X hits on this document





14 / 156

The present system is in a poor condition and is unable to provide an assured water supply to the town. The main problem is it has very little reservoir storage (only 5% of current requirements), it only has one pump dependent on an erratic electricity supply without standby pump or generator. Moreover the source of water is a well which risks pollution from nearby septic tanks or possible salt water contamination if over pumped (it is only 100 m from the sea). Finally, the reticulation is old and uncertain and unlikely to be able to meet peak demands if major hotels are to be built.

Storage is much too small for pumping rates of 20m3/hour representing only 2 hours storage at this rate (40m3). Moreover, projected water consumption can be expected to reach 400m3/day within a year or two if rehabilitation of existing houses and development continues and if water is made available to the existing inland villages as required. A water demand of 4000m3/day in 20 to 25 years is considered satisfactory for efficient supply of 25,000 tourists/residents.

In order to meet this demand and to supply good pressure to proposed development along the coast up to the 60m contour it is necessary to build new reservoirs at a common top water level of 88m close to the areas of consumption. The elevation of 88m has been chosen as top water level as there are a number of sites where reservoirs can conveniently be built at this elevation close to the areas they serve. Placing all the storage at the same level allows interconnection of distribution systems between reservoirs without overflow problems, as well as economical extension of the system over the years. Because the supply of water is from aquifers found in the sand all along this coast the supply to reservoirs can be drawn from wells sited as required next to the reservoirs. Such wells should be carefully sited away from habitations to reduce the possibility of pollution.

A new water distribution system (reticulation) will be required to suit the new higher reservoirs with resulting higher pressures in pipelines. Reservoirs set at 88m top water level also coincide with piping designed to work at this pressure as piping is designed in steps of 30m.


In normal urban areas volumes of the order of 70% of water consumed finds its way back into the sewers as wastewater. Along the coast with a narrow, widely dispersed pattern of population growth a smaller percentage is likely to be collected in sewers. On the basis of projected water consumption of 4000 m3/day in about 20 years time the amount of waste water likely to reach the sewers can be expected to be of the order of 2500 m3/day and we should plan accordingly.

Initially all pre-treatment could be in the form of suitably sized septic tanks with a minimum retention period of 24 hours each. Tanks to be fitted with a filter device to preclude solids from entering the system. The only pumping or mechanical plant required would be a vacuum tanker to empty septic tanks occasionally as required and to transport the residual solids and sludge to a properly managed and designed land disposal area. A specialised team could be trained to operate and maintain the vacuum tanker and equipment and to dispose of the sludge.

In the case of the larger sources of waste water, such as hotels, more sophisticated pre-treatment package plants could be installed such as totally enclosed biodisk plants, etc. Again the plant would have to operate so only liquid was discharged into the sewer pipes.

It is noted that further research will be required to check levels and position a possible collecting sewer laid at flat grades. Also to be investigated is the best position for an submarine outfall sewer into the sea near Praia Velha.

Sites for possible development for sewerage treatment have been identified in both the west and central areas and these can be further investigated once the level and possible position of the collecting sewers along the coast have been more accurately determined.

Solid Waste

With the limited budget available it is best to use the area that has already been quarried as a landfill site despite the limitations previously mentioned. From a location point of view it is a bit close to the people and the road. From a hydrogeological point of view it is relatively high and away from the sources of water. All the soil is sandy so pollution of ground water will take place. It is, however, already being used at present so pollution would continue to be limited to one area.

There is a limited amount of space available, but it is hard to determine its volume without an accurate survey and with uncertain rate of growth. In the longer term they will have to use another site or the site at Xai-Xai town. This site is 14 km away (28 km return trip) which is not economical for a tractor and trailer as it is too far.


Document info
Document views466
Page views466
Page last viewedTue Jan 17 05:14:35 UTC 2017