A Look at the Basics
A wastewater collection system is the network of pipes, manholes, pump stations, and related structures that convey sewage from homes, businesses, and industry to a treatment plant.
A sanitary sewer carries only wastewater (sewage) and no stormwater. A storm sewer carries rain and snowmelt runoff, while a combined sewer conveys both sewage and stormwater in the same pipe.
A CSO is a discharge of a mixture of stormwater and untreated wastewater that occurs when the capacity of a combined sewer system is exceeded during wet weather.
An SSO is a discharge of raw or partially treated sewage from a separate sanitary sewer system before it reaches the treatment plant, often due to blockages, equipment failure, or overloading.
Stormwater runoff is rain or snowmelt that flows over land surfaces, streets, and roofs instead of soaking into the ground, often entering storm drains and sewers.
Level is the depth or height of water above a reference point (such as the invert of the pipe or the channel bottom), often used with a known relationship to calculate flow.
Data, software, and analytics
Flow monitoring data are time-series records of depth, velocity, flow rate, and sometimes rainfall, collected to analyze system performance, I&I, and capacity.
A scattergraph is a plot of measured depth versus velocity from a flow monitor that helps evaluate data quality, sensor performance, and hydraulic behavior such as surcharge or backwater.
Real-time monitoring is the continuous collection and transmission of sensor data with minimal delay, enabling near-instant visibility into sewer conditions and faster response.
Real-time control uses live data and automated controls (such as gates, pumps, or valves) to actively manage flows and storage in the sewer system to reduce overflows and optimize capacity.
In wastewater monitoring, DaaS refers to turnkey programs where a provider supplies equipment, installs and maintains it, collects and validates data, and delivers results and reports to the utility.
- The d/D ratio is the ratio of flow depth (d) in the pipe to the internal pipe diameter (D) for gravity sewers.
- It is a standard hydraulic performance indicator used to assess remaining capacity and identify potentially undersized or overloaded pipes.
• Many design standards limit dry-weather d/D to about 0.5 for small sewers (≤ 15 in) and 0.75 for larger sewers to ensure reserve capacity and ventilation.
• Peak or wet-weather design is often limited to d/D between about 0.7 and 0.83; for example some agencies require d/D ≤ 0.7–0.83 at peak flow to avoid chronic surcharge.
• In circular pipes, velocity at partial flow can exceed the full-flow velocity, with maximum velocity typically occurring around d/D ≈ 0.8.
• Discharge is not maximized at exactly full flow; for circular sections, maximum discharge usually occurs at about d/D ≈ 0.95.
• High d/D values indicate low residual capacity; agencies flag pipes with high d/D and low reserve capacity for further evaluation or upgrades.
• Sustained high d/D (near or above 1.0) implies surcharging, raising risks of basement backups, manhole overflows, and structural or infiltration/inflow problems.
• Brief, infrequent wet-weather events may be allowed to approach d/D ≈ 1.0, but many guidelines still recommend that even wet-weather d/D not routinely exceed 1.0.
• Existing systems with high d/D are commonly monitored, modeled, and prioritized for capacity relief projects rather than immediately replaced, especially in trunk sewers.
Flow, level, and hydraulic concepts
Flow rate is the volume of water or wastewater passing a reference point in a specific period of time, typically expressed in liters per second, gallons per minute, or cubic feet per second.
Open-channel flow is water movement in a conduit that is not completely full and has a free surface exposed to the atmosphere, such as most gravity sewers and ditches.
Closed-channel flow occurs when a pipe runs full under pressure, such as in a force main or pressurized water line, where flow is usually measured with in-line meters.
A diurnal pattern is the repeatable daily variation in sewer flow caused by human activity, such as morning and evening peaks and lower nighttime flows.
Monitoring equipment and sensors
A flow monitor is an instrument installed in a pipe or channel to measure flow rate and related parameters like depth and velocity over time.
A level monitor is a device that continuously measures water depth in a sewer, manhole, or tank, often used to provide early warning of blockages and overflows.
An ultrasonic level sensor emits sound pulses toward the water surface and measures the time it takes for the echo to return, converting this distance into water level.
A submerged pressure transducer is a sensor placed under water that converts the pressure exerted by the water column into a level reading.
A magnetic flow meter uses a magnetic field and electrodes to measure the voltage induced as conductive fluid flows through the meter, which is proportional to velocity and used to compute flow.
A weir or flume is a hydraulic structure installed in open-channel flow that creates a known relationship between water level and flow rate, allowing indirect flow measurement.
An ultrasonic level sensor measures only distance to the liquid surface, converting that to depth, while an ultrasonic flow sensor uses ultrasound to derive flow velocity (often with transit‑time or Doppler methods) and then computes volumetric flow. In open channels, a level sensor is often paired with a separate velocity or area‑velocity sensor, whereas ultrasonic flow meters for full pipes directly calculate flow from velocity and known pipe area.
An area‑velocity sensor measures both water depth and velocity at a point in an open channel or sewer, and multiplies the cross‑sectional area (from depth and geometry) by the velocity to calculate flow. These sensors typically use ultrasonic or electromagnetic methods for velocity and ultrasonic or pressure methods for depth, making them popular for portable sewer flow studies and permanent open‑channel sites.
A radar level sensor emits microwave (radio) pulses toward the liquid surface and measures the time of the reflected signal to determine distance and level. In harsh wastewater sites with heavy foam, vapors, or temperature swings, radar is often chosen instead of ultrasonic because it is less affected by these conditions.
Hydrostatic (submersible pressure) sensors are used where non contact sensors struggle, such as deep wet wells, narrow chambers, or locations with strong turbulence, steam, or obstructions. They sit in the liquid and convert water pressure into level, providing robust measurement but they need to be protected from debris and regularly inspected.
Mag meters generate a magnetic field and measure the voltage induced as conductive fluid flows through, while ultrasonic meters use sound waves and either transit‑time or Doppler principles. Mag meters are widely used on full, pressurized pipes with conductive liquids (e.g., force mains), whereas ultrasonic meters can be clamp‑on, inline, or open‑channel and are often chosen where non‑intrusive or retrofittable installations are needed.
Yes, some modern open channel sensors combine non contact radar for velocity with ultrasonic or other methods for level, providing area velocity measurements without contacting the wastewater. These systems are designed for challenging locations with high solids, shallow flows, or corrosive conditions where traditional in pipe AV sensors are hard to maintain.
Optimizing Conditions, Capacity, and Asset Health
Hydraulic capacity is the maximum flow that a sewer, manhole, or facility can convey without causing unacceptable surcharging, flooding, or structural stress.
A bottleneck is a location where limited pipe size, slope, or condition restricts flow, causing upstream levels or surcharging to increase during higher flows.
Surcharge occurs when the water level in a gravity sewer rises above the pipe crown, meaning the pipe is running full and pressurized, often during wet weather or downstream restrictions.
A hotspot is a location prone to problems such as blockages, high I&I, or frequent overflows, which often receives targeted monitoring or maintenance.
Asset criticality reflects how important an asset is to system performance, safety, and compliance, often driving which locations receive more monitoring, inspection, and rehabilitation.