Among the topics reviewed were questions such as the storage requirements for water in order to safeguard supplies in case of breakdowns in desalination plants, the use of storage as a means of lowering the cost of desalination, and possibilities of various types of storage facilities, including the technical and economic feasibility of storing desalinated water underground.

Organized by the Resources and Transport Division of the United Nations Secretariat, the meeting of experts was a followup to the 1965 United Nations International Seminar on the Economic Application of Water Desalination, held at UN Headquarters in 1965. The seminar recommended that the proper relationship between desalination and water storage capacity deserved careful attention and should be further studied.

The finding of the experts who completed the examination of this specific subject will be contained in their final report, to be published as a United Nations document early next year.

Water Shortage

Water is a precious commodity in many parts of the world. The amount of water that goes to make anything whether edible or not, is at first sight fantastic. An Australian Professor of Agriculture has calculated that to produce a kilo of meat, a hundred tonnes of water are required. For a kilo of rice or wheat, anything between five and seven tonnes. By the time a hen has laid her first egg, she has already consumed about a tonne.

The thirst of industry is seemingly unquenchable. It takes approximately 45,000 gallons of water to make one motor car. About 120,000 gallons are needed for a tonne of finished aluminium. A tonne of newsprint sucks up 240,000 gallons.

At present rates of water consumption, the amount of it available unless drastically supplemented, may be enough for only about a third of the world population of the year 2000 A. D.

Still, vast quantities of water go waste every year. A recent UNESCO study dis

closed that almost one million cubic miles of underground water, located in the upper half mile of the earth's surface, lies unutilised. This amount is 3,000 times more than all the water in the world's rivers.

In Europe and America much research is in progress about what is to be done when the inevitable shortage of fresh water arrives.

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The city must renew and modernise its physical structure and infrastructure to withstand the contemporary demands and must adequately be equipped to ensure adaptability to the atomic transition. Contemporary conditions require rationalisation and order of activities of man. Whereever desirable and possible, the city must readopt the traditional order to complement the multi-activated socio-economic matrix.

The dilemma of perpetual pregnancy of the city calls for proportionate and adequate expansion of socioeconomic potentials and certain basic services and utilities essential to cope with the demands in normal time and periods of emergency.

Must we wreck when we build ?

Outright adoption of contemporary technology, many times, demands the wreckage of the existing natural assets of the city. But the effort of the city planners must orient towards preser ving the natural assets of the city, as far as possible. This will add to the individuality, personality and philosophy of the city.

Contemporary technology has manifested a mechanical life for the city dweller.

Should a man become a robot and act on the radio signals of the technology? This requires the city planners to envisage urban life not only on the momentum of technology, but to conceive it on the ethical forces of human emotion and social attitude.

The constantly increasing populations of the world today are creating an urgent need for higher standards of hygiene and sanitation. As a result, more efficient treatment of sewage and disposal of sludge assumes an importance greater than ever before.

In some countries increasing shortage of available land, and lack of manpower for sewage work due to the attraction of more congenial occupations, have been additional incentives to the development of new processing techniques.

Most methods introduced are mechanical, in which there is no attempt to change the structure of the sludge; the moisture content of primary and digested sludge is reduced from about 95 per cent to 65 or 70 per cent. There are other mechanical processes, however, in which the physical structure of the sludge is altered. One being widely adopted in Britain and other countries employs the Porteous (1) of heating sludge by steam injection.

Heat Treatment Of Sludge

Long experiments and trials were involved in the evolution of the present Porteous sludge heat treatment process. They clearly demonstrated the permanent effect of heat treatment on sludge. Advantages included a great saving in the land required as compared with conventional drying systems; reduction of labour requirements to a minimum; complete sterilisation of the product without the use of chemicals; possibility of continuing treatment throughout the year, in any weather; and practicability of continuous processing.

To achieve satisfactory conditioning, it is necessary to raise the temperature of the sludge to above 150°C, and to maintain it at this temperature for a predetermined period. The temperature and time will vary with the nature of the sludge.

The Porteous Process

It can be assumed that raw sludge has been delivered from the sedimentation tanks

to a storage chamber. From there it is pumped at high pressure into a heat exchanger, in which its temperature is raised as far as is economically possible before eatering the reaction vessel.

Steam is injected into the reaction vessel at the same time as pre heated sludge is admitted, a specially designed steam jet circulator being used to ensure complete mixing of steam and sludge. The injection of steam not only raises the temperature but avoids physical damage to the sludge; furthermore, all the steam's latent heat is utilized, making the process very economical.

On leaving the vessel, treated sludge passes back through the heat exchanger, giving up most of its heat to the raw sludge. After leaving the exchanger the sludge, now cooled to about 30°C, passes into one or more decanting vessels.

The decanter is, in effect, a sedimentation tank in which solid material in the treated sludge settles to the bottom. The supernatent liquid is drawn off by means of weirs, and may be re-introduced into the sewage flow.

The thickened sludge will now have been reduced to about one third of one half of its original volume, its density having increased correspondingly. This sludge is drawn by a pump into a filter press where firm, hard cakes may be produced in as little as two hours, the actual time depending on the type of sludge being processed and the type and condition of the filter press

Disposal Of Sludge Cakes

35

cloths.

The cakes frequently contain as little as per cent of moisture. They are not only hard, firm and easily handled but completely sterile, as all seeds of weeds and pathogenic germs have been destroyed. Once dried, the cakes will not re-absorb moistrue; they can therefore be safely used for filling in lowlying ground or, after disintegrating, as fertilizer.

Although filter presses are normally used, excellent results may be obtained from vacuum filters.

A constant aim in development has been to evolve a mechanical process which will not only be continuous in operation, but which will be automatic and so require little, if any, labour, Oil-fired boilers are normally recommended for steam generation as they can be completely automated, but coal-or gasfired boilers can be used in some circumstances.

Boilers can be fired by sludge gas from the digesters or on the dried sludge press cake produced by the plant, thus reducing operating costs. Only raw primary sludge is normally suitable for this purpose-digested or biological sludges do not show sufficient calorific value.

The advantages of this patented Porteous process have been adequately proved by its adoption for large and small plants, for populations from 10,000 to 800,000.

Sludge Lifting By The Sludgemaster

Another method of sludge disposal (2) has been evolved for situation where adequate land area is available. This is a technique of lifting sludge from drying beds by mechanical means.

It has been recognized for many years that where sufficient land is available one of the most economical ways of drying sludge is to pump it on to drying beds. The beds. are normally rectangular areas of sand or gravel, surrounded by low walls of brick or concrete, so that free water can drain downwards while the top surface is exposed to the weather.

Until recent years, the dry sludge cake was usually lifted off the sand bed by labour. ers with large, multipronged forks, and loaded into vehicles. Today it is difficult to recruit labour for sludge lifting. Further more, the ever-increasing value of land has precluded the acquisition of more for drying beds, in spite of the growing need due to increase in population.

The only solution to the problem has been to utilize the drying bed areas with

greater efficiency, which means by mechnical methods. This has been attained very effectively with the Sludgemaster.

Advantage of this equipment is that more sludge can be dried in a given time over a given area. This is achieved by not only reducing the time for drying but also the period during which each bed is out of use while one layer of dried sludge is being lifted and replaced by another layer of wet sludge. Economies have been attained by applying thinner layers of wet sludge, which dry more quickly and can be lifted by mechanical

means.

Lifting The Sludge

To lift sludge from the surface of the bed an inclined slat elevator is used. This is supported from a travelling bridge spanning the drying bed, each end of the bridge being mounted on a carriage with a rack and pinton drive-to eliminate wheel slip-on the walls at each side of the bed.

Transportation of sludge from the lifter to a stockpile may involve traversing several-perhaps six or seven-beds, each possibly 60 feet (18,29 m) wide. For this purpose a transverse travelling belt conveyor is used, which is built into a series of light alloy bridges or gantries supported on wheeled carriages on the wall of each bed, each carriage having a rack and pinion drive. To drive all the pinions at the same speed and yet eliminate an excessively long shaft, electrically interconnected "looked rotor" motors are used at intervals along the gantriThe gantries carrying the conveyor therefore move very slowly the length of the beds, which may be nearly 2,000 feet (609,6 m) long. Tney move in perfect line with each other, the only moving parts visible being the carrying wheels.

es.

Laying Wet Sludge And Coultering Sand

The system is highly successful in its primary purpose of enabling quickly dried sludge to be lifted in thin layers, but the system has been made even more efficient by further developments.

By attaching pipes to the gantries from which wet sludge is to be poured, a constantly homogeneous layer can be ensured that (Continued on page 29).