Monitoring Flow to Full Treatment and Pass Forward Flow at wastewater treatment works

Date: 05/06/19 | In: wastewater management

As we enter June 2019, UK water companies may well be reminded of the press release issued last June by DEFRA. “5 billion investment by water companies to benefit the natural environment”. In this communication published by DEFRA, Michael Gove, Secretary of State for Environment, Food & Rural Affairs, made a bold statement by challenging water companies to increase investment and improve environmental outcomes by 2025.

The Environment Agency (EA) set out the ambitious measures in WINEP (Water Industry National Environmental Programme).

What does this mean to those responsible for managing WwTW?

There are around 9,000 wastewater treatment works (WwTW) across the UK. Each and every one of those WwTW has been planned, designed and constructed to accept, handle and treat a specific volume of wastewater.

All well and good. But what if that works was built in 1989 and intended to treat sewage from 500 households, trade effluent two factories and handle two significant storm events a year? Fast forward 30 years and it now has to treat sewage from 1,000 houses, industrial wastewater from four factories and handle multiple storm events thanks to climate change. The works is, more than likely, no longer fit for purpose.

If you’re tasked with managing that WwTW, how on earth do you ascertain this is actually the case. How can you prove that the works just isn’t capable anymore? How do you build a solid, unarguable solid proposal to present to your board that urgent investment is required to improve your WwTW? The answer? Implement Flow to Full Treatment (FFT) and Pass Forward Flow (PFF) monitoring.

 What is Flow to Full Treatment (FFT)?

Flow to Full Treatment (FFT) is the maximum rate of flow that a WwTW can accept for settlement and biological treatment.

All WwTWs have an inlet that allows flow into the works. This inlet uses a Formula A Weir to protect flow into the works. If there is too much pressure on the inlet, perhaps due to increased surface water following heavy rainfall, the flow can be diverted and stored in the plant’s storm water retention tanks. The water can be held there until it can be safely processed.

Explains Dave Walker, commercial director here at Detectronic: “The EA allows the use of storm water retention tanks for emergency purposes. They are not intended to be used prematurely or to as a ‘just in case we might exceed capacity’ measure. They are not a safety net for daily use!”

A key PR19 driver

States Dave: “One of the key PR19 drivers set by the Environment Agency is the measurement of compliance with pass forward flow (PFF) to WwTW. It demands that the WwTW provides MCERTS flow measurement and Event Duration Monitors (EDMs). The objective of this driver is the prevention of premature discharges from storm water retention tanks and thus reduce the number of discharges from storm water retention tanks into the receiving waters.”

What is Pass Forward Flow (PFF)?

Pass Forward Flow (PFF) is the instantaneous upstream flow that a Combined Sewer Overflow (CSO) or pumping station can accept.

When do I need to monitor FFT and PFF?

Outlines Dave: “As part of this PR19 driver, the EA has stipulated that any daily flow that exceeds 50m3 must be monitored by flow monitoring equipment. Any monitoring equipment installed must reach the MCERTS standard, however the EA does not specify which particular monitoring system you must implement, rather it refers to the overall ‘performance’ of the system.

“If your permit states that you must monitor the flow of liquid waste, your arrangements to monitor flow will need to be independently assessed against the MCERTS minimum requirements standard. This assessment will always be carried out by an MCERTS inspector. For example, where the fundamental requirement for the flow monitoring is to measure the total daily volume of treated effluent discharged to the receiving water, then the flow monitoring system must be capable of measuring the total daily volume with an uncertainty of no more than 8%.”

Continues Dave: “Looking at the PR19 driver in more detail, there are a couple of requirements – U_MON3, U_MON4 and U_MON5 – outline what you need to take into account to achieve compliance”


  • Record the operation of the WwTW overflow that limits the flow passed for full treatment.
  • Overflow may discharge to storm tanks, off-line storage, on-line storage or direct to the environment.
  • Record overflow operation/discharge start and end times and report along with flow data to demonstrate compliance with PFF


  • USE MCERTS flow monitoring to record overflow PFF within +/- 8% combined uncertainty.
  • Where practical existing flow monitors can be used.
  • Sign off from MCERTS inspector


  • Provide MCERTS flow monitoring for the first time at WwTW where permitted Dry Weather Flow (DWF) or maximum daily flow is greater than 50m3/d.

 What’s the best way to monitor FFT and PFF?

Since the Environment Agency does not identity any specific monitoring system, choosing the right system for your WwTW can seem a daunting task. It needn’t be.

The Detectronic MSFM MCERTS flow meter is ideal for monitoring both FFT and PFF to enable your works to achieve U_MON4 / U_MON5compliance AND create a Costing & Economics proposal for U_IMP5 & U_IMP6 improvement Scheme reporting requirements.

Highly versatile, the MFSM MCERTS doesn’t require mains power which means it can be placed almost anywhere within the network. With telemetry added, data can be transmitted to the Detectronic data centre where it will be expertly analysed. If flow rates breach pre-determined limits or there are anomalies in the data, our data technicians will notify your site engineers.

With battery powered portability, the MSFM MCERTS will also measure velocity, depth and temperature and it is suitable for deployment in high temperature environments.

To find out more or to organise a site visit to discuss your specific FFT and PFF requirements in relation to PR19, U_MON4, U_MON5 and U_IMP5 & U_IMP6 qualifying criteria, please call us on 01282 449124 or email: