(44) sludge screening or maceration prior to sludge pressing

2.4.2 Hand Raked Screens

Hand raked screens were amongst the first methods of removing solids from sewage, initially used mainly to protect pumps, their

development from the early gratings and sieves has been detailed by

S t a n b r i d g e a n d Dunbar and Calvert The earliest types of

screen were perforated wooden plates, first reported in 1853, with the first mention of an iron grating being in 1864. These early screens were placed at various angles in the flow, even horizontal. They were

also placed on hinges, like doors, in the sewers and perforated baskets:. were also used. These screens were either cleaned within the flow with

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rakes or as in the case of cage screens ’ were removed from the

Mechanical screenings plant has mostly superceded hand raked screens, and now apart from very coarse screens (100mm to 150mm openings) placed upstream of mechanical screens to prevent damage from large

o b j e c t s h a n d raked screens are only recommended for use on

very small treatment works, small pumping stations and in by-pass channels on larger treatment works. Though the tendency from them to be superceded by mechanical plant even on very small treatment works

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has been noted . It has also been suggested that at very small

works screens are more of a nuisance than a help and instead scum boards should be used purely to arrest floaters. In a recent

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publication the I.W.P.C. consider that the use of hand raked

screens should be restricted to works treating less than lOOOnrVday whereas in the early fifties C a s t e r w a s recommending their use on

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works treating flows from populations up to 10 000. Lang recommended

that if the incoming sewer is small (150mm to 225mm diameter) then the hand raked screen which would normally be installed for such a small works should be omitted. He suggested that in order to provide

sufficient area of screen for narrow channels a 1 two-slope1 bar could

be used. This has a bottom section of low inclination and a top section of steep inclination.

Opinions differ regarding both bar spacing and slope. On the question of spacing recommendations vary from 15mm to 50mm with a concensus of opinion favouring 25mm. S c o t t s u g g e s t e d that the spacing should match that of the common garden rake as this, regardless of the rake originally supplied, was the impliment the operator ended

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up with. With regard to slope the I.W.P.C. recommended an angle

of 60° to the horizontal whilst in the U.S.A. the W . P . C . F . ^ ^ stated that the most common angle of existing hand raked screens was 60° though the present tendancy was to use a slope of 30° with a maximum of 45° usually accepted. It is pointed outf^"**"*^ that a flatter slope makes cleaning easier, and further that the action of the flow will push the screenings up the bars rather than cause a blockage. Round

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bars are being used though it is recommended that screen bars

should be designed with the leading edge slightly wider than the trailing edge to minimise the jamming of the large solids between the bars.

A drainage platform should be provided for fresh screenings

preferably extending over the edge of the screening chamber to

facilitate loading of skips.

Problems of deposition of grit within the screenings chamber (13)

are mentioned by I.W.P.C. who recommend that the velocity through

the bars should not drop below 0.3m/sec to prevent this deposition, at least 0.45m/sec should be reached under normal flow conditions and the maximum velocity should not exceed 0.9m/sec or screenings which have already been intercepted may be dislodged. Other design figures

suggested^^,52) approach velocity of 0.3m/sec to 0.6m/sec or

a velocity through the chamber of 0.4m/sec to 0.8m/sec. Various enpirical design figures have been suggested for very small works; a

2 (53)

maximum area irrespective of flow of 0.7m , area of twice the

area of the incoming s e w e r 0 . 1 4 m ^ of submerged area per 1000

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population and 0.58m /1000m /d. These areas assume frequent

cleaning of the screen and in the latter case a maximum of 3 times during the day is suggested with the screen left clean at night.

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Merz states that it is particularly important that a definite

routine is established for the normal cleaning of screens because it is not a very desirable job and if neglected can effect the operation of the treatment works. To reduce the amount of soft organic solids included in the screenings, the solids accumulated on the screen may be rolled with a light roller prior to raking so that the soft organic

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