In decentralised systems, wastewater from individual houses is collected, treated and disposed / reused at or near the point of its origin. The most important benefits of this system compared to the centralised system are:

• there is no need of laying sewers for the transportation of sewage as in the centralised treatment plant, which is normally located far from the point of the origin of the sewage; construction, maintenance and operation of sewers are very costly parts of sanitation systems;

• there is far lower dilution of sewage than in the centralised system, which creates possibilities to reuse treated wastewater and nutrients.

Therefore, decentralised wastewater treatment technologies will play a significant role, if they are low-cost and allow reuse. There are many existing decentralised wastewater treatment systems which have been widely used worldwide. However, all of them cause pollution i.e. nutrients and pathogens seeping from these systems contaminate the groundwater and nearby surface water, they cannot destroy pathogens and deprive agriculture of valuable nutrients and soil conditioner from human excreta. Moreover, some systems require expensive tanker-trucks to pump and transport the sludge deposited at the bottom of the system far away. In large cities, transportation distances are normally long, since suitable sites for treatment and disposal can mostly be found at the outskirts of cities. Transportation of relatively small faecal sludge volumes (5 - 10 m³ per truck) through congested roads over long distances in large urban agglomerations is not suitable, neither from an economical nor from an Ecological point of view (Montangero and Strauss, 2002).

Most of the people in urban and peri-urban areas of Asia, Africa and Latin America and peri-urban areas of industrialised countries use conventional decentralised sanitation systems (On-site sanitation systems), notably septic tank systems. Even in the USA, 25 percent of the houses are served by septic tank. Basically septic tanks are designed only to collect household wastewater, settle out the solids and anaerobically digest them to some extent, and then leach the effluent into the ground, not to destroy pathogens contained in wastewater. Therefore, septic tank systems can be highly pathogenic, allowing the transmission of disease causing bacteria, viruses, protozoa and intestinal parasites through the system. It is reported that there are 22 million septic system sites in the USA issuing contaminants such as bacteria, viruses, nitrate, phosphate, chloride, and organic compounds into the Environment (Jenkins, 1994). Another problem is home chemicals with hazardous constituents which are discharged to toilets and contribute to severe groundwater contamination in sanitation using septic tanks. According to the EPA, states of the USA reported septic tanks as a source of groundwater contamination more than any other source, with 46 states citing septic systems as sources of groundwater pollution (figure 5), and nine of them to be the primary source of groundwater contamination in their state. It has to be noted that occasionally problems with broken septic tanks occur leading to infiltration of nearly untreated wastewater.

Figure 5: Reported sources of groundwater contamination in the United States (Jenkins, 1994)

The incomplete anaerobic decomposition in septic tanks results in unpleasant odour that spreads in the surrounding. Many households often add chemicals into septic tank to reduce odour. These chemicals have adverse effects on the decomposition process and ultimately in Environment.