kg/cm An average weight reduction of 67% was achieved together with an average volume reduction of 80% A full-scale press has since

been constructed to handle the screenings from about 150 000 people and results similar to those from the pilot press have been obtained. Included in the system is a bag filler to provide for the automatic packing of pressed briquettes.

The Northumbrian Water Authority (NWA) has recently

investigated six screenings presses. The Harleyford Hydrosand Screenings Press, consists of a horizontal flat conveyor belt supported on a

variety of drum centres, above which is another separate driven smooth pressure belt. The pressing action is achieved by forcing the screenings

through the constriction formed by the two belts. The press has a '

capacity of 1 tonne/h and a belt speed of 6 m/min. The weight and

volume reductions achieved during the investigation were 50% and 30% respectively. It was considered to be rather messy in operation and possibly to have problems with large solids; however, the manufacturers were said to be rectifying these faults. The NWA also tested a Sludge Dewatering Limited press which was developed from a refuse compactor. In this system a hydraulically operated ram compressed the screenings in a chamber.. Its capacity is 2.7 m /h and it reduced the moisture content to 45%. A Danish Bias press, also considered, produced'bales of screenings on a similar principle. The moisture content was reduced to between 60% and 70%, and the volume reduction was 50%. This press has a smaller capacity of 0.2 - 0.8 m /h. The fourth device to be investigated was a German Geiger press. Initial pressing is by a drum at the entry to the press from a feeding chute, and final pressing

achieved through a mangle action between two drums. The moisture content of screenings was reduced to 60%; the two standard models can handle 5.0 and 2.5 m /h. It was found that this press successfully crushed

a brick and passed a four metre length of timber without difficulty. Another German press manufactured by Passavent was also tested. This employed the principle of a low pressure hydraulic chamber followed by a high pressure chamber, after which the screenings were forced into a final dewatering chamber for repeated pressings. The moisture content of the screenings was reduced to between 50 and 60%. Systems

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with a handling capacity of 0.7m /h to 6.6m /h are available. The

last press investigated was that of Hawker Siddeley Water Engineering. This consists of a hydraulic ram which pushes screenings into a

perforated pressing area. The ram pressure was 105 kg/cm and the weight and volume reductions attained were 40 to 60% and 30 to 40% respectively. The NWA noted its easy maintenance. All the presses surveyed by the NWA may be used with a continuous feed system.

2.5.4 Washing and Centrifuging

Little has been written regarding screenings washers Cogg a n ^ ^ ^ noted the installation of one at Rodboume STW in 1953

and in the same year it was reported that one was installed at

Budds Farm STW. The washer was described as a machine which macerated organic matter under high pressure water sprays, the organic matter and washwater being returned to the flow and the debric, etc., being

discharged from the machine in a clean state.

Centrifuges have been used for the dewatering of screenings. S u t h e r l a n d d i s c u s s e d the use of centrifuges in sewage treatment and considered that a suitable application would be the dewatering of

screenings. Centrifugal screenings dewatering plants have been installed at Jamaica, New York, Minneapolis and M i l w a u k e e H O ) . F^ood^^ also described tests with various types of centrifuge which again gave

(C\

average reductions in moisture content to about 65%. Allen reported

that when 1250 kg of fine screenings which had been allowed to drain briefly and had a moisture content of 85%, were placed in bags and dewatered in.a Laundryette centrifuge the moisture content was reduced to 65% and the screenings could be incinerated by the addition of 25%

(14) .

by weight of coal. Keefer reporting on the installation at

Milwaukee stated that with a load of about 320 kg it took from 10 - 25

minutes to reduce the moisture content 88 to 61% for coarse screenings

and from 92 to 78% for fine screenings.

2.6 LANDFILL

2.6.1 Disposal of Screenings

Since screenings have a high moisture content, Bloodgood^0^. considered that they should be drained in perforated cans before burial. Problems with this method of disposal may occur in the handling of

screenings due to their objectionable nature, and from birds and (27)

rodents being attracted to the disposal area . To prevent the

nuisance of flies and rodents before the screenings are coveres, sprinkling with calcium chloride or lime has been s u g g e s t e d .

F l o o d ^ suggests that screenings shall not be buried too deeply, as they will decompose slowly if removed from the upper layers of earth in which bacterial activity is greatest. Instances of very slow decay of the buried screenings have been found and R a w n ^ ^ ^ reported that four years after burial, screenings showed little

deterioration. Similarly, A p p l e t o n f o u n d that in 2.4m deep pits

covered with 0.6m of sand, there was little evidence of decay after

27 months.

(13)

The IWPC sugghst in their recent publication that this

method of disposal is often unsatisfactory on larger works, although bagging of the screenings before burial reduces the likelyhood.bf nuisance. Furthermore, with the bags used being .usually synthetic and with the unscreened amounts of plastics and synthetic fibres in

screenings, they consider that this method is less desirable than it used to be.

The alternative to burial on site, is burial with refuse on municipal refuse tips, mentioned by various authors(73,111,114)^

2.6.2 Composting

Treatment of screenings to produce a compost or similar has been widely attempted with varying results. Fine screenings have been composted at New Jersey, USA, in open c e l l s T h e s e cells were cleaned out annually and were not particulary offensive. Composting at Low Angeles has been carried out in open pits 3.8mm square by 3m de e p ^ " ^ . Screenings and skimmings were treated at a rate of 0.57 -

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