Feature Report
Odor · lssues and Solutions for Wastewater Treatment
Increasingly, wastewater treatment plants must address odors due to volatile contaminants. This guide provides an overview of odor causes and possibl_e remedial actions
chemical process industries (CPI).
In' many cases, including wastewa ter treatment plants ryvwTPs), paper mills, asphalt refineries and other op erations, large volumes of odor-caus ing compounds are generated, and residents of the areas surrounding the facility can be exposed.
In the past, these odors were less of an issue because most industrial facili ties tended to be located at consider-
FIGURE 1. Wastewater treatment plants can be sources of odor-caus
ing gases, such as hydrogen sulfide, ammonia, mercaptans and others
abledistances from large residential areas. In more recent times, however, there has been an increase in the number of industrial op erations that are located within or near small towns and even large cities. When local resi dents constantly smell these industrial pro cesses, unhappy neighborhoods are gener ally the result.
The collective voice of the public is now often strong, and has resulted in new regu lations regarding odors and odor control at industrial facilities. Many industries can no longer produce odors without facing the consequences of either constant com plaints, or fines from federal, state and local authoritiesIn addition to being a nuisance, these foul odors can pose a health and safety concern for the community, especially at high concentrations.
Furthermore, increases in population and associated industrial growth have driven an increase in the amount of waste requiring processing, and a subsequent increase in the odor problem.
This article provides information about common odor-causing compounds and about possible methods for controlling those odors, with a particular focus on WWTPs.
Odors and health hazards
In general, odors are subjective, so that a
particular type of odor may bother one per
son while having little or no impact on an other. The odor threshold is the minimum concentration of an odorous compound that is necessary to be detected by 50% of the population [1].
In principle, all chemicals can theoreti cally be toxic at high enough concentra tions. For instance, water is not normally considered toxic. However, drinking an ex cessive amount of water under certain cir cumstances can create a condition called hyponatremia [2]. A similar situation applies to the concentration of odorous gases. At low concentrations (parts per billion; ppb), some gases can be detected by most hu mans. However, those gases are not toxic
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Laura Haupert
OMI Industries
IN BRIEF ODORS AND HEALTH HAZARDS
ODOR REMEDIATION METHODS
APPROACHES
| Substance |
Odor threshold (ppm) |
Characteristic odor | TWAc (ppm) | IDUfd (ppm) |
| Ammoniaa | 46.8 | Pungent | 25 | 300 |
| Benzeneb | 37 | Solvent | 1 | 500 |
| Ethyl mercaptana | 0.001 | Earthy, sulfide | 0.5 | 500 |
| Hydrogen sulfidea | 0.00047 | Rotten egg | 10 | 100 |
| Methyl mercaptana | 0.0021 | Sulfide, pungent | 0.5 | 150 |
| Styrene" | 1.9 | Aromatic | 50 | 700 |
| Toluenee | 2.9 | Sour, solvent | 10 | 500 |
| Trichloroethyleneb | 21 | Solvent | 25 | 1,000 |
| a.http://dx.doi.org/10.1080/00022470.1969.10466465 b.https://cfpub.epa.gov/ncea/risk/recordisplay.cfrn?deid=4061O&CFID=79236647&CFTOKEN=90216829 c.https:/lwww.osha.gov/dsg/annotated-pels/tablez-2.html d.https://www.cdc.gov/niosh/idlh/longdev/oldfiles/idlhabb3.html e.http$:/lwww.epa.gov/sites/production/files/2016-09/documentsltoluene.pdf | ||||
until the concentrations reach low parts-per-million (ppm) levels. Unfor tunately, many industrial processes produce odorous gases at concen trations that are both a nuisance and that can present real health hazards. Ammonia, hydrogen sulfide, mer captans and volatile organic com pounds (VOCs) are examples of odorous gases that may be pro.: duced in the processes of several types of industrial facilities, including (but not limited to) WVVTPs, asphalt refineries, hot mix plants, landfills,
farms and others.
Table 1 lists several common odorous gases associated with these industry sectors. The table in cludes odor thresholds, characteris tic odor, time-weight average (TWA) and immediately dangerous to life or health (IDLH) levels for each gas. The TWA represents the concentra tion of a given chemical that will not cause adverse effects when the ex posure reaches eight hours per day. In most cases, the TWA is higher than the odor threshold; meaning that you can smell the odor before there is any potential hazard. The IDLH is the level at which a chemical can cause death or permanent ad verse health effects. Notice that the
IDLH is a higher concentration than the odor threshold for most of these chemicals. However, in the case of hydrogen sulfide, the human nose can become desensitized to the gas at concentrations lower than the IDLH. This means that humans may not smell the hydrogen sulfide at lev els where rt can cause severe harm or death.
The following descriptions cover several of the most common odor ous gases found in the CPl. Ammonia. Ammonia is a colorless gas that is highly irritating with a pungent odor at room temperatures. It is soluble in water, corrosive, and has alkaline properties. It can cause hoarseness, violent coughing, pain ful breathing, impaired vision, dys pnea and cyanosis when exposed to high levels [3].
Hydrogen sulfide. Hydrogen sulfide
is a highly flammable, extremely haz ardous gas that has a characteristic "rotten egg" smell. Hydrogen sulfide has an odor threshold of 0.00047 ppm, meaning that 50% of a human panel can smell it at that low of a concentration [4]. At a concentration of around 150 ppm, olfactory nerves can become paralyzed, resulting in humans having the sense that the
smell disappears. Hydrogen sulfide is lethal at 70G-1,000 ppm within minutes [5-7]. A complication with hydrogen sulfide is that your sense of smell disappears at only 150 ppm. Therefore, one could be· exposed to higher levels of hydrogen sulfide and not know that it is present. It is sol uble in water, corrosive and toxic to both humans and the environment. Mercaptans . Mercaptans, such as ethyl mercaptan, are compounds that contain a carbon-bonded sulfhy dryl group (-G-SH or R-SH, where R represents an alkane, alkene or other carbon-containing group of atoms). Mercaptans are flammable, have strong odors, and can be detected by the human nose at concentrations in the parts per billion range. Ethyl mercaptan is intentionally added to propane at low concentrations in order to help warn of a potential gas leak. At higher concentrations, ac cording to OSHA (U.S. Occupational Safety and Heatth Administration; Washington, D.C.; www.osha.gov) regulations, ethyl mercaptan is clas sified as an acute toxic material for oral ingestion and inhalation, as well as for skin sensitization [8].
VOCs. VOCs, such as styrene, tolu
ene and trichloroethylene, are or ganic chemicals that have high vapor pressures at room temperature. Their high vapor pressure allows them to either evaporate or sublime, respec tively, from a liquid or solid to a gas. Hazardous VOCs present safety con cerns, because they can be flamma ble, can cause cancer and chronic health effects, and may be lethal at certain concentrations. For example, toluene is a VOC found in many in dustrial applications, including both wastewater treatment plants and asphalt refineries. It has a high odor threshold of 2.9 ppm, which indi cates that the presence of the odor may not be a good judge of safety for this chemical. Toluene poses a safety concern, as it presents acute toxicity hazards for oral ingestion. It is also a skin and eye irritant, and it may cause damage to organs through prolonged or repeated exposure. With an odor similar to paint thinner, it is an aromatic hydrocarbon, and is insoluble in water [9].
40 CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM SEPTEMBER 2017
FIGURE 2. Odorcremoval methods, such as the vapor-line system shown here, involve treating the air that contains the odor-causing species
causing compounds. Industrial sources often cause concern because the waste can contain a number of toxic chemicals. Other odor-causing chemicals do not di rectly come into a VVWTP from house holds or industrial sources. They are generated by anaero bic biological activity that consume the or ganic material found in the wastewater.
leased from the wastewater [10]. In addition, chemical reagents used at WWTP for treatment may be odor ous and can produce odors during the treatment process. Common emissions from WWTP include hy drogen sulfide, mercaptans, VOC, ammonia and some nitrogen con taining compounds.
At WWTPs, compounds are only
noticeably odorous if the compound is volatilized into the air. For exam ple, hydrogen sulfide dissolved into wastewater has little odor. How ever, turbulence in the wastewater, increased temperature, solubility changes and pH changes can all cause compounds, such as hydro
Odors at WWTPs
Wastewater treatment plants
0/VWTPs) generate odors during many of their normal processes, in cluding collection, treatment and disposal (Figure 1). Incoming waste includes both household and indus trial sources, which both contain a number of different types of odor-
common odorous gases found in WWTP is hydrogen sulfide (H2S). It is produced in wastewater by the re
duction of sulfate to hydrogen sulfide gas by bacteria. At a pH above nine, the hydrogen sulfide is in the form of hydrosulfide ion (HS-), which does not have an odor. However, at pH below nine, hydrogen sulfide is re-
gen sulfide, to volatilize to the gas phase, thereby producing a higher level of odor. In addition, the sur rounding community often considers the odors coming from WWTPs as being problematic. The WWTPs have to evaluate the source of the odors. Many studies have determined the main sources of odors at WWTPs
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to be the primary settlers, sludge
digestions tanks, sludge thickening, dewatering areas, raw wastewater, centrifugation and disposal systems [11]. Overall, two of the main contrib utors to odors are the primary treat ments and sludge-handling areas. At this point, WWTPs have to consider what type of odor control to use.
Odor remediation methods WWTPs can use a variety of meth ods to remove odors. The main odor control categories into which these methods fall are the following: oper ation control methods, liquid-phase addition and vapor-phase methods (Figure 2).
Operation control methods. Op
eration control methods involve changing a process or operation in order to reduce the odors. Operators can help prevent offensive odors by maintaining the correct dissolved oxygen concentration, for example, or by preventing excess sludge from degrading, preventing overloading and many other simple maintenance techniques. However, operators are often limited in what changes they can make and may lack the resources to make the necessary changes. If operators ·are unable to make changes to these processes, or the changes do not fix the odor problem, then they may look to other methods of odor control.
Liquid-phase addition. Liquid
phase addition involves adding a chemical to the wastewater in order to control the odors. The chemical addition may either prevent odor causing chemicals from forming or may react with the odorous com pounds to produce a safer, non odor-causing species. Engineers may be able to take advantage of other benefits with this technique; namely, corrosion control. For in stance, hydrogen sulfide will react with many of the liquid-phase chemi cals, resulting in hydrogen sulfide not being released into the air stream. Therefore, hydrogen sulfide will not corrode the WWTP equipment.
Liquid-phase chemicals include,
but are not limited to, oxidizers, pH modifiers and bactericides. Oxidizers typically react with hydrogen sulfide and other sulfur compounds. Bac-
tericides work by either inactivating
or killing anaerobic bacteria. The pH modifiers tend to prevent the hydro gen sulfide from volatizing into the air. The most common chemicals added to the wastewater are iron salts, ozone, nitrates, chlorine and hydrogen peroxide.
Vapor-phase approaches
Vapor-phase methods involve the treatment of the air or vapor that con tains the odorous compounds. There are many different vapor-phase tech nologies that WWTPs utilize, includ ing, but not limited to, scrubbers, bio filtration, activated carbon, masking agents and natural odor control using plant oils as the active ingredients to neutralize the odors. Below is a brief summary of the basic way each of these methods works.
Wet scrubbers. Wet scrubbers
pump the contaminated air into an aqueous solution before it goes to the ambient air. The odorous com pounds go into the liquid, and then the odor-causing chemicals react with the solution. Wet scrubbers typically use sodium hypochlorite, potassium permanganate, hydrogen peroxide or sodium hydroxide in the aqueous solution. If hydrogen sulfide is the main odor ·concern, sodium hydroxide is often used. However, if mercaptans or ammonia are also present, then a multi-stage scrubber is used, with both sodium hydroxide and sodium hypochlorite utilized in separate stages.
Biofiltration. Biofiltration uses soil,
compost or some other material, such as a substrate for baCteriologi cal population. The microbes remove odors from the air through the media. In order for the microbes to interact with the odorous compounds, there must be a large residence time in the media. By lowering the velocity of the air going through the media, longer residence times result. Biofil tration works on odors that are both biodegradable and water-soluble, including hydrogen sulfide and other sulfur-containing compounds. They do not work effectively on chemicals containing nitrogen.
Activated carbon. Activated car
bon works by having the contami
nated air .stream pass through this
adsorbent material. Activated car
bon is porous, with a large surfac area, which allows it to adsorb the odorous chemicals in the air stream. While activated carbon works wel on sulfur-containing compounds, it is not as effective at treating nitrogen based compounds [12].
Masking agents. Masking agents
are chemicals that are sprayed into the air to cover up the odors pro duced at the WWTP. Masking agents use fragrances to cover up the odors. They often work by adhering to the outside of the odor molecule. After a short period, the fragrance and the odor separate, leaving the odor be hind. While masking agents give an immediate cover to the odor, the odor will eventually return. As a result, not everyone in nearby communities will be happy with the overall outcome. Natural solutions. Finally, natura odor control solutions can fully neu tralize odors, and are atomized or vaporized into the air. These odor control products contain natural plant oils, food-grade surfactant and water, and are both safe for the environment and the local community. When the natural odor control product is atom ized into the air, small droplets are produced that cover a large surface area and attract odorous gas mol ecules through elec::trostatic charges. The electrostatic charge also facili tates the attraction of malodor mol ecules to the droplet surface whereby they absorb into the droplet. Once the malodor is in the droplet, odor neutralization occurs. The malodor then either naturally biodegrades in the droplet or an acid/base reaction takes place, producing a non-hazard ous organic-satt and water.
Natural odor-control solutions often
use plant oils. These products have the capability of neutralizing a broad spectrum of odorous chemicals, in cluding hydrogen sulfide, mercap tans, VOCs, ammonia and amines. These solutions can be atomized irito the air at WWTPs via air atomization systems, hollow bladed fans or by va por-phase ·systems. The air atomiza tion systems use water and nozzles to atomize the product into the air. The nozzle system can be suited to most applications at a WWTP. ·
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