GB2336361A - Biological treatment of waste gases - Google Patents

Abstract

A treatment plant for the biological treatment of waste gases containing volatile organic compounds and/or odoriferous compounds uses a biotrickling filter having microorganisms on an inert support material and a biofilter (3) containing organic media for supporting microorganisms, wherein the biotrickling filter and the biofilter (3) are separated by distinct baffles (4,5,6). Preferably, the organic media comprises Norfolk reed (11) and hemp shive (12). The gaseous effluent to be treated may comprise an air stream containing acetone, methyl ethyl ketone, toluene and hexane. The microorganisms are typically selected from bacteria, fungi, and protozoa which are able to metabolise the compounds into carbon dioxide and water.

Description

2336361 Biological, Baffled VOCI0dour Treatment Plant

Introduction

The invention relates to a treatment plant (1) and controlloperation of a biological treatment process of off gases containing odoriferous andlor volatile organic compounds (VOCs) and their efficient removal by passing them through a combined and baffled biological treatment plant (1) in which the contaminated air is passed through a biotrickling filter (2) followed by an organic biofilter (3) bed. Both sections have selected and enriched micro-organisms (5) for the biodegradation of the contaminants.

Biological purification of off gases is a well known technology. Biofiltration for the removal of volatile organic compounds (ViOCs) relies on microorganisms (biomass), immobilised on media or suspended in liquid, to biodegrade the VOC compounds in off gases to harmless salts and carbon dioxide and water.

There are several biological treatment systems currently applied for VOC treatment including biofilter, bioscrubber and biotrickling filter. Biofilter contain an organic packed bed ( e.g. heater, compost, peat etc. ). The packing material acts as a support material on which microorganisms are attached within a thin liquid film or biofilm. The gaseous effluent passes through the bed where the VOC components are utilised by the microorganisms as their carbon source. Application range of biofiltration is below 0.5 91M3 Of VOC target compounds.

Bioscrubber consists of a scrubber unit ( i.e. a spraying tower filled with an inert packing material), through which the VOC off gas is forced through and a separate bioreactor containing the microorganisms. The aqueous phase is continuously recirculated between the two separate units. Bioscrubbing are mainly suitable for easily water soluble VOC compounds at concentrations of 1 - 5 glm'.

Biotrickling filterconsists of an inert packed bed through which the gaseous effluent is passed. The inert material is covered with an active microbial biofiim. The aqueous phase is sprayed on the packing material and continuously recycled. Blotrickling filters are very effective for easily soluble and poorly soluble compounds at concentration ranges between 0. 1 - 5 g1M3.

These generic biotechniques have been successfully applied for treatment of VOC/odours, but still have limitations. r- Biofilters are generally suitable for poorly water solublelinsoluble compounds, but generally only at low concentrations due to clogging of excess biomass growth at higher concentrations. Bioscrubbers are most efficient on VOC components with a high or moderate water solubility, but are limited for the treatment of VOCs above 1 g1M3 due to the high costs of irrigation liquid requirement. Biotrickling filters are suitable for VOC 1 components with a high, moderate and poor water solubility over a large range of -=centrations, but are not suitable for insoluble VOCs/oclours.

The organic media in biofilters has three main functions.

Provide surface area for attachment of microorganisms ii. Temporary absorption of VOC components at peak loads iii. Storage of water and source of nutrients Common materials used as biofilter media have the common disadvantages that they..

i.

Age very quickly ii.

Lead to drying out iii.

Rewetting is difficult iv, Formation of lumps causing channelling V.

Rapid utilisation of available nutrients vi.

Restriction of bed height to 0.5 - 1.0 m, requiring large footprints.

The main disadvantage of biotrickling filters is said to be clogging of the inert support media with the production of excess biomass, leading to an increase in back pressure and reduction of treatment efficiency. However, different types of inert media have been shown to have insignificant problems with clogging.

Biotrickling filters are quoted to be less capable in dealing with variation in peak VOC loadings. The recirculation liquid velocity in biotrickling filters ranges between 2 - 50 m/h in order to provide effective mixing of the solvent laden air and the liquid and is sprayed over the inert support material via different types of nozzles generating either coarse, fine or very fine droplets. It has been demonstrated that liquid droplet size influences the absorption capacity of poorly water soluble compounds. To engineer a blotrickling filter with this large range of liquid velocities would require expensive pump capacities and electricity and also, once specified, the liquid velocity is fixed, making it inflexible.

This invention now has the objective of providing an apparatus, method and control system for biological treatment of VOC'slodours in order to improve the above mentioned disadvantages and provide a more efficient biotechnique. The extent of the invention can be subdivided into the following categories..

2 i. Design and configuration of a biological VOClodour treatment plant i 1. Application of a new type of organic media for a biofilter Method of inoculation of biofilters iv.

V.

Operation of the biotreaterrit process Innovative process control by feedback control loops.

The design and configuration of biological VOCIodour treatment plant is an important aspect for a successful treatment, but some important engineering aspects were overlooked in previous publications. Since in most industrial or manufacturing off gases the composition of VOCsIodours will be a complex mixture of compounds containing hydrophobic and hydrophilic compounds, to combine two suitable generic biotechniques into one combined system is a logical step. Combinations of a BTF followed by a 13F in separate have been previously published. This invention has combined a biotrickling filter (2) and biofilter (3) in such a way that the treatment process can be constructed into one treatment plant (1). The sizing of the combined treatment plant (1) and the size of each individual section will be determined and calculated by means of concentration and composition of VOCslodours present in the waste gas. Each section is separated by means of an inserted baffle (4), providing means of re-direction of the air stream. Additional baffles (5,6) are inserted into the biotrickling filter (2) and biofilter (3) in order to provide further sections and increase the serpentine air flow within the treatment plant (1). By this mean the VOCslodour components are treated in the air stream in the first baffled section by specialised microorganisms (7) and the air reappears below the first baffle, where remixing of the residue VOC components can occur. The remixed air is then reentering the second baffle of the biotrickling filter (2) where biological treatment of VOCsIodours proceeds. The baffle which separates individual sections are constructed in this way that the first baffle (5) closes the top air space and the second baffle (4), penetrates 0.2 m below the BTF sump (13) liquid level to prevent air passing through. This baffled approach may be similar to that of the SHEGOBIOCLEAN system. However, the system of this invention claims to have overcome the problems of increasing the number of baffles to direct the air flow in a multi-serpentine mode. Also this invention claims to have determined the optimum distance between baffle sections. The distance between baffles were determined in this invention to be proportional to the total air flow not exceeding an air velocity of more than 0.25 mls in each section during steady state operation. For start up of the system a staged air velocity approach has to be used, starting with air velocity in the order of 0.05, 0.1 and 0.25 m/s dependent on the VOC/odour concentration and composition of the off gas.

The biotrickling filter (2) sections contain an inert support media (13) for the attachment of specialised microorganisms (7), an irrigation system with nozzles (14) and has a sump (15) in order to contain the liquid for continuous recirculation over the inert support media (13). Construction of current BTF plant has the disadvantage that excess blomass settles and accumulates on the floor which may cause blockage and 3 may give rise to malodours. The construction of biotrickling filter sump (15) of this. nvention includes a false floor (16) with a slope of 5' to 400 at its base, which faci I itates that any excess biomass is sliding towards the channel of the biological treatment plant (1). A pipework structure (17) is evenly distributed throughout the channel in the sump (15) in order to enhance biomass removal using pump P4 (18) at intermittant intervals to remove the liquid sludge.

The rate of irrigation of the BTF is quoted to range between 2 - 50 mlh. Research has revealed, and this constitutes a major aspect of this invention, that regulation of the pump speed by means of an on-line feedback control loop (8) can provide a more economic means of regulating the recirculation pump (19). This method also contributes to remove efficiently peak VOC concentrations in the BTF which were up to now less effective to cope with peak VOC loadings. The control system employed involves the on-line monitoring of the VOC concentration in the treated off gas by means of a PID, FID or olefactometic instrument (20). Once a set VOCIodour concentration threshold level has been reached or overstepped (i.e. 50 mg CIM3), the recIrculation pumping rate is increased by means of an invertor (10). This invertor is controlled and combined in the programmable logic control software programme (9). By this feedback control loop (8) the size of the pump P 1 (19) and liquid velocity can now be determined and adjusted according to VOC components and concentration in the off gas and sprayed over the inert media (13) through the irrigation system (14).

For the biodegradation of VOCIodour components the specialised microorganisms (7) require nutrients including N, P and S as well as trace elements. Addition of nutrients is either carried out in the form of a liquid, premixed, concentrated nutrient stock solution from a concentrated nutrient liquid tank (22) or by means of a hopper using granulated nutrients. Concentrated nutrients are either stored in a tank (22) and dosed via a peristaltic pump P2 (23) into a dilution tank (24) or directly into the recirculation liquid stream or added via a hopper directly into the recirculation liquid stream. These nutrients are added in proportion to the expected carbon load. Production of the dilute nutrient solution in dilute nutrient solution tank (24) is produced by the accurate addition of a known volume of the concentrated nutrient solution (22) and a known addition of water from the water tank (25). Addition of water from the water tank (25) is regulated by level switches L1 (26) and L2 (27) via opening of solenoid valve S5 (28).

Water loss from the purified air through the exhaust stack is reduced by means of a demister unit (29), installed at the base of the treatment plants' off gas outlet ports. The liquid droplets are returned to the sump (15) liquid by means of a plate (30) with a slope of 40 0. Replenishment of water in the BTF sump (15) liquid is accomplished by means of adding water through a pump P3 (31) from the water storage tank (25), involving opening of solenoid valve S1 (32) and solenoid vale S3 (33) when reaching a predetermined sump liquid level. The sump liquid level and water refill volume is controlled by two level switches, level switch L3 (34) and level switch L4 (35), connected to the PLC soft ware programme (9). Dilute nutrient solution from tank (24) is added to the BTF sump (15) at regular intervals controlled by the PLC (9) and added proportion to the expected carbon load. This involves pump P3 (31) and opening of 4 solenoid valve S2 (36) and solenoid valve S3 (33).

Efficient removal of VOCsIodours is dependent on tight control of pH of the sump liquid. Measurement of pH in the biotrickling filter sump is achieved by means of a pH probe connected to an electronic pH control unit (37). The pH control unit (37) doses acid or alkali into the BTF sump (15) liquid to a set pH of 7.0 via a peristaltic pump (37).

The invention of the treatment plant to remove VOCsIodours from off gases comprises a section of a conventional biofilter (3). The organic media used in this invention is different to the traditionally used materials.. In this invention a mixture of Norfolk reed (11) and hemp shive (12) is used as organic media in the biofilter (3). Both materials have the advantage over others including:

i. Provide high surface area for bacterial growth ii. Have an excellent absorption/desorption capacity for VOCsIodours iii. Very good intrinsic buffering capacity iv. Generate little back pressure comparable to inorganic media V. Efficient movement and transport of microorganisms, water and nutrients Since hemp shive (12) is in addition a waste product and readily available it has additional economic advantages.

The Norfolk reed (11) and hemp shive (12) are placed into the biofilter in a layered fashion, placing the Norfolk reed (11) on the bottom of a plastic lattice (38). The Norfolk reed (11) layer should range between 0. 1 to 0.5 m. The hemp shive (12) layer is added on top of the Norfolk reed (11) at a thickness of 0. 1 - 0. 5 m. These layers are repeated till reaching the desired biofilter depth of between 0.5 to 3.0 m. Duetothe very low air resistance of the material, the bed depths can reach up to 3. 0 m, reducing the foot print required by conventional biofilters. Fifty percent of Norfolk reed (11) and hemp shive (12) each will be used forfilling.the biofiter (3). in general, the air entering a biofilter is humidified to 95 - 100% before entering a conventional biofilter. In this invention humidification is not required due to the pre-humidification in the blotrickling filter (2) section of the air. Additional moisture is added through a separate irrigation system (39) using water from water tank (25) or dilute nutrients from dilute nutrient tank (24). Addition of water is accomplished by means of a pump P3 (31) and opening of solenoid valve S1 (32) leading to the water tank (25). Nutrients are added proportional to expected carbon loading after treatment in the BTF section. Addition of nutrients to the biofilter (3) is carried out by means of pump P3 (31) and opening of solenoid valve S2 (36) and regulated by PLC (9). The carbon/nutrient ratio will depend on the VOC off gas composition and concentration and will have to be determined case by case. Examples from research during this invention indicate that the carbon load to the BF is about 25% of the total carbon load.

For inoculation of the biological VOCloclour treatment plant (1) specially selected and,nriched microorganisms (7) comprising bacteria, fungi and protozoa are used which are capable of metabolising compounds into carbon dioxide and water. For inoculation of the biotrickling filter (2) it was determined in this invention that an inocula of 10% (vlv) is required to reduce start up of the biological apparatus. This is accomplished by adding the microbial culture broth directly the BTF sump (15) liquid which is recirculated and sprayed over the bare cross flow modular media in the biotrickling filter (2). Inoculation of the biofilter section is accomplished by intermittent irrigation of the biofilter (3) section with liquid from the biotrickling filter sump (15). This is accomplished by opening solenoid valve SW (40) and pump P 1 (19). This inoculation procedure is continued for the period of 2 weeks to establish a steady-state microbial population in the organic filter material.

The efficiency of the new invention will be demonstrated in the following application examples.

Example 1

Air stream containing acetone, methyl ethyl ketone, toluene and hexane ranging between 300 mg Carbon/ M3 to 1000 mg Claf was treated in the combined. baffled treatment plant. The volumetric loading rate ranged between 40 to 120 g VOCIM3 /h and a 100% VOC removal efficiency achieved. Increasing the volumetric to 185 g CIM3 /h and at a VOC concentration of 3.2 g C1M3 the biological combinded treatment plant achieved a 66% removal efficiency. The biotrickling section (2) removed between 45% to 90% of total VOCs and biofilter (3) section contributed between 10% to 25% to overall VOC removal.

Example H

Air stream containing acetone, methyl ethyl ketone and toluene ranging between 200 Mg CIM3 to 950 mg Clnf was treated. In Table 1 removal efficiencies of individual VOC components are listed and the contribution of the biotrickling filter (2) and biofilter (3) section are shown, indicating that combination of the biotrickling filter (2) and biofilter (3) increases overall VOC removal efficiency by between 22% to 91 %.

Solvent type Inlet concentration Profile of removal efficiency Biotrickling filter Biofilter Combined Acetone 12.5 10% 86% 96% MEK 119.4 95% 4% 99% Toluene 14.7 15% 37% 52% The recirculation liquid velocity was varied and the inlet VOC concentration kept constant. The results in Table 2 show that effects of liquid velocity to VOC removal efficiency and demonstrate that the VOC removal efficiency was increased by between 20% 25%, demonstrating the efficiency of the feed back control loop (8) described 6 in this invention.

rhe back pressure of the inert media of the biotrickling filter section was determined to be 95 Palm and the air resistance of the layered Norfolk Reed (11) and Hemp Shive (12) was determined to be 185 Palm at an air velocity of 0. 1 mls.

Table 2.. Effect of increasing liquid velocity of recirculation liquid on VOC removal efficiency Liquid velocity mls Removal efficiency case A 2 7 3 8 71 89 66 88 case B -7

Claims (1)

1. Claims

   6.

   7.

   8.

   9.

   12.

   13.

   1 A treatment plant (1) for the biological treatment t usmg specially selected and ermiched microorganisms (7) of waste gases containing VOCs/odours by a treatment plant which combines a blotrickling filter (2) on which are attached microorganisms on inert support material and a blofilter (3) containing organic media for support of nuicroorganisms, but are separated within and between with distinct baffles (4-6).

   2. A treatment plant of claim 1, characterized in that the BTF and BF are separated by means of a baffle (4) penetratmig between 0. 1 m to 0. 5m into the sump (15) liquid.

   3. A treatment plant of claim 1, characterized mi that the BTF and BF sections are flu-ther separated by baffles (5,6) determined by the volume of air to be treated. A treatment plant of claim 3, characterized in that the distance between baffles should not exceed an air velocity of 0.25 ni/s. A treatment plant of claim 1, characterized in that the floor of the apparatu. has a sloped floor (16) at an angle of 40' leading to a channel. A treatment plant of clanin 1, characterized 'm that the floor has additional eqUI-distance drainage tubing (17) for means of excess biomass removal. A treatment plant of claim 1, characterized in that the biotricklmig filter recirculation rate is dependent on the outlet VOC/odour concentration. A treatment plant of claim 7, characterized in that the recirculation pump (19) speed is controlled by a PLC (9) controlled feed back control loop (8). A treatment plant of claim 8, characterized in that the recirculation pump (19) is controlled by a PLC (9) connected invertor (10).

   10. A treatment plant of claim 1, characterized in that water replenishment and nutrient addition in the biotrick]. Mig filter is automatically replenished to preset volumes involving (9, 31, 33, 36).

   11. A treatment plant of claim 10, characterized in that this is accomplished by level switches (34, 35) involving (9).

   A treatment plant of claim 1, characterized in that the organic material 'm the blofilter consists of two types of organic materials.

   A treatment plant of claim 12, characterized in that the two orgarnic materials are Norfolk reed (11) and hemp shiver (12).

   A treatment plant of claim 13, characterized mi that Norfolk reed and hemp shive is added 50% by 50% to the blofilter.

   15. A treatment plant of clanin 14, charactenizedmi that the Norfolk reed (11) and hemp shive (12) is added as distmict layers.

   16. A treatment plant of claim 15, characterized in that each layer ranges between s 0. 1 to 0.25m.

   17. A treatment plant of clanin 13, characterized in that the Norfolk reed (11) and hemp shive (12) depth can be up to 3. Oni.

   18. A treatment plant of claim 1, characterized in that the orgamic media in the blofilter (2) is inoculated interinittently from the BTF sump (15) liqwd.