Guidelines on Sewer Flow Monitoring

main sewer systemCollecting the data and measuring the flow of combined storm, sanitary and sewer discharge is important to determine the amount of incoming wastewater at various stages of the sewer collection system so that studies can be carried out for future development and town planning purposes. However, as there is presence of sediments, solid debris in the discharge coupled with high water turbulence along the variable pipe sizes and slopes, this has made the task difficult and detailed studies need to be carried out to select the right equipment and install it at the proper location. Most of the time this can only give an estimate figure although technology has advanced over the years to provide a better and more accurate flow monitoring.

There are several reasons on why having the data can prove to be important. First, it provides the information for operators to manage the incoming wastewater through the use of sewer control system (RTC) which is equipped with control gates in order to prevent over loading of the downstream treatment facilities. Also the information can help the team to schedule a plan for cleaning and maintenance works to be carried out based on the amount of wastewater that flows through the system. This is to avoid unnecessary breakdowns that can disrupt the operation. More over, with the data collected, it can also pinpoint and identify which section along the collection system that generates the highest amount of wastewater particularly how much that comes from those industrial areas. This relates back to the regulatory requirements on whether there is under declaration on the waste generations.

There are basically two common types of flow measuring techniques. One which is normally used is based on the Faraday’s law electromagnetic principle to estimate the average velocity and this is then multiplied by the cross-sectional area. Another type which is also commonly used is based on the relation between state and discharge for example the Manning’s equation and flumes. Manning’s equation is often used for discharge calculations in long uniformed channel whereby there is uninterrupted flow without sudden contractions or expansions taking place. Flumes on the other hand, rely on flow-constriction structures in order to make the flow to be directly related to head. Regardless of which instrument, selection on the ideal measuring device must take into account several considerations such as the maximum and minimum flow rate, the operating range, the accuracy level needed (higher particularly on RTC systems) and nevertheless the overall costs of purchase and maintenance. In all, the flow measuring technique should be applicable and match towards the flow and channel characteristics in order to get the best representation.

Choice of location is also another factor to consider. Usually in order to select the best site for installation that can provide the most accurate measurement, the location must has uniform flow occurring on a straight pipe and is not subjected to backwater effect which will affect the accuracy. Site that is selected should also represent the overall average flow and must not deviate towards the maximum or minimum level all the time. For example, a good engineering practice when it comes to installation of flow meter is to select a point whereby there must be at least 30 to 50 diameters length in straight pipe, measured upstream from the installation point. Similarly, whereas when it comes to downstream the measurement should be at least 5 diameters length of straight pipe so that overall this will have uninterrupted flow.

These days, new developments have seen the introduction of Doppler Area–Velocity Flow Meters (AVFM) and it has become the most commonly used device for flow measurement in sewers. There are basically two different types of these these units using either electromagnetic or ultrasonic beams, one which is a narrow beam and the other is the wide beam. A narrow beam Doppler AVFM will measure the velocity in a small volume of the flow and the mean flow velocity is then estimated based on a specified ideal velocity profile. This is usually taken at the center of the flow and normally a single velocity reading may not give a good representation especially in large sewers. A wide beam Doppler AVFM works differently as it hopes to address the limitation in a narrow beam system. It will instead measure the total velocity of the flow and then comes up with the average figure and provided that the angle of the beam is sufficient to get the total flow, else it won’t be able provide an accurate average number.

Accuracy of sewer flow measurement will become a problem especially in wastewater which is highly turbid and vicious which will have a marked effect on the velocity particularly as in the case of a wide beam Doppler AVFM. Thus, it is very important that there should be an active discussion to try to understand as much as possible about the device before getting one. Most important is to try to understand on how the whole mechanism works and check also whether field calibration is part of the package as this need to be periodically carried out by a trained and certified personnel.

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