Waste water and the Sewage Network

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Introduction / Sewage Network / Drainage /

Domestic Effluent / Industrial Effluent / Hotel Effluent

Treatment

Future Developments

Bibliography

Introduction

All liquid wastes from factories, hotels and residential premises are included in the term sewage. It can be harmful in many ways, depending on its source.

Sewage can be a source of nutrients for microbial activity, it can contain pathogenic organisms and toxic materials. Treatment and disposal of sewage is carried out to neutralise its harmful effects.

The Sewage Network

There are four main sewer networks in operation. Port Louis is served by two networks and the whole municipal area is covered. The first sewer network in Port Louis dates from 1880. In the 1920's and from the mid 1960's to the mid 1970's the system was extended to serve the whole of Port Louis.

In the district of Plaines Wilhems, the first network was laid out in the early 60's. It has been extended since on a few occasions and presently covers the central part of Beau Bassin, Rose Hill, Quatre Bornes, Curepipe and part of Phoenix. Its capacity is already exceeded.

The Coromandel system was constructed to cater for the industrial effluents coming from the Coromandel industrial estate. Effluent from Beau Bassin North is also routed via this network.

Only about 20% of the population ( in the urban areas ) is connected to the sewage mains and the systems are severely overloaded in places. There is no sewer system in the rural districts.

Recently though, very important public works have been undertaken to connect Grand Baie, in the north, to the sewerage system. It should be operational soon.

Less than 15% of industries are connected to the sewerage system and it is not possible for the rest, specially those industries found in rural areas to be connected up in the near future.

The Drainage System

In Mauritius, the drainage system, used to carry away storm water from man made impervious surfaces such as roof tops and roads, is entirely separate from the sewerage system. This is due to cost and practical considerations. Sewage should not enter the drainage system and vice versa. Unfortunately, at times storm water does infiltrate the sewage system causing dangerous sewage overflows.

Storm water eventually finds its way to the lagoon where large influxes of turbid and polluted freshwater can cause significant environmental damage to the lagoon ecology. Changes in salinity, temperature and oxygen levels, siltation, nutrient enrichment of lagoonal waters can particularly affect coral communities.

It is not possible, in Mauritius, to collect and treat surface waters. It is preferable to control the causes of siltation and pollution and minimise the impact of storm water by

  1. the re-afforestation of areas prone to erosion.
  2. better agricultural practices like the terracing of steep slopes, the planting of appropriate crops to bind loose soils for example.
  3. regular street cleaning and disposal of refuse in landfills.
  4. the prevention of run off entering the sewer system and vice versa.
  5. the discharge of storm waters into natural watercourses or over wider parts of the lagoon.

The last measure points to the importance of marshes or wetlands for they act as natural buffers or holding points from which excessive run off can evaporate or can slowly infiltrate the lagoon. The impact of storm water run off is thus minimised.

In Mauritius along the northern, eastern and western coastlines wetlands or marshes are found and thus they act as natural buffers. Unfortunately, over the years considerable property development has occurred close to or on wetlands, especially in the Grand Baie region ( in the north) and in the Flic en Flac region ( in the west). It is feared that wetlands in these areas may no longer carry out their ecological functions.

As in those regions the reefs are relatively close to the shore the impact of storm waters on local coral communities may well be under way.





Domestic Waste Water

Over 97% of the population receive the water supply through pipes, the few percent remaining receive water through tank wagon, wells or springs. It has been estimated that the domestic sewage per capita per day is between 60 to 140 litres.

As only 20% of the population is connected to the sewerage system it means that over 80% of the population discharge domestic sewage into absorption pits or pit latrines. The discharge of untreated wastes into the subsoil may prove to be hazardous especially where the water table is near the surface. It is thought that waste discharged in this manner in the Plaines Wilhems carries a high risk of contamination of the Curepipe aquifer.

On the coastal regions, it is probable that domestic sewage from pits seeps into the surrounding lagoon contributing to nitrate and phosphate pollution.

Hotel Effluent

The current legislation requires that all hotels of more than 75 rooms be equipped with a treatment plant. This figure of 75 rooms was chosen because of technical reasons, as it appears that it is not possible to have an activated sludge treatment plant that works at lower loads.

Hotels with less than 75 rooms use cesspits. The danger is that effluent can rapidly percolate to the lagoon and pollute it through nutrient enrichment.

Treatment

Sewage from the Port Louis area is treated at two plants situated at Roche Bois and Fort Victoria. There the treatment consists of screening, removal of grit and disintegration of solids before the sewage is pumped into the sea through an 800 metres out fall.

In the Plaine Wilhems system, sewage is sent to the Saint Martin treatment plant. There the treatment consists of screening, disintegration of solids and removal of grit. Then the effluent is discharged out to sea at Pointe Moyenne through a 600 metres out fall.

In the Coromandel system, the treatment consists of screening, disintegration of solids and removal of grit before discharge into the sea at Pointe aux Sables through an out fall.

All the out falls discharge at shallow depth or on the surface. Little dilution takes place before the waste waters reach the surface.

As a result the reef opposite the Pointe Aux Sables discharge is dead.


Future Developments

The government has elaborated a National Sewerage Master Plan for Mauritius with the help of the Severn Trent water authorities and Gibb Partners. Though the report is confidential it appears that in the long term a proliferation of sea out falls around the coast line is not envisaged. At most three or four deep water out falls will be commissioned: one or two for Port Louis; one for the Rose Belle/Mahebourg/Beau Vallon Industrial park; and possibly one for the northern tourist zone. The four existing shallow out falls will eventually be decommissioned.

In the aforesaid plan, two levels of sewage treatment is being considered.

(1) Preliminary and primary treatment with marine disposal of effluent by sea out fall pipes;

  1. Preliminary, primary and secondary treatment with effluent re-use for crop irrigation.

Preliminary and pre treatment with marine disposal consists of screening, grit removal, comminution of solids followed by primary settlement in sedimentation tanks. The resulting effluent, which is still considered to be raw sewage is then pumped out to sea via out falls.

Secondary treatment consists of the above treatments followed by a biological treatment to remove some suspended solids and faecal bacteria. The resultant effluent could be used in agriculture to irrigate crops destined for industrial processing such as sugar cane but not crops grown for direct human consumption.

Primary and secondary sewage treatment produces sludges which can be further processed to reduce its volume and then dried. It can then be used as an organic fertiliser.

At the present time it is not known which type of treatments is being favoured by the government.

Bibliography
  1. Government Of Mauritius: State Of The Environment Report 1991
  2. Government Of Mauritius: National Physical Development Plan 1994

Last Update: Thursday, January 22, 1998