La aplicación y estudio del régimen del DIH en Perú

reported that rotary screens gave better performance than stationary

screens for treating grease-laden waste flows. Allen reported

that water, steam, compressed air jets or sodium hydroxide have usually

(

)

been found to be effective for screen cleaning. Roebuck showed m

his tests that the optimum.. washwater pressure for cup screens was

2 bar.

' 2.4.4.3 Performance

The fracturing of large solids can occur on the screen medium (6 3 )

such that they eventually pass through the openings. Allen pointed

out that in order to prevent this happening screen designs such as the shovel vane, which provides a gentle uplifting of solids, were developed.- He concluded that with openings of 2.5mm, removals of at least 30%

suspended solids and 20% suspended organic matter should be achieved

from domestic sewage and held that the best fine screening could give removals of 30 to 50% as compared with 50 to 65% for sedimentation.

(23) . . .

Metcalf and Eddy gave efficiencies for a number of different types

of fine screen quoting figures of around 20% for removal of suspended

solids from screens with opening sizes of around 1mm. Whereas

(14) . •

experiments on fine screens quoted by Keefer gave efficiencies for

removal of suspended solids of between 2.7 and 7.5%.

'2.4.5 Special Screens

The Hydrasieve is a development of the sidehill screen in which sewage is delivered to a headbox above the screen from where it

flows over a weir and cascades down the face of the screen. It is intended to replace bar screens, primary sedimentation tanks and grit

(73)

removal devices. Whittenmyer reported BOD and suspended solids

removal of 20 to 35% at Dayton, Ohio, USA. In the Hydrasieve the screen plate is made up from horizontal wedge shaped stainless steel bars, and is set at three distinct angles of 25°, 35° and 45° to the vertical. Screened sewage flows through the screen whilst the solids travel down the face of the screen and are discharged at the bottom. A 1.5mm bar

opening is common for sewage applications. The Hydrasieve has no moving parts and is therefore relatively maintenance free apart from occasional washing down with a stream jet or high pressure hose to remove fat and

grease. The frequency of this cleaning depends on the nature of the

(

)

sewage but an average figure of once every 48 hours has been reported (72)

although Benkovitch considered that cleaning might be necessary

every few hours for some sewages.

The Rotostrainer, consists of a fine stainless steel cylindrical screen and headbox; sewage enters the headbox and passes through the

slowly rotating cylindrical screen. Solids are retained on the surface of the screen and are removed by a wiper mechanism as the screen

rotates. The continuous washing action of the falling water prevents grease and fibrous material blinding and clogging the screen surface.

(72)

Benkovitch reported that this self cleaning action proved to be so

effective that manual cleaning of the screen was only necessary every six to eight weeks.

Tests were carried out in 1975 on a raw sewage sea outfall to the Mediterranean^^. The screen used had a 0.75mm opening, and it was found that during the six week test period no manual cleaning of

the screen surface was necessary. The reductions achieved were: suspended solids. 40%; BOD 35%; COD 65%.

Sweco Europe and Sweco USA manufacture two types of fine screen suitable for dealing with sewage flows and storm flows: a

separator which is a form of vibrating sieve; and a concentrator which

( 761

is a form of centrifuge with backwashing facility. Sauer described

tests carried out in California using a 1.52m diameter concentrator which was found to effect reductions in suspended matter by 80% and settleable solids by 98%. From tests carried out at Portland, USA,

(

)

.

.

.

Marske concluded that high rate fine screening with the concentrator

is an economical method of treating combined storm overflows. The effectiveness of this screen is significantly reduced by the build-up of oil and grease on the surface of the screen and frequent backwashing is necessary when this occurs. M a r s k e d i d not recommend the use of the Separator at sewage works because of its lower hydraulic capacity and the low levels of removal achieved.

2.5 TREATMENT OF SCREENINGS

2.5.1 Maceration

Maceration is the combined action of pumping and disintegrating sewage screenings. The disintegration is by shearing and cutting actions in the pump. The screenings are then usually returned to the sewage flow. This method of screenings disposal dates back to the beginning of the twentieth century and is considered by some to be the most

(78)

hygenic and, usually the simplest means of disposal . One of the

first known installations was at a Moscow Pumping Station

In the UK, macerators have been developed from the 1 Stereophagus1

type of pump which was used to disintegrate gross solids by a cutting action before discharge to sea. The original patent for this type of pump was granted in 1910 but the first recorded use was at Bournemouth where a ’Gargantua’ disintegrator was installed at the sea outfall in

1 9 2 0 ^ ^ . Townend^"^ recorded that a ’Stereophagus1 pump was installed

at Broadstairs in 1928. These early macerators were for the whole sewage (79)

flow rather than just the screenings. In fact Homewood considered

that liquid/solids separation defeated one of the principal objects of maceration i.e. the disintegration of screenings and ultimate dispersal and degradation; whereas, L u p t o n ^ ^ considered that the maceration of separated screenings and return to flow would supercede

the practice of putting the whole flow through a macerator.

In the USA in 1926 a hammer mill type of macerator was tested Most screenings were ground to a fine pulp but some stringy material varying from 125 to 300mm in length remained. The power requirements

for this machine was 8,8 kWh/m of screenings at a throughput of 1.5

to 2 tonnes/h. A screenings macerator with cutting^teeth was tested