Bio-deodorization technology for waste gas treatment: principles and technical characteristics

This project is a wastewater treatment plant, which belongs to an explosion-proof area. When selecting a waste gas treatment and deodorization technology, the actual operating conditions must be fully considered, and a safe, reliable, and stable deodorization technology should be selected accordingly.

Comparison and selection of common waste gas treatment and deodorization technologies:

According to the comparison, our company chooses the "biological washing + biological filtration" combined biological deodorization technology for this project. In addition, considering the high concentration of benzene and hydrocarbon substances in the waste gas of the petrochemical wastewater treatment plant, our company has adopted the oil-water separation flotation technology before the biological treatment to ensure that the tail gas meets the emission standards.

1, Waste gas treatment process

After being collected, the malodorous gas enters the pretreatment section of the malodorous pollution control system for oil-water separation, temperature adjustment, dust removal, and humidification, and then enters the biological treatment section. The pollutants in the waste gas contact with the microorganisms on the filler, are captured, degraded, and oxidized by the microorganisms, so that the pollutants are decomposed into harmless substances, and the harmless gas is discharged to meet the standards.

Process flow control chart

The odor treatment equipment adopts a multi-stage treatment process; the multi-stage treatment process is a combined deodorization process consisting of a pretreatment section, a biological treatment section 1, and a biological treatment section 2.

 Pretreatment section

The pretreatment has an independent pretreatment tower; oil-water separation technology is used to enhance the ability of the deodorization system to remove oily substances; to ensure the effectiveness of subsequent biological treatment. Supplementary water sources can be the site's own water as needed.

After being washed by high-pressure water, the oil in the special filter material of the pretreatment part is collected into the circulating liquid. The oil-containing circulating liquid is de-oiled by gas-liquid mixed pump flotation, and the oil is concentrated in the overflow tank. The overflow wastewater enters the wastewater treatment system.

The role of the pretreatment stage of the deodorization unit

ØBy removing dust from the odor, it prevents dust from entering the subsequent biological trickling filter, causing an increase in pressure drop, to avoid increasing operating costs or even operational failure.

ØThrough washing, the humidity of the odor entering the subsequent biological treatment device reaches saturation, meeting the strict requirements of biological treatment for humidity.

ØThrough water washing, some soluble waste gas components can also be removed, and to a certain extent, the impact load caused by the sudden change of odorous components can be reduced, ensuring the relative stability of the subsequent biological treatment odor load.

If the pretreatment is forHydrocarbon gasesWithout treatment, on the one hand,Hydrocarbon gasesoily droplets are easily formed in the biological treatment section, depositing on the surface of the filler and thus affecting the metabolism of microorganisms; on the other hand, the oily smell of VOCs cannot be removed.

      Biological treatment section: Includes biological treatment section 1 and biological treatment section 2.

ØBiological treatment section 1

Biological treatment section 1 uses a biotrickling filter process, mainly targeting sulfur-containing odorous substances.

ØBiological treatment section 2

Biological treatment section 2 uses a biofilter process to enhance the treatment of benzene, hydrocarbons, and other hydrocarbon compoundsand prolongs the contact time between odor and biological filler.The pH, temperature, and other conditions in this section ensure a suitable environment for the growth and reproduction of microorganisms that treat benzene, hydrocarbons, and other hydrocarbons.

     Subsequent treatment section: Use a catalyst to catalytically absorb odor again.

The HT-708 room-temperature high-efficiency composite metal oxide desulfurization catalyst is independently developed by Dalian Institute of Chemical Physics. It is a room-temperature high-efficiency desulfurizer prepared by a special process. Its main components are αM2O3 and γM2O3. It has the characteristics of developed micropores, large specific surface area (100-200m3/g), high bulk density, fast reaction speed, high sulfur capacity (≥25%), good strength (≥60N/cm), and good water resistance. It is widely used in the fine desulfurization of various gases such as natural gas, water gas, semi-water gas, air gas, coke oven gas, liquefied gas, CO2 regeneration gas, food CO2, synthesis raw gas, biogas, and waste gas, and the use effect has reached or exceeded the design requirements.

Ø2.2、Reaction principle

ØFe₂O₃ ·H₂O.+3H₂S=Fe₂S₃ ·H₂O +3H₂O

ØFe₂O₃ ·H₂O.+3H₂S =2 FeS+S+4H₂O

ØZnO·H₂O.+H₂S = ZnS+S+2H₂O

Ø2.3、Main performance and quality indicators

Ø2.4、Main features

Ø1) High desulfurization accuracy: Outlet H₂S≤1mg/m³。

Ø2) High strength, good water resistance: After 50 impact tests of rapid pressurization and depressurization under 10 MPa pressure, the strength remains unchanged.

ØAfter the subsequent treatment section, the highest emission standard value of "Emission Standard for Volatile Organic Compounds and Odorous Pollutants in Wastewater Treatment Plants (Stations) of Organic Chemical Enterprises" DB37-3161-2018 is reached.

2 Waste gas treatment process principle and technical characteristics

2.1 Biological filler

The biofilter used is artificial filter material and specially treated high-quality natural materials. The biofilter has a tough structure, strong acid and alkali resistance, and a large specific surface area. It can improve the biochemical reaction efficiency, and the porosity between the filter materials is large, so the pressure drop of the equipment is low. Due to its unique material, it is resistant to biodegradation and has high acid resistance. After contact with acidic odorous gases, it will not undergo qualitative changes, so compaction and agglomeration will not occur.

The shape of this filter material is suitable for the passage of odorous gases, and the filter material is graded in the filter body, with the following functions:

ØAs a carrier for microorganisms

ØProvide nutrients for microorganisms

ØProvide a humid ecological environment for microorganisms

ØProvide an adsorption surface for odorous pollutants

ØMaintain an appropriate pH value

Microorganisms exist in the form of populations, and multiple microorganisms coexist in one environment. The characteristics of microorganisms are both similar and different. Different polluting substances can find microorganisms that degrade them in nature. Therefore, multiple pollutants can be treated and purified simultaneously in one device.

Biological strains degrade odorous pollutants into carbon dioxide and water, without causing secondary pollution.

The biodegradation reaction formula is:

                         Microorganism

 Odd (smelly) pollutants + O2 ————→ Cell substance + CO₂ + H₂O

After environmental conditions change, some microorganisms will die, while others will continue to survive. The surviving microorganisms, after a short period of reproduction, can develop into dominant bacteria. Therefore, this device can withstand shock loads. When the concentration of pollutants increases, the treatment effect decreases in a short period, but it can quickly recover to normal.

2.2 Biopurification Device for Waste Gas Treatment

The total gas volume processed by the odor control project is 18000 m³/h. The deodorization system consists of a blower, main deodorization equipment, and corresponding water pumps, pipes, pipe fittings, valves, and electrical control systems.

The main equipment shell of the deodorization unit in this project uses corrugated 304 stainless steel, which enhances equipment strength, improves aesthetics, and meets the requirements of equipment corrosion resistance.

The biopurification device is the main equipment in the complete set of odor pollution control equipment. It is the place where microorganisms degrade gaseous organic pollutants and is the latest deodorization technology equipment. Odorous gases are drawn into the biopurification device by a blower, and after passing through the biofilter material, they are biopurified by the strains attached to the biofilter material.

This equipment has insulation function. The inner layer material is fiberglass reinforced plastic, and the outer layer is 304 stainless steel. The top of the shell has a manhole for maintenance and inspection, the middle has an inspection port for the spray system, and the bottom has an air inlet connected to the air intake pipe.

Many kinds of microbial strains are fixed inside the biopurification equipment. The complete set of biopurification equipment uses specially selected biofilter materials, which fix the selected microbial strains on the biofilter materials. The biofilter materials are supported by corrosion-resistant plastic grid plates to achieve uniform air distribution. Microorganisms are attached and fixed on inert high-efficiency fillers with a large specific surface area. The biopurification equipment provides suitable temperature, humidity, and pH conditions for microbial growth, as well as sufficient oxygen and nutrients. A suitable environment for microbial growth can be created through effective control in this device. The organic matter in the waste gas can provide the carbon, nitrogen, phosphorus, and some trace elements needed by microorganisms. When a certain nutrient element is lacking, it can be added selectively in appropriate amounts.

When odorous gas passes through the biofilter material, the selected high-efficiency strains on the filter material carry out biological oxidation of the malodorous components in the odorous gas, and the gas is purified. The metabolic products such as sulfuric acid and nitrous acid produced during the oxidation and decomposition of odorous gas enter the circulating water device and are finally discharged together with the metabolic products.

Performance advantages of biopurification equipment:

(1) Low equipment operating cost.                             `nbsp_tag

(2) High degree of equipment automation (considering the need to ensure that the deodorization equipment operates under optimal conditions, it is still recommended to configure an automatic control system), stable operation, and unattended operation.

(3) Microorganisms grow and regenerate quickly, always maintaining good activity. The restart speed is fast after stopping use.

(4) The biopurification equipment has strong resistance to shock loads and can adapt to the treatment of waste gases of various concentrations.

(5) High-efficiency bio-filler is used, with a long service life, generally up to more than ten years, without replacement;

This type of bio-filler has good structural stability and strength, so the equipment has low wind resistance and low pressure drop.

2.3 Waste Gas Treatment: Recirculating Spray Demisting System

(1)　Recirculating water pump

Each section of the treatment process of the complete set of odor pollution control equipment is equipped with a recirculating water pump, which uses corrosion-resistant materials.

Because a combined bio-deodorization process is used, its humidification function and biodegradation function are combined into one, and the humidification pipeline is arranged in the same shell, resulting in a simple structure. The humidification system is equipped with a full set of components: including nozzles, circulating water pumps (with level switches), filters, valves, flanged pipes, etc.

The shell, main shaft, and impeller of the circulating water pump are all made of corrosion-resistant materials, with strong acid corrosion resistance. The sealing uses mechanical seals, and the protection level is IP55. The insulation class of the circulating pump is F.

(2)　Spray system

A spray sprinkler device is set above the filling filter material to maintain a certain humidity on the surface of the filling material and aerobic microorganisms. The water pump is installed on the filter connected to the bottom of the bio-deodorization equipment. Ensure that the circulating water pump is filtered before entering the water.

1)  The humidification pipe is arranged in a mesh shape to ensure that the circulating liquid is evenly sprayed.

2)  This system is located inside a closed filter tank, and its installation method allows installation personnel to inspect and maintain without disassembling the pipeline, without affecting the operation of the treatment system.

3)  This system can meet automatic or continuous operation.

4)  A drain pipe is set at the bottom of the nutrient solution circulation tank to drain rainwater and metabolic residues. PVC pipes are used.

(3)　Demisting system

A demisting device is installed at the outlet of the deodorization equipment, and the demisting efficiency is no less than 90%.

2.4 Waste Gas Treatment: Temperature Control System

The biopurification of gaseous pollutants is a comprehensive process of diffusion, adsorption, and biochemical reaction. An increase in temperature is conducive to the biochemical process. The optimal temperature for microbial growth in the bio-deodorization system is 20-40℃. Microorganisms can metabolize at low temperatures, but the reaction rate is slower. In winter, the weather is cold. To ensure that the entire system has a good deodorization effect under any conditions, and to prevent a large number of microbial deaths due to a sudden drop in environmental temperature.

This project is located in northern China, where winter temperatures are relatively low. To ensure the deodorization effect of the bio-deodorization equipment, the deodorization system is equipped with a temperature control system in the scheme. When the temperature is below the specified value, steam is used to heat the circulating liquid to maintain a heat balance; ensuring a suitable growth environment for microorganisms, keeping the microorganisms in optimal condition, thereby achieving the best effect of removing pollutants.

2.5 Waste Gas TreatmentElectrical and Automatic Control

This project is equipped with on-site electrical control cabinets, PLC control systems, and cables within the scope of supply.

ØControl System and Equipment

The control system can complete all the measurement, monitoring, control, and interlocking functions within the odor treatment system.

ØPLCControl System

     The control system provided by our company includes hardware, software, and corresponding engineering services.

Brief Description of the Control Process

(1)    The entire process includes a pre-washing section and a biological treatment section. Each section can operate independently, automatically judging its operation based on the conditions of that section. When all conditions are met, the entire equipment will operate.

(2)    Startup sequence: Circulation pump----Fan

(3)    Shutdown sequence: Fan stop----Circulation pump stop------Valve closure

(4)    Detection parameters (including settings): Liquid level, PH value

2.6 Waste Gas TreatmentEquipment Material and Corrosion Protection

Appropriate anti-corrosion treatment is carried out during equipment selection and manufacturing:

1. Considering corrosion protection, the main material of the deodorization equipment uses UV-resistant stainless steel that does not require external maintenance, with fiberglass anti-corrosion on the inside.

2. Fiberglass is used for external packaging at the internal support welding points of the deodorization equipment.

3. The materials of the main equipment are as follows:

Fan: FRP

Circulation pump: Horizontal centrifugal pump, corrosion-resistant, mechanical seal used for sealing

Connecting pipes inside the deodorization device and connecting pipes and pipe fittings between the deodorization device and the fan: Fiberglass FRP or FRP+PVC

Fan inlet and outlet pipes: Fiberglass

Exhaust stack:Fiberglass，Equipped with lightning protection and grounding facilities. The exhaust stack is 15 meters high.

Supplementary water: Secondary sedimentation tank effluent or site self-supplied water is used as supplementary water