Flow and Level Measurement at Wastewater Works: MCERTS, Primary Devices, Uncertainty and the Flow Settings They Protect

What MCERTS flow certification requires

The Environment Agency's Monitoring Certification Scheme (MCERTS) sets minimum requirements for self-monitoring of effluent flow where a permit requires it. Its essentials are:

• A total measurement uncertainty within ±8 percent of the actual flow across the permitted flow range, demonstrated for the installation as a whole, not just the instrument.
• An installation that follows the relevant standard for the primary device (ISO 1438 for thin-plate weirs, ISO 4359 for flumes, ISO 4064 and manufacturer practice for closed-pipe meters) so that the theoretical discharge relation applies.
• Inspection by an accredited MCERTS inspector, who checks the installation against the standard, the secondary (level) instrument and data handling, and issues a certificate; re-inspection every five years and after significant modification.
• Daily total flow records retained and reported, with data availability targets.

That 8 percent figure is a combined uncertainty. It is reached by adding, in quadrature, the uncertainty of the primary device (the flume or weir, from its standard), the level measurement, the zero datum, the flow computation and the data logging. A well-installed flume with a good ultrasonic level sensor sits at 4 to 6 percent; a weir with an uncertain crest datum or an approach channel that breaks the standard can exceed 8 percent at low flows regardless of how accurate the transmitter is.

Open-channel primary devices: flumes and weirs

Open-channel devices convert flow to a head measurement by forcing critical flow at a control section. Its two families behave differently.

Thin-plate weirs. A sharp-crested plate; the nappe must spring clear and be aerated. For a 90° V-notch:

A V-notch's 2.5 exponent gives it a very wide turndown, which is why it dominates small works, but it also amplifies level error: dQ/Q = 2.5 dh/h, so a 1 mm level error at 100 mm head is a 2.5 percent flow error, and at 40 mm head it is 6.25 percent. Weirs also pond flow upstream, collecting solids and biasing the datum, and the discharge relation assumes a still, deep approach pool that many retrofit chambers lack.

Flumes. A contraction in width (and often a raised throat) that forces critical depth within the structure. Rectangular long-throated flumes and the trapezoidal and Parshall forms share a head-discharge relation of the form Q = C b hⁿ with n of about 1.5 for rectangular throats, so level error propagates at 1.5 dh/h, better than a V-notch. Flumes pass solids, have a lower head loss and are the usual choice for final effluent at larger works. They require a straight, uniform approach channel (typically ten channel widths), free discharge (no drowning from downstream) and a precisely levelled throat. A flume that drowns at high flow, or whose downstream channel silts up, reports a flow it is not passing.

Level sensing: where most of the uncertainty lives

Secondary devices measure the head h at the point the standard specifies (three to four times the maximum head upstream of a weir; a defined distance upstream of a flume throat). Options and their failure modes:

• Ultrasonic: non-contact, the UK default. Errors from temperature gradients in the air column (sound speed varies 0.17 percent per degree), foam and surface ripple, and a transducer not mounted above the correct point. A temperature-compensated sensor in a stilling well or shaded position is standard.
• Radar: immune to temperature and air movement; preferred in open-air chambers and where foam is persistent.
• Pressure (submerged): robust against surface effects but fouls, drifts and needs density correction; usually the backup rather than the primary.
• Bubbler: the pressure method with the sensor kept clean by an air purge; still used where surface sensing is impossible.

The single largest error in practice is the zero datum: the relation between the sensor reading and the true head above the crest or throat invert. It should be set by direct measurement against the crest with the channel at a known level, recorded, and re-verified at every inspection. A datum error of 5 mm is a flow error of 12 percent at 100 mm head on a V-notch, and it is invisible in the telemetry.

Closed-pipe measurement: electromagnetic meters

Where flow can be measured in a full pipe, an electromagnetic flowmeter (EMF) is simpler and more accurate than any open-channel device: typically ±0.5 percent of reading over a 1:50 turndown with no moving parts and no head loss. Faraday's law gives an induced voltage proportional to the mean velocity across the diameter, so the meter needs:

• A full pipe at all times (a siphon or vertical rising leg guarantees it);
• Straight pipe upstream and downstream (five and three diameters are typical minimums; more after pumps and partly open valves);
• Fluid conductivity above about 5 µS/cm, which all wastewater exceeds;
• Correct earthing and a liner compatible with the effluent.

Pumped works, works with a final effluent rising main, and inlet flows after pumping are therefore best metered by EMF, with verification by a certified clamp-on or reference check rather than a wet calibration. Ultrasonic transit-time meters serve where the pipe cannot be cut, with greater sensitivity to installation.

The flow settings: DWF, flow to full treatment and Formula A

Flows that a works is permitted and designed to treat are defined from population and infiltration. In UK practice (originating in the 1970 Technical Committee report and carried into Environment Agency permitting):

Flow up to FFT receives full (secondary and any tertiary) treatment. Between FFT and Formula A, excess flow is diverted to storm tanks (sized to give around two hours of retention at Formula A minus FFT, which with standard assumptions gives the familiar 68 litres per head of population), and only flow above Formula A may discharge via the storm overflow after screening. The dry weather flow itself is defined in permits as the flow exceeded on 80 percent of days (Q₈₀) and is the value the works must not exceed in the permit's DWF condition.

Worked example. A works serves 20,000 people with G = 150 L/head/d, infiltration of 1.2 Ml/d and trade effluent of 0.3 Ml/d.

• PG = 20,000 × 150 = 3.0 Ml/d; DWF = 3.0 + 1.2 + 0.3 = 4.5 Ml/d
• FFT = 3 × 3.0 + 1.2 + 3 × 0.3 = 11.1 Ml/d (about 2.5 × DWF)
• Formula A = 4.5 + (1,360 × 20,000)/10⁶ + 2 × 0.3 = 4.5 + 27.2 + 0.6 = 32.3 Ml/d
• Storm tank volume at 68 L/head = 20,000 × 0.068 = 1,360 m³; at the maximum diversion rate of 32.3 − 11.1 = 21.2 Ml/d (883 m³/h) the tanks fill in 1,360 / 883 = 1.5 hours, which is why a storm lasting longer than that spills.

Every one of those settings is enforced through a flow measurement: the FFT setting is a control on the inlet flow meter and a penstock or pump; the storm overflow setting is a weir at Formula A; the permit DWF is assessed on the MCERTS final effluent meter. A 10 percent flow measurement error at FFT is a 1.1 Ml/d error in how much sewage gets treated on a wet day, which is why the regulators care about the 8 percent.

Designing an installation that will pass inspection

In effect, the inspection checklist is the design specification. Points most often failed:

1. Approach conditions: bends, drops or a change of section too close to the device; velocity distribution not uniform; surface waves at the head measurement point.
2. Drowning: downstream level rising into the control section at high flow, usually from a long outfall or a tidal receiving water; check the modular limit at peak flow and tide.
3. Device geometry out of tolerance: crest not level, throat not vertical or not the dimension on the drawing, V-notch angle wrong, sharp crest rounded by corrosion.
4. Datum: not recorded, not verifiable, or drifted after a transducer replacement.
5. Range: device sized for peak flow and measuring at a few millimetres of head for most of the year; a compound weir or a smaller flume with a bypass is the answer.
6. Data: flow computed with the wrong equation or exponent in the telemetry, no totaliser validation, gaps in the daily record.

Designing the hydraulics upstream of the device is usually the hardest part on a retrofit site, and it is where process and hydraulic design support earns its fee. A related calculation, weir flow from head, is available in our calculator, and the wider permit context is in the water quality and compliance hub.

How to specify an MCERTS-compliant effluent flow installation

1. Define the flow range. Minimum night flow to peak wet-weather flow at the measurement point; the device must hold its uncertainty across it, which may require a compound or two-stage device.
2. Choose closed pipe where possible. If the flow is pumped or can be routed through a full pipe, specify an electromagnetic meter with the required straight lengths; it is the lowest-uncertainty option.
3. Otherwise select the open-channel device. Flume for solids-bearing or large flows; V-notch weir for small, clean flows with wide turndown; design to the ISO standard dimensions and approach conditions.
4. Check the modular limit. Confirm free discharge at peak flow and worst downstream level, including tide-lock and outfall surcharge.
5. Specify the level instrument and datum procedure. Temperature-compensated ultrasonic or radar at the standard measurement point, with a documented, repeatable datum check.
6. Prepare the uncertainty budget. Combine primary device, level, datum, computation and logging uncertainties in quadrature and confirm the total is within 8 percent across the range before the inspector does.