THE FCT URBAN WATER SCHEME PHASE 1 & 2 PART 1

THE FCT URBAN WATER SCHEME PHASE 1 & 2

PART 1

Introduction

Lower Usuma Dam

Lower Usuma dam is located at about the highest point in the Federal capital territory in the North western part of the territory and feed the treatment plant by gravity. Sited on a virgin location where human activities is minimal. Thereby ensuring non pollution of the environment and free from industrial impurity.

The dam is homogeneous earth fill with an upstream face of rock fill while the downstream is grassed with a grout out wall with vertical and horizontal fitters. The dam is provided with a pumping station in order to pump raw water from main dam reservoir to the water works in the event of extreme drought.  

The sources of the raw water come from three outlet: The two major rivers: River Usuma where it takes its name from, River Gidna and Gurara transfer outlet.

It is made up of two system of dams: the main and secondary saddle dam with a spillway in the rock in-between.

The characteristic of main dam:

Dam height- 49m

Dam crest length- 1300m

Dam crest width-        10m

Reservoir capacity- 107m3

Maximum width- 250m

Free board-      5m

Area at full supply level- 8.5KM

The saddle dam is located on the right of the right abutment of the main dam. The construction of the saddle dam was born out of the need to maintain a maximum supply level of 570m above sea level which correspond to maximum reservoir capacity of 107m3.

The saddle dam is an earth dam with a zoned laterate with impervious core.

Saddle dam characteristic:

Dam height-    25m

Dam crest width-        10m

Free board-      5m

Construction time 1980-1984

Designer/constractor- Spic- Batignolles Nig. Ltd (with Guff as original designer).

The dam is covered with gravel while the upstream slope has a rip-rap protection and the down-stream slope is protected with grass.

. The intake structure is a concrete, circular shape, has eight intake points at four different levels these convey raw to the treatment plant. It has height of 40m and diameter of 10m. The construction time for the dam was from 1980 to 1984 and the reservoir impoundment commenced on 1984, it was designed and constructed by Spie Batignolles Nigeria Ltd. With Guff as original designer.

PRECAUTION AT DAM SITE WITH DYKES/ EMBANKMENT

The instrumentation in the dam are instruments set on the embankment to protect the dam, these include pore pressure cells and piezometers.

Pore pressure cells

Pore pressure cells in saddle dam are based on the vibrating wire strain gauge principle, the changes in pore pressure acting on the diaphragm causes changes in the tension and frequency of vibration of a fine steel wire.

There are ten cells in all, two each located in profile 50, 95, 105, 175 and 245, and are located in the fill. The pore pressure obtained from these cells may use to establish the saturation line or the phreatic surface and the flow within and underneath the dam.

Piezometers

In dam piezometers are installed in a row parallel to the dam axis down of the in 50, 100, 150, 200, 250, 300 and 340, running from left to right abutment. Water levels in the piezometers are measured to obtain the ground water level and raising of pore water in the embankment.

GENERAL PLANT STRUCTURE

Raw Water Inlet Structure

The raw water inlet structure is the place where the treatment of the incoming raw water starts. The raw water is obtained from a dam 2km away from the treatment plant. It is supplied via a pipeline from the dam to the inlet structure, with the flow rate averaging about 5000 m3/hr.  Raw water inlet Structure is divided into three sections, namely the aeration chamber, distribution chamber and dosing chamber. 

Raw water inlet structure

The Aeration chamber                                          
The aeration chamber is that section of the raw water inlet structure, which consists of a series of four weirs which forces the raw water to flow downwards, upwards and back down again in a turbulating manner. This is the first step in the treatment process and is aimed at removing the foul odor/smell that comes along  with the raw water in the form of H₂S (Hydrogen sulphide), as well as oxiding the ions present in the raw water, with Iron (II) oxide (Fe²⁺) being the most notable. It also improves the dissolved oxygen content of the raw water. 

Aeration Chamber

The Distribution chamber 

This chamber ensures the even distribution of the aeration water into the two sections of the dosing chamber that leads to the different clarifiers (settling tanks).

The Dosing chamber 

 After distribution, the aerated water flows to the dosing chamber (which is the third and last section of the raw water inlet structure). This is where the treatment with chemicals begins. Here Aluminum sulphate (Al₂ (SO₄)₃), chlorine (Cl₂) and calcium hydroxide (Ca (OH)₂) are added to the aerated water. 

Aluminum sulphide (Alum for short), is added to cause coagulation of the minute particles that cannot be seen with the naked eyes, as well as the visible dirt particles. These particles are positively charged and therefore repel each other. Thus, the introduction of a negatively charged substance Alum is used to bring these particles together and make them more visible and heavier, so that they can be dealt with easily.          Chlorine is added to the aerated water in the form of chlorinated water. That is, gaseous chlorine is dissolved in water and this form chlorinated water. Chlorine is used as a disinfectant in water treatment i.e. it destroys the microbes present in water; it serves as a coagulant to extent. Addition of Alum and chlorine to the aerated water reduces the PH of the water thus makes the aerated water acidic. Powdered Lime (CaCO₃) is dissolved in water, to form hydrated lime (Ca(OH)₂) and is used to bring PH of the aerated water to more suitable level for further treatment.

These three chemicals will react with the aerated water in the dosing chamber, which is characterized by turbulent flow, for thought mixing. The amount of Alum and Lime to be added to the aerated water is determined by a jar (flocculation) test, carried out in a laboratory.

The Pulsator Clarifier (Settling Tank)

After the aerated water has been treated with the chemicals mentioned above, it flows through an underground pipe, to the clarifier, where further treatment takes place. The clarifier is basically a large settling tank, where the clog formed as a result of coagulation, come together to form floccs (which are bigger particles) and settle. Floccs are bigger and more visible particles, as compare to clogs and are formed by the process the process known as flocculation. In the clarifier, the biggest particles formed settle to the bottom of the tank, while the clear water remains at the top of the tank. In between, there are several settling zones/layers, which consist of flocc particles of different sizes. The floccs, which settle to the bottom of the tank form what is known as Sludge. It is essential that the sludge blanket (sludge covering the base of settling tank) be present during flocculation. This because the sludge blanket attracts other flocc particles to its self and thus improves the flocc settling rate i.e. improves flocculation.

Clarifier showing troughs

From time to time, the level of the sludge blanket increases and needs to be reduced. The operation by which excess sludge is removed from the clarifier is known as de-sludging and takes place in the de-sludging chamber.

The Pulsator

If the sludge is left undisturbed, it will solidify to form sludge cakes, which breaks up, floats to the top of the clarifier and makes the clarifier appears dirty. It will also reduce flocculation efficiency, as the sludge cakes do not attract floccs to themselves. Even when they do, they tend to reverse the process of flocculation, as the floccs will be rising up instead of settling downwards. The pulsator is thus used to disturb the sludge regularly and keeps it in molten state.

The pulsator is a suction device that works on the principle of suction. It consists of a flat-bottomed tank with a series of perforated pipes uniformly distributed over the entire bottom of the clarifier.

The principle consists of removing the air by suction, with aid of fans from the vacuum chamber. As a result, the water level rises gradually inside vacuum chamber. When it reaches a level between 0-6 and 1.0m above the clarifier water level, a contact sudden opens an air inlet valve. Atmospheric pressure is therefore immediately supplied to the water stored in the vacuum chamber and the water flows into the distribution pipes at high speed.

The Filter

After settling the clarified water flows into the filters, where filtration takes place. Filtration in this case, is the movement of water through a sand bed (permeable medium), which allows the clear water (filtrate) to pass through the permeable medium and retain the dirt or residual particles that did not settle in the clarifier, as well as other minute substances, leaves etc from the surroundings, as the residue. The sand bed consists of different sizes of sand particles, from fine to coarse particles. The filter (clear water in this case) is collected by a structure known as siphons, from which it flows to the treated tank.

Backwashing

From time to time, as the sand bed continues to filter the clarified water, the residue will accumulate to an extent that it will block the surface of the sand bad and thus filtration can no longer take place. This process is known as filter clogging. The operation used to correct this problem is known as back washing. It involves the addition of air under pressure, from beneath the sand bed, through the nozzles as well as water for rising.

The air supplied causes the sand particles to rub against each other, thereby removing the dirt attached to them, while the water supplied, and

rinses the dirt off. The whole process is control on a back washing desk- a device with a number of buttons and switches to control the flow of water and air into and out of the filer bed, during back washing.

The water recovered during back washing is collected in a recovery tank. The clearer part of the water is recycled to the aeration chamber at the raw water inlet structure, while the very dirty part of the water is discharged as sludge waste.

Back washing of a Filter in progress

The Treated water tank 

The treated water tank is divided into two sections, the tank the storage tank:                          

The Contact tank

This section receives the filtered water from the siphon and chlorine in the form of chlorinated water, is added to finally disinfect the water. This tank consists of baffles that cause the filtered water to meander its way through the tank. This is aimed at holding the filtered water for a period of 20-30 minutes for proper chlorine contact because, it is expected that no living microorganism can survive after contact with chlorine within that time range. There is always some residual chlorine in the treated water to make up for the organisms that will be encountered by the treated water on its journey to the consumers, along the pipe network.

The store tank 

After 20-30 minutes, the treated water flows into the sludge tank, where it is stored and lime is added for PH correction. The water is distributed under gravity to area of low altitude and by pumps to higher altitude or distant locations.

The Lime Saturator

Lime produces a milky color when dissolved in water, which tends to give the impression that the water is impure/dirt even if it is not. Lime is added to water at the treated water tank, which is the final treatment stage of water purification. If the lime is just dissolved in water and used like that for treatment, it will make the treated water colorful. To prevent this, the lime needs to be dissolved in water to a micro level, so that its color is not visible when used for treatment. The lime saturator is equipment used to achieve this.

The Alum tank

There are two alum tanks of 52 m3 capacity each. The tanks are used to dissolve the solid Aluminum sulphate (Al₂(SO₂)), as this is the form in which the alum is dosed. About 80 bags of solid Aluminum sulphate is dissolved in water treatment per two days each weighing 50 kg. Only one of the tanks is used at a time.

The tank has gauze on which the Alum is packed. The tank is then flooded with water to dissolve the alum. An air blower, to cause agitation is applied, this makes the solid alum pieces to rub against each other, wear themselves out and thus enhance dissolution.

The Lime tank

The lime tank is a close tank with a hopper on top, through which the powered lime CaCO3 is fed. The tank is the flooded with water, to dissolve the CaCO3 and convert it to Ca(OH)₂, as this is the form in which is used in water treatment.

CaCO₃(s) + H₂O (l)                     Ca(OH)₂(aq) + CO₂(g)

CONSTRAINTS/CHALLENGES

Inadequate Funding

The Federal Capital Territory Water board was created specifically among other things to control, manage, install, and maintain all water works and service vested on it by the Minister of Federal Capital Territory. Therefore all funds generated from the sale of water go to the government purse. Running cost is given out monthly to carryout minor maintenance works. In cases where the replacement of broken or faulty parts, pumps and other plant facilities excess the monthly subventions, requests will need to go through procurement procedures which take more time, during this period the quality and quantities of water may be compromised.            

The summary of the entire water treatment process is shown below.