GB2282828A - Extracting peat fibre for use in biofiltration - Google Patents

Abstract

Peat fibre for use as at least part of a filtration medium in a biofiltration system is extracted from milled peat. Feedstock milled peat is broken up in a hammer mill 3 and then screened at a first screening station 5. Oversize material is further screened using a finger deck screen 7. The screened fibre is further processed by passing it in series along three inclined belt conveyors 8, 9, 10 which remove higher density fibres, the lighter fibres being received in a silo 12. The fibres to be used have an average length of 2 - 10cms. The peat fibre is typically mixed with heather in a 50:50 volume ratio to form an effective biofiltration medium. <IMAGE>

Description

"Improvements in and relating to biofiltration" The invention relates to a biofiltration medium and in particular to a biofiltration medium based on peat.

Biofiltration is a method of treatment for cleaning gas streams originating from a number of industrial and waste treatment activities. The removal of odorous components from the gas takes place by the activity of microorganisms (mainly bacteria) which are present on an organic carrier material or medium.

Waste gases are passed via ducting into a space located beneath or above the filter medium. As the gas passes through the filter the odorous compounds are absorbed onto an aqueous layer and are degraded by microorganisms which reside within the organic filter material. The endproducts of the oxidation process are odourless compounds, mainly carbon dioxide, water, nitrates and sulphates.

Oxidation restores the absorption capacity of the filter material and provides the microorganisms with the nutrients necessary for their survival and growth. Thus the entire biofiltration process is self generating.

One of the main advantages of biofiltration in comparison with other techniques is that many chemically different compounds can be eliminated with a high degree of efficiency due to the adaptation of the microorganisms to the compounds present in the foul gas stream. Biofilters require little maintenance and have low energy requirements. The running costs of biological odour treatment technologies have been found to be 2-10 times lower than for other treatment methods. As there are no secondary disposal problems, biofiltration is the most "environmentally friendly" option for odour control.

Biofiltration is a proven odour control technology for a number of applications as described in the VDI guideline of biofilters 3477 (1991).

Biofiltration of gases is a result of physical, chemical and biological processes. The main physical process are adsorption and absorption. Odourless compounds are adsorbed onto the filter medium and absorbed onto the layer of water surrounding the medium. The chemical reactions that take place on the surface of the biofiltration medium includes precipitation, ion exchange and chemisorption. The biological processes are the breaking down of waste gases by microorganisms.

The efficiency of a biofilter depends on a large number of factors. One of the main factors is the biofiltration medium that is used in the bio=1ter.

According to the invention there is provided a method for extracting peat fibre comprising the steps of: preparing milled peat; blending the milled peat to provide a feedstock milled peat; breaking up the milled peat; and screening the broken up milled peat to extract peat fibres having an average length of from 2 cm to 10 cm and at least 60% of the fibres having an average dimension greater than 5 mm.

Preferably at least 70%, most preferably at least 80% and ideally approximately 82.5% of the fibres have an average dimension greater than 5 mm.

In a preferred embodiment of the invention, the milled peat is broken up in a hammer mill.

In one arrangement the broken up milled peat is screened by passing the peat over at least two screens arranged in series. Preferably the screens comprise finger deck screens.

In a particularly preferred embodiment of the invention, the screened peat is further processed to extract peat fibre having a loose bulk density at 50% moisture of from 100 to 150 Kg/m3, preferably from 110 to 130 Kg/m3.

In an especially preferred embodiment of the invention, the screened peat is passed up an inclined conveyor means.

Typically the inclined conveyor means comprises at least two inclined conveyors arranged in series, preferably at least three conveyors arranged in series.

In a particularly preferred arrangement, each inclined conveyor comprises an inclined belt conveyor having a friction belt driven at high speed, fibre being delivered from one conveyor to the next and fibre which is of sufficiently low density to travel along the conveyors being collected.

The invention also provides peat fibre whenever extracted by the method of the invention.

Preferably the fibre has a length from 2 to 10 cm, a moisture content of from 40 to 60% w/w, a loose bulk density at 50% moisture of from 110 to 130 Kg/m3 and a size range of 75% greater than or equal to 10 mm and 82.5% greater than or equal to 5 mm.

Most preferably the fibre is primarily the root residues of eriophorum (cottongrass plants).

The invention further provides a biofiltration medium including the peat fibre. Preferably the medium comprises a mixture of peat fibre with heather. Typically, the mixture is approximately 50:50 by volume. The heather generally consists mainly of the species Calluna Vulgaris.

The invention also provides a biofiltration system including a biofiltration medium according to the invention.

The invention will be more clearly understood from the following description thereof given by way of example only xith reference to the accompanying drawing which is a schematic diagram of a Set45d for extractLr.s eat fibre according to the invention.

Referring to tre drawing, there is illustrated a method for extracting peat fibre according to the invention. The method comprises the step of preparing milled peat and delivering the milled peat into an intake silo 1 in which the milled peat is blended to produce a feedstock with a specific range of moisture contents and densities. The feedstock milled peat is delivered to the silo 1 along a conveyor 2 to a hammer mill 3 where the milled peat is broken up and delivered to a conveyor 4. The broken up milled peat is then delivered from the conveyor 4 to the first screening station 5 in which the broken up peat passes over perforated plate and finger deck screens. The undersize material from the finger deck screens is used to produce peat briquettes and the oversize is delivered to a conveyor 6 to a further finger deck screen 7. The fines from the second screen 7 are typically used as an energy source.

Approximately 75% of the fibre from the screening station 7 has an average dimension of greater than or equal to 10 mm and approximately 82.5% of the fibre has an average dimension of greater than or equal to 5 mm. The fibre is further processed to extract peat fibre having a loose bulk density of 50% moisture of from 110 to 130 Kg/m3 by passing the fibre along an inclined conveyor means which in this case comprises three inclined belt conveyors 8, 9, 10 arranged in series. Fibre which is of sufficiently low density to ascend the conveyors 8, 9, 10 is delivered to a conveyor 11 for storage in a silo 12. The higher density peat which falls from the conveyors 8, 9, 10 is collected on a conveyor 13 and is typically used as an energy source.

the peat fibre consists mainly of root residues of eriophorum (cottongrass plants) and has the following distinctive properties for use as a biofiltration medium: Length of fibre 2-10 cm Moisture content 60-60% w/w Loose Bulk Density 110-130 Kg/m3 (at 50% moisture) Size Screening 75% > 10 mm 82.5% 2 5 mm Organic Matter Content > 95% w/w (anhydrous basis) Botanical Composition typically > 50% eriophorum (v/v) In order to maintain an open structure and to provide a support skeleton, the peat fibre is mixed with mature heather. Heather is mixed with peat fibre in a 50:50 (vol:vol) ratio. It must be snfficiently lignified for structuring and supporting the peat fibre. It consists mainly of Calluna Vulgaris.

The biofiltration medium produced by the method of the invention provides a suitable environment for microbial growth. The medium has sufficient voids to achieve a low pressure loss, small specific filter resistance and low energy requirements with good drainage and oxygenation.

The structure of the medium is regular and even resulting in a uniform penetration by waste gases and a low pressure drop across the medium. The medium is suitable for providing a large surface area for physical, chemical and biological reactions to occur. The medium further provides good buffering against pH fluctuations in the waste gas or oxidation products. The medium also provides good water retention with a very low depreciation rate.

The rate of humidification of the peat fibre is extremely low, resulting in a ong flltratior oed lifrime.

The volume of a typical biofiltration bed is a function of the loading rate of the waste gas to be treated. The residence time of the gas in the filter should be such that sufficient time is available to allow absorption of chemical components into the aqueous phase and subsequent degradation. Typically the size of the filter bed is designed dependent on the gas volume flow requiring treatment and the concentrations of individual compounds in the malodorous gas. The medium is typically inoculated with microorganisms. A biofilter is typically ventilated by a forced draught fan and a water sprinkler system consisting of a network of spray nozzles is located above the biofiltration bed. This is used to ensure that the moisture content of the medium is maintained between 50 to 70%.

EXAMPLE 1 Using a fibre/heather mixture in a 50:50 vol:vol ratio as a biofiltration medium, the following results were obtained.

Application: Animal By-product Rendering Source of foul gas: Factory air and non condensible process gases Pretreatment: Acid scrubber (non condensibles) Humidification Total air flow: 70,000 m3/hr Biofilter volume: 370 m3 inlet odour concentration: 40,000 ou/r Performance: 95-98% odour removal* * determined by Forced Choice Dynamic Olfactometry.

EXAMPLE 2 Using a fibre/heather mixture in a 50:50 vol:vol ratio as a biofiltration medium, the following results were obtained.

Application: Fishmeal Processing Source of foul gas: Process gas and factory ventilation air.

Pretreatment: Bag filters for dust removal.

Humidification Total gas flow: 130,000 m3/hr Biofilter volume: 600 m3 Inlet odour concentration: > 65,000 ou/m Performance: Odour removal efficiency > 99% * determined by Forced Choice Dynamic Olfactometry EXAMPLE 3 Using a fibre. heather mixture in a 50:50 vol::ol ratio as a biofiltration medium, the following results were obtained.

Location: Sewage Treatment Plant Application: Treatment of emissions from primary sludge storage tank Date of installation: December 1992 Biofilter size: 20m3 (2 stage) Total gas flow rate: 306m3/hr Removal efficiency for individual compounds:

Compound Inlet % removal Concentration H2S (PPM) 40-100 98-100 Mercaptans (PPM) 4 Dimethyl 20 95 Sulphide (PPM) EXAMPLE 4 Using a fibre/heather mixture in a 50:50 vol:vol ratio as a biofiltration medium, the following results were ~Dtalned .

Location: Sewage Treatment Plant Application: Treatment of emissions from primary sludge storage tank.

Date of installation: September 1992 Biofilter size: 50 m Total gas flow rate: 3500 m3/hr Inlet odour concentration: '20,000 OU/m3 Odour removal efficiency: 97% Removal efficiencies for individual compounds: Dimethyl disulphide: 84% 1,2,3, trimethyl benzene: 99.9% 1,2,4, trimethyl benzene: 99.9% Dimethyl trisulphide: 99.5% Diethyl, methyl benzene: 99.9% Cg hydrocarbon: 72.2% C10 hydrocarbon: 73.5% Total other hydrocarbons: 94.3% The invention is not limited to the embodiments hereinbefore described which may be varied in both construction and detail.

Claims (26)

1. A method for extracting peat fibre comprising the steps of: preparing milled peat; blending the milled peat to provide a feedstock milled peat; breaking up the milled peat; and screening the broken up milled peat to extract peat fibres having an average length of from 2 cm to 10 cm and at least 60% of the fibres having an average dimension greater than 5 mm.

2. A method as claimed in claim 1 wherein at least 70% of the fibres have an average dimension greater than 5 mm.

3. A method as claimed in claim 1 or 2 wherein at least 80% of the fibres have an average dimension greater than 5 mm.

4. A method as claimed in any of claims 1 to 3 wherein approximately 82.5% of the fibres have an average dimension greater than 5 mm.

5. A process as claimed in any preceding claim wherein the milled peat is broken up in a hammer mill.

6. A method as claimed in any preceding claim wherein the broken up milled peat is screened by passing the peat over at least two screens arranged in series.

7. a method as claimed in claim 6 wherein the screens comprise finger deck screens.

8. A method as claimed in any preceding claim wherein the screened peat is further processed to extract peat fibre having a loose bulk density at 50% moisture of from 100 to 150 Kg/m3.

9. A method as claimed in claim 8 wherein the peat fibre has a loose bulk density at 50% moisture of from 110 to 130 Kg/m2.

10. A method as claimed in claim 8 or 9 wherein the screened peat is passed up an inclined conveyor means.

11. A method as claimed in claim 10 wherein the inclined conveyor means comprises at least two inclined conveyors arranged in series.

2. A method as claimed in claim 11 wherein there are at least three inclined conveyors arranged in series.

13. A method as claimed in any of claims 10 to 12 wherein each inclined conveyor comprises an inclined belt conveyor having a friction belt driven at high speed, fibre being delivered from one conveyor to the next and fibre which is of sufficiently low density to travel along the conveyors being collected.

14. A method substantially as hereinbefore described with reference to the drawing and Examples.

15. Peat fibre whenever extracted by a method as claimed in any preceding claim.

16. Peat fibre as claimed in claim 15 having a length of from 2 to 10 cm.

17. Peat fibre as claimed in claim 15 or 16 having a moisture content of between 40 and 60% w/w.

18. Peat fibre as claimed in any of claims 15 to 17 having a loose bulk density at 50% moisture of from 110 to 130 Kg/m3.

19. Peat fibre as claimed in any of claims 15 to 18 at least 75% of which has an average size greater than or equal to 10 mm.

20. Peat fibre as claimed in any of claims 15 to 19 in which 82.5% of the fibre has an average size greater than or equal to 5 mm.

21. Peat fibre as claimed in any of claims 15 to 20 wherein the fibre IS primarily the root residue of eriophorum (cottongrass plants).

22. A biofiltration medium including a fibre as claimed in any of claims 15 to 22.

23. A biofiltration medium as claimed in claim 22 wherein the medium comprises a mixture of the peat fibre with heather.

24. A biofiltration medium as claimed in claim 23 wherein the peat fibre heather mixture is approximately 50:50 by volume.

25. A biofiltration medium as claimed in claim 23 or 24 wherein the heather consists mainly of the species Calluna Vulgaris.

26. A biofiltration system including a biofiltration medium as claimed in any of claims 22 to 25.