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3.3 Treatment (Topic c)

3.3.1 Small-scale treatment (household on-site treatment)

The minimum facilities required for most of the households in the Asia Pacific developing countries are latrines and suitable on-site or other means of satisfactory disposal of waste from these households.

Most households use so called "unsealed" septic tanks. This is a two chamber tank with a volume of 2-4 m³ with open bottom and without overflow pipe. In areas where the ground is too impermeable to allow infiltration, or the groundwater level is too close to the ground surface, there is an overflow pipe that allows the wastewater to flow into the street drains, or elsewhere. The share of households that use on-site disposal in South Vietnam is two to three times as high as in North Vietnam, which underscores the fact the sanitary conditions are better in urban areas in the South than in the North.

It is a common misconception of public officials that septic tanks provide satisfactory sewage treatment and that STE (Septic Tank Effluent) is a satisfactory end product. Septic tanks are meant as pre-treatment for the removal of suspended solids prior to disposal by means of a sub-surface leaching field. Septic tanks remove about 60% of the suspended solids and 30% of the BOD of raw sewage. The effluent still contains most of the organic pollutants and pathogens. The suspended solids that settle in septic tanks are partially stabilised by anaerobic digestion. Biological activities reduce the mass of sewage solids in the tank; however, the septic tank still accumulates solids over time. Typically, in Asia septic tanks are relatively small and should be cleaned every two to three years. If not, solids will pass through the septic tank and into drainage canals.

An estimated 70% of households in developing countries, which have flush toilets, also have septic tanks. Most of these tanks are in very poor condition due to insufficient maintenance. In some cases, solids pass through the septic tank and into the drainage system. In contrast, a fairly large number of households do not have septic tanks, probably in the range of 10%. There are some houses which still use the double vault and bucket latrines. The number of households that are without toilet facilities or rely on such devises as overhang toilets, remains at 20%. This means that the equivalent of primary treatment is provided to only 40-50% of the domestic wastewater flow.

Among Jakarta’s 1.4 million poor people, the greatest environmental threat still occurs at the household and neighbourhood level. One recent survey found that in the poorest socio-economic quartile, 31% of the households have neither a piped water connection nor neighbourhood access to a private well, compared with 12% for the city as a whole. In addition, the poorest households were less likely to have neighbourhood waste collection, more likely to share toilets and to have problems with flies both near toilets and in food-handing areas.

Table 3.7 shows toilet types in the surveyed cities of Vietnam as a percentage of households.

In developing countries only 8% of urban low-income dwellers have a house sewer connection, compared with 62% of the urban high-income dwellers. Low-income families often share latrines with 100 or more other community members, and long lines or overflowing tanks deter residents from using them at all.

For household wastewater, the available technologies fall into on-site and off-site categories, with a large number of potential technologies in both categories. The three most relevant on-site technologies for urban conditions in the developing countries are summarised below:

The most relevant on-site technologies

• Ventilated improved pit latrine (no water needed)
• Pour flush latrine or flush toilet with septic tank (large capacity septic tanks are required to include both sullage water and excreta)
• Soakaways or soakage pits for septic tank effluent, where soil permeability is reasonable and where distance to ground water table is tolerable.
• Communal or shared facilities for squatter areas (although case-by-case solution will be needed in some locations, for example over waterways)

The most common off-site technologies

• Small-bore sewer (accept septic tank effluent)
• Septage (septic tank sludge, scum and liquid) cartage and treatment in multi-stage lagoon
• Simplified or condominial (low cost) local sewers
• Dry-weather-flow interceptors (to remove septic tank overflow from open drains)
• Conventional trunk sewers and pumping stations
• Treatment of collected or intercepted wastewater by low-cost means, including where appropriate, multi-stage lagoon and/or aquatic plant systems.
• Basic primary treatment and disposal through marine outfalls with diffusers or on land

Criteria for technology selection include

• Effectiveness and operability
• Affordability and cost recovery possibility
• Acceptability to the user
• Availability of trained personnel for operation and maintenance
• Sustainability

In most of the developed countries, 100% of households have flush toilets. These systems function well and are adequately operated and maintained.

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