The DWMP blog – Episode 22. FFT, ffs

This is the latest in a series of blogs discussing the development of Drainage and Wastewater Management Plans (DWMPs).?If you haven’t already seen the earlier episodes in this series, they are all here (https://tinyurl.com/MartinOsborneArticles) I suggest that you start from Episode 1 (https://tinyurl.com/DWMP-blog).

This episode considers how the first cycle of DWMPs will respond to the current concern over spills of untreated sewage at wastewater treatment works (WwTW).

On 18 November 2021 OFWAT and the Environment Agency announced investigations into all water and wastewater companies in England and Wales.?This was after several water companies explained that they might not be treating as much sewage at their wastewater treatment works as they should be, and that this could be resulting in sewage discharges into the environment at times when this should not be happening.

The overflows at the entry to WwTWs are the final step in managing the excess flows in the system during rainfall.?There are typically two overflows. ?

• The first operates at high flow rates and spills very dilute but untreated sewage to the receiving water.?This is governed by the same rules as a combined sewer overflow in the sewerage network.
• The second operates at lower flow rates and spills dilute sewage into storm tanks that capture much of the spill for later treatment and also provide some treatment themselves through settlement of solid particles.?FFT stands for Flow to Full Treatment and is the minimum flow that must be passed forward to the treatment process when the second overflow is spilling.?This is generally written into the operating permit for the works.?

FFS stands for something completely different.

So, achieving FFT whenever flows are high is an important operating parameter for the WwTW. ?It controls how much flow goes into the storm tanks and therefore how often the storm tanks spill to the receiving water.

Another important operating parameter is how the storm tanks are drained back for treatment when the incoming flow reduces.?They should be drained back as quickly as possible so that there is space in them to store the next spill flow and also to stop the settled solids in the tanks fermenting and increasing the pollutant load in the stored sewage.?In an ideal case they would be emptied using a variable flow pump that exactly matched the gap between incoming flow and FFT so that they are emptied as quickly as possible.?In practice there will be operating rules that emptying should begin as soon as the incoming flow drops below a certain proportion of FFT and be emptied at a specified flow rate.?These should also be written into the operating permit.

So, the combination of FFT and draining practice governs how often the storm tanks spills, although this is, of course, heavily dependent on the pattern of incoming flow and therefore the pattern of rainfall.?Assessing whether the works is operating correctly by working back from the spill frequency of the storm tanks is a complex process as it would require modelling of long-term rainfall timeseries to determine the average and range of annual spill frequencies and then monitoring of several years’ spills to determine if they were within range.

However, almost all WwTW will have monitoring of the flow that goes to treatment and it is relatively easy to analyse this data to determine if FFT is being achieved.?See this really good article in the Guardian https://tinyurl.com/2p8fz49w, which is more balanced than some of their coverage of sewage discharge issues.

So, achieving the required FFT is a good first step to showing that the integrated system of sewerage and WwTW is operating correctly.

DWMPs are long term strategic plans for the integrated system of sewerage and WwTW.?They will therefore obviously check whether FFT is being achieved and what investment is required to ensure that it is and will continue to be.?Wrong.?Compliance with FFT is not one of the standard measures used in assessing performance and developing options.?It is one of the suggested optional extra measures in the risk-based screening process but not for the full risk assessment.?There is also a hint against adopting such extra measures with the requirement:

“However, companies are encouraged to share proposals for bespoke indicators, aiming for consistency/commonality of approach where possible, where similar indicators are proposed.”

I did suggest to one water company, early in the DWMP process, that this was a key measure and should be added to the standard list, but they were reluctant to step out of line with what all of the other water companies were doing.?

Different companies may have had different approaches to assessing required investment at the interface between the sewerage system and the WwTW.?Some may have assumed that the treatment works was meeting its required FFT and modelled performance based on that assumption.?Some may have assessed the actual FFT from flow measurements and modelled performance based on that real world situation.?I think that the former is more likely.

There is therefore a significant risk that the full cost of the investment required to achieve the required integrated performance will have been underestimated.?

So, is there one simple measure that we can add to the list of performance measures for DWMPs that will give a better assessment of the integrated performance of sewerage and WwTW??

Yes, it’s FFT ffs!