DONATI, Gianni (Loc. Cassinazza di Baselica, Giussago, I-27010, IT)
NATTA, Giuseppe (Loc. Cassinazza di Baselica, Giussago, I-27010, IT)
DONATI, Gianni (Loc. Cassinazza di Baselica, Giussago, I-27010, IT)
CLAIMS
1. Method for the elimination of aqueous effluent, characterised in that it provides for the feeding of this aqueous effluent onto a pile of biodegradable and digestible waste, to cause evaporation thereof.
2. Method according to claim I 5 characterised in that said aqueous effluent is fed onto said pile of waste at the end of the digestion of solid waste until the initial humidity is restored in whole or in part and digestion reactivated.
3. Method according to claim 1 , characterised in that said aqueous effluent is fed onto said pile of waste during digestion of the solid waste, and feeding is stopped before final bio-drying.
4. Method according to any one of the previous claims, characterised in that it provides for the conveying of air through said pile of waste to promote digestion. 5. Method according to claim 4, characterised in that said air carrying the evaporated water is subjected to biofiltering before the release into the environment.
6. Method according to any one of the previous claims, characterised in that said solid waste is collected in a substantially closed digestion chamber containing a support surface divided into sectors and having apertures distributed evenly for the passage of the air.
7. System for the elimination of aqueous effluent, comprising a substantially closed digestion chamber (1), suitable for containing solid waste (4) and having apertures for the passage of the air, characterised in that it also provides means (8) for dispersion of aqueous effluent on solid waste to allow the evaporation thereof following digestion of the biodegradable substances contained both in the solid waste and in the aqueous effluent.
8. System according to claim 7, characterised in that said digestion chamber (1) has a support surface (2) divided into sectors (3), for the placing of waste (4), said apertures for the passage of air distributed evenly on said sectors (3).
9. System according to claim 8, characterised in that means (5) are provided, in particular in the form of cranes, for the layered depositing of the shredded waste on said support surface (2) at each of said sectors (3) and for the transfer of the waste between the various sectors and between the various areas of the system.
10. System according to claim 8 or 9, characterised in that means (6) are provided for aspiration or injection of the air and for its conveying through the apertures of said support surface (2) so as to traverse the layered waste on said surface and promote digestion.
11. System according to any one of claims from 7 to 10, characterised in that means (7) are also provided for the treatment of the air carrying the evaporated water by biofiltering before release into the environment. |
METHOD AND SYSTEM FOR THE ELIMINATION OF AQUEOUS EFFLUENT
Field of application
The present invention relates to a method and a system for the treatment of effluent water containing toxic substances and organic material in solution or suspension such as for example municipal or agricultural effluent, percolate of landfills and the byproducts of food or chemical industrial plants.
This effluent contains toxic or organic substances which cannot be released into the environment and therefore require treatment procedures that are onerous both due to the investments in plants and the running of the same. Moreover these treatments may produce other effluent such as sludge or concentrates that must in turn be treated or destroyed.
It has been found that existing plants for bio-drying municipal solid waste or in some cases composting plants can perform this task without additional effluent and investments.
State of the art The aqueous effluent from municipal, agricultural and industrial activities or from environmental reclamation and treatment represent a serious problem that often can only be resolved at the cost of high investments and/or onerous procedures for its removal.
The technologies for solving the problem of aqueous effluent rank second in fact in terms of volumes and economic interest, after those of municipal solid waste.
The main methods implemented by the community and by industry are the following: purification in tanks or reactors in which treatments are performed with chemical substances, adsorbents and flocculants followed by
processes of aerobic/anaerobic digestion and filtering; concentration through distillation of the water, ultrafiltration or reverse osmosis, often combined one with the other; combustion in plants for incineration of solid and liquid waste with high heating value.
The first of these methods is the most widespread, is applied to usually high volumes and requires considerable plant investments, running and energy costs, and chemical products, and generates sludgy waste which must in turn be treated. Examples of this type of treatment are in the public domain and are known for example from US2005279710, US2005284811, WO2005123613, US2006000784 and WO2005107930.
The second method is applied to small volumes and typical applications can be found at landfills of municipal solid waste for the treatment of percolate. In the most common version, ultrafiltration or microfiltration is performed, followed by two stages of reverse osmosis, although in some cases it is possible to carry out evaporation in place of the ultrafiltration.
This is a costly technology in terms of plant and maintenance due to soiling and changing of the membranes, not lacking in problems due to the production of a concentrate which has to be incinerated and due to the presence of ammonia which is difficult to resolve.
This technology has also been seen to have been applied to the treatment of vegetation waters of olives, as reported in WO2005123603.
Incineration constitutes the fastest and most satisfactory method although possibly the most costly, given the low heating value of the effluent and the requirement of auxiliary fuel which, should effluent with high heating value not be available, must be gasoil or methane.
Even in the case where a balanced mix of waste is available, the
combustion of aqueous effluent does not make a positive contribution to the economy of the incinerator so that the costs of incineration are high.
Moreover the economy of present incinerators of municipal and industrial solid waste is uncertain due to their low efficiency and due to the small scale as described in literature.
This has persuaded the Applicant in the past to develop a new technology that allows the energy contained in municipal solid waste to be concentrated, improving the logistics of transport and allowing the construction of large incinerators with high efficiency. In EP-A-706839 in the name of the same Applicant a method
(BIOCUBES ® ) is described for the recovery of energy from municipal solid waste by means of the preparation of non-conventional fuel, comprising the phases of rough shredding of the waste, accumulation of the same on a porous bed in a closed chamber, forced digestion with temperatures of around 60 - 65°C until the waste is dried by means of the passage of air through the waste itself and removal of the odours from the output air using biofilters.
The waste is placed in the digestion chamber in layers, also of considerable thickness, traversed by a flow of air which gives rise to spontaneous digestion with combustion at a low temperature of part of the waste, production of heat and evaporation of the water present in the same waste. The distribution in the country of plants for the processing and transfer of waste (ITS ® - Intelligent Transfer Station) allows the combustion of waste in large plants with economies of scale and improved efficiency.
It has now been found that the BIOCUBES ® process and the ITS ® are also suitable for evaporation and the full disposal of aqueous effluent.
Disclosure of the invention
The general object of the present invention is that of eliminating the disadvantages of the technologies for treatment of aqueous effluent reported
above, by making available an economical method fully integrated in the life cycle of the waste.
One particular object is that of making available at the sites, where there is a plant for bio-drying of municipal solid waste, the possibility of disposal of the aqueous effluent that may be produced.
A further object is therefore that of cancelling the specific investments for the treatment of wastewater and of reducing the energy consumption for evaporation or separation of the water, yielding a dry effluent in optimal conditions for incineration. A special object is that of the complete exploitation of the combustible part contained in the effluent, both for the purpose of evaporation of the water and during the final incineration phase.
These objects and others which are to be explained in greater detail herein below are achieved by means of a method and a system having the features of independent claims 1 and 7, respectively.
Preferred embodiments of the invention are disclosed by the dependent claims.
Substantially, according to the invention, a system is used for the biological stabilisation of solid waste containing putrescible fractions, in particular municipal solid waste, to which are added, during or after bio- drying of solid wastes, the aqueous effluent.
More particularly, the system comprises a substantially closed digestion chamber containing a support surface divided into sectors whereon piles of solid waste are deposited and having openings evenly distributed for the passage of the air.
At the end of or during digestion the aqueous effluent is fed onto the pile of waste and, simultaneous to bio-drying of the solids, the aqueous effluent is evaporated.
The heat of evaporation is supplied by the digestion of the biodegradable substances contained both in the solid waste and in the aqueous effluent and the water is removed by the hot air saturated with humidity which traverses the pile. This air is sent to a series of biofilters before being released into the environment.
In this way it is possible to remove, simply and economically, aqueous effluent that would otherwise require complex, costly and not always satisfactory treatments. Further features of the invention will be made clearer by the following detailed description, referring to one of its embodiments which is purely an example and therefore non-limiting, illustrated in the accompanying drawings, in which:
- Figure 1 is a simplified diagram of a plant (BIOCUBES) for the treatment of solid waste used for eliminating the aqueous effluent according to the invention;
- Figures 2 to 5 are graphs showing the temperature profile and the percentage of water evaporated, referred to the initial weight of the solids as a function of time, in examples of plants for the elimination of aqueous effluent according to the invention.
Referring to Fig. 1, the system comprises a substantially closed digestion chamber 1 of predetermined size, a support surface 2 divided into sectors 3 positioned inside said chamber for the layered deposition of waste 4, said support surface being provided with apertures distributed evenly on said sectors for the passage of air, means 5 for the laying of shredded waste on said support surface at each sector and for the transfer of waste between the various sectors and between the various areas of the plant, means for the aspiration of air 6 and for its conveying through the apertures of said support
surface so as to traverse the layered waste on said surface and promote digestion thereof, and means for the treatment of the air 7 by biofiltering before the release into the environment.
This method and this system allow the obtaining of a dry and hygienised fuel, with sufficient net energy recovery for repaying investments. It has now been found that the waste bio-dried in this way, if wetted, becomes active again and can be bio-dried again and that therefore it is possible to use the same waste as an energy source to evaporate not only the water originally contained therein but also the aqueous solutions of different origin.
An energy source is therefore made available at zero cost for the removal of aqueous effluent through evaporation.
The evaporated water, carried with the air and containing malodorous substances, is treated by the biofilters 7 and the residues contained in the effluent are either digested in the pile during digestion, producing further heat, or are treated by the biofilters or are deposited on the waste and sent with it to combustion.
The system described above can easily be provided with means 8 for dispersion of the effluent on the piles and then converted to receive and treat not only solid effluent but also aqueous effluent.
In brief the method that is the object of the present invention functions as follows: each pile 4 of solid waste, after having been deposited by an automated crane 5 and having a content of water of around 40% in weight undergoes its process of digestion and bio-drying which ends when the content of water of the waste reaches a critical value so that the microorganisms contained in the waste are no longer active and typically around a value of humidity between 17 and 20%;
the aqueous effluent is sprayed, with a content of residues and organic matter of around 5%, onto the pile until all or part of the initial humidity is restored and the digestion is reactivated in this way; a new bio-drying cycle is carried out in which the evaporated water is mainly that added with the aqueous effluent until achieving the final humidity of the solid waste as required. the cycle can continue until the digestible organic matter is finished; alternatively it is possible to add the aqueous effluent during actual digestion and stop the feeding of the effluent before the final bio-drying.
In the bio-drying process the solid waste typically undergoes a loss of weight of around 25% due to the water evaporated and with the method that is the object of the present invention all the water contained in the aqueous effluent is also evaporated and there is not normally an increase due to the aqueous effluent, the residues contained therein being compensated by the combustion of the organic matter present in the waste itself.
Moreover the aqueous waste is completely removed as it follows the cycle of the solid waste.
The only apparent advantage is the prolonging of the operation of bio- drying which is controlled by the kinetics of aerobic digestion and which can easily be compensated by an appropriate oversizing of the plant.
The results obtained with the method of the present invention are reported in the following examples of application performed using the ITS ® in Montanaso (LO). In these demonstration examples reference is made to the experimental tests performed on one of the piles 6 during service of the same system. Example 1 Consideration was made of a pile of waste 4, deposited in one of the
sectors 3 of the building, 6 m in height, 7.5 m in width and 22.5 m in length and therefore having a surface area of approximately 170 m 2 and volume of 1.000 m 3 .
The weight of the waste accumulated in the sector by the automated crane 5 is approximately 500 t and the content of water approximately 40% and therefore 200 m 3 .
Normal bio-drying was carried out with a constant flow of air at the corresponding fan 6 and equal to 3.000 m 3 /h which corresponds to a surface speed of traversing of the pile of 0.005 m/s. With knowledge of the average temperature of the air in the building
(20 0 C) and the temperature of the air, assumed to be saturated, in output from the pile, it is possible to calculate the evaporated water and the humidity of the solids.
Fig. 2 shows the temperature profile and the percentage of evaporated water referred to the initial weight of the solids, i.e. a value close to the drop in weight of the solids, the quantity of solids burnt being very small.
The evaporated water is therefore around 130 m 3 in relation to the estimate of the 200 m 3 initially contained in the solids.
120 m 3 of percolate from a dump and with a dry residue of 2% and a content of organic matter and ammonia of around 1 % was at this point sprayed over the pile via a temporary system acting as dispensers 8, and the operation of bio-drying continued for a further 15 days.
The result obtained is shown in Fig. 3 where the evaporated water is always referred to the initial weight of the solids. A quantity of evaporated water of 258 m 3 was calculated, of which 120 due to the percolate which was found to be totally volatilised.
The organic and malodorous part of the percolate was in part destroyed in the pile and in part treated in the biofilters, at the output of which anomalies
in relation to normal service were not found.
The extracted bio-dried solid with humidity of around 19% was, together with the rest of the bio-dried product, screened and the iron removed, and it was then finely ground and sent for combustion in the fluidised bed incinerator at Corteolona (PV).
Example 2
On another pile at the plant of Montanaso (LO), similar to that of the previous example and traversed by the same flow of air after the first two days of operation and up to the nineteenth day of bio-drying, a daily flow of 15 m 3 of percolate was fed continuously so as to maintain in this range virtually constant humidity of the solids for a total of effluent fed of approximately 260 m 3 .
Starting from the twentieth day feeding of the percolate was stopped and the operation of bio-drying was continued up to the thirtieth day. Fig. 4 shows the first 15 days of operation and highlights a temperature profile and a constant rate of evaporation, indicative of constant reactivity in the digestion process.
Fig. 5 shows the continuation of the process with a drop in the temperature after the twentieth day and corresponding reduction in the rate of evaporation.
The final humidity of the solids was around 22% and total evaporated water 390 m 3 .
The solids were sent for incineration as in the previous example.
It can be seen how in this case, having fed a greater quantity of aqueous effluent, the final humidity is greater, the evaporating capacity being exclusively linked to the reactivity of the digestion process.
On average this evaporating capacity can be estimated at around 0.3 - 0.5 m 3 H 2 O per tonne of solids according to the reactivity of the same
solids.
If such a system as the ITS of Montanaso, with capacity of 60.000 t/a of municipal solid waste, were made to operate at 50% it could treat from 9.000 to 15.000 m 3 /a of percolate, representing the annual production of a landfill of average size.
The versatility of the ITS distributed in the area therefore allows the problem of solid and liquid waste produced in the same area to be tackled in a global, efficient and integrated manner.