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3.2 Simplified sewerage

Simplified sewerage is also known as shallow sewerage. Again the term reflects the nature of the shallower placement of the pipes in contrast to the conventional or deep sewerage. The purpose of simplified sewerage is to reduce the cost of construction and the corresponding cost of operation and maintenance. Simplified sewerage is designed based on hydraulic theory in the same manner as for conventional sewerage. Its design assumptions are, however, less conservative. Smaller diameter pipes are used when water use per person is known to be less. Minimum depth of cover of pipes can be as low as 0.2 m when there is only light traffic. Manholes can be replaced by inspection cleanouts because of the shallow pipes. Design planning horizon can be 20 instead of 30 years, because population projection may be uncertain. In a variation of the simplified sewerage the pipe layout passes through property lots (condominial) rather than on both sides of a street (conventional). Figure 2.12 shows a comparison between sewerage layout in conventional sewerage and in condominial sewerage, while Table 2.3 shows a comparison of length of pipes required. Cost of construction can be 30 to 50 % less than conventional sewerage depending on local conditions.

Figure 2.12: Pipe layout for (a) conventional and (b) condominium sewerage

Shallow sewerage is also conducive to local community participation. This is because of sewer pipes having to cross property boundaries and hence the need for the community to agree to this arrangement. This arrangement needs to be in place not only during construction, but also for maintenance (e.g. unblocking of sewer pipes). The shallow pipe, and hence the shallow trenches, also allow members of the community to participate by for example providing labour for digging the trenches. This is in contrast to conventional sewerage where specialised machinery is required for the deep trenches. Figure 2.13 contrasts the two approaches.

Table 2.3. Comparison of length of pipes required for conventional and condominium sewerage

Figure 2.13: Contrast in community participation between conventional and condominial sewerage

There has been considerable experience with simplified sewerage (Regional Overview of Central & South America) and manuals have been produced to assist engineers with its design. Developed initially in Brazil it is beginning to be used in many parts of the world.

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