A System for Monitoring Extreme Sewer Flow Conditions

Excessive infiltration and inflow reduce the cost-efficiency and capacity of sewer systems to manage and treat wastewater.

Infiltration enters a sewer system through defective sewer pipe joints, broken pipes, or manhole defects or degradation. It also occurs when sewer lines are poorly designed and constructed.

Inflow normally occurs when rainfall enters the sewer system through direct connections, such as roof leaders, yard drains, catch basins, sump pumps, defective manhole covers, and frame seals, or indirect connections with storm sewers (MassDEP, 2017).

Infiltration and inflow (I/I) comprise 40.2% of wastewater inflow to the wastewater treatment plant (Figure 1). I/I can overload the sewer system. The real water flow and quality can be quite different from the parameters used for the capacity design of the treatment plant (Choi and Chung, 2019)

Figure 1: Relative Amount of Wastewater and I/I

I/I = infiltration and inflow, WWTP = wastewater treatment plant.
Source: Park et al., 2006.

The biological process of wastewater treatment is sensitive to the characteristics of wastewater, including the concentration of nutrient organics, pH, and temperature. I/I changes the characteristics of wastewater inflow into the treatment process (Figure 2), which decreases the efficiency of the process (and increases the cost and possibility of sanitary safety risk).

Figure 2a: I/I Ratio (SEW) and BOD₅ in Sewage Discharged

I/I = infiltration and inflow, SEW = extraneous water share, BOD₅ = biological oxygen demand.
Note: Kaczor et al. defines SEW as 'Q_di/Q_dw x 100' where Q_di is daily inflow penetrating into a sewer system [m³/day] and Q_dw is daily quantity of a mixture of actual sewage and an inflow delivered to a sewerage system during wet weather [m³/day].
Source: Kaczor et al., 2017.

Figure 2b: Defects in Sewer Pipe and I/I

Source: Park et al., 2006.

Another problem is sewer pipe overflow. When the overflow is combined with 1) heavy stormwater due to climate change and 2) increased impervious surface area in urbanized environments, this results in an urban flash flood. Optimizing the design and operation of the sewer system can reduce the risk of flash flood in a city.

A recent report from the World Resource Institute measures water-related flood risks around the world. It finds that by 2030, 15 million people and $177 billion in urban property will be impacted annually by coastal flooding, while 132 million people and $535 billion in urban property will be affected by riverine flooding (WRI, 2020).

The simulator used for the report, Aqueduct Floods, finds that every $1 spent on flood protection infrastructure in India results in $248 in avoided damages and reduces the likelihood of these areas being flooded by half. In Bangladesh, every $1 spent on flood protection infrastructure results in $123 in avoided damages and reduces the likelihood of floods to 4% from 20% (WRI, 2020).

The mitigation of I/I by sewer system rehabilitation and inflow source removal, combined with an ongoing operation and maintenance program, are essential in protecting the environment and the significant capital investment in sewers and wastewater treatment facilities (MassDEP, 2017).

Efficient and agile decision-making for sewer systems requires an accurate and resilient flow monitoring system.

The challenge starts from water flow measurement itself as the velocity of water in a sewer pipe is much less than 0.3 meter/second without rain. With heavy rain, the pipe is full of water with enormous pressure and velocity. Both these extreme conditions make accurate measurement of water flow in the sewer pipe difficult (Figure 3).

Figure 3: Fluctuating Sewer Flow Rates

Left: Without rain. Right: When raining.

JAIN Technology developed a flow monitoring system that considers the extremely harsh conditions in the sewer pipe. The system uses 4-path transducers to measure the flow velocity and a level transmitter to receive the level data. The dimension of the system is very flexible to cover the sewer pipe with the diameter between 150 and more than 1,200 millimeters (Figure 4).

The transducer measures the flow velocity of each channel based on the technology of transit-time cross correlation and ultrasound. Transit-time difference is proportional to the velocity of fluid when the fluid is homogeneous, like clean water, with a constant flow pattern. However, the flow of a fluid with inhomogeneity is hard to measure. The cross correlation method, which is commonly used in signal processing, enables a much more accurate transit-time difference estimation as it is little affected by gas bubbles and particles in the fluid.

Figure 4: The Sewer Flowmeter

By applying the transit-time cross correlation technology, the system can measure a wide variation in sewage flow rates over the course of a day—from the lowest flow velocity at midnight, which is almost zero (Figure 5), to the highest velocity of 10 m/sec with heavy stormwater (Figure 6). Even under harsh conditions, the percentage of error is around 2%.

Figure 5: Field Data at Midnight at a Pumping Station

m³/hr = cubic meter per hour, m/sec = meter per second, cm = centimeter.

Figure 6: Field Data from Normal Conditions to a Full Pipe with Heavy Rain

m³/hr = cubic meter per hour, m/sec = meter per second, mm = millimeter.

The Doppler-type flow meter, one of the popular technologies in the market, can hardly measure the water flow in the sewer pipe at midnight when the velocity is lower than 0.3 m/sec (Table 1).

Table 1: Comparison of Flow Metering Technologies in the Market

m/sec = meter per second.

The flowmeters in the market are installed inside the pipe either at the bottom or at the top. Both have limitations. The sensor installed at the bottom cannot function if the water level is lower than the sensor. The system with the sensor installed at the top cannot measure the velocity of the water if the sensor is submerged in water.  These sensors are also easily broken by debris with the extremely fast velocity and high pressure of wastewater and stormwater.

The JAIN Technology system is designed to avoid sediments and debris. It can detect sediment deposits, and it sounds off an alarm when the pipe needs to be cleaned. The system corrects the value of water flow, taking into account the sediments in the sewer pipe (Figure 7). This design reduces the need for repair and maintenance. One of the sites in the Republic of Korea has been operating the system for 3 years without any maintenance work, but some sites with a harsher environment may need to be checked and cleaned one or two times a year. In comparison, the other solutions in the market need frequent or even weekly maintenance to check if the flow path is blocked with debris or sediments, or if the sensor is broken and not working correctly, or if the flow path needs to be cleaned.

Figure 7: Sediment-Proof Design and Alarm Function

As the system is fully digitized, data is transferred to the cloud or the server via telecommunication network, such as LTE and/or Internet of Things (NBIoT, Lora, and CAT M1).

The system has been installed at 10 sites in seven cities in the Republic of Korea. Four out of the 10 sites are stormwater drainage systems, four are sewer systems, and two are for the effluent discharge from water treatment plants. The system will also be tested in 10 more sites in five big cities in the country, including five sites in Seoul this year. The installation will be completed by March, and the systems will be operating from April.

The proposed monitoring system is most recommended for sewer systems with extreme flow conditions (from no flow to raging torrent).

There are two types of sewer system. When wastewater and stormwater from houses and buildings are separated and flow through different sewer and drainage system, the system is called “separated sewer system.” When they use the same sewer and drainage line, it is called “combined sewer system.” The proposed monitoring system is much more useful for the combined sewer system although the system is also beneficial for the separate sewer system to measure the water flow in the stormwater drainage pipes.

G.B. Kaczor, K. Chmielowski, and P. Bugajski. 2017. Influence of Extraneous Waters on the Quality and Loads of Pollutants in Wastewater Discharged into the Treatment Plant. Journal of Water and Land Development. No. 33. pp. 73–78.

L. Choi and J.D. Chung. 2019. Analytical Evaluation of Influent Depending on the Occurrence of Rainfall by Case Study of Wastewater Treatment Facility. Journal of Korean Society of Disaster & Security. Vol. 12(3). pp. 33–49.

MassDEP. 2017. Guidelines for Performing Infiltration/Inflow Analyses and Sewer System Evaluation Surveys. Department of Environmental Protection, Commonwealth of Massachusetts.

M.G. Park et al. 2006. A Quantitative/Qualitative Study of Infiltration/Inflow for Order Decision of Sewer Pipe Maintenance. Journal of the Korean Society of Water and Wastewater. Vol. 20 (1). pp. 53–62.

World Resources Institute. 2020. New Data Shows Millions of People, Trillions in Property at Risk from Flooding—But Infrastructure Investments Now Can Significantly Lower Flood Risk. News release. 23 April.