Flow measurement in the sewer network: Less construction work with advanced metering

Understanding what is happening in the sewer system is crucial for the efficient operation of sewer facilities. The volume of wastewater flowing through the sewer network directly affects environmental impact, as well as the capacity of wastewater treatment plants and their energy requirements. Flow data can be used to identify necessary structural measures and detect damage or malfunctions in the sewer network in good time.

Level and flow measurements in wastewater networks provide valuable insights into their condition and functionality. For instance, level measurements taken via radar can be used to monitor heavy rainfall events, extraneous water inflows, and other significant occurrences. This information may be sufficient for certain applications and existing structures.

The change in flow measurement

Traditionally, accurately measuring wastewater flow required complex construction work to obtain reliable data. However, recent advances in sensor and transmission technology are opening up more and more areas of application where simple level measurements are insufficient or extensive construction work would be required.

Until now, many sewer system operators have relied on structural measures to calculate flow rates. The construction of Venturi flumes or weir edges has been used to influence the flow of wastewater specifically, enabling flow rates to be determined based on level measurements and mathematical calculations.

Modern measurement systems: Less complexity, more precision

In order to minimise the need for structural measures, more and more sensor manufacturers are offering advanced measuring systems that directly and accurately determine the flow rate. These systems measure the flow area (A) in square metres (m²) and the flow velocity (V) in metres per second (m/s) in order to calculate the flow rate in cubic metres per second (m³/s) or litres per second (l/s).

Magnetic inductive flowmeters (MID)

Magnetic inductive flow sensors are considered to be one of the most accurate and reliable methods of measuring flow. They measure flow based on changes in the magnetic field of the flowing liquid. However, this method requires considerable structural and energy expenditure, and it only works when the sensor is completely filled with fluid. MID systems offer high measurement accuracy and are primarily used in areas where precision is paramount.

Measurement methods with low structural and energy requirements

Measurement methods that require less construction and energy fall into two categories: medium-contact and non-contact. Lower accuracy for these flow measurements is accepted in order to significantly reduce installation and operating costs. Consequently, the same or a lower budget can be used to monitor not just one point in the sewer network, but a large number of measuring points with minimal construction effort. Battery-powered devices are a significant factor in achieving these cost savings.

Measurement in the medium

In this method, ultrasonic sensors are installed directly in the wastewater stream to measure the average flow velocity (v). The flow area (A) is then determined by measuring the water level using a pressure probe and taking the channel’s geometry into account. However, this technique has its limitations, particularly at low water levels when flow velocity cannot be reliably measured, or when the sensors are heavily contaminated, which affects accuracy.

Non-contact measurement

Non-contact measurement uses radar or laser systems to measure the level and surface velocity of wastewater flows. The level is used alongside the sewer geometry to calculate the flow area (A). Surface velocity provides information about flow velocity, which is converted into average flow velocity using manufacturer-specific algorithms. This is then used to determine the flow rate. This method is advantageous because it avoids direct contact with the wastewater, making it less susceptible to contamination and reducing maintenance costs.