Sanitation: Controlling problems at source:
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What are the options for control of excreta?
For practical purposes sanitation can be divided into on-site and off-site technologies. On-site systems (e.g. latrines), store and/or treat excreta at the point of generation. In off-site systems (e.g. sewerage) excreta is transported to another location for treatment, disposal or use. Some on-site systems, particularly in densely populated regions or with permanent structures, will have off-site treatment components as well.
In many places, particularly in areas with low population densities, it is common to store and treat wastes where they are produced - on-site. There are a number of technical options for on-site waste management which if designed, constructed, operated and maintained correctly will provide adequate service and health benefits when combined with good hygiene. On-site systems include: ventilated improved pit (VIP) latrines, double vault composting latrines, pour-flush toilets, and septic tanks. Dry sanitation or eco-sanitation is an onsite disposal method that requires the separation of urine and faeces (box 3). Building and operating these systems is often much less expensive than off-site alternatives. Some on-site systems (e.g. septic tanks or latrines in densely packed urban areas) require sludge to be pumped out and treated off-site. Composting latrines allow waste to be used as a fertilizer after it has been stored under suitable conditions to kill worm eggs and other pathogens.
Box 3: Eco-sanitation
Dry sanitation or ecosanitation as it is called, can provide viable alternatives to water-borne sewerage in some areas. Dry sanitation approaches usually require the separation of urine and faeces. Urine which generally poses little threat to human health, also contains most of the nutrients (88% of the nitrogen, 67% of the phosphorus, and 71% of the potassium) (Wolgast, 1993; Swedish Environmental Protection Agency, 1995). Separation of urine allows it to be used safely as a fertiliser after minimal treatment. Similarly, faeces which contains most of the pathogens, also can be safely used as a fertiliser after storage at ambient temperatures for two years or composting at high-temperatures for six months (WHO, 1996; Mara and Cairncross, 1989). Several types of dry-sanitation systems have been used in China, central America, Sweden and elsewhere.
In more densely packed areas sewerage systems are frequently used to transport wastes off-site where they can be treated and disposed. Conventional centralized sewerage systems require an elaborate infrastructure and large amounts of water to carry the wastes away. This type of approach may work well in some circumstances but is impractical for many other locations. The cost of a sewerage system (which can be as much as 70 times more expensive than on-site alternatives (Carr and Strauss, 2001)) and its requirement of a piped water supply preclude its adoption in the many communities in less-industrialised countries that lack adequate sanitation (Franceys, Pickford, & Reed, 1992). In specific circumstances, cost-effective alternatives to conventional sewerage systems have been developed including small diameter gravity sewers, vacuum and pressure sewers. Simplified sewer systems have been successfully used in Brazil, Ghana and other countries.
Wastewater and Excreta Treatment
Waste needs to be treated to remove or inactivate pathogens before it can be safely reused or disposed of safely. Many on-site waste disposal methods treat excreta by storing it for enough time to kill the pathogens. Most off-site strategies (and some on-site systems) require wastes to be treated at a facility before it can be safely used or released into the environment. In industrialised countries, one approach has been to use mechanical and biological processes (primary and secondary treatment) to remove suspended solids, biological oxygen demanding substances (BOD) and other pollutants. Pathogens and nutrients are typically only minimally removed in these processes. The problem is that these conventional or mechanical processes are expensive to operate: they require energy, skilled labour, infrastructure, and maintenance. To further reduce the pathogens and nutrients requires additional processes, which pushes up the cost still further. In efforts to reduce the cost and complexity of waste treatment, experiments have been conducted with smaller decentralised treatment units. For example in Durban, South Africa local sewerage networks have been connected to small treatment plants (baffled aerobic reactors) to cost-effectively treat more waste. In other areas where offsite treatment is required, and land is available at low cost, waste stabilization ponds have proven to be cost effective methods for treating wastewater (Box 4).
Box 4: Waste stabilization ponds
In warm climates where land is available at low cost, waste stabilization ponds (WSP) are an excellent method for treating wastewater. When designed properly, WSP are more effective at removing pathogens - particularly intestinal worm eggs, than most conventional secondary treatments. Moreover, WSP remove pathogens without the addition of chemicals such as chlorine, are simple to operate and maintain, and promote the use of the water and nutrient resources in the wastewater.
The pathogen removal ability of WSP has been well documented. For example, the use of inadequately treated wastewater in irrigation is especially associated with elevated prevalence of intestinal worm infection. In one area studied in Mexico, irrigation with untreated or partially treated wastewater was responsible for 80% of all intestinal worm infections and 30% of diarrhoeal disease in farm workers and their families. However, when wastewater was retained longer in a series of retention ponds there was minimal risk of either intestinal worm infection or diarrhoeal disease (Cifuentes et al., 2000).
Sanitation: Controlling problems at source:
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