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6.3 Treatment (Topic c)

6.3.1 On-site systems

It follows from Table 6.6 that with the exception of few countries in Western Europe (e.g. Greece, Ireland and Spain), most people are connected to sewerage systems. Yet, on-site systems still exist in all countries to some degree. However, in contrast to many developing countries, the population served by on-site systems is basically rural or isolated; on-site systems in the "urban fringe" are the exception rather than the rule and will disappear after the EU's Directive 91/271 has been implemented (see Section 6.6.1).

After World War II, permits to built a dwelling were made depended on the existence of either on-site or off-site disposal of the wastewater. Where an on-site system is appropriate, the requirement is in most cases for a septic tank. Yet, other systems are in use in older dwellings and isolated places: pit privies and latrines of various types. There are also several types of package plants on the market for both individual buildings, neighborhoods, and smaller establishments, e.g. hotels, and camp-sites (IFAT 1999).

Most septic tanks are prefabricated. Mostly, they have two compartments and are followed by filtration of the effluent into the ground. When desludging is foreseen (twice yearly) they may have a capacity of 300 L/person and their minimum size should not be less than 3000 L. Without desludging, their capacity may be as high as 1500 L/person (Imhoff 1985). A typical design in shown in Figure 6.3. Infiltration systems use clay or PVC piping of 10 mm diameter laid in a trench filled with gravel or crushed stone. They should have a gradient of 1:400 to 1:500 and between trenches there should be a space of 2 to 3 m. Surface loads are between 10 m/person in sandy soil or gravel to 20 m/person in sandy clays (Imhoff 1985).

Figure 6.3: Typical design of septic tank

Source: DIN, 1991

Some of the national standards organizations have issued norms for septic tanks but these are superseded by the new norm elaborated by the European Standard Organization (CEN). As will be outlined in Section 6.6.1, five norms will be forthcoming for wastewater treatment for less than 50 pe. They are:

• EN 12566-1: Prefabricated septic tanks.
• EN 12566-2: Soil infiltration systems:
• EN 12566-3: Packaged and/or site assembled domestic wastewater treatment plants.
• EN 12566-4: Septic tanks built in situ from prefabricated kits.
• EN 12566-5: Filtration systems (including sand filters).

EN 12566-1 is already available in draft and should be published soon. The four other norms will follow. EN 12566-1 specifies the requirements for prefabricated septic tanks and ancillary equipment used for the partial treatment of domestic wastewater for a population of less than 50 persons. Pipe sizes, loads, watertightness, marking and quality control are specified.

It should be noted that these norm are not design standards but rather product norms with the aim of contributing to the elimination of trade barriers (which was one of the initial main objectives of the EU), to give orientation to manufactures, purchasers, public tenders and planers, and to give orientation to meet legislative requirements as regards, safety, health and environmental protection, among others. As regards design, EN 12566-1 merely states that depending on the end use, one or more of the following criteria may need to be stated (by the manufacturer):

• Population; total load.
• Minimum sizing criteria including sludge storage capacity.
• Additional sizing criteria for domestic wastewater flows from sources such as hotels, restaurants or commercial premises.

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