FR2675386A1 - Biological process for the treatment of gas, biofilters, and application to the deodorisation of gas - Google Patents

Abstract

Biological process for the treatment of gas, in which the gas is treated with the aid of a biofilter comprising wet sludges of a sewage purification plant, and biofilters consisting of wet sludges of sewage purification plants. The biofilters according to the invention do not require any adaptation period and permit the treatment of organic or inorganic odours.

Description

 The present invention relates to a biological process for gas treatment, biofilters and their applications, in particular to the deodorization of gases.

Many agricultural, industrial and even domestic activities are sources of olfactory nuisance. Arbitrarily, it is possible to bring them together according to their origin in two categories
- odors from fermentation,
- odors emitted by industrial activity.

Another classification is based on the chemical or biological reactions of odorous emissions
- thermal decomposition of organic compounds: incineration of waste from smelters, the petrochemical industry, thermal power stations and the like,
- anaerobic decomposition of organic materials: manufacture of food, yeasts,
anaerobic decomposition of animal products: rendering, manufacture of gelatin, fishmeal and the like
- animal manure: pig slurry, poultry droppings, ...

 Various methods are used today to eliminate odors. These methods are generally based on absorption, adsorption or combustion and vary according to the physical and chemical properties of the gases to be treated and according to their concentration in the carrier gas.

At present, the most used are the following
- catalytic combustion,
- thermal combustion,
adsorption on activated carbon,
- condensation and wet absorption,
- ozonation,
- biological processes.

 The use of masking agents, dilution and dispersion are also used to minimize the perception of an odor.

 Biological processes are characterized by the oxidation of biodegradable odorous compounds to neutral or odorless compounds. For many microorganisms (bacteria, yeasts, molds ...) these odorous compounds are a source of substrates (carbon, nitrogen, oxygen ...) essential to develop their cellular constituents and thus allow a multiplication of microorganisms to the detriment of said odorous compounds / substrates. The oxidation products of the substrates are generally carbon dioxide and water.

 In the case of inorganic odorous compounds such as hydrogen sulphide, biological oxidation provides the chemical energy essential for microorganisms and leads to the formation of olfactory neutral compounds (examples: sulfur, sulphate).

 The biological processes use biofilters comprising soil, compost garbage but especially peat biofilters watered by an aqueous phase consisting of a solution of nutrients.

Peat contains few assimilable carbons and therefore requires nutrients; it is necessary to sow microorganisms, and a period of adaptation is also necessary.

There are generally three types of biological processes
scrubbers, in which the microflora and the aqueous phase are mobile,
bacterial beds, in which the microflora is immobile and the aqueous phase mobile,
- biofilters, in which the microflora and the aqueous phase are immobile.

 An efficient and economical method is still sought to treat the discharges responsible for olfactory nuisances.

 In addition, the disposal of sewage sludge remains a problem because it is a waste, usually thrown to the landfill.

 This is why the present invention relates to a biological gas treatment process in which the gas is treated with a biofilter, characterized in that said biofilter comprises wet sludge treatment plant.

 The gas to be treated may contain both organic and inorganic compounds. It can come for example from refineries and in this case include telluric gases, cracking, catalysis of fluids, boilers or warehouses. It may also come from the mineral chemical industry, including the production of fertilizers or phosphates, phosphoric acid, soda or nitric or sulfuric acid. It can still come from the organic chemical industry, including the development of paints, plastics. Finally, it can come from rendering stations or from the agri-food industry.

 By "biofilter" is meant a product permeable to gases and containing a number of microorganisms of bacteria or fungi type or a mixture thereof. The biofilter according to the invention comprises wet sludge treatment plant. As a biofilter the sludge below can be used alone or mixed, for example up to 60% with organic materials: peat, bark, straw or sawdust, or plastics.

 Said materials can be intimately mixed or arranged in alternating layers with the sludge, distributed in succession of filtering units and gaseous volume (gaseous sky) so as to improve the liquid-solid gas transfer (biomass fixed to the biofilter).

 The sludges above are advantageously used essentially alone, that is to say they represent more than 70% and preferably more than 90% of the active ingredient of the biofilter, which does not exclude that the we can add to them various adjuvants as will be seen below.

 The sludge can come from biological biological water treatment plants, but preferably from wastewater treatment plants of dairies and particularly from urban wastewater.

 The purification plants of biological type are well known, they consist in subjecting wastewater treatment by aerators and then digesters, namely in agitated and heated tanks, using sludge activated by aerobic bacteria.

 the activated sludges above, after having been conventionally used, can be used according to the present invention.

 The above sludge is used wet, raw or partially digested. Their essential characteristics are to contain a large variety of viable microorganisms and a large proportion of assimilable carbon.

As a purely illustrative example, digested sludge may contain, on average, a percentage by weight expressed as dry matter
- 45-55% carbon C
4-7% hydrogen H
1-3% N N
- 1-2% sulfur S
- 20-50% oxygen O
0.4-0.6% iron Fe
- [manganese (50-100 ppm) Mn
- Of zinc (600-900 ppm) Zn
- 0.1 to [copper (250-300 ppm) Cu
- 0, 2% E of lead (80-160 ppm) Pb
- [chromium (30-60 ppm) Cr
Carbon is essentially present in organic form. Sludge is in itself a source of nutrients for the growth of microorganisms.

After drying (sand bed) or dewatering (centrifugation), the sludge contains a heterogeneous and dense population of microorganisms essentially constituted by
aerobic or aerobic tolerant bacteria (the majority of strict anaerobic populations have disappeared) which are bacilli, shells or filaments,
- microscopic fungi.

 The enumeration of these microorganisms on complete medium shows that these sludges contain a large bacterial microflora.

By nutrient agar culture (or PCA), 0.5 to 1 × 10 10 CFU / g of dry sludge is obtained. (CFU: Colony forming unit)
Sludge is not only the carrier on which many microorganisms are immobilized, but is also an important potential source of nutrients for these microorganisms.

The sludge used will preferably have a microorganism content greater than 107 Forming Units
Colony (CFU) per gram of dry sludge.

Sludge can be conditioned in different ways
grinding after drying, leading to a particle size distribution between 0.2 and 10 mm in diameter,
the sludges can be used after having been packaged, in various forms: beads, granules, platelets, and the like. They may have undergone a physical treatment (centrifugation), chemical (addition of poly-electrolytes ...) or thermal (400-500 C) in order to dehydrate them. They may have been dried, dehydrated, compacted, extruded. The sludge obtained after these different treatments may possibly be subject, prior to their use as a biofilter in air treatment, rehumidification.

 They can also be seeded with fresh liquid activated sludge for 24 hours, for example, 1 liter of liquid sludge per kg of dewatered sludge.

Sludge can be added with chemicals
- metal salts which can serve as trace elements,
- nitrogenous elements,
- Salts such as phosphates or carbonates, which can serve as carbon sources for autotrophic microorganisms or improve the buffer effect of the biofilter vis-à-vis any significant changes in pH during operation of the biofilter.

 The said chemicals can be added directly to the sludge or diluted in the feedwater of the biofilter.

 The addition, for example of solid particles of a buffer salt such as a phosphate or a carbonate, for example alkaline or alkaline-earth such as potassium acid phosphate, magnesium carbonate or preferably calcium carbonate, especially the maerl, to limit the possible problems of settlement of a column. In addition, it buffers the biofilter, thus preserving its effectiveness over time. The calcium carbonate will represent for example 10 to 15% by weight of the biofilter; it may be present in particles ranging from 0.5 to 10 mm, and in particular from 2 to 3 mm.

 Under preferential conditions of implementation of the method described above, the biofilter will be used in a pH range from 4 to 9 and in particular from 6 to 8.

 The process according to the invention can be carried out with an ascending, descending or even horizontal gas flow. It can work in the pit, but preferably in reactors, isolated or in battery.

 Given the flow of gas to be treated which causes some of the moisture of the biofilter, the sludge is advantageously rewetted during the implementation of the process, both continuously and sequentially.

 Figure 1 shows schematically an installation usable for the implementation of the method described above.

The process according to the invention comprises a large number of advantages listed below
The gases which can be treated according to the invention may contain organic or inorganic compounds or a mixture thereof. The results obtained show that the efficiency of the process is total in both cases and this from the beginning of the feed of the biofilter with the polluted gas.

 Compared to other peat-type substrates or biofilters, soils and the like, the biofilter used in the above process consisting of sewage sludge has the peculiarity and the advantage of being naturally rich in microorganisms, 108 to 1010 CFU / g dry sludge depending on the duration of drying.

 The biofilter further has the advantage of being a source of nutrients for microorganisms.

 The biofilter, because of its intrinsic qualities mentioned above, is likely to face significant fluctuations in flow rates and concentrations of pollutants, this particularly low Cv load <0.3 kg / m3 / day.

 The biofilter does not require thermal energy to operate: placed outside, it remains effective in a wide temperature range (-20 C, +50 C) with a gas temperature to be treated ranging from 50C to 40 C.

 The biofilter as such represents a small financial investment.

 The biofilter presented here can work with gases to be treated containing little oxygen, up to only 5% by volume.

 The biofilter does not move pollution from a gas phase to a liquid or solid phase. Indeed, the treatment according to the invention leads to the formation of olfactory neutral compounds and non-toxic (for example, SO, SO 4, CO 2, H 2 O).

Once the biofilter has been used up, it constitutes a material presenting interesting characteristics among which
- a high concentration of organic matter in the form of biomass,
- an enrichment in certain mineral elements; this is particularly the case for the treatment of gases containing sulphides and / or mercaptans which make it possible to enrich the biofilter with sulfur up to a content of 50%.

 Such characteristics give the spent biofilter material interesting agronomic properties to improve the soil quality of certain soils (supply of organic matter, sulfur, etc.).

 The present invention therefore also relates to biofilters characterized in that they comprise sludge treatment plants.

 As already indicated, the biofilters according to the invention may comprise dried or moist sludge, therefore ready for use, alone or mixed, in particular with peat, bark, straw, sawdust, plastics or plastics. still some glass.

 The sludge may be in the various forms mentioned above, and may have received adjuvant treatment.

 The moisture content of the biofilters and sludges, according to the present invention, may range, for example, from 10 to 70% during operation of the steady state process, and preferably from 10 to 40%.

 The present application finally relates to the application of sludge treatment plants, including dairies or urban deodorization gas.

The following examples illustrate the present invention, but without limiting it.
EXAMPLE 1
Deodorization of a gas
The equipment used is shown in FIG.

 The gas to be treated arrives at 1, sucked by a fan 2. A rotameter 3 makes it possible to measure the gas supply rate of the reactor 4. The gases pass vertically through the biofilter 5.

 The biofilter is rehydrated with water from a reservoir 6, aspirated by a pump 7, for example peristaltic. The treated gases, deodorized out 8. 8. A differential pressure gauge 9 is also provided between the gas supply to be treated and the output of the treated gases.

The characteristics of the biofilter used are the following
Diameter 8.5 cm, height 40 cm, ground mud, 700 g dry matter. The sludge comes from the sewage treatment plant in the city of Alès (Gard - France).

 Bio-deodorization tests have been carried out on gases polluted with inorganic (for example H 2 S) or organic (for example acetone) compounds, in a range of pollutant concentrations varying from 10 ppm to 2500 ppm.

 The flow of polluted air is fixed at 90 l / h, a gas velocity of 16 m / h; the residence time in the biofilter is 90 s.

 Humidification with water is performed by a peristaltic pump, so as to maintain the moisture content between 30 and 40%. The rate was determined by sampling, weighing, and passing to the oven.

The tests carried out, without the addition of nutrients, have shown that the treatment unit as presented in FIG. 1 makes it possible to obtain total effectiveness at low, medium or high load, as shown in the table below. after:

<tb><SEP> Load <SEP> Cv <SEP> Time <SEP> Yield
<tb><SEP> kg <SEP> H2S / m3 <SEP>
<tb><SEP> biofilter / d
<tb> Low <SEP> load <SEP> (200 <SEP> ppm <SEP> H2S) <SEP> 0.27 <SEP> 6 <SEP> months <SEP> 100 <SEP>%
<tb> Strong <SEP> Load <SEP> (2500 <SEP> ppm <SEP> H2S) <SEP> 3.33 <SEP> 10 <SEP> days <SEP> 100 <SEP>%
<Tb>
The biofilter is efficient and effective from the beginning of the process feed by a polluted gas (no lag phase or acclimation).

 The biofilter according to the invention can be indifferently effective for purifying a gas containing an inorganic gaseous compound such as hydrogen sulfide or an organic gaseous compound such as acetone.

 On the other hand, tests carried out with low oxygen concentrations (5% v / v) in the gas to be treated show that the biofilter retains an efficiency comparable to that observed in the presence of air (21% oxygen).

EXAMPLE 2
Deodorization of a gas containing acetone
The characteristics of the biofilter used are identical to those of Example 1. The tests were carried out on a polluted gas with 400 vpm of acetone, according to a charge of 0.985 kg of acetone per cubic meter of biofilter and per day.

 The test performed without the addition of nutrients shows a total effectiveness at the load considered: 100% yield over a period of 6 months.

 It should be noted that the biofilter is effective from the beginning of its diet by the polluted gas and did not require acclimation phase or latency phase.

EXAMPLE 3
Deodorization of a mixture
Using a biofilter analogous to that of Example 1 was treated with an odorant mixture containing - H2S 60 vpm - NH3 10 vpm - Ethyl mercaptan 0.5 vpm - Ethylamine 0.5 vpm - Ethanol 1 vpm
The effectiveness of the treatment was 100% for a speed of 200 meters per hour.

Claims (9)

 1. Biological treatment process of gas in which the gas is treated with a biofilter, characterized in that said biofilter comprises wet sludge treatment plant.  2. Method according to claim 1, characterized in that the sludge has a microorganism content greater than 107 Colony Forming Unit (CFU) / g of dry sludge.  3. Method according to claim 1 or 2, charac terized in that the biofilter further contains a salt selected from the group of calcium carbonate and magnesium carbonate.  4. Method according to any one of claims 1 to 3, characterized in that the sludge moisture is between 10 and 70 t.  5. Method according to any one of claims 1 to 4, characterized in that the biofilter is remoistened during its implementation.  6. Biofilter characterized in that it comprises sludge treatment plant.  7. Biofilter according to claim 6, characterized in that it contains particles of a buffer salt.  8. Biofilter according to claim 7, characterized in that the buffer salt is selected from calcium carbonate and magnesium carbonate.  9. Application of sewage sludge to gas deodorization.