Understanding efficiencies of total solids retention

Screen purchasers must consider all data that each screen manufacturer supplies relating to total solids retention with a critical eye, says Huber Technology.

The current legislation driving the design of storm overflow chambers comes in the form of the AMP 2 guidelines, NRA (1993) which relates CSO chamber performance to the retention of a significant quantity of solids greater in size than 6mm in two dimensions.

Extensive testing of propriety screens designed with this in mind (for example, at the National CSO Testing facility at Wigan, UKWIR 1999) has shown that in most cases where perforated plate is employed as a screening component, the screens allow this requirement to be achieved. In most cases the screen will actually prevent all or virtually all solids greater than 6mm in two dimensions from passing to spill.

If this requirement is achieved, what then is the need to relate the effectiveness of each device to anything else?

Even where a screen stops all solids greater than 6mm in 2D, there appears to be a growing trend to examine the efficiency of each screen relative to the total solids entering the CSO. With the fact that the screen already achieves the requirements set upon the CSO designer by legislation, just what emphasis is there on the total solids efficiency of the chamber and screen? And what does that term actually mean?

Testing of this nature will invariably involve the capture of solids in the continuation flow to treatment, the spill flow to the receiving water and possibly, if applicable, the subsequent collection of any solids deposited on the chamber floor or on the screening device.

The efficiency of the screen and chamber will then be determined from finding the ratio of the solids passing to continuation to the total solids that entered the chamber during the test. (That is the total of the continuation load, the spill load and the solids deposited in the chamber and screen where applicable.)

The mesh sacks employed are usually formed by squared mesh with the sides of the squares being 6mm in length. This is assumed to equate to a 6mm perforation.

Aperture reduction
As the test progresses, paper and fibrous solids will wrap around the mesh and gradually reduce the aperture size such that the mesh soon begins to capture solids smaller than 6mm in two dimensions.

But there is a limit to this - silt and other small solids are more than likely going to find their way through the gaps and not be included in any weight comparison later. The term total solids, if used in the determination of how effective a screen and chamber are, and any data that is subsequently derived from such, is therefore somewhat ambiguous.

Significant emphasis has been placed on solids retention efficiencies by elements of the wastewater industry.

These include manufacturers who have achieved high values for their products through testing. And they include far-sighted industry experts, who are aware
that the guidelines in the AMP 4 period are likely to start focusing more on river quality in terms of oxygen demand, for instance, rather than aesthetic pollution,
which by that point should (theoretically) have been addressed successfully.

There can be no doubt that a reduction in the total solids passing to spill will be beneficial in terms of river quality. But given that the overflow must still succeed in acting as a relief valve for the sewer network, are storm overflow screens the point of the puzzle where this future problem should be tackled?

Arguably, a screen solution that reduces the impact on the river quality will result in the need for less storage so as to allow fine solids to settle out and be retained in the sewer network.

But a high efficiency value relating to a CSO screen will typically be associated with a relatively smaller aperture size on the screening component. It can also be due to a mat of solids building up on the screenings component. Both are likely to result in a reduced total area of opening such that less flow is able to spill through the screen.

If the hydraulic capacity of the overflow is therefore drastically reduced by the screen due to a reduced area of opening, then the chamber will need to be larger regardless. This is to allow more screen units to be installed. And it is to increase the hydraulic capacity back up to the design flow rate.

Any perceived hydraulic capacity reduction due to the use of a reduced effective aperture size (as built or through blinding as part of the process methodology) may be countered by locating the screen on the dry side of the weir.

Generally, however, when comparing two screens with a similar screening component area, the unit employing smaller apertures or that allows blinding to occur is likely to have the lower hydraulic capacity of the two.

Insight
Huber Technology now has more than 700 units of its RoK1 and 60-plus of its RoK2 installed in the UK. Its experience with urban overflows in particular has granted insight into the ramifications of pausing a cleaning mechanism in order to allow a mat of solids to build up.

At present, this is unnecessary as the RoK devices have been found at the Wigan test facility to stop all solids greater than 6mm in 2D from passing to spill. This therefore allows overflows to achieve regulatory standards as required.

Where running costs and maintenance costs have been investigated, though, control philosophies have been employed on some sites that permit the screening mechanism to pause intermittently during its run cycle.

These will, as a side effect, have increased the total solids retention of the chamber and screen. But, in some instances where first foul flushes have been exhibited, we have found that it can be crucial to initiate the cleaning mechanism constantly from the start of the storm event.

This is so the subsequent persistent induction of silt and concentrated solids on to the screen is removed as quickly as possible. And it is such that the overflow and screen can handle the solids load without becoming compromised.

There are overflows, particularly in urban areas, where it would not be prudent to employ a screening device that induced a relatively high total solids retention within the chamber (by either having relatively smaller apertures or by inducing a mat of screenings to build up so the same effect was achieved).

Given the number of overflows that must be tackled within the current AMP period, the number of locations where this is the case is likely to be significant.

Screen purchasers must therefore consider all data that each screen manufacturer supplies relating to total solids retention with a critical eye.

What will be the hydraulic impact of using such a machine particularly when put through its paces during high solids loadings?

The Rotamat RoK1 storm screen from Huber Technology, for example, when installed at 0ยบ, sits on the dry side of the weir and can even be lowered below the weir level if employed with its bulk-head design. Even in the rarest of high storms, the water level could theoretically be kept down to what it would be if the weir alone were in place.

Huber has focused solely on the regulatory requirements, and the purpose of a CSO. As such, it offers maximum hydraulic capacity while ensuring that no solids greater than 6mm in 2D pass to spill with their standard RoK1 and RoK2 screens.

But, in the event of needing to screen down to a smaller solids size in the future, a RoK unit fitted with a 4mm DIA perforated plate achieved an average total solids retention efficiency of 71.4%. When examined in the same format as that used in the 1999 UKWIR report Screen Efficiency - Proprietary Designs, this equates to a screenings retention value of 64%. This value is higher than that achieved by all the screens discussed in that report.

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