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gravity; so terms such as small-bore sewerage, small diameter gravity sewerage or shallow sewerage are unclear. Following the Portuguese terminology developed in Brazil (see Guimarães, 1986), I wish to propose the following: |
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1. Settled sewerage, to describe the system in which wastewater from one or more households is discharged into a single-compartment septic tank (usually termed in this context a solids interceptor tank), the settled (or solids-free) effluent from which is discharged into shallow, small-bore gravity sewers. Settled sewerage is thus small-bore sewerage sensu Otis and Mara (1985), small diameter gravity sewerage sensu Otis (Chapter 9) and common effluent drainage sensu South Australian Health Commission (1982). In Portuguese it is called redes de esgotos decantados; in French, réseaux d'eaux usées decantées; and in Spanish (Rizo Pombo, Chapter 10), acantarillado sin arrastre de sólidos. |
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2. Simplified sewerage, to describe shallow sewerage sensu Sinnatamby (1986) and its in-block variant sometimes called backyard or condominial sewerage (Rodrigues de Melo, 1985). This system does not convey presettled sewage, but is essentially conventional sewerage without any of its conservative design requirements that have accrued over the past century or so; it can be considered as the latter stripped down to its hydraulic basics. In Portuguese, it is called redes de esgotos simplificadas; in French, réseaux d'eaux usées simplifiés; and in Spanish, accantarillado simplificado. |
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2.2.2
The Case for On-Site Systems |
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The development of simplified sewerage in northeast Brazil in the early 1980s (see Sinnatamby, 1983) has really changed our views on the role of on-site systems. Despite their undoubted technical feasibility (see Roy et al., 1984; Middleton, 1995), they can now only be considered appropriate in urban/peri-urban areas if they are less expensive than simplified sewerage. The reason for this is that above a certain population density (160 persons per ha in the case of Natal in northeast Brazil on which Figure 2.1 is based) simplified sewerage is cheaper than on-site systems. Thus sanitation engineers should now always ascertain in every case |
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