Conservation & Recycling, Vol. 3, pp. 337 - 342. Pergamon Press Ltd., 1980. Printed in Great Britain.

A COMPOSTING PLANT FOR THE ORGANIC FRACTION OF MUNICIPAL SOLID WASTE*

ANTONIO CIPRIANI Marini s.p.a., 48011 Alfonsine, Italy

INTRODUCTION

The conference of the United Nations, held in Stockholm in 1972 included an introduction of which we report the most significant passages:

‘Man is the creator and the creature of the environment he lives in, which supplies him with physical sub- sistence and offers him the possibility of intellectual, moral, social and spiritual development. In the long and laborious evolution of the human race on the earth, the moment has come when, thanks to the pro- gress of science and technology, growing faster and faster each day man has attained the power of transforming his environment in the most different ways and to an unprecedented extent. Two elements of his environment, the natural environment and the one he creates for himself, are both essential for his welfare and for the complete enjoyment of his fundamental rights, including the right to live . .

The protection and improvement of the environment is a most important matter which affects peoples’ welfare and the economic development of the whole world, conforming to the will of mankind and is a must for all governments . . .

Man must constantly take note of his past experiences and continue to discover, invent, create and im- prove. Today, this power that man has to transform the environment in which he lives, if used with in- telligence, can give him the benefits of development and the possibility of improving his standard of liv- ing. If practiced abusively and inconsiderately, this same power can cause an incalculable damage to human beings and the environment. Examples of damages, destructions and devastations caused by man, can be seen multiplying under our very own eyes each day in many parts of the world: we witness dangerous rates of pollution in water, air, land and living organisms; serious disturbances of the equilibrium of the biosphere; the destruction and the depletion of unreplaceable resources; lastly, serious deficiencies are dangerous to man’s physical, mental and social health in the environment he creates and especially in his working and living medium . .

Men of all levels and organizations of any kind can, by means of their function and their actions, deter- mine the environment of tomorrow . . .“[ l]

The situation becomes dramatic, the warning is clear and so is the invitation to consider, with more respect, the not unlimited resources of our planet. Nature in particular must be assisted in its continuous work of construction and renewal by putting back all that man can return to it. Thus, the balance of ecological systems is not disturbed; on the contrary, it is restored where it has been upset.

Let us turn our attention especially to the soil, its degradation and the positive action that ur- ban solid wastes can have on it, if suitably treated, and on the technology used for this treat- ment.

All agricultural land suffers from an ever-increasing impoverishment of organic matter, worsened by the massive use of chemical fertilizers which have been erroneously regarded as able to replace manuring, for centuries the only form of fertilizer used.

To look back to the past is unthinkable, because of the enormous extension of land used to- day for agriculture and also because of the absurdity of disqualifying what science has made available for us. The most reasonable suggestion is that of supplementing the techniques prac- ticed since the beginning of agriculture with the ones available today and, in particular, to reject *paper presented at the Second Recycling World Congress, Manila, Philippines, 20 - 22 March 1979.

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indiscriminate wastage. From waste heaps accumulated in our towns we must recuperate all that originates from the countryside in order to bring it back to it thus closing one cycle and opening another. That is to say, we must use compost instead of manure, to the advantage of agriculture and of man’s economy.

Compost is an industrial product deriving from the transformation and the stabilization, by bacteria in an aerobic environment, of the organic matter contained in urban wastes.

The reasons why the process of aerobic processing of wastes is preferred, are several: in the first place, no noxious gases are produced; furthermore, during the fermentation process, such temperatures are reached that the product is pasteurised, thus eliminating pathogenic germs, seeds of any kind, spores, larvae and polluting agents.

The biologically transformable wastes can be divided into the following categories: (a) the organic part of domestic and public wastes. (b) paper, cardboard and similar products coming from domestic, public and industrial

wastes. (c) sludges of organic origin coming from water purifying systems. (d) dead animals and rotten meat. (e) wastes from agriculture and cattle-breeding. (f) tissues of biodegradable organic origin. Once they have been transformed into compost and put into the soil these substances do not

harm the natural environment; on the contrary, the regular supply of organic matter to the soil contributes to maintain and increase its ‘humus’ content.

Humus is a bioenergetic dynamic mixture which, in a land with spontaneous vegetation, renews itself in an autonomous way, thanks to leaves, dead roots, and animal carcasses.

An agricultural land must, by contrast, be continuously restored in its humus content by sup- plying it with manure or modern substitutes. In the recent past we thought we could neglect humus without doing any harm, since its function was thought to be restricted to the mineralization of organic matter and to the solubilization of the minerals already contained in the ground. By giving the soil substances that were already saline (soluble chemical fertilizers), humus could have been omitted since it was no longer necessary. However, we realized later that the mineral elements, within the complicated mechanism which governs their assimilation by flora, always reach crops through humus, which is therefore an irreplaceable centre of pro- cessing. Land lacking humus and sprayed with chemical fertilizers can be shown to absorb only a small quantity, the rest is washed away by rain, with the consequent frequent phenomenon of eutrofication of surface and sea waters.

Compost, in the form in which it derives from wastes, contributes greatly to the humus supp- ly in the ground, though providing a scant amount of mineral salts. Moreover, it performs the very important action of improving the physical and biochemical conditions of the ground, by conditioning and making it more suitable for roots and the many living organisms that make the very ground a living organism and a creator of life.

The quantity of mineral salts contained in the compost can easily be increased and graded in order to transform it into as real and optimum ad hoc manure to be used for specific crops.

COMPOSTING - RECUPERATING - INCINERATING PLANT

The plant we will describe has been designed to process urban solid wastes with a low percent- age of industrial wastes, and to recycle the sludges coming from purification plants for sewage water.

In particular, preference has been given to the recovery of organic matter and its aerobic

A COMPOSTING PLANT FOR THE ORGANIC FRACTION OF MUNICIPAL SOLID WASTE 339

transformation into compost, keeping in mind that in the wastes of the Mediterranean area, Africa and Asia, vegetable refuse, paper included, exceed 50 per cent by weight, of refuse. The other materials must therefore be regarded as incidental to the main recovery of organic matter or as being justified by the low investment costs in equipment and by the high re-sale price of the materials recovered.

In its basic elements, the plant is described by the following scheme: (see Fig. 1 for diagrams and captions).

BLUDGE FEED PUMP

WATER BACK TO THE WATER TREATMENT PLANT

Fig. 1. (1) Refuse storage bin. (2) Grab bucket and bridge crane. (3) Cornposting line feed hopper. (4) Rotating drum. (5) Organic material conveyor. (6) Magnetic drum. (7) Air classifier. (8) Conveyor of inorganic refuse. (9) Mineral salt silo. (IO) Doser. (II) Sewage sludge thickener. (12) Thickened sludge silo. (13) Mixer. (14) Fermentation tower. (IS) Screen. (16) Refining unit. (17) Incineration furnace. (18) Slag extinguish tank. (19) Slag conveyor. (20) Magnetic separator. (21) Iron baling press. (22) Water conditioning tower. (23) Electrostatic precipitator. (24) Chimney stack.

The wastes discharged into the refuse storage bin (1) are taken by a grab bucket (2) suspended from a bridge crane. The grab bucket discharges the refuse into a composting line feed hopper (3) which, by means of a plate extractor with variable speed, feeds the rotating drum (4). A series of teeth placed on the top of the extractor rips the plastic bags and breaks the bigger refuse pieces. In the first section of the drum the organic wastes undergo a crushing process due to a smashing pressure exerted by the rotation of the heaviest and hardest parts. In this phase, water is added which helps the tearing of paper and fabrics and brings the percentage of humidity to the most suitable level for aerobic fermentation. The end section of the drum is a screen which draws from the mass the greater part of the organic substances which, at this stage, have already been reduced to small pieces. (Some plants reach the same objective by pro- cessing the raw wastes by means of the action of a hammer mill, which breaks up the whole mass with no selection, thus rendering the subsequent separation of the organic parts more dif- ficult. Moreover, such machines, besides involving sizeable capital investment, have very high operating costs.)

The materials that do not pass through the rotating drum screen are thrown back into the col- lection pit for the next incineration. The wastes that pass through the screen are sent, by means of an organic material conveyor (5), to an air classifier (7) which separates glass and pottery fragments from the organic matter, using the different physical mechanical characteristics of these materials. Before entering the air classifier, a magnetic drum (6) separates the small

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metallic parts that are subsequently put into the iron baling press (21). Glass and pottery fragments extracted from the air classifier are sent back to the waste collection pit, or treated separately.

The organic material purified in the air classifier can be enriched with mineral salts and mixed with the sludges coming from the purification plants for sewage water. The mineral salts, con- tained in the mineral salt silo (9) previously measured in the doser (lo), can be added to the organic material, in order to improve the final characteristics of the compost as fertilizer. The recuperated sludges are thickened in the sewage sludge thickener (11) or in centrifugal machines, in order to reduce their water content. They are subsequently stored in a thickened sludge silo (12) provided with an extractor-feeder with variable speed to check the dosage.

The use of recuperated sludges is useful to improve the nitrogen content of the organic matter and, at the same time, solves the serious problem of disposal.

Organic material, the mineral salts and the sludges are homogenized in a mixer (13) and the mixture is then passed to the fermentation tower (14). The fermentation tower is not a digestor but a closed chamber into which air is passed and where the first phase of fermentation starts. The material remains in the inside of the tower for a certain period of time so as to reach a high temperature and to achieve complete sterilization. Inside the tower the material is continuously moved and agitated so as to expose the entire surface of the particles to oxygen. The fermenta- tion tower is particularly recommendable when the plant treats recuperated sludges that are notoriously rich in pathogenic germs. A small percentage of the material coming out of the tower is taken back to the input to facilitate the immediate multiplication of the bacterial col- onies and therefore accelerate the fermentation process. At the tower outfall the material is passed through a screen (15) which selects the most suitable size that will facilitate aerobic fermentation. The material rejected by the screen passes across a refinery unit (16) that reduces its size.

The compost is now ready to be stored in the fermentation area where, in about 8 - 12 weeks, it will complete its aerobic fermentation. For this final phase, the possible technologies are dif- ferent and take into consideration the available area, the local climate and other collateral situa- tions. The easiest fermentation method consists of arranging the compost in rather small heaps or in rows and periodically turning the mass to favour maximum aerobic fermentation, thus avoiding the formation of gaseous areas and the production of noxious gases.

The materials rejected by the rotating cylinder and taken back into a separate section of the collection pit are then taken again by a grab bucket and discharged into the feeding hopper of the incinerating furnace (17). The choice of incinerating the materials that are not used for the compost is suggested by many factors:

(a) To obtain completely sterile residues that can be used as filling material. (b) The sizeable reduction in volume of about 90 070 achieves enormous reductions in

transport costs, and a reduction in the necessary space for waste pits. (c) The reduction in weight is about 70 Vo. (d) The possibility for large plants to recover the energy developed in the combustion. The furnace has been designed to obtain the complete combustion of the refuse until

scarification is achieved. The combustion is supported by the ingress of primary air under the grid. The air is drawn from the refuse pit with the aim of keeping it air tight, thus avoiding the emission of bad odours.

The primary air can also be fed preheated, thus favouring the predesiccation of refuse and improving its calorific value until a point is reached where combustion is autogenous.

The secondary air has the main aim of regulating the temperature in the combustion chamber, keeping it between 700°C and 1 000°C.

At lower temperatures the complete oxidation of fumes could not be guaranteed. At the

A COMPOSTING PLANT FOR THE ORGANIC FRACTION OF MUNICIPAL SOLID WASTE 341

higher temperatures, partial fusion of the ashes with blocking and damaging of the grid may OC-

cur. The grid, with mobile steps, assures good turning of the refuse and the exposure of all com- bustible material to heat, by means of its movements.

The slags are discharged into a slag extinguisher tank (18) and then passed, by means of a slag conveyor (19), to a collection point for the final disposal. A magnetic separator (20) collects all metal parts that are then transferred to the iron baling press (21) to be reduced into bales.

In furnaces of medium and small power, the heat produced by combustion as a general rule, is not recovered, only a small part being drawn off for the internal use of the plant. The out- coming fumes of the furnace have a content of suspended solids of 3 - 5g/Nm3, which is inac- ceptable by international regulations. It is therefore necessary to provide for their purification by means of suitable collecting systems.

Modern technology makes several systems available: multi-cyclones, multi-cyclones with wet dust collectors, bag filters, electrofilters, and combinations of the above systems. In this specific case, an electrofilter has been used that brings the dust content in fumes to levels lower than 150 mg/Nm3. Due to the high temperature of the gases coming out from the furnace (about 950°C) it is necessary to reduce the temperature to about 300°C before letting the gases into the dust collector. Also for this procedure several systems are available that suggest the mixing of air at ambient temperature, water atomisation, heat exchanges or, lastly, a combina- tion of them. The choice of the cooling method is a compromise among the following factors:

(a) an increase in the final volume of gases and consequent size of the dust collector and the suction fan.

(b) availability of water. (c) final humidity of fumes. Of course, where a boiler is installed, the withdrawal of heat by the latter often suffices to

bring the temperature of the fumes to the desired level. In this case, a water conditioning tower with dry bottom has been sugested. At the inlet to the tower the gases meet finely atomized water that, evaporating until a dry saturated vapour state is reached, withdraws heat from fumes, thus reducing their temperature. The tower is also used as a sedimentation chamber, and at its base a certain amount of dust is already collected.

The dust collected at the base of the electrostatic precipitator is conveyed to the slag collector together with the dust collected at the base of the conditioning tower. The fumes purified in this way are released to atmosphere without danger of pollution.

The plant, as is described above, can be provided with accessory units, which range from the neutralization of possible polluting agents to the fumes originating from the combustion of par- ticular substances, to the subsequent processing of the slags to enrich the compost with potassium salts. These are, in any case, solutions that must be examined one by one and that cannot act as the frame of a general treatise.

CONCLUSION

To conclude the plant can, by virtue of its modular characteristics, serve areas from 150 000 inhabitants or more, with the utmost operating economy.

The existence of neighbouring agricultural land, which makes use of compost, is not strictly essential since the compost is also used for covering waste pits, for growing plants on the slopes next to the roads, and to reclaim marshlands.

To manufacture compost from wastes not only brings about the above benefits, but also per- mits the incineration, by self-combustion, of what is left after the selection of the organic parts, while the combustion of the original wastes would have required the use of an auxiliary fuel.

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Therefore, to these benefits the economy of operation is added. We can be sure that a plant designed according to recycling criteria, with the recovery of

organic substances, metallic materials, and possibly heat with the production of a very small quantity of slag, represents, for the carrying out of a social service, a saving in both economic and ecological terms for the community, and a possible source of income for the municipalities.

REFERENCE

1. Declaration SW L’Environment - Conference des Nations Unies, Stockholm (June 1972).