case study of water engineering

8/9/2019 Case Study of Water Engineering

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Acknowledgements

I would like to express my gratitude and appreciation to, firstly, Mrs Reshma Rughooputh for her

guidance and precious help in the monitoring of this mini-project and enlightening me whenever I had

doubts on certain related issues.

I am also grateful to the Technicians at La Marie and Piton du Milieu treatment plants and to the Officers

at the CWA Sub Office at Quartier Militaire, who have thoroughly guided me throughout the site visits

conducted and devoted much time in providing an adequate explanation as to how water treatment

processes are carried out industrially.

Last but not the least, a special thanks goes to all my classmates who have shared every information

and knowledge they acquired prior to writing this report.


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Table of contents

Abstract ................................................................................................................................................... 1

1.0 Introduction ....................................................................................................................................... 2

2.0 Aims and Objectives ......................................................................................................................... 3

3.0 Literature Review .............................................................................................................................. 4

3.1 Topography of Mauritius .............................................................................................................. 4

3.2 Climate of Mauritius ..................................................................................................................... 4

3.3 Water management in Mauritius ................................................................................................... 6

3.4 Water resources in Mauritius ........................................................................................................ 7

3.5 Water treatment in Mauritius ........................................................................................................ 9

3.6 Water distribution networks ........................................................................................................ 10

3.7 General water costs. .................................................................................................................... 11

3.8 Conclusion .................................................................................................................................. 13

4.0 Methodology ................................................................................................................................... 14

4.1 Locality and distribution network ............................................................................................... 14

4.2 Desk study ................................................................................................................................... 15

4.3 Water treatment processes and distribution at Piton du Milieu treatment plant. ........................ 19

4.3.1 Intake of water and raw water screening.............................................................................. 19

4.3.2 Water purification processes ................................................................................................ 21

4.3.2.1 Coagulation ................................................................................................................... 22

4.3.2.1.1 Addition of coagulant and coagulant aids. ............................................................. 22

4.3.2.1.2 Rapid mixing .......................................................................................................... 24

4.3.2.2 Flocculation ................................................................................................................... 26

4.3.2.3 Sedimentation ............................................................................................................... 27

4.3.2.4 Filtration ........................................................................................................................ 28

4.3.2.5 Backwashing ................................................................................................................. 31

4.3.2.6 Chlorination .................................................................................................................. 33

4.3.2.7 Storage of treated water ................................................................................................ 37

4.2.3 Water quality control ........................................................................................................... 38


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4.2.3.1 Raw water entering treatment plant from Piton du Milieu reservoir ............................ 39

4.2.3.2 Coagulated raw water .................................................................................................... 39

4.2.3.3 Chlorinated filtered water ............................................................................................. 40

4.2.3.3.1 Turbidity ................................................................................................................ 40

4.2.3.3.2 pH ........................................................................................................................... 40

4.2.3.3.3 Odour ..................................................................................................................... 41

4.2.3.3.4 Chlorine residual .................................................................................................... 41

4.2.3.3.5 Conductivity ........................................................................................................... 42

4.2.4 Treatment during special circumstances .............................................................................. 43

4.2.5 Health and Safety aspects .................................................................................................... 43

4.2.6 Distribution of potable water within eastern network to my locality ................................... 45

4.3 Costing ........................................................................................................................................ 46

4.4 Limitations .................................................................................................................................. 47

4.4.1 Limitations of the general water distribution network in Mauritius .................................... 47

4.4.2 Limitations of the study ....................................................................................................... 47

5.0 Conclusion ...................................................................................................................................... 48

References ............................................................................................................................................. 49

Appendix ............................................................................................................................................... 51

Item 1 – Water bodies that provide water for production of potable water. ..................................... 51

Item 2 – Main aquifers of Mauritius ................................................................................................. 52

Item 3 – Boreholes in Mauritius ....................................................................................................... 53

Item 4 – Man-made reservoirs in Mauritius ...................................................................................... 54

Item 5 – Diary of activities (corresponding to ECSA ELO 8) .......................................................... 55

Item 6 – Resource Persons (corresponding to ECSA ELO 8) .......................................................... 55

Item 7 – Dissemination of knowledge (corresponding to ECSA ELO 6) ......................................... 55


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List of figures

Figure 1 - Location of Mauritius. ............................................................................................................ 2

Figure 2 - Mean annual rainfall distribution over the last 5 years. ......................................................... 5

Figure 3- Approximate location of Saint Julien D’Hotman. ................................................................. 14 Figure 4 - Location of Piton du Milieu reservoir. ................................................................................. 17

Figure 5 - View of Piton du Milieu reservoir. ....................................................................................... 17

Figure 6 - Long term and current mean water levels of Piton du Milieu reservoir (2009 - 2013). ....... 18

Figure 7 - Diversion of incoming free-flowing water to hydro generators. .......................................... 19

Figure 8 - Hydro generator. .................................................................................................................. 20

Figure 9 - Representative diagram of the rapid sand filters found at Piton du Milieu treatment plant. 21

Figure 10 - A 25-kg industrial alum bag. .............................................................................................. 22

Figure 11 - Apparatus to carry out jar tests. .......................................................................................... 23

Figure 12 - Mechanical pumps which feed alum and polyelectrolyte to the raw water........................ 23

Figure 13 - Rapid mixing chamber sign board at Piton du Milieu treatment plant. .............................. 24

Figure 14 - Rapid mixing chamber in operation. .................................................................................. 25

Figure 15 - Hopper through which lime is prepared and pumped to the raw water. ............................. 26

Figure 16 - Two of the four available flocculators at the Piton du Milieu treatment plant. .................. 27

Figure 17 - Formation of flocs. ............................................................................................................. 27

Figure 18 - Sedimentation tank. ............................................................................................................ 27

Figure 19 - Discharging of sludge into side canals. .............................................................................. 28

Figure 20 - Sludge tank. ........................................................................................................................ 28

Figure 21 - Raw water being fed to the filter beds after sedimentation. ............................................... 29

Figure 22 - Filtration in process. ........................................................................................................... 29

Figure 23 - System of nozzle cleaning to clean the rapid sand filters. .................................................. 30

Figure 24 – Backwashing taking place at Piton du Milieu treatment plant. ......................................... 31

Figure 25 - Pressure gauge to monitor clogging of the filter bed. ........................................................ 32

Figure 26 - Electronic board to operate the backwashing process. ....................................................... 32

Figure 27 - Wash water tank at Piton du Milieu treatment plant. ......................................................... 33

Figure 28 - Toners containing chlorine. ................................................................................................ 34

Figure 29 - Chlorine toners connected to a manifold. ........................................................................... 35

Figure 30 - Representative diagram of the vacuum chlorinator used at the Piton du Milieu treatment

plant. ..................................................................................................................................................... 35

Figure 31 - Actual chlorinator unit at Piton du Milieu treatment plant. ................................................ 36

Figure 32 - Chlorine leakage detector. .................................................................................................. 36

Figure 33 - Chlorine Residual Analysers at Piton du Milieu treatment plant. ...................................... 37

Figure 34 - Representative diagram of a storage tank found at Piton du Milieu treatment plant. ........ 38

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Figure 35 - Turbidimeter. ...................................................................................................................... 40

Figure 36 - pH meter. ............................................................................................................................ 41

Figure 37 - DPD (No 1) tablets. ............................................................................................................ 42

Figure 38- Comparator apparatus. ........................................................................................................ 42

Figure 39 - Emergency eye and body wash in case of exposure to alum or polyelectrolyte. ............... 44

Figure 40 - Emergency outdoor shower in case of exposure to chlorine. ............................................. 44

Figure 41 - Established procedures for the handling of chlorine toners or cylinders. .......................... 45

Figure 42 - Rivers, rivulets, boreholes, wells and reservoirs used in potable water production. .......... 51

Figure 43 - Main aquifers of Mauritius. ................................................................................................ 52

Figure 44 - Boreholes in Mauritius. ...................................................................................................... 53

Figure 45 – Man-made reservoirs in Mauritius..................................................................................... 54


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List of abbreviations

1. CWA – Central Water Authority

2. WMA – Wastewater Management Authority

3. IA – Irrigation Authority4. WRU – Water Resources Unit


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Abstract

Potable water is not a major issue in Mauritius due to the existing and upcoming infrastructure to purify

raw water and distribute it effectively around the island. The aim of this report is to investigate thedifferent processes involved in the treatment of raw water and the distribution of the resulting potable

water in the eastern part of Mauritius. The supply of potable water to the inhabitants of the eastern

localities is done from the treatment plant of Piton du Milieu which operates rapid sand filters only. Site

visits were conducted both at the afore-mentioned treatment plant and that of La Marie so as to have a

broader and more practical view of the purification processes. Furthermore, it was deemed necessary to

also consider the cost implications of water treatment and its distribution. This has been done through

available data from the Central Statistics Office, the Central Water Authority and from information

obtained from officers at the treatment plants. Lastly, the distribution network was analysed to evaluate

its efficiency. The latter was deemed as satisfactory.

As part of the ‘water studies’ comprising this module, this assignment enabled me to better understand

the theoretical aspects of water treatment processes and to get a practical feel of how the latter is carried

out industrially. Moreover, as engineering also includes an economic point of view, the cost

implications involved allowed me to ponder over the vast amounts of money that the Government needs

to spend each day in order to provide the population with potable water and realise why under-

developed countries are not as lucky as Mauritius.

Key words: water treatment, distribution, eastern region, rapid sand filters, cost implications.


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1.0 Introduction

Mauritius is situated in the South-West of the Indian Ocean between longitudes 57°18’ and 57°46’ East

and latitudes 19°59’ and 20°32’ South. Its position is shown in Figure 1 below.

Figure 1 - Location of Mauritius.

Source: Proag. V, 2006.

It can be clearly seen from the above figure that the country is surrounded by sea. Despite this fact, the

main source of water is rainfall since the desalination of water is far from being a reality, owing to the

current economic and political situation. The rainy season occurs mainly in summer and is usually from

December to April. However, according to statistics from the Meteorological Station, situated at

Vacoas, February and March are the wettest months.

The main fresh water resources in the island include reservoirs, rivers, rivulets, man-made and natural

lakes, boreholes, wells and aquifers. Mauritius is no exception to the rule that each and every country

in the world requires an adequate and safe supply of water. However, water from these afore-mentioned

resources cannot be directly supplied to the public, owing to the undesirable organic matter, suspended

particles and unwanted chemicals which may be found in raw water. The latter needs to be treated and

hence is the need for water treatment plants.

Water treatment plants are involved in the purification of raw water to make it potable and palatable, as

well as meeting the local water quality standards set by the Environmental Protection Act 1996 (revised

in 2002) and derived from the World Health Organisation guideline values. The Central Water

Authority (CWA), acting under the aegis of the Ministry of Renewable Energy and Public Utilities, is

the main corporate body which is involved for the treatment, quality control and distribution of potable

water on the mainland.


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2.0 Aims and Objectives

The primary aim of this investigation was to understand the concept of water distribution and water

treatment processes for domestic use as well as having an overview of the costing associated.

The underlying aims following this investigation were to be as follows:

1. To be able to define the different processes that take place on an industry scale to treat raw

water.

2. To determine what methods are mainly used by the CWA to distribute water round around the

whole island, with particular emphasis to the one used for my locality.

3. To determine to costs involved in the storage, treatment and distribution of water along piping

networks.

4. To determine the cost of distributed water.

The objectives associated with the above-mentioned aims were as follows:

1. To identify the different water resources and their respective distribution networks around the

island.

2. To locate the raw water, its treatment plant and distribution system to my house.

3. To conduct an in-depth site visit of a water-treatment plant.

4. To look at the different methods used to purify water in Mauritius.

5. To understand the underlying reasons for the choice of a distribution method.

6. To understand how the pricing of distributed water is done.


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3.0 Literature Review

3.1 Topography of Mauritius

Mauritius is about 58 km in length from North to South and about 47 km from East to West. Its total

land surface area is about 1865 square kilometres (Reef Conservation, 2014). It is a picturesque island

nation with rugged volcanic features and a large fertile plain area. The compact mainland is the worn

and eroded base of an extinct volcano. It stands on a mostly undersea feature called the Mascarene

Plateau, which, itself, sits on the African Tectonic Plate.

Being of volcanic origin, the topography of the island has, as main feature, a Central Plateau. The latter

is about 670 metres above the mean sea level and it rises dramatically to the south-west region, with the

highest point being the Piton de la Petite Riviere Noire peak at 828 metres (Encyclopaedia of the

Nations, 2014)

The Central Plateau is surrounded by the remnants of the primary volcanic crater system, which founded

the island. The remnants includes chains of mountains which are not very high, as exemplified by the

Pieter Both mountain which stands at a length of 323 metres, and isolated peaks such as Corps De Garde

and Candos Hill. These small mountains and peaks rise above the level of the plateau, thus forming the

rugged landscape of the island.

To the North, the island drops and flattens out to form the extensive Northern Plains, which comprises

of the Riviere du Rempart and Pamplemousses districts. These plains further extend to the western coast

as a coastal plain. The eastern coast has a very narrow to an almost non-existent coastal plain with a

relatively steep topography (Britannica, 2014).

3.2 Climate of Mauritius

Being found in the tropics, Mauritius enjoys a mild tropical maritime climate, which is mostly humid

with prevailing south-east winds, throughout the year. The country has 2 seasons, namely:

a warm humid summer, which extends from November to April.

a cool dry winter from June to September.

The months from October to May are commonly known as the transition months (Mauritius

Meteorological Services, 2014).


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A summary of the climatology of Mauritius, in terms of temperature, is shown in the following table.

Table 1- Climatology of Mauritius in terms of temperature.

Source: Mauritius Meteorological Services website (2014).

Mean summertemperature/°C 24.7

Mean wintertemperature/°C 20.4

Warmest monthsJanuary and February, with an average daily mean temperature of

29.2 °C.

Coolest Months July and August, with an average daily mean temperature of 16.4 °C.

Although there are no marked rainy season, most rainfall occurs in the summer months. The long term

mean annual rainfall, measured over the period of 1971 to 2000, over the island has been measured to

be 2010 mm. The wettest months are February and March, while the driest one is October. It is to be

noted that most rainfall occurs on the Central Plateau (Mauritius Meteorological Services, 2014).

The mean summer and winter rainfall, measured over the same time period mentioned previously, are

1344 mm and 666 mm respectively (Mauritius Meteorological Services, 2014). Due to the climate

changes occurring because of global warming, the annual mean rainfall has been varying significantly

during the last few years and the trend can be mainly seen to be a decreasing one.

The mean annual rainfall distribution for the last 5 years is shown below.

Figure 2 - Mean annual rainfall distribution over the last 5 years.

Source: Mauritius Meteorological Services website (2014) (adapted data).

2397

18061945

1609

2049

0

500

1000

1500

2000

2500

3000

2009 2010 2011 2012 2013

M e a n a

n n u a l r a i n f a l l / m m

Year

Mean annual rainfall distribution over the last 5 years


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The main source of water in Mauritius is rainfall, with the latter contributing to water collection in

surface and ground water bodies. Despite receiving much rain, with an average annual amount of about

2050 mm (Ministry of Energy and Public Utilities, 2013), the water situation in the island is not always

good. The main reason which can explain this discrepancy is perhaps the amount of rain water which

goes uncaught or is lost. In 2013, Mauritius received a gross total of about 3821 cubic metres of rainfall.

However, only 10 % of that amount (about 382 Mm³) went as water recharge in water storage bodies

such as reservoirs, rivers and aquifers, while evapotranspiration and surface runoff into seas accounted

for 30 % (1146 Mm³) and 60 % (2293 Mm³) respectively (Water and Energy Digest, 2013).

Thus being said, potable water in the island is derived from those afore-mentioned water storage bodies

and the latter can be classified into two types, namely:

1. Surface water resources, which include reservoirs, rivers and rivulets amongst others.

2. Ground water resources, which include boreholes.

Due to climatic change, the average annual rainfall does not remain almost constant throughout the

years and thus the CWA has been investing more and more in the digging of boreholes. In 2013, the

average production of potable water from the boreholes has been almost equal to that from reservoirs

and other surface water resources, with a gross amount of 49.6 % and 50.4 % respectively (CWA, 2014).

3.3 Water management in Mauritius

The Ministry of Renewable Energy and Public Utilities (MREPU) is the parent body for the

management of water resources in the island. This power has been bestowed to the Water Resources

Unit (WRU), established in May 1993, which is responsible for the assessment, management,

development and conservation of the country’s water resources (CWA, 2014).

The water sectors in Mauritius can be classified into 5 main categories, namely:

Domestic

Commercial

Industrial

Agricultural

Wastewater

Acting under the aegis of the WRU, there are 3 subsidiary bodies for managing the above-mentioned

water sectors.


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1. According to the Central Water Authority Act 1971, the Central Water Authority (CWA) is the

sole legal undertaker for the distribution of water for domestic, commercial and industrial

purposes.

2. The Irrigation Authority (IA), under the Irrigation Authority Act 1978, has the responsibility

for managing water required for irrigation and other agricultural purposes.

3. The Wastewater Management Authority (WMA) is responsible for all matters relating to the

collection, treatment and disposal of wastewater according to the Wastewater Management

Authority Act 2000.

The water supply for the previously-named sectors is carried out through piped distribution networks.

While the distribution networks of water required for domestic, commercial and industrial purposes are

universal, those for wastewater are clearly distinct and do not cover the whole country. Further

development is currently being carried out to extend the wastewater network to all households and properties of the island.

The treatment of water for the water sectors managed by the CWA is carried out at the different water

treatment plants, which are described later in this section, while wastewater is mainly treated at the

Saint Martin Wastewater Treatment Plant, found in the western region of the island.

The construction of this wastewater treatment plant was completed in 2005 and has been in operation

since then. It treats a daily wastewater flow of 69,000 cubic metres, coming mainly from Plaines

Wilhems, having a total population of about 233,000 (BerlinWasser International, 2014) .

3.4 Water resources in Mauritius

Piped potable water is universal in the island. More than 99 % of the local households have access to

potable water within their premises. The current distribution network supplies an average of 545,000

cubic metres of water per day, along a total pipeline length of about 3985 km (Mauritius Science Portal,

2014).

The main storage reservoirs used by the CWA for potable water supply are listed below:

Mare Longue reservoir

La Ferme reservoir

La Nicoliere reservoir

Mare aux Vacoas reservoir

Midlands Dam

Piton du Milieu reservoir


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The water storage capacity of each of the above-named reservoir and its respective uses are given in the

following table:

Table 2- Storage capacities and uses of the main reservoirs in Mauritius.

Source: CWA website (2014).

Reservoir Storage Capacity (Mm³) Use

Mare Longue 6.3 Hydro-power production and irrigation

La Ferme 11.5 Irrigation

La Nicoliere 5.3 Domestic, industrial and irrigation

Mare aux Vacoas 25.9 Domestic

Midlands Dam 25.5 Domestic, industrial and irrigation

Piton du Milieu 3 Domestic

These reservoirs are not the only surface water resources found in the island. There are other ones such

as rivers, rivulets, dams and lakes which feed the main reservoirs or from which water is directly

supplied to inhabitants.

Some of these water surface resources include the following:

Valetta Lake

Municipal Dykes Dam

Riviere du Poste

Grand River North West

Bagatelle Dam

Dagotiere Reservoir

A figure outlining all rivers and rivulets having feeder characteristics or from which water is directly

supplied to inhabitants is annexed under Item 1.

Ground water resources consist mainly of aquifers from which water is airlifted through boreholes

pumping stations (Mauritius Science Portal, 2014).

The five main aquifers are listed below:

1. Aquifer of Curepipe-Vacoas-Flic en Flac

2. Aquifer of Northern Plains

3. Aquifer of Phoenix-Beau Bassin-Albion/Moka-Coromandel

4. Aquifer of Rose Belle-Nouvelle France-Plaisance

5. Aquifer of Nouvelle Decouverte-Plaine des Roches/Midlands-Trou d’Eau Douce


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The secondary aquifers are:

1. Aquifer of Chemin Grenier / Frederica

2. Aquifer of Chamarel

3. Alluvial aquifers of Grande Riviere Noire, Yemen and Vallee des Pretres4. Fractured aquifers of Chamarel and Bambous Virieux.

5. Carbonated aquifers of Mt Bambous and Case Noyale

A figure outlining the main aquifers of Mauritius is annexed under Item 2.

Geological and hydrogeological investigations have resulted in the drilling of about 900 boreholes of

diameter ranging from 150 mm to 300 mm till now. Out of these 900, about 115 are administered by

the CWA for potable water supply (Proag, 2006). These boreholes account for an average of 285,000

cubic metres of potable water supply per day. All the boreholes meant for potable water supply areshown in an annexed figure under Item 3.

3.5 Water treatment in Mauritius

The purification process of water follows the following scheme:

The process of filtration involves the flow of water through a bed containing granular media, which can

be sand or any other suitable material. Currently, there are 2 types of filters which are used worldwide

to treat raw water from various sources to potable water standard. These are:

1. Slow sand filter

2. Rapid sand filter.

While rapid sand filters consist of both chemical and physical unit processes, slow sand filters englobe biological and physical treatment only.

The CWA operates 7 water treatment plants around the island, out of which, 2 use slow sand filters

while the rest use the rapid sand filters.

Raw water

from source

Coagulation,

flocculation and

sedimentation

Filtration Disinfection Distribution

of treated


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The different treatment plants and their water treatment capacity are summarised in the following table.

Table 3 - Water treatment plants in Mauritius.


Treatment plantType of filtration process

adoptedTreatment capacity /

m³/day

Pailles Treatment Plant Slow sand filtration 60,000

La Nicoliere Treatment Plant Rapid sand filtration 66,000

Piton du Milieu TreatmentPlant Rapid sand filtration 37,000

Riviere du Poste TreatmentPlant Rapid sand filtration 13,500

Mont Blanc Treatment Plant Rapid sand filtration 11,000

La Marie Treatment Plant Rapid sand filtration 70,000

La Marie Treatment Plant Slow sand filtration 60,000

It is to be noted that water extracted from ground water resources does not undergo the filtration process.

This can be explained by the fact that the extracted groundwater is of excellent quality.

Following filtration, the filtered water undergoes a disinfection process before it is distributed. The

latter can be done using ozone or chlorine. In Mauritius, chlorine is the only disinfectant used. Both

surface and ground waters undergo the chlorination process.

3.6 Water distribution networks

It is virtually impossible to supply water around the whole island from a single water treatment plant.

Hence, the CWA has divided the country into 6 autonomous water supply districts such that each district

receives water from at least one treatment plant and in such a way, it is ensured that there will be no

preferential treatment over water supply, thus ensuring a quality service.

The profiles of the 6 water supply systems are summarised in the table on the following page.


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Table 4- Different water supply systems in Mauritius.


Water Supply

System (W.S.S)

Service Area

covered / km²

Population

served

No. of

service

reservoirs

Treatment

plants

Normal

production /

m³/day

Port Louis W.S.S 36 200,000 16 Pailles 100,000

North W.S.S 337 307,000 19 La Nicoliere 123,000

East W.S.S 232 150,000 13Piton duMilieu 72,000

South W.S.S 220 210,000 22 Mont Blanc 13,500

Riviere duPoste 11,000

Mare aux VacoasUpper W.S.S 220 220,000 18 La Marie 114,000

Mare aux Vacoas

Lower W.S.S 200 227,000 18 - 95,500

It is to be noted that the Mare aux Vacoas Lower Water Supply System receives water primarily from

the Curepipe Aquifer. Thus, mostly groundwater is supplied in that district and as mentioned earlier,

the ground water does not require to be treated conventionally. Only chlorination is carried out for that

water (Mauritius Science Portal, 2014).

3.7 General water costs.

Water supply costs in Mauritius are based on a minimum-tariff system, i.e., irrespective of the amount

of water consumed, there is a minimum fee to be paid. The following table gives the minimum water

rates for some of the consumers.

Table 5 - Minimum tariff rates for different consumers in Mauritius.

Source: CWA website, 2014 (adapted data).

Types of supply Minimum charge / Rs

Domestic 45.00

Business consumers 1122.00

Commercial consumers 391.00

Industrial consumers 450.00

Agricultural consumers 220.00

Religious & Charity institutions 60.00

Public Sector Agencies 391.00


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For domestic consumers, based on the amount of water consumed, the water charges are as follows:

Table 6 - Water charges.


Amount of water consumed / m³ Water charges / Rs

First 10 m3 6.00

Next 10 m3 8.00

Next 30 m3 17.00

Every additional m3 32.00

Along with the minimum and water charges, all consumers are also legally accounted to pay for the

meter rent charges. The latter is as follows:

Table 7 - Meter rent charges.


Meter size/ mm Domestic & Non-domestic / Rs

12 or 18 10.00

25 30.00

37 45.00

50 60.00

75 90.00

100 150.00

> 100 200.00

The above mentioned costs are those costs which are payable due to the amount of water consumed.

However, there are additional costs which are incurred prior to connecting domestic water supplies to

distribution networks. These additional costs are summarised in the table below.

Table 8 - Additional payable fees.

Source: Government Gazette of Mauritius, 2014 (adapted data).

Additional payable fees Cost / RsProcessing fee for new water supply 500

Fee for new water supply without road reinstatement:

(a) minimum (up to 7 metres) 2000

(b) every additional metre (over 7 metre) 75 / metre

Fee for new water supply with road reinstatement:

(a) minimum (up to 7 metres) 2750

(b) every additional metre (over 7 metre) 250 / metre

Examination fee for testing metre 500

Re-establishment fee 500


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3.8 Conclusion

With all of the above being said, it can be deduced that Mauritius has an efficient water supply system

to cater for the needs of all its inhabitants. However, the water supply is primarily dependent on rainfall,

which is an unpredictable parameter. But, with a good administrative planning involving the proper

storage, construction of more reservoirs in rain catchment areas and adequate use of water, which is a

valuable resource, giant strides can be made to further enhance the quality of service provided by the

CWA.


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Figure 3- Approximate location of Saint Julien D’Hotman.

4.0 Methodology

4.1 Locality and distribution network

The locality under investigation is Saint Julien D’Hotman, a village found in the eastern part of the

island, more specifically in the district of Flacq. The approximate location of the village is marked with

a yellow box in the figure below.

Source: Google Images (2014).


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In terms of water supply districts, the above-named village is found in the East Water Supply System

zone and piped potable water is supplied from Piton du Milieu water treatment plant.

4.2 Desk study

The East Water Supply System caters for the distribution of potable water to domestic and other

economic operators of the eastern region. The characteristics of this water supply system are

summarised in the following table.

Table 9 - Characteristics of East Water Supply System.

Source: CWA, 2014.

Service area covered / km2 232

Population served 160,000

Number of subscribers 40,400

Number of service reservoirs 12

Length of pipeline / km about 540

The distribution of potable water occurs from the Piton du Milieu treatment plant to the service

reservoirs, from where the water is distributed along the different networks within the region. This

treatment plant is a rapid gravity one and was constructed in 3 phases, as shown in the table below.

Table 10 - Construction phases of Piton du Milieu water treatment plant.

Source: CWA, 2014.

Construction phase - Year Total filtration capacity / m3

Phase I - 1955 17,000

Phase II - 1987 30,000

Phase III - 2000 37,000

The main water sources which feed the East Water Supply System are:

1. Piton du Milieu reservoir.

2. Groundwater from boreholes.

3. Nearby rivers and rivulets such as Riviere Vacoas and Riviere Bateau.


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The average monthly potable water production from the treatment plant and boreholes for the different

months of 2013 are as follows:

Table 11 - Average monthly production of potable water for the different months of 2013.

Source: Energy and Water Digest, 2013 (adapted data).

Month Surface water / Mm3 Ground water / Mm3 Total / Mm3

January 0.7 1.5 2.2

February 0.7 1.4 2.1

March 0.9 1.8 2.7

April 0.8 1.7 2.5

May 0.8 1.6 2.4

June 0.7 1.6 2.3

July 0.8 1.7 2.5

August 0.8 1.7 2.5

September 0.7 1.7 2.4

October 0.8 1.7 2.5

November 0.8 1.7 2.5

December 0.9 1.6 2.5

The normal daily water production from the above-named sources is approximately 72,000 m 3 per day.

Water from the surface bodies account for about 28 % (mean daily production derived from Table 11)

of the daily production while the remaining 72 % is obtained from abstracted groundwater.

The abstracted ground water are usually of good quality and thus are not required to be channelled to

the treatment plant to be treated. The only treatment process that the groundwater undergoes is

chlorination before being fed to service reservoirs. However, it is to be noted that water abstracted from

agricultural and industrial boreholes, which have 3 and 4 accounts respectively, do not undergo any

treatment at all (Mauritius Science Portal, 2012).

On the other hand, water from the aforementioned reservoir and rivers need to undergo purification

processes before being fed to service reservoirs. Most of the raw water that is treated is mainly supplied

from the Piton du Milieu reservoir. The latter is situated on the Central Plateau, with more than 66 %

of the catchment area covered with sugarcane while the rest is forest. The total catchment area is

approximately 6.30 km2. The mean annual precipitation in that region is about 3529 mm, indicating that

it is a highly humid area (Liedholm, 2007).

Piton du Milieu reservoir has been constructed by erecting an 825-metre long earth dam over the Vacoas

River, with a height of 13.5 metres above foundation level (Hydrology Data Book, 2010). It is one of

the smallest reservoirs of the island, but also the deepest with a maximum depth of 15 metres.


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Figure 4 - Location of Piton du Milieu reservoir.

Source: Google Images, 2014.

Figure 5 - View of Piton du Milieu reservoir.

Note: The red arrow shows the dam wall mentioned previously.


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According to the Central Statistics Office, the percentages of water level during the months of 2013 is

as follows:

Table 12 - Water levels during the months of 2013 for the Piton du Milieu reservoir.

Month Mean % Minimum % Maximum %January 48 27 61

February 84 61 100

March 99 98 100

April 100 98 100

May 95 89 99

June 84 82 89

July 79 75 83

August 71 69 74

September 68 64 70

October 58 51 64

November 53 50 60

December 61 56 64

During the last 5 years, it has been observed that, due to climatic change, there has been considerable

decrease in the long term mean water level of the Piton du Milieu reservoir, as illustrated by the

following figure.

Figure 6 - Long term and current mean water levels of Piton du Milieu reservoir (2009 - 2013).

Source: Water and Energy Digest, 2013.

The decrease in the water production capacity of the Piton du Milieu reservoir and the corresponding

treatment plant have had a drastic change in the distribution of water in the eastern region. To cater for

this decrease, one of the initiated projects of the Central Water Authority has been to connect the


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Midlands Dam reservoir to the Piton du Milieu treatment plant and this project is expected to be

completed by the end of 2015. The main aim of this project is to provide an alternative water source

and improve the water supply in the east of Mauritius.

From all of the illustrated data, it can be deduced that the East Water Supply System cannot be solelydependent on the Piton du Milieu Reservoir and its treatment plant due to the decreasing average water

production capacity. The abstraction of ground water is a must and must be an on-going practice.

Furthermore, water leakages in pipes in the eastern region should be located and remedied.

4.3 Water treatment processes and distribution at Piton du Milieu treatment plant.

4.3.1 Intake of water and raw water screening.

As previously mentioned, raw water entering the Piton du Milieu treatment plant is derived mainly from

the Piton du Milieu reservoir. The latter is found at a higher level above mean sea level than the

treatment plant. Hence, the collected raw water flows to the treatment plant by gravity in a pipe having

a diameter of 900 mm. The latter is then diverted to a hydro generator, through pipes having a diameter

of 800 mm, which pumps the free flowing incoming water to the different rapid sand filters.

Figure 7 - Diversion of incoming free-flowing water to hydro generators.


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Figure 8 - Hydro generator.

It is to be noted that the incoming raw water may contain rocks, large materials and other debris, which,if carried through the hydro generator and into the treatment plant, may cause serious structural damage.

Thus, the entry of these debris need to be prevented. At the Piton du Milieu treatment plant, large bar

racks with openings of 2.5 cm and 7.5 cm apart, are used to remove the coarser materials in the raw

water.

The hydro generator, costing approximately Rs 25 M, is the result of agile engineering ideas. Firstly, it

increases the rate at which water is being fed to the filter beds. Furthermore, energy wise, it is self-

sufficient. It uses the flow of the very incoming raw water to turn a turbine, from which electricity is

produced to run the generator. The resulting energy consumption is decreased by as much as 30 kWh

equivalent to about Rs 500,000.

However, this system of bar racks is not efficient enough to cater for the finer particles which occur in

far greater numbers than the coarser ones. Hence, there has been a recent upgrading in the previous

system of bar racks to accommodate intake screens together with the latter. These intake screens have

smaller openings which vary from 0.5 cm to 2.5 cm and are thus able to prevent the entry of the finer

particles to a high degree of efficiency. Moreover, it has been brought to my knowledge that these

screens are fitted with a mechanical cleaning system, which prevent them from clogging. Furthermore,

according to the Technicians working at the treatment plant, there has been a significant decrease in the

turbidity level of the raw water since the installation of the intake screens. This has led to a decrease in

the production costs of the treatment plant, albeit a minor one.


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4.3.2 Water purification processes

As previously mentioned, the Piton du Milieu treatment plant consists only of rapid sand filters. At the

end of the construction of Phase III at the start of the twenty first century, there has been a total of 4

filter beds which have been installed and all of them are fully operational till date.

Rapid sand filtration is a purification method whereby only physical processes are applied. It allows the

rapid removal of relatively large suspended particles with a high degree of efficiency. For the provision

of safe drinking water, rapid sand filtration requires 2 types of treatments, namely:

1. Pre-treatment.

2. Post-treatment.

Pre-treatment of the raw water intended to undergo filtration in rapid sand filters include coagulation,

flocculation and the removal of these flocs by sedimentation. On the other hand, post-treatment of the

filtered water includes disinfection by chlorine (Schmitt.D, 2001). These different processes making up

both treatments are discussed in the following sections.

Figure 9 - Representative diagram of the rapid sand filters found at Piton du Milieu treatment plant.

Source: D.Schmitt, 2001.


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4.3.2.1 Coagulation

Coagulation is the first process normally carried out at the afore-mentioned treatment plant. Coagulation

refers to the destabilization of colloid particles by the addition of chemicals which neutralize the

negative charges of the colloids and allow them to stick together and eventually settle by gravity.

The industrial process of coagulation at the said treatment plant takes place in 2 consecutive stages.

These are:

1. Addition of coagulant and coagulant aids.

2. Rapid mixing.

4.3.2.1.1 Addition of coagulant and coagulant aids.

Nowadays, there are various primary coagulants available. One of the earliest used coagulants and still

the most commonly used at the Piton du Milieu treatment plant is aluminium sulfate, industrially known

as alum.

Figure 10 - A 25-kg industrial alum bag.

According to sources at the said treatment plant, alum is mainly imported from Indonesia, India and

sometimes Singapore and are usually packed in bags of 25 kg, as illustrated in Figure 10. The cost of

alum per 25-kg bag is around Rs 125.


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The addition of alum is not random. Laboratory technicians at the Piton du Milieu treatment plant

regularly carry out jar tests to determine the dosage of alum to be added and it was found that the use

of alum for the pre-treatment of water is quite extensive. The Treatment Plant Technician specified a

daily use of about 30 25-kg bags.

Figure 11 - Apparatus to carry out jar tests.

The use of alum alone is not sufficient to create large enough flocs which are able to settle by gravity.

To cater for this problem, a coagulant aid is used. The one used at the above-mentioned treatment plant

is synthetic polyelectrolyte. The main use of the latter is to add toughness and density (after rapid

mixing) to the coagulated particles and it is used in smaller quantities as compared to alum (less than

one 25-kg bag per day). It is to be noted that, for the same mass, the synthetic polyelectrolyte can cost

more than 10 times the cost of alum.

Figure 12 - Mechanical pumps which feed alum and polyelectrolyte to the raw water.


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The above figure illustrates the mechanical pumps used to feed alum and polyelectrolyte to the raw

water. There are 3 such pumps at the Piton du Milieu treatment plant. Two are operational and one is

on stand-by mode in case the other pumps encounter any mechanical difficulty. Each one of them is

believed to cost approximately Rs 200,000.

4.3.2.1.2 Rapid mixing

The addition of alum and polyelectrolyte takes place in a rapid mixing chamber as illustrated below.

The purpose of the rapid mixing is to evenly distribute the primary coagulant and the coagulant aid

through the raw water.

Figure 13 - Rapid mixing chamber sign board at Piton du Milieu treatment plant.


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Figure 14 - Rapid mixing chamber in operation.

According to the Technicians at the treatment plant, the rapid mixing time typically lasts for about 60

to 65 seconds and the mixing rate can be as much as 400 litres per second during summer time.

After the rapid mixing, alum, containing high valence cations, will react with the alkalinity in the water.

The Zeta potential of the colloidal particles will be reduced and the latter will be able to coagulate.

The problem with alum is that the reaction between the aluminium sulfate and alkalinity in the water

will tend to decrease the pH of the water, such that it is well below the specified limits for safe drinking

water. To cater for this serious problem, prepared liquid lime is added to the water through a hopper

and will neutralise the excess acidity of the water.


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Figure 15 - Hopper through which lime is prepared and pumped to the raw water.

Prior to and following the addition of lime, the pH of the raw water is tested at least twice per day. This

activity forms part of the quality control of the raw and treated waters at the treatment plant.

4.3.2.2 Flocculation

Flocculation is a process whereby the raw water, containing the coagulant particles, is gently stirred

and mixed such that these particles come into contact with each other and aggregate to form larger flocs.

Slader blades are used for the mixing.

There are 4 flocculators at the Piton du Milieu treatment plant and the flocculation time of the large

flocs lasts for about 40 minutes during normal days. It has been brought to my attention that during

heavy rainfall or cyclonic weather, the flocculation time is increased to about 45 to 50 minutes so as to

allow more particles to be removed.

The flocculators are directly connected to the settling tanks and the flocs are removed at regular 3-hour

intervals.


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Figure 16 - Two of the four available flocculators at the Piton du Milieu treatment plant.

Figure 17 - Formation of flocs.

4.3.2.3 Sedimentation

Once the flocs grow in size, they are allowed to settle by gravity in sedimentation tanks, each one having

a depth of 10 metres.

Figure 18 - Sedimentation tank.


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There are 4 settling tanks which are fully operational at the previously-named treatment plant. The

length of each settling tank is 15 feet (about 9 metres) and the retention time varies between 4 to 6

hours.

Once the particles are down in the settling tanks, the sludge deposits are discharged into side canalswhich carry them to the sludge tank as illustrated in Figure 18. While in the tank, the sludge is allowed

to be dried by the sun. When sufficiently dried, the sludge is removed, both manually and mechanically,

and disposed off.

Figure 19 - Discharging of sludge into side canals.

Figure 20 - Sludge tank.

4.3.2.4 Filtration

The filter beds at the Piton du Milieu treatment plant are all made up of reinforced concrete. Each one

of them has the following dimensions:

Table 13 - Dimensions of one filter bed.

Length / m 6.0

Width / m 3.5

Depth / m 6.0


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The beds are filled with gravels up to a height of 0.30 to 0.50 metres from the underdrain system and

the silica sand is filled from the top to a height varying from 0.45 to 0.75 metres above the level of the

gravels.

The raw water, after undergoing coagulation, flocculation and sedimentation, is supplied to the filter beds from the top and as the water passes through the layers of graded silica sand and gravels, it gets

filtered at a rate of 1.3 to 4.0 litres/ second/ m2.

Figure 21 - Raw water being fed to the filter beds after sedimentation.

Figure 22 - Filtration in process.

The filtering process is determined by 2 basic principles, namely:

1. Mechanical straining.

2. Physical adsorption.


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During the mechanical straining process, large suspended particles, which have not been removed by

the combination of coagulation flocculation and sedimentation processes, get trapped between the silica

sand grains as they pass through while during the physical adsorption process, smaller particles adhere

to the surface of the sand grains due to the effect of Van der Waal forces (Brikke, F, 2003).

The rapid sand filters is cleaned at least twice every year through a mechanical process of nozzle

cleaning.

Figure 23 - System of nozzle cleaning to clean the rapid sand filters.

The characteristics of the rapid sand filtration process at the Piton du Milieu treatment plant are

summarised in the following table.

Table 14 - Characteristics of rapid sand filters at Piton du Milieu treatment plant.

Characteristic Description

1 Filter medium Silica sand

2 Thickness of filter medium 0.45 to 0.75 metres

3 Grain size of filter medium 0.4 to 1.0 mm

4 Filtration rate 1.3 to 4.0 litres/sec/m2

5 Head loss

(i) Initial about 0.3 metres

(ii) Final about 2.0 to 2.5 metres6 Support for medium 0.3 to 0.5 metres of graded gravels.

7 Method of cleaningUsed filter media is discarded and replaced by new

material.

Spent material is removed by reversed flow.

8 Length of operation between cleaning. 6 - 40 hours

9 Penetration of impurities Almost through entire depth of filter bed.

10 Major processes taking place. Mechanical straining

Physical adsorption.

11Tolerable turbidity limit for incoming

raw water. High


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4.3.2.5 Backwashing

In the course of the processes taking place during filtration, more and more particles are trapped in the

silica sand and this leads to the clogging of the filters, which decreases the efficiency of the filter bed.

To allow the filter bed to gain its initial efficiency, it needs to be backwashed. The latter is a process

whereby the flow of the treated water is reversed through the filter medium. The silica sand becomes

suspended and the solid particles are separated in the surface water. Compressed air is usually injected

to aid the backwashing process (WHO, 1996).

As soon as most trapped particles are washed out and the backwash water becomes clear again, the filter

bed is put back into operation.

Figure 24 – Backwashing taking place at Piton du Milieu treatment plant.

At the Piton du Milieu treatment plant, the backwashing cycle takes place every 6 to 40 hours,depending on the time when clogging is at its maximum in the filter bed. The clogging process is

monitored through a pressure gauge. According to the Treatment Plant Technician at the said plant, it

is known that clogging of the filter bed is at its maximum when the pressure gauge reads between -20

to -25 kPa. Actually, the pressure being measured refers to the head loss of the filtered water. The

pressure range mentioned previously is the range during which the head loss is so high that water cannot

be filtered at the desired rate.


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carries the suspended particles in a backwash trough and is carried away in wash water drains

which eventually connects to a wash water tank.

5. After the elapsed 7 minutes, the backwash pump is stopped and the backwash inlet valve is

closed.

6. After some 3 to 4 minutes (this time period is based on judgement), the filter bed is put back

into operation.

Note: The pressure at which air is blown and the velocity of the reverse water flow is controlled so as

not to let the filter medium be washed away.

Figure 27 - Wash water tank at Piton du Milieu treatment plant.

The wash water is treated before being discharged into neighbouring rivers.

4.3.2.6 Chlorination

Chlorination is usually the penultimate step in the purification of water during either slow sand or rapid

sand filtration. During the latter process, it is normally used as a post treatment for the filtered water. It

can also be used as a pre-treatment in some circumstances.

Chlorine is added to water to kill pathogens and other microorganisms that are present in the filtered

and that can prove to be harmful to the human health if consumed. The addition of chlorine is a good

disinfection technique for that it is a relatively easy and inexpensive process (as compared to the use

and application of other disinfectants such as ozone) and yet it is very effective in destroying most

microorganisms that may be present in the filtered water.


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At the Piton du Milieu treatment plant, liquid chlorine from gas cylinders or toners is added to the

filtered water in the amount of 1 to 1.2 parts per million. It is to be noted that despite the fact that the

chlorine is stored in its liquid state, it returns to its gaseous state when leaving the cylinders as it absorbs

heat from the surrounding atmosphere. According to the technicians at the treatment plant, the

percentage of chlorine present in the toner or cylinder in the liquid state is 85 % while the remaining 15

% exists in the gaseous state.

Figure 28 - Toners containing chlorine.

One toner of chlorine weighs 900 kg and such a toner is completely consumed in 3 days. If the said

treatment plant falls short of these chlorine toners, the disinfection process is done using chlorine from

cylinders and one such cylinder is completely consumed in a single day.

According to the Treatment Plant Supervisor, the draw off rate of chlorine from the toners or cylinders

need to be carefully controlled. This is so because of the toxicity of chlorine and due to the fact that if

chlorine is drawn off too quickly, there is a possibility that the surrounding temperature may suddenly

drop, causing frosting and eventually leading to a lower gas flow rate. To cater for this problem, the

draw off rate is limited using a control valve.

If a greater feed rate is required, several toners are connected to a manifold and chlorine is drawn off

the connected toners simultaneously.


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Figure 29 - Chlorine toners connected to a manifold.

The type of chlorinator used at Piton du Milieu is a vacuum one. The figure below shows a

representative diagram of the one used there.

Figure 30 - Representative diagram of the vacuum chlorinator used at the Piton du Milieu treatment plant.

Source: Damal. C, 1991.

According to the technicians operating the chlorine plant at Piton du Milieu treatment plant, the vacuum

chlorinator works as follows:

1. Filtered water passes through the injector and the constricting tube causes a decrease in

pressure.

2. The decreased pressure creates a vacuum and opens the regulating valve. Chlorine gas is flown

out of the toner into the chlorinator.


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3. Once inside the chlorinator, the chlorine flow rate is measured and is adjusted accordingly.

4. The chlorine gas is then pulled into the injector and chlorinates the filtered water.

5. The running unchlorinated filtered water keeps flowing and allows the drawing off of more

chlorine to disinfect water.

Figure 31 - Actual chlorinator unit at Piton du Milieu treatment plant.

The vacuum chlorinator is preferred due to its safety features. Till date, there has never been any

chlorine gas leaks at the named treatment plant.

Figure 32 - Chlorine leakage detector.


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The actual chlorination technique applied at the said treatment plant is breakpoint chlorination. One of

the benefits of applying this technique, in addition to the fact that it acts as a germicide and may control

tastes and odours, is that it leaves a chlorine residual. The latter constitutes as a safeguard against

additional microbial contamination that may occur in distribution pipes. Thus, this chlorine residual

needs to be regularly monitored by technicians at the treatment plant. This is done through the use of

chlorine residual analysers.

Figure 33 - Chlorine Residual Analysers at Piton du Milieu treatment plant.

The average concentration of chlorine residual usually measured lies between 0.2 and 1.5 g/ L.

Technicians estimate that the ideal chlorine residual concentration should lie between 1.0 and 1.5 g/ L.

4.3.2.7 Storage of treated water

This is the last stage in the water purification process at Piton du Milieu treatment plant before

distribution. The potable water are stored in reservoirs made up of reinforced concrete. There are 11

storage reservoirs at the above-named plant. According to sources there, the depth of each reservoir is

nearly 3.5 metres, while the lengths and widths varies from 11 to 13.5 metres and 7 to 8 metres

respectively. Water flows into the tank through an inlet pipe of diameter 800 mm found above the

maximum water level of the storage tank. This prevents backflow of water through the inlet pipe. Water

is released through an outlet pipe having the same diameter as the inlet one. The outlet pipe, connected

to the main distribution network, is found at the bottom of the reservoir, along a pitched reinforced

concrete slab.


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Figure 34 - Representative diagram of a storage tank found at Piton du Milieu treatment plant.

Source: WHO, 2006.

4.2.3 Water quality control

Water quality refers to the physical, chemical, microbial and radiological characteristics of water. It is

most commonly used by reference to a set of standards against which compliance can be assessed.

At the Piton du Milieu treatment plant, water quality control is done by 2 Technicians in a laboratory

found on site. The control tests are carried out on samples of the following:

1. Raw water entering the treatment plant from the reservoir (after raw water screening).2. Raw water during the aftermath of the rapid mixing of alum and polyelectrolyte.

3. Chlorinated filtered water.

The sampling and testing of the above is done twice per day, at 10 a.m. and at 14 p.m. The standards

used for the quality control of the water are:

1. Environmental Protection Act (EPA) 2002.

2. World Health Organisation (WHO) Guidelines for drinking water.

The following table gives the different tests carried out on the different samples mentioned previously.


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4.2.3.3 Chlorinated filtered water

Most emphasis is laid on the chlorinated filtered water since the latter is meant for human consumption

and the latter’s health cannot be compromised in any way.

4.2.3.3.1 Turbidity

Turbidity refers to the cloudiness of a solution. In this case, turbidity refers as to what extent is the

purified water clear. It is measured using a turbidimeter as shown below.

Figure 35 - Turbidimeter.

According to the EPA 2002, the acceptable range of turbidity for drinking water is between 0 and 5

NTU. However, 0 NTU is not desired. According to the laboratory Technician, the expected and most

commonly obtained value for turbidity lies between 1 and 1.3 NTU, which can be considered as an

ideal value.

4.2.3.3.2 pH

pH refers to the degree of acidity or alkalinity of the purified water. It is simply measured using a pH

meter as illustrated in Figure 35.

The supplied potable water neither needs to be acidic or alkaline. However, a water at neutral pH 7 is

not desired since it is not palatable and hence not appealing to the consumer. The EPA 2002 specifies

a range of 6.5 to 8.5. This range is considered to be safe and caters for the problem of palatability as

well.


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Figure 36 - pH meter.

The pH range of the tested potable water at Piton du Milieu treatment plant normally lies between 6.5

and 7.8. According to the Technician, there has been very few cases whereby a pH value of 8 has been

exceeded.

4.2.3.3.3 Odour

Odour control is not always done at the above-named treatment plant. It is occasionally done, especially

in circumstances when it is known that the turbidity of the incoming raw water from the reservoir is

abnormally high.

There are no specific procedures to carry out this test at the laboratory of the Piton du Milieu treatment

plant. The sense of smell is used to detect any odour. This is done by taking a sample of 50 to 100 ml

of potable water, warming it to a temperature of 40 to 50 ° C and smelling the vapour.

4.2.3.3.4 Chlorine residual

This is a fundamental test carried out at the treatment plant. It is a visual test and hence is subjective to

one’s eyesight. The procedures for this test are as follows:

1. Dissolve 2 DPP (diethyl-p- phenylenediamine) tablets (No 1) and dissolve in the potable water

sample.

2. The sample is shaken and compared to a standard pink colour in a comparator.

3. The intensity of light required to match the colour of the sample to the standard pink colour

gives a measure of the residual chlorine present in the potable water.


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Figure 37 - DPD (No 1) tablets.

The ideal concentration of residual chlorine required in the potable water is 1.0 to 1.5 mg/ L and usually,

this is the same range within which most test results lie.

Figure 38- Comparator apparatus.

4.2.3.3.5 Conductivity

Conductivity refers to the ability of a solution to sustain the passage of an electric current, which is

carried by the dissolved inorganic ions.

The main intention behind carrying out conductivity tests on potable water produced at Piton du Milieu

treatment plant is that it is an indirect measurement of the amount of dissolved inorganic ions in the

water and the presence of these ions have an effect on the palatability of the water.

Conductivity tests at the said treatment plant is carried out by using a pH/ conductivity meter. It is the

same apparatus used to determine the pH of potable water, as illustrated in Figure 36. According to the

laboratory Technician, the usual conductivity range detected in the potable water produced there lies

between 150 and 200 µScm-1. The EPA 2002 or WHO Guidelines for Drinking Water Quality does not


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specify any limiting values for conductivity but the Technician finds the usual range satisfying

palatability requirements.

4.2.4 Treatment during special circumstances

During rainy or cyclonic weather, the quality of water stored in the Piton du Milieu reservoir is

extremely poor due to the very large amount of undesired materials which can get carried away in

running water. Sometimes the turbidity of the raw water can rise to as much as 750 NTU. In these

circumstances, the conventional purification process at the Piton du Milieu treatment plant may not be

effective enough to produce potable water. Hence, some changes are made to the current system of

rapid sand filtration to adapt to the situation. These changes are summarised in the following table.

Table 16 - Changes in conventional treatment processes in special circumstances.

Normal conditions Special circumstances

Raw water

screening

Carried out at entry

point of treatment plantonly.

Carried out both at reservoir and treatment plant.

Coagulant and

coagulant aid

dosages. Normal conditions Increased dosage.

Pre-chlorination Not carried out. Carried out.

Post-treatment.

Normal dosage (1.0 mg/L) Increased dosage (1.5 mg/L)

Sampling time for

quality control

tests

Twice a day (usually at3 hour intervals) Every 30 minutes

Quality control

tests

Simple ones such asturbidity and pH.

More complex ones such as arsenic content andmercury content in addition to the simple ones.

On the other hand, during drought seasons, there is no change in the conventional water purification

process.

4.2.5 Health and Safety aspects

The Piton du Milieu treatment plant, as well as all other water treatment plants worldwide, is a place

where there are my health hazards and potential dangers due to the large amount of toxic chemicals

used in the purification of water. Workers on site are thus at risk and need to be safeguarded against

these dangers. In this context, several protective measures have been established and enforced to make

sure that there is no harm done to any worker. According to the Treatment Plant Supervisor, there is no

worker who has suffered any health risk on site till now. Some of the measures established are illustrated

below.


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Figure 39 - Emergency eye and body wash in case of exposure to alum or polyelectrolyte.

Figure 40 - Emergency outdoor shower in case of exposure to chlorine.

Despite the safety features of the vacuum chlorinator in use at the treatment plant, provision has been

made to cater for the problem of chlorine leakage. A neutralising tower has been installed. Whenever

there is chlorine leakage, this leaked chlorine is absorbed by a pump, which feeds to a small chamber

containing caustic soda. The latter reduces the chlorine to sodium chloride (NaCl) and it is then

expelled.


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Figure 41 - Established procedures for the handling of chlorine toners or cylinders.

Furthermore, all workers dealing with the different chemicals in use at the treatment plant are

encouraged to make use of gloves and safety shoes as far as possible.

4.2.6 Distribution of potable water within eastern network to my locality

At its maximum level, water in the storage reservoirs at Piton du Milieu treatment plant is found at 428

metres above mean sea level (Hydrology Data Book, 2010). The eastern part of Mauritius is found on

lower plains. Thus being said, water from the treatment plant flows solely by the action of gravity, in

pipes having a diameter of 800 mm, to the service reservoirs.

Some of these service reservoirs include the following.

Constance Reservoir

Saint Julien Reservoir

Mont Ida Reservoir

Quartier Militaire Reservoir

Alma Reservoir

Constance Reservoir

According to the officers at the CWA Sub Office at Quartier Militaire, the locality of Saint Julien

D’Hotman is connected to 3 reservoirs, namely those at Saint Julien, Mont Ida and Quartier Militaire.

The latter is at a higher level than those at Saint Julien and Mont Ida respectively. The village is divided

into 3 sections, and each section obtains water from a reservoir that is found at a higher reduced level


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than itself, such that the whole distribution system operates by the action of gravity only and the use of

pumps is thus restricted.

The pipes connecting the service reservoirs to the domestic houses have the following dimensions.

8 inches (200 mm)

12 inches (300 mm)

14 inches (350 mm).

4.3 Costing

The only possible costing data about the items used, which could be provided by officers at the Piton

du Milieu treatment plant are summarised in the following table.

Table 17- Costs of items used at treatment plant.

Item Unit price / Rs

Chlorine 25,000 per toner

9000 per cylinder

Silica sand 3500 per tonne

Solid lime 8100 per tonne

Alum 9150 per tonne

Poly-electrolyte 375 per kilogram

Furthermore, according to some sources, the electricity cost for the running of the treatment plant alone

is approximately Rs 1.9 Million and the total treatment cost per month is about Rs 15 Million.

Unfortunately, the individual costs for the different treatment processes and those of water distribution

could not be provided.


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4.4 Limitations

4.4.1 Limitations of the general water distribution network in Mauritius

According to the Central Statistics Office, more than 99.6 % of the total Mauritian population has access

to potable water supply nowadays. However, this figure does not reflect the actual water situation in

the island. The current system has many limitations and despite massive investments from the

Government to try to remedy the situation, it is unfortunate to note that there has been no real progress.

Some of the limitations of our current potable water supply system are listed below.

1. The current piping network dates back to very long ago. According to some sources, some

localities have pipes which date more than 40 years back. These pipes are damaged and there

are leakages. Due to these leakages, the supply of potable water is often disturbed.

2. There is a lack of storage reservoirs in catchment areas. Some of these catchment areas may

obtain adequate amount of rainfall throughout the year and most of the water goes is lost.

3. Some regions of the island, especially the northern and western regions, may receive potable

water only twice a day for a total time period of 4 hours. Inhabitants thus do not receive an

adequate amount of water for their daily needs and the use of water storage trucks is often

solicited.

4. Some of the existing reservoirs, like Mare aux Vacoas, loses some of its water into the ground.

This is a problem which has to extensive testing on the soil properties there. However, no

appropriate solution has been proposed till date.

5. Some of the existing water treatment plants are under-designed and are not able to produce the

required amount of potable water to satisfy the needs of the population on a daily basis.

4.4.2 Limitations of the study

The investigation carried out was largely dependent on the provision of relevant data and information

from relevant authorities. However, despite all possible efforts, this did not happen. The following is a

list of the limitations encountered during the carrying out of the study.

1. Officers at the treatment plants (at La Marie and Piton du Milieu) could not provide us with

some of the technical specifications of the individual treatment processes. The technical

specifications included: dimensions of the weirs in place, costs related to the individual

processes, detention times, and exact dimensions of storage tanks amongst others.

2. The costings related to the supply of potable water and of the piping network were deemed

confidential by relevant authorities and could not be communicated to us.

3. With time playing a huge constraint in the schedule of the officers at the treatment plants,

complete explanations were often missed.


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References

1. Proag.V (2006), European Water Resources Association, Water Resources Management in

Mauritius, European Water Publications, Vol. 15/16 p. 45 – 57.

2. Reef Conservation (2014), About Mauritius, [Online], Available at:

http://www.reefconservation.mu/about-mauritius/environmental-profile/ [Accessed on:

24.11.2014].

3. Encyclopaedia of the Nations (2014), Mauritius, [Online], Available at:

http://www.nationsencyclopedia.com/Africa/Mauritius.html, [Accessed on: 24.11.2014]

4. Encyclopaedia Britannica (2014), Mauritius, [Online], Available at:

http://www.britannica.com/EBchecked/topic/370153/Mauritius, [Accessed on: 24.11.2014].

5. Mauritius Meteorological Services (2014), Climate of Mauritius, [Online], Available at:

http://metservice.intnet.mu/climate-services/climate-of-mauritius.php, [Accessed on:

24.11.2014].

6. Mauritius Meteorological Services (2014), Climate Info and Data, [Online], Available at:

http://metservice.intnet.mu/climate-services/climate-of-mauritius.php, [Accessed on:

24.11.2014].

7. Ministry of Finance and Economic Development (2013), Water and Energy Digest – Statistics:

Statistics Mauritius.

8. Central Water Authority (2014), Water Production in Mauritius, [Online], Available at:

www.cwa.gov.mu/info/water-services.php, [Accessed on: 24.11.2014]

9. BerlinWasser International (2014), Saint Martin Wastewater Treatment Plant, [Online],

Available at: http://www.bwi.com.mu/, [Accessed on: 24.11.2014].

10. Mauritius Science Portal (2014), Water in Mauritius, Rajiv Gandhi Science Centre, [Online],

Available at: www.gov.mu/portal/sites/nsp/popular/index.htm, [Accessed on 24.11.2014].

11. Google Images (2014), Map of Mauritius, [Online], Available at:

https://www.google.mu/search/ maps of Mauritius. php [Accessed on: 24.11.2014].

12. Ministry of Energy and Public Utilities (2010), Mauritius Hydrology Data Book 2006 – 2010,

Chapter 4, p. 56 – 72, Department of Hydrology, CWA.

13. Guidelines for Drinking Water Quality (2006), Vol. 1 – Recommendations, World Health

Organization.

14. Mauritius Research Council (2001), Thematic Working Group Water Resources Final Report.

15. The Biosand Filter (2004), Rapid & Slow Sand Filtration [Online], Available at:

http://www.biosandfilter.org/biosandfilter/index/php/item (229), [Accessed on 04.12.2014].

16. Di Bernado, L and Isaad R.L (2001), Upflow direct filtration – A review, CIWEM International

Conference on Advances in Rapid Granular Filtration in Water Treatment, London, England.


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17. John.T.O’Conor and Thomas.l.O’Conor (2001), Water Engineering – Removal of

Microorganisms by Rapid Sand Filtration, Columbia, Missouri.

18. Baker, Moses.N (1981), The Quest for Pure Water – The History of Water Purification from

the Earliest Records to the 20th century (2nd Edition), Benter: American Water Works

Association, p. 336.

19. Jeanette Liedholm (2007), Primary Production of Freshwater in Mauritius, p. 56-64.


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Appendix

Item 1 – Water bodies that provide water for production of potable water.

Figure 42 - Rivers, rivulets, boreholes, wells and reservoirs used in potable water production.

Source: CWA (2014)


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Item 2 – Main aquifers of Mauritius

Figure 43 - Main aquifers of Mauritius.

Source: CWA (2014)

Note: The aquifers mentioned in the main report are numbered by correlation to this map.


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Item 3 – Boreholes in Mauritius

Figure 44 - Boreholes in Mauritius.

Source: CWA, 2014.


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Item 4 – Man-made reservoirs in Mauritius

Figure 45 – Man-made reservoirs in Mauritius.

Source: Berg.L (2004).


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Item 5 – Diary of activities (corresponding to ECSA ELO 8)

The following table illustrates the activities carried out prior/ during the conduction of this investigation.

Table 18 - Diary of activities.

Date Description of activity

27-Nov-14 Site visit at La Marie Treatment Plant (in group)

28-Nov-14 Site visit at La Nicoliere Treatment Plant (in group)

30-Nov-14 Site visit at Piton du Milieu Treatment Plant (individu

case study of water engineering

Documents