest canopy (sylvan) cycle between primates and mosquitoes in South America that does not involve humans. When trees are felled by loggers, however, the yellow fever–infected mosquitoes then bite the workers, who carry the infection to cities, where the cycle is maintained in humans via Aedes aegypti mosquitoes (the urban cycle).
Other examples are plentiful. The damming of the Nile River at Aswan in Egypt led to an explosion in the incidence of schistosomiasis, as did the damming of the Volta River in Ghana. Schistosomes have difficulty penetrating the skin of human hosts in rapidly flowing waters, but the damming led to placid waters and greatly increased transmission. The introduction of irrigation in Puerto Rico for sugarcane production led to extremely high rates of schistosomiasis at the turn of the last century.26 The several decades long increase in the incidence of Lyme disease in the densely populated northeastern United States is considered by most biologists to be the result of an exploding deer population (after elimination of their natural predators) and the desire of humans to live in suburban or semirural areas. Both of these factors increase the likelihood of exposure to the tick vector, which normally feeds on deer and mice. Thus environmental risks for acquiring infectious diseases often are linked to both disturbed or changed ecosystems and increasing human presence in the involved area.
As delineated by the WHO, three approaches to the environmental management of mosquito-borne diseases such as malaria, Japanese encephalitis, and dengue are as follows1:
Modification of the environment to reduce vector habitats
Manipulation of the environment on some periodic basis
Modification of human behavior or habitation
Draining swamps, leveling land, filling in pools, modifying river boundaries, lining irrigation canals to prevent water loss, and avoiding stagnant waters are examples of the first approach. In urban environments, these methods include the construction of drains, improving house design so that water does not pool in gutters, and providing wastewater and sanitation facilities to remove mosquito breeding sites.
The second approach is represented by efforts such as changing the levels of reservoirs. The third includes simple methods such as fine screens in household windows to decrease contact with mosquitoes, and the use of bednets. Bednets are an interesting tool because they incorporate both an environmental barrier between the vector and humans, and, if insecticide treated, a chemical defense as well. Treated bednets have been found to reduce overall mortality in children younger than 5 living in malaria-endemic regions by as much as 40%.27
Climate, the Environment, and Human Health
The linkage between climate, alterations in the environment, and specific diseases is regarded as well founded. The incidence of cholera in western South America and of diseases such as Oroya fever in the Andes has been linked to the sea temperature of the Humboldt Current, especially during El Niño phenomena.28 The details and mechanistic explanations for these relationships are still being delineated, but it is not hard to imagine that sea temperature affects land conditions, which in turn affect vegetation and humidity, and in turn the density of insect vectors of disease. By way of example, my colleagues and I have shown that cases of Salmonella and Campylobacter infection reported to the Massachusetts Department of Public Health are tightly linked to the ambient temperature, whereas reported cases of Cryptosporidium, Shigella, and Giardia infections peak some weeks after the peak in summer temperature.29 Salmonella and Campylobacter are known to reproduce in foodstuffs, and the coinciding peaks of temperature and infection with these two bacterial pathogens probably represent the product of maximal bacterial growth during the hottest days of the year. In contrast, it can be argued that the triad of Cryptosporidium, Shigella, and Giardia infections in Massachusetts represents transmission via recreational water exposure. Surface waters used for recreational purposes (ponds, rivers, outdoor pools) achieve their highest temperatures some weeks after ambient air temperatures peak, explaining the lag period, because people are most likely to swim when the water is warmest.
Environmental factors also include rainfall. In many cities and towns in the United States, surface water runoff is drained into the sewage treatment system because separate runoff and sewage treatment systems are more expensive than a combined system. However, heavy rainfall can overwhelm the capacity of sewage systems, leading to the discharge of untreated sewage into rivers or lakes. Indeed, Curriero and colleagues have shown that waterborne disease epidemics in the United States tend to follow periods of very heavy rainfall.30 In the developing world, it has been noted that epidemics of diseases such as cryptosporidiosis tend to occur at the beginning of the rainy season when rainfall is likely to sweep human and animal wastes into waters eventually used for drinking and cleaning.31
Infectious disease is also the final mechanism by which other physical environmental factors cause human disease. For example, air pollution both decreases lung function and increases the risk of pneumonia.