ITCR20090003A1 - PHOTOCATALYTIC TREATMENT SYSTEM FOR REDUCTION OF NITROGEN CONTENT IN LIVESTOCK AND RELATIVE PLANT. - Google Patents

Description

DESCRIPTION

PHOTOCATALYTIC TREATMENT SYSTEM FOR THE REDUCTION OF THE NITROGEN CONTENT IN THE ZOOTECHNICAL WASTE AND RELATED PLANT

DESCRIPTION

The invention is aimed at the sewage treatment sector for the reduction of nitrogen content.

More in detail, the invention relates to a photocatalytic treatment system for reducing the nitrogen content in sewage, and is particularly (but not exclusively) addressed to the treatment of livestock waste.

The invention also relates to a plant for making said treatment system.

The high nitrogen content in the sewage, more or less treated with systems to reduce the content of pollutants, can cause eutrophication and acidification of the waters of the receiving water bodies and, if reused on the soil as fertilizers, can generate emissions in atmosphere of gaseous ammonia, molecular nitrogen and nitrous oxide, as well as the pollution of deep and surface waters following the leaching of nitrates.

The European Economic Community has addressed these problems by adopting the so-called Nitrates Directive (Directive 1991/676 / EEC), which aims to reduce and prevent water pollution, direct or indirect, caused by nitrates from agricultural sources. It requires Member States to identify polluted (or potentially polluted) waters, both surface and underground, and designate Vulnerable Zones; it also requires that they establish codes of good agricultural practice and apply action programs for Vulnerable Zones, imposing a limited nitrogen load on them, going from an annual quantity of 340 kgN / ha to 170 kgN / ha.

The Nitrates Directive was implemented in Italy with Legislative Decree 152/2006, but the Lombardy Region had already anticipated its implementation since 1993 with the L.R. 37 and its implementing regulation.

The techniques currently in use for the reduction of nitrogen content in livestock waste are essentially based on the containment of nitrogen excreted through the reduction of protein nitrogen in the diet or on the reduction of nitrogen in the slurry with physical, chemical and physical treatments and biological.

With regard to nitrogen reduction treatments in sewage, preferably associated with a rational management of the effluents, the following techniques are generally adopted:

1. Separation of coarse and fine solids from the liquid phase;

2. Stabilization of effluents;

3. Biological removal of nitrogen;

4. Extraction of nitrogen as a mineral fertilizer;

5. Energy production combined with nitrogen removal; 6. Constructed wetlands.

The defects of the techniques currently in use are the following:

1. Separation of coarse and fine solids from the liquid phase

It reduces nitrogen and phosphorus only if the paiable solid fraction is exported outside the company.

It requires storage tanks for the separated liquid and a platform for the storage of the pairable fraction. It also requires equipment for the distribution of palatable solids.

The removal yields depend on the solids content of the incoming slurry.

The separated product is not always coupled and therefore, especially for the separation of fine solids, it may be necessary to use additives to improve the effectiveness of the separation, with an increase in cost to be carefully considered.

Nitrogen in ammonia form is not separated.

Obtainable nitrogen reduction: 4-35%.

2. Stabilization of effluents

The goal is to improve the manageability of the effluents, but not specifically the removal of nitrogen. The possible reduction of nitrogen occurs by volatilization of ammonia and therefore it is necessary to minimize the quantity emitted into the atmosphere, possibly filtering the exhausted air.

In these treatments in which ammonia and other volatile compounds can be released, it is necessary to check the compatibility with the regulations for the emissions and the necessary authorizations.

Energy consumption can be high. The reduction of the humidity of the effluent is rarely feasible for liquid effluents. In the production of compost, the exhaust air must be filtered by removing ammonia.

Obtainable nitrogen reduction: 0-20%.

3. Biological removal of nitrogen

This type of treatment is suitable for livestock farms that do not find other solutions to the management of nitrogen surpluses. The high investment and management costs make these systems very expensive and their choice must be evaluated wisely and with qualified technical support.

Energy consumption is high. Overall nitrogen removal efficiencies do not exceed 70%. The remaining portion must be managed agronomically or exported outside the company. Obtainable nitrogen reduction: 50-70%.

4. Extraction of nitrogen as a mineral fertilizer

Conservative techniques are adopted: nitrogen and phosphorus are separated and concentrated, but not eliminated.

Preliminary separation produces organic paiable solids that must be managed and enhanced.

The efficiencies of nitrogen removal in mineral form are 45-65%. The remaining portion is partly in the liquid effluent and partly in the paiable fraction which must be appropriately managed.

Obtainable nitrogen reduction: 45-65%.

5. Energy production combined with nitrogen removal The only production of biogas and energy from renewable sources is obtainable both with sewage and with paiable effluents. However, it is a technique aimed at obtaining energy and not at reducing the nitrogen content.

Thermal exploitation is only feasible with dry material. Attention must be paid to the emissions into the air that must be controlled, both in thermal processes and in the production and use of biogas.

Therefore, combined techniques are adopted, such as the stripping of ammonia from the previously heated sewage thanks to the insufflation of air.

The ammonia is then salified and captured with an acid solution in a washing tower, to prevent its dispersion in the environment and produce ammonium sulphate.

The production of paiable material requires careful evaluation and management which may include the eventual export outside the company.

Obtainable nitrogen reduction: 45-70%.

6. Constructed wetlands

Constructed wetlands cannot be fed with sewage as such but only after an effective treatment to reduce the organic load and nitrogen.

It is therefore suitable for sewage that has undergone a preliminary treatment. It requires large surfaces for the installation of the system. Nitrogen reduction achievable: 20-40%

More innovative liquid treatment systems are also known to obtain a reduction in the content of organic and inorganic pollutants in the same, such as, for example, the photocatalytic treatment of wastewater, based on a process that has recently had extensive development also for air purification.

Photocatalysis with semiconductors is based on the absorption of light radiation by the semiconductor which, thanks to its peculiar electronic structure, leads to the formation on its surface of strongly oxidizing and strongly reducing species, able to decompose the organic and inorganic substances present in the atmosphere and in the water.

The photocatalysis process takes place without the photocatalyst (for example, titanium dioxide, Ti02) undergoing chemical transformations; this guarantees a continuous and constant effectiveness of the process over time.

The photocatalytic process therefore begins with the exposure of a semiconductor, which acts as a photocatalyst, to light radiation having an energy equal to or greater than the band gap of the semiconductor itself. Following absorption of photons, electrons in the semiconductor are photopromoted from the valence band to the conduction band, leaving behind vacancies or holes in the valence band. Thus pairs of mobile charge carriers are formed, the electrons in the conduction band and the holes in the valence band. One of the most important characteristics of ΊΊΟ2, the semiconductor oxide that is most widely used as a photocatalyst, given its peculiar ability to absorb radiation without undergoing photocorrosion phenomena, is that the oxidizing power of the photogenerated holes in the valence band, which tend to be filled by species adsorbed on the surface of the photocatalyst and which in turn oxidize - for example water or hydroxyl anions, which give strongly oxidizing OH radicals - is greater than the reducing power of the electrons excited in the conduction band (A.Fujishima, T.N.Rao, D.A.Tryk, J. Photochem. Photobiol. C: Photochem. Rev.1 (2000) 1). In contact with air, the latter typically interact with molecular oxygen, to give the superoxide anion, in turn a weak oxidant, capable of combining with organic compounds.

Numerous research groups, worldwide, in the last two decades have been involved in the kinetic study of the photodegradation processes of organic and inorganic compounds in gaseous medium or in solution. As pollutants, phenols, nitrophenols, anilines, free and complexed cyanides, pesticides, drugs and various volatile organic compounds were examined, for example. The speed of photo-oxidation of organic substrates has been extensively studied as a function of some experimental parameters, such as the initial concentration of the substrate, the amount of photocatalyst, the initial pH, the power of irradiation, the concentration of oxygen present. . Rate expressions, reaction paths and kinetic models derived from experimental data can be used to predict the feasibility of a process.

Japanese researchers from the Department of Applied Chemistry of the University of Tokyo have recently developed a pilot reactor for the degradation of non-ionic surfactants - such as, for example. polyoxyethylene - based on the photocatalytic reaction generated by Ti02 nanoparticles activated by ultraviolet light. The reaction, whose speed is correctable with the surface of the titanium dioxide reached by the radiation, has shown that high concentrations of surfactants present in the water can be totally eliminated (photomineralized) even by irradiation with the ultraviolet light fraction normally present in the spectrum. solar.

The remarkable ability to disinfect waste water using photocatalytic processes has been amply demonstrated: the irradiation for 30 minutes of water containing high concentrations of coliforms in the presence of photocatalytic surfaces based on Ti02 has led to the complete inactivation of microbial strains.

Very few studies have been carried out on the photocatalytic abatement of ammonia both in aqueous solution and in the gaseous phase. In particular, ammonia can be photo-oxidized on Ti02 in aqueous solution (Murgia et al., 2005) and in the atmosphere (Levine and Calvert, 1976; llâ € ™ chenko and Golodets, 1975), in the latter case with formation of N2, N20 or NO.

In a recent experimental study, J.Lee, H. Park, W.Chai, Envirom. Sci. Technol. 36 (2002) 5462, argue that while the photocatalytic oxidation in aqueous phase of NH3a N02 <'> / N03 <'> is the only reaction path for NH3 on unmodified Ti02, on modified titanium dioxide on the surface by deposition of platinum a new reaction path opens which leads to the selective oxidation of ammonia to molecular nitrogen. Furthermore, the photocatalytic conversion of NH3a to N2 greatly increases when the system is saturated with N20, with conversions of ammonia nitrogen into N2 higher than 80%. Therefore the selective photocatalytic oxidation of NH3a N2gaseous has considerable potential as a method for controlling the levels of ammonia in the aqueous phase.

However, no one has studied a photocatalytic treatment system with modified titanium dioxide to reduce the nitrogen content in livestock manure. This also derives from the fact that in the past livestock slurry has always been considered a source of nutrients for crops. With the introduction of the Community directive on nitrates and the high concentration of animal husbandry in vulnerable areas, livestock sewage has been transformed from a resource into a problem to be addressed in order to fall within the parameters indicated by the Nitrates Directive.

The purpose of the invention is to define a photocatalytic treatment system for the reduction of nitrogen content in livestock manure that can lead to a selective photocatalytic oxidation of NH3a N2, transforming nitrogen compounds into nitrogen gas, minimizing the formation of nitrogen oxides in the gaseous phase and of nitrites / nitrates in the aqueous phase, to constitute a very effective solution to the problem of excess nitrogen in animal husbandry.

A further purpose of the invention is to create a treatment plant that can easily adapt to the reality of farms, which is modular, which can be integrated with anaerobic treatment systems for sewage used to produce energy from biogas, which is with limited investment, energy and management costs.

These purposes are achieved with a photocatalytic treatment system for reducing the nitrogen content in zootechnical waste and relative plant as claimed in the characterizing portion of the independent claims.

Variants and alternative embodiments of the invention are specified in the dependent claims.

The advantages of the invention essentially consist in the fact that a significant reduction of the nitrogen content in livestock manure is obtained, and in particular of ammonium hydroxide, with a simple, economical, easy to manage system, suitable to be combined with other treatment systems in use, such as those for the production of biogas and energy.

Another advantage consists in the fact that, thanks to the selectivity of the process, the risk of moving the environmental problem from one resource (water) to another (air) is eliminated.

These and other advantages of the invention will be more evident in the following, in which a preferred embodiment is described, by way of non-limiting example, and with the help of the figures, where:

Fig. 1 represents the graph, obtained in the laboratory under exemplary conditions, of the trend of the percentage conversion of ammonia, of the selectivity to N2, nitrites and nitrates during the irradiation time with an external lamp that emits light in the visible spectrum according to the invention;

Fig. 2 represents a diagram of a plant for reducing the nitrogen content in livestock waste which uses a photocatalytic treatment system according to the invention, in which the reactor is illustrated according to a cross section in the vertical plane;

Fig. 3 represents a variant of the diagram of Fig. 1.

With reference to Fig. 1, ammonia abatement tests in aqueous suspensions containing a suitably modified titanium dioxide-based photocatalyst were carried out in the laboratory, starting from an initial concentration of ammonium hydroxide equal to 121 ppm (corresponding to 100 ppm of nitrogen). In particular, platinum was used in the experimentation to modify the titanium dioxide.

More specifically, titanium dioxide (active phase, or catalyst) has been modified by surface deposition of platinum metallic nanoparticles (cocatalyst) in a weight ratio between the two equal to 0.5%.

The synthesis was obtained in a single stage by means of a pyrolysis technique in a spray flame.

The modified titanium dioxide was deposited in a thin film on the surfaces to be made active, subjected to a thermal treatment of calcination and subsequent reduction in hydrogen.

In particular, in order to fix the film on the support surface and reactivate the platinum, the film itself was subjected to a thermal treatment of calcination at 400 ° C and subsequent reduction to 150 ° C in hydrogen. In general, the photoactive phase can be comprised of a superficially and / or massively modified semiconductor, or combined with other semiconductors.

As an alternative to platinum it is possible to use ruthenium, gold, palladium or other pure noble metals or as an alloy.

The treatment system for reducing the nitrogen content in livestock manure according to the invention therefore provides for the following phases: â € ¢ separation of the coarse solids from the sewage as such by filtration;

â € ¢ thermostating of the separated sewage at a predetermined temperature;

â € ¢ ventilation with forced air flow;

â € ¢ contact of the aerated sewage for a predefined time with a photoactive phase composed of a modified semiconductor;

â € ¢ exposure of the slurry to light radiation.

The light radiation can be of the ultraviolet type or in the visible spectrum originating from lamps, or directly from sunlight.

The photocatalytic tests were carried out at a constant temperature of 30 ° C. The process can work up to 60 ° C.

The working pH is essentially that typical of livestock slurry, that is, between 7 and 12, in particular close to the value of 10.

In particular, the influence of the following parameters has been studied: - type of light source;

- concentration of oxygen in the bubbling gas.

During the irradiation time, the concentration in the aqueous phase of the ammonium (NH4 <+>), nitrite (NO2 <'>) and nitrate (NO3 <'>) ions was determined by ion chromatography, the latter being possible products of photo-oxidation, in addition to molecular nitrogen (desired product).

Two different types of lamps were used: visible light (Vis) and ultraviolet light (UV).

The UV lamp is preferably used in immersion, and is of the mercury vapor type with monochromatic emission at 250 nm.

The Vis lamp is used as an external lamp and emits polychromatic white light with an emission spectrum similar to the solar one (solar light simulator). In principle, sunlight can also be used as an external source.

Table I shows the results obtained after 4 hours of irradiation with the two lamps, in terms of percentage conversion of ammonia (CNH3), selectivity to molecular nitrogen (SN2), selectivity to nitrites (SNO2 <'>) and selectivity to nitrates (SNO3 <'>).

Table I.

Gas lamp CNH3SN2SN02 <'> SN03 <'>

(%) (%) (%) (%)

UV 0238 6.0 56.2 37.8

pure

UV Aria 36 23.5 42.1 34.4 Vis Aria 20 85.3 8.4 6.3 The results obtained using the UV lamp show that in these conditions a higher conversion of ammonium is obtained compared to the Vis lamp, while the concentration of oxygen in the bubbling gas (and consequence of the dissolved oxygen in the irradiated suspension) does not significantly influence the conversion of ammonia, but it has been surprisingly verified that on the contrary it greatly influences the selectivity to molecular nitrogen. In fact, for example, by using the UV lamp and bubbling pure oxygen, only 6% of selectivity to nitrogen was obtained, while bubbling air (which contains only 20% of oxygen in nitrogen) the selectivity increased to 23.5%. Significantly higher nitrogen selectivities were instead obtained with the use of the Vis lamp.

It is important to underline that from the analyzes carried out at various times of irradiation of the suspensions, the results of which are illustrated in Fig. 1, it is evident that the conversion of ammonia increases linearly during irradiation, while the selectivity to nitrogen, nitrites and nitrates remain substantially constant over time.

With reference now to Fig. 2, a plant is shown that uses the photocatalytic treatment system for reducing the nitrogen content in livestock manure according to a first way of realizing the invention, comprising:

â € ¢ a reactor 1 provided with means for generating light radiation 2 and with photocatalyst means composed of a modified semiconductor, able to contain for a predetermined time the slurry to be treated, containing nitrogen in organic form, previously separated from the coarse solids by filtration;

â € ¢ means 3, 4, 5 for the insufflation of air inside the reactor; â € ¢ heat exchanger means 6 connected to heat generation means 7;

â € ¢ means of discharge 8, 9 of nitrogen and gaseous compounds;

â € ¢ means for feeding 10 and discharging 11 the slurry to be treated. The reactor consists of a cylindrical, parallelepiped or other suitable shape tank, preferably closed at the top, so as not to allow the uncontrolled exit of gases and odors. The upper part of the tank is connected to a feed pipe 10 for the sewage to be treated and the lower part is connected to a discharge pipe 11 of the treated sewage, for conveying to storage tanks, so that the reactor always remains full of sewage.

The means for generating light radiation comprise a UV lamp 12, of the mercury vapor type with emission below 400 nm, connected to an electric power supply circuit by means of a cable 13. The lamp is placed below the level of the sewage and is contained in a transparent protective tube 14 made of glass or quartz, so that it does not come into direct contact with the sewage itself.

The reactor 1 can comprise a plurality of means for the generation of light radiation arranged according to a suitable grating.

The photocatalyst means comprise titanium dioxide, also modified on the surface or massively (for example by selective doping), or combined with other semiconductors and prepared according to different techniques.

Said photocatalyst means are deposited in thin film on the surface of a rigid support by means of different techniques and subjected to thermal treatment of calcination and subsequent reduction in hydrogen.

Said support can be constituted directly by the glass wall of the protective casing 14 of the lamp 12,

filling material of the reactor 1, such as spheres, polyhedra or the like. The means for blowing in air essentially comprise a compressor 3, transport pipes 4 and diffusion means 5 of a known type.

The heat exchanger means comprise a coil 6 placed in the reactor 1, inside which circulates a heat transfer fluid which is heated by heat generation means 7, fed with traditional fuel or biogas. The fluid is set in motion by a pump 15 inserted in the heating circuit 16.

The same result can be obtained both with external heat exchanger means and, if the slurry temperature is sufficient, in the absence of heating means.

The means for discharging the nitrogen and gases released from the sewage comprise an exhalation pipe 8 and a filter 9 for treating ammonia that may be stripped from the sewage due to the insufflation of air. The filter can in turn be of the type with a photocatalytic effect, in which a lamp 17 activates a photocatalytic cell 18 operating in the gas-solid phase, containing for example titanium dioxide placed on a rigid support.

In a variant of the plant illustrated in Fig. 3, downstream of reactor 1 there is a further reactor 100, comprising:

â € ¢ a channel 101 coated on the bottom with a photoactive film, comprising photocatalyst means composed of a semiconductor modified on the surface by deposition of a noble metal, able to remain in contact for a predetermined time with the sewage to be treated containing nitrogen in organic form, previously separated from coarse solids, aerated and thermostated;

â € ¢ means for generating light radiation 102;

â € ¢ means 108 for discharging nitrogen gas;

means for feeding 110 and discharging 111 of the sewage to be treated.

In a further variant not shown, reactor 100 can be used without reactor 1.

Naturally, the details of construction and the embodiments may be widely varied with respect to what has been described and illustrated, without thereby departing from the scope of the present invention, as described, illustrated and claimed.

Claims (17)

CLAIMS 1. Photocatalytic treatment system for the reduction of nitrogen content in livestock manure characterized by the fact that it includes the following phases: â € ¢ separation of the coarse solids from the sewage as it is by filtration; â € ¢ thermostating of the separated sewage at a predetermined temperature; â € ¢ aeration with air flow; â € ¢ contact of the aerated sewage for a predefined time with a photoactive phase composed of a modified semiconductor; â € ¢ exposure of the slurry to light radiation. 2. Treatment system according to claim 1, characterized in that said semiconductor is modified superficially and / or massively, or by combination with other semiconductors. 3. Treatment system according to claim 1, characterized in that said photoactive phase comprises a titanium dioxide-based catalyst. 4. Treatment system according to claim 1, characterized in that said photoactive phase comprises a titanium dioxide-based catalyst also doped with suitable metals or non-metals, as well as combined with other semiconductors, prepared according to any technique. 5. Treatment system according to claim 2, characterized in that said surface modification consists in the deposition of a noble metal selected from platinum, ruthenium, gold, palladium or other pure noble metal or as an alloy. 6. Treatment system according to claim 5, characterized in that said deposition process is obtained in a single stage by means of a pyrolysis technique in a spray flame. 7. Treatment system according to claim 1, characterized in that said photoactive phase is deposited in a thin film on surfaces to be made active. 8. Treatment system according to claim 7, characterized in that said thin film is subjected to a thermal treatment of calcination and subsequent reduction in hydrogen. 9. Treatment system according to claim 1, characterized in that said light radiation is contained in the spectrum of sunlight. 10. Use of the photocatalytic treatment system according to the preceding claims for reducing the nitrogen content to treat livestock slurry 11. Photocatalytic treatment plant for the reduction of nitrogen content in livestock manure comprising: a reactor (1) equipped with light radiation generating means (2) and photocatalyst means composed of a modified semiconductor, able to contain for a predetermined time the slurry to be treated, containing nitrogen in organic form, previously separated from the coarse solids by filtration ; â € ¢ means (3, 4, 5) for the insufflation of air inside the reactor; â € ¢ heat exchanger means (6) connected to heat generation means (7); â € ¢ means of discharge (8, 9) of nitrogen and gaseous compounds; â € ¢ means for feeding (10) and discharging (11) the sewage to be treated. 12. Plant according to claim 11, characterized in that said light radiation generating means comprise a UV lamp (12), located below the level of the sewage, contained in a transparent protective sheath tube (14). 13. Plant according to claim 11, characterized in that said modified photocatalyst means are deposited in thin film on the surface of the protective casing (14) of the lamp (12). 14. Plant according to claim 11, characterized in that said means for discharging the nitrogen and gases released by the sewage comprise a filter (9) of the type with a photocatalytic effect. 15. Photocatalytic treatment plant for the reduction of nitrogen content in livestock manure comprising: â € ¢ a channel (101) coated on the bottom with a photoactive film, comprising photocatalyst means composed of a modified semiconductor, able to remain in contact for a predetermined time with the slurry to be treated containing nitrogen in organic form, previously separated from the coarse solids , ventilated and thermostated; â € ¢ means for generating light radiation (102); â € ¢ means of discharge (108) of the gaseous nitrogen; â € ¢ means for feeding (110) and discharging (111) the sewage to be treated. 16. System according to claim 15, characterized in that said radiating means for generating light radiation (102) comprise an external lamp Vis with reflector, which emits polychromatic white light with an emission spectrum similar to that of the sun. 17. Plant according to claims 11 and 15, characterized in that it comprises a reactor (1) and a channel (101) arranged in series.