WO2010086891A1

WO2010086891A1 - Photocatalytic treatment system and plant for reducing the nitrogen content in livestock waste

Info

Publication number: WO2010086891A1
Application number: PCT/IT2010/000023
Authority: WO
Inventors: Gian Luca Chiarello; Annamaria Costa; Marcella Guarino; Elena Selli
Original assignee: Rota Guido Srl; Regione Lombardia; Universita' Degli Studi Di Milano
Priority date: 2009-01-30
Filing date: 2010-01-28
Publication date: 2010-08-05
Also published as: IT1398144B1; ITCR20090003A1

Abstract

The invention relates to a photocatalytic treatment system for reducing the nitrogen content in livestock waste comprising the following steps: • separation of the gross solids from the sewage as is by filtration; • temperature control of the separated sewage at a preset temperature; • aeration with air flow; • contact of the aerated sewage for a preset time with a photoactive phase composed of a modified semiconductor; • exposure of the sewage to light radiation. The invention also relates to a photocatalytic treatment plant for reducing the nitrogen content in livestock waste comprising: • a reactor (1 ) provided with light radiation generating means (2) and photocatalyst means composed of a modified semiconductor, suitable to contain for a preset time the sewage to be treated, containing nitrogen in organic form, previously separated from the gross solids by means of filtration; • means (3, 4, 5) for the insufflation of air into the reactor; • heat exchanger means (6) connected to heat generating means (7); • discharge means (8, 9) of the nitrogen and of the gaseous compounds; • feed (10) and discharge (11) means of the sewage to be treated.

Description

PHOTOCATALYTIC TREATMENT SYSTEM AND PLANT FOR REDUCING THE NITROGEN CONTENT IN LIVESTOCK WASTE

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The invention relates to the sector of sewage treatment for reducing the nitrogen content.

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

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

The high content of nitrogen in sewage, treated to a greater or lesser extent with systems for reducing the content of polluting substances, can cause phenomena of eutrophication and acidification of the waters in the receiving water bodies and, if re-used on the ground as fertilisers, can generate emission into the atmosphere of gaseous ammonia, of molecular nitrogen and of nitrous oxide, as well as pollution of groundwaters and surface waters due to nitrate leaching.

The European Community has tackled these problems by adopting the Nitrates Directive (Directive 1991/676/EC), the objective of which is to reduce and prevent direct or indirect water pollution caused by nitrates from agricultural sources. It requires member States to identify waters that are polluted (or that might be polluted), both surface water and groundwater, and to designate Vulnerable Zones; it also requires them to establish codes of good agricultural practice and to implement action plans for the Vulnerable Zones, setting nitrogen loading limits for these Zones decreasing from an annual quantity of 340 kgN/ha to 170 kgN/ha.

The Nitrates Directive was incorporated in Italy with Legislative Decree 152/2006, but the Lombardy Region had already incorporated it in advance since 1993 with Regional Law 37 and its implementing regulation.

The techniques currently in use for reducing the nitrogen content in livestock waste are in substance based on containing the nitrogen excreted through reducing protein nitrogen in the diet or reducing the nitrogen in sewage with physical, chemical-physical and biological treatments.

In relation to treatments for reducing the nitrogen in sewage, preferably associated with rational effluent management, the following techniques are generally adopted:

1. separation of the gross and fine solids from the liquid phase;

2. effluent stabilisation;

3. biological removal of nitrogen;

4. extraction of nitrogen as mineral fertilizer; 5. energy production combined with removal of nitrogen; 6. phyto purification.

Prior art techniques have the following defects: 1. Separation of the gross and fine solids from the liquid phase

This reduces nitrogen and phosphate only if the solid fraction is removed from the farm. It requires storage tanks for the separated liquid and a concrete bed to store the solid fraction. It also requires equipment for distribution of the solids.

The removal yields depend on the solid content of the incoming sewage.

The separated product is not always solid and therefore, above all for separation of fine solids, it may be necessary to use additives to improve the efficacy of separation, with a cost increase to be carefully considered. Nitrogen in ammonia form is not separated.

Nitrogen reduction obtainable: 4-35%. 2. Effluent stabilization

The aim is to improve effluent management, but not specifically to remove nitrogen. Any reduction in nitrogen occurs through volatilization of the ammonia and therefore it is necessary to minimize the quantity emitted into the atmosphere, optionally by filtering exhaust air.

In these treatments in which ammonia and other volatile compounds can be released, compatibility with the regulations for emissions and the necessary authorizations must be verified.

Energy consumptions can be high. Reduction of the humidity of the effluent is rarely feasible for liquid effluents. In the production of compost the exhaust air must be filtered to remove ammonia.

Nitrogen reduction obtainable: 0-20%. 3. Biological removal of nitrogen This type of treatment is suitable for livestock farms that cannot find other solutions to manage nitrogen in excess. High investment and management costs make these plants very costly and their choice must be carefully evaluated with qualified technical support. Energy consumptions are high. Overall effectiveness of nitrogen removal does not exceed 70%. The remaining percentage must be agriculturally managed or removed from the farm.

Nitrogen reduction obtainable: 50-70%.

4. Extraction of nitrogen as mineral fertilizer Conservative techniques are adopted: the nitrogen and phosphate are separated and concentrated, but not eliminated.

Preliminary separation produces organic solids that must be managed and valorised.

Effectiveness of nitrogen removal in mineral form is of 45-65%. The remaining percentage is partially in the liquid effluent and partly in the solid fraction, which must be appropriately managed. Nitrogen reduction obtainable: 45-65%.

5. Production of energy combined with nitrogen removal Production of only biogas and energy from a renewable source is obtainable both with sewage and with solid effluents. However, this is a technique aimed at obtaining energy and not at reducing the nitrogen content.

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

Consequently, combined techniques are adopted, such as ammonia stripping from sewage previously heated by insufflation of air. The ammonia is then salified and trapped with an acid solution in a scrubbing tower, to prevent dispersion thereof into the environment and to produce ammonium sulphate.

The production of solid material requires careful evaluation and management which if necessary can include removal from the farm. Nitrogen reduction obtainable: 45-70%. 6. Phyto purification

Phyto purification cannot be fed with sewage as is but only after effective treatment to reduce organic load and nitrogen.

Therefore, it is suitable for sewage that has been subjected to a preliminary treatment. It requires extensive areas for installation of the plant.

Nitrogen reduction obtainable: 20-40%

More innovative systems for liquid treatment are known to obtain a reduction of their content of organic and inorganic polluting substances, such as photocatalytic treatment of waste waters, based on a process that has recently been widely developed also for air purification.

Semiconductor photocatalysis is based on the absorption of light radiation by the semiconductor which, due to its particular electronic structure, causes the formation on the surface thereof of highly oxidizing and greatly reducing species, capable of decomposing organic and inorganic substances present in the atmosphere and in water.

The photocatalysis process takes place without the photocatalyst (for example titanium dioxide, TiO₂) undergoing chemical transformations; this ensures continuous and constant process efficacy.

The photocatalytic process therefore starts by exposing a semiconductor, acting as photocatalyst, to light radiation with energy equal to or greater than the band gap of the semiconductor. After absorbing photons, electrons in the semiconductor are photopromoted from the valence band to the conduction band, leaving behind vacancies or "holes" in the valence band. In this manner, 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 features of TiO₂, the semiconductor oxide most widely used as photocatalyst, given its particular capacities 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 adsorbed species on the surface of the photocatalyst and which are in turn oxidized - for example water or hydroxyl anions, which give highly oxidizing OH radicals - is greater than the reducing power of the excited electrons 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, these 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, on a global level, in the last two decades have been involved in the kinetic study of photodegradation processes of organic and inorganic compounds in a gaseous medium or in solution. As pollutants, phenols, nitrophenols, anilines, free and complex cyanides, pesticides, pharmaceuticals and various volatile organic compounds have, for example, been considered. The photo- oxidation speed of organic substrates has been widely studied as a function of some experimental parameters, such as initial concentration of the substrate, quantity of photocatalyst, initial pH, irradiation power, oxygen concentration present. The velocity expressions, reaction paths and kinetic models inferred from the 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 recently devised a pilot reactor for degradation of non ionic surfactants - such as polyoxyethylene - based on the photocatalytic reaction generated by TiO₂ nanoparticles activated by UV light. The reaction, whose velocity can be correlated to the surface of the titanium dioxide reached by the radiation, has shown that high concentrations of surfactants present in the waters can be totally eliminated (photomineralized) also by radiation with the fraction of ultraviolet light normally present in the solar spectrum.

The considerable capacities for disinfection of waste waters through photocatalytic processes has been widely proven: irradiation for 30 minutes of waters containing high concentrations of coliforms in the presence of photocatalytic surfaces based on TiO₂ has resulted in complete inactivation of the microbial strains.

Very few studies have been performed on photocatalytic reduction of ammonia, either in aqueous solution or in gaseous phase. In particular, ammonia can be photo-oxidized on TΪO2 in aqueous solution (Murgia et al., 2005) and in the atmosphere (Levine and Calvert, 1976; ll'chenko and Golodets, 1975), in the latter case with the formation of N₂, N₂O or NO. In a recent experimental study, J. Lee, H. Park, W.Chai, Envirom. Sci. Technol. 36 (2002) 5462, maintain that while photocatalytic oxidation in aqueous phase of NH₃ to NO₂VNO₃ ^" is the only reaction path for I¹NH₃ on unmodified TiO₂, a new reaction path opens on surface modified titanium dioxide by platinum deposition, which leads to selective oxidation of ammonia to molecular nitrogen. Moreover, photocatalytic conversion of NH₃ to N₂ considerably increases when the system is saturated with N₂O, with ammonia nitrogen conversions to N₂ of over 80%. Therefore, selective photocatalytic oxidation of NH₃ to gaseous N₂ has considerable potential as method to control ammonia levels in aqueous phase.

However, no-one has studied a system for photocatalytic treatment with modified titanium dioxide for reducing the nitrogen content in livestock waste. This is also due to the fact that in the past livestock waste was always considered a source of nutrients for crops. With the introduction of the EC nitrates Directive and due to the high concentration of livestock farms in vulnerable zones, livestock waste has been transformed from a resource into a problem to be tackled in order to fall within the parameters indicated in the Nitrates Directive.

Aim of the invention is to define a photocatalytic treatment system for reducing the nitrogen content in livestock waste that can lead to selective photocatalytic oxidation of NH₃ to N₂, transforming nitrogen compounds into gaseous nitrogen, minimizing the formation of nitrogen oxides in gaseous phase and of nitrites/nitrates in aqueous phase, to provide a very effective solution to the problem of excessive nitrogen in livestock farming.

A further aim of the invention is to produce a treatment plant which can be easily adapted to the situations found on farms, which is modular, can be integrated with anaerobic treatment systems of sewage used to produce energy from biogas, and with limited investment, energy and running costs.

These aims are achieved with a photocatalytic treatment system for reducing the nitrogen content in livestock 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 is obtained of the nitrogen content in livestock waste, and in particular of ammonium hydroxide, with a simple, inexpensive and easily managed system, suitable to be combined with other treatment systems in use, such as those for the production of biogas and energy.

A further advantage consists in the fact that, due 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 made clearer below with the description of a preferred embodiment, provided as a non-limiting example, and with the aid of the figures, wherein:

Fig. 1 represents the graph, obtained in the laboratory in exemplificative conditions, of the trend of the conversion percentage of ammonia, of the N₂, nitrite and nitrate selectivity during the irradiation time with an external light 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, wherein the reactor is shown 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 , tests were conducted in the laboratory to reduce the ammonia in aqueous suspensions containing a photocatalyst based on suitably modified titanium dioxide, starting from an initial concentration of ammonium hydroxide of 121 ppm (corresponding to 100 ppm of nitrogen). In particular, in the experiments platinum was used to modify the titanium dioxide.

Even more specifically, the titanium dioxide (active phase, or catalyst) was modified by means of surface deposition of metallic platinum nanoparticles (co-catalyst) in a ratio in weight between the two of 0.5%.

Synthesis was obtained in a single step by means of a flame spray pyrolysis technique. The modified titanium dioxide was deposited in a thin film on the surfaces to activate them, subjected to calcination heat treatment and subsequent reduction in hydrogen.

In particular, to fix the film to the support surface and reactivate the platinum, the film was subjected to calcination heat treatment at 400 ⁰C and subsequent reduction in hydrogen at 150 ⁰C.

In general, the photoactive phase can comprise a semiconductor, modified on the surface and/or in bulk, or combined with other semiconductors.

Alternatively to platinum, ruthenium, gold, palladium or other noble metals, pure or in an alloys, can be used.

The treatment system for reducing the nitrogen content in livestock waste according to the invention, thus comprises the following steps:

• separation of the gross solids from the sewage as is by filtration;

• temperature control of the separated sewage at a preset temperature;

• aeration with forced air flow;

• contact of the aerated sewage for a preset time with a photoactive phase composed of a modified semiconductor;

• exposure of the sewage to light radiation. The light radiation can be of ultraviolet type or in the visible spectrum generated by lamps, or can be sunlight.

The photocatalytic tests were carried out at a constant temperature of 30⁰C. The process can operate up to 60 °C.

The operating pH is substantially that typical of livestock waste, i.e. between 7 and 12, in particular in the vicinity of 10.

In particular, the influence of the following parameters was studied:

- type of light source;

- concentration of oxygen in the bubbling gas.

During the irradiation time, the concentration in the aqueous phase of ammonium (NH₄ ⁺), nitrite (NO₂ ^") and nitrate (NO₃ ^') ions was determined by means of ion chromatography, as these are possible products of photooxidation, in addition to molecular nitrogen (the desired product).

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

The UV lamp is preferably used immersed, and is of the mercury vapour type with monochromatic emission at 250 nm.

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

Table 1 shows the results obtained after 4 hours of radiation with the two lamps, in terms of percentage of conversion of the ammonia (CNH₃), molecular nitrogen selectivity (SN₂), nitrite selectivity (SNO₂ ^") and nitrate selectivity (SNO₃ ^"). Table I

The results obtained using the UV lamp show that, in these conditions, a greater conversion of ammonium is obtained with respect to the Vis lamp, while the concentration of oxygen in the bubbling gas (and consequently of the oxygen dissolved in the radiated suspension) does not significantly influence ammonia the conversion of ammonia, but it was surprisingly found that it instead greatly influences molecular nitrogen selectivity. In fact, for example, using the UV lamp and bubbling pure oxygen, nitrogen selectivity of only 6% was obtained, while when bubbling air (which contains only 20% of oxygen in nitrogen) selectivity increased to 23.5%. However, much higher nitrogen selectivities were obtained using the Vis lamp.

It is important to underscore that from the tests conducted at various irradiation times of the suspensions, the results of which are shown in Fig. 1 , it is evident that the conversion of ammonia increases linearly during irradiation, while the nitrogen, nitrite and nitrate selectivities remain substantially constant in time.

With reference now to Fig. 2, this shows a plant that uses the photocatalytic treatment system for reducing the nitrogen content in livestock waste according to a first embodiment of the invention, comprising:

• a reactor 1 provided with light radiation generating means 2 and photocatalyst means composed of a modified semiconductor, arranged for containing for a preset time the sewage to be treated, containing nitrogen in organic form, previously separated from the gross solids by means of filtration;

• means 3, 4, 5 for the insufflation of air into the reactor;

• heat exchanger means 6 connected to heat generating means 7;

• discharge means 8, 9 of the nitrogen and of the gaseous compounds;

• feed 10 and discharge 11 means of the sewage to be treated. The reactor is composed of a cylindrical, parallelepiped or other appropriately shaped tank, preferably closed at the top, to prevent uncontrolled leakage of gases and smells. The top part of the tank is connected to a feed pipe 10 of the sewage to be treated and the bottom part is connected to a discharge pipe 11 of the treated sewage, to convey it towards storage tanks, so that the reactor remains full of sewage at all times.

The light radiation generating means comprise a UV lamp 12, of the mercury vapour type with emission below 400 nm, connected to a power supply circuit through a cable 13. The lamp is positioned below the level of the sewage and is contained in a transparent protective sheath 14 made of glass or quartz, so that it does not come into direct contact with the sewage. The reactor 1 can comprise a plurality of light radiation generating means arranged according to an appropriate grid.

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

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

Said support can be composed directly of the glass wall of the protective sheath 14 of the lamp 12, or of filling material of the reactor 1 , such as spheres, polyhedrons or the like.

The air insufflation means essentially comprise a compressor 3, transport pipes 4 and diffusion means 5 of known type.

The heat exchange means comprise a coil 6 positioned in the reactor 1 , circulating inside which is a heat carrying fluid heated by heat generating means 7, fed with conventional fuel or biogas. The fluid is conveyed by a pump 15 arranged in the heating circuit 16.

The same result can be obtained both with external heat exchanger means and, if the temperature of the sewage is sufficient, without heating means.

The means for discharge of the nitrogen and of the gases released by the sewage comprise a outlet pipe 8 and a filter 9 for treatment of any ammonia stripped from sewage due to insufflation of air. The filter can in turn be of the type with photocatalytic effect, in which a lamp 17 activates a photocatalytic cell 18 operating in solid-gas phase, for example containing titanium dioxide positioned on a rigid support.

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

• 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, arranged for remaining in contact for a preset time with the sewage to be treated containing nitrogen in organic form, previously separated from the gross solids, aerated and temperature controlled; • light radiation generating means 102;

• discharge means 108 of the gaseous nitrogen;

• feed 110 and discharge 111 means of the sewage to be treated. In a further variant not shown, the reactor 100 can be used without the reactor 1. Naturally, the constructional details and the embodiments can be widely varied with respect to those described and illustrated, without however departing from the scope of the present invention, as described, illustrated and claimed.

Claims

1. Photocatalytic treatment system for reducing the nitrogen content in livestock waste characterized in that it comprises the following steps: • separation of the gross solids from the sewage as is by filtration;

• temperature control of the separated sewage at a preset temperature;

• aeration with air flow; • contact of the aerated sewage for a preset time with a photoactive phase composed of a modified semiconductor;

• exposure of the sewage to light radiation.

2. Treatment system according to claim 1 , characterized in that said semiconductor is modified on the surface and/or in bulk, or by combination with other semiconductors.

3. Treatment system according to claim 1 , characterized in that said photoactive phase comprises a catalyst based on titanium dioxide.

4. Treatment system according to claim 1 , characterized in that said photoactive phase comprises a catalyst based on titanium dioxide also doped with appropriate metals or non-metals, and 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 chosen from platinum, ruthenium, gold, palladium or other noble metal, pure or in an alloy.

6. Treatment system according to claim 5, characterized in that said deposition process is obtained in a single step by means of a flame spray pyrolysis technique.

7. Treatment system according to claim 1 , characterized in that said photoactive phase is deposited in a thin film on surfaces to be activated.

8. Treatment system according to claim 7, characterized in that said thin film is subjected to calcination heat treatment and subsequent reduction in hydrogen.

9. Treatment system according to claim 1 , characterized in that said light radiation is contained in the sunlight spectrum.

10. Use of the photocatalytic treatment system according to the previous claims for reducing the nitrogen content to treat livestock sewage.

1 1. Photocatalytic treatment plant for reducing the nitrogen content in livestock waste comprising:

• a reactor (1) provided with light radiation generating means (2) and photocatalyst means composed of a modified semiconductor, arranged for containing for a preset time the sewage to be treated, containing nitrogen in organic form, previously separated from the gross solids by means of filtration;

• means (3, 4, 5) for the insufflation of air into the reactor;

• heat exchanger means (6) connected to heat generating means

(7); • discharge means (8, 9) of the nitrogen and of the gaseous compounds;

• feed (10) and discharge (11) means of the sewage to be treated.

12. Plant according to claim 11 , characterized in that said light radiation generating means comprise a UV lamp (12), positioned below the level of the sewage, contained in transparent protective sheath (14).

13. Plant according to claim 11 , characterized in that said modified photocatalyst means are deposited in a thin film on the surface of the protective sheath (14) of the lamp (12).

14. Plant according to claim 11 , characterized in that said discharge means of the nitrogen and of the gases released from the sewage comprise a filter (9) of the type with photocata lytic effect.

15. Photocatalytic treatment plant for reducing the nitrogen content in livestock waste comprising: • a channel (101) coated on the bottom with a photoactive film, comprising photocatalyst means composed of a modified semiconductor, arranged for remaining in contact for a preset time with the sewage to be treated containing nitrogen in organic form, previously separated from the gross solids, aerated and temperature controlled;

• light radiation generating means (102);

• discharge means (108) of the gaseous nitrogen;

• feed (110) and discharge (111) means of the sewage to be treated.

16. Plant according to claim 15, characterized in that said light radiation generating means (102) comprise an external lamp Vis with reflector, which emits polychromatic white light with emission spectrum similar to sunlight.

17. Plant according to claims 11 and 15, characterized in that it comprises a reactor (1) and a channel (101) disposed in series.