Pollutants index

Aerosols and mists are extremely fine liquid droplets that cannot be removed using traditional packed scrubbers. These droplets can be formed from gas phase hydrolysis of halogenated acids (HCl, HF, HBr), metal halides, organohalides, sulfur trioxide (SO₃), and phosphorous pentoxide (P₂O₅). Proper equipment selection for this type of application depends on the specific contaminant and exhaust volume and includes venturi scrubbers, fiber-bed mist eliminators, High Efficiency Aerosol Filtration (HEAF) Systems, and ionizing wet scrubbers (IWS®).

Emissions from chrome plating and chromic acid anodizing operations produce hexavalent chrome (sometimes called "Chrome 6") mist that cannot be effectively removed in a conventional packed tower. A modified type of vertical or horizontal scrubbing device using specialized mesh filter pads with periodic flushing is required to meet regulatory guidelines for this contaminant.

Entrained liquid droplets are much larger than aerosols and mists and are usually produced by drag-off or collection from a processing tank involving dipping or mixing operations. Typical examples are sulfuric acid (H₂SO₄), nitric acid (HNO₃), and sodium hydroxide (NaOH) from metal finishing/cleaning operations. Shallow packed bed scrubbers or stand-alone entrainment separator modules are typically used on this type of application.

Ethylene (C₂H₄O) and propylene oxide (C₃H₆O) react with water to form ethylene and propylene glycol. However, by normal scrubbing standards, the reaction is very slow and an acid catalyst such as H₂SO₄ is recommended to reduce the contact time required. Even then, a larger reaction tank is typically recommended to maximize the hydrolysis reaction. Packed tower scrubbers are the standard equipment type for this application.

For some applications, such as peak shavers prior to thermal-oxidizers, water only scrubbing can be used. Peak shavers are devices designed to reduce large fluctuations in contaminant levels prior to another processing stage. The units act as a chemical capacitor absorbing contaminant during high concentration cycles then discharging through desorption during low concentration periods.

Arsenic, Beryllium, Cadmium, Chromium, Nickel, Lead, Zinc.

High temperature oxidation of hazardous waste vaporizes heavy metals, which then condense on the ash during cooling. High efficiency removal of the finer particles is critical to meet emission limits for the volatile metals. An ionizing wet scrubber (IWS) is the most effective device to insure the highest efficiency removal at the lowest possible energy usage.

Hydrogen Chloride (HCl), Hydrogen Fluoride (HF), Hydrogen Bromide (HBr), Chlorine (Cl₂), Bromine (Br₂), Fluorine (F₂), Sulfuric Acid, Sulfur Dioxide (SO₂), Hydrogen Sulfide (H₂S), Nitric Acid (HNO₃), and Hydrogen Cyanide (HCN).

This class of pollutants is typically removed using an aqueous solution in a packed scrubber (vertical or horizontal). Depending on the specific contaminant and concentration, additional neutralizing chemicals such as NaOH (sodium hydroxide), KOH (potassium hydroxide), NaOCl (sodium hypochlorite), and Na2S (sodium sulfide) may be required to aid absorption and create more desirable by-products. Other scrubbing equipment such as sieve tray towers and eductor venturis can also be used alone or in conjunction with packed scrubbers for special applications requiring product recovery or small exhaust volumes.

Although highly water soluble, these compounds exhibit a high vapor pressure that makes recycled water scrubbing impractical. Once-through water can be used, but the amount of water required can be excessive, especially at elevated temperatures. These compounds are typically scrubbed with acidic water solutions containing low vapor pressure acids such as H₂SO₄. Higher vapor pressure acids such as HCl are not recommended due to the potential for gas phase reaction that can result in the formation of submicron salt particulate. Packed scrubbers are the most common equipment choice, however, other types of contacting devices such as eductor venturis can be used due to the rapid kinetics of the acid-base reaction along with the high solubility of the by-products.

Acetic Acid, Acetone, Acetaldehyde, Citric Acid, Cyclohexanol, Dimethyl Formamide, Formic Acid, Formaldehyde, Glycol, n-Methyl Pyrrilidone, Methanol, Ethanol, Propanol, Tetrahydrofuran, and Toluene Di-isocyanate.

Depending upon the solubility and vapor pressure of the specific pollutant, water scrubbing can be a very effective method for removal of the contaminants listed above. In some cases, chemical additives such as sodium hydroxide and/or oxidizing agents are recommended to enhance removal efficiency. For soluble ketones and alcohols, once-through water scrubbing may be required due to their higher vapor pressure. Packed scrubbers are typically the best choice for these applications but sieve tray towers can also be used reduce water consumption for once-through water designs.

Arsine, Phosphine, Chlorine dioxide, Cyanogen Chloride, Cyanogen Bromide, Sulfur Monochloride, Phosgene, Perchloric Acid, and Thionyl Chloride.

Packed towers are generally the most effective method for removal of reactive inorganic compounds. Eductor venturis are sometimes used prior to the packed tower for high concentration streams and/or proper hydrolysis before entering the packed scrubber. Scrubbing solutions utilizing alkaline chemicals (NaOH) alone or with the addition of a oxidizing and reducing agents can generally achieve high removal efficiencies.

The most prevalent oxides of nitrogen are nitrogen oxide (NO) and nitrogen dioxide (NO₂). NO is colorless but NO₂ emissions can create a visible orange-brown plume. Removal of these compounds can be accomplished using packed scrubbers with chemical addition. The number of scrubbing stages required will vary depending on the ratio of NO to NO₂ in an exhaust stream. Aqueous chemical and metal finishing operations typically produce a higher concentration of NO₂ versus combustion processes that normally have higher NO levels. In many cases, removal of NO₂ and the resulting visible plume can be accomplished in a single scrubber. For high efficiency NO removal, multiple scrubbing stages are required.

The solution for solving particulate emissions can vary greatly based on the size and characteristics of the specific particulate. Dusts are generally defined as larger particulate greater than 1 micron in aerodynamic diameter and are fairly easy to collect. Fumes are solid particles with diameters below 1 micron (submicron) requiring more energy intensive equipment for high efficiency removal. Current EPA regulations are aimed at PM10, which is defined as particulate matter 10 microns in size and smaller. Future regulations will be targeting PM2.5 or particulate matter 2.5 microns and smaller.

Large particulate is typically formed from mechanical operations such as mixing and blending or low temperature drying. Finer particulate normally results from specific chemical reactions or higher temperature thermal processes. High temperature thermal destruction creates the finest particulate. Incineration of different hazardous wastes can produce submicron particles containing various metal oxides, salts, and silicon dioxide.

Larger particulate (>10 micron) can be removed with eductor venturis, low pressure drop venturis, or in some cases, sieve tray towers. Finer particulate (1-10 microns) can be effectively removed with low to medium pressure drop venturis and fluidized bed scrubbers. Submicron particulate in the 0.5 to 1.0 micron range can be effectively removed by high-pressure drop venturis, fluidized bed scrubbers, ionizing wet scrubbers (IWS), or possibly a combination of equipment. The proper equipment choice depends on the specific application. Submicron particulate below 0.5 microns is the most difficult to remove and typically requires an ionizing wet scrubber (IWS) due to its low energy usage relative to other types of scrubbers.

Boron Trichloride, Boron Trifluoride, Dichlorosilane, Germanium Tetrachloride, Phosphorous Oxychloride, Silicon Tetrachloride, Titanium Tetrachloride, and Tungsten Hexafluoride.

Halides of this type typically decompose readily upon contact with water forming HCl gas and mist plus an insoluble oxide particulate (except for Phosphorous Oxychloride). The oxide particulate can be very small in size requiring venturi scrubbers or ionizing wet scrubbers (IWS) for effective removal. Most oxides formed can be readily dispersed in water as in the case of B₂O₃ and TiO₂. Some, such as Silica Tetrachloride, Silica Tetrafluoride, Germanium Tetrachloride, and Chlorosilanes, can form a more gelatinous substrate, but this can be controlled by maintaining proper scrubber solution pH levels.

Hydrogen sulfide, Methyl Mercaptan, Ethyl Mercaptan, Dimethyl Sulfide, and Dimethyl Disulfide.

Sulfides are emitted from a variety of industrial processes including wastewater treatment and petrochemical processing. They are highly odorous even at very low concentrations. Wet scrubbing using packed towers has proven to be the most reliable means of removal and odor reduction handling wide variations in inlet concentration. Alkaline (NaOH) scrubbing can achieve high efficiency removal of hydrogen sulfide (H₂S), but the solution pH must be maintained at a high level to prevent release of the H₂S. In most cases, an oxidant such as NaOCl is added to prevent this from occurring. Highly volatile sulfides such as Dimethyl Sulfide (DMS) and Dimethyl Disulfide (DMDS) typically require two stages of scrubbing to insure removal below detectable odor thresholds.

Most volatile organic compounds, or VOCs, are not appreciably soluble in water or reactive with standard aqueous chemical solutions. This class covers a wide range of compounds including Toluene, Styrene, and many chlorinated solvents. Wet scrubbing is not considered an effective means for removal of these compounds and technologies such as thermal oxidation or carbon adsorption are required for effective removal. However, when halogenated compounds are oxidized, the resultant stream contains hydrogen chloride gas that needs to be removed using wet scrubbing technology.

Due to their low water solubility, VOCs can be desorbed or stripped from contaminated liquid solutions. Packed towers are an effective equipment choice for this application.