Articles & Case Studies


Posted: Monday 15th December 2008

The installation of an innovative and very effective mixing system, as part of a refurbishment project at a Severn Trent Water sewage treatment facility, has resulted in a significant reduction of ‘rag wrapping’ problems which had been causing expensive and time-consuming maintenance. Huntingdon based solids mixing specialists, System Mix Ltd, (P&M Pumps mixing system design and supply company), were asked to design a system which would overcome the problems experienced at Strongford Sewage Treatment Works, near Stoke-on-Trent.

The original anoxic zone mixing design was suffering from rag wrapping on submersible mixers and their locating shafts, resulting in frequent outage. Due to restricted access and lifting equipment issues, these outages required the use of an 80 ton crane because of the reach involved, making it a costly exercise. As a result of this, System Mix Ltd was asked to design a mixing system for the activated sludge plant’s 12 Anoxic zones, each 22 metres long, 9 metres wide and 3 metres deep, which would combine the benefits of both “completely mixed” and “plug flow” systems. The key requirements were:

• Dead volume less than 10%

• Equivalent of minimum 2 “tanks-in-series” – higher retention time for biological processes

• Minimal short circuiting (t10 time > 10% of HRT)

• Mixing velocities to keep grit in suspension

To understand the benefits of both ‘completely mixed’ and ‘plug flow’ systems, the following applies. Two different extremes of open systems are (1) “completely mixed” or (2) “completely plug flow”. In a completely mixed system, any fluid entering the system is assumed to be completely mixed instantaneously throughout the contents of the zone.

As a result, some fluid particles could be immediately drawn out of the outlet of the zone, while others circulate for sometime before leaving.

If one looks at the characteristics of fluid leaving a zone, it comprises of particles that have spent varying amounts of time within the anoxic zone. In contrast, with a perfectly plug flow system, every fluid element entering a system spends exactly the same amount of time in the anoxic zone. By definition, ideal plug flow should consist of “no mixing” of fluid particles as they travel from entry to exit a zone. However, plug flow results in deposition which would be detrimental to the biological process, so the need to create a compromise between the two to provide sufficient mixing to prevent deposition is important, whilst creating adequate recirculation to achieve the desired retention. Every real-life system will fall somewhere in between these two extremes.

A common way to understand the behaviour of a given system is to apply a tracer of a given concentration at the inlet to a system and then measure concentrations of tracer at the outlet over a given time. By plotting these measurements, it is possible to obtain a “residence time distribution” (RTD) curve which helps describe the system. An RTD plot can then be used to characterize a system as being close to perfectly mixed, perfectly plug flow, or some more realistic system which falls in between the two. The shape of curve reveals information about how long particles that entered the system will be expected to remain in the system. Deviation from the “ideal” curve, whether the goal is completely mixed or perfect plug flow, can then give clues about short-circuiting, dead areas, etc. Although RTD plots can be useful for generalizing systems and understanding the extent of trouble zones, they cannot provide detailed information about the location and severity of these trouble areas.

By using CFD simulations System Mix developed a picture of what would be theoretically produced to achieve the goal of a ‘fully mixed plug flow’

The resulting CFD models provided a basis for the overall system design and allowed System Mix to press forward and offer Severn Trent Water a trial system on a full working scheme; on a sale or return basis.

From samples taken and visual inspection of Anoxic Zones it was clear that the flocculated sludge particles were unaffected by mixing and were in fact better that those of neighbouring traditionally mixed lanes. These results were enough to convince Severn Trent that System Mix’s design worked and provided the degree of nitrification they required.

Armed with initial tracer tests System Mix analysed these results and ascertained that the design had a slight shortfall in retention and deposition due to entry flow path and exit wall baffle. Modifications were made to these designs and a second lane commissioned under the trail and developments implemented. The results matched retention time and dead spotting design criteria.

The initial system is still in operation and is nitrifying well, a further 3 systems are now in place with the modification in place and these too are working well. The first system commissioned has now run uninterrupted for over 18 months with no failures and no signs of wear adding weight to the 5 year maintenance free operation System Mix predicted from the start. In addition as a result of working with System Mix’s design engineer the Anoxic zones are now being optimised to run intermittently saving energy costs during low flow periods.

Tel: 01487 830123

Read the magazine online

March 2021

About the magazine »
Magazine archive »


Information for advertisers »

Cranfield University Pulsar Button June 13 Huber British Water Harvey Communications Water Aid buttonwood marketing wateractive
Pulsar New Banner