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close this book Surface water treatment by roughing filters - A design, Construction and Operation manual (1996)
close this folder Annexes
View the document Simple methods for water quality analysis
View the document Simple methods for discharge measurements
View the document Salient data and features of slow sand filters
View the document Roughing filter theory
View the document Pilot plant design examples
View the document Roughing filter design examples
View the document Outline for caretaker training
View the document Monitoring of filter operation
View the document Acknowledgements and credits

Simple methods for discharge measurements

1. Introduction

Discharge measurements are necessary to control the flow through the treatment plant. The total flow has to be distributed evenly amongst the different filter units running in parallel. Unequal flow distribution will usually reduce the overall performance of the filters. Flow adjustments are required to cope with the weekly and seasonal demand fluctuations. Furthermore, flow adjustments are also necessary before and after cleaning and maintenance work.

Fixed installations or mobile equipment are used for discharge measurements. Since flow control plays an important part in treatment plant operation, the use of fixed installations is recommended.

2. Fixed Installations

Flow meters are relatively sophisticated and mechanically sensitive. Solid matter (sand, silt) carried by the water, can easily damage the device. It is therefore strongly recommended not to use such equipment in water treatment plants. Flow measurements at the outlet of a clear water tank might be the exception.

V-notch weirs are simple, strong and cheap installations, and, therefore, most suitable for flow control in water treatment plants. Weirs can be made from wooden boards or preferably steel or plastic plates. The weir's discharge is measured by recording the water height above the deepest point of the weir's crest.

A gauging rod, fixed at a distance of minimum 30 cm from the inlet weir and marked with different colours (e.g. green in the range of the design capacity, red for the zone above design capacity and yellow for the one below design capacity), will ease measurements. Compared to a 90° angle weir, V-notch weirs with a 60° angle will increase the accuracy of the readings. Slot-shaped holes in the weir's plate and in the gauging allow and accurate adjustment of the horizontal position. Fig. 2/1 gives more details on the possible dimensions of a weir's plate. The relation between water height and weir's discharge is listed in Table 2/1 and is shown as a graph in Fig. 2/2.

Table 2/1 Discharge over a 60° V-notch weir

Height of water hW (cm) above weir crest

I/s

flow rate l/min

m³/h

1

0.01

0.6

0.036

2

0.05

3.0

0.180

3

0.13

7.8

0.470

4

0.27

16

0.970

5

0.46

28

1.7

6

0.73

44

2.6

7

1.08

65

3.9

8

1.50

90

5.4

9

2.02

121

7.3

10

2.63

158

9.5


Fig. 2/1 Details of a 60° V-notch weir - View Plan


Fig. 2/1 Details of a 60° V-notch weir - Section View


Fig. 2/2 Calibration Curve

3. Mobile Devices

The simplest method to measure water flow is to record the filling time of a determined bucket volume. This procedure is inaccurate for high flow rates as filling time becomes very short and easy handling is hampered by the weight of the filled bucket.

Therefore, SANDEC has developed a more suitable flow control device which is illustrated in Fig. 2/3. The overfalling water flows into a bucket whose lower end is equipped with a calibrated nipple through which the water is discharged. An equilibrium between in and out flow will soon be established. The water height from the centre of the nipple is recorded and the discharge read from the graph as presented in Fig. 2/4. This method does not require a watch nor special material. A commonly used bucket or a small drum can be used as vessel. The nipple is assembled with standard pipe fittings and does not require great accuracy with respect to its length as shown by the graph. A separation wall with an opening of approx. 2 cm above the vessel's bottom creates a turbulence-free water level in the effluent's compartment. Finally, the distance from the centre of the nipple is marked on a half cm scale in the inner wall of the bucket. Flow rates between 6 and 30 I/min can be measured accurately with this simple device equipped with a 1/2" nipple. Larger nipple sizes can be used for higher flow rates and to reduce the water level difference required by the measurement.


Fig. 2/3 Simple Flow Control Device


Fig. 2/4 Calibration Curve for 1/2" nipple

4. Flow Control and Distributor Box

V-notch weirs are also installed in special structures used for flow distribution and possibly also for maximum flow limitation. An example of such a structure is illustrated in Fig. 2/5. This illustration shows a flow control box used in the raw water supply line and placed in front of the treatment plant. The flow which runs through the outlet pipe to the treatment plant is measured by the V-notch weir and gauging rod. A rectangular overflow weir in the inlet chamber limits the maximum flow through the treatment plant. The surplus water is discharged through the overflow pipe.

The controlled total flow through the treatment plant must be evenly distributed to the treatment units running parallel. This is achieved by a distributor box equipped with several V-notch weirs. Since such a box concentrates the flow control in one installation, it simplifies the hydraulic layout of a treatment plant and increases the operational flexibility. The inlet weirs of the subsequent treatment units can be omitted with such a layout.

Details of a Flow Control Inlet Box


Fig. 2/5 Details of a Flow Control Box