CA2235963C - Process for the biological-thermal treatment of waste - Google Patents

Abstract

A process is used for the biological-thermal treatment of organic constituents-containing wastes in a tank. In said process the exhaust air drawn off from the tank is again fed back to the tank. In order to improve said process, the exhaust air is heated before it re-enters the tank (Fig. 2).

Description

= - :~, -PROCESS FOR THE BIOLOGICAL-THERIMAL
TREATMENT OF WASTE

The invention relates to a process for the biological-thermal treatment ) of organic constituents-containing wastes in a tank, wherein the exhaust air drawn off from the tank is again fed back to the tank. In other words, a circulating air system is used, wherein all or part of the exhaust air leaving the tank is again fed back to the tank. The process can be used for the biological-thermal composting or stabilising of organic constituents-containing wastes.

The tank is a closed tank with forced ventilation. Normally, in the bottom part of the tank an essentially horizontally extending perforated bottom is provided, on which the waste rests as rotting material or rotting mixture in the form of a pile or heap. By means of an air supply system, which normally consists of several fans, a gas mixture consisting mainly of air or re-circulated air is supplied to the area underneath the perforated bottom. The gas mixture flows upwards through the holes or other openings of the perforated bottom and from there into the rotting = - 2 -mixture. It flows through the rotting mixture substantially from the bottom upwards and is then drawn off from the upper part of the tank as exhaust air. The exhaust air can be treated before all or part of it is again fed back to the rotting material. It_is possible, for example, to ~? .
separate water from the circulating air, e.g. by cooling th e circulating air and separating the water that occurs as condensate.

When composting bio-wastes, i.e. wastes that contain organic constituents, for the purpose of making compost or stabilising material, the rotting mixture is 'ventilated more or less for so long until the biologically easily decomposable organic substances contained therein are decomposed. It is decomposed mainly by bacteria which under aerobic conditions produce the greatest possible metabolic effect. With the known processes for the composting by an intensive rotting process (i.e. a process in a closed tank with forced ventilation), to speed up the composting the supply of oxygen is optimised. With this, from solid, amorphous or liquid substances the gaseous substances C02, water vapour and ammonia are produced. These metabolic products together with the air flowing through the rotting mixture are discharged as (..

exhaust air, wherein the temperature and oxygen content in the rotting mixture are controlled by way of the air (i.e. the condition of the inlet air and of the exhaust air) in such a way that microbial life can unfold unhindered and can decompose the existing potential of easily -decomposable organic substance in the shortest -possible time.

Organic substances that are difficult to decompose are in the first instance decomposed mainly by fungi which - the same as most bacteria take the oxygen required for the metabolism from the water of the substrate on which they grow (see Schlegel, General Microbiology, 5th edition, page 169). During the decomposition of this potential new bio-~ mass occurs in the form of a microbial cellular substance which in turn consists of easily decomposable hydrocarbon compounds. The newly formed cellular substance is under aerobic conditions approximately 20 times larger than under anaerobic conditions. It consists - the same as the easily decomposable organic substance of the basic substrate - also of approximately 85 % biologically easily decomposable organic substance and at the end of the decomposition of the easily decomposable organic substances present in the substrate represents a decomposable potential the quantity of which should not be under-estimated. The reduction of the substrate mass as well as of the newly formed bio-mass (i.e. the biological decomposition) takes place according to a declining exponential function, i.e. it will not lead to a complete dissolving of the entire bio-mass (consisting of basic substrate _ and formed cellular substance), but it is possible, after a suitable period, to decompose the organic part of the basic substrate (i.e. of the bio-waste) completely.
Most present composting works operators complain that on completion of the technical treatment most composts display :, it is true, a self-heating according to the rotting degree 4 or 5, but that after a dry storage a self-heating according to the rotting degree 1 can again be observed. This can be attributed to the fact that micro-organisms of organic substances that are difficult to decompose change info easily decomposable organic substances (cellular substances) which, when moistened again, then react immediately and, the same as fresh substrate, in a short while release large quantities of heat (as metabolic product), wherein the temperature in the compost substrate increases sharply within a short time. Until now the time and the temperature maximum reached in this time is the main criterion for determining the rotting degree. This, it is true, is only correct when the test conditions with regard to the substrate mass and ambient conditions are always the same, but until now as a simple standard method it determines the quality of the compost. The correct testing method must accurately determine the quantity of heat generated during a specific time or another metabolic product (see DE-OS 43 36 497). This is the only way in which the rotting degree or decomposition degree of an organic substance can be determined.

Methods for carrying out a composting process are known from the prior publications DE-PS 36 37 393, DE-PS 40 21 865, DE-PS 43 34 435, EP-PS 322 424, DE-PS 38 29 018, DE-PS 41 24 880, DE-PS 43 22 688, DE-PS 41 07 340, DE-GM 93 00 127 and DE-PS 42 15 267, to which reference is made here. All these prior publications have in common the use of atmosptieric oxygen from the air flowing through the rotting mixture.

In the German patent application No. 195 13 262.9-41 a completely closed circulation of the air and of the gases generated during the fermentation is disclosed, wherein the oxygen required for the breathing is taken from the water content of the waste, the chemical compounds or from fresh water fed in from the outside, and not from the air. Here the drying of the rotting mixture after the decomposition of the easily ~ decomposable organic substances takes place by feeding in fresh air after separating water from the circulating air. However, this method of operation has the disadvantage that by feeding in fresh air the relative moisture of the circulated gases is reduced and accordingly the water absorption capacity increased, but the heat required for the evaporation of residual water in the substrate is missing. If - due to the absence of easily decomposable organic substances - the biologically generated heat decreases, then, due to a lack of thermal energy of the rotting mixture, a complete extraction of moisture and accordingly an environmentally neutral storage stability of the end product will not be ensured. Furthermore, the method of operation used until now has the disadvantage that the moisture extraction from the exhaust air takes place in the condenser at the dew point limit. As a result, when the temperature in the rotting mixture drops, the water absorption capacity of the circulating air is limited, and the final water content values obtainable in the rotting mixture are detrimental for ensuring the long-term storage stability as well as for the calorific value.

Proceeding from this it is the object of the invention to eliminate the shortcomings that occur according to the prior art, and to propose an improved process for the biological-thermal treatment of organic constituents-containing wastes of the type described at the outset.
According to the invention this object is achieved in that the exhaust air is heated before it re-enters the tank. The exhaust air or circulating air is heated in such a way that in the rotting mixture a residual moisture content is obtained which leads to a diffusion equilibrium with the air which under normal circumstances surrounds the rotting mixture when it is stored.

As heat sources which can be used for this re-heating, in particular the following can be used: the feeding back of the condensation heat energy drawn off from the circulating air, preferably by means of air/air heat exchangers; changing the condition of the air by increasing the pressure;
sluicing the heat of condensation out of the air circulation system, which heat by means of a heat pump is brought up to a higher temperature level; the heat from the combustion process of a dry stabilising product or other intermediate product produced from the wastes; the heat from the combustion process of screen overflows; the heat from combustion processes of other energy carriers; the heat from solar installations; the heat from electrical transformation processes. The heat sources can be operated as central plants or also as decentralised energy stations which supply every tank in a plant comprising several tanks.

Advantageous further developments are described below.

Preferably the composition of the circulating air, i.e. the exhaust air, which is fed back to the tank, is controlled in dependence on the partial pressures of its components. The composition of the circulating gas volume is, therefore, controlled in dependence on the partial pressure of the gas components. As a result thereof it is possible to ensure an aerobic breathing without having to supply gaseous oxygen from outside {'it is possible, however, to supply gaseous oxygen from outside). During the mass balancing of rotting processes it was found surprisingly that no significant consumption of gaseous oxygen could be detected, although all characteristics of an aerobic metabolism were present. From this it can be concluded that the oxygen required for the aerobic breathing for the decomposition of easily decomposable organic substances in the sub-strate is already present and suffices for the microbial phase of the aerobic decomposition or can be obtained by feeding in water .

Another advantageous further development is characterised in that the gaseous metabolic products are separated from the circulating air. This is done preferably by molecular sieves. To control the partial pressures of the circulating scavenging gases, the gaseous metabolic products are, therefore, separated from the gas mixtures, which preferably takes place by suitable molecular sieves.

After coming out of the rotting mixture the gases, which have absorbed moisture, are cooled so that the condensable andlor sublimatable {~ -substances carried along in same, e.g. water, ammonia and the like, can be separated and removed in a liquid from the circulation system. The partial pressure ratio of the "carrier gases" (nitrogen and oxygen) in the _ climatic environment of the micro-organisms to the "metabolism gases"
(carbon dioxide and water vapour) preferably is controlled in that, when the overall pressure increases to above the atmospheric pressure (of 1013 mbar = hectopascal), excess quantities of CO2, ammonia and water vapour are drawn off. This can take place by a washing operation.

Subsequently the gases are not, as before, mixed with oxygen-containing, -dry air to increase the water absorption capacity, but they are heated from the outside by a heat source. As a result the partial pressure ratios of the gas components are changed. Furthermore, the water and carbon dioxide absorption capacity of the gases is increased and the thermal energy required for the evaporation of the water is supplied. This is in particular extremely advantageous when the microbial decomposition capacity drops drastically due to a slowly occurring dryness rigidity. If bio-masses are to be stabilised in such a way that microbial decomposition comes to a complete stop, the water content at the surfaces of the waste particles must be brought down to values below the normal condition of the air (H20 < 3,9 g/kga, corresponding to approximately 15 % relative substance and air moisture), and the easily decomposable compounds, -e.g. carbohydrates, must be biologically - -~~ -decomposed. With the methods used until now (composting with circulating air without re-heating or mechanical thermal drying) this is not possible in a short time (i.e. in less than about seven days), but it can be done with the process according to the invention.

Another advantageous further development is characterised in that aerobic and anaerobic gas mixtures flow alternately through the rotting ~ mixture, in dependence on the nitrogen content.

Furthermore it is advantageous when a water-saturated and a water-absorbable gas mixture flow alternately through the rotting mixture, in dependence on the prevailing type of micro-organisms.

In certain cases it may be advantageous to enrich the circulating air with _ 12 _ pure oxygen.

Another advantageous further development is characterised in that condensate is separated from the exhaust air and this condensate is i~ controlled with respect to its volume and/or _ its constituents. The condensate may be separated from the exhaust air by cooling. It may be collected and measured and following this be controlled with respect to its volume and/or constituents. Subsequently the circulating air is heated.

The process control preferably takes place in dependence on the conductivity, the chemical oxygen requirement and/or the pH-value of the condensate.

The circulated gases are preferably moved by several inlet air fans and without exhaust air fan. To further reduce the newly forming bio-mass, the rotting mixture can be removed from the tank after a fermentation cycle, preferably re-comminuted, preferably moistened again and fed back again to the tank (fermenter) and subjected to another rotting cycle. When doing so, as a result of a rapidly occurring heat generation an increase in temperature can be observed, the progression of which is the greater, the greater the as a result of the preceding biological decomposition newly formed bio-mass of strictly aerobic micro-organisms. -The invention is based on the following considerations:
~

~--' 14 -Bacteria, fungi and yeasts which carry out the biological decomposition, unfold their metabolic maximum under different conditions of life.
Bacteria prefer a high aw,-value of 0,98 (the aw-value represents the - relative air moisture over water with oxygen and nutrients dissolved in same). Mould fungi with a preferred aa,-value of 0,8 and yeasts with a preferred aw-value of 0,6, on the other hand, can live at a considerably lower relative moisture, i.e. less water and accordingly also oxygen in their environment.

In an atmosphere witli a low oxygen content less new cellular substance is formed, which therefore also need not be decomposed to produce a ~ matured or storable stable compost.

The formation of the bio-gas CH4 can be prevented when an acidification of the substrate is avoided by controlling the circulating gas composition in respect of its moisture (aw,-value < 85 %), C02-content (>
10%), oxygen content (< 10%) and ammonia content (< 1%) in such a way that for fungi and fungi-like micro-organisms (actinomycetes) advantageous intermediate decomposition products and climatic ~
.

conditions occur.

An intermittent method of operation with a change in the substrate moisture has the advantage that bio-masses in the dryness rigidity are more easily broken up into smaller pieces and accordingly can be attacked more easily by bacteria. After the fungi phase comminution measures are, therefore, carried out and the climatic moisture required for an optimum decomposition metabolism is produced. According to the invention this is achieved in that in chronological sequence, by changing the oxygen partial pressure in conjunction with water, facultative anaerobic micro-organisms are given the opportunity to utilise organically bound oxygen, for example by de-nitrification.

Another advantageous further development is characterised in that water is fed to the exhaust air before it re-enters the tank. Preferably fresh water is fed in. The feeding in of water preferably takes place after heating the exhaust air re-entering the tank. The water is preferably sprayed into the air flow.

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According to another advantageous further development several tanks are provided, the exhaust air pipes of which open out into an exhaust air manifold and the inlet air pipes of which branch off from an inlet air manifold. As a result thereof the process can be carried out particularly_ effectively. Furthermore, there is the added advantage that as a result of the manifolds the C02-content of various tanks can be balanced out.
If, for example, the C02-content in the circulating air of a specific tank is very high, air with a lower C02-content can be fed to this tank from the manifold. As a result thereof the exhaust air volume can be reduced further, possibly down to zero.

It is advantageous when the air of the exhaust air manifold is first fed to a heat exchanger, preferably an air/air heat exchanger, by which the -heat given off by the exhaust air manifold is transferred to the inlet air manifold or to the air or circulating air flowing through same. It is furthermore advantageous when the heat from the exhaust air manifold, instead thereof or in addition thereto, by an optional further heat exchanger, preferably an air/water heat exchanger, is given off to a cooling system, preferably a cooling water circulation system.

It is furthermore advantageous when the air or circulating air in the inlet air manifold is heated by a preheating device, which preferably takes place in the direction of flow behind the aforementioned air/air heat exchanger.

The invention further more relates to an apparatus for implementing the process according to the invention with several tanks, inlet air and exhaust air pipes leading to and from the tanks and an exhaust air manifold as well as an inlet air mani fold.

In accordance with a first aspect of the present invention, there is provided a process for biothermal treatment of waste materials containing organic constituents in a container, the process comprising the steps of:

(i) removing exhaust air from the container;

(ii) retaining at least a partial amount of the exhaust air as circulating air for reuse;

(iii) separating condensate from the circulating air;

(iv) collecting and monitoring the condensate to determine at least one of quantity and content of the condensate;

(v) heating the circulating air; and (vi) returning the circulating air to be circulated through the waste materials in the container.

- 17a-In accordance with a second aspect of the present invention, there is provided an apparatus for biothermal treatment of waste materials containing organic constituents comprising at least one waste material container tank, having (i) an air exhaust means and an air supply means; and (ii) an air recirculating system connected to the air exhaust means and the air supply means, the system comprising a condensate separation means and at least one air reheating means.

Exemplified embodiments of the invention will be explained in greater detail in the following with reference to the attached drawing, wherein:

Fig. I is a diagrammatic view of two tanks including the other components for implementing the process, '_.

- Z'8 -Fig. 2 is a diagrammatic view of a tank with the other components for implementing the process, and Fig. 3 shows an apparatus for implementing- the process, _comprising several tanks. -The bio-mass to be fermented is filled daily into the tanks (rotting boxes) RB1 and RB2 shown in Fig. 1, seeing that also the waste disposal cycles display a daily rhythm. Still further tanks may be present in a plant. Every tank is fitted in its bottom part with an essentially horizontally extending perforated bottom 1, in which holes or other ~ openings are provided. Underneath the perforated bottom 1 a plurality of smaller air chambers 2 are provided, to which air can be admitted, i.e.
which can be controlled. The air is fed by fans 4 into the air chambers

2 and flows through the holes in the perforated bottom 1 into the rotting mixture 3 that rests on the perforated bottom 1. Because of the multitude of individual air chambers 2 an uneven gas flows through the rotting mixture 3 with the attendant risk of a breaking through of the entire gas volume at one point or in a small area is avoided. The C.~

perforated bottom 1 may consist of solid perforated plates, perforated bricks, pendulum bottom profiles or air-permeable belt conveyors.
On the perforated bottoin 1 rests the ratting mixture 3 in gas-permeable form. The fan 4 circulates the gas volume enclosezl in the system via the inlet air heat exchanger 5 through the rotting mixture and the exhaust air heat exchanger 6.

When in special cases CO2 must be discharged and oxygen let in, the valves 7 and 8 are opened.

~ The condensate drawn off from the heat exchanger 6 by cooling is fed through a pipe 9 to a condensate treatment plant with oxygen enrichment. Thereafter it can be used to again moisten the rotting material for a further rotting cycle.

The heat carried off from the heat exchanger 6 is fed via the storage unit 10, the heat pump 11 and the other storage unit 12 to the heat exchanger 5 and there is transferred to the inlet air. The drying process r-.

is at an end when the water absorption of the circulating gas is close to zero glkg. If heat is available from thermal processes, e.g. from the combustion of the stabilising material, this can be fed into the heat exchanger 12 at point 13.

t...! _ As can be noted from Fig. 1, fresh water 15 can be fed in. This fresh water is sprayed into the air flow after it has been heated in the heat exchangers 5 and 6. The air moistened in this manner then flows into the tanks.

The tanks are filled daily, corresponding to the waste disposal cycle of the refuse disposal industry, which means that the individual tanks at daily intervals and in the course of the decomposition kinetics reach a maximum in the release of the metabolic product "heat". This is illustrated in the time-heat diagram 14. Every day a tank Rbn is filled.
The tank RB1 reaches the heat maximum after approximately one day, the tank RB2 after two days, the tank RB3 after three days and so on.
Seeing that after about seven days the heat release has reached a minimum, the process can be ended here. From the diagram 14 it can {

also be noted that for the summation of the quantities of heat to be eliminated daily, not the respective daily maximums should be taken as a basis, but a diminishing pattern. From this it follows that when up to seven rntting tanks are connected to a heat exchanger combination with heat pump, a favourable cost-usefulness ratio can be obtained.

Figure 2 illustrates an exemplified embodiment with a rotting tank which basically is constructed in the same way as that of Fig. 1. The rotting tank is supplied with air by a fan, which air passes through the perforated bottom and the rotting mixture resting on same. Subsequently the exhaust air is drawn off by another fan and fed to an air/air heat ~ exchanger. There it gives off heat to the air to be fed to the rotting mixture. Subsequently the air is fed to a water/air heat excfianger, where it gives off further heat to an air-cooled cooling tower. The air then flows through the other side of the air/air heat exchanger and as circulating air is again fed to the rotting mixture. If required, fresh air can be fed into the circulation system through an adjustable valve. It is furthermore possible to remove exhaust air from the circulation system by way of a valve and connected filter. Water of condensation can be drawn off from the heat exchangers.

The closed rotting tanks preferably are thermally insulated. They are -preferably made of reinforced concrete. In the tank, in a period of about one week all substances that form biologicalI}T easily decomposable compounds are released in gaseous form by oxygen dissolved and bound in the substrate liquid, by aerobic breathing, and for the greater part are removed from the rotting mixture with a circulating, continually moving gas volume, the gas volume being fed through the rotting mixture from the bottom upwards through a perforated bottom, which preferably is provided with air chambers positioned underneath to which the air can be supplied individually. The heating of the circulating air can take place by the condensation heat of the condensate separated from the exhaust air (see Fig. 2). It can take place by increasing the pressure in the circulating gas flow or by a heat exchanger with known heat carriers.
With the process according to the invention the composting of bio-masses can be carried out or the stabilising of wastes. The overall plant for the implementation of the process can consist of inlet air fans, rotting tanks (rotting boxes), exhaust air/inlet air heat exchangers, exhaust air/heat carrier heat exchangers, exhaust air valve, exhaust air purification plant, inlet air valve, water spraying nozzle, condensate measuring device, temperature measuring device, programme regulators and switch cabinet. It is possible to work mdth or without exhaust air fans as well as with or without COZ measuring device The rotting tanks may consist of reinforced concrete housings with and without thermal insulation, a door that can be closed gas-tight, air-permeable bottom plates and underneath same air boxes to which air can be supplied individually. The air ducts underneath the perforated plates may run in the longitudinal direction underneath the rotting boxes, extending conically from the blowing-in point and/or can be supplied with different air volumes, so that it is possible to influence edge flow losses on the walls of the rotting tank.

Fig. 3 illustrates an apparatus with several tanks 21, 22, 23, an inlet air pipe 24, 25, 26, in each of which a fan is arranged, leading to each one of these tanks and an exhaust air pipe 27, 28, 29 leading away from each tank. The exhaust air pipes 27, 28, 29 each lead via a slide valve to an exhaust air manifold 30. The inlet air pipes 24, 25, 26, each of which is -also provided with a slide valve, branch off from an inlet air manifold 31.

The exhaust.air manifold 30 runs through an air/air heat exchanger 32, ~ by which the heat of the air flowing through the exhaust air manifold 30 is given off to the air flowing through the inlet air manifold 31.
Subsequently the air in the exhaust air manifold 30 passes through three air/water heat exchangers 33, 34, 35, by which the heat is given off to a cooling water circulation system 36.

By way of a slide valve 37 in the branch pipe 38 from the exhaust air ,-~ manifold 30, air can be given off to a bio-filter.

The exhaust air manifold 30 changes over via a connecting pipe 39, in which a slide valve 40 is arranged, to the inlet air manifold 31. A branch pipe 41, in which a slide valve 42 is provided, which can be controlled for the feeding in or mixing in of inlet air, opens out in the inlet air manifold 31. Following this in the direction of flow a COZ measuring device 43 is provided in the inlet air manifold 31.

~.

-The inlet air manifold then leads agdin to the air/air heat exchanger 32.
A branch pipe 44 by-passes this air/air heat exchanger 32, so that the .inlet air or part thereof can also be fed to the tanks 21, 22, 23 without being heated in this heat exchanger 32, i.e. through other inlet air pipes '-~ 45, 46, 47, each of which is also provided with a-slide valve.

From every exhaust air pipe 27, 28, 29 an individual circulating air pipe with a slide valve leads to the respective inlet air pipe 24, 25, 26. As a result thereof it is possible to individually circulate part of the circulating air for each tank and to let only part of the circulating air flow through the manifolds.

_.~ .

In the inlet air manifold 31, in the direction of flow after the air/air heat exchanger 32, a preheating device 48 is provided which consists of an air/water heat exchanger to which outside heat. can be supplied.

Claims (16)

What is claimed is:

1. A process for biothermal treatment of waste materials containing organic constituents in a container, the process comprising the steps of:
(i) removing exhaust air from the container;

(ii) retaining at least a partial amount of the exhaust air as circulating air for reuse;

(iii) separating condensate from the circulating air;
(iv) collecting and monitoring the condensate to determine at least one of quantity and content of the condensate;
(v) heating the circulating air; and (vi) returning the circulating air to be circulated through the waste materials in the container.

2. A process according to Claim 1, wherein a composition of the circulating air is controlled as a function of partial pressures of its components.

3. A process according to Claim 1 or Claim 2, further comprising the step of separating gaseous products released during metabolic decomposition of the waste materials from the circulating air.

4. A process according to Claim 3, wherein the gaseous products are separated by molecular sieving.

5. A process according to any one of Claims 1 to 4, further comprising the step of determining nitrogen content of the waste materials at intervals during decomposition, and alternately passing aerobic and anaerobic gas mixtures through the waste materials based on the nitrogen content.

6. A process according to any one of Claims 1 to 5; further comprising the step of determining a prevailing type of micro organism contained in the waste materials at intervals during decomposition and alternately flowing a water-saturated gas mixture and a water-absorbable gas mixture through the waste materials based on the prevailing type of micro organism.

7. A process according to any one of Claims 1 to 6, further comprising the step of enriching the circulating air with pure oxygen.

8. A process according to any one of Claims 1 to 7, wherein the condensate is controlled as a function of one member selected from the group consisting of conductivity, biochemical oxygen demand and pH value of the condensate.

9. A process according to any one of Claims 1 to 8, further comprising the step of adding water to the circulating air after heating in step (v).

10. An apparatus for biothermal treatment of waste materials containing organic constituents comprising at least one waste material container tank, having (i) an air exhaust means and an air supply means; and (ii) an air recirculating system connected to the air exhaust means and the air supply means, the system comprising a condensate separation means and at least one air reheating means.

11. An apparatus as claimed in Claim 10 comprising a plurality of container tanks, wherein the air exhaust means comprises an exhaust manifold constructed and arranged to receive at least part of the exhaust air from each container tank, and the air supply means comprises a supply manifold connected to an air inlet means provided to each container tank.

12. An apparatus according to Claim 11, wherein the supply manifold comprises a pre-heating device.

13. An apparatus according to any one of Claims 10 to 12 wherein the air reheating means is an air-air heat exchanger.

14. An apparatus according to Claim 13, wherein the air recirculating system further comprises at least one air-water heat exchanger connected to a cooling circuit.

15. An apparatus according to Claim 14, wherein the cooling circuit is a cooling water circuit.

16. An apparatus according to any one of Claims 10 to 15, wherein the air recirculating system further comprises a water input means.