ISSUE 5

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INSIDE THIS ISSUE

•• Tshwane water sources and

treatment plants• Rietvlei water treatment plant• Water waste treatment

Water cycles p 1

p 2p 2, 3 & 4p 5 & 6

Water Cycles

WATER CYCLES

A - NATURAL WATER CYCLE

The natural water cycle (A) is the process of water moving on the earth. When humans use water for drinking, irrigation, sanitation and industrial use, they take the water out of the natural cycle and it becomes part of the human or man-made water cycle – the consumer water cycle (B).

Evaporation

Cloud formation

Rainfall

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B - CONSUMER WATER CYCLE

Source extractionWater used for human consumption comes from rivers, dams, boreholes and springs.

Water treatment worksOnce the water has been extracted, it undergoes several types of treatment processes in order to make it safe to drink.

Storage of clean waterOnce it is made safe to drink, the water is transported via a network of pipes for storage at reservoirs.

Distribution of clean water to consumersWater is supplied to consumers using a complex system of pipes. Reservoirs, water

flow, water pressure and water quality are constantly monitored to ensure that enough water reaches all consumer connected to the network.Consumers/Users:

Commercial UsersDomestic UsersMining UsersIndustry UsersAgricultural Users

Waste water collectionWastewater is collected using a sewerage system. This network consists of a collection of pipes made from cement, concrete or PVC which carry the polluted water to be sent to a waste water treatment plant.

Waste water treatment worksSewage treatment, or domestic wastewater

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treatment, is the process of removing contaminants from wastewater and household sewage, both runoff (effluents) and domestic. It includes various processes to remove physical, chemical and biological contaminants. The objective of sewage treatment is to produce a disposable effluent without causing harm to the surrounding environment, and also to prevent pollution.

Returning clean water to the natural environmentOnce treated in a wastewater treatment plant, water is released into the natural environment. It continuous its natural treatment process in rivers or in dams, without adversely affecting the natural balance of the environment.

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A - NATURAL WATER CYCLE B - CONSUMER WATER CYCLE

Users

Supply

Source extraction

Storage

Water treatment works

Waste water treatment works

Returning clean water to the natural environment

Waste water collection

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Rietvlei Water Treatment Plant

National Award Winning Project for Engineering Excellence

DWAF Award: Most Presentable Medium Size Water Works 2009

* It was constructed between 1932 and 1934 in the Sesmyl Spruit near situated in Pretoria, which is the Rietvlei Nature covers 3 200 hectares

Reserve, which

8%

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* The first 18 Ml/d Plant was built in 1934* The treatment capacity of the Plant was

increased in 1988 to 40 Ml/d* The Plant saw the first full-scale application of the Dissolved

Air Flotation and Filtration (DAFF) process in South Africa* The new Granular Activated Carbon (GAC) filtration system

was completed in 2000 and is the first operational application of open bed gravity filters for drinking water treatment in South Africa

The Plant was established to meet Tshwane’s drinking water requirements

Rietvlei Water Treatment Plant

Springs & Boreholes

Rand Water87%

5%

Facts about the Rietvlei Dam

Facts about the Treatment Plant

Tshwane Daily Water Requirements

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* Dam wall : 32 metres high3Capacity : 12 000 000m

Surface area : 1,87 km²Yacht and canoe club

Water Treatment

Tshwane receives 81,3% of its water from Rand Water and Magalies Water. The City of Tshwane supplies the remaining 18,7% from its own dams, boreholes and springs.

The City of Tshwane has three water treatment plants, namely the Tembawater treatment plant at the Leeukraal Dam, the Roodeplaat water treatment plant at the Roodeplaat Dam, and the Rietvlei water treatment plant at the Rietvlei Dam.It also has ten waste-water treatment plants (Babelegi , Temba , Rietgat , Klipgat , Sandspruit , Rooiwal , Zeekoegat , Baviaanspoort ,

Daspoort and Sunderland Ridge ) where all the waste water is treated before it is discharged into the river system. The discharged water must comply with a standard specified in the licence of each plant and issued by the Department of Water Affairs.

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Water is abstracted from the Rietvlei Dam by means of an inlet tower. Three different abstractions points at varying levels in the tower can be used, depending on the water quality at those points.

Chemicals called flocculants (ferric chloride and aluminium sulphate), are added to the water to make the particles in the water bind together and form flocs. The water in the dam is characterised by a very low turbidity (cloudiness of the water) but has extremely high algal activity, including blue-green algae, which is known to produce toxins and can cause taste and odour problems in the drinking water. Lime is also added at this stage to stabilise the water. Stabilisation (buffering) ensures non-corrosive conditions in the structures of the unit processes.

The water is flash mixed and flows slowly and evenly through a series of baffled channels called flocculation channels, which allow the flocs to grow.

From the flocculation channels the water enters the Dissolved Air Flotation/Filtration (DAFF) process, which removes all the particles that have just been formed in the flocculation channels. This process can be divided into a flotation and filtration process. Because the water in the dam is eutrophic (high algal concentrations) it was decided to use a flotation instead of a sedimentation process. Algae floats rather than sinks and therefore its natural properties are used to clean the water.A stream of clean water containing large quantities of air (supersaturated) is passed through a bank of nozzles at the inlet of the filter.

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The flocculated water is then passed over thesupersaturated water and thisreleasing the airfrom the liquid phase, causing bubbles to attachto the flocs and rise - this is the flotation process. This process is so efficient that the flotation process alone removes about 70% of the suspended particles in the water. hese particlesform a scummy brown layer on the surface of thefilter area, which is all that can be seen fromabove. Underneath there is a filter bed madeup of various sizes of sand. The water passesthrough the sand filter, which removes the restof the particles left in the water - this is calledthe filtration process.

Water Treatment Steps

Water Treatment

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Although the DAFF process is very effective at removing the flocculated particles from the water, it is unable to remove dissolved organic matter (this includes colour, odours and tastes caused by the algae present in the water). Granular Activated Carbon (GAC) filtration is used to remove the organic matter by adsorption. Since GAC filtration has been introduced, the colour, odour and taste of the water have improved noticeably.

Although the water has passed through both the DAFF and the GAC process and seems to be clean, it still requires disinfection, which is accomplished by means of chlorination. Chlorine gas is used to kill all the remaining bacteria and viruses still present in the water. Chlorine is the most effective disinfectant because it has a residence time of several hours. (This means that the water is protected from contamination and growth of bacteria while it moves through the pipelines towardsthe storage reservoirs and eventually to the consumer).

The water flows into an on-site storage reservoir where water from fountains in the nature reserve is added. This reservoir serves as a chlorine contact chamber where the chlorine has time to work and ensure effective disinfection.

From the reservoir, the water is pumped to two of the largest storage reservoirs in Pretoria. At the storage reservoirs the water is blended with water provided by Rand Water and water from other boreholes. The water is now ready for consumption by the consumer.

The DAFF and GAC filters are cleaned by regular backwashing. The backwash water flows into settling tanks. These settling tanks act as holding tanks for the dirty water and allow settling of the particles to take place. After a retention time of two hours the settled sludge is pumped away into the sewerage system and the relatively clean water (supernatant) gravitates back into the river downstream of the dam.

STEP 6

STEP 7

STEP 8

STEP 9

STEP 10-12

For more information on the Rietvlei Water Treatment Plant,

please contact :-

leannec@tshwane.gov.za

Leanne Coetzee

Tel (012) 358 1800

Water Treatment

STEP 5

Owing to the hydraulics of the older sectionof the Plant, some height was neededfor the waterto gravitate through the next section of the Plant. Two 10m Archimedean screw pumps have beeninstalled and lift the water some 4,5m. From herethe water gravitates through the remainder of thePlan.

WHY IS IT NECESSARY TO TREAT (CLEAN) WASTE WATER?

HOW DOES WASTE WATER GET TO THE WASTE WATER TREATMENT WORKS?

WHAT NEEDS TO BE TAKEN OUT FIRST?

HOW DO WE TAKE IT OUT?

WHAT HAPPENED TO THE SOLIDS WE TOOK OUT?

WHAT HAPPEN TO THE WATER PART OF THE WASTE WATER?

* There are materials and pathogens in waste water that will cause pollution.* It will take the available oxygen out of the water.* It will destroy the friendly life forms in the rivers and dams.* Treated water is put back in the rivers, because SA has limited water resources.

Through a network of pipelines under gravitation. Small pipes from the different users such as industries, houses and businesses. These pipes connect to bigger pipes, which connect to an outfall sewer that end at the Waste Water Treatment Works.

All the solids:* Material bigger than 25mm eg. cloth, plastic bags , plastic cups and bottles etc. * Material smaller than 25mm eg. rice, maize, carrots, etc.* Settled organic solids.

The water flows through a Screen which catches the big solids, to the Degritters, which catches the smaller solids, to the Primary Settling Tanks where the solid organic matter (sludge) settles out.

* The solids bigger than 25mm are dried and burned in the Incinerator. The Incinerator uses methane gas which is produced in the Anaerobic Digestors.

* The Solid organic matter (sludge) is removed to the Anaerobic Digestors. In the Anaerobic Digestor organisms digest some of the organic matter to methane and carbon dioxide.

* The sludge from the Anaerobic Digestors is pumped into the sludge drying beds where it is left to dry out. The dried sludge is used by the Parks Department to make compost for their gardens.

* Only the solids have been removed at this stage!* Waste water is purified by micro-organisms.

From the Primary Settling Tanks the settled sewage (water) flows either to the Biofilter or Activated Sludge Reactor.

In the Biofilter the organisms attached themselves to stones; to live they take oxygen directly out of the atmosphere. When the water flows over the stones the organisms digest the organic matter from the water. These Biofilters were build in 1913 and are still being used today, at the Daspoort Waste Water Treatment Works.

In the Activated Sludge Reactor the water is aerated, to get oxygen into the water, which is necessary for the organisms to live and digest the organic matter. This method is the more modern method used on all the works.

The cleaned water from the Activated Sludge Reactor and Biofilters flow to the Secondary Sedimentation Tanks where the fattened organisms settle out.

They are then pumped back via screw pumps to the Activated Sludge Reactor so they can restart the process.

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Waste water treatment

Waste Water Treatment

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HOW DO WE KILL THE REMAINING MICRO-ORGANISMS?

INSPECTION OF THE WATER?

IS THE WATER NOW CLEAN?

DEFINITIONS OF WATER QUALITY PARAMETERS:

* Colour & turbidity:

* Taste & Smell: * Hardness: * pH:

* Fluoride & chlorides:

* Manganese:

HOW MANY WASTE WATER TREATMENT WORKS ARE THERE IN THE TSHWANE AREA?

* The most common method used by most of the Waste Water Treatment Works is by making use of Chlorine Gas. * The newer method used at the Daspoort Waste Water Treatment Works is UV.

With the Ultraviolet (UV) process the effluent (clean water) passes the UV-lights and harmful bacteria are killed.

Throughout the process samples of the water are taken and monitored in the Laboratory for compliancy to the water act.

Yes, only now is this water ready to be received in the public streams. It is now safe for downstream users and the water can be reclaimed to be treated as potable water.

Water that is turbid is cloudy or hazy, because it has sediment or foreign particles floating in it. The colour of water is influenced by the presence or absence of dissolved chemicals in the water.

Drinking water must not have any strange taste or smell. Hardness in water is caused by calcium and magnesium compounds, and occurs naturally.

The pH level of water is a measure of how acidic or basic the water is. If the pH levels are high, the water may taste bitter.

Fluoride is an essential nutrient that we need in our diet. Fluoride taken in through food and water during the tooth-forming years protects against dental cavities. Fluoride is found in drinking water. However, if drinking water with high fluoride concentrations is consumed over a long period of time, it may be harmful.

Manganese is a metal that is often found in solids and is associated with water. It is responsible for the dark tea-like colour of some water sources.

Ten in total named:

POTABLE OR DRINKING WATER

* Totally dissolved solids:

Totally dissolved solids are inorganic substances dissolved in water. These usually include chloride, sulphate, calcium, magnesium and bicarbonates.

The basic requirements for safe water are that the water must be- * Free from pathogenic organisms. * Free from compounds having an adverse effect on human health. * Clear with acceptable turbidity and colour levels. * Reasonably palatable and odourless. * Colourless and does not stain clothes during washing.

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Zeekoegat , Sunderland RidgeBaviaanspoort ,Daspoort

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Klipgat ,Sandspruit ,Rooiwal ,

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Babelegi ,Temba , Rietgat ,

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REPORT –no water supply; water pipes burst; leaking meters; and blocked street sewers

to 012 358 2111/9999 or 080 1111 556 (toll-free)Industrial effluent enquiries: 012 358 9067/9078/9999

Pollution spill response service: 012 358 9067/9078/9999

Waste Water Treatment