FR2674844A1 - Process and plant for the purification treatment of liquid animal manure - Google Patents

Abstract

According to the invention the screened raw residues stored in a vessel (1) are centrifuged (2) in the presence of a flocculant, with removal of the solid residues (4); the centrifuged products are then subjected to a biological purification in an anaerobic reactor (6) and then in an aerobic reactor (7) and then to a clarification (8). The mixed liquors from the reactors (6, 7) are recycled (9) and the sludge (10) from the clarification (8) is also recycled. An additional decoloration of the liquids is advantageously carried out with a suitable agent. Application to the treatment of liquid pig manure.

Description

 The present invention relates to the field of the treatment of animal manure consisting, as we know, of mixtures of litter and solid and liquid stains and excrement strewn on the floor of stables, stables and other places of animal husbandry.

It especially relates to a process and an installation making it possible to operate the solid / liquid separations and to purify the liquid fractions until fluids capable of being discharged, without pollution, into the natural environment are obtained.

 We know that the production of animal manure, which is considerable in regions with intensive farming, poses crucial problems in certain geographical sectors, as it is the case, for example in France in the province of Brittany or is concentrated near 50 % of national pork production.

 These slurries, generally spread on agricultural soils, in a more or less controlled manner, cause serious damage to the environment, by pollution of surface water and groundwater, in addition to obviously odor nuisances. Surplus manure must be treated to eliminate or greatly reduce carbon, nitrogen and phosphorus pollutants, sources of many nuisances, the main ones being the eutrophication of rivers and coastlines as well as the increase in nitrate contents of waters underground.

 Among the treatment methods already known, apart from the physicochemical processes which entail high and often prohibitive investment and operating costs, mention may be made of biological treatment, including in particular anaerobic fermentation, that is to say controlled anaerobic fermentation. .

 This process, which makes it possible to remove part of the organic matter, has practically no effect on nitrogen and phosphorus pollution for which an anaerobic post-treatment with “low charge” activated sludge must be considered and, often a physico-chemical co-precipitation of phosphorus compounds. To date, the results obtained by such treatments are still not sufficient to be able to consider that the effluents are capable of being sent, without risk, into the natural environment.

 Starting from these considerations and following series of tests aimed at optimizing series of treatment processes, in particular by using the biological route, the invention aims to propose an overall method and a adapted installation, making it possible, from raw slurry of various origins - and in particular from pig slurry, to obtain waste streams whose pollutant contents are sufficiently minimal to be able to eliminate any risk in the environment.

According to the general process of the process of the invention, the following successive steps are combined a) storage in tank of raw screened manure; b) liquid-solid separation by centrifugation with
addition of flocculating agent, the solid residues being
recovered as agricultural fertilizer; c) biological purification of liquids from
centrifugation by treatment according to an anoxic phase
then according to an aerobic phase, followed by a
clarification with recirculation of the mixed liquor
from the second phase to the first and recycling of
sludge settled from clarification to
anoxic phase, excess sludge from clarification
being eliminated; d) possibly, additional discoloration of
the effluent from the clarification.

 The conventional screening operation consists, as is known, in the coarse separation of the largest solid elements, in the present case of large straws or other stalks of cereals, from the liquid fraction and the solid particles of small diameter. Storage in tanks is a compulsory step in any treatment process and it makes it possible to periodically collect various mixtures of liquid manure taken from animal production prefosses. Generally in these tanks we find "fresh" manure which is less than a week old, and "aged" manure which can be several weeks or even several months old.

In the case, for example, of raw manure from pig farming, the average values of the different representative parameters out of a number of 37 samples of fresh manure and 9 samples of old manure, are summarized in Table 1 below. below or TAC is the full alkalimetric title. MES is the concentration of suspended matter COD corresponds to the Chemical Oxygen Demand NTK the total nitrogen content (Kjehldahl). N-NH4 the nitrogen content from ammonia. P the phosphorus content (total or from
phosphorus)
TABLE 1
pH TAC Extract MeS COD NTK H-NH4 P total P-P04
dry
gCaO / L g / lg / tg / lg / L g / lg / lg / l
"Fresh" manure 7.73 ~ 1.48 76.55 68.91 103.6 9.28 3.8 1.94 1.34 "ases" manure 8.17 11.38 61.93 58.37 96.07 9.47 1.67
In accordance with the first operational phase of the process of the invention, a raw slurry is centrifuged with the addition of a flocculating agent in order to optimize the capture rates of the particulate fractions. Among the flocculants which can be used, it is advantageous to cite polyelectrolytes such as, for example, cationic polymers; this of course without limitation. The quantity of flocculants varies, of course, depending on the case, but it is generally between 3 and 7 kg per tonne of dry extract. The conditions of centrifugation (speed, residence time, etc.) are obviously suitable for case in point and within the scope of those skilled in the art.

After the aforementioned operation, the elimination yields or abatements of the descriptive parameters of table 1 above are already very significant since of the order of 77% for the dry extract; 95% for
MY; 72% for COD; 65% for NTK; 32% for nitrogen from
NH4; 93% for total phosphorus and 91% for P of P04.

 As for solid residues or centrifugation lees, they can be advantageously used as agricultural fertilizers.

 In accordance with an advantageous embodiment, one proceeds during the centrifugation and / or during the commissioning of the installation, to an initial dilution of the slurries by surface water or waste water having a low concentration, or even zero in ammoniacal nitrogen . In this way, the inhibitory effects of the high ammonia concentrations of the centrifuged manure can be overcome in the seeding and the development of the purifying biomass during the subsequent phase of biological treatment.

 According to the second step of the process, a biological treatment of the centrifuged slurry is carried out in two successive phases, the first in anoxic medium or the slurries are used as carbon source and where the reduction of nitrates to nitrogen gas takes place, the second in medium ventilated to ensure nitrification.

 According to an operating variant, the anoxia phase can optionally be preceded by an anaerobic phase to cause increased biological dephosphatation.

The volume load, applied to all anoxic and aerobic capacities and expressed in terms of
DB05, is generally between 0.3 and 0.65 kg / m3 / day.

 During the aerobic phase, the oxygenation mode advantageously adopted, by blowing air into the reactor, makes it possible to avoid drops in temperature of the mixed liquor, a phenomenon observed when surface aeration devices are used. according to known techniques, which lead to falls in purification efficiency. In practice, according to the invention, it is advantageous to use deep aerators of the "oxytubes" type which allow the sequencing of the air blowing without causing clogging on the air diffusers. This sequencing of the aeration makes it possible to improve the supply of the recirculated nitrates by putting into partial anoxia of the normally aerated zone. This sequencing can also be regulated by a continuous measurement of the redox potential of the mixed liquor.

 Thanks to the recirculation of the decanted sludge and the mixed liquor, almost all of the nitrates to be denitrified can be removed at the outlet of the aerobic tank and the clarifier to bring it, by recycling, to the anoxic zone at the head of the treatment. . In certain cases, it may be necessary to add certain reagents during this biological treatment step such as for example lime to accelerate the nitrification process or even ferric chloride to facilitate the coprecipitation of phosphorus.

However, it turned out that in general, during the experiments carried out on pig manure, the residual concentrations of biological effluents in total phosphorus and soluble phosphorus were generally not greater than 20-25 mgl and 10 to 15 mg / l.

The sludge produced during biological treatment, which can for example correspond to 0.4 kg per Kg of
DB05 eliminated or 600 to 700 liters per m3 of treated slurry, are treated by centrifugation, the centrifuged products being sent to the biological stage while the rejects are mixed with the solids from the first centrifugation stage to give fertilizers as explained above.

 According to a particularly advantageous final step to be implemented in the process of the invention, it is possible to proceed with the elimination or at least the attenuation of the intense brown color of the effluents, attributable to the presence of non-biodegradable oxidized bile pigments in animal droppings. After experimenting with several methods and a fairly large number of reagents, it has been found that the addition to treated slurries of products chosen from the group consisting of: pulverulent activated carbon, sodium hypochlorite and ferric chloride , provided the best results.

In particular, FeC13 makes it possible not only to ensure the discoloration of the treated manure but to provide a significant refinement of the quantity of the effluents.

 The method according to the invention can be implemented in a general treatment installation the phases or devices of which are illustrated in the block diagram of the single figure of the appended drawing which will simultaneously make it possible to describe an exemplary embodiment, corresponding to the average results obtained over six months of experiments in a pig manure processing unit.

 A treatment assembly in accordance with the invention essentially comprises: a tank 1 for recovering and storing raw manure; a centrifuge 2, where at least one flocculating agent 3 is advantageously added and from which the lees or cakes of solids are discharged in 4 which can be used as soil spreading materials, as fertilizer; optionally, a storage tank 5 for centrifugal slurry; a biological purification unit comprising an anoxic reactor 6, the aerobic reactor 7 and a clarifier 8.

 In normal operation and as said above, recycling is carried out, according to 9, of the mixed liquor from the reactors, while the sludge removed from the clarifier 8 is partly sent according to 10 to the anoxic reactor and partly recycled, according to ll in the centrifuge system 2; they then magnify the semi-solid part or lie 4 which is removed from the centrifuge.

 According to a preferred implementation, the effluent 12 from the clarifier is subjected to discoloration to attenuate or even eliminate the dark brown color which is difficult to accept when discharged onto the site.

 The addition, advantageously, of ferric chloride at doses of 3 to 5 g / l, quickly leads to the insolubilization of non-biodegradable oxidized bile pigments. The precipitates obtained do not sediment but float on the surface.

By operating in an installation of the aforementioned type for the treatment of pig manure for a period of 6 months at a rate of 0.5 m3 of manure / day, the results given in table No. 2 below were obtained.
TABLE 2
TAC Extract MES COD NTK N (NH4) P total
(in CaO) dry raw slurry (g / l) 11.48 76.55 68.9 103.60 9.28 3.80 1.94 centrifuged slurry (g / l) 7.33 17.60 3.99 29.70 3.28 2.60 0.15 clarifier effluent <g / t) 1.70 8.30 0.43 2.11 0.14 0.03 0.03 d yield '' purification (X) 84.20 88.43 99.33 97.83 98.39 99.24 98.57
During the centrifugation, the cationic polymer type F0 4290 (Floerger) was used as flocculant at the rate of 5.7 kg per tonne of MES. In the biological treatment, volume loads corresponding to 0.5 kg of BOD per m3 per day and mass loads of 0.1 kg BOD / kg of MES per day were used.

Sludge production and oxygen consumption, in the biological treatment stage, were respectively equal to 0.43 kg and 1.51 kg per kg of BOD eliminated.

 As can be seen from Table 2, the abatement rates for the various types of pollution were entirely satisfactory, the effluents obtained being capable of being discharged into the river, especially after discoloration.

Claims (9)

1. Process for treating animal manure, in particular pig manure with a view to eliminating the various forms of pollution and obtaining a liquid effluent of quality compatible with discharge into the natural environment, characterized in that one operates according to the combination of the following successive stages a) storage in tank of raw screened slurry; b) liquid-solid separation by centrifugation with  addition of flocculating agent, the solid residues being  recovered as agricultural fertilizer; c) biological purification of liquids from  centrifugation by treatment according to an anoxic phase  then according to an aerobic phase, followed by a  clarification, with recirculation of the mixed liquor  from the second phase to the first and recycling of  sludge settled from clarification to  anoxic phase, excess sludge from clarification  being eliminated; d) optionally, additional discoloration of  the effluent from the clarification. 2. Method according to claim 1, characterized in that an anaerobic treatment phase precedes the anoxia phase. 3. Method according to any one of claims 1 and 2, characterized in that, during centrifugation, the slurries are diluted with surface water or waste water with a low content of ammoniacal nitrogen. 4. Method according to any one of claims 1 or 2, characterized in that, during the aerobic phase, the blowing of air is done in a sequenced manner. 5. Method according to any one of claims 1 to 4, characterized in that part of the sludge from the clarification is recycled to centrifugation. 6. Method according to any one of claims 1 to 5, characterized in that the discoloration of the treated effluent is carried out with activated carbon, sodium hypochlorite or, advantageously, ferric chloride. 7. Installation for the treatment of slurry according to any one of claims 1 to 6, characterized in that it comprises a container (1) for receiving and storing raw slurry; a centrifuge (2) from which solid or lees residues (4) are eliminated; optionally, a storage tank (5) for centrifuged liquid manure; a biological purification unit combining an anoxic reactor (6) followed by an aerobic reactor (7), and a clarifier (8).  8. Installation according to claim 7, characterized in that it is provided with means for recycling (9) of the mixed liquors from the reactors (6, 7) of biological treatment, thus by means for sending a portion (10) clarification sludge (8) in the anoxic reactor and part (11) of this sludge in the centrifuge (2). 9. Installation according to any one of claims 7 and 8, characterized in that the aerobic reactor (7) is provided with means for blowing air in depth and sequentially.