EP3965965A1

EP3965965A1 - A hydrothermal process and plant for treating solid waste

Info

Publication number: EP3965965A1
Application number: EP20722214.2A
Authority: EP
Inventors: Nazareno GIORGI; Lucio SANASI
Original assignee: Tpa Srl
Current assignee: Tpa Srl
Priority date: 2019-05-06
Filing date: 2020-05-06
Publication date: 2022-03-16
Also published as: WO2020225748A1; IT201900006578A1

Abstract

A process for treating OFMSW is disclosed, comprising an initial step in which leachate resulting from discharging and handling OFMSW is collected and sent to a fast biodigester plant. OFMSW is then submitted to a hydrothermal treatment by means of pressurised hot water. The organic pasty material resulting from the operations of treating and dividing the material leaving the hydrothermal treatment of OFMSW is collected and sent to third-party plants for the production of biogas. A plant for carrying out the process of the present invention is also proposed.

Description

A HYDROTHERMAL PROCESS AND PLANT FOR TREATING SOLID WASTE

Description

Technical Field

The present invention relates to a process for treating OFMSW (Organic Fraction of Municipal Solid Waste). In particular, the invention relates to a process capable of improving the recycling efficiency of OFMSW, with the aim of developing a circular and sustainable economy. More particularly, the present invention relates to a process for the hydrothermal treatment of OFMSW for producing material suitable for biogas production by means of anaerobic digestion, with reduced production of solid digestate and greatly reduced odorous impact.

The present invention also concerns a plant for carrying out the process of the present invention.

Prior Art

According to Italian law, OFMSW denotes the organic fraction of municipal solid waste resulting from the differentiated waste collection. Disposal of OFMSW is a highly felt socio-environmental problem. Interest in OFMSW as a source of biofuels for sustainable development originates from both the need of managing increasing amounts of such a differentiated waste, and the need of replacing crops preferably intended for animal nourishment, such as cereals, hay, maize silage, in feeding plants for biogas production.

Whereas domestic OFMSW mainly contains food residues, such as residues of meet, fish, pasta, bread and food used in preparing meals, grass, prunings as well as residues of fruit and vegetables and residues of liquids such as milk and fruit juices, and it is collected in bio-degradable bags, OFMSW coming from the collection by means of skips may contain very different materials, such as metals, plastics, glass, inorganic inert materials such as gravel, and non bio -degradable materials such as bones, fruit stones, nut shells, crustacean shells and mollusc shells.

Several processes for treating municipal solid waste are known in the art, which are based both on their thermal conversion into energy (incineration) and on their transformation into final matter suitable for being conveyed to a dump or utiisable in other applications. Said processes are under continuous development in order to reduce costs, times, volumes, odorous emissions and other polluting emissions associated with the treating process.

It is known that OFMSW essentially contains substances of organic nature, with a high availability of carbon-to-carbon chemical bonds that makes OFMSW potentially suitable for biological processes of valorisation and recovery, such as fermentation, in order to obtain biogas, as well as a stabilised solid residue that can be used in agriculture as soil amender or fertiliser.

At present, OFMSW is treated in aerobic composting plants for compost production or in anaerobic digestion plants with biogas production. Said plants originate some serious practical problems as far as valorisation and recovery are concerned, which problems are caused by the lack of qualitative uniformity and the poor structuring capability of the waste to be treated.

The organic fraction of OFMSW has a high fermentability that facilitates arising of putrefactive phenomena with strong odorous emissions, which phenomena are accentuated by the prolonged permanence of great masses of material undergoing fermentation. The prior art plants treating OFMSW generate environmental impacts of various kinds resulting from the release and the natural percolation, particularly evident in summer months, of the drainage liquid fraction originating from the organic matter, the leachate. The leachate produced, due to its intrinsic characteristics, is a source of unpleasant smells and of high processing and/or disposal costs. Moreover, the conventional composting and bio-gasification plants occupy very wide surfaces and are scarcely compact.

Thus, in the presence of an increase in the amount of OFMSW to be treated, the main difficulty in building new aerobic or anaerobic plants is related with environmental factors, having a great odorous impact, which jeopardise their acceptance by the neighbouring communities.

The prior art also teaches mechanically or hydraulically separating the organic fraction from the non-digestible fraction including metals, plastics, glass, inorganic inert materials and non-biodegradable material, in order to increase the biological potential of OFMSW. Yet, the presence of metals, plastics, glass, inorganic inert materials and non- digestible materials in the mass to be treated can cause serious drawbacks and damages to the selecting and sorting machines. More particularly, a disadvantage consists in pump and pipe abrasion, as well as in the clogging of reservoirs.

Moreover, as far as feeding of OFMSW to anaerobic digesters for biogas production in third-party plants is concerned, in order to increase the reaction speed it is useful to reduce the size of the organic faction upstream of the plant by grinding or hashing, and to dilute it by mixing it with sufficient process water to create a sludge in which bacteria responsible for anaerobic fermentation can diffuse and act. Yet, a drawback is that hashed and ground organic materials tend to become separated from the dilution water, by settling on the bottom on the one hand and by floating at the surface as a fibrous layer on the other hand, thereby slowing down the evolution of the anaerobic fermentation process.

Besides those drawbacks, the prior art processes lastly do not allow optimising matter and energy recovery, with the aim of achieving a circular and sustainable economy.

Thus, the need is still felt to have at disposal a process that, besides overcoming the disadvantages mentioned above, is also capable of maximising the waste recovery without being further prejudicial to the environment, by converting the waste into further exploitable products, such as fuels and secondary raw matters.

WO 2006/048008 A2 concerns a method for decomposing biologic material, in which the material is conveyed to a tank in order to make it percolate by means of sieving. The leachate thus obtained is subsequently introduced again in order to modulate biogas production and thus to obtain more electric power in periods in which such power is more valuable, typically in order to meet peaks of electric power demand during the day.

Yet, the need is felt to have at disposal a process for treating organic material, which can be performed just in time and in which all obtainable liquids having energetic capability since they contain organic matter, are used in place in order to produce biogas to be used for energetic needs.

Description of the invention

It is the main object of the present invention to overcome the technical disadvantages mentioned above, by providing a process capable of improving the recycling efficiency of OFMSW, with the aim of developing a circular and sustainable economy.

In order to achieve the aim, the present invention provides a process for the hydrothermal treatment of OFMSW for the production of a pasty material that can be handled, is uniform, has reduced volume and weight, is sanitised and sterilised and has low odorous impact, which material can advantageously be used, upon a treatment for eliminating inorganic inert materials and non-digestible substances, in order to feed third- party plants for the production of biogas.

Another aspect of the process of the present invention is heat recovery performed by steam tapped from the hydrothermal treatment of OFMSW. Moreover, being such steam rich in micronized nutrients, it can advantageously be fed, after heat recovery, to a fast biodigester plant for the production of power necessary to the economy of the process of the present invention. A further aspect of the process of the present invention is the collection and forwarding of the leachate, obtained from the unloading and the handling of OFMSW, to the fast biodigester plant mentioned above, thereby avoiding the prolonged permanence of the leachate with consequent arising of putrefactive phenomena and odorous emissions.

Yet another aspect of the process of the present invention concerns the transformation time of the arriving OFMSW, including a cycle of 8 - 10 hours, advantageously shorter than the permanence time of OFMSW, typically 20 - 30 days per cycle, as is the case for instance in the conventional anaerobic digestion plants.

Lastly, the present invention concerns a plant for carrying out the process of the present invention, the diagram of which will be illustrated by the description below.

Some terms used in the present invention have the meaning defined hereinafter.

The term "OFMSW" refers to the putrescible organic material with high humidity content coming from the differentiated collection of municipal waste and consisting of food residues or waste. Collection usually takes place at domestic users or at selected users (such as canteens, restaurants, food industries and so on) according to management models referable to the use of specific street containers or to the collection at the premises of the concerned users (regulatory references: Decree of the Regional Council of 15 June 2006, No. 76, and Decree of the Regional Government of 25 February 2005, No. 568).

The term "fast biodigester plant" refers to an equipment of the plant of the present invention intended for the production in situ, according to techniques known per se, of biogas necessary to the economy of the process of the present invention.

The term "about" includes the ranges of operating conditions and parameters that can be normally employed in the process of the present invention.

The term "carbon-to-carbon chemical bonds" refers to an intrinsic characteristic of the organic matter in OFMSW and such bonds are a feeding source for methanogenic bacteria responsible for the production of biogas in anaerobic digestion.

The term "SC (Separation Chamber) device" refers to the equipment where the hydrothermal treatment of OFMSW in the process of the present invention takes place. The SC device is an equipment derived from Patent of Invention IT-0001416920 and Patent for Utility Model IT-0000280277.

All other terms used in the description of the present invention, unless otherwise specified, are to be intended according to their usual meaning known to the skilled in the art.

Advantageously, the process according to the invention allows building a complete hydrothermal and mechanical industrial plant, with very low environmental impact, for the fast treatment of putrescible organic substances, even in admixture with various solids, such as inert materials, plastics, metals etc. Thus, the plant according to the invention is capable of treating such a material according to a continuous process, without creating accumulations, even in great amounts and by producing materials directly utilisable as clean organic matter, clean metals, inert materials and plastics.

Brief Description of the Figures

A preferred embodiment of the invention, given by way of non-limiting example, will be described with reference to the accompanying Figure, which shows a diagram of the process of the present invention.

Description of a Preferred Embodiment

With reference to the diagram of the accompanying Figure, the process of the present invention includes a first aspect in which material 1, in loose condition or contained in bags, after having been weighted, is discharged on the ground in the suitable receiving yard 2 for the compulsory visual inspection. In this discharging step, a great amount of drainage liquid, leachate 3, produced by OFMSW during the steps of permanence in the municipal skips, loading onto the transport means, constipation and, generally, in all steps of OFMSW handling, pours down onto receiving yard 2.

Thanks to the slope of yard 2 receiving the material, the leachate is collected and sent, with the aid of suitable pumps, to pre-tank 5 feeding fast biodigester plant 6. The leachate can optionally be submitted to a physical treatment, such as filtering through a fine mesh grid 4, in order to retain solid particles 7. The latter can optionally be sent to dosing system 8.

Receiving yard 2 and dosing system 8, when provided, are preferably confined within a suitable restricted space with reduced size, preferably a size slightly exceeding the size necessary for containing waste and for allowing loading and unloading operations. That space is kept under a small negative pressure to prevent bad smells from diffusing to the outside. The reduced size advantageously allows a rapid, cheap and effective depuration of the odorous atmosphere existing inside said space. By way of example, the restricted space can be obtained by means of a structure made of plasterboard or the like, having an opening for the material introduction, which can be closed.

Digestate 9 leaving fast biodigester plant 6 is divided into solid digestate 10 and liquid digestate 11. A fraction 12 of the liquid digestate, after the depuration treatment in 13 by means of techniques known per se, provides water that can be sent to accumulation tank 14 and used for the needs of the plant of the present invention for treating OFMSW.

In a second aspect of the process of the present invention, part of the water depurated in 13 is sent, after thermal exchange in exchanger 15, to boiler 16 for production of pressurised hot water 19 to be used for the hydrothermal treatment in SC devices in 17.

In a third aspect of the process of the present invention, a fraction 13a of the depurated water is used for washing OFMSW receiving yard 2 at the end of the daily shift in order to eliminate residues of organic material and leachate. The resulting liquid is recycled to pre-tank 5 feeding fast biodigester plant 6.

Biogas 35 produced in fast biodigester plant 6 is used as combustible gas for feeding boiler 16 intended for producing pressurised hot water 19 necessary for the OFMSW hydrothermal treatment in 17.

In a fourth aspect of the process of the present invention, the material from receiving yard 2 is loaded onto a dosing system 8 which loads it onto a conveyor belt in order it can be forwarded to SC devices in 17 in a predetermined amount each said SC device is capable of receiving. This prevents from having putrescible material staying on the conveyor belt, thereby preventing, especially in summer months, odorous emissions annoying the staff in the department and people living in the neighbourhood. Further leachate 18 resulting from the handling of OFMSW in the dosing system and on the conveyor belt is recycled to pre-tank 5.

In a fifth aspect of the process of the present invention, the material coming from dosing system 8 is submitted to a hydrothermal treatment with pressurised water 19 in the SC devices in 17, at suitable temperatures and for suitable times.

Hydrothermal treatment with pressurised water 19 in the SC devices, combined with the hydraulic and mechanical shearing forces acting in said devices, besides separating inert fractions from the organic matter, produces a pasty material that can be handled, is uniform, has reduced volume and weight, is sanitised and sterilised and has a high potential of carbon-to-carbon chemical bonds and a low odorous impact. That material can be advantageously used, upon a treatment for eliminating inorganic inert materials and non-digestible substances, in order to feed third-party plants for the production of biogas through anaerobic digestion

As described in the accompanying Example, the methanogenic potential of the material treated according to the hydrothermal process of the present invention has proven to be significantly higher than the methanogenic potential of the same, unprocessed material. Due to its characteristics, such a material can replace food crops for the production of biogas, either alone or co-digested with other matrices, such as sewage and dejects coming from farms for intensive animal breeding.

Moreover, hydrothermal treatment of OFMSW with pressurised hot water 19 produces saturated steam that can be extracted and condensed in exchanger 15, thereby providing a liquid rich in nutrients such as carbohydrates, proteins, alcohols, fats, organic acids and other micronized organic substances, which can be advantageously used for feeding fast biodigester plant 6. Laboratory tests have shown that such a liquid is excellent in terms of yield and quality of the anaerobic fermentation: the net production of biogas takes place in few days, preferably in about five days.

In an embodiment of the process of the present invention, treatment of OFMSW in the SC devices in 17 includes the following steps:

a. loading, in which a predetermined amount, in weight, of OFMSW is transferred from the conveyor belt into the SC device;

b. closing, in which the SC device automatically closes, once loading is complete;

c. introducing pressurised hot water, in which pressurised hot water 19, preferably at 250°C and about 40 bars, coming from boiler 16, is introduced into the SC device; d. hydrothermal treatment, in which the mass is treated at a temperature preferably in the range 150°C to 170°C for about 30 minutes; under such conditions, also sterilisation and stabilisation of the treated material, which no longer can undergo putrefactive phenomena, are achieved;

e. venting, in which pressure is restored down to atmospheric pressure and temperature is lowered, preferably below 100°C, thereby enabling the subsequent safe opening of the SC device; the saturated steam extracted, rich in micronized nutrients; yields heat in exchanger 15 and, after condensation, it is sent to pre-tank 5 feeding fast biodigester plant 6;

f. opening, in which the SC device automatically opens;

g. unloading the treated mass; once unloading is complete, the cycle starts again from step a.

In a preferred embodiment of the process of the present invention, loading takes place automatically, through the following steps:

1. loading the predetermined amount of OFMSW into the first SC device in accordance with the diagram shown in the accompanying Figure;

2. once the material has been loaded into the first SC device, diverting the conveyor belt in order to supply the second SC device with the same amount;

3. going on with the sequence according to the operations of steps 1 and 2 until the other SC devices have been loaded.

More particularly, in a preferred embodiment of the process of the present invention, six SC devices are sequentially fed in the period of about 1 hour elapsing between the loading and the unloading of the first SC device in 17.

In a preferred embodiment of the process of the present invention, the loading and the unloading of the SC devices take place by keeping the blades in a slight rotation in order to assist in spreading and handling the mass. Optionally, unloading of the SC chambers can be facilitated by means of a small jet of recovery water coming from accumulation tank 14.

In a preferred embodiment of the process of the present invention, the predetermined OFMSW amount, in weight, transferred to each SC device is one ton

The SC device only needs recycled water, coming from water treatment in 13. The water is then pre-heated in exchanger 15 and converted into pressurised hot water by means of boiler 16.

In a sixth aspect of the process of the present invention, the material unloaded from the SC devices is sent, by means of a conveying and lamination plate, to a conveyor belt that carries it to solid-liquid separator 20, through which the material being processed is divided into a liquid-pasty flow 21 and a solid flow 22.

In a preferred embodiment of the process of the present invention, liquid-pasty flow 21, comprising mixed organic materials and fine inert materials, is divided by means of a settler 23 into fine solid inert materials 26, e.g. glass, destined for a subsequent collection, and organic pasty material 24, having a high methanogenic potential thanks to the hydrothermal treatment it has been submitted to, which is collected by means of pumping, e.g. by means of a lobe pump, and sent, by means of tank trucks 25, to third-party plants for the production of biogas. Excess water 34 from settler device 23 is sent to pre-tank 5 and hence destined for fast biodigester plant 6.

In a preferred embodiment of the process of the present invention, solid flow 22, containing non-biodegradable material, metals, plastics and inorganic inert materials, is collected in 27 and divided into its merchandising components by means of techniques known per se. Solid flow 22 is transferred to a conveyor belt, and ferrous and non-ferrous metals are recovered by means of ferrous metal separator 28 and non-ferrous metal separator 29 and sent to collection skips 30 and 31 for being recycled. Remaining material 29a, essentially including inorganic inert materials and plastics, is sent to waste material containers 32 and hence to collection 33.

Example

A heterogeneous organic material has been prepared by mixing 2.5 kg of bread, 2.5 kg of vegetables, 2.5 kg of fruit waste and 2.5 kg of cooked pasta. The matrix produced in this manner has been suitably divided into portions of 2 kg each, packaged in ultra-light and compostable bags. The bags have been kept at 28 - 30°C and 60 - 80% relative humidity (values that are typically attained in summer season) and then used for the tests in the subsequent 48 hours,

The results of the tests of methanogenic potential, carried out in anaerobic digestion according to standard UNI EN ISO 11734/2004, are reported in the following Table, which shows a higher methanogenic potential in the material treated according to the present invention.

More particularly, in case of hydrothermal treatment at 150°C for 15 min, without separation of the phases obtained (liquid material from organic pasty material), a 43% increase in methane production has been detected.

Industrial applicability

The plant for carrying out the process of the present invention in accordance with the preferred embodiment thereof has a capacity of about 30,000 tons/year of OFMSW treated, corresponding to 100 tons/day, sufficient to serve a urban conglomeration of about 250,000 inhabitants, considering a per capita OFMSW production of about 110 Kg/year per inhabitant. The plant occupies about 7,000 square metres, i.e. about one fifth of a prior art composting plant.

Claims

Patent claims

1. A hydrothermal process for treating and recycling a solid waste including a wet and putrescible organic fraction, said process comprising an initial step of collecting and sending the leachate, obtainable from the discharging and handling of said waste, to a fast biodigester plant (6), capable of generating biogas.

2. The process according to Claim 1, wherein said waste is the organic fraction of municipal solid waste (OFMSW).

3. The process according to Claim 1, wherein the hydrothermal treatment of said waste comprises the following steps:

a. loading, in which the waste is transferred into a treatment device;

b. closing, in which the treatment device automatically closes, once the loading is complete;

c. introducing pressurised hot water, in which pressurised hot water is introduced into the treatment device at about 250°C and about 40 bar;

d. hydrothermal treatment, in which the waste mass in the treatment device is treated at a temperature in the range 150°C to 170°C for about 30 minutes;

e. venting, in which the pressure is restored down to atmospheric pressure and the temperature is lowered below 100°C;

f. opening, in which the treatment device automatically opens;

g. unloading the treated mass.

4. The process according to any one of Claims 1 to 3, wherein hydraulic and mechanical shearing forces working in combination with the hydrothermal treatment of said waste promote the separation of organic pasty material from solid inert material.

5. The process according to any one of Claims 1 to 3, wherein saturated steam generated during the hydrothermal treatment of said waste is condensed and sent to the fast biodigester plant.

6. The process according to any one of Claims 1 to 3 and Claim 5, wherein the biogas produced by the fast biodigester plant is used as energy source for producing pressurised hot water aimed at the hydrothermal treatment of said waste.

7. The process according to Claim 6, wherein the net production of biogas from the fast biodigester plant occurs in about five days.

8. The process according to Claim 6, wherein the water used for producing pressurised hot water comes from the depuration of liquid digestate leaving the fast biodigester plant.

9. The process according to Claim 8, wherein the water used for producing pressurised hot water is pre-heated, by thermal exchange with the saturated steam coming from the hydrothermal treatment of said waste.

10. The process according to Claim 1 or Claim 4, characterized in that the material leaving the hydrothermal treatment of said waste undergoes handling and separation operations, thus generating excess liquid, and in that said excess liquid is sent to the fast biodigester plant.

11. The process according to Claim 4, characterized in that the material leaving the hydrothermal treatment of said waste undergoes handling and separation operations, thus generating said organic pasty material having high methanogenic potential.

12. The process according to Claim 11, characterized in that said organic pasty material is collected and sent to third-party plants for the production of biogas.

13. The process according to any preceding claim, wherein the waste is discharged in a receiving area confined in a restricted space with reduced size kept under a small negative pressure to prevent smells from diffusing to the outside.

14. The process according to any preceding claim, wherein the waste is loaded onto a dosing system (8) which supplies it to a conveyor belt in order it is forwarded to SC (separation chamber) devices (17) in a predetermined amount each said SC device is capable of receiving, and wherein the leachate (18) resulting from the handling of OFMSW in the dosing system and on the conveyor belt is recycled to a pre-tank (5) feeding the digester plant (6).

15. A plant for the hydrothermal treatment of OFMSW according to the process as claimed in claims 1 to 14.