FR2795398A1 - Treatment of pig slurry and other liquid agricultural wastes to remove nitrogen and phosphorous by use of bacteria and plants - Google Patents

Abstract

Liquid agricultural wastes are given a biological nitrification and denitrification, then phases are separated in a vessel (31) containing macrophytic plants (36). Liquid filters through a substrate (35) and permeable membrane (34) into a gravel layer (32) and is drained away (33 and 37). Solids (38) collect on the surface of the substrate and are dried by circulating air.

Description

The present invention relates to a method and an installation for the treatment of agricultural effluents. Although the description relates essentially to animal manure, in particular pig slurry, it will be understood that the present invention also applies to any type of agricultural effluent, for example, green and / or white water from livestock cattle, or wine effluents, etc.

Agricultural effluents, even those less loaded with nutrients or nutrients, can not be discharged directly into the natural environment. To be spread or irrigated on small surfaces, they must first be treated to reduce their pollutant load.

If slurry is considered, in particular pig slurry discharged by livestock facilities which is a highly charged effluent, this treatment can be carried out biologically, which makes it possible to eliminate some of the nitrogenous compounds which it contains, and this, depending on the overall amount of slurry to be spread and the spreading area available.

Such a biological treatment of heavily loaded agricultural effluents, in this case pig manure, consists of a process which is implemented in two distinct stages. On the one hand, a nitrification stage where the ammoniacal compounds contained in the manure are transformed, in the presence of aerobic microorganisms, into nitrates and, on the other hand, a subsequent denitrification stage where the nitrates resulting from the previous stage are now transformed, in the presence of anaerobic microorganisms, nitrogen gas which can then escape into the ambient air.

Such a treatment process provides a very satisfactory response to the resorption of nitrogen, but does not allow the removal of phosphorus content in relatively large quantities in agricultural effluents, including pig manure. As a result, this treated effluent, because of its phosphorus concentration, can not be spread in sufficient quantity so that the crops can value the overall amount produced.

The object of the invention is therefore to propose a method for treating agricultural effluents which not only allows the resorption of excess nitrogen, but also the resorption of phosphorus so that the purified liquid resulting from this treatment can be spread in relatively large amount.

The object of the invention is also to allow the use of a co-product containing phosphorus as an amine. In the case of the least loaded agricultural effluents, the purpose of the invention is somewhat different and consists in allowing filtration of the effluent, so that its necessary spreading surface is reduced significantly.

According to one characteristic of the invention, a method of treating agricultural effluents consists in a) treating said effluents in a drying bed consisting of a pit in which are superimposed a draining layer and a substrate in which are planted macrophyte plants (b) recovering the percolation water from the draining layer; and (c) removing the sludge layer which decays above the substrate at the end of treatment.

According to another characteristic of the present invention, said method consists in implementing, prior to step a) above, a nitrification-denitrification treatment.

According to another characteristic of the present invention, said method consists in allowing said effluents to decant before their treatment on said drying bed and for using for step a) only the biological sludges resulting from said settling.

The present invention also relates to an agricultural effluent treatment plant for carrying out the method.

According to one characteristic of the invention, it comprises a drying bed consisting of a pit filled with a draining layer and a substrate in which are planted macrophyte plants.

According to another characteristic of the invention, the substrate consists of a mixture of earth and sand.

According to another characteristic of the invention, between the draining layer and the substrate, there is provided a water-permeable insulation membrane.

According to another characteristic of the invention, the draining layer consists of gravel, for example, with a particle size of a few millimeters.

According to another characteristic of the invention, in the draining layer is provided at least one drain.

According to another characteristic of the invention, above the pit is mounted a greenhouse provided to allow air circulation above the drying bed. According to another characteristic of the invention, it comprises a biological reactor in which a nitrification-denitrification treatment carried out before the treatment is carried out in said drying bed.

According to another characteristic of the invention, said installation comprises a pit which is located upstream of said bed and which is provided to allow said effluent to settle.

The characteristics of the invention mentioned above, as well as others, will appear more clearly on reading the following description of an exemplary embodiment, said description being made in connection with the attached drawings, among which: FIG. . 1 is a schematic view of an agricultural effluent treatment plant according to the invention, and FIG. 2 is a sectional view of a drying bed of an agricultural effluent treatment plant according to the invention.

The description which follows essentially concerns an installation of agricultural effluents heavily loaded with nutrients or nutrients. It can for example be pig manure.

The installation of the agricultural effluent treatment shown in FIG. 1 therefore essentially comprises a pit 10 for receiving said agricultural effluents in which they are stored and in which they are stirred by means of a stirrer 11 to ensure good homogeneity.

This plant also comprises a biological reactor 20 in which the agricultural effluent pumped from the receiving pit 10, by means of a submerged pump 12, is subjected to a biological process for removing the ammonia it contains. The reactor 20 is provided with aeration means, such as slow aeration turbines 21 which are provided to float on the surface of the effluent being treated.

In the biological reactor 20, two distinct steps are carried out successively. On the one hand, a nitrification step which is carried out by specific aerobic microorganisms in the presence of oxygen supplied by the turbines 21 put into operation and which will make it possible to transform the ammoniacal compounds contained in the agricultural effluent into nitrates (N03 ) and, on the other hand, a denitrification step subsequently carried out by anaerobic microorganisms, this time in the absence of oxygen (the turbines 21 are then off) and which will make it possible to transform the nitrates (N03-) from the nitrification step nitrogen gas N2 which is then emitted into the ambient air.

Thus, after treatment in the reactor 20, 70% of the nitrogen initially present in an agricultural effluent, such as pig slurry, was removed. In addition, this treated effluent has the property of decanting into two distinct by-products: on the one hand, biological sludge which concentrates most of the nutrients and which represent approximately 40% of the initial volume and, on the other hand, the supernatant which is a weakly charged water and which represents approximately 60% of the initial volume.

The plant also comprises a drying bed 30 in which the effluents treated and pumped from the biological reactor 20 by means of a pump 22 are separated into its solid and liquid elements. The liquid elements are recovered in a lagoon 40. As for the solid elements, they are dried on a drying bed 30.

Note that in the case of less loaded agricultural effluents, especially nitrogenous materials, the biological reactor 20 may not be necessary and therefore be removed.

It is shown in FIG. 2 a section of a drying bed 30 according to the invention. This drying bed 30 essentially consists of a pit 31 whose walls may be made of a waterproof material, such as concrete or a plasticized membrane. In the bottom of the pit 31, there is a draining layer 32, for example, consisting of gravel, with a grain size of a few millimeters, through which circulates at least one drain 33 which makes it possible to collect the percolates. Above the gravel layer 32, there is a water permeable insulation membrane 34, for example made of geotextile on which a substrate 35 is based. The membrane 34 establishes the interface between the draining layer 32 and the substrate 35 by preventing them from mixing with each other, but allowing the passage of the water of percolation.

The substrate 35 consists for example of silty clay soil mixed with sand. It simultaneously serves as a filter medium and a support for the rooting of macrophyte plants 36 such as phragmites or typhas, etc.

At the outlet of the drain (s) 33, there is provided a collection port 37 which is connected via a pipe suitable for the lagoon 40 for storing the liquids (see FIG. As for the sludge that settled on the substrate 35, they constitute a dried layer 38 during treatment.

The operation of the drying bed 30 is as follows. The water contained in the effluent treated and pumped from the reactor 20 percolates through the substrate 35 slowly along the stems of the plants 36 and then reaches the draining layer 32. There, it is recovered by the drain (s) 33 where it is located. flows towards the collecting eye 37. As for the solid parts of the treated effluent that have settled so as to form the layer 38, they dry and mineralize under the combined effect of a drainage phenomenon, a phenomenon of evapotranspiration and plant-induced oxygen transfer 36 planted in the substrate 35.

 The treated and pumped effluent from the reactor 20 is discharged into the bed about once a week. After some contributions have been made, it turns out that the surface of the substrate 35 is clogged by the treated parts which have decanted and that it then becomes impermeable to water. However, it has been observed that for subsequent additions, the sludge is wiped off and the water is percolated by means of the stems of the plants 36 planted in the substrate 35. In addition, the plants 36 contribute to the mineralization. sludge by promoting the transfer of oxygen.

This method in the bed 30 can also be applied to biological sludge which would result from a settling phase carried out in a specific pit (not shown) situated between the reactor 20 and the bed 30, subsequent to the nitrification treatment. denitrification which takes place in the reactor 20.

The following results were obtained on an installation as described above. During a first phase of one month, the plants 36 developed in the drying bed 30. Then over a period of about <B> 100 </ B> days, the bed 30 was fed with 35 m 3 of slurry previously treated in the biological reactor 20 as indicated above. It was possible to obtain a solid product containing 15% dry matter and 0.9% P 2 O 5. A very clear liquid at 0.4% dry matter and only 0.03% P 2 O 5 was also obtained. It has thus been possible to record a phosphorus capture rate estimated at about 80%.

To increase the drying capacity of the drying bed 30, a greenhouse 39 is provided which makes it possible, on the one hand, to collect the rainwater and prevent it from reaching the sludge layer 38 and, on the other hand, to increase the temperature inside the pit 31. This rise in temperature facilitates the growth of the plants 36 and also increases the evapotranspiration phenomenon induced by the plants 36. An air circulation is provided under the greenhouse 39 to evacuate the water generated by this evapotranspiration. The greenhouse 39 is for example open at both ends The greenhouse 39 is shown in dashed lines in FIG. 2 to the extent that it is optional. Furthermore, the greenhouse 39 comprises means to be retracted so as to allow the extraction of sludge from the layer 38 at the end of the treatment process.

Claims (11)

1) A method of treating agricultural effluents, characterized in that it consists in a) treating said effluents in a drying bed (30) consisting of a pit (31) in which are superimposed a draining layer (32) and a substrate (35) in which macrophyte plants are planted (36), b) recovering the percolation water from the draining layer (32), and c) recovering the sludge layer that settles over the substrate ( 35) at the end of treatment. 2) Process for treating agricultural effluents according to claim 1, characterized in that it consists, prior to said treatment on said drying bed (30), to implement a nitrification-denitrification treatment. 3) Process for the treatment of agricultural effluents according to claim 1 or 2, characterized in that it consists of allowing the effluents to settle before their treatment on said drying bed, and to use for step b) only the biological sludge from said settling. 4) Plant for treating agricultural effluents for carrying out the method according to one of the preceding claims, characterized in that it comprises a drying bed (30) consisting of a pit (31) filled with a draining layer (32) and a substrate (35) in which macrophyte plants (36) are planted. 5) Installation according to claim 3, characterized in that the substrate (35) consists of a mixture of soil and sand. 6) Installation according to claim 4 or 5, characterized in that between the draining layer (32) and the substrate (35) is provided an insulating membrane (34) permeable to water. 7) Plant for treating agricultural effluents according to one of claims 4 to 6, characterized in that the draining layer (32) consists of gravel, for example, particle size of a few millimeters. 8) Plant for treating agricultural effluents according to one of the preceding claims 4 to 7, characterized in that in the draining layer (32) is provided at least one drain (33). 9) plant for treating agricultural effluents according to one of claims 4 to 8, characterized in that above the pit (31) is mounted a greenhouse (39) provided to allow air circulation beyond- above the drying bed (30). 10) Installation according to one of claims 4 to 9, characterized in that it comprises a biological reactor (20) in which is carried out a nitrification-denitrification treatment before treatment on said drying bed (30). 11) Plant for treating agricultural effluents according to one of claims 3 to 8, characterized in that it comprises a pit which is provided to decant the effluent and which is located upstream of said bed (30).