Keeping water systems clear of solids

Current legislation driving the design of storm overflow chambers comes in the form of the AMP2 guidelines, NRA (1993). It 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 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 is the need to relate the effectiveness of each device to anything else?

Even where a screen stops all solids greater than 6mm in two dimensions, 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 imposed on 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?

To begin with, testing of this nature will invariably involve the capture of solids in the continuation flow to treatment and the spill flow to the receiving water. It will also possibly involve, 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. And, where applicable, the total of the continuation load, the spill load, and the solids deposited in the chamber and screen.

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.

As the test progresses, paper and fibrous solids will wrap around the mesh and gradually reduce the aperture size, so 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 is used in the determination of how effective a screen and chamber are. Any data that is subsequently derived is therefore somewhat ambiguous.

Significant emphasis has been placed on solids retention efficiencies by elements of the wastewater industry. Primarily, these are manufacturers who have achieved relatively high values for their products through testing. And farsighted industry experts who are aware that the guidelines in the AMP4 period are likely to start focusing more on river quality. This will be in terms of oxygen demand rather than aesthetic pollution, which by that point should 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 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, or with allowing a mat of solids to build up on the screenings component so that the same effect is achieved. Both are likely to result in a reduced total area of opening, so 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 will be needed to allow more screen units to be installed, and to increase the hydraulic capacity backup to the 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 to some extent by locating the screen on the dry side of the weir. But, generally, 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.

Huber Technology now has more than 700 units of its RoK1 and 60 plus of its RoK2 installed in the UK. Its experience, it says, with urban overflows has granted it 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, because Huber says the RoK devices have been found, at the Wigan test facility, to stop all solids greater than 6mm in two dimensions from passing to spill. This allows overflows to achieve regulatory standards.

But where running costs and maintenance costs have been investigated, 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.

In some instances where first-foul flushes have been exhibited, Huber has found that it can be crucial to initiate the cleaning mechanism constantly from the start of the storm event.

This is so that the subsequent persistent induction of silt and concentrated solids onto the screen is removed as quickly as possible, so that the overflow and screen can handle the solids load without becoming compromised.

In short, there are overflows, particularly those located 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. This retention would be caused by either having relatively smaller apertures, or by inducing a mat of screenings to build up so that the same effect is achieved. Given the number of overflows that must be tackled within the AMP period, the number of locations where this is the case is likely to be significant.

Screen buyers must consider all data that each screen manufacturer gives them, relating to total solids retention, with a critical eye. What will be the hydraulic impact of using such a machine particularly be when put through its paces during high solids loadings?

Huber says the Rotamat RoK1 storm screen, for example, when installed at 0 degrees sits on the dry side of the weir and can be lowered below the weir level if employed.

Its bulk-head design means, the firms says, that even in the rarest of high storms the water level could theoretically be kept down to where it would be if the weir alone were in place.

Huber says: “We have focused solely on the regulatory requirements and the purpose of a CSO. We offer, with our RoK1 and RoK2 screens, maximum hydraulic capacity, while ensuring no solids greater than 6mm in two dimensions pass to spill.

“But in the event of there being a need to screen down to a smaller solids size in the future, we have a unit fitted with a 4mm perforated plate.

“When examined in the same format as that used in the UKWIR report Screen Efficiency – Proprietary Designs, this equates to a screenings retention value of 64%. This value is higher than that achieved by screens discussed in that report.”

www.huber.co.uk