Xai-Xai IDP – INFRASTRUCTURE PRELIMINARY PROJECTS
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 4000m3/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 2500m3/day and we should plan accordingly.
5.2.2 Method of Collection
Because all sewerage drains downhill towards the sea the collection of sewerage along the coast has always provided problems. With all the sewerage collecting at the same level, namely sea level, there is no fall available and the only way of moving waste water along the coast is using a series of pump stations to lift the sewage artificially so it may drain along the coast. Such pump stations are expensive to build and operate and require careful expert maintenance and uninterrupted power supplies. Under present conditions of development and reconstruction with limited resources a series of pump stations are not regarded as a feasible solution.
Sewers contain a mixture of liquid and solids and sewers, therefore, have to be laid at steep grades of the order of 1 in 200 in order to provide self cleansing velocities of the order of 0,7m/second minimum in the pipes.
If, however, the sewerage is pre-treated to remove the solids lower velocities of flow would be permissible and pipes could be laid at much flatter gradients, of the order of 1 in 1000 to 1 in 1500 for 200 mm diameter pipes. This would allow sewerage to be transported along the coast for up to 1,5 km with only 1m fall and would allow most of the areas of concentrated sewage flows from hotels or dense developments, such as camp sites, group housing, etc. to be moved up or down the coast without pumping. Such a collecting sewer could also easily be utilised by all development on the hills above it.
On this basis a sewer could be laid from the existing hotel east of the town past the central area and the old existing hotel and proposed new hotel to a point opposite Praia Velha where it could drain into the sea beyond the reef. Eventually, in years to come, sewerage could be treated on land before sea discharge by introducing one pump station and treatment facilities. The extent of such treatment will be determined by both quantities and the effectiveness of dispersion and dilution in the sea at the discharge point. In other words, the system could be improved on an add-on basis as required without having to abandon or lose the benefit of any of the original pipe system.
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.
5.2.4 Assessment of Possibilities
It is noted that further research will be required to check levels and position a possible collecting sewer laid at flat grades. A survey technician is to made available by the Xai-Xai Water Section to do surveys in mid September 1998. Also to be investigated is the best position for an outfall sewer into the sea near Praia Velha.