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Feasibility study for sewerage and drainage improvement in Phuket city, Thailand.

Like other big cities in Thailand, in Phuket Municipality (the City), there exists a public storm water drainage system which is also used for domestic wastewater removal. However, there is no public sewage disposal system. (Source: Project for Phuked Municipality in Kingdom of Thailand. Sewage Works in Japan)

In the city centre, the sullage from kitchens, washing and bathing is discharged to rivers or canals through the existing stormwater drainage system, and approximately one-third of the volume of septage from cesspools and septic tanks is simply allowed to infiltrate into the ground. Therefore, the existing stormwater drainage system is practically refered to as the combined sewage collection system, even though it was not originally planned for such a purpose. This has caused pollution not only of river and sea water, but also of groundwater.

The evaluation of alternatives for planning the sewage collection system for the study area was undertaken. Experience has shown that, for surrounding environmental impact and water pollution control, a separate collection system is advantageous compared with a combined system. However, further analysis shows that under the following conditions, a combined system may be more desirable to promote the diffusion of sewerage more economically.

1. The degree of pollution of the receiving watercourses during rains is acceptable.
2. The existing public drainage system, which is collecting both surface runoff and sewage can be intercepted to combined sewers to be constructed thus resulting in a lesser need for lateral sewers.

Based on a comparative cost study, the combined system is less expensive than the separate system in terms of construction cost and operation and maintenance (O & M) cost. Construction cost and O & M cost in the combined system are only 30% and 40% of those in the separate system, respectively.

Indonesian sewerage development plan

Because open drainage networks drain stormwater in some areas, two types of sewerage collection systems are applicable depending on the terrain in the collection area. One is the conventional separate system, and the other is the interceptor collection system (Wastewater Disposal Jakarta 1994).

As a general rule, the conventional system is applied for the following areas.

1. Commercial and institutional areas located along main roads.
2. Residential areas where redevelopment has been completed and the existing road is wider than 2m, which is the minimum width required for laying sewer lines and other dimensions.

The interceptor system is applied for high population density areas, which cannot be covered by a conventional system. This system will collect only grey water through the existing roadside ditches and other drains to the proposed interceptor. Toilet waste is treated by septic tank systems and its overflow flows into the interceptor. The proposed interceptor however, will not receive storm water collected through ditches. Wastewater exceeding the dry weather flow will overflow into rivers or canals in the vicinity.

Jakarta water quality suffers from combination of domestic and industrial pollution. The backbone of the sanitation system is still an open ditch system that serves as a conduit for all wastewater. While this system may have been adequate for the city with a population of less than half a million, which was the size of the city when the system was planned, it can not cope with the wastes of the current 11.5 million residents.

Sewerage systems in more developed countries

Currently, more developed countries such as Australia, Hong Kong, Japan, Korea, Singapore and Taiwan normally have separate systems for sewage, with two types of collection system, one for stormwater and another for sanitary sewage in the new development areas. As development and urbanisation has continued to increase, the pollution load due to stormwater overflow has been found to be significant. Many communities in the developed countries, therefore, provide some degree of treatment to stormwater overflows or store stormwater for treatment at the sewage treatment facilities.

Sewer systems in Australia

In Australia, the community collection system comprises a reticulation system serving individual household properties. The sizes of the pipes, generally gravity pipes, are such that the system can readily accommodate changes in the residential development densities. More recently vacuum collection systems have been introduced in Western Australia.

For local communities, the reticulation system conveys the wastewater to the central treatment and reuse disposal facilities. For large community systems there is generally a main sewer system that conveys wastewater to a large central treatment and reuse/disposal facility. The main sewer system is developed as catchment grows and its capacity can be increased to accommodate flows.

Drainage systems in Japan.

There are two different systems for draining wastewater; namely, the combined system and the separate system. The combined system was first developed in Japan, but to prevent contamination of the receiving water, at present the separate system is primarily used. In Japan, it is common to establish a five-year medium term plan for structures such as sewers. The current five- year plan is the seventh such plan.

The proportion of sewered population in Japan increased very quickly. In 1965, only 8% of population was sewered. By 1970 the sewered population had increased to 16%, in 1985 to 36% and in the year 1993 to 49%. Based on the progress made during previous successive five-year plans the sewered population in Japan is now estimated to be about 65%. Total length is about 11,858 km, with size from 200 to 5000mm.

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