FR2648309A1 - Method for treating liquid pig manure as well as installation allowing implementation of this method - Google Patents

Abstract

The invention relates to a method for treating liquid pig manure characterised in that a coagulant agent is added to a liquid manure to be treated, preferably at least 5 days old, the solid phase and the liquid phase thus obtained are separated and the solid phase is stored for the purpose of subsequent spreading. It also relates to an installation allowing implementation of the method according to any one of claims 1 to 6, characterised in that it includes a tank 1, 3 for prior activating treatment, a pump 2 allowing the activated liquid manure to be transferred to a vessel 5 for separating the phases, as well as a pump 7, particularly of the peristaltic type, for supplying the vessel 5 for separating the phases with coagulant agent, and, if required, a bacterial filter with fixed culture 6.

Description

"Process for treating pig manure
as well as installation allowing the setting
of this process "
The present invention relates to a method of treating pig manure.

 National pork production is around 26.5 million head per year, representing a permanent herd of 11 million pigs.

 Depending on how it is fed, each pig produces about 1 m3 of slurry in its lifetime. Pigs are actually much more polluting than humans: each animal generates a load equivalent to 5 inhabitants.

 The purification of such a pollution is technically complicated and this, all the more, that one observes a great disparity and an important atomization of the breeding.

 It is, however, well known that pig manure, which consists of a mixture of organic and mineral materials more or less diluted, has a very interesting fertilizing value.

This property is confirmed in particular by the table below which indicates the average concentrations of the main elements present in pig slurry.

<tb> Designation <SEP> Maxi <SEP> Average <SEP> Mini
<tb>% <SEP> dry matter <SEP><SEP> 11.26 <SEP> 7.43 <SEP> 3.60
<tb>% <SEP> Minerals <SEP> 2.76 <SEP> 1.94 <SEP> 1.12
<tb>% <SEP> organic matter <SEP> organic <SEP> 8.54 <SEP> 5.50 <SEP> 2.46
<tb>% <SEP> N <SEP> total <SEP> 0.74 <SEP> 0.55 <SEP> 0.37
<tb>% <SEP> N-NH4 <SEP> 0.43 <SEP> 0.33 <SEP> 0.23
<tb>% <SEP> P2 <SEP> OS <SEP> 0.64 <SEP> 0.40 <SEP> 0.15
<tb>% <SEP> K20 <SEP> 0.51 <SEP> 0.39 <SEP> 0.27
<Tb>
In view of this composition, the most frequent destination of pig manure is spreading on the soil, the ammoniacal nitrogen being essen- tially oxidized to nitrate and absorbed in this form by the plants following the cycle.

<tb> 2NH <SEP> Nitrosornonas <SEP> + <SEP> 3 2 <SEP>><SEP> 2N02- <SEP> + <SEP> 2H20 <SEP> + <SEP> 4H +
<tb> 2NH
<tb> 2NO2- <SEP> nitrobacter <SEP> + <SEP> 2 <SEP>><SEP> 2NO3- <SEP>
<tb> 2NO2
Such spreading on the ground is however not without many disadvantages
The pollutant load of pig manure can be represented by the following average values obtained from a mixture coming from different farms taking into account criteria such as the exploitation size, the storage time or the type of production.
COD (chemical oxygen demand) in mg of O 2/1: 29,663
DB05 (biological oxygen demand) in mg of 02 / l: 8,750 pH: 7.06
Conductivity in pS / cm2: 39,866
COD
Biodegradability index (0B05: 3s45 02 dissolved <1% of saturation
These measures, in particular the biodegradability index (representing the ratio of the amount of oxygen required to transform the molecules present in the manure into water and carbon dioxide, to the quantity of oxygen consumed for five days by the bacteria present in slurry) prove that we are in the presence of conditions to consider biological purification.

 However, and despite the importance of organic matter, this possibility is hampered by the high value of the conductivity which is directly related to the quantity of dissolved salts; it is well known that the latter induce variations in the osmotic pressure which slow down the development of the bacteria of the common flora making it possible to obtain reactions with normal kinetics.

 This characteristic, linked to the low content of dissolved oxygen slurry, thus limits the development of usual bacteria and participates in the establishment of anaerobic bacteria (therefore slow kinetics) inducing fermentations belonging to the group of putrid fermentations producing very volatile substances. and olfactory whose threshold of perception is sometimes lower than the ppm. This nuisance to the environment is particularly troublesome during the drop in atmospheric pressure creating an imbalance between the internal and external pressures and thus causing a greater release of dissolved gases. This disadvantage means that manure spreading is subject to limiting or even constraining standards for farmers: it is especially forbidden to spread manure during the summer months.

Moreover, it is generally accepted that, in order to be in ideal conditions for biological purification, the ratio C / N / P must be equal to 100/5/1. By convention, the factor C which represents the total organic carbon is assimilated to the DB05, which, considering the average value of the BOD5 in pig manure could be written
DBO5 - N - P
8,750 mg 437 mg 88 mg
However, in the case of an average pig manure, these values actually become
DBO5 - N - P
8,750 mg 3,330 mg 1,778 mg
These values correspond to a ratio C / N / P equal to 100/38/20, which means that, compared to the ideal values, pig slurries contain about 7.5 times too much nitrogen and 20 times too much phosphorus .

 This high nitrogen content leads to the formation of a large quantity of highly soluble nitrates that settle more or less well in the soil and that, during heavy rainfall, are entrained and stored in the underlying aquifers, making the water unsuitable. for domestic consumption; these large quantities of nitrates also have the disadvantage of having a fertilizing effect of a nature to modify the flora of the watercourses by aiding the planting of plants and thus reducing the availability of oxygen, which causes the disappearance of some fish: this is the well-known phenomenon of eutrophication, which, in the case of closed reserves such as lakes, can quickly lead to their transformation into peat bogs.

 The presence of an excess of phosphorus is not without inconvenience, even if it is less troublesome than the presence of nitrates: indeed, under the action of heavy precipitation, there is a slight leaching of phosphates which found stored and released in time, which represents a stimulating effect on the growth of plants.

 It should also be mentioned that manure also has relatively large quantities of potash, the solubility of which is comparable to that of nitrates, and which, therefore, induce the same problems.

 The object of the present invention is to overcome the aforementioned drawbacks by proposing a novel process for treating pig manure which does not entail any constraint for the breeders, and which is safe for the environment but makes it possible to profit in a certain way. economically acceptable for their fertilizing value.

 This process is characterized in that a coagulating agent is added to a slurry to be treated, preferably less than five days old, in order to prevent the start of the natural process of transforming nitrogen into nitrates under the action of bacteria in the slurry, separating the concrete phase and the liquid phase thus obtained and the concrete phase is stored for later spreading.

 In fact, we have seen by studying the relationship between the density and the concentrations of the different fertilizer values of slurry that we could consider, despite the significant solubility of nitrogen and potash, to separate this last in a liquid phase representing approximately 80% of the initial volume and a concrete phase representing approximately 20% by volume, while retaining in this last phase, an interesting proportion of the fertilizing value.

One can, indeed, thus obtain a concrete phase containing, compared to the initial values
- 59% of BOD5
- 80% of the COD
- 80% of the phosphorus,
- 25% of the ammoniacal and organic nitrogen.

 This result is particularly interesting since it makes it possible to obtain, under a reduced volume (approximately 20% of the initial volume), a fraction that is useful for the development of crops and that can be stored without heavy investment and used at will depending on the rainfall. plant needs. This reduces the cost of spreading to a large extent.

According to the invention, the chosen coagulant must be capable of creating in the slurry a protein network; satisfactory tests could be carried out using polyaluminum chloride; however, a high molecular weight biodegradable coagulant agent such as, for example, a polyacrylamide, is preferably chosen.

The important polarity of the amide function by its ability to interact on Van bonds
Der Waats, allows bridging between suspended solids and thus forces their separation from the dissolved phase.

 The choice of this coagulating agent, which is generally used in an amount of the order of 10 ppm should, however, be in no way considered to be limiting of the invention.

 Despite its advantages, the above-mentioned method does not make it possible to remedy all the drawbacks associated with the traditional spreading of manure, in particular with the odor problem.

In addition, the dissolved phase has the following values
DBO5 - N - P
3587 mg 2 497 mg 355 mg and therefore a C / N / P ratio of 100/69/10, corresponding to a much too much nitrogen amount. Given its polishing charge, the liquid phase can not, of course, be discharged directly to the sewer.

 In order to improve these conditions, and according to a preferred characteristic of the invention, prior to phase separation, the slurry is subjected to a prior activation treatment that makes it possible to convert part of the ammoniacal nitrogen into organic nitrogen, in particular in the form of proteins by increasing the biomass in order to increase the effectiveness of the coagulating agent and to act continuously on the quantities of dissolved nitrogen.

 According to another characteristic of the invention, this prior activation treatment is carried out in the presence of oxygen and rapidly kinetic bacteria capable of promoting an aerobic degradation process while blocking the nitrification process.

The addition to the slurry of bacteria in the presence of oxygen makes it possible, in fact, to modify the starting equilibria according to the following balance sheet

<tb> Substrate <SEP> + <SEP> Oxygen
<tb><SEP> t <SEP>><SEP> biomass <SEP> +
<tb> products of + H2 O + CO2 degradation
It is clear that this reaction may involve a rapid increase in the biomass of the general formula and, depending on the yield of the bacteria, a significant protein manufacture; this reaction continues as long as the substrate has no limiting factors. The number of bacteria is in fact, on average, multiplied by two every twenty minutes.

 This modification, by transforming a part of the ammoniacal nitrogen into proteins, makes it possible to prepare the ground for a better efficiency of the coagulant (van der Waals bonds). Indeed, it also reduces the development of anaerobic reactions approximately 20 times slower than aerobic reactions, to intervene strongly on the development of odors whose amount produced depends on the space left available to anaerobic fermentations.

 However, in this process of prior activation, these two types of reaction are necessarily in direct competition: in fact, the choice of bacteria and the amount of oxygen used must take into account the desire to delay the production of nitrates by Nitrosomas and Nitrobacter. Directing reactions to a strict aerobic pathway is therefore dangerous.

According to the invention, the formation of foul odors during spreading is, moreover, slowed down by the transformation of a large part of the celluloses, essential elements of slurry according to the following equation

 It is indeed known that for a slurry to not smell bad for too long, it must penetrate quickly into the soil; however, the penetration rate is a direct function of the particle size. It is therefore essential to transform the cellulose which is in the form of grains or straw.

 It should also be noted that there are known products such as for example dicyandiamide which are capable of inhibiting Nitrosomas and Nitrobacters responsible for nitrification The use of this molecule is, however, not economically interesting, and, may lead to the poisoning of other bacteria; it can however be considered in case of accident.

As already indicated, the method that is the subject of the invention leads not only to obtaining the concrete phase having the above-mentioned characteristics, but also to obtaining a liquid phase that can be represented by the following data. : DC0 (chemical oxygen demand) in mg of 02 / l: 1,400
DB05 (biological oxygen demand) in mg of O 2 / i: 400 pH: 4
NH4 content in g / l: 1.05
P205 content in mg / l 75
K20 content in mg / l: 135 constituting a very significant improvement in the purification efficiency.

 The C / N / P ratio corresponding to this phase is equal to 100/262/33, but can be very substantially modified by acting on the working time of the bacteria on the slurry during the activation treatment.

 This liquid is of course not completely purified and can not be rejected directly into a receiving environment; However, after studying the soil, it can serve as a watering liquid: the polluting load corresponding to the BOD5 is divided by 20 with respect to its initial value in the slurry, and this liquid does not normally contain nitrates. and, the amount of ammoniacal nitrogen was significantly reduced from its initial value.

 As has already been indicated, it is essential to obtain these results from treating slurry less than five days old.

In order to directly discharge a liquid into a watercourse, the Administration requests the obtaining of a level of loads such that the level E corresponding to:
COD: 90 mg / l
BOD5: 30 mg / l
MES (suspended solids content): 30 mg / l
The invention also provides for the liquid phase to undergo additional treatment to meet these obligations.

 For this purpose, and according to another characteristic of the invention, the liquid phase is filtered on a fixed culture bacterial bed in particular formed by a mixture of bacteria and a support composition consisting of about a portion of fixer with a high specific surface area. for example lithothamme 150 p for about 2 parts of an available carbon source, especially sphagnum or peat moss to bring the ratio C / N / P to about 100/5/1.

 Such bacterial filters, in which the effluent is likely to pass and discharge, are constituted by supports having very large specific surface areas of up to 40 m 2 / g and a very large number of cavities in which the Bacteria can bind to transform carbon (which is in a rapidly assimilable form), and eliminate nitrogen by assimilation and biomass increase; an excellent purification is thus obtained.

 The respect of the amounts of carbon source, in particular peat to be added, is extremely important since this is the way to overcome the low percolation capacity of the lithothamme as a result of its particle size. In addition, this carbon contributes to the production of biomass thus making it possible, after a certain time, to obtain a compost that can also be spread on the soil at the desired moment by the farmer.

 It should also be noted that peat beds are, in general, not very effective after 40 cm of depth; two dies can be used, including a deep distribution using a countercurrent of oxygen in the form of compressed air.

 The invention also relates to an installation for carrying out the aforementioned method.

 According to the invention, this installation is characterized in that it comprises a prior activation treatment tank, a pump for transferring the activated slurry to a phase separation tank and a pump, particularly of the peristaltic type. supplying the coagulating agent with the phase separation tank and, if appropriate, a fixed culture bacterial filter.

 According to the invention, and depending on the size of the farm and the type of building, the pre-activation treatment can be carried out directly in the pre-pits above which the pigs are raised, by adding the necessary bacteria, and, to associate these pre-pits with pits into which air can be blown by means of a compressor. Such pits should have a maximum storage time of five days in order to avoid starting the nitrification process. To avoid the risk of favoring anaerobic reactions at the bottom of the pit, it is preferable to use a compressor rather than a surface aerator that has a poor O2 transfer coefficient (O2 / kW consumed).

 According to another characteristic of the invention, the phase separation tank comprises a substantially cylindrical upper part defining a mixing chamber of the activated manure and of the coagulating agent, and a frustoconical lower part at the end of which can be drawn off. the concrete phase.

 This tank must, of course, have a useful volume in relation to the time required for sedimentation.

 It is difficult to provide a continuous working installation given the fact that the phase separation is fast and can be carried out in a tank of several cubic meters.

 When sizing the plant, it is also necessary to take into account the amount of liquid phase produced so that the fixed culture bacterial filter is continuously watered; the treatment cycle by this filter is indeed very long to get back on track if it is stopped.

 According to another characteristic of the invention, a by-pass is also provided making it possible to reduce a portion of the liquid leaving the fixed culture bacterial bed to the phase separation tank.

 In conclusion, the method and the facility for treating pig manure in accordance with the invention are based on the transformations of the original qualities of these manures under the action of bacteria and enzymes preparing the medium for phase separation. The liquid phase, which is very lightly charged, can be easily purified on a fixed bacteria filter, while the destination of the concrete phase remains spreading, in particular because of its high content of fertilising material.

 The characteristics of the method and the installation which are the subject of the invention will be described in more detail with reference to the attached figure which is a diagram of this installation.

 According to the figure, the manure treatment is initiated directly in the pre-pools 1 above which P pigs are raised by the incorporation at this level, and according to the arrow a, of bacteria likely to guide the biological transformations. produced in manure towards mixed aerobic / anaerobic / anoxic reactions with relatively fast kinetics. It is thus possible to increase the biomass, and thus the protein slurry content, at the expense of the ammoniacal nitrogen so as to improve the efficiency of the subsequent coagulation reaction.

 Slurry 1 from these pre-pits is then pumped by a pump 2 and introduced according to the arrow b in a pit 3 into which air is blown from below according to the arrow c with the aid of a compressor 4 in order to promote aerobic reactions at the expense of anaerobic reactions, and consequently to prevent, as much as possible, the formation of troublesome odors.

 It is imperative that the residence time of the slurry in the pre-pit 1 and in the prior activation pit 3 is less than five days, that is to say the time required for the implementation of nitrification reactions.

 The activated slurry from the pit 3 is then pumped along the arrow d by the pump 8 to be introduced into a separation tank 5 having a cylindrical upper portion 50 and a frustoconical lower portion 51. The cylindrical upper part 50 defines a chamber in which the activated manure 3 is mixed with a coagulant introduced along the arrow e by means of a peristaltic pump 7; the separation of the slurry is observed almost immediately in a concrete lower phase A occupying about 20% of the initial volute and a liquid upper phase B occupying about 80% of this volume. The concrete phase A can directly be drawn at the lower part of the frustoconical portion 51 along the arrow f to be stored for spreading on the ground. This operation can be carried out at the time chosen by the farmer, that is to say in dry weather and when crops need inputs. The invention thus makes it possible, in fact to manage the time of the spreading, this is particularly interesting, on the one hand, for the farmer and, on the other hand, to avoid the risks of eutrophication and pollution streams and groundwater.

 The liquid phase B is in turn withdrawn according to the arrow g to be purified on a fixed culture bacterial filter 6 before emerging in the arrow h can be directly discharged to the sewer in a manner satisfactory to the standards in force. A by-pass pipe i makes it possible to return a portion of the purified liquid leaving the ba-terien bed 6 along the arrow h in the chamber of treatment 50.

 The support of the fixed culture bed 6 also constitutes a compost capable of being spread on the ground.

Claims (10)

 1 *) Process for treating pig manure, characterized in that a coagulating agent is added to a slurry to be treated, preferably less than five days old, and the concrete phase and the liquid phase thus obtained are separated off and stores the concrete phase for later spreading.  2) Process according to claim 1, characterized in that a high molecular weight biodegradable coagulating agent is used, in particular a polyacrylamide or a copolymer derived from acrylamie.  3) Process according to any one of claims 1 and 2, characterized in that, prior to the separation of the phases, the slurry is subjected to a prior activation treatment for converting part of the ammoniacal nitrogen to organic nitrogen, especially in the form of proteins by increasing the biomass.  4) Method according to claim 3, characterized in that it implements the prior activation treatment in the presence of oxygen and rapidly kinetic bacteria capable of promoting an aerobic degradation process while blocking the process nitrification.  5 *) Method according to any one of claims 1 to 4, characterized in that the liquid phase is filtered on a fixed culture bacterial bed before rejecting directly to the network.  6) Process according to claim 5, characterized in that the fixed culture bacterial bed is formed by a mixture of bacteria and a support composition consisting of about a portion of fixer with a high specific surface area for about two parts of a source carbon.  7) Installation for carrying out the method according to any one of claims 1 to 6, characterized in that it comprises a tray (1, 3) of prior activation treatment, a pump (2) for transferring activated slurry to a phase separation tank (5) and a pump (7), in particular of the peristaltic type for supplying the coagulating agent with the phase separation tank (5), and, if appropriate, a fixed culture bacterial filter (6).  8) Installation according to claim 7, characterized in that the pretreatment tank consists of the combination of a pre-pit above which the pigs (P) are raised and a pit (3) in which air can be blown in using a compressor (4).  9) Installation according to any one of claims 7 and 8, characterized in that the phase separation tank (5) comprises a substantially cylindrical upper portion (50) defining a mixing chamber of the activated slurry and the coagulating agent , and a frustoconical lower portion (51) at the end of which can be withdrawn the concrete phase (A).  10 ') Installation according to any one of claims 7 to 9, characterized in that it comprises a bypass (i) for bringing a portion of the liquid exiting the fixed culture bed bacterial (6) to the tank of phase separation (5).